The Mechanisms of Reactions Influencing Atmospheric Ozone [1 ed.] 0190233028, 9780190233020

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THE MECHANISMS OF R E AC T I O N S I N F L U E N C I N G AT M O S P H E R I C   O Z O N E

THE MECHANISMS OF R E AC T I O N S I N F L U E N C I N G AT M O S P H E R I C   O Z O N E JAC K G.   CA LV E R T, J O H N J.   O R L A N D O, WILLIAM R. STOCKWELL, AND T I M OT H Y J.   WA L L I N G T O N

1

1 Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trademark of Oxford University Press in the UK and certain other countries. Published in the United States of America by Oxford University Press 198 Madison Avenue, New York, NY 10016

© Oxford University Press 2015 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, by license, or under terms agreed with the appropriate reproduction rights organization. Inquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above. You must not circulate this work in any other form and you must impose this same condition on any acquirer. Library of Congress Cataloging-in-Publication Data Calvert, Jack G. ( Jack George), 1923– author. The mechanisms of reactions influencing atmospheric ozone / Jack G. Calvert, John J. Orlando, William R. Stockwell, and Timothy J. Wallington. pages cm Includes bibliographical references and index. ISBN 978–0–19–023302–0 (alk. paper) 1. Atmospheric ozone. 2. Hydroxyl group—Reactivity. 3. Atmospheric chemistry. I. Orlando, John J. ( John Joseph) 1960– author. II. Stockwell, William R., author. III. Wallington, Timothy J., author. IV. Title. QC879.7.C35 2015 551.51’12—dc23 2014030522

9 8 7 6 5 4 3 2 1 Printed in the United States of America on acid-free paper

CONTENTS

Acknowledgments  About the Authors  I. Ozone in the Atmosphere  I-A. Introduction  I-B. Historical Perspective of Atmospheric Ozone  I-C. Properties of the Earth’s Atmosphere  I-C-1. Chemical and Physical Properties  I-C-2. Meteorology and Its Effects on Atmospheric Ozone  I-D. Summary of Ambient Measurements of Tropospheric Ozone  I-D-1. Measurements of Surface Ozone in Background and Remote Locations  I-D-2. Measurements of Surface Ozone in Polluted Urban Areas  I-E. Summary of Measurements of Stratospheric Ozone 

I-H-1. Contribution of Ozone to the Natural Greenhouse Effect 

54

1

I-H-2. Contribution of Ozone to Radiative Forcing of Climate Change 

57

1

I-H-3. Impact of Climate Change on Ozone 

58

xiii xv

3 7 7 14 16 16 25 31

I-F. Ozone and Its Mechanisms of Formation and Destruction in the Atmosphere 

33

I-F-1. Ozone Formation and Destruction in the Unpolluted Stratosphere 

33

I-F-2. Stratospheric Ozone Depletion: The Role of Halocarbons 

38

I-F-2.1. Stratospheric Ozone Depletion and the Montreal Protocol 

40

I-F-3. Ozone Formation in the Troposphere 

44

I-F-4. Ozone Destruction in the Troposphere  47 I-G. Major Sources of the Atmospheric Trace Gases (NOx and VOCs) 

48

I-H. Ozone and Climate  

54

I-I. About the Material Covered in This Book 

II. Mechanisms of Ozone Reactions in the Troposphere  II-A. Introduction  II-B. Ozone Photodecomposition and the Mechanism of HO Radical Formation  II-B-1. Absorption Cross Sections of Ozone  II-B-2. Quantum Yields of O(1D) Formation in Ozone Photodecomposition  II-B-3. Estimated Photolysis Frequencies j[O(1D)] for Ozone Photodecomposition  II-B-4. Comparison of Measured and Calculated j[O(1D)] Values  II-C. Mechanisms of the Ozone Reactions with Organic Compounds  II-C-1. Ozone Reactions with the Acyclic Mono-Alkenes  II-C-2. Ozone Reactions with the Cyclic Alkenes  II-C-3. Ozone Reactions with the Dienes  II-D. Mechanisms of O3 Reactions with Unsaturated Oxygenates 

59 61 61 61 62 63 65 65 67 67 84 86 87

II-D-1. Ozone Reactions with the Unsaturated Alcohols 

87

II-D-2. Ozone Reactions with the Unsaturated Ethers 

88

vi

Contents II-D-3. Ozone Reactions with the Unsaturated Aldehydes 

93

II-D-4. Ozone Reactions with the Unsaturated Ketones 

94

II-D-5. Ozone Reactions with the Unsaturated Organic Acids 

95

II-D-6. Ozone Reactions with the Unsaturated Esters 

96

II-E. The Mechanisms of Reactions of the Criegee Intermediates 

96

II-E-1. The Fragmentation Reactions of the Criegee Intermediates 

96

II-E-1.1. Suggested Fragmentation Modes of the CH2OO‡ Intermediate 

97

II-E-1.2. Suggested Fragmentation Modes of the CH3CHOO‡ Intermediate 

97

II-E-1.3. Suggested Fragmentation Modes of the (CH3)2COO‡ Intermediate  97 II-E-2. The Mechanism of HO Radical Formation in the Reactions of Ozone with the Alkenes  II-E-3. Reactions of the Stabilized Criegee Intermediates 

98 99

II-E-3.1. The CH2OO and CH3CHOO Reactions with Water Vapor  107 II-E-3.2. Reaction of Criegee Intermediates with Other Trace Gases 

108

II-F. Use of SARs in Ozone-Alkene Reactions 

108

II-F-1. Estimation of Rate Coefficients for the Reactions of Ozone with the Alkenes 

108

II-F-2. An SAR Method of Estimating the Yield of HO Radical Formed in the Alkene-Ozone Reactions 

III.The Oxides of Nitrogen: Their Relation to Tropospheric Ozone  III-A. Overview of Tropospheric “Odd Nitrogen” Chemistry  III-B. Reactions of NO with O3 and with HO2  III-C. The Photodecomposition of NO2 and j(NO2) Determination  III-D. Evaluation of the Mechanisms of the Major Reactions of NO2 with Trace Gases in the Troposphere  III-D-1. Reaction of NO2 with HO: HONO2 Formation 

110 114 114 116 117

III-D-2. Reactions of NO2 with HO2, RO2, and RC(O)O2 Radicals: Reversible Peroxynitrate and Peroxyacetyl nitrate Formation 

121

III-D-2.1. Loss Processes for HONO2, Organic Nitrates, Peroxynitrates, and Peroxyacyl Nitrates : Photolysis, Reaction with HO, and Deposition 

121

III-D-3. The Heterogeneous Formation of HONO and Its Subsequent Losses 

125

III-D-4. The Reaction of NO2 with O3: The Formation of NO3 Radical 

126

III-D-5. NO2 + NO3 Reactions: N2O5 Formation and Dissociation 

126

III-D-5.1. Heterogeneous Reactions of N2O5 with H2O and Halide Ions 

126

III-E. Evaluation of the Mechanisms of NO3 Radical Reactions 

127

III-E-1. The Photodecomposition of NO3 [j(NO3)] 

127

III-E-2. Mechanism of the Reaction of NO3 with NO 

130

III-E-3. Rates and Mechanisms for Reactions of NO3 with the Alkanes 

130

III-E-4. Rates and Mechanisms for Reactions of NO3 with Haloalkanes 

131

III-E-5. Rates and Mechanisms for Reactions of NO3 with Alkenes, Haloalkenes, and Alkynes 

133

III-E-5.1. Rates and Mechanisms for Reactions of NO3 with Alkenes 

133

III-E-5.2. Rates and Mechanisms for Reactions of NO3 with Haloalkenes 

142

III-E-5.3. Rates and Mechanisms for Reactions of NO3 with Alkynes 

144

III-E-6. Rates and Mechanisms for the Reactions of NO3 with Aromatic Hydrocarbons 

146

III-E-7. Rates and Mechanisms for Reactions of NO3 with the Oxygenates 

146

III-E-7.1. Rates and Mechanisms for the Reactions of NO3 with Alcohols 

148

120

III-E-7.2. Rates and Mechanisms for the Reactions of NO3 with Ethers 

151

120

III-E-7.3. Rates and Mechanisms for the Reactions of NO3 with Aldehydes 

154

Contents III-E-7.4. Rates and Mechanisms for the Reactions of NO3 with Ketones 

IV-D-3. HO Reactions with Dienes  157

III-E-7.5. Rates and Mechanisms for the Reactions of NO3 with Organic Acids  159 III-E-7.6. Rates and Mechanisms for the Reactions of NO3 with Esters  III-E-7.7. Rates and Mechanisms for the Reactions of NO3 with N-Atom-Containing Oxygenates  III-E-8. Use of SARs to Estimate NO3 Rate Coefficients  III-E-8.1. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with the Alkanes and Haloalkanes  III-E-8.2. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with Saturated, Oxygenated Compounds (Aldehydes, Alcohols, Ethers, Ketones, Esters, and Saturated Multifunctional Species)  III-E-8.3. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with the Alkenes, Haloalkenes, and Alkynes  III-E-8.4. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with Unsaturated Oxygenated Compounds (Alcohols and Ethers) 

160

160 161

162

IV-A. Introduction 

206 208

IV-E-1. Reaction of HO with Acetylene (Ethyne, HC≡CH) 

208

IV-E-2. Reaction of HO with Propyne (Methyl Acetylene, CH3C≡CH) 

208

IV-E-3. Reaction of HO with 1-Butyne (HC≡CCH2CH3) and 2-Butyne (CH3C≡CCH3) 

208

IV-E-4. The Mechanism of HO Addition Reaction with Alkynes 

208

IV-F. Mechanisms of the HO Reactions with Aromatic Hydrocarbons 

209

IV-G. Mechanisms of the HO Reactions with the Alcohols 

211

IV-G-1. HO Reactions with Acyclic Alcohols  211 163

166

167

III-E-8.5. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with Unsaturated Carbonyl Compound (Aldehydes, Ketones, Esters)  169

IV. The Hydroxyl Radical and Its Role in Ozone Formation 

IV-E. Mechanisms of the HO Radical Reactions with the Alkynes  

vii

172 172

IV-A-1. Measurement of HO Radical Concentrations 

173

IV-B. Mechanisms of HO Radical Reactions with Alkanes 

178

IV-C. Mechanisms of HO Radical Reactions with Haloalkanes 

185

IV-D. Mechanisms of HO Radical Reactions with Alkenes 

191

IV-G-2. HO Reaction with Diols 

227

IV-G-3. HO Reactions with Saturated Alcohols 

228

IV-G-4. HO Reactions with Aromatic Alcohols 

228

IV-G-5. HO Reactions with Unsaturated Alcohols 

228

IV-G-6. HO Reactions with Halogen-Atom-Substituted Alcohols 

229

IV-H. Mechanisms of Reactions of HO with Ethers  

230

IV-H-1. HO Reactions with Acyclic Ethers 

230

IV-H-2. HO Reactions with Difunctional Ethers 

251

IV-H-3. HO Reactions with Vinyl Ethers 

251

IV-H-4. HO Reactions with Cyclic Polyethers  251 IV-H-5. HO Radical Reactions with Halogen-Atom-Substituted Ethers 

251

IV-I. Mechanisms of Reaction of HO Radical with the Aldehydes 

253

IV-I-1. HO Radical Reactions with Acyclic Aldehydes 

263

IV-I-2. HO Radical Reactions with Unsaturated Aldehydes 

263

IV-D-1. HO Reactions with Mono-Alkenes  191

IV-I-3. HO Radical Reactions with Aromatic Aldehydes 

265

IV-D-2. Mechanism of HO Addition to Alkenes 

IV-I-4. HO Radical Reactions with Halogen-Atom-Substituted Aldehydes 

265

205

viii

Contents

IV-J. Mechanisms of HO Radical Reactions with Ketones  

265

IV-J-1. HO Radical Reactions with Acyclic Ketones 

266

IV-J-2. HO Radical Reactions with Hydroxyketones 

276

IV-J-3. HO Radical Reactions with Unsaturated Ketones 

276

IV-J-4. HO Radical Reactions with Halogen-Atom-Substituted Ketones 

276

IV-K. Mechanisms of Reaction of HO Radical with Organic Acids and Acid Anhydrides 

276

IV-L. Mechanisms of Reaction of HO Radicals with Esters 

277

IV-L-1. HO Radical Reactions with Acyclic Esters 

V-H-1. Introduction 

336

V-H-2. Kinetics of RO2 Reactions with RO2 and RO2 with R′O2 

337

V-H-3. Products of the RO2 Reaction with RO2 and RO2 with R′O2 

340

V-I. RO2 Unimolecular Reactions 

343

V-I-1. RO2 → QOOH Isomerization 

343

V-I-2. Decomposition Reactions of the α-Hydroxy-Peroxy Radicals 

346

VI. Mechanisms of Reactions of the RO Radicals 

347

VI-A. Introduction 

347

VI-B. Modes of Alkoxy Radical Reactions in the Atmosphere 

348

277

VI-B-1. Alkoxy Radical Reactions with O2 

348

279

VI-B-2. Unimolecular Decomposition Reactions of the Alkoxy Radicals 

349

279

VI-B-2.1. Unimolecular Decomposition of RR′R″CO• Radicals 

349

280

VI-B-2.2. Unimolecular Decomposition of Acyloxy Radicals [RC(O)O•] 

352

IV-N. Summary of HO Radical Reactions with Organic Compounds 

296

VI-B-2.3. Unimolecular Isomerization Reactions of the Alkoxy Radicals 

352

IV-O. Structure-Activity-Relations (SARs)for Estimating HO Rate Coefficients 

296

VI-B-2.4. Unimolecular Decomposition of RCHClO• Radicals 

357

VI-B-2.5. Unimolecular Decomposition of RC(O)OCH(O•)R′ Radicals 

357

VI-B-2.6. Chemical Activation in the Chemistry of Alkoxy Radicals 

358

IV-L-2. HO Radical Reactions with Unsaturated Esters  IV-L-3. HO Radical Reactions with Halogen-Atom-Substituted Esters  IV-M. Mechanisms of Reactions of HO Radical with N-containing Oxygenates 

V. Mechanisms of Reactions of HO2 and RO2 Radicals  V-A. Introduction  V-B. Reactions of HO2 with NO and RO2 with NO  V-B-1. Kinetics of the HO2 + NO and the RO2 + NO Reactions   V-B-2. Products and Mechanisms of the HO2 + NO and RO2 + NO Reactions   V-C. Reactions of HO2 + NO2 and RO2 + NO2  V-D. Reactions of HO2 + HO2 and HO2 + RO2  V-E. Reactions of HO2 with NO3 and RO2 with NO3  V-F. Reactions of HO2 with RO2 and HO2 with ClO, BrO, IO 

315 315 317

VI-C. Reaction Rate Coefficients and Atmospheric Fate of Selected Alkoxy Radicals 

360

317

VI-C-1. Reaction of the Methoxy Radical (CH3O•) 

360

320

VI-C-2. Reaction of the Ethoxy Radical (C2H5O•) 

361

326

VI-C-3. Reactions of the 1-Propoxy Radical (n-Propoxy Radical, n-C3H7O•) 

361

331

VI-C-4. Reactions of the 2-Propoxy Radical [iso-Propoxy Radical, CH3CH(O•)CH3] 

362

VI-C-5. Reactions of the 1-Butoxy Radical (n-Butoxy Radical CH3CH2CH2CH2O•) 

363

VI-C-6. Reactions of the 2-Butoxy Radical [sec-Butoxy Radical, CH3CH(O•)CH2CH3] 

364

324

332

V-G. Reactions of HO2 with O3 and RO2 with O3  

335

V-H. Reactions of RO2 with RO2 and RO2 with R′O2 

336

VI-C-7. Reactions of the 2-Methyl-2-Propoxy Radical [tert-Butoxy Radical, (CH3)3CO•]  VI-C-8. Reactions of the 2-Methyl-1-Propoxy Radical [iso-Butoxy Radical, (CH3)2CHCH2O•)] 

365

365

VI-C-9. Reactions of the 1-Pentoxy Radical (n-Pentoxy Radical, CH3CH2CH2CH2CH2O•) 

366

VI-C-10. Reactions of the 2-Pentoxy Radical [CH3CH2CH2CH(O•)CH3] 

366

VI-C-11. Reactions of the 3-Pentoxy Radical [CH3CH2CH(O•)CH2CH3] 

367

VI-C-12. Reactions of the 2,2-Dimethyl-1-Propoxy Radical [Neopentoxy Radical, (CH3)3CCH2O•]  VI-C-13. Reactions of the 2-Methyl-2-Butoxy Radical [CH3CH2C(O•)(CH3)2]  VI-C-14. Reactions of the 2-Methoxy-1-Butoxy Radical [CH3CH2CH(CH3)CH2O•] 

367

368

368

VI-C-15. Reactions of the 2-Hexoxy Radical [CH3CH2CH2CH2CH(O•)CH3] 

368

VI-C-16. Reactions of the 3-Hexoxy Radical [CH3CH2CH(O•)CH2CH2CH3] 

369

VI-C-17. Reactions of the 1-Hexoxy Radical [n-Hexoxy Radial, CH3CH2CH2CH2CH2CH2O•] 

369

VI-C-18. Reactions of the 2-Methyl-2-Pentoxy Radical [CH3CH2CH2C(O•)(CH3)2] 

369

VI-C-19. Reactions of the 4-Methyl-1-Pentoxy Radical [(CH3)2CHCH2CH2CH2O•] 

370

VI-C-20. Reactions of the 5-Methyl-2-Hexoxy Radical [(CH3)2CHCH2CH2CH(O•)CH3] 

370

VI-C-21. Reactions of the 2-Methyl-2-Hexoxy Radical [CH3CH2CH2CH2C(O•)(CH3)2] 

370

Contents

ix

VI-C-24. Reactions of the Cyclohexoxy Radical 

371

VI-C-25. Reactions of the 4-Methyl-Cyclohexoxy Radical 

371

VI-C-26. Reactions of Other Five- and Six-Carbon Alkoxy Radicals 

372

VI-C-27. Reactions of the 2-Hydoxy-Ethoxy Radical(HOCH2CH2O•)  372 VI-C-28. Reactions of the CH3OCH2O• Radical 

372

VI-C-29. Reactions of the CH3CH2OCH(O•)CH3 Radical 

373

VI-C-30. Reactions of the CH3C(O)CH2O• Radical 

374

VII. The Impact of Inorganic Trace Gases on Ozone in the Atmosphere  VII-A. Introduction  VII-B. Sources and Sinks of Atmospheric Carbon Dioxide (CO2) and Carbon Monoxide (CO)  VII-B-1. Levels of Atmospheric CO2 and the Global Carbon Cycle: A Brief Overview  VII-B-2. Atmospheric Budget and Chemistry of Carbon Monoxide   VII-B-2.1. Sources of CO to the Atmosphere  VII-B-2.2. Loss of CO via Reaction with HO: A Significant Source of HO2  VII-C. Tropospheric Chemistry of Inorganic Halogen Species  VII-C-1. Sources of Inorganic Halogens to the Troposphere  VII-C-2. Tropospheric Inorganic Halogen Cycles  VII-C-3. Reactions of Cl Atoms with the Hydrocarbons  VII-C-3.1. Reactions of Cl Atoms with Alkanes  VII-C-3.2. Reactions of Cl Atoms with Haloalkanes 

375 375

375 375 376 376

376 376 377 377 380 380

VI-C-22. Reactions of the 2,5-Dimethyl-2-Hexoxy Radical [(CH3)2CHCH2CH2C(O•)(CH3)2] 

370

VII-C-3.3. Reactions of Cl Atoms with Alkenes 

383

VI-C-23. Reaction of the Cyclopentoxy Radical 

371

VII-C-3.4. Reactions of Cl Atoms with Aromatic Hydrocarbons 

384

380

x

Contents VII-C-4. Reactions of Cl Atoms with Oxygenates  VII-C-4.1. Reactions of Cl Atoms with Alcohols 

385 385

VII-C-4.2. Reactions of Cl Atoms with Ethers 

388

VII-C-4.3. Reactions of Cl Atoms with Aldehydes 

392

VII-C-4.4. Reactions of Cl Atoms with Ketones 

392

VII-C-4.5. Reactions of Cl Atoms with Organic Acids 

394

VII-C-4.6. Reactions of Cl Atoms with Esters 

396

VII-C-4.7. Reactions of Cl Atoms with N-Atom-Containing Oxygenates 

399

VII-C-5. Structure-Activity Relations (SARs) for Reactions of Cl Atoms with Organic Species  VII-C-5.1. SARs in the Reaction of Cl Atoms with Alkanes  VII-C-5.2. SARs in the Reaction of Cl Atoms with Saturated Alcohols and Ethers 

VII-D. Atmospheric Reactions of Sulfur Compounds 

411 412

VII-D-1. Sources of Atmospheric Sulfur Compounds 

412

VII-D-2. Loss Processes for SO2 and SO3 

412

VII-D-3. Atmospheric Oxidation of Reduced Sulfur Species: H2S, CH3SH, CH3SCH3, CH3SSCH3 

414

VII-D-3.1. Atmospheric Oxidation of H2S 

414

VII-D-3.2. Atmospheric Oxidation of CH3SH (Methanethiol, Methyl Mercaptan) 

414

VII-D-3.3. Atmospheric Oxidation of CH3SCH3 (Dimethyl Sulfide, DMS) 

418

400

VII-D-3.4. Atmospheric Oxidation of CH3SSCH3 (Dimethyl Disulfide) 

420

400

VII-D-4. Atmospheric Chemistry of COS and CS2 

421

VII-D-4.1. Atmospheric Chemistry of Carbonyl Sulfide (OCS) 

421

VII-D-4.2. Atmospheric Chemistry of Carbonyl Disulfide (CS2) 

423

400

VII-C-5.3. SARs in Reactions of Cl Atoms with Saturated Carbonyl Compounds 

401

VII-C-5.4. SARs in Reaction of Cl Atoms with Unsaturated Compounds 

405

VII-C-6. Reactions of Br Atoms with Organic Compounds: An Overview 

VII-C-7. Reaction of Iodine Atoms with Organic Compounds 

405

VII-C-6.1. Reactions of Br Atoms with Alkanes 

406

VII-C-6.2. Reactions of Br Atoms with Alkenes and Alkynes 

406

VII-C-6.3. Reactions of Br Atoms with Aromatic Hydrocarbons 

407

VII-C-6.4. Reactions of Br Atoms with Alcohols and Ethers 

407

VII-C-6.5. Reactions of Br Atoms with Aldehydes 

407

VII-C-6.6. Reactions of Br Atoms with Ketones, Acids, and Esters 

409

VII-C-6.7. Polar Surface Ozone Depletion Events: A Special Case 

409

VIII. Photodecomposition of Light-Absorbing Oxygenates and Its Influence on Ozone Levels in the Atmosphere  VIII-A. Introduction  VIII-B. Photolysis Frequency (j-Value) of Molecules and Their Photochemical Lifetimes (1/j)  VIII-C. The Mechanisms of Photodecomposition of Aldehydes  VIII-C-1. Mechanism of Photodecomposition of Formaldehyde  VIII-C-2. Mechanisms of Photodecomposition of Higher Acyclic Aldehydes  VIII-C-3. Photodecomposition Pathways of Aldehydes Containing Additional Functional Groups  VIII-C-3.1. Photodecomposition of Unsaturated Aldehydes 

425 425

425 428 428

433

435 435

Contents VIII-C-3.2. Photodecompositions of HO- and Halogen-Atom-Substituted Aldehydes  VIII-C-3.3. Photodecomposition of Acyclic Dials (Dicarbonyls) 

Development, and an Overview of the Types of Mechanisms Available 

465

437

IX-A-2. Computer-Generated and Detailed Explicit Mechanisms 

468

438

IX-A-3. Condensed Mechanisms (EKMA, Carbon Bond, SAPRAC, RADM/RACM)  469

VIII-D. Mechanisms of Photodecomposition of Ketones in the Troposphere 

440

VIII-D-1. Mechanisms of Photodissociation of Acetone within the Troposphere 

442

VIII-D-2. Mechanisms of the Atmospheric Photodecomposition of Larger Acyclic Ketones 

446

VIII-D-2.1. RC(O)R′ + hν → RCO + R′: Process (I)  VIII-D-2.2. Ketone Photodissociation into Enol and Alkene Products: Process (II)  VIII-D-3. Photodecomposition of Ketene (CH2=C=O) 

VIII-D-6. Photodecomposition of Carbonyl Halides, Formyl Halides, and Acetyl Halides  VIII-E. The Photodecomposition of N-Atom-Containing Compounds 

IX-B-2. O3, PAN, HNO3, H2O2, HO, and HO2 Isopleths: VOC and NOx Sensitivity 

476

IX-B-3. Methods of Sensitivity and Process Analysis for Mechanism Assessment 

477

447

IX-B-4. NOx and VOC Sensitivity and Indicator Ratios for NOx and VOC 

480

448

IX-C. Computer Assessment of the Effects of [H2O], Temperature, and Clouds on O3 Generation 

481

IX-D. Simulation of the Effects of CH2O, CO, SO2, NO3, and N2O5 on O3 Generation 

484

446

449

451 451

VIII-E-2. Photodecomposition of the Peroxyacyl Nitrates (RC(O)OONO2) 

453

VIII-E-3. The Role of HONO in Ozone Generation in Urban Atmospheres 

454

IX.Chemical Mechanisms for Air Quality Modeling and Their Applications  IX-A. Development of Mechanisms for Air Quality Modeling  IX-A-1. Requirements for Mechanisms for Air Quality Modeling, Their

IX-E. Measures of Ozone Formation Reactivity  487 IX-E-1. Incremental Reactivity: MIR and MOIR 

452

VIII-F. Summary of Photochemical Processes in the Troposphere 

473

473

VIII-E-1. Photodecomposition of the Alkyl Nitrates 

VIII-E-4. Photodecomposition of Other N-Atom-Containing Oxygenates and Other Photochemically Active Trace Gases 

IX-B. Methods of Assessing the Influence of VOCs and NOx on Ozone Generation Using Computer Models  IX-B-1. Process Analysis and the Sensitivity of Ozone Formation to VOC and NOx 

VIII-D-4. Photodecomposition of Some HO- and Halogen-Atom-Substituted Ketones  448 VIII-D-5. Photodecomposition Pathways for Some Difunctional Ketones 

xi

455 457

487

IX-E-1.1. Maximum Ozone Incremental Reactivity: MOIR 

487

IX-E-1.2. Maximum Incremental Reactivity: MIR 

487

IX-E-1.3. Comparison of MOIR and MIR Values 

488

IX-E-2. Photochemical Ozone Creation Potential (POCP) 

489

IX-F. Modeling of Secondary Inorganic Aerosol and Organic Aerosol Formation 

490

IX-G. Potential Deficiencies of Atmospheric Chemistry Mechanisms as Implemented in Air Quality Models 

493

IX-H. Future Development of Atmospheric Chemical Mechanisms for Air Quality Modeling  496

465 465

References  Author Index Subject Index

497 549 575

AC K N OW L E D G M E N T S

The authors thank several agencies and their members for support of this research effort: Brent K.  Bailey, Executive Director of the Coordinating Research Council (3650 Mansell Road, Suite 140, Alpharetta, GA, 30022), and the members of the Coordinating Research Council’s Atmospheric Impacts Committee who gave the authors their support and monitored this study: Daniel C. Baker, Shell Global Solutions (US), Houston, TX; Robert Cassidy, Nissan Technical Center North America, Irvine, CA; Susan Collet (co-chair), Toyota Technical Center, Ann Arbor, MI; Alan M. Dunker (retired), General Motors R&D Center, Warren, MI; Coleman Jones, General Motors, Warren, MI; Rory S.  MacArthur (co-chair), Chevron Energy Technology Company, San Ramon, CA; Scott Mason, Phillips 66 Company, Bartlesville, OK; Mani Natarajan, Marathon Petroleum Co. LLC, Findlay, OH; David Patterson, Mitsubishi Motors R&D of America, Cypress, CA; Brigitte Postel, BP, Naperville, IL; Jenny Sigelko, Volkswagen Group of America, Auburn Hills, MI; Timothy J. Wallington, Ford Scientific Research Laboratory, Dearborn MI; Matt Watkins, ExxonMobil Research & Engineering, Paulsboro, NJ. The authors thank Dr.  Steven M.  Japar for his careful reading of the manuscript and his helpful suggestions. The authors acknowledge their research organizations and educational institutions for the support of their participation in this research:  Howard University, chemistry department and its program in atmospheric sciences, Washington D.C.; Oak Ridge National

Laboratories, Environmental Sciences Division, Oak Ridge, TN; National Center for Atmospheric Research (NCAR), Boulder, CO; Ford Motor Company, Systems Analytics and Environmental Sciences Department of the Research and Innovation Center, Dearborn, MI. The authors thank their many co-workers who provided them with important new information and other assistance:  Geert Moortgat and Hannelore Keller-Rudek, Max-Planck Institute, Mainz, Germany, for providing spectral data from the files of the Mainz Spectral Atlas; James Burkholder, Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, for providing original spectral data; William H.  Brune and S.  Chen for their permission to use their data from figures in their publication [Atmos. Environ., 44, 4166 (2010)]; J.  P. Burrows for providing his high-resolution spectral data for ozone. W.  R. Stockwell thanks Howard University graduate students Charlene V.  Lawson and Tamil Maldonado-Vega for help with library research and useful discussions. We thank Andre Prevot, David Parrish, Martin Steinbacher, and E Brunke for sharing their data and Kenshi Takahashi and Nadine Allemand for helpful discussions. The authors acknowledge the help of the library staffs in obtaining reprints of published data that were reviewed and referenced in this study:  National Center for Atmospheric Research, Boulder, CO; Oak Ridge National Laboratories, Oak Ridge, TN; the Ohio State University, Columbus, OH; Howard University, Washington, DC.

A B O U T T H E AU T H O R S

Jack G. Calvert studied at the University of California, Los Angeles, where he received his BS (Chemistry) in 1944 and his PhD in 1949 with Professor F. E. Blacet. He continued postdoctoral studies in photochemistry with Dr. E. W. R. Steacie of the National Research Council, Ottawa, Canada (1949–1950). He joined the faculty of the Ohio State University in 1950 and served as Kimberly Professor of Chemistry until 1981. He studied with Drs. R. A. Cox and S. A. Penkett at the Harwell Laboratory of the United Kingdom’s Atomic Energy Research Establishment (1978). He was appointed Senior Scientist at the National Center for Atmospheric Research, NCAR (Boulder, Colorado) in 1981, where he led the Atmospheric Kinetics and Photochemistry Group of the Atmospheric Chemistry Division until his retirement when he was appointed Emeritus Senior Scientist at NCAR (1993–present). He was a Visiting Scientist in the Environmental Sciences Division of the Oak Ridge National Laboratory (2002–2011). Professor Calvert has authored or co-authored approximately 300 publications in the scientific literature that relate to various aspects of photochemistry, reaction kinetics, and atmospheric chemistry. His honors include the Distinguished Research Award, the Ohio State University (1981); the Simon Guggenheim Memorial Fellowship (1978); the American Chemical Society, Columbus Section Award (1981); the American Chemical Society Award for Creative Advances in Environmental Science and Technology (1982); the Chambers Award of the Air Pollution Control Association (1986); and the Haagen-Smit Award (2011). He has been a member and chair of many national and international scientific committees organized to solve recognized problems in atmospheric chemistry. Calvert is co-author of the four preceding books in this series published by Oxford University Press and devoted to the mechanisms of atmospheric oxi¬dation of the alkenes, the aromatic hydrocarbons, the alkanes, and the oxygenates.

xvi

About the Authors

John J. Orlando was born in Timmins, Ontario, Canada, in 1960. He received both his BSc (1982) and PhD degrees (1987) in chemistry from McMaster University in Hamilton, Ontario, Canada, with Professors D. R. Smith and J. Reid. He then spent two years as a postdoctoral fellow at the National Oceanic and Atmospheric Administration’s Aeronomy Laboratory under the supervision of Drs. C. J. Howard and A R. Ravishankara, where he worked on problems related to the effects of halogens on stratospheric ozone. Dr. Orlando is presently a senior scientist in and deputy director of the Atmospheric Chemistry Division of the National Center for Atmospheric Research (NCAR where he has been employed since 1989 as an active member of the laboratory kinetics group. His research at NCAR has focused largely on the determination of the mechanisms of the atmospheric oxidation of carbon-containing species, on halogen chemistry of relevance to the stratosphere and troposphere, and on the atmospheric chemistry of nitrogen oxide species. He is author or co-author of more than 100 scientific publications related to atmospheric chemistry. Dr. Orlando was the recipient of the American Meteorological Society Special Award (2001), the Velux Foundation Visiting Professor Fellowship at the University of Copenhagen (2002), and the NCAR Special Achievement Award in 2003. Dr. Orlando was co-editor and co-author of the book Atmospheric Chemistry and Global Change (Oxford University Press, 1999). He is also co-author of two previous books in this series, Mechanisms of Atmospheric Oxidation of the Alkanes and The Mechanisms of Atmospheric Oxidation of the Oxygenates (Oxford University Press, 2008 and 2011, respectively).

William R. Stockwell received his BS in chemistry from Bowling Green State University (1975). He performed research with J. G. Calvert for his MS (1977) and PhD (1981) degrees in chemistry from The Ohio State University. He continued his research on atmospheric chemistry with J. G. Calvert as an NCAR Advanced Study Program postdoctoral fellow (1982–1983). Since 1984 he has held positions at NCAR, the Atmospheric Sciences Research Center of the State University of New York at Albany, the Forschungszentrum Jülich, the Fraunhofer Institute for Atmospheric Environmental Research, the National Oceanic and Atmospheric Administration, and the Desert Research Institute. He was appointed to the faculty of Howard University in 2005 where he is now an associate professor of chemistry and atmospheric science. He has authored or co-authored 78 peer-reviewed publications and more than 100 other publications. Dr. Stockwell’s research interests include the development of atmospheric chemistry mechanisms, air quality modeling, field measurements for model evaluation, and the effects of toxic agents on biologically important molecules. He has served on a number of international and national panels including the United Nations Environment Program. Dr. Stockwell is the editor for atmospheric chemistry of the Bulletin of the American Meteorological Society. His honors include the American Meteorological Society Editor’s Award (2009), a fellowship from the Oak Ridge Institute for Science and Education (2008–2009), the Editors’ Citation for Excellence in Refereeing from the American Geophysical Union (1998), and the Research Prize of the Association of the Friends and Sponsors of the Fraunhofer Institute for Atmospheric Environmental Research (1995). The Alfred P. Sloan Foundation recognized him for advancing underrepresented minority students in mathematics, science, and engineering (2008).

About the Authors

xvii

Timothy J. Wallington was born and educated in England. He received BA (1981), MA (1982), DPhil (1983), and DSc (2007) degrees from Corpus Christi College, Oxford University, where he studied with Professor R. P. Wayne and Dr. R. A. Cox. He has carried out extensive research on various aspects of atmospheric chemistry and the kinetics and mechanisms of many different transient atmospheric species. He carried out postgraduate research studies at the University of California, Los Angeles (1984–1986) with Professor J. N. Pitts and Dr. R. Atkinson. He was Guest Scientist at the US National Bureau of Standards (1986–1987) with Dr. M. J. Kurylo. He joined the research staff at the Ford Motor Company in 1987, where he is currently the senior technical leader in the Environmental Science in the Systems Analytics and Environmental Sciences Department. Dr. Wallington has studied the atmospheric chemistry of vehicle and manufacturing emissions and their contribution to local, regional, and global air pollution and global climate change. He is co-author of more than 400 peer-reviewed scientific publications dealing with various aspects of air pollution chemistry. He is the recipient of 16 Ford Research Publication Awards (1991–2013); the Ford Motor Company Technical Achievement Award (1995); the Henry Ford Technology Award (1996); a Humboldt Research Fellowship, Universität Wuppertal (1998–1999) with Professor K. H. Becker; the American Chemical Society Award in Industrial Chemistry in 2008; and a USEPA Montreal Protocol Award in 2011. He has an honorary doctorate in science from Copenhagen University (2006). Dr. Wallington was co-author on the four previous books on the mechanisms of atmospheric oxidation of the alkenes, aromatic hydrocarbons, alkanes, and oxygenates published by Oxford University Press.

I Ozone in the Atmosphere

I-A. INTRODUCTION The importance of ozone to life on Earth and to atmospheric chemistry cannot be overstated. Nucleic acids and other macromolecules essential to life absorb strongly in the ultraviolet (UV) and are damaged by UV radiation with wavelengths of less than approximately 300  nm. For proper functioning, such biological macromolecules need to be shielded from the full intensity of solar radiation. Molecular oxygen (O2) absorbs strongly and blocks solar radiation with wavelengths below 230–240 nm from reaching the Earth’s surface. However, oxygen is transparent at wavelengths above approximately 245 nm. Fortunately, absorption of UV radiation of wavelengths of less than 242 nm by molecular oxygen (O2) yields oxygen atoms that add to O2 to form ozone which has a very strong absorption band at 200–300  nm (see Figure I-A-1). Even though it is present in only trace amounts in the atmosphere, absorption by ozone effectively blocks harsh solar UV radiation from reaching the Earth’s surface. There is no other molecule in the atmosphere that provides protection from solar UV radiation in the 250–300 nm region. The development of the ozone layer is intimately connected to the development of life on Earth. Oxygen levels in the prebiotic atmosphere were less than 5  × 10−9 of the current level. Photosynthesis after the appearance of life on the planet more than

3.5 billion years ago led to increased oxygen levels in the atmosphere. By approximately 600 million years ago, the O2 concentration had exceeded 10% of the current level, and the corresponding layer of ozone was sufficient to offer an effective UV shield for the migration of life onto land (Wayne, 1991). Life on Earth as we know it would not have developed without the protection offered by the ozone layer, and, equally, the ozone layer would not have developed without life on Earth. In addition to its obviously important physical role in shielding biota from the damaging effects of harsh UV radiation, ozone plays an essential chemical role as a photolytic source for HO radicals.1 The driving force for most of the chemistry that occurs in the lower atmosphere is the formation of HO radicals via photolysis of ozone to form O(1D) atoms that react in part with water vapor: O3 + hν (λ < 320 nm) → O(1D) + O2(1Δg) O(1D) + H2O → 2 HO

(1) (2)

The flux of UV light, O3, and H2O vapor combine to give a potent source of HO radicals. HO radicals react with almost every molecule emitted into the atmosphere. The atmospheric lifetimes of most pollutants are determined by their reactivity toward HO radicals. As an example, HO radicals react with

2

the mechanisms of reactions influencing atmospheric ozone 10−17

Absorption cross section, cm2 molecule−1

Ozone

10−18

10−19

10−20

DNA Absorption Maxima Wavelength Range

10−21

10−22

O2

10−23

10−24 200

220

240

260

280

300

320

340

360

Wavelength, nm FIGURE I-A-1.

Absorption spectra of ozone and oxygen and range of significant absorption of sunlight by DNA.

NO2 to give nitric acid (HNO3), which is relatively unreactive within the atmosphere and is removed by wet and dry deposition: HO + NO2 + M → HNO3 + M where M is a third molecule (usually N2 or O2) that removes energy and promotes coupling of HO and NO2. The generation of HO radicals is the primary mechanism by which the atmosphere is cleansed of pollutants. Although ozone is a natural component of the lower atmosphere (troposphere), it is also an important component of urban and regional photochemical air pollution. Reactions involving reactive organic compounds and nitrogen oxides (NOx) emitted from human activities (e.g., vehicle exhaust, power stations, industrial and residential activities) and driven by sunlight lead to the formation of ozone and other oxidants that are characteristic of urban areas in many parts of the world. At elevated concentrations, it is well established that exposure to ozone has adverse impacts on human and ecosystem health. Recognition of these adverse impacts has led to strict regulation of the emissions of organic compounds and nitrogen oxides that, over the past few decades, have generally led to decreased levels of

ozone and other air pollution in major urban centers around the world. Although impressive progress has been made, hundreds of millions of people still live in areas that do not meet national and international air quality standards and guidelines for ozone levels. A relatively new area of interest is the contribution to climate change of increased levels of ozone in the lower atmosphere. There is clear evidence that the current background ozone levels at mid-latitudes in the Northern Hemisphere are substantially higher (perhaps a factor of 2) than in the preindustrial era. It is now recognized that, in terms of contribution to radiative forcing of climate change since the industrial era began in 1750, ozone is the third most significant gas behind carbon dioxide and methane. The physical structure of the atmosphere and meteorology are profoundly impacted by the presence of ozone. Absorption of solar UV by ozone in the stratosphere (20–50 km) provides a substantial fraction of the heating that is responsible for the existence of the stratosphere. The presence of ozone to a large degree dictates the structure of the atmosphere: the first 10–15 km (troposphere) is a rather turbulent region heated from below by the Earth’s surface and capped by a warmer layer of air at 15–50 km (stratosphere). The stratosphere is heated by the absorption of solar UV radiation by both oxygen and ozone.

Ozone in the Atmosphere It is abundantly clear that ozone is an immensely important trace component of the atmosphere. The aim of this book is to provide an overview of the chemical processes associated with the formation and loss of ozone in the atmosphere and of the importance of ozone in atmospheric chemistry. We focus most of our attention on processes related to tropospheric ozone. I-B. HISTORICAL PERSPECTIVE OF AT M O S P H E R I C   O Z O N E Interest in ozone and atmospheric chemistry has a rich and intertwined history (Rubin, 2001; Stolarski, 2001). Ozone was discovered by Christian Friedrich Schönbein in 1840 (Schönbein, 1840). Schönbein noted that the odor at the positive electrode during the electrolysis of water was similar to that produced in the arc through air between two electrodes, and he suggested that this odor was attributable to a new compound. Schönbein proposed the name ozone for the new compound after the Greek ozein, meaning “to smell.” Experiments demonstrating the formation of ozone on arcing electricity through pure oxygen showed that ozone was an allotrope of oxygen. The molecular formula for ozone (O3) was established by Soret (1863) by taking the ratio of the change in volumes when O3/O2 mixtures were either heated or exposed to turpentine and cinnamon oil. Heating leads to the decomposition of O3 into O2 and provides a measure of how much oxygen is contained in the ozone in the O3/O2 sample. Exposure to natural oils leads to loss of ozone via reaction with the unsaturated >C=C< bonds present in the oils and provides a measure of how much ozone is present in the O3/O2 sample. From the ratio of the volume changes and the molecular formula of oxygen (O2), it was deduced that ozone has the molecular formula O3 (Soret, 1863). The association of ozone with atmospheric chemistry can be traced back to Schönbein’s original paper in 1840. Schönbein recognized the odor in the air after lightning and suggested that ozone was present in the atmosphere (Rubin, 2001). Schönbein showed that starch-iodide paper exposed to air developed its characteristic color due to the formation of iodine and concluded that ozone was present in the atmosphere. The chemistry is summarized in equation (l) 2O3 + 4KI + 2H2O → 2O2 + 4KOH + 2I2.

(1)

3

Schönbein developed a commercial kit based on starch-iodide paper using a chromatic scale of 0–10 for the semiquantitative determination of ozone concentration. The dry paper strips after exposure to the atmosphere developed a brown color and were then moistened and compared to the chromatic scale (Rubin, 2001). Schönbein recognized the potential for the physiological effects of ozone and conducted some of the first experiments in this area. Exposure to ozone irritates mucous membranes, and it was shown that breathing ozone (at very high concentrations!) in air is fatal to mice and rabbits (Schönbein, 1851). Interest in ozone, combined with the availability of the Schönbein paper measurement method, led to measurements of ozone in ambient air in Europe, Asia, Africa, and the Americas by the late 1800s. Cornu studied the UV solar spectrum and observed that the intensity of radiation reaching the surface drops off rapidly below approximately 300 nm (Cornu 1879a, 1879b). Cornu noticed that the cutoff wavelength shifts to the red (longer wavelengths) with increasing solar zenith angle and to the blue (shorter wavelengths) at higher altitudes, and he hypothesized that the cutoff was due to the presence of an absorbing species in the atmosphere. Hartley conducted laboratory studies of UV absorption by ozone. By comparing his observations with the cutoff of the solar spectrum, Hartley concluded that attenuation of UV by the atmosphere was attributable to absorption by ozone and that this mainly occurred in the upper atmosphere (Hartley, 1881). Fabry and Buisson (1913) made precise measurements of the cutoff in the solar spectrum and calculated that the total amount of ozone in the atmosphere was equivalent to a layer approximately 5  mm thick at normal temperature and pressure; modern measurements show it is typically 3–4 mm thick. Fowler and Strutt (1917) demonstrated the presence of absorption bands in the solar spectrum near the cutoff just below 300  nm that matched those from ozone. Strutt (1918) was unable to observe absorption bands of ozone when making observations of a light source 4 miles across a valley and concluded that the majority of ozone was present at high altitudes, not near the ground. In the 1920s, Dobson invented a spectrometer that was designed to measure the ozone column (amount of ozone in a path directly overhead) and the altitude profile of ozone (Dobson and Harrison, 1926; Dobson et al., 1927). The Dobson

4

the mechanisms of reactions influencing atmospheric ozone

spectrometer compares the relative intensity of solar flux at pairs of UV wavelengths (e.g., 305.5 and 325.4  nm) chosen to be relatively close together, such that the solar flux entering the atmosphere is similar but at wavelengths where there is a large difference in the absorption by ozone. From the ratios of transmitted intensity of the pairs of UV wavelengths and knowledge of the UV spectrum of ozone, the amount of ozone in the path can be calculated. Ozone concentrations are expressed in Dobson units (DU); one DU corresponds to a layer of ozone that, at standard temperature and pressure (STP), is 0.01 mm thick (i.e., 100 DUs = 1 mm of ozone at STP = 2.69 × 1018 molecule cm−2). Typical overhead ozone columns at mid-latitudes are in the range of 300–400 DUs. Dobson spectrometers have proved to be remarkably robust and have provided accurate measurements of ozone since the 1920s. Dobson spectrometers form the basis of the current ground-based ozone observing stations. A Dobson spectrometer was the instrument used in the 1980s to detect what is now known as the Antarctic ozone hole (Farman et al., 1985). By the late 1920s, it was well established that column ozone levels were several mm and that most of this ozone was located in the upper atmosphere. Chapman (1930) was the first to propose a mechanism capable of explaining the existence of the layer of ozone in the upper atmosphere. Chapman recognized that UV photolysis of molecular oxygen would produce oxygen atoms that could then add to molecular oxygen to give ozone. The rate of production of oxygen atoms is a function of the product of the UV intensity and the O2 concentration. Absorption by O2 attenuates the UV flux as it passes through the atmosphere. At high altitudes, the intensity of UV radiation is high, but the concentration of O2 is low. At low altitudes, the O2 concentration is high, but the UV radiation is low. At intermediate altitudes, the product of UV intensity and O2 concentration reaches a maximum, the rate of oxygen atom formation reaches a maximum, and hence we have an ozone layer. This feature is discussed further in Section I-F. Based on his photochemical mechanism, Chapman estimated that the ozone layer was probably located at 40–50 km altitude. Throughout the 1930s and 1940s, measurements were made using Dobson spectrometers around the world, and the data provided a wealth of information on the vertical, diurnal, seasonal, and geographic distribution of ozone. Observations

that the maximum ozone concentrations typically occur at somewhat lower altitudes (25–30 km) than initially predicted by the Chapman mechanism (Götz et  al., 1933)  led to suggestions that reactions in addition to the oxygen-only chemistry hypothesized by Chapman were important. It was established that the greatest ozone concentrations (400–440 DU) are found at high latitudes in the winter and early spring, and the lowest ozone levels (260–300 DU) are found in the tropics. These observations, together with observations of water vapor distribution, led to recognition of the importance of meteorology in ozone distribution. It was suggested that there was a slow upward motion of air into the stratosphere in the tropics, a slow downward and poleward motion at the middle latitudes, and a return of air to the troposphere at middle and high latitudes (Brewer, 1949; Dobson, 1956). This circulation brings tropospheric air with low concentrations of ozone into the stratosphere in the tropics and moves air from the ozone production region high in the low-latitude stratosphere downward and poleward. This is known as Brewer-Dobson circulation. The UV flux at high latitudes is lower than that near the tropics, and ozone descending into the troposphere can accumulate to greater concentrations than observed in the tropics. This is particularly pronounced in the winter when the sun is low, resulting in a maximum concentration at the end of winter or in early spring at high latitudes. In the 1950s and 1960s, observations of emissions from the atmosphere (airglow) established the presence of trace amounts of chemically exited species such as HO radicals and NO in the stratosphere. In 1957, as part of the International Geophysical Year, a global network of Dobson spectrometers was established to monitor stratospheric ozone levels. Bates and Nicolet recognized the importance of the photodissociation of water vapor in the stratosphere and the fact that reactions of hydrogen atoms with ozone produce vibrationally excited HO radicals whose emission gives the Meinel bands, which are a prominent feature in the dayglow (Bates and Nicolet, 1950). Hampson noted that HO and HO2 radicals would be present in the stratosphere at significant levels, and reactions with these species could reduce the levels of ozone (Hampson, 1964). Hunt developed a mathematical model of stratospheric ozone formation and destruction and showed that inclusion of reactions involving HO

Ozone in the Atmosphere and HO2 radicals brought the ozone profiles into better agreement with observations (Hunt, 1966). In 1970, Crutzen pointed out that chain reactions involving nitrogen oxides could also play an important role in determining stratospheric ozone concentrations. In 1971, Johnson showed that nitrogen oxide emissions in the stratosphere by a fleet of supersonic aircraft could lead to chain reactions that would deplete stratospheric ozone by a factor of approximately 2.  In 1974, both Molina and Rowland and Stolarski and Cicerone came to the conclusion that, like NOx, chlorine oxides could catalytically deplete stratospheric ozone and that the catalytic cycle involving ClOx radicals was more efficient than that involving NOx (Molina and Rowland, 1974; Stolarski and Cicerone, 1974). Molina and Rowland (1974) further pointed out that chlorofluorocarbons (CFCs) were being added to the atmosphere and that, although these compounds were inert in the troposphere, they would be transported into the stratosphere where UV photolysis would release chlorine atoms that would then deplete stratospheric ozone. For their work in atmospheric chemistry, particularly that concerning the formation and decomposition of stratospheric ozone and in helping to highlight the vulnerability of the ozone layer to pollution from industrial activities, Crutzen, Molina, and Rowland were awarded the Nobel Prize in Chemistry in 1995. In 1985, Farman et al. published their observations of a dramatic decline in stratospheric ozone levels above the British Antarctic Survey site at Halley Bay. Measurements at Halley Bay had been made since 1957 using a Dobson spectrometer. Ozone levels in October in 1957–1965 averaged approximately 320 DU with no discernible trend; however, from 1975 to 1985, there was a very large decrease in the average ozone levels in October to approximately 200 DU in 1985. This large decrease of stratospheric ozone in the Austral spring was confirmed by satellite measurements that showed the continental scale and extent of ozone loss (Stolarski et al., 1987). The springtime Antarctic ozone loss was explained by surface reactions on polar stratospheric aerosols promoting the efficiency of chlorine-based catalytic loss of ozone (Crutzen and Arnold, 1986; McElroy et  al., 1986; IPCC, 2007). Observations of the anticorrelation of the concentrations of ClO radicals and ozone within the Antarctic stratosphere during ozone depletion events provided compelling evidence for enhanced chlorine-based catalytic loss

5

of ozone (Anderson et al., 1989). The observation of large losses of stratospheric ozone in the springtime in the Antarctic became known as the “ozone hole” and galvanized public interest and concern regarding potential global loss of the ozone layer. International regulations were formulated and resulted in the adoption in 1989 of the Montreal Protocol on Substances Which Deplete the Ozone Layer. The Montreal Protocol (1989) set a timetable for the phase-out of global production and use of compounds such as CFCs that deplete stratospheric ozone. The Montreal Protocol led to large reductions in global emissions of ozone-depleting substances, and this prevented further damage to the ozone layer. Given the approximately 100-year atmospheric lifetime of the most abundant CFC in the atmosphere (CFC-12), it will be decades before the ozone hole diminishes substantially. The Montreal Protocol of 1989, including its London (1990), Copenhagen (1992), Montreal (1997), and Beijing (1999) Amendments, is widely recognized as one of the most successful international treaties of all time and a major environmental success. Returning to our considerations of the chemistry in the troposphere, in the 1940s and 1950s, Los Angeles experienced severe air pollution episodes in which it was not uncommon for visibility to be cut to just a few city blocks. For example, the Los Angeles Times reported in 1941 that “a pall of smoke and fumes descended on downtown, cutting visibility to three blocks” and that “striking in the midst of a heat wave, the gas attack was nearly unbearable, gripping workers and residents with an eye-stinging, throat-scraping sensation.” In the early 1950s, Haagen-Smit and co-workers recognized that photochemical formation of ozone from the reactions of nitrogen oxides and hydrocarbons in sunlight was a significant contributor to Los Angeles air pollution (Haagen-Smit, 1952; Haagen-Smit et al., 1953) and that this ozone could have significant human and ecological health impacts. In 1952, Blacet recognized that the photodecomposition of NO2 in the polluted atmosphere could generate ozone (Blacet, 1952). In the 1950s, the relative contribution of ozone and other oxidants such as NO2, H2O2, and peroxyacetylnitrate (PAN; [CH3C(O)O2NO2]) to photochemical air pollution and their associated deleterious impacts were unknown. Total oxidants in the air as measured using potassium iodide (KI) solution were found to correlate with eye irritation,

6

the mechanisms of reactions influencing atmospheric ozone

visibility effects, and crop damage. In 1955, the Los Angeles Air Pollution Control District established a three-stage alert system that was designed to prevent a “possible air pollution disaster in Los Angeles County.” The first alert, which was considered “relatively safe,” was triggered when total oxidants exceeded 0.5 ppm over an averaging period of 1 hour. Second and third alerts were triggered at oxidant levels of 1 ppm and 1.5 ppm, respectively. In 1959, the Department of Public Health developed and published statewide ambient air quality standards for California. There were three levels in the standards:  adverse, serious, and emergency. The adverse level was reached if total oxidants (measured chemically using KI) exceeded 0.15 ppm over a 1 hour averaging period. The standard was reduced in 1969 to 0.10 ppm of oxidant over a 1 hour averaging period, and a frequency requirement was added:  “occurring either seven or more days in 90 consecutive days, or in three or more consecutive days.” In 1967, the Air Resources Board (ARB) was established and authorized to set air quality standards for the state of California. In 1970, the frequency component of the oxidant standard was removed, but the concentration of 0.1 pm over a 1 hour average was retained. In 1975, the ARB revised the 1 hour ozone standard to define “oxidant” as ozone, changed the measurement method to UV photometry, but retained the 0.10 ppm 1 hour standard. In 1988, the 1 hour standard was lowered to 0.09 ppm, and, in 2006, an 8 hour average of 0.07 ppm was added (California Air Resources Board [CARB], 2012). Under the Clean Air Act, the US Environmental Protection Agency (EPA) issued National Ambient Air Quality Standards (NAAQS) for ozone of 0.12 ppm (1 hour) in 1979, 0.08 ppm (8 hour) in 1997, and 0.075 ppm (8 hour) in 2008. The NAAQS is violated when the annual fourth-highest daily maximum 8 hour concentration, averaged over 3 years, exceeds 0.075 ppm. At the time of writing (2014), the EPA is reconsidering the ozone standard. The World Health Organization (WHO) has set an 8 hour average interim target of 160 μg m−3 (80 ppb), which it characterizes as having “important health effects; does not provide adequate protection of public health” and an air quality goal of 100  μg m−3 (50 ppb), which it believes “provides adequate protection of public health, though some health effects may occur below this level” (WHO, 2006).

Over the past 60  years, air quality regulations and guidelines have evolved from levels up to 500 ppb of total oxidants being considered by the California Department of Public Health to be “relatively safe” to levels over 50 ppb being considered by the WHO to be “not providing adequate protection of public health.” Interestingly, although the importance of ozone formation in urban areas was well established in the 1950s, it was believed through the 1960s that the main source of background tropospheric ozone was transfer from the stratosphere (Regener, 1941; Junge, 1962). In the early 1970s, it became recognized that oxidation of hydrocarbons, such as methane and α-pinene, in the presence of NOx represented a large source of ozone in the background troposphere (Ripperton et  al., 1971; Chameides and Walker, 1973; Crutzen, 1973). It was shown that the springtime ozone maximum observed in background Northern Hemisphere air was associated with a maximum in PAN, which is a precursor to photochemical ozone formation (Penkett and Brice, 1986). Prior to 1986, it had been believed that stratospheric injection was the cause of the springtime ozone maximum, but the observation of a PAN maximum showed that photochemical production of ozone could also contribute substantially. Based largely on modeling studies, it is now recognized that, on a global basis, the photochemical production of ozone in the troposphere is many times larger than influx from the stratosphere (Wild, 2007; Royal Society, 2008). Observations of surface ozone levels have shown that mid-latitude tropospheric background levels of ozone in the Northern Hemisphere have increased significantly during the past century, perhaps by a factor of 2 over the period 1900–2000 (Staehelin et  al., 1994; Parrish et  al., 2012). Concentrations appear to have leveled off and perhaps even started to decline over the period 2000–2010, although it is difficult to detect long-term trends over such a relatively short period. The interest in establishing the trends in background ozone levels can be traced to three considerations. First, tropospheric ozone is a greenhouse gas (Fishman et al., 1979). As stated earlier, in terms of contribution to radiative forcing of climate change, ozone is the third most significant gas behind carbon dioxide and methane (IPCC, 2007). It has been estimated that increased tropospheric ozone levels have resulted in a radiative forcing of 0.3–0.5 Wm−2 (Shindell et  al., 2006; IPCC,

Ozone in the Atmosphere 2007; Skeie et  al., 2011; Søvde et  al., 2011)  and could have contributed approximately 0.3oC of the 0.5oC observed warming over the period 1890–1990 (Shindell et  al., 2006). There is interest in addressing climate change and understanding the past and likely future contribution from changes in tropospheric ozone. Decreases in stratospheric ozone have led to a small negative radiative forcing of approximately −0.05 W m−2 (IPCC, 2007) since the Industrial Revolution. Second, with increasingly stringent air quality standards and guidelines, the ozone in the background air arriving at urban locations constitutes an increased fraction of the air quality standards (UNECE, 2010). During springtime, the air arriving on the west coast of North America contains ozone at levels that are approximately 50–60% of the current US ozone air quality standard and 75–90% of the WHO ozone guideline. The ozone levels in air arriving on the west coast of Ireland in springtime are at 75–80% of the European ozone standard and 88–94% of the WHO standard (Parrish et  al., 2009; UNECE, 2010). Third, there is interest in whether the oxidizing capacity of the global atmosphere is changing (Thompson, 1992)—that is, whether levels of ozone (and hence HO) are changing such that the atmosphere’s capacity to cleanse itself is being altered. In the 170 years since the discovery of ozone by Schönbein, a vast body of scientific data has been amassed concerning its chemistry and physics and its vital role in atmospheric chemistry. The main research motivations today would be familiar to the original researchers. There is a substantial and ongoing research effort to understand the sources and fates of ozone in the atmosphere, its concentrations and trends, and the health and ecosystem impacts of exposure to ozone (albeit at much lower levels than those investigated by the original researchers). A relatively new area of interest is the contribution of increased levels of ozone in the lower atmosphere to climate change. I-C. PROPERTIES OF THE E A R T H ’ S AT M O S P H E R E I-C-1. Chemical and Physical Properties The atmosphere can be viewed as a gigantic inhomogeneous photochemical reactor in which temperature, pressure, radiation flux, and composition vary widely. All of the energy for atmospheric chemistry comes from solar radiation, which is absorbed

7

by various components of the atmosphere. Figure I-C-1 shows the direct solar flux over the wavelength range 120–360  nm at the top of the atmosphere (labeled solar), and at 80, 40, 20, and 0 km altitudes. The radiation emitted by the Sun corresponds to a first approximation to emission from a black body at approximately 6,000 K. Wavelengths shorter than 100 nm are absorbed by O2, O, and N2 and do not penetrate to altitudes below 100 km. As shown in Figure I-C-2, molecular oxygen absorbs strongly at 130–175  nm in the Schumann-Runge continuum and blocks wavelengths of less than 175 nm from penetrating below 80 km. In the altitude range 20–80 km, absorption by O2 in the Schumann-Runge bands filters out UV at 175–200  nm. Because of the structured nature of the Schumann-Runge bands, wavelengths near the band minima penetrate more deeply into the atmosphere, as indicated in the bottom panel of Figure I-C-1. Absorption at wavelengths of less than 242 nm leads to photodissociation of O2 to give two O atoms. The O atoms add to molecular oxygen to form ozone. As shown in Figure I-C-3, ozone has a strong absorption in the Hartley bands in the 210–290 nm region. Absorption by ozone is responsible for the decrease in UV flux in this wavelength region with decreasing altitude, until altitudes of approximately 50 km where ozone levels become significant. Absorption by ozone in the stratosphere at 15–50 km shields the Earth’s surface from UV radiation of wavelengths of less than 295  nm. At wavelengths of approximately 200  nm, neither O2 nor O3 have strong absorption, and this allows solar UV at 190–220 nm to penetrate to quite low altitudes in the stratosphere. Photolysis of many key atmospheric trace gases such as CFCs and nitrous oxide occurs at relatively low altitudes in the stratosphere as a result of the availability of this radiation. Absorption by ozone falls off rapidly at wavelengths above 295  nm, and the weak absorption in the Huggins bands at 310–350 nm and the weak visible Chappuis bands at 400–850 nm does not substantially attenuate the solar radiation from reaching the Earth’s surface. Figure I-C-4 shows the sensitivity towards solar zenith angle and overhead ozone columns of the actinic flux reaching the Earth’s surface. The contributions of the different absorption bands to ozone photodissociation as a function of altitude are given in Figure I-C-5 (Goody, 1995).

8

the mechanisms of reactions influencing atmospheric ozone Wavelength, nm

1015

100

200

300

1014

Solar 0 km

1013

1012

Actinic Flux, quanta cm−2 s−1 nm−1

1011

40 km Solar

1010

20 km

20 km

80 km 109 1012 1011

Solar 40 km

1010

80 km

109 108 160

170

20 km

180

190

200

Wavelength, nm FIGURE I-C-1.

Direct solar flux over the wavelength range 120–370 nm at top of atmosphere (labeled solar) and at 80, 40, 20, and 0 km altitude. The bottom panel is an expanded view of the 160–200 nm region showing the result of absorption by O2 in the Schumann-Runge bands. Reprinted with permission from Schneider et al. (1995). Copyright 1995 American Chemical Society.

The mean composition of the atmospheric gases is given in Table I-C-1. N2, O2, and Ar make up 99.9% of the atmosphere, and, for all practical purposes, the relative proportion of these gases is constant in the lower 100 km of the atmosphere. The total dry mass of the atmosphere is approximately 5.1  × 1018 kg (Trenberth and Guillemot, 1994). We are concerned here with ozone, which is present at levels of 10–200 ppb in the lower atmosphere (troposphere) and 0.5–10 ppm in the background upper atmosphere (stratosphere). Compared to the main atmospheric constituents,

ozone is present at relatively low concentrations. As will be discussed, the levels of ozone in turn are dependent on the presence of compounds such as nonmethane hydrocarbons (NMHC), halocarbons, and nitrogen oxides, which are present at even smaller concentrations (see Table I-C-1) and whose concentrations vary significantly both spatially and temporally. The atmosphere is a thin envelop of gas held around the Earth by gravitational forces. The vertical pressure profile can be predicted by considering the change in overhead force exerted by the

Oxygen cross section, cm2 molecule−1

Ozone in the Atmosphere

9

10−17

10−18

10−19

130

140

150

160

170

Wavelength, nm

Cross section, cm2 molecule−1

10−18 10−19 10−20 10−21 10−22 10−23 10−24

180

185

190

195

200

Wavelength, nm FIGURE I-C-2.

Absorption spectrum of oxygen showing the Schumann-Runge continuum (upper panel) and the Schuman-Runge bands as measured by Yoshino et al. (1992). The high-resolution spectrum of the Schumann-Runge bands contains 94,384 data points (lower panel).

atmosphere (Wayne, 1991)  for an area A over a given vertical distance dz: dF = − g ρ A dz

(1)

in a column of gas whose density is ρ and area A. Hence, the change in pressure, dp = − g ρdz.

(2)

ρ = Mp/RT

(3)

For an ideal gas

where M is the molar mass and R is the molar gas constant. Substitution of (3)  into (2)  and rearrangement gives

dp / p = −dz / ( Mg / RT )

(4)

which, upon integration, gives p = p0 exp (− gz / RT ) = p0 exp ( − z / H )

(5)

where p0 is the pressure at zero altitude, which by definition is 1 atmosphere and H  =  (RT/g). As indicated by the form of expression (4), this simple analysis shows that the atmospheric pressure decreases exponentially with altitude. The distance over which the pressure decreases by a factor of 1/e is given by H, which is called the scale height. From the universal constant for ideal air R = 287 J K−1 kg−1 (Brasseur et al., 1999), T = 288 and 217 K at zero and 20 km altitude (NOAA, 1976) and g = 9.8 m s−2,

the mechanisms of reactions influencing atmospheric ozone 1.2 x 10−17

6.0 x 10−21

10−17

5.0 x 10−21

8.0 x 10−18

4.0 x 10−21

6.0 x 10−18

3.0 x 10−21

4.0 x 10−18

2.0 x 10−21

2.0 x 10−18

10−21

0

Ozone cross sections (σ), cm2 molecule−1

Ozone cross sections (σ), cm2 molecule−1

10

0 250

300

350

400

450

500

550

600

650

700

750

800

Wavelength, nm FIGURE I-C-3.

Absorption spectrum of ozone showing the intense ultraviolet Hartley band and the weak visible Chappuis band; data from Voigt et al. (2001) was provided to the authors.

10−18

1.5 x 1014 10−19

1014

AF Z = 0; 350 DU AF Z = 0; 300 DU AF Z = 60; 400 DU AF Z = 60; 350 DU AF Z = 60; 300 DU Ozone Cross Section

10−20 5.0 x

1013

0 290

300

310

320

330

O3 Cross sections, cm2 molecule−1

Actinic flux (AF), quanta cm−2 nm−1 s−1

2.0 x 1014

10−21

Wavelength, nm FIGURE I-C-4.

Effect of solar zenith angle (Z) and overhead ozone columns (DU) on the actinic flux (AF) reaching the

Earth’s surface.

the scale height is approximately 8.5 and 6.4 km at these two altitudes, respectively. A value of 7 km can be used as an approximate scale height throughout the atmosphere to estimate the decrease in pressure. Commercial aircraft cruise at approximately 11 km, at which altitude the pressure has dropped by a factor of 1/e−11/7 = 5 from that at sea level. Figure I-C-6

shows the pressure as a function of altitude for the US Standard Atmosphere (NOAA, 1976). In discussions of atmospheric processes, it is useful to divide the atmosphere into different regions. Temperature profiles provide the most convenient basis for this division. Figure I-C-6 shows the altitude profile of atmospheric temperature (NOAA, 1976).

Ozone in the Atmosphere

11

50 Huggins Chappuis

Altitude, km

40

30

Total

λ < 240 nm

20 Hartley

10 10−6

10−5

10−4

10−3

10−2

O3 Photolysis rate, s−1 FIGURE I-C-5.

Contributions of the different absorption bands for ozone photodissociation. Reprinted with permission from Goody (1995). Copyright 1995 Oxford University Press.

TABLE I-C-1 . AVER AGE COMPOSITION OF DRY AIR

Gas

N2 O2 Ar CO2 Ne He CH4 H2 N2O CO O3 (troposphere) O3 (stratosphere) Nonmethane hydrocarbons Halocarbons Nitrogen oxides (NOx)

Average Mixing Ratio (by volume) Percent (%)

Parts per million (ppm)

78.1 20.9 0.9 0.039 0.0018 0.00052 0.00017 0.00006 0.00003 0.00001 0.000001–0.00002 0.00005–0.001 0.0000005–0.000002 0.0000001 1 × 10−9–2 × 10−5

781,000 209,000 9,000 390 18 5.2 1.7 0.6 0.3 0.1 0.01–0.20 0.5–10.0 0.005–0.02 0.001 0.00001–0.2

The troposphere is characterized by a temperature profile in which colder air overlays warmer air. This situation is caused by the fact that the predominant heat source for this region of the atmosphere is the warm surface of the Earth. Reflecting the available heating trend from the surface, the troposphere extends to approximately 18 km in the tropics, 12 km at mid-latitudes, and 6–8 km near the poles. There

are often large changes in the Earth’s surface temperature during the day that lead to heating of the lower atmosphere. The troposphere is often, but not always, dynamically unstable, exhibiting the vertical convective mixing from which the region derives its name (tropos is Greek for “turning”). Intense tropical thunderstorm systems can transport molecules from close to the Earth’s surface to the top of the

12

the mechanisms of reactions influencing atmospheric ozone Atmospheric pressure, atmospheres 10 100

−6

10

−5

10−4

10−3

10−2

10−1

100

Thermosphere

90

Mesopause

80 Mesosphere

Altitude, km

70 60 50

Stratopause

40 Stratosphere

30 20

Tropopause

10 Troposphere

0 180

200

220

240

260

280

300

Temperature, K FIGURE I-C-6.

Vertical profile of temperature and pressure in the US Standard Atmosphere (NOAA, 1976) and the various regions of the atmosphere as defined by the temperature.

troposphere within a few minutes. However, more typical mixing times are of the order of days to weeks. More than 90% of the mass of the atmosphere is located in the troposphere, and it is here that the vast majority of anthropogenic molecules are degraded. The first approximately 0.5–1.0 km of the troposphere is the planetary boundary, which is a highly turbulent mixing region close to the Earth’s surface. Air movement in the boundary layer is hampered by surface drag and obstruction by local topology. In urban areas, buildings offer resistance to air flow. Winds in the troposphere above the boundary layer are not impacted by surface drag or obstacles on the surface (buildings, trees, etc.), and the region above the boundary layer is termed the free troposphere. In contrast to the troposphere, the stratosphere, which lies at approximately 20–50 km altitude, is heated principally from above by the absorption of solar UV radiation by oxygen and ozone. In the stratosphere, warm air lies on top of cooler air, which is an inherently stable situation resulting in the layered structure that gives the region its name (stratos is Latin for “layered”). Vertical mixing in the stratosphere proceeds slowly, typically on a time scale

of several years. The region that marks the boundary between the troposphere and the stratosphere is called the tropopause. The stratosphere contains approximately 90% of atmospheric ozone. At the top of the stratosphere, at an altitude of approximately 50 km, the atmospheric pressure is a factor of 1/ e−50/7  =  1,000 times less than that at sea level. The region marking the boundary between the stratosphere and mesosphere is called the stratopause. Above the stratopause, the concentrations of O2 and O3 and the absorption of solar radiation decrease, and the temperature once again decreases with increasing altitude. The layer above the stratosphere between 50 and 85 km is called the mesosphere and is dynamically unstable with respect to vertical convective mixing. Within and above the mesosphere, ionization of N2, O2, NO, and O by extreme UV radiation (e.g., Lyman-α at 121.5 nm) and X-rays leads to the formation of electron layers. The electron layers are of practical significance because they enable radio wave propagation. The region of the atmosphere containing significant quantities of electrons and ionized species above approximately 80 km is called the ionosphere. The

Ozone in the Atmosphere top of the mesosphere is called the mesopause and is located at 85–90 km. Above the mesopause, in a region called the thermosphere, the pressures drop to very low levels (at 100 km the pressure is less than one-millionth that at sea level), collisions between molecules become infrequent, and energy is not efficiently equilibrated between the available translational, rotational, and vibrational degrees of freedom within molecules. Solar radiation in the extreme end of the UV spectrum, with wavelengths of less than 100 nm, is absorbed within the thermosphere and leads to photodissociation and photoionization. There is a bottleneck of energy transfer, and the cooling mechanism via radiation from infrared active vibrational modes becomes less effective. Figures I-C-7 and I-C-8 show the concentrations of some important trace gases as a function of altitude. For species such as CH4, CF2Cl2, CCl4, and N2O that are emitted at the Earth’s surface and are not produced in any reactions in the atmosphere, the concentrations decrease with increasing altitude, reflecting both the decrease in atmospheric pressure and chemical and/or physical losses as the species are transported to higher altitudes. CF2Cl2 and CCl4 do not undergo chemical reactions in the

troposphere and are transported into the stratosphere where they undergo UV photolysis rather efficiently within the lower stratosphere; hence, their concentrations decrease dramatically above approximately 30–40 km. CH4 does not undergo photolysis but does react with HO radicals that are present throughout the atmosphere; hence, its concentration decreases with increasing altitude less abruptly than does CF2Cl2 and CCl4. Ozone is not emitted from the Earth’s surface but is formed following O2 photodissociation in the stratosphere and as a result of photochemical reactions in the troposphere. The concentration of ozone increases with altitude, reaches a maximum, and then decreases. The ozone layer is approximately 20 km thick. In terms of absolute concentration, the maximum ozone levels are at 25–30 km (Figures I-C-7 and I-C-8), whereas in terms of mixing ratio, the maximum ozone levels are at 30–35 km (Figure I-C-9). The reaction of O atoms with O2 produces ozone, whereas the absorption of UV by ozone produces O atoms and O2. The interconversion of O atoms and O3 in the sunlit stratosphere is rapid, occurs on a timescale of minutes, and it is convenient to refer to the sum of O atoms and ozone as

60

H2 O 50

N2O CH4

Altitude, km

40

O3 30

CO

O(3P) 20

10

0 107

108

109

1010

1011

1012

1013

−3

Concentration, molecule cm FIGURE I-C-7.

13

Typical vertical profiles for some important trace gases at 30oN in March (Brasseur et al., 1999).

14

the mechanisms of reactions influencing atmospheric ozone 60

NO2

HO2

OH NO

CF2Cl2

50

NO2 CCl4

Altitude, km

40

30

20

Cl

ClO

10 OH

ClO

0 103

104

105

CCl4

HO2

106

107

Concentration, molecule FIGURE I-C-8.

108

109

cm−3

Typical vertical profiles for some important trace gases at 30oN in March (Brasseur et al., 1999).

“odd oxygen.” The O + O2 addition reaction forming ozone requires the presence of a third body to remove the excess energy in the nascent O3 product; hence, it has a rate coefficient that depends linearly on the total pressure. With increasing altitude, both the total pressure and the partial pressure of O2 decrease, and thus the rate of conversion of O into O3 decreases. With increasing altitude, the solar UV flux increases and hence the rate of conversion of O3 into O increases. Hence, the partitioning of odd oxygen between O3 and O atoms is a sensitive function of altitude, as shown in Figure I-C-7. I-C-2. Meteorology and Its Effects on Atmospheric Ozone Meteorology plays a critical role in determining atmospheric ozone concentrations. Table I-C-2 lists meteorological parameters that affect air quality. Meteorology affects the background ozone transport from the stratosphere, air pollutant emission rates, the mixing and transport of emissions and their products, and chemical reaction rates, along with dry and wet deposition. A synoptic scale (order of 1,000 km) weather forecast is essential to accurately forecast tropospheric ozone concentrations.

The availability of sunlight is obviously a key factor that drives ozone photochemistry and influences ozone concentrations. Clouds influence both UV intensity and the maximum temperatures. Clear to scattered cloudy skies are required for significant ozone formation. The atmospheric lapse rate or stability (temperature change by height) controls the amount of vertical mixing that takes place. Strong stability tends to reduce mixing (i.e., reduce dilution) and confine emissions and ozone closer to the ground. This is important because higher concentrations of precursors are needed to form higher ozone concentrations. In addition, aloft temperature inversions can act to trap pollutants below the inversion and inhibit vertical mixing. Surface wind speeds control the degree of ventilation. Calm or light winds produce weak ventilation and allow more emissions to accumulate in a given volume of air, resulting in higher precursor concentrations. Aloft winds are important because they transport ozone and precursors into a given region overnight and in the early morning hours or transport locally formed ozone out of a region during the afternoon. For example, low-level jet streams with winds of 10–20 m s−1 form throughout

Ozone in the Atmosphere

15

60 Ozone Mixing Ratio 50

Altitude, km

40

30

20

10

0

0

2 x 10−6

4 x 10−6

6 x 10−6

8 x 10−6

10−5

Ozone mixing ratio 60 [O3], Molecules cm−3

50

Altitude, km

40 30 20 10 0

0

1e + 12

2e + 12

3e + 12

Ozone concentration, molecule FIGURE I-C-9.

4e + 12

cm−3

Vertical profile of ozone mixing ratio (top panel) and concentration (bottom panel) 30°N in March (Brasseur

et al., 1999).

the United States shortly after sunset and remain through the night (Blackadar, 1957). Low-level jets are efficient at transporting ozone and its precursors several hundred kilometers during the night (Samson, 1978). Synoptic-scale meteorological patterns (i.e., atmospheric circulation on scales of approximately 1000 km) control the mixing, ventilation, sunlight, and temperature and have a large impact on surface ozone levels (Pagnotti, 1987; Comrie and Yarnal, 1992). High pressure is typically associated with the highest ozone concentrations. This pattern occurs

about 1–2  days after a cold front and associated trough has passed through an area. As the surface high pressure develops, winds become weak allowing ozone and its precursor emissions to build up. Warming temperatures lead to increased biogenic and evaporative emissions of volatile organic carbon compounds (VOCs), and lower humidity produces clearer skies that promote photochemistry. Sinking air (subsidence) warms and stabilizes the lower atmosphere, which suppresses cloud development and mixing. An aloft temperature inversion may form that inhibits vertical mixing and reduces

16

the mechanisms of reactions influencing atmospheric ozone TABLE I-C-2 . IMPORTANT METEOROLOGICAL VARIABLES THAT

AFFECT AIR QUALIT Y

Meteorological Variable Temperature Air temperature Surface temperature Vertical temperature structure Clouds Humidity Absolute Relative Wind Surface Aloft

Effect Chemical reaction rate constants, anthropogenic and biogenic emission rates Convection and mixing through heat transfer from Earth’s surface to surface layer Atmospheric stability, mixing layer height, and deposition velocities Distribution of ultraviolet radiation in atmosphere, minimum and maximum temperatures, vertical distribution of pollutants and aqueous phase chemistry Gas phase reactions involving water, in particular the formation of HO from O(1D) The presence or absence of liquid water coating on aerosol particles; the rate of reactive nitrogen loss to the particles Amount of ventilation within the mixed layer, atmospheric stability and turbulence Long-range transport of ozone and ozone precursors

dilution of ozone and ozone precursors. As the surface high pressure moves out of the region, the accumulated ozone can be transported to downwind locations. Warm air may be advected into the region, and winds may increase. This pattern typically continues to produce warm temperatures and relatively clear skies even with a low pressure system approaching. Ozone levels can remain high on these types of days, and the potential for longer range ozone and precursor transport is significant until a cold front arrives. A  cold front pattern is characterized by a low pressure system at the surface and associated cold and warm fronts. A  cold front weather pattern produces clouds and precipitation that reduce photochemistry while stronger winds and mixing reduce ozone concentrations by diluting ozone and its precursors. I - D. S U M M A RY O F A M B I E N T MEASUREMENTS OF TROPOSPHERIC OZONE I-D-1. Measurements of Surface Ozone in Background and Remote Locations Ambient measurements of ozone date back to the end of the 19th century when the Schönbein method was used in a variety of sites around the world to provide semiquantitative measurements of ozone concentration (Pavelin et  al., 1999). As described previously, the Schönbein method consisted of exposing paper impregnated with starch and iodide

to air for several hours. Iodide within the paper was oxidized to iodine causing a characteristic change in the color of the paper, which could be compared to a color scale provided with the measurement kit. As with many wet chemical techniques, this technique suffers from chemical interferences. Oxidizing species such as NO2, H2O2, and PAN cause a positive interference and result in an overestimation of the ozone level, whereas reducing species such as SO2 and NH3 produce a negative interference and result in an underestimation of the ozone level (Anfossi et al., 1991). The most comprehensive and significant of ozone measurements in the 19th century were those conducted by Albert-Lévy in 1876–1910 at the Parc de Montsouris. At the time of his measurements, this park was located just beyond the southwestern limits of Paris. Albert-Lévy (1877) bubbled air at a carefully measured flow rate through a bubbler containing a solution of potassium arsenite and potassium iodide. Ozone oxidizes iodide ion (I−) to iodine (I2), which then oxidizes arsenite (AsO33−) to arsenate (AsO43−), the net reaction being: O3 + AsO33− → O2 + AsO43− After bubbling air through the solution for a set time, the amount of arsenite remaining was determined by titration using a solution containing iodine: I2 + AsO33− + 2OH− → 2I− + AsO43− + H2O

Ozone in the Atmosphere The measurements by Albert-Lévy are the only known quantitative ozone measurements from the preindustrial era. Albert-Lévy recognized that the method was subject to positive interferences by oxidizing gases such as H2O2 and negative interferences by reducing gases such as SO2. In the modern era, Volz and Kley (1988) conducted laboratory experiments to study the likely accuracy of the measurements reported by Albert-Lévy. Volz and Kley replicated the apparatus and method described by Albert-Lévy and reported that when applied to O3-air mixtures the method was accurate to within approximately 2% as calibrated using a modern UV absorption ozone analyzer. Volz and Kley studied potential interferences by NO2, SO2, H2O2, and CH2O. H2O2 and NO2 caused a positive interference, whereas SO2 caused a negative interference with a stoichiometry of 1 (i.e., each ppb of SO2 present leads to an underestimation of the ozone concentration by a ppb). CH2O showed no significant interference. Volz and Kley examined approximately 3,000 daily measurements of ozone reported by Albert-Lévy in the years 1876–86 and 1905 and correlated the results with wind direction. The area to the southwest of Montsouris had no significant urban centers in 1876–1905, and Volz and Kley

17

concluded that air from the southwest would be free of urban air pollutants that might interfere with the analysis. It was found that when the wind was coming from the southwest, the average ozone levels measured in 1876–86 and 1905 were approximately 8 and 10 ppb, respectively, and when the wind was coming from directions other than southwest (i.e., from Paris), the ozone levels were lower by 2 and 5 ppb, respectively. The generally lower ozone levels observed when the air direction was from Paris were attributed to the presence of SO2 in the Paris urban plume. Based on the difference in the ozone readings between air masses from the southwest and from other directions, Volz and Kley made an SO2 correction that increased in a linear fashion from 2 ppb in 1881 to 5 ppb in 1905. No correction was made for positive interferences such as from H2O2 or NO2 because the magnitude of the concentration of these species was assessed as being likely small. The corrected data derived by Volz and Kley range from 5 to 16 ppb, with an average of 11 ppb over the period 1876–1910; these estimates are shown in Figure I-D-1. It can be seen from the Figure that the ozone concentrations in Montsouris in 1876–1910 derived from the data reported by Albert-Lévy are

60 Jungfraujoch Annual average ozone, ppb

50 40

Mace Head

30

Pacific MBL

20 Arkona 10 Montsouris 0 1880

FIGURE I-D-1.

1900

1920

1940 1960 Year

1980

2000

2020

Annual average surface ozone concentration measured at Montsouris (France), Arkona (Germany), Mace Head (Ireland), Jungfraujoch (Switzerland), and for the Pacific marine boundary layer (MBL) on the west coast of North America. Symbols are individual day or shorter averages from Alpine stations:  Grands-Mulets, square; Chamonix, stars; Arosa, circles; Lauterbrunen, inverted triangles; Jungfraujoch, diamonds; Pfänder, circles; obtained using either chemical (open symbols) or spectroscopic (filled symbols) methods. Data sources: Montsouris, Volz and Kley (1998); Arkona, Feister and Warmbt (1987); Mace Head, Derwent et al. (2007b); Alpine stations, Staehelin et al. (1994); Pacific MBL, Parrish et al. (2009); Jungfraujoch, Steinbacher (2012).

18

the mechanisms of reactions influencing atmospheric ozone

lower by approximately a factor of 3 than those observed in current background rural sites. In addition to importance in their own right, the observations at Montsouris are important because Albert-Lévy conducted simultaneous experiments using the Schönbein paper method in an attempt to provide an absolute calibration of the Schönbein color scale. As discussed by Linvill et  al. (1980), Bojkov (1986), Marenco et al. (1994), and Pavelin et  al. (1999), these simultaneous measurements can be used to convert Schönbein paper readings into absolute units. Conversion is complicated by the fact that the response of the Schönbein paper is dependent on the relative humidity, so that corrections need to be applied to account for the relative humidity at which the measurements were taken. Pavelin et al. (1999) evaluated the Schönbein readings of surface ozone levels taken in Adelaide, Coimbra, Hiroshima, Hobart, Hong Kong, Luanda, Mauritius, Montevideo, Mont-Ventoux, Nemuro, Tokyo, and Vienna over the period 1880–1928. In the data evaluated at all these sites, the ozone levels are consistent with those reported at Montsouris and are in the range of 10–15 ppb, much lower than current levels. However, as noted by Pavelin et  al. (1999), there is substantial scatter in the ozone values retrieved, and, in some cases, the seasonal variation is opposite expectations based on both recent measurements and preindustrial simulations. Given the significance of the quantitative measurements conducted at Montsouris by Albert-Lévy in 1876–1910, it is of interest to address the question of whether a negative interference by either SO2 or NH3 could have led to an underestimation of the ozone levels. As noted earlier, based on a difference in apparent ozone concentrations when Montsouris was upwind and downwind of Paris, Volz and Kley inferred the presence of 2 ppb (5.4 μg m−3) SO2 in the Paris urban area in 1881 rising to 5 ppb (13.5 μg m−3) in 1905. As with London in the past and Beijing presently, coal was an important fuel in Paris in 1900. In comparison with annual average levels of SO2 of approximately 50–400  μg m−3 in London in 1950–1990 (Greater London Authority, 2002), 40–60  μg m−3 in Beijing in 2007–2009 (Lin et  al., 2012), and 120–190 μg m−3 in Paris in 1956–1970 (AIRPARIF, 2012), the levels of SO2 of 5–15 μg m−3 estimated in Paris by Volz and Kley in 1876–1910 seem rather low. The emissions of SO2 in France increased by approximately a factor of 3 from 1905 to 1960 (Mylona, 1996; Smith et al., 2011). Assuming

a proportional increase in the Paris area and based upon an SO2 concentration in Paris in 1960 of approximately 150 μg m−3, we would expect an SO2 concentration of approximately 50 μg m−3 (20 ppb) in 1905. This value is substantially greater than the value of 13 μg m−3 (5 ppb) in 1905 estimated by Volz and Kley. It seems reasonable to conclude that the SO2 concentrations in the air in Montsouris were higher than estimated by Volz and Kley and hence the ozone level was underestimated. Furthermore, a possible interference by NH3 was not considered by either Albert-Lévy or Volz and Kley. Ammonia is a negative interference in the Schönbein paper method (Pavelin et al., 1999) and presumably also in the arsenite/arsenate method used by Albert-Lévy. Ammonia emissions associated either with agricultural activities around Paris, activities within Paris, or both might reasonably be expected to be present in the air in Montsouris in 1900. The atmospheric concentration of ammonia in Montsouris in 1900 is unknown, but typical concentrations over continents are currently 0.1–10 ppb (Brasseur et  al., 1999). Concentrations of ammonia in this range or higher may have been present in Montsouris in 1900 and may have contributed to an underestimation of the ozone concentration. Given the surprisingly small correction for SO2 deduced by Volz and Kley noted earlier and the lack of a correction for a possible NH3 interference, we believe that the ozone levels derived from the measurements at Montsouris are underestimates of ozone levels in the 19th century. Support for this view comes from modeling studies that are not able to replicate the very low ozone levels estimated by Volz and Kley (Mickley et al., 2001; Lamarque et al., 2005). Feister and Warmbt (1987) reported long-term measurements of surface ozone concentrations during the period 1952–1984 in the former German Democratic Republic. A wet chemical technique was employed with ozone bubbled through a KI solution. The resulting iodide loss was determined by conductivity titration using an AgNO3 solution. Interference by SO2 was recognized, and, beginning in 1972, this was eliminated by passing the air samples through a chromium trioxide (CrO3) filter to remove the SO2. Feister and Warmbt report ozone records from five monitoring stations. The average ozone readings 3 years before (1969–1971) and after (1972–1974) installation of the filter were compared to assess the likely SO2 interference at the site. Only one of the stations was determined to have a negligible interference

Ozone in the Atmosphere by SO2 (< 2%) and hence be suitable for determining long-term trends; this was the remote site at Arkona on the island of Rügen in the Baltic. The surface ozone concentrations measured at Arkona in 1956–1984 by Feister and Warmbt are given in Figure I-D-1. As seen in Figure I-D-1 there is considerable variability in the annual average ozone concentrations, with values increasing from approximately 15 ppb in the late 1950s to approximately 25 ppb by the late 1970s and falling to approximately 18 ppm in 1984. Despite the year-to-year fluctuations there is a distinct trend in the data, and a linear regression gives a rate of ozone increase of 0.08 ppb yr−1 over the period 1956–1984 in Arkona. The individual data points indicated by symbols in Figure I-D-1 are the available data from measurements at Alpine stations over the period 1896–1941 as tabulated by Staehelin et  al. (1994). The measurements were made using either the KI chemical method or via UV absorption spectroscopy, and they represent short-term averages typically over periods of less than 1 day. Unlike the situation for the measurements at Montsouris, the Alpine stations are remote and the chemical measurements (indicated by open symbols in Figure I-D-1) are very unlikely to have been influenced significantly by interferences from impurities such as SO2. Staehelin et  al. (1994) compared the ozone concentrations measured in 1989–1991 at sites around Arosa (Switzerland) with measurements in the 1950s and concluded that the surface ozone concentrations had increased by approximately a factor of 2. As seen from Figure I-D-1, comparison of the data taken at the Jungfraujoch (Switzerland) in the 1930s with the current levels also reveals an increase by approximately a factor of 2. Beginning in 1973, long-term measurements of surface ozone concentrations based on UV absorption began at several remote background sites around the world. Figures I-D-2 to I-D-5 show the monthly average surface ozone concentrations measured at Barrow in Alaska, Mauna Loa in Hawaii, the South Pole, and Tutuila in American Samoa (NOAA, 2012); these data provide essentially continuous measurements since the early 1970s. Figures I-D-6 to I-D-14 show equivalent although more temporally limited datasets for observations at Storhofd, Vestmannaeyjar (Iceland), Tudor Hill (Bermuda), Ragged Point (Barbados), Niwot Ridge (Colorado), Summit (Greenland), Mace Head (Ireland), Cape Grim (Australia), Cape Point (South Africa),

19

and Jungfraujoch (Switzerland) (NOAA, 2012; Steinbacker, 2012; Brunke, 2012; Spain, 2012). A couple of points are immediately clear from Figures I-D-2 to I-D-14. First, at all sites there is a pronounced seasonal variation of ozone. Highest values are generally observed in spring and summer and lowest values in the fall and winter. Second, there are large differences in the ozone levels at the various locations. The average annual concentrations of ozone over the more than 30 years of monitoring at Barrow, South Pole, Mauna Loa, and American Samoa are 26, 29, 39, and 13 ppb, respectively. Close inspection of Figure I-D-2 shows negative spikes associated with springtime ozone decreases discussed later in Sections I-F-4 and VII-C-6.7. Figures I-D-15 and I-D-16 show the annual average data for sites with the longest records (Mauna Loa, Barrow, American Samoa, and the South Pole). The lines through the data are linear least squares regressions that show small increases in ozone at the sites in Mauna Loa and Barrow, but little or no trend at American Samoa or the South Pole. Logan (1985) examined trends in tropospheric ozone around the world and provided convincing evidence of an increase in background ozone levels associated with industrial activity. The available historical data for tropospheric ozone observations around the globe were analyzed, and it was concluded that summertime levels of ozone near the surface at rural sites in Europe and the eastern United States had increased by 6–22 ppb from the 1940s to the 1980s. This large increase in tropospheric ozone was attributed to photochemical production from anthropogenic precursors whose emissions had increased substantially over the same period. Chan and Vet (2010) studied surface ozone levels in North America using data from 97 nonurban monitoring sites over the period 1997–2006 and reported annual average baseline mixing ratios for Continental Eastern Canada (30  ± 9 ppb), Continental Eastern United States (30  ± 10 ppb), Coastal Eastern Canada (27  ± 9 ppb), Coastal Western Canada (19  ± 10 ppb), Coastal Western United States (39 ± 10 ppb), Continental Western Canada (28  ± 10 ppb), and Continental Western United States (46 ± 7 ppb) (uncertainties are 1 standard deviation). Increasing baseline ozone trends (temperature adjusted) were observed in all seasons along the Pacific coasts of Canada and the United States, although the trends in California were not statistically significant. In the coastal zone of Pacific

20

the mechanisms of reactions influencing atmospheric ozone 50

Monthly average ozone, ppb

Barrow, Alaska 40

30

20

10

0 1970

1980

1990

2000

2010

Year FIGURE I-D-2.

Monthly average surface ozone concentrations observed at Barrow, Alaska, USA (NOAA ESRL, 2012).

Monthly average ozone, ppb

50

40

30

20

10

0

South Pole

1980

1990

2000

2010

Year FIGURE I-D-3.

Monthly average surface ozone concentrations observed at the South Pole (NOAA ESRL, 2012).

Canada, positive trends were found with increases of 0.28  ± 0.26, 0.72  ± 0.55, and 0.93  ± 0.41 ppb yr−1 in spring (MAM), summer ( JJA), and winter (DJF), respectively (Chan and Vet, 2010). In the Atlantic coastal region, the trends were positive in three of the four seasons (but only significantly so in spring). In the high ozone precursor emission areas of the Eastern United States, decadal trends in baseline ozone were, in general, negative in the spring, summer, and fall and appear to be controlled by the strong within-region changes induced by decreasing ozone precursor emissions.

Cooper et al. (2012) analyzed rural ozone trends across the United States over the period 1990–2010. A  distinct difference in trends in the western and eastern sites was noted. In the east in summertime, there were statistically significant decreases in the 95th, 50th, and 5th percentile ozone values for 83%, 66%, and 20% of the sites, respectively. In spring, 43% of sites in the east had a significant decrease, whereas no sites had a significant increase. In the west in summertime only, 17% (two sites) and 8% (one site) had statistically significant decreases in the 95th and 50th percentile ozone levels. In spring,

Ozone in the Atmosphere

21

70

Monthly average ozone, ppb

60 50 40 30 20 Mauna Loa, Hawaii

10 0

1980

1990

2000

2010

Year FIGURE I-D-4.

Monthly average surface ozone concentrations observed at Mauna Loa, Hawaii (NOAA ESRL, 2012). 30

Tutuila, American Samoa

Monthly average ozone, ppb

25 20 15 10 5 0 1980

1990

2000

2010

Year FIGURE I-D-5.

Monthly average surface ozone concentrations observed at Tutuila, American Samoa (NOAA ESRL, 2012).

no sites in the west had decreased ozone over the 1990–2010 period. The level of springtime ozone in air above western North America increased at a rate of 0.41 ± 0.27 ppb yr−1 over the period 1995–2011 (Cooper et  al., 2010, 2012). The trends in ozone levels in rural sites in the United States reflect the impacts of reduced emissions of ozone precursors within the United States and increased emissions in Asia. Logan et  al. (2012) used data from sondes, aircraft, and alpine sites to assess changes in ozone levels above 2 km over Europe over the period

1990–2009. Using data from the mountain site of Zugspitze for 1978–1989, it was concluded that ozone increased by 6.6–10 ppb in 1978–1989 and by 2.5–4.5 ppb in the 1990s and then decreased by 4 ppb in the 2000s in the summer. There were no significant trends observed in seasons other than in summer. Parrish et  al. (2012) analyzed the ozone trend at mid-latitude background stations in the Northern Hemisphere. The data were analyzed by season, and it was found that, prior to 2000, the average increase in ozone at all sites was

22

the mechanisms of reactions influencing atmospheric ozone

Monthly average ozone, ppb

50

40

30

20 Storhofd, Vestmannaeyjar, Iceland

10

0 1990

1995

2000

2005

2010

Year FIGURE I-D-6.

Monthly average surface ozone concentrations observed at Storhofd, Vestmannaeyjar, Iceland (NOAA

ESRL, 2012). 70

Monthly average ozone, ppb

60 50 40 30 20

Tudor Hill, Bermuda

10 0 1985

1990

1995

2000

2005

2010

Year FIGURE I-D-7.

Monthly average surface ozone concentrations observed at Tudor Hill, Bermuda (NOAA ESRL, 2012).

approximately 1% yr−1 relative to the concentrations in 2000. There were small differences by season, with average increases of 1.08 ± 0.09, 0.89 ± 0.08, 0.72 ± 0.12, and 1.22 ± 0.12 % yr−1 increases in spring, summer, autumn, and winter, respectively. A linear change of 1% yr−1 relative to the 2000 intercept corresponds to a doubling of surface ozone from 1950 to 2000. Parrish et  al. (2012) noted that the surface ozone levels measured at Mt. Happo in Japan since 1991 were generally increasing at a faster rate than for other stations. This larger increase is likely attributable

to the recent rapid increase of emissions of NOx from East Asia (Parrish et al., 2012). Oltmans et al. (2013) examined changes in tropospheric ozone over the period 1970–2010. At mid-latitudes in the Northern Hemisphere there was a significant increase in ozone concentrations from 1970 to 2000, followed by little or no growth in 2000–2010. The change in trend was attributed to controls of ozone precursors. Satellite measurements are now being used to monitor tropospheric ozone on a global scale (Ziemke et  al., 2011). The Tropospheric Ozone

Ozone in the Atmosphere

23

Monthly average ozone, ppb

40

30

20

10

Ragged Point, Barbados

0

FIGURE I-D-8.

1990

1995

2000 Year

2005

2010

Monthly average surface ozone concentrations observed at Ragged Point, Barbados (NOAA ESRL, 2012). 70

Monthly average ozone, ppb

60 50 40 30 20 Niwot Ridge, Colorado

10 0 1990

1995

2000

2005

2010

Year FIGURE I-D-9.

Monthly average surface ozone concentrations observed at Niwot Ridge, Colorado, USA (NOAA

ESRL, 2012).

Residual method uses ozone measurements from two separate satellites, with the tropospheric component being deduced by subtracting the stratospheric component from the total ozone column measurement (Fishman et  al., 1990). Detailed global tropospheric ozone level data for 2004–2010 have been reported by Ziemke et  al. (2011). Consistent with surface-based measurement discussed earlier and ozone sonde data, the satellite data show large temporal and spatial variability in the tropospheric ozone levels. The satellite data show ozone accumulation zones

year-round in the tropical south Atlantic and in the summer months across the Mediterranean, East Asia, and over the North Pacific and North Atlantic following the outflow from East Asia and North America. The satellite data record is at present too short to determine a trend in tropospheric ozone levels. Whereas it is clear that there has been a trend of increased ozone concentrations at background locations at mid-latitudes in the Northern Hemisphere over the past several decades, the trend is not uniform (Vingarzan, 2004; Oltmans et al., 2006). The

24

the mechanisms of reactions influencing atmospheric ozone 70

Monthly average ozone, ppb

60 50 40 30 Summit, Greenland

20 10 0 2000

2005

2010

Year FIGURE I-D-10.

Monthly average surface ozone concentrations observed at Summit, Greenland (NOAA ESRL, 2012). 60

Monthly average ozone, ppb

50 40 30 20 Mace Head, Ireland

10 0 1990

1995

2000 Year

2005

2010

FIGURE I-D-11.

Monthly average surface ozone concentrations observed at Mace Head, Ireland (Department for Environment Food and Rural Affairs, 2012; Spain, 2012).

trends were most pronounced in the 1970s through the 1990s and have leveled off in the past decade. Model calculations suggest that the increase in NOx emissions over the 1970–1990s in the Northern Hemisphere, especially in Asia, was responsible for the increase in background ozone (Vingarzan 2004; Royal Society, 2008; Parrish et  al., 2012). Anthropogenic emissions are relatively small in the Southern Hemisphere, and the troposphere has not been impacted by industrialization to the

same degree as in the Northern Hemisphere. As seen in Figures I-D-5, I-D-12, I-D-13, I-D-14, and I-D-15, it is difficult to discern a significant long-term trend in background ozone levels in the observational record for monitoring stations in the Southern Hemisphere. Modeling studies indicate that, as the result of human activities since the Industrial Revolution (beginning in approximately 1750), there may have been a 10–20 ppb increase in background surface ozone levels in the Northern

Ozone in the Atmosphere

25

Monthly average ozone, ppb

40

30

20

10 Cape Grim, Australia

0 1980 FIGURE I-D-12.

1985

1990

1995 2000 Year

2005

2010

Monthly average surface ozone concentrations observed at Cape Grim, Australia (WMO GAW, 2012).

Monthly average ozone, ppb

40

30

20

10

Cape Point, South Africa

0 1995

2000

2005

2010

2015

Year FIGURE I-D-13.

Monthly average surface ozone concentrations observed at Cape Point, South Africa (Brunke, 2012).

Hemisphere and a 0–5 ppb increase in the Southern Hemisphere (Royal Society, 2008). I-D-2. Measurements of Surface Ozone in Polluted Urban Areas There has been a strong and consistent trend of urbanization over the past century, and this trend is expected to continue over the coming decades. In 2012, the United Nations estimated that 3.6 billion people, more than half the world’s population, live in urban areas (United Nations, 2012). Historically, urban areas have been associated with dense

population, concentrated economic activity, and high levels of emissions and air pollution (Brimblecombe, 1995). In the 1940s, a new type of air pollution became evident in Los Angeles, as mentioned earlier in this chapter. Haagen-Smit was the first to suggest that reactions of hydrocarbons with NOx in sunlight would lead to photochemical formation of ozone (Haagen-Smit, 1952; Haagen-Smit et al., 1953) and that this was an important component of the photochemical air pollution in Los Angeles. Blacet (1952) first suggested that the photodecomposition of NO2 was the reaction that created the ozone: NO2 + hv →

26

the mechanisms of reactions influencing atmospheric ozone 80

Monthly average ozone, ppb

70 60 50 40 30 20 Jungfraujoch, Switzerland 10 0

1990

1995

2000

2005

2010

Year FIGURE I-D-14.

Monthly average surface ozone concentrations observed at Jungfraujoch, Switzerland (Steinbacher, 2012). 50

Annual average ozone, ppb

Mauna Loa, Hawaii 40

Barrow, Alaska

30

20

10

0 1970

American Samoa

1980

1990

2000

2010

Year FIGURE I-D-15.

Annual average surface ozone concentrations observed at Mauna Loa, Barrow, and American Samoa (NOAA ESRL, 2012).

NO + O; O + O2 + M → O3 + M. Concerns over the impacts on human and ecological health of elevated ozone concentrations within and downwind of urban areas has led to a large research effort to understand the mechanism of ozone formation and its environmental impacts and health impacts. Ozone reacts rapidly with rubber, leading to visible cracks. Haagen-Smit showed that the peak ozone levels (as deduced from the time taken for cracks to become visible in fresh samples of stretched rubber

exposed to ambient air) occurred during periods of severe smog in the early afternoon. In 1955, the Los Angeles Air Pollution Control District established a three-stage smog alert system for four air pollutants including ozone. The KI method (using a 2% neutral buffered KI reagent) was used to measure oxidant. Figure I-D-17 shows peak oxidant levels measured using a 1 hour averaging time. The values from 1955 to 1974 were measured using the KI method. Data from 1975 and later were

Ozone in the Atmosphere

27

Annual average ozone, ppb

50

40

30

20 South Pole

10

0 1970

1980

1990

2000

2010

Year FIGURE I-D-16.

Annual average surface ozone concentrations observed at the South Pole (NOAA ESRL, 2012).

700 600

[O3], ppb

500 400 300 200

Los Angeles

100 0 1950

1960

1970

1980 Year

1990

2000

2010

FIGURE I-D-17.

Peak ozone levels in Los Angeles area. Data for 1955–1972 are peak 1 hour oxidant levels in the city of Los Angeles measured using the potassium iodide wet chemical method; data for 1973–2010 are maximum 1 hour averages in the South Coast Air Basin measured using ultraviolet photometry (CARB, 2012).

measured using UV photometry. As discussed earlier, the KI method is not specific to ozone because other strong oxidants such as peroxides and peroxyacylnitrates give a positive interference, whereas reducing gases such as SO2 give a negative interference. The UV photometry technique is a spectroscopic method and is specific for ozone. The broad consistency of the trend in Figure I-D-17 before 1975 using the KI method, and after 1975 using

UV photometry, suggests that the KI technique provides a reasonable estimation for the ozone concentrations in the air in Los Angeles prior to 1975. In addition to the UV absorption and KI methods, techniques based on gas-phase and liquid-phase chemiluminescence reactions have been developed for measuring ambient ozone levels. In particular, the gas-phase reaction of NO with ozone has been used extensively to provide ozone measurements

28

the mechanisms of reactions influencing atmospheric ozone 400

Max. 8-hr Average

[O3], ppb

300

200

South Coast Air Basin

100

O3 NAAQS 0 1970

1980

1990 Year

2000

2010

Maximum 8 hour ozone concentrations in the South Coast Air Basin (CARB, 2012). The dashed line is the National Ambient Air Quality Standard (NAAQS) of 75 ppb.

4th Maximum 8-hour average ozone, ppb

FIGURE I-D-18.

140 120 100 80 60

Houston-Sugar Land-Baytown

40 20 0

1990

1995

2000

2005

2010

Year FIGURE I-D-19.

Fourth maximum 8 hour average ozone concentrations in Houston-Sugar Land-Baytown, Texas (US EPA, 2012). The dashed line is the National Ambient Air Quality Standard (NAAQS) of 75 ppb.

when fast response times are required (Stedman et  al., 1972; Ridley et  al., 1992). Ozone monitoring techniques are discussed in detail by Parrish and Fehsenfeld (2000). As understanding of the photochemistry of the sources of ozone improved, regulations were developed and implemented that controlled the emissions of NOx and hydrocarbons and hence the photochemical formation of ozone. As suggested by the trend in Figure I-D-17, these emission regulations have been very successful in reducing the levels of ozone in the

air in Los Angeles. The improvement is even more impressive considering the approximately 2.4-fold increase in population in Los Angeles County from 1950 to 2012 (US Department of Commerce, 2012a). The trend of maximum 1 hour ozone is matched by that of the maximum 8 hour average as seen by comparison of Figures I-D-17 and I-D-18. Figures I-D-19 and I-D-20 show data for the Houston and New York urban areas; these also show a significant trend of decreased ozone levels over the past two decades. Throughout the United States, there have been

Ozone in the Atmosphere

29

4th Maximum 8-hour average ozone, ppb

140 120 100 80 60

New York-Northern New Jersey-Long Island

40 20 0 1990

1995

2000

2005

2010

Year FIGURE I-D-20.

Fourth maximum 8 hour average ozone concentrations in the New York-Northern New Jersey-Long Island area (US EPA, 2012). The dashed line is the National Ambient Air Quality Standard (NAAQS)of 75 ppb.

significant reductions in the levels of ozone in urban areas over the past 20–30 years. Data from the US EPA (US Environmental Protection Agency, 2012) for 247 sites in the United States are shown in Figure I-D-21. The solid symbols show the average of the annual fourth maximum 8 hour ozone readings in the 255 sites. The average decreased from approximately 100 ppb in 1980 to approximately 75 ppb in 2009. In addition, the range of the readings has decreased substantially, with heavily polluted locations in 1980 making the most progress in reducing the ozone levels. Although the decrease in ozone levels evident in Figures I-D-17–I-D-21 is a remarkable achievement, it should be noted that further progress is required for all urban areas to meet the US NAAQS of 75 ppb. Figure I-D-22 shows the maximum 1 hour average ozone concentrations measured in Mexico City from 1988 to 2011. Similar to the trends in US cities, there has been a substantial decrease in the level of ozone in Mexico City over the past two decades reflecting progress in reducing the emissions of ozone precursors NOx and hydrocarbons (Parrish et al., 2011). Surface ozone data from 13 rural sites (over the period 1990–2006) and five urban sites (over the period 1993–2006) show that surface ozone levels over the United Kingdom have been influenced by global (hemispheric), regional, and local factors ( Jenkin, 2008). These factors are a long-term and gradual increase in the hemispheric background that determines the ozone in the air arriving in the United Kingdom, short-term increases in ozone

during summer episodes from regional-scale photochemical processing of VOC and NOx emissions over northwest Europe, and a decrease in local-scale removal of ozone via reaction with emitted NO as a result of emissions controls ( Jenkin, 2008). Derwent et al. (2010) reported that in central England over the period 1990–2007, the episodic peak ozone levels declined strongly (−1.4 ppb yr−1) whereas annual mean daily maximum ozone levels increased slightly (0.02–0.09 ppb yr−1). Derwent et al. (2010) ascribed the decrease of episodic peak ozone levels to regional reductions in VOC and NOx precursors and the increase in the annual mean maximum values to intercontinental trans-Atlantic transport. Wilson et al. (2012) have quantified and modeled European ozone annual and seasonal trends from 158 rural background monitoring sites over the period 1996–2005. Overall, there was reported to be a small net positive annual trend of 0.16 ± 0.02 ppb yr−1 in the annual mean ozone concentration. The average of the annual average ozone levels in the 158 rural monitoring sites was approximately 30 ppb. With the exception of Austria-Hungary, Wilson et  al. (2012) did not find that reductions in anthropogenic NOx and VOC emissions had a significant effect on the observed annual mean ozone levels in Europe. Wilson et  al. (2012) concluded that, over the period 1995–2006, the expected negative ozone trend associated with decreased NOx and VOC emissions in Europe was

30

the mechanisms of reactions influencing atmospheric ozone 160

National annual 4th maxium 8-hour average [O3], ppb

140 120

90% Percentile

100 80 60

10 % Percentile

40 20 0 1980

1985

1990

1995 Year

2000

2005

2010

FIGURE I-D-21. Annual average 8 hour ozone concentrations from 247 sites in the United States (the 90th and 10th percentiles are indicated by open symbols). The dashed line is the National Ambient Air Quality Standard (NAAQS) of 75 ppb (US EPA, 2012).

600

Maximum 1-hr Average 500

[O3], ppb

400 300 200

Mexico City

100 0

1990

2000

2010

Year FIGURE I-D-22.

Maximum 1 hour average ozone concentration in Mexico City (Sistema de Monitoreo Atmosférico, 2012).

masked by a number of factors including meteorological variability and changes in background hemispheric ozone. Data concerning the historical ozone trends in Asia are less available than for North America and Europe. Ghude et  al. (2008) and Kulkarni et  al. (2010) have assessed the ozone levels measured at the surface and in the troposphere above Delhi, Hyderabad, and Bangalore. Ghude et  al. (2008) reported surface ozone levels measured using UV photometry in Delhi from 1997 to 2004 and found

that the daily maximum ozone levels had increased at an average rate of 1.7 ± 0.7 ppb yr−1. The maximum ozone concentration observed over the 7 years reported occurred in the month of April and was 64 ppb. Kulkarni et  al. (2010) used satellite measurements to derive estimates of tropospheric ozone by subtracting stratospheric ozone levels from the total ozone column over Delhi, Hyderabad, and Bangalore in 1979–2005. Kulkarni et  al. (2010) report that there was a statistically significant increase in tropospheric ozone levels over these cities after 1990.

Ozone in the Atmosphere Ohara and Sakata (2003) analyzed the trend in photochemical oxidant (O3 + NO2) concentration from monitoring stations across Japan from 1985 to 1999. The oxidant concentration was found to have increased in 82% of the monitoring stations, with an average rate over Japan of 0.33 ppb yr−1 (1.1% yr−1). The increases in oxidant levels over the period 1985–1999 were slightly larger in spring and summer. Increasing photochemical oxidant levels over Japan reflect increased transport of ozone and precursors associated with economic development in East Asia (Ohara et al., 2007). Although pollutant measurement data for sites in China are rather difficult to obtain for researchers outside China, it is clear that high ozone concentrations are observed in many Chinese cities in the Beijing, Pearl River Delta, and Yangste River Delta areas (Wang and Hao, 2012). Researchers at Peking University measured the diurnal variations of episodic ground-level ozone in Beijing over 1982–2003 and reported that, since the 1990s, there has been a sharp increase in ozone concentrations, which now often exceed 100 ppb (200 μg m−3; Shao et al., 2006). Ran et al. (2009) measured daily 1 hour ozone maxima as high as 128 ppb in an urban site in Shanghai in May 2007. Ran et al. (2012) report an average daily maximum 1 hour ozone level of approximately 110 ppb in a study conducted in suburban Tianjin during July–August 2010. Wang et al. (2006) reported measurements of surface ozone in a mountainous area approximately 50 km north of Beijing in 2005. Of the 39 days of observation, there were 13  days when the 1 hour average exceeded 120 ppb, 8 days when it exceeded 180 ppb, and the highest 1 hour reading was 286 ppb (Wang et al., 2006). Measurements at Miyun (100 km northeast of Beijing) in 2006 showed that ground-level ozone concentrations often exceed 120 ppb in summer. Zhao et al. (2009) analyzed an episode of highly elevated ozone concentration on June 9–14, 2004, and concluded that an area of high ozone with maximum 8 hour averages in the range 80–140 ppb extended over more than 1  million km2 of the East China plains (home to more than 800 million people). Although yields of rice, wheat, and soybeans increased by a factor of approximately 1.5–2.0 over the period 1980–1995, Aunan et al. (2000) and Wang et al. (2005) have suggested that surface ozone have led to lower yields (perhaps by 20–30% for winter wheat) than would otherwise be the case.

31

The introduction of emission control technologies on vehicles and other sources has resulted in substantial progress in reducing ozone precursors in North America and Europe. NOx emissions in the US and Europe have declined by approximately 40–50% since their peaks in the 1980s–1990s (Granier et al., 2011; US Department of Commerce NEI, 2012b). In contrast, the recent economic development in East Asia has led to large increases in the emissions of ozone precursors. For example, NOx emissions in China increased at an average annual rate of 7% between 2001 and 2006 (Ohara et  al., 2007)  and approximately doubled from 2000 to 2008 (Wang and Hao, 2012). As a result of improved emission control technologies, the ozone levels in many cities in the developed nations have decreased substantially. In contrast, high ozone levels are being experienced in urban areas in many developing nations. Ozone and its precursors can be transported over large distances, and changes in emissions on one continent can have impacts on other continents, as evidenced by the increased ozone levels in the background air arriving on the Pacific coast of North America from Asia. I - E . S U M M A RY O F MEASUREMENTS OF S T R AT O S P H E R I C   O Z O N E Measurements of stratospheric ozone have been performed since the 1920s with Dobson spectrometers (Dobson and Harrison 1926; Dobson et  al., 1927). As described earlier in this chapter, Dobson spectrometers are located on the ground and compare the relative intensity of solar flux at pairs of UV wavelengths. The UV wavelength pairs are chosen such that the solar flux entering the atmosphere is similar, but the absorption cross-sections for ozone are significantly different. Using the absorption cross-sections for ozone the column, concentration can be calculated. Since 1957 (International Geophysical Year) a network of Dobson spectrometers has been recording total ozone columns around the globe. In the 1970s, the Brewer instrument was developed. It operates on the same principle as the Dobson spectrometer but has more modern components (e.g., holographic gratings instead of quartz prisms to disperse the UV spectrum and modern electronics enabling automatic readings). The instrumental precision of well-maintained Dobson and Brewer instruments has been estimated to be 0.5% and 0.15%, respectively (Scarnato et al., 2010).

the mechanisms of reactions influencing atmospheric ozone

Satellite measurements of ozone date from 1970, with the launch of the NIMBUS 4 satellite that carried the Backscatter UV (BUV) instrument. This was augmented in 1978 with the launch of the NIMBUS 7 satellite that carried the Total Ozone Mapping Spectrometer (TOMS) and the Solar Backscatter UV 2 (SBUV/2). BUV, TOMS, and SBUV/2 observe reflected UV radiation, and total ozone can be retrieved by comparing the relative intensity of scattered UV radiation at wavelengths near 300 nm. In 2004, the Aura satellite was launched that carries the Ozone Mapping Instrument (OMI). Total ozone columns measured by satellites are validated with measurements from ground observations as the satellite passes over. When comparing ground-based total ozone measurements with satellite overpass data, the standard deviation of monthly differences was on average about 1.5% and within 0.6–2.6% for 90% of Dobson and Brewer network stations (WMO, 2011). Data from the BUV, TOMS, SBUV/2, and OMI satellite instruments have been merged to provide a total column ozone dataset (Stolarski and Frith, 2006; Douglass et  al., 2011). Column ozone measured using ground stations during 1964–1980 did not display a significant trend (Douglass et al., 2011). Over the period 1980–1995, there was an approximately 3–4% decrease in the total column ozone. However, since about 2000, there has been no discernible further decrease in ozone levels. Ground-based measurements show that erythemal (“sunburning”) irradiance over mid-latitudes has increased since the late 1970s, in qualitative agreement with the observed decrease in column ozone

(WMO, 2011). Erythemal irradiance monitored at eight sites in Europe increased by approximately 3–5% from the 1970s to 2010 (WMO, 2011). One of the more dramatic developments in our understanding of ozone chemistry and stratospheric ozone levels occurred as a result of a series of measurements of the ozone column above the British Antarctic Survey research stations at Halley Bay in Antarctica and on the Argentine Islands (Farman et  al., 1985). Measurements have been made using a Dobson spectrometer since 1957 and show large decreases in the ozone levels in spring in the 1970s and early 1980s. There were no changes in the circulation patterns, and Farman et  al. concluded that the very low temperatures in the Antarctic stratosphere from mid-winter through the spring equinox, coupled with the UV irradiation available in the spring, made the region uniquely sensitive to chemical loss of ozone associated with the increasing chlorine from the build-up of CFCs in the atmosphere. As discussed in Section I-F-2, heterogeneous reactions on polar stratospheric clouds play a significant role in this chemistry. The observations of Farman et al. were confirmed by satellite measurements that revealed the continental size extent of the ozone loss over Antarctica. The springtime loss of ozone over Antarctica has become known as the “ozone hole.” Figure I-E1 shows the area and minimum mean ozone for the period 21 September–16 October from 1979 to 2011 (NASA GSFC, 2012b). The Antarctic ozone hole is defined as the region with ozone values below 220 DU located south of 40°S. As seen from Figure I-E-1, both the area and depth of the ozone hole increased

Ozone hole area, millions km2

30

25

300 Area Ozone Minimum

250

20

200

15

150

10

100

5

50

0

1980

1990

2000

2010

0

Year FIGURE I-E-1.

Area and minimum ozone concentration in the Antarctic ozone hole (NASA, 2012b).

Minimum ozone concentration, dobson units

32

Ozone in the Atmosphere from 1979 through to the mid-1990s. Fortunately, since the mid-1990s, there has been no further deterioration of the ozone layer over the Antarctic (closure of the ozone hole will take many decades given the long atmospheric lifetime of CFCs). A similar but more variable and less pronounced seasonal decrease in ozone levels is observed in the Arctic. The topology of the Northern Hemisphere, with its sizable mountain ranges and land masses located close to the Arctic circle, renders the polar vortex much less stable. Very cold conditions in the Arctic stratosphere are much less stable and shorter lived than in the Antarctic where there is a strong and persistent polar vortex. The ozone loss in the Arctic is generally not as extensive or persistent as that in the Antarctic, although in some years the magnitude of the ozone loss might reasonably be described as an Arctic ozone hole (Manney et  al., 2011). In the Antarctic, large ozone losses produce a clear increase in surface UV radiation. Ground-based measurements show that the average spring erythemal irradiance for 1990–2006 is up to 85% greater than the modeled irradiance for 1963–1980, depending on site (WMO, 2011). The Antarctic spring erythemal irradiance is approximately twice that measured in the Arctic for the same season (WMO, 2011). I - F. O Z O N E A N D I T S M E C H A N I S M S O F F O R M AT I O N AND DESTRUCTION IN THE AT M O S P H E R E I-F-1. Ozone Formation and Destruction in the Unpolluted Stratosphere As we have seen from the earlier discussions in this chapter, Chapman (1930) was the first to propose a chemical mechanism explaining the formation of ozone in the stratosphere. The reactions proposed by Chapman were: O2 + hν → O + O

(1)

O + O + M → O2 + M

(2)

O + O2 + M → O3 + M

(3)

O + O3 → 2O2

(4)

O3 + hν → O2 + O

(5)

In these reactions, M represents a third body, such as N2, that can remove the excess energy released during the formation of the chemical bond. Measurements of the rate coefficient for

33

reaction (2)  have shown that it is too slow to be of significance in the stratosphere. The rate of reaction (2)  increases with the square of the O atom concentration, and it is important in the mesosphere where the O atom concentration is higher (see Figure I-B-7). The photolysis reactions (1)  and (5)  can produce O atoms in the ground state, O(3P), or the electronically excited, O(1D) state. For O2 photolysis, the formation of O(1D) is energetically possible for wavelengths of less than 175.9 nm. For O3 photolysis, the formation of O(1D) is energetically possible for wavelengths of less than 310 nm. The main fate of O(1D) atoms in the stratosphere is collisional relaxation to O(3P): O(1D) + M → O(3P) + M

(6)

Reactions (3) and (5) interconvert O and O3. The rates of these reactions are such that, in the sunlit stratosphere, the lifetimes of O and O3 are of the order of a few minutes. After sunset, the production of O atoms in reactions (1) and (5) ceases, but loss via reactions (3) and (4) continue, so the concentration of O atoms collapses. In contrast, there is very little diurnal variation of O3 concentration in the stratosphere because both the source and sink reactions drop to essentially zero at night. Reaction (1)  followed by (3)  is the source of stratospheric ozone. The absorption spectrum and photolysis quantum yields of O2 are known, so the rate of production of odd oxygen (and hence ozone) following molecular oxygen photodissociation can be calculated. Figure I-F-1 shows the zonal average rates of ozone production as a function of altitude calculated for the Northern Hemisphere spring equinox (March 22). The solar flux is greatest in the tropics, and the partial pressure of O2 decreases exponentially with altitude, which leads to a maximum in ozone formation at approximately 40 km in the tropics. Figure I-F-2 shows the zonal average ozone concentrations observed on March 22 as a function of altitude. Comparison of Figures I-F-1 and I-F-2 shows that regions with the highest observed ozone concentrations (lower stratosphere over Arctic) do not correspond to the locations where the ozone production rate is the greatest (upper stratosphere over tropics). The explanation for the difference between the regions of maximum ozone production and the regions of maximum concentration and the asymmetry in latitudinal distribution evident in Figure I-F-2 lies in transport effects and different loss

34

the mechanisms of reactions influencing atmospheric ozone

40

Altitude (km)

30

3 x 106 1 x 106 3 x 105 1 x 105

20

1 x 104 1 x 102 1 x 101 10−2 10−4 10−6

10

0 North pole

60°

30°

0

30°

60°

South pole

Latitude −3 −1

FIGURE I-F-1.

Zonally averaged rate (molecule cm s ) of ozone formation from oxygen photolysis as a function of altitude. Reprinted with permission from Johnston (1975). Copyright 1975 American Geophysical Union.

50 0.1 0.5 1

Altitude (km)

40

2 3 4

30

4 3 2 1

5 20 7

6

0.5 10 0.5 0 −90

−45

0

45

90

Latitude

Zonally averaged observed ozone concentrations (in units of 1012 molecule cm−3) as a function of altitude. Reprinted with permission from Johnston (1975). Copyright 1975 American Geophysical Union.

FIGURE I-F-2.

rates in the different regions. Air enters the stratosphere driven by powerful convection in the tropics. Vertical movement within the stratosphere is slow, and the air is stratified. Air within the stratosphere moves toward the poles, cools, and subsides. The photochemical lifetime (concentration divided by photochemical formation rate) of ozone in the tropics ranges from 1 to 2 days at 40 km to several years

at 15 km. The time scale for transport from the tropics to the poles is several months; hence, ozone-rich air at 15–25 km survives transport to the poles, but ozone at 30–40 km does not. Applying a steady-state analysis to O3 gives d[O3]/dt = k3[O][O2][M] – k4[O][O3] – k5[O3] = 0 [O]/[O3] = k5/(k3[O2][M] – k4[O3])

Ozone in the Atmosphere The concentration of O2 is many orders of magnitude greater than that of O3, and, under stratospheric conditions, k3[O2][M] is substantially greater than k4[O3]; hence, to a good approximation, [O]/[O3] = k5/(k3[O2][M]) With increasing altitude, both [O2] and [M] decrease exponentially; hence, the ratio [O]/[O3] increases very rapidly. However for altitudes below 50 km, the concentration of ozone exceeds that of O atoms by at least an order of magnitude (see Figure I-C-7). Because O and O3 interconvert rapidly in the daytime stratosphere, the two species can be conveniently lumped together as “odd oxygen,” Ox. Applying a steady-state analysis for odd oxygen gives d[Ox]/dt = 2k1[O2] – 2k4[O][O3] = 0 and substituting for [O] using [O] = k5[O3]/(k3[O2] [M]) (see earlier) and rearranging gives [O3] = (k1k3[M]/k4k5)0.5 [O2] As noted by Jacob (1999), the simplicity of this expression for [O3] provides the misleading impression that calculation of the ozone profile is straightforward from laboratory measurements of the rate coefficients for reactions (1, 3, 4, and 5)  and the atmospheric pressure profile. In reality, the estimation of the rate of photolysis of O2 and O3 at a given altitude relies on knowledge of the ozone column above that altitude. The ozone concentration must be calculated numerically in a model starting at the top of the atmosphere and working downward in incremental steps, calculating the ozone concentrations for each step and making allowance for the resulting attenuation in UV in the subsequent step. The resulting profile of ozone is given in Figure I-F-3. As seen in Figure I-F-3, the simple Chapman mechanism oxygen-only chemistry correctly predicts the formation of a layer of ozone at an altitude of approximately 25 km. However, although the shape of the ozone layer matches well with observations, the magnitude of the ozone concentration predicted by the Chapman chemistry is greater by approximately a factor of 2 than the

35

observations. Clearly, there are ozone loss processes missing from the Chapman scheme that need to be considered. With regard to physical loss processes, photolysis is included in the Chapman scheme, and it is difficult to imagine other physical loss mechanisms in the stratosphere. With regard to chemical loss processes, given the large concentration of ozone relative to other species in the stratosphere (see Fis I-C-7 and I-C-8), the processes must not be simple reactions (ozone would easily titrate out any other reactant), but must be catalytic in nature. In laboratory experiments, it was observed that ozone levels could be impacted by the presence of trace amounts of water, which led Bates and Nicolet (1950) to suggest that catalytic cycles could be important in stratospheric ozone chemistry. The essence of the catalytic cycle is reaction of a catalyst species X with ozone to give XO, which can react with O atoms to regenerate X: X + O3 → XO + O2 XO + O → X + O2

(7) (8)

Net: O + O3 → 2O2 The net effect of reactions (7) and (8) is loss of odd oxygen without any loss in X and is equivalent to speeding up the rate of reaction (4) in the Chapman mechanism. Bates and Nicolet (1950) recognized the importance of photolysis of water vapor and suggested that the resulting hydrogen-containing radicals can catalyze ozone loss H + O3 → HO + O2 HO + O → H + O2 Net: O + O3 → 2O2 HO + O3 → HO2 + O2 HO2 + O → HO + O2 Net: O + O3 → 2O2

(9) (10) and (11) (12)

Inclusion of the HO and HO2 radical chemistry into models of stratospheric chemistry brings the ozone profiles into better agreement with observations (Hunt, 1966). Reactions (9) to (12) involve species of the so-called HOx family (H, HO, HO2) and are thus known as HOx cycles. An additional O3-destroying cycle involving NOx was identified by Crutzen (1970) and Johnston (1971); they recognized the importance of the formation of NO in the stratosphere by

36

the mechanisms of reactions influencing atmospheric ozone 70

60

Balloon Goddard JPL Microwave SAGE Calculated, O3, O2, O(1D), O3P) Chemistry Only

Altitude, km

50

40

30

20

10 1010

1011

1012

[O3], molecule

1013

1014

cm−3

FIGURE I-F-3. Comparison of several different measurements of the [O3] vertical profile during an instrument intercomparison at the Mauna Loa Observatory during August 1995 (McPeters et al., 1999). Also shown for comparison are the results of a Chapman-like calculation of the profile using current rate coefficients for oxygen-only chemistry. Although the altitude of the maximum [O3] and the shape of the profile are predicted qualitatively, catalytic cycles involving other trace components (Ox, NOx, HOx) as described in the text are required to match the [O3]max.

reaction of O(1D) atoms with N2O and also by aircraft emissions into the stratosphere and suggested that reactions (13–16)—involving nitrogen oxides can deplete stratospheric ozone: NO + O3 → NO2 + O2

(13)

NO2 + O → NO + O2

(14)

Net: O + O3 → 2O2

and

NO + O3 → NO2 + O2

(13)

NO2 + O3 → NO3 + O2

(15)

NO3 + hν → NO + O2

(16)

Net: 2O3 → 3O2 Molina and Rowland (1974) and Stolarski and Cicerone (1974) suggested that a catalytic cycle based on chlorine oxides could also contribute. Furthermore, Molina and Rowland (1974)

suggested that photolysis of CFCs in the stratosphere could become an importance source of chlorine atoms, and, if the CFC emission trends continued, this would threaten the ozone layer: CFCl3 + hν → CFCl2 + Cl Cl + O3 → ClO + O2 ClO + O → Cl + O2 Net: O + O3 → 2O2 A similar concern was raised based on a catalytic cycle based on bromine released from the photolysis of bromine-containing compounds such bromofluorocarbons and bromochlorofluorocarbons, which are known as “Halons” (e.g., CF2ClBr, Halon-1211, CF3Br, Halon-1301; and CF2BrCF2Br, Halon-2402):

Ozone in the Atmosphere CF2Br2 (Halon-1202) + hν → CF2Br + Br Br + O3 → BrO + O2 BrO + O → Br + O2 Net: O + O3 → 2O2 The HOx, NOx, ClOx, and BrOx catalytic ozone destruction cycles given here do not operate in isolation but are interconnected by coupling reactions and common reservoir species. Examples of coupling are the reactions of ClO radicals with BrO radicals and NO that give rise to the cycles: ClO + BrO → Cl + Br + O2 Cl + O3 → ClO + O2 Br + O3 → BrO + O2 Net: 2O3 → 3O2

and

ClO + NO → Cl + NO2 Cl + O3 → ClO + O2 NO2 + O → NO + O2 Net: O + O3 → 2O2 An example of a reservoir species is chlorine nitrate (ClONO2) formed in the reaction of ClO radicals with NO2. ClONO2 is unreactive towards ozone, and its formation removes ClO and NO2 that would otherwise be free to participate in ozone destruction reactions. ClONO2 is a particularly important reservoir species because it removes species from both the ClOx and NOx cycles. The removal is only temporary because there are processes such as photolysis that return ClONO2 to more active chemical forms that can continue to destroy ozone: ClONO2 + hν → Cl + NO3 Cl + O3 → ClO + O2 NO3 + hν → NO + O2 NO + O3 → NO2 + O2 ClO + NO2 + M → ClONO2 + M Net: 2O3 → 3O2 As seen in Figures I-C-7 and I-C-8, ozone is present at concentrations many orders of magnitude greater than any of the HOx, NOx, ClOx, and BrOx species. The cycles described here are effective in converting ozone back into oxygen because of their catalytic nature. Under typical stratospheric conditions, chlorine atoms loop through the Cl → ClO → Cl cycle approximately 105–106 times before the cycle is terminated by chlorine being converted into an inactive form. Hence, although the

37

concentrations of HOx, NOx, ClOx, and BrOx are very small, their impact on ozone is large. The cycles are interrupted when the reactive chain carrying species are converted into less reactive compounds. The most important chain breaking reactions are the formation of nitric acid (HNO3) and hydrochloric acid (HCl): HO + NO2 + M → HNO3 + M Cl + CH4 → HCl + CH3 There are processes, such as reaction with HO radicals, that convert HNO3 and HCl into the more active ozone-depleting species NO3 radicals and Cl atoms. However, the rates of such processes are typically rather slow, and HNO3 and HCl act as effective sinks for HOx, NOx, and ClOx species. HNO3 and HCl are transferred from the stratosphere to the troposphere where they are subject to removal from the atmosphere by rain out. Interestingly, the sinks for BrOx radicals (HBr, BrONO2) are not very effective because these species are reactive and rapidly photolyzed. As a result, on a molecule-for-molecule basis, the ozone-destroying ability of BrOx radicals is much greater than for HOx, NOx, or ClOx. When released into the stratosphere, Br atoms are 30–50 times more effective than chlorine atoms in removing ozone (Danilin et  al., 1996; WMO, 2011). Fluorine atoms are produced in the oxidation of halocarbons but are present at very low levels because they are rapidly converted into HF, a stable reservoir species that does not react with ozone. Iodine atoms participate in catalytic ozone destruction cycles, but most iodine-containing halocarbons do not survive transport through the troposphere to the stratosphere. In contrast to C–F and C–Cl bonds, C–I bonds absorb strongly at tropospherically relevant wavelengths and iodine-containing haloalkanes have an atmospheric lifetime of days (or less) with respect to photolysis in the troposphere (e.g., with overhead sun, 4.9  days for CH3I, 4.3  days for C2H5I, 4.9 hours for CH2ICl, and 4.9 minutes for CH2I2 (Calvert et al., 2008; see Table VIII-F-1). The combined effect of the HOx, NOx, ClOx, and BrOx reactions accounts for approximately 75% of the loss of ozone within the stratosphere. The remaining 25% is attributable to the O + O3 reaction (4) in the oxygen-only Chapman mechanism. The combined impact of the HOx, NOx, ClOx, and BrOx cycles can be viewed as having the effect of quadrupling

38

the mechanisms of reactions influencing atmospheric ozone

the rate of reaction (4). Recalling from earlier that [O3] = (k1[O2]/ k4)0.5, the impact of a fourfold increase in k4 is a twofold decrease in [O3], which brings the ozone profile calculated using photochemical models into agreement with observations. The situation is somewhat more complex than implied earlier because the contribution of the different cycles varies with altitude. However, the general principle that inclusion of the impacts of the cycles brings the modeled profile into agreement with observations holds. I-F-2. Stratospheric Ozone Depletion: The Role of Halocarbons The ClOx and BrOx ozone destruction cycles discussed earlier play an important role in determining the levels of stratospheric ozone. Natural emissions of CH3Cl and CH3Br from vegetation and the oceans are substantial, and they are the most abundant chlorineand bromine-containing organic compounds in the atmosphere—present at levels of approximately 550 ppt and 7 ppt, respectively (WMO, 2011). The atmospheric oxidation of CH3Cl and CH3Br in the troposphere is initiated by reaction with HO radicals, and these compounds have atmospheric lifetimes of approximately 1.0 and 0.7  years, respectively. With such short lifetimes, the majority of CH3Cl and CH3Br emitted into the troposphere does not survive transport to the stratosphere; however, significant amounts of CH3Cl and CH3Br do reach the stratosphere. Once in the stratosphere, CH3Cl and CH3Br can undergo photolysis or reaction with HO radicals leading to the

liberation of Cl and Br atoms that can then participate in ClOx and BrOx ozone-destruction cycles. Molina and Rowland (1974) recognized the threat posed by the release of chlorofluorocarbons (CFC) into the atmosphere. In the 1970s, CFCs were finding increased use in a variety of consumer products such as the propellants for aerosol cans, foam blowing agents, and refrigerants for air conditioning and domestic refrigeration units. Lovelock and Maggs (1973) measured CFCl3 (CFC-11) in air samples and showed that the amount observed in the atmosphere (50 ppt global average in 1971–1972) was indistinguishable from that expected from the estimated global emissions up to that date. The implication was that CFCs were inert in the troposphere and that with continued emissions their atmospheric concentration would continue to rise. Molina and Rowland (1974) pointed out that although there were no loss processes for CFCs in the troposphere, when CFCs entered the stratosphere, they would undergo photolysis when they reached an altitude above approximately 20–30 km and that chlorine atoms produced would catalyze stratospheric ozone depletion. Furthermore, Molina and Rowland (1974) noted the sharp increase in industrial production of CFCs (growing in the United States at 9% annually over the period 1961–1971) and the potential for substantial future ozone loss. In the 1970s and 1980s, the use, emissions, and atmospheric concentrations of CFCs increased substantially, as shown in Figure I-F-4.

600

CF2Cl2(CFC-12)

Concentration, ppt

500 400 300

CF3Cl (CFC-11)

200 CCl4

100 CFCl2CF2Cl (CFC-113)

0 1950

1960

1970

1980 Year

1990

2000

2010

FIGURE I-F-4. Mean global surface mixing ratios of CFC-11 (CFCl3), CFC-12 (CF2Cl2), CCl4, and CFC-113 (CFCl2CF2Cl) (Bullister, 2011).

Ozone in the Atmosphere From 1971 to 1990, the atmospheric concentration of CFCl3 increased from 50 to 250 ppt (Lovelock and Maggs, 1973; Bullister, 2011; WMO, 2011). The suggestion by Molina and Rowland that increased emissions of CFCs would lead to stratospheric ozone loss led to a large-scale research effort to understand better the stratospheric ozone chemistry and search for evidence of changes in the ozone layer. Legislation was introduced in the United States in the 1970s banning CFCs from uses in aerosol cans, but there was resistance to a complete phaseout of CFC use in all applications. By the early 1980s, photochemical models were predicting that use of CFCs was causing a few percent loss in stratospheric ozone; however, it was difficult to discern such a small change in the observation record. Perhaps the most dramatic change in our understanding of the chemistry of stratospheric ozone occurred with the report of the observation of large springtime losses of ozone in the 1970s–1980s over Antarctica by Farman et al. (1985): the ozone hole. The hole in the ozone layer over the Antarctic is deep, with almost total loss of ozone at some altitudes; large, with an area of approximately 20 million km2; and predictable, with holes being reported every September–October for the past 25 years (see Figure I-E-1; Hofmann et al., 1987; Stolarski et al., 1987; Solomon, 1988; WMO, 1991; Solomon et al., 2003; NASA, 2012a). The observation of the Antarctic ozone hole came as a total surprise to the atmospheric chemistry community and led to expeditions to the Antarctic to gather data on the chemical species present; some of the first of these were Airborne Antarctic Ozone Experiment in 1989, and the second Airborne Arctic Stratospheric Experiment in1991–1992 (https:// www.espo.nasa.gov/missions.php). Laboratory experiments were also conducted to study gas-phase and heterogeneous processes that could potentially explain the phenomenon—as just a couple of examples, see Molina and Molina (1987) and Hanson and Mauersberger (1988). The ozone hole occurs in springtime when sunlight first returns after the winter. The circulation in the stratosphere did not change in the 1970s and 1980s, leaving chemistry as the likely explanation for the ozone loss. As in the discussions earlier regarding explaining deficiencies in the Chapman mechanism, the magnitude of the ozone loss requires an explanation involving catalytic processes. The O atom concentration in the Antarctic stratosphere is low and cycles involving O atoms are probably not important.

39

As stratospheric air travels from the tropics toward the pole, the Coriolis effect results in a strong circumpolar circulation over Antarctica. This polar vortex isolates the air, and this isolation coupled with the absence of the sun results in extremely low temperatures (Brasseur et al., 1999). The low temperatures enable the uptake of nitric acid into H2SO4/H2O background stratospheric aerosol to form HNO3/H2SO4/H2O aerosols, solid particles of ice, and solid hydrates of nitric acid (WMO, 2011). This mixture of aerosols and solid particles forms what are called polar stratospheric clouds. These stratospheric clouds can sometimes be observed from the ground, particularly when they scatter sunlight at sunrise and sunset. HCl can adsorb onto the surface of the polar stratospheric clouds. Heterogeneous reactions on these clouds convert relatively unreactive chlorine reservoir species ClONO2 and HCl into the more active forms, Cl2 and HOCl, and sequester NOx as HNO3 (Brasseur et al., 1999): ClONO2(g) + HCl(s) → HNO3(s) + Cl2(g) ClONO2(g) + H2O(s) → HNO3(s) + HOCl(g) N2O5(g) + HCl(s) → ClNO2(g) + HNO3(s) The levels of Cl2, ClNO2, and HOCl build up during the dark Antarctic winter, and, with the arrival of the sun in the spring, these species are photolyzed to give Cl atoms, which will react rapidly with ozone and be converted into ClO radicals. In the springtime Antarctic lower stratosphere, the usual loss mechanisms for ClO radicals (reaction with O atoms and NOx) are ineffective, and ClO radicals build up to very high levels. At sufficiently high concentrations, self-reaction2 of ClO radicals becomes important and leads to the catalytic ozone destruction cycle (Molina and Molina, 1987): ClO + ClO + M → ClOOCl + M ClOOCl + hν → Cl + OOCl ClOO + M → Cl + O2 + M 2Cl + O3 → 2ClO + O2 Net: 2O3 → 3O2 A flight of the NASA ER-2 from Punta Arenas, Chile, down into the Antarctic vortex on September 16, 1987, provided proof of the importance of ClO radical chemistry (Anderson et al., 1989). As shown in Figure I-F-5, flying at an altitude of approximately 20 km, the ER-2 observed large ozone loss in locations with high ClO concentrations. The ClO concentrations observed in the Antarctic ozone hole

40

the mechanisms of reactions influencing atmospheric ozone 3500 ClO O3

3000

ClO Mixing ratio, ppt

1400 1200

2500

1000

2000

800

1500

600

O3 Mixing ratio, ppb

1600

1000 400 500

200 0

62

63

64

65

66

67

68

69

70

71

72

0

Latitude, degrees South FIGURE I-F-5.

Mixing ratios of ClO and ozone observed during a flight on September 16, 1987, from Punta Arenas, Chile, south into the Antarctic polar vortex in the lower stratosphere at an altitude of approximately 20 km. Reprinted with permission from Anderson et al. (1989). Copyright 1989 American Geophysical Union. This Figure was replotted using the original data files provided by Professor William Brune at Pennsylvania State University.

are remarkably high and range up to 1 ppb, approximately 500 times that observed in the unperturbed stratosphere (Anderson et  al., 1989; Brasseur et  al., 1999; Seinfeld and Pandis, 2006). By the late 1980s, it was well established that human activities were resulting in increased levels of chlorine and bromine in the stratosphere and that this was having a dramatic effect on the protective stratospheric ozone layer. I-F-2.1. Stratospheric Ozone Depletion and the Montreal Protocol The observation of the Antarctic ozone hole provided considerable impetus to international negotiations to ban the use of CFCs and other ozone-depleting substances. It was clear that emission of CFCs from the industrialized countries was having a profound impact on stratospheric ozone and that the large-scale ozone depletion observed over Antarctica would spread to mid-latitudes if coordinated international action was not taken. The negotiations led to the signing of the Montreal Protocol in 1989. The Montreal Protocol and subsequent amendments that have now been signed by all UN-recognized nations provide for reductions in the use and eventual elimination of substances that deplete stratospheric ozone. In the United States, the production of Halons, CFCs, CCl4, hydrobromofluorocarbons, CH3CCl3,

CH2ClBr, and CH3Br was phased out by 1994–2005, and the production of hydrofluorochlorocarbons (HCFCs) had been reduced by more than 65% by 2010 and will be reduced by more than 99.5% by 2020. As a result of the Montreal Protocol, the emissions of CFCs and other ozone-depleting substances has decreased dramatically over the past 20 years and a catastrophic global collapse of stratospheric ozone levels has been avoided (Garcia et al., 2012). As illustrated in Figure I-F-4 the levels of CFCs in the atmosphere plateaued in the 1990s and are now beginning to decline. The influence of the atmospheric lifetime of the different gases is clear from Figure I-F-6. Methyl chloroform (CH3CCl3) has a relatively short lifetime (5 years), and its atmospheric concentration responds rapidly to changes in emissions. In contrast, CFC-12 and CFC-113 have atmospheric lifetimes of 100 and 85 years, respectively, and their atmospheric concentrations respond more slowly to changes in emissions. CFC-11 has an atmospheric lifetime of 45 years, and its atmospheric concentration is more responsive than CFC-12 but less responsive than methyl chloroform. The fact that the atmospheric concentrations of carbon tetrachloride (35  year lifetime) have only declined modestly over the past 30 years reflects continued emissions from developing nations (WMO, 2011).

Ozone in the Atmosphere 1000

41

CH3Cl

Concentration, ppt

CF2ClH CCl4

100

CF3CFH2 CH3CCl3

10

Halon-1211

1

1995

2000

2005

2010

Year FIGURE I-F-6. Mean global surface mixing ratios of CH3Cl, CF2ClH (HCFC-22), CCl4, CF3CFH2 (HFC-134a), CH3CCl3 (methyl chloroform), and CF2BrCl (Halon-1211) (NOAA, 2012).

Industry has followed two general approaches in addressing the need to phase-out the use of ozone-depleting substances:  either redesigning the process or product to remove the need for the functionality provided by the CFC, or replacing CFCs with molecules that have similar physical properties but are more environmentally friendly. An example of the former approach is a shift from aerosol propelled consumer products to hand-pumped or roll-on alternatives (e.g., for deodorants and antiperspirants). An example of the latter approach is the shift in air conditioning (AC) and refrigeration systems from CFCs to hydrofluorocarbons (HFCs) or HCFCs. In the 1990s, vehicle AC systems used CFC-12 (CF2Cl2) as the working fluid. By 2000, most manufacturers had switched from CFC-12 to HFC-134a (CF3CFH2) in vehicle AC systems. HFCs do not contain chlorine and hence do not contribute to the well-established chlorine-based catalytic ozone destruction cycles. Whereas HCFCs such as HCFC-22 (CF2ClH) contain chlorine, they also contain hydrogen. The presence of hydrogen in these species provides an Achilles heel that allows for HO radical-initiated oxidation in the troposphere. Reaction with HO radicals limits the tropospheric lifetime of HCFCs and limits the amount of HCFCs that can enter the stratosphere. For example, the atmospheric lifetime of HCFC-22 is 12  years compared to 100  years for CFC-12 (CF2Cl2). To place discussions of the

relative contributions of individual compounds on stratospheric ozone and climate change on a quantitative basis, the metrics of ozone depletion potential (ODP) and global warming potentials (GWP) were developed (IPCC 2007; WMO, 2011). These metrics approximate the integrated impact of the emission of a given gas relative to that for the emission of the same mass of a reference compound (CFC-11 for ODP and CO2 for GWP). Uncertainties in translating emissions into absolute environmental impacts tend to cancel, and the relative benefits of controlling emissions of different gases are apparent. ODPs and GWPs have found widespread use in international agreements (e.g., the Montreal Protocol and Kyoto Protocol) and in national regulatory discussions. ODP is defined as change in global ozone per unit mass emission of a specific compound relative to the change in global ozone per unit mass emission of CFC-11 (CFCl3; Fisher et  al., 1990; Solomon et al., 1992; Wuebbles, 1983). CFC-11 was a widely used industrial compound in the 1970s and 1980s and so has been chosen as a convenient reference gas (Wuebbles and Chang, 1982; Wuebbles, 1983; Fisher et  al., 1990). The ODP of a well-mixed ozone-destroying species x is defined by: ODP(x) =

ozone loss per unit massemission of x ozone loss per unit mass emission of CFC-11

42

the mechanisms of reactions influencing atmospheric ozone

Ozone loss is calculated using chemistry-transport models. Calculation of the GWP of a compound requires knowledge of its radiative efficiency and global lifetime. Radiative efficiency is the change in net radiation at the tropopause for a given change in greenhouse gas concentration or mass. Radiative efficiency is usually specified in units of W m−2 ppb−1 but it can be given in units of W m−2 kg−1; it is calculated using radiative transfer models and depends on the strength and spectral position of a compound’s absorption bands. The absolute global warming potential (AGWP) for time horizon t′ is defined as: t′

AGWPx (t ′ ) = ∫ Fx [x(t )]dt 0

where Fx is the radiative efficiency per unit mass of species x, x(t) describes the decay with time of a unit pulse of compound x, and t′ is the time horizon considered. The AGWP has units of W m−2 kg−1 yr and quantifies the future integrated radiative forcing to the time horizon for a 1 kg pulse emission of a greenhouse gas. The GWP is the ratio of the AGWP of species x to the AGWP of CO2 for a given time horizon t′ (IPCC, 2007; WMO, 2011):

()

GWPx t ′ =

()= (t ′ )

AGWPx t ′ AGWPCO2

∫

 −t  Fx exp   dt  τx 

t′

0

∫

t′

0

FCO 2 R (t ) dt

where FCO2 is the radiative efficiency of CO2, R(t) is the response function that describes the decay of an instantaneous pulse of CO2, and the decay of the pulse of compound x is written above assuming that it obeys a simple exponential decay curve with a response time of τx. The pulse response terms lead to a dependence of GWPs on the integration time horizon; compounds that decay more quickly (slowly) than the reference (CO2) have GWPs which decrease (increase) with increasing time horizon (WMO, 2011). Radiative efficiencies, lifetimes, ODPs, and GWPs for selected gases are listed in Table I-F-1. Interim replacements such as the HCFCs have substantially lower ODPs than the CFCs they replace. Longer term replacements such as the HFCs do not contain chlorine or bromine and hence do not deplete stratospheric ozone. Typically, the replacement compounds have much shorter atmospheric lifetimes than CFCs and hence have much lower GWPs. However, as indicated in Table I-F-1 there are HFCs such as HFC-23

(CHF3) and HFC-125 (CF3CHF2) that have GWPs comparable to those of CFCs (WMO, 2011). Unsaturated HFCs (also known as hydrofluoroolefins or HFOs) and hydrofluoroethers (HFEs) are also either under consideration or in use as replacements for CFCs and other ozone-depleting substances. Given the commercial importance of such replacements, there has been a large body of research conducted on their atmospheric chemistry and environmental impacts. The reaction of OH radicals with unsaturated HFCs proceeds rapidly, and, as a consequence, unsaturated HFCs have short atmospheric lifetimes (typically 1–20  days). With such short lifetimes, the contribution of unsaturated HFCs to radiative forcing is negligible. As an example, HFC-1234yf (CF3CF=CH2) has a GWP of approximately 4 (Nielsen et al., 2007; Papadimitriou et al., 2008). The atmospheric removal mechanism for HFEs is reaction with HO radicals. The reactivity of HFEs toward HO radicals decreases with increasing degree of fluorination of the ether. Highly fluorinated HFEs (e.g., CHF2OCF3) can have substantial atmospheric lifetimes and large GWPs. HFEs bearing several hydrogen atoms (e.g., C2H5OC4F9) have short atmospheric lifetimes and small GWPs. The GWPs for HFCs and HFEs span a large range, and it is not possible to make any general statement regarding their GWPs. The climate benefits of each compound need to be assessed on a case-by-case basis. Hayman and Derwent (1997) assessed the potential contribution of HCFCs and HFCs to the formation of tropospheric ozone and concluded that these compounds do not have a large potential to contribute to ground-level ozone formation. The reactivity of HFEs toward OH radicals is comparable to those of analogous HCFCs and HFCs. As with HCFCs and HFCs, HFEs are not expected to make a significant contribution to ground-level ozone formation. HO radicals react rapidly with >C=C< double bonds, and unsaturated HFCs are typically much more reactive than HCFCs, saturated HFCs, and HFEs. The photochemical ozone creation potential (POCP) concept ranks compounds by their ability to form ozone in the troposphere over periods of up to 5  days in northwest Europe (see Chapter IX). POCP values are determined along an idealized straight-line trajectory using a photochemical trajectory model. Using the approach outlined by Derwent et al. (1998) and Jenkin (1998), Wallington et  al. (2010) have estimated POCPs for representative unsaturated HFCs. The results

Ozone in the Atmosphere

43

TABLE I-F-1 . OZONE DEPLETION POTENTIAL (ODP), R ADIATIVE EFFICIENCIES,

ATMOSPHERIC LIFETIMES, AND GLOBAL WARMING POTENTIAL (GWP) FOR SELECTED GA SES

Species

Formula

ODP

Carbon dioxide Nitrous oxide Methane CFC-11 CFC-12 CFC-113 HCFC-22 HCFC-141b HCFC-142b HFC-23 HFC-125 HFC-134a HFC-143a Methyl chloroform Methyl chloride Methyl bromide Halon-1301

CO2 N2O CH4 CFCl3 CF2Cl2 CCl2FCClF2 CHClF2 CH3CCl2F CH3CClF2 CF3H CF3CF2H CF3CFH2 CH3CF3 CH3CCl3 CH3Cl CH3Br CBrF3

1.0 0.82 0.85 0.04 0.12 0.06 0 0 0 0 0.16 0.02 0.66 15.9

Radiative Lifetime GWP 20 GWP 100 GWP500 Efficiency (years) (W m−2 ppb−1) 1.38 × 10−5 3.03 × 10−3 3.7 × 10−4 0.25 0.32 0.30 0.20 0.14 0.20 0.19 0.23 0.16 0.13 0.06 0.01 0.01 0.32

114 12 45 100 85 11.9 9.2 17.2 222 28.2 13.4 47.1 5.0 1.0 0.8 65

1 289 72 6,730 11,000 6,540 5,130 2,240 5,390 11,900 6,290 3,730 5,780 506 45 19 8,480

1 298 25 4,750 10,900 6,130 1,790 717 2,220 14,200 3,420 1,370 4,180 146 13 5 7,140

1 153 8 1,620 5,200 2,690 545 218 678 10,700 1,070 416 1,440 45 4 2 2,760

From IPCC, 2007; WMO, 2011.

TABLE I-F-2 . PHOTOCHEMICAL OZONE CRE ATION POTENTIAL (POCP)

FOR SELECTED HYDROFLUOROOLEFINS AND REL ATED ALKANES, ALKENES, AND HYDROFLUOROCARBONS

Compound

POCP

Compound

POCP

CH2=CH2 CH3CH=CH2 CH3CH2CH=CH2 CH4 C2H6 C3H8 n-C4H10

100 112.3 107.9 0.6 12.3 17.6 35.2

CH2F2 CH3CF3 CH2FCF2 CH3CHF2 CF3CHFCF3 CH2FCHFCF3 CHF2CHFCF3

0.2 0.0 0.1 1.0 0.0 0.2 0.0

Compound CH2=CF2 CF2=CF2 CH2=CHCF3 CH2=CFCF3 CF2=CFCF3 Z-CHF=CFCF3 CH2=CHCF2CF3

POCP 18.0 12.5 10.7 7.0 5.4 5.6 6.6

From Wallington et al., 2010.

are presented in Table I-F-2 together with values for selected alkanes, alkenes, and HFCs. As seen from this Table, the POCPs for unsaturated HFCs (HFOs) are larger than those for longer lived HFCs, smaller than those for the parent alkenes, and, for many compounds (including the commercially significant HFC-1234yf), lie between those for methane and ethane. Methane and ethane are oxidized sufficiently slowly that they do not contribute to any

appreciable degree to local air quality issues and are generally exempt from air quality regulations. Luecken et  al. (2010) conducted an atmospheric modeling study of the impact of replacing all HFC-134a currently used in vehicle AC systems in the United States with HFC-1234yf. They concluded that such large-scale use of HFC-1234yf would result in a less than 0.01% increase in total ozone formed. The use of unsaturated HFCs in mobile AC will not

44

the mechanisms of reactions influencing atmospheric ozone

make a significant contribution to tropospheric ozone formation. The atmospheric degradation of HCFCs, HFCs, HFEs, and unsaturated HFCs is initiated by reaction with HO radicals leading to the formation of halogenated carbonyl compounds that undergo further oxidation to HF, HCl, CO2, and, in some cases, trifluoroacetic acid (TFA; Wallington et  al., 1994b; Calvert et al., 2008; WMO, 2011). Assuming that the combined global emissions of HCFCs, HFCs, HFEs, and unsaturated HFCs (HFOs) are of the order of 100 ktonnes per year, that they are uniformly distributed in the atmosphere, and have an annual global precipitation of 5.79 × 1017 liters (Legates and Willmott, 1990), the concentrations of HF and HCl in precipitation from degradation of HCFCs, HFCs, HFEs, and HFOs will be of the order of 2 × 10−9 molar. Although HF and HCl are strong acids, the concentration of fluoride and chloride and the additional acidity in precipitation resulting from the atmospheric oxidation of HCFCs, HFCs, HFEs, and HFOs would be minor. TFA is a persistent, potentially toxic degradation product of some HCFCs (e.g., HCFC-123, -124), HFCs (e.g., HFC-134a, -227ea), and unsaturated HFCs (e.g., 1234yf, 1225ye [CF3CF=CHF]). Its sources (natural and anthropogenic), sinks, and potential environmental effects have been reviewed by Tang et  al. (1998), Solomon et  al. (2003), and IPCC/TEAP (2005). The World Meteorological Organization (WMO, 2007), concluded that “TFA from the degradation of HCFCs and HFCs will not result in environmental concentrations capable of significant ecosystem damage.” The available data suggest that the same conclusion is applicable to unsaturated HFCs and HFEs. It has been shown that TFA is ubiquitous in precipitation and ocean water even in remote areas (Berg et al., 2000; Von Sydow et  al., 2000; Frank et  al., 2002; Scott et  al., 2006). Frank et  al. (2002) estimated that the oceans contain 268 million tonnes of TFA. At a global level, the natural environmental loading of TFA greatly exceeds that expected from the atmospheric degradation of HCFCs, HFCs, and unsaturated HFCs (Kotamarthi et  al., 1998). Although Tromp et  al. (1995) have argued that TFA will accumulate to high levels in seasonal wetlands, Boutonnet et  al. (1999) showed that the assumptions made by Tromp et  al. were highly improbable. Benesch et  al. (2002) showed that TFA does not adversely affect the development of soil microbial communities and pool plant species in vernal ponds. Luecken et al. (2010) assessed

the TFA concentrations following replacement of all HFC-134a currently used in vehicle AC systems in the United States with HFC-1234yf. Their model predicted peak concentrations in rainfall of 1,264 ng L−1. This level is similar to peak concentrations currently observed (Scott et  al., 2006)  and is approximately two orders of magnitude lower than the level considered safe for the most sensitive aquatic organisms (Luecken et al., 2010). In summary, HCFCs, HFCs, HFEs, and HFOs are not expected to contribute significantly to the formation of tropospheric ozone. In the concentrations expected in the environment, the degradation products of HCFCs, HFCs (saturated and unsaturated), and HFEs do not pose an environmental threat. I-F-3. Ozone Formation in the Troposphere In the early 1950s, Haagen-Smit and co-workers recognized that photochemical formation of ozone from reactions involving NOx and hydrocarbons was an important source of air pollution in Los Angeles (Haagen-Smit, 1952; Haagen-Smit et  al., 1953). However, it was believed by many through the 1960s that the main source of background tropospheric ozone was transfer from the stratosphere (Regener, 1941; Junge, 1962)  in what are called tropospheric folding events. In the early 1970s, it was recognized that oxidation of hydrocarbons such as methane and α-pinene in the presence of NOx represented a large source of ozone in the background troposphere (Ripperton et  al., 1971; Chameides and Walker, 1973; Crutzen, 1973). Computer simulations of ozone generation in polluted atmospheres suggested strongly that ozone in polluted areas was generated by NO2-RH-sunlight interactions (Demerjian et  al., 1974); atmospheric measurements supported this view (e.g., see Calvert, 1976a, 1976b). It is now understood that photochemical formation of ozone occurs throughout the troposphere and that photochemistry involving NOx, hydrocarbons, and CO is the dominant source of ozone in the troposphere. On a global basis, photochemistry accounts for the formation of approximately 4,500 Tg of ozone per year in the troposphere as compared to an annual flux of approximately 500 Tg that arrive from the stratosphere (Wild, 2007; Royal Society, 2008). These production terms are balanced by a chemical loss within the atmosphere of approximately 4,100 Tg and a surface deposition flux of approximately 900 Tg ozone per year (Stevenson et al., 2006; Wild, 2007; Royal Society, 2008).

Ozone in the Atmosphere The key reactions in the formation of tropospheric ozone are shown in schematic form in Figure I-F-7. The chemistry begins with the photolysis of ozone. At wavelengths below approximately 320  nm, the photolysis of ozone gives electronically excited oxygen atoms O(1D). The main fate of the excited O(1D) atoms is collisional relaxation to ground state O(3P) atoms, but a small fraction (approximately 10% under typical tropospheric conditions) react with water vapor to give HO radicals:

OH + CO → H + CO2 H + O2 + M → HO2 + M HO2 + NO → HO + NO2 NO2 + hν (λ < 420 nm) → NO + O O + O2 + M → O3 + M net: CO + 2O2 → CO2 + O3 The HO radical-initiated oxidation of CH4 can be represented by: HO + CH4 → H2O + CH3 CH3 + O2 + M → CH3O2 + M CH3O2 + NO → CH3O + NO2 CH3O + O2 → CH2O + HO2 HO2 + NO → HO + NO2 2 × (NO2 + hν (λ < 420 nm) → NO + O(3P)) 2 × (O + O2 + M → O3 + M) net: CH4 + 4O2 → H2O + 2O3 + CH2O

O3 + hν (λ < 320 nm) → O(1D) + O2(1Δg) O(1D) + H2O → 2 HO O(1D) + M → O(3P) + M O(3P) + O2 + M → O3 + M where M is a third body that removes energy to allow stabilization of the energy-rich O3 molecule initially formed. HO radicals play an enormously important role in initiating the atmospheric oxidation of organic and inorganic air pollutants. Reaction with HO radicals determines the atmospheric lifetimes of the majority of air pollutants. The reaction of HO radicals with CO and hydrocarbons (RH) leads to the formation of HO2 and alkyl peroxy radicals (RO2). These peroxy radicals can undergo reaction with NO leading to the formation of NO2, which then can undergo photolysis to generate oxygen atoms and hence ozone. The HO radical-initiated oxidation of CO can be represented by

The formaldehyde formed in the oxidation of CH4 can react with HO or photolyze, thus leading to further NO2 formation and ozone production: CH2O + HO → H2O + HCO CH2O + hν → H + HCO HCO + O2 → HO2 + CO H + O2 + M → HO2 + M 2 × (HO2 + NO → HO + NO2) net: CH4 + 8O2 → CO2 + 2H2O + 4O3 R CHO

Ozone

hv

hv,

CO, RH

H2O

RO2NO2 NO2

∆ RO2

HO HO2

hv,

H2O2 ROOH

O2

HNO3

45

from HO2

from RO2

NO2

NO HO2, RO2 hv

FIGURE I-F-7.

Schematic of reactions responsible for tropospheric ozone formation. Species in circles are the active species in the cycle of reactions leading to ozone formation. Species in boxes are longer lived sinks or reservoirs for the active species.

46

the mechanisms of reactions influencing atmospheric ozone

As indicated earlier, in principle, the atmospheric oxidation of one CH4 molecule can lead to the formation of four molecules of ozone. In reality, there are processes that are dependent on the NOx concentration and that limit the photochemical production of ozone. There is a complex interplay of reactions involving HO, HO2, and RO2 radicals and NOx in the atmosphere. Some of these reactions lead to ozone formation, and some lead to ozone loss. The balance between these reactions determines whether there is net ozone formation or destruction. Three chemical regimes characterized by the concentration of NOx can be identified. First, at very low NOx concentrations (below approximately 20 ppt) the self-reaction of HO2 radicals and the cross-reaction of HO2 with RO2 radicals compete with NO for the available HO2 and RO2 radicals: HO2 + HO2 → H2O2 + O2 HO2 + RO2 → ROOH + O2 HO2 + NO → HO + NO2 As illustrated in Figure I-F-7, the formation of H2O2 and ROOH removes reactive free radicals from the cycle, limits the conversion of NO into NO2, and hence limits the formation of ozone. In one atmosphere of air at 298 K saturated with water vapor k(HO2 + HO2)  =  7.7  × 10−12 and k(HO2 + NO)  =  8.5  × 10−12 cm3 molecule−1 s−1 (Atkinson et al., 2004). The concentration of HO2 radicals in the marine boundary layer is typically in the range of 1–10 ppt (Stone et al., 2012). Given the similarity in the k(HO2 + HO2) and k(HO2 + NO) rate coefficients, it follows that NO concentrations greater than approximately 20 ppt are required for reaction with NO to match the self-reaction as a loss mechanism for HO2 and RO2 radicals in the marine boundary layer. Large areas of the troposphere, such as that over the South Pacific, are remote from substantial emission sources and have levels of NO that are typically below 20 ppt. In such very low NOx environments there is a net loss of odd oxygen following photolysis of ozone because loss via reaction of O(1D) atoms with H2O exceeds formation by photolysis of NO2 (HO2 and RO2 radicals are removed from the cycle before they can oxidize NO into NO2). In addition to ozone loss attributable to the inefficiency of the cycle shown in Figure I-F-7, in very low NOx environments, direct loss of ozone via reaction with HO2 radicals becomes significant: HO2 + O3 → HO + 2O2

Although the rate coefficient for reaction of HO2 radicals with O3 is rather small—2.0  × 10−15 cm3 molecule−1 s−1 at 298 K (Atkinson et al., 2004)—the O3 concentration is typically more than 1,000 times that of NO (e.g., 20–30 ppb O3 vs. < 20 ppt NO over the South Pacific), and loss of O3 via reaction with HO2 radicals is important in the remote troposphere. Finally, loss of O3 via reaction with HO radicals can play a minor role in remote pristine environments ( Jacob, 1999). The loss of HO radicals in the troposphere is dominated by reaction with CO and, to a lesser extent, with CH4. CO is not well mixed, and its concentration varies geographically and seasonally. CO concentrations can be as low as 50 ppb in pristine locations and, given the rate coefficient ratio k(HO + O3):k(HO + CO) = 7.3 × 10−14/2.3 × 10−13 = 0.3 and the concentration ratio [O3]/[CO] = 20 ppb:50 ppb = 0.4, a small fraction (approximately 10%) of HO radicals will react with O3: HO + O3 → HO2 + O2 Second, at intermediate NOx concentrations of approximately 20–2,000 ppt, the loss of HO2 and RO2 radicals via self- and cross-reactions become of minor importance. As the NO concentration increases, the dominant fate of HO2 and RO2 radicals becomes reaction with NO, which occurs on a time scale of approximately 2 seconds in the presence of 2 ppb (2,000 ppt) of NO. The NO2 formed in these reactions is photolyzed to generate ozone, and HO radicals are reformed that initiate the oxidation of more CO and VOCs to generate more HO2 and RO2 radicals, which continues the ozone formation cycle shown in Figure I-F-7. The lifetime of HO radicals with respect to reaction with VOCs and CO is of the order of seconds (less in polluted regions with higher VOC concentrations); the lifetime of HO2 and RO2 radicals in the presence of 2 ppb of NO is 2.5 seconds; and the rate determining step in ozone formation is photolysis of NO2, which occurs on a timescale of minutes (1.7 minutes for overhead sun at 40o latitude on a cloudless day; see Table VIII-F-1). At intermediate NOx concentrations of approximately 20–2,000 ppt, increased NOx leads to increased rate of NO2 and hence O3 formation. Such conditions are representative of rural areas in industrialized countries. The rate of ozone formation is not dependent on the VOC concentration because conversion of HO radicals into peroxy radicals is already rapid and not rate determining. The rate of formation of ozone is limited by the NOx concentration,

Ozone in the Atmosphere and the chemical environment is often referred to as “NOx-limited” or “NOx-sensitive.” Third, at high NOx concentrations (above ~2 ppb) the reaction of HO radicals with NO2 starts to become a significant loss mechanism for both HO radicals and NO2 and inhibits the formation of ozone: HO + NO2 + M → HNO3 + M The importance of the reaction of HO with NO2 can be appreciated by studying Figure I-F-7. This reaction removes two key species from the ozone-generating cycle (HO and NO2) and produces nitric acid, which is essentially inert to further gas phase atmospheric chemistry. In one atmosphere of air at 298 K, the rate coefficient for the reaction of HO radicals with NO2 is 1.2  × 10−11 cm3 molecule−1 s−1 (Atkinson et al., 2004) and is comparable to those for reactions of HO radicals with reactive organic species (see Chapter IV). In urban areas close to pollution sources, the concentrations of NOx are typically in the range 10–100 ppb, and in highly polluted environments, the NOx concentration can exceed 100 ppb. In polluted environments, the reaction with NO2 becomes an important loss for HO radicals, and this slows down the cycle of reactions leading to ozone formation (see Figure I-F-7). Under such conditions, the addition of more NOx to the system actually decreases the rate of ozone formation. Conversely, the addition of more VOCs provides additional reaction partners for HO radicals, mitigates loss of HO via reaction with NO2, and increases the rate of ozone formation. The rate of formation of ozone is limited by the VOC concentration, and the chemical environment is often referred to as “VOC-limited” or “VOC-sensitive.” In urban areas with high levels of NOx emissions, the concentration of ozone can be depleted because of the reaction with NO: NO + O3 → NO2 + O2 

(1)

This reaction occurs with a rate coefficient of 1.8 × 10−14 cm3 molecule−1 s−1 at 298 K (Atkinson et al., 2004). In the presence of 1 ppb of NO, the lifetime of ozone with respect to reaction with NO is 40 minutes, whereas in the presence of 100 ppb of NO, the ozone lifetime with respect to reaction with NO is just 20 seconds. Photolysis of NO2 occurs on a timescale of a few minutes (see Table VIII-F-1) in the sunlit troposphere and gives NO and an oxygen atom, which adds O2 to regenerate ozone:

NO2 + hν (λ < 420 nm) → NO + O O + O2 + M → O3 + M

47 (2) (3)

The effective second-order rate coefficient for reaction of oxygen atoms with O2 is 1.8 × 10−14 cm3 molecule−1 s−1 at 298 K in one atmosphere of air (Atkinson et al., 2004) and [O2] = 5.2 × 1018 molecule cm−3; hence, the lifetime of oxygen atoms with respect to addition of O2 to give ozone is 14  μs. A photostationary steady state can be assumed for NO2: d[NO2]/dt = k1[NO][O3] − j2[NO2] = 0. Rearrangement leads to an expression for the ozone concentration that is known as the Leighton relationship, recognizing Phillip Leighton’s pioneering work (Leighton, 1961): [O3] = j2[NO2]/k1[NO] This expression provides a useful description of the relationship between [O3], [NO2], and [NO] observed in the daytime in urban areas (e.g., see Clapp and Jenkin, 2001), but significant deviations are expected for certain conditions (Calvert and Stockwell, 1983b). Reactions (1)–(3) rapidly interconvert NO2 into ozone but do not alter the total oxidant in urban air masses, and it is convenient to refer to the sum of [NO2] + [O3] as “total oxidant.” I-F-4. Ozone Destruction in the Troposphere Ozone is removed from the troposphere both by chemical processes within the troposphere and by deposition to surfaces. The main loss processes are chemical in nature. Photolysis of ozone to give O(1D) atoms that then react with water vapor to give HO radicals is an important loss mechanism. At a latitude of 40o at the equinoxes (March 22, September 22), the diurnal cycle average lifetime of ozone with respect to photolysis to give O(1D) atoms is approximately 1.5 days (Calvert et al., 2011, Table VIII-F-1). Under typical tropospheric conditions, the majority (~90%) of the electronically excited O(1D) atoms are quenched to ground state O(3P) atoms by collisions with atmospheric constituents (N2 and O2, mainly). Ground state O(3P) atoms lack sufficient energy to react with water vapor and instead combine with O2 to reform ozone. Hence the chemical lifetime of ozone in the troposphere is typically an order of magnitude greater than the rate of photolysis suggests. As discussed in the previous section, in pristine environments, loss of ozone via

48

the mechanisms of reactions influencing atmospheric ozone

reaction with HO2 radicals and, to a lesser extent with HO radicals, can be important. Ozone reacts rapidly with unsaturated organic compounds (recall that the rate of cracking of rubber was one of the first methods of estimating ozone concentrations). Isoprene (2-methyl-1,3-butadiene) and the related terpenes (isoprene dimers) and sesquiterpenes (isoprene trimers) are emitted in large amounts by vegetation, particularly forests. Ozone reacts rapidly with dienes with rate coefficients of the order of 10−17–10−16 cm3 molecule−1 s−1 at 298 K (see Chapter II) and close to and within forest canopies, the loss of ozone via reaction with isoprene, terpenes, and sesquiterpenes is a locally significant chemical loss mechanism. On a global basis, the loss of tropospheric ozone via the reactions O(1D) + H2O, HO2 + O3, HO + O3, and O3 + alkene, and these account for approximately 40%, 40%, 10%, and 10%, respectively, of the chemical loss of ozone ( Jacob, 1999). Whereas on a global scale the chemical removal processes for ozone exceed the surface deposition by approximately a factor of 4 (Wild, 2007), surface deposition is important in the boundary layer. Ozone is a reactive gas and is lost via reaction with vegetation, soils, and other surfaces. The loss rate is expressed as a deposition velocity; it is 2–15 mm s−1 over land and is controlled by the opening and closing of stomata in plant leaves, wind speed and turbulence, and the presence of surface water on the vegetation (Royal Society, 2008). Ozone is not very soluble in water, and deposition to water surfaces is slower than to land. However, given the fact that oceans cover 70% of the Earth’s surface and that reactions of ozone with halide ions (Br− and I−) and chlorophyll on water surfaces have been reported and augment the loss rate (Chang et al., 2004; Hunt et al., 2004; Clifford et al., 2008; Reeser et al., 2009), loss of ozone to the surface of the world’s oceans is a significant mechanism for ozone removal from marine environments. Halogen chemistry associated with bromineand iodine-containing compounds emitted from open-ocean sources is responsible for significant ozone loss in the tropical marine boundary layer (Read et al., 2008). The importance of halogen chemistry (primarily bromine and iodine) for tropospheric ozone on a global basis has been investigated by Saiz-Lopez et al. (2012). It was reported that the inclusion of chemistry associated with emissions of CHBr3, CH2Br2, CH2BrCl, CHBr2Cl, and CHBrCl2, CH2I2, CH2IBr, and CH2ICl in a global atmospheric chemistry model led to an annually averaged decrease of approximately

10% (2.5 DUs) of the tropical tropospheric ozone column. The largest effects were found in the middle and upper troposphere. This ozone loss corresponds to a decrease of approximately 0.10 Wm−2 in the radiative flux at the tropical tropopause. Parrella et  al. (2012) have noted that the inclusion of bromine chemistry improves the ability of global models to simulate the levels of ozone reported in the 19th century. Finally, rapid ozone destruction has been observed in many studies in the marine boundary layer in polar regions in the spring (e.g., Barrie et  al., 1988; Fan and Jacob, 1992; Kreher et  al., 1997; Tarasick and Bottenheim, 2002; Frieß et al., 2004; Simpson et al., 2007, and references therein; see Section VII-C6.7 for a more detailed discussion). The ozone loss is correlated with an increase in the concentration of bromine (and possibly iodine) compounds in the air (e.g., Hausmann and Platt, 1994; Platt and Lehrer, 1996; Liao et  al., 2011). These compounds are released from the ocean or as a result of reactions on sea-salt aerosols or ice/snow surfaces. Bromine atoms react rapidly with ozone to give BrO radicals (iodine atoms give IO radicals). Depending on the subsequent fate of the BrO and IO radicals, there can be significant ozone depletion. If BrO radicals react with NO, the result is the formation of NO2, which will photolyze to regenerate odd oxygen. However, if BrO radicals react with HO2 or undergo self-reaction, there is a loss of odd oxygen; for example: Br + O3 + M → BrO + O2 + M BrO + HO2 → HOBr + O2 HOBr + hν → HO + BrO Net: HO2 + O3 → HO + 2O2 A key process in these depletion events is the heterogeneous reaction of HOBr with halide ions (Cl− and/ or Br−) to form BrCl and /or Br2, an autocatalytic process that can substantially increase levels of inorganic halogen available in the gas phase for ozone depletion (Fan and Jacob, 1992; McConnell et al., 1992). I-G. MAJOR SOURCES OF THE AT M O S P H E R I C T R A C E G A S E S ( N O X A N D   VO C S ) Formation of tropospheric ozone requires sunlight, NOx, and VOCs. To understand the current trends in ozone levels and to predict the likely future requires knowledge of the sources and emission trends of NOx and VOCs. On a global scale, there is a fundamental difference between the relative magnitude

Ozone in the Atmosphere

NOx emission, Tg N yr−1

of anthropogenic and natural emissions of NOx and VOCs. For NOx, the anthropogenic emissions from fossil fuel combustion, biomass burning, and agriculture are significantly larger than the natural sources from soils, lightning, and NH3 oxidation (Jacob, 1999). In contrast, the global emission of biogenic nonmethane organic carbon compounds (NMOCs) is approximately seven times larger than total anthropogenic emissions (Guenther et al., 1995). Although on a global scale the anthropogenic emissions of VOCs are not significant, on local scales, such as in urban areas, they can be very significant in promoting ozone formation. Figure I-G-1 shows the anthropogenic emissions of NOx estimated for 1980–2010 for the United States, Western Europe, Central Europe, China, and India from the Emissions for Atmospheric Chemistry and Climate Model Intercomparison Project and the Representative Concentration Pathways project (RCP8.5) as described by Granier et al. (2011). In developed nations such as the United States and in Europe there has been a large decrease in NOx emissions over the past couple of decades, whereas in developing nations such as China and India there has been a large increase in NOx emissions. The net result on a global basis is little or no significant change in the overall emissions. The decrease in emissions in the developed nations reflects the introduction of emission control equipment on vehicles and stationary sources. The increase in emissions

in developing nations reflects the large economic growth in these areas. Figure I-G-2 shows the emissions of CO, NMHC, and NOx in the EU-27 countries from 1990 to 2010 (EEA, 2012). Over the past 20 years, there has been a trend of decreasing emissions of the ozone precursors CO, NMHC, and NOx from the EU nations. Figures I-G-3 to I-G-5 show the emission trends from the top five emission sources for the various pollutants. Comparing Figures I-G-3 to I-G-5 to Figure I-G-2, it can be seen that the driving force for emission reductions has been a large decrease in emissions from on-road passenger vehicles. This trend reflects the development and implementation of effective emission control technologies for light-duty vehicles. There is a 10–20 year lag between the implementation of new emission control technology and its diffusion into the on-road fleet because of the time required for the turnover of the on-road fleet. Hence, the emission trend from cars shown in Figures I-G-3, I-G-4, and I-G-5 is expected to continue even in the absence of additional regulations. Over the past 20 years, there has been an interesting transition in the EU from a situation in which cars were clearly the dominant source of CO, NMHC, and NOx emissions in 1990 to a situation in which, in 2010, vehicles are no longer clearly the dominant single source of emissions. However, it should be noted that because of the proximity of

Global Western Europe Central Europe China India US

10

1

1980

1990

2000

2010

Year FIGURE I-G-1.

49

Anthropogenic emissions of NOx in 1980–2010 from various regions (Granier et al., 2011).

50

the mechanisms of reactions influencing atmospheric ozone 100

NOx

Emissions, Mt yr −1

NMHC CO

10

1990

1995

2000 Year

2005

2010

FIGURE I-G-2.

Emissions of CO, nonmethane volatile organic compounds (NMVOC), and NOx in EU-27 countries, 1990–2010 (EEA, 2012).

Residential: Stationary Plants Road Transport: Passenger Cars

CO emissions, kt yr −1

30000

Iron and Steel Production Stationary Combustion in Manufact. Industry Road Transport: Mopeds & Motorcycles

20000

10000

0 1990

1995

2000

2005

2010

Year FIGURE I-G-3.

Emissions of CO by sector in EU-27 countries over period 1990–2010 (EEA, 2012).

vehicles to human activities, emissions from vehicles can lead to significant population exposure. Figure I-G-6 shows the emissions of CO, NMHC, and NOx in France over the period 1960–2011 (CITEPA, 2012). The overall trend evident in Figure I-G-6 is that CO and NOx emissions peaked around 1980 and have declined significantly over the past couple of decades. Figures I-G-7, I-G-8, and I-G-9 show the emissions from the major economic sectors in France. Inspection of Figures I-G-7, I-G-8, and I-G-9 and comparison with Figure I-G-6 shows that the large decrease in emissions from vehicles is the main factor responsible

for the overall national trend of decreased emissions of CO, NMHC, and NOx in France. Figure I-G-10 shows the emissions of CO, VOCs, and NOx in the United States over the period 1970–2012 from the US Department of Commerce National Emissions Inventory (2012b). There is a significant downward trend in US emissions of CO, VOCs, and NOx ozone precursors from the 1980s onward. The overall trend is that CO and NOx emissions peaked around 1980 and have declined significantly over the past couple of decades. Figures I-G-11, I-G-12, and I-G-13 show the emissions of CO, VOCs, and discontinuity in the NHMC values from 2001 to 2002 that reflects a

5000 Heavy-Duty Vehicles Light-Duty Vehicles Electricity Generation Combustion in Manufacturing Agriculture

NOx emissions, kt yr−1

4000

3000

2000

1000

0 1990

1995

2000

2005

2010

Year FIGURE I-G-4.

Emissions of NOx by sector in EU-27 countries over period 1990–2010 (EEA, 2012).

NMVOC emissions, kt yr −1

4000 Residential: Stationary Plants Domestic Solvent Use Industrial Coating Application Road Transport: Passenger Cars Other Product Use

3000

2000

1000

0 1990

1995

2000 Year

2005

2010

FIGURE I-G-5.

Emissions of nonmethane volatile organic compounds (NMVOC) by sector in EU-27 countries over period 1990–2010 (EEA, 2012). NOx NMVOC CO

Emissions, kt yr−1

10000

1000 1960

1970

1980

1990

2000

2010

Year FIGURE I-G-6.

Emissions of CO, nonmethane volatile organic compounds (NMVOC), and NOx in France 1960–2011 (CITEPA, 2012). The data base for NMVOC is less extensive than for CO and NOx.

51

52

the mechanisms of reactions influencing atmospheric ozone

CO emissions, kt yr−1

10000

1000 Energy Transformation Manufacturing Industry Residential Road Transport 100

1960

1970

1980

1990

2000

2010

Year FIGURE I-G-7. Emissions of CO in France 1960–2011 from road transport, energy transformation, industrial manufacturing, and residential sectors (CITEPA, 2012).

NOx emissions, kt yr −1

1000

100

Energy Transformation Manufacturing Industry Residential Road Transport 1960

1970

1980

1990

2000

2010

Year FIGURE I-G-8.

Emissions of NOx in France 1960–2011 from road transport, energy transformation, industrial manufacturing, and residential sectors (CITEPA, 2012).

change in measurement methods employed. Figure I-G-12 shows trends in NOx emissions by the major contributing economic sectors in the United States. As in Europe, there has been a large decrease in the emissions of CO, VOCs, and NOx from the road transport sector, and, as a result, the national emissions are decreasing significantly. Figure I-G-14 shows the concentrations of CO and several VOCs measured in ambient air in the Los Angeles area over the past 50 years (Warneke et al., 2012). As indicated from the Figure, there has been a major decrease in the ambient concentration of

VOCs in the Los Angeles area over the past several decades. Efforts to control urban ozone over the past five decades have led to a decrease in the concentrations of VOCs in ambient air in the Los Angeles area by factors of 30–50. The EPA’s Mobile Source Air Toxics Program has led to even greater reductions in benzene (down by a factor of approximately 65). Formaldehyde and acetaldehyde have decreased at a rate similar to other VOCs. Looking back over a longer time period, Figure I-G-15 shows the NOx emissions from the United States over the past century. There was a large increase

Ozone in the Atmosphere Energy Transformation Manufacturing Industry Residential Road Transport

1000 NMHC emissions, kt yr−1

53

100

1990

1995

2000

2005

2010

Year

Emissions of nonmethane volatile organic compounds (NMVOC) in France 1988–2011 from road transport, energy transformation, industrial manufacturing, and residential sectors (CITEPA, 2012).

Emissions, mtons yr −1

FIGURE I-G-9.

100 NOx NMVOC CO

10 1970

1980

1990 Year

2000

2010

FIGURE I-G-10.

Emissions CO, nonmethane volatile organic compounds (NMVOC), and NOx (reported as NO2) in the United States 1970–2012 (US Department of Commerce, National Emissions Inventory, 2012b).

in emissions of NOx from 1900 through approximately 1980. The emissions peaked during the 1980s and began falling rapidly in the 1990s. The emission pattern shown in Figure I-G-15 for NOx has been rationalized as that expected for a society undergoing economic development. The pollution levels initially increase with development; as society becomes more affluent, the levels of pollution plateau; and, with further affluence and education of society, the pollution levels decrease. This progression is known as the Kuznets curve, and is illustrated in Figure I-G-16 (Kuznets, 1955; Selden and Song, 1994).

It seems clear that the developed nations are on the decreasing emissions side of the Kuznets curve and that further diffusion of emission control technology for mobile and stationary sources will lead to further reductions in the emissions of NOx and VOCs from the developed nations. The position of the developing nations on the Kuznets curve is less clear. Aggressive actions are being undertaken in China to limit future emissions, such as the target to reduce national emissions of SO2 by 8% and NOx by 10% from 2010 to 2015, called for in the Twelfth Five-Year plan. This suggests that China and perhaps

54

the mechanisms of reactions influencing atmospheric ozone

CO emissions, ktons yr −1

200 Waste Disposal Highway Vehicles Off Highway Vehicles Fuel Combustion Metal Processing

150

100

50

0 1970

1980

1990

2000

2010

Year FIGURE I-G-11.

Emissions of CO from highway vehicles off-highway vehicles, fuel combustion (electricity generation, industrial, and other), metal processing, and waste disposal in the United States 1970–2012 (US Department of Commerce, National Emissions Inventory, 2012b).

NOx emissions, ktons yr −1

15000

Fuel Combustion Industrial Highway Vehicles Off-Highway Vehicles Fuel Combustion Electricity Generation

10000

5000

0 1970

1980

1990

2000

2010

Year FIGURE I-G-12.

Emissions of NOx from highway vehicles (measured as NO2), off-highway vehicles, fuel combustion in electricity generation, and fuel combustion in industry in the United States 1970–2012 (US Department of Commerce, National Emissions Inventory, 2012b).

other developing nations may be on, or near, the plateau region of the emission curve at the time of this writing. From the viewpoint of emissions from vehicles, research sponsored by the global automotive industry and conducted by the International Energy Agency suggests that the absolute magnitude of CO, VOC, particulate matter (PM), and NOx emissions from the on-road vehicle fleet will decrease by factors of 10, 10, 6, and 6, respectively, from 2000 to 2050 (WBCSD, 2004). In conclusion, it seems likely that

global emissions of ozone precursors will decline in the coming decades. I - H . O Z O N E A N D C L I M AT E I-H-1. Contribution of Ozone to the Natural Greenhouse Effect Fourier (1827) suggested that the presence of the atmosphere warms the Earth’s surface by transmitting short wavelength solar radiation that heats the Earth but blocking emission of longer wavelength

VOC emissions, ktons yr −1

20000 Chemical Industry Highway Vehicles Off-Highway Vehicles Solvents

15000

10000

5000

0 1970

1980

1990 Year

2000

2010

FIGURE I-G-13.

Emissions of volatile organic compounds (VOCs) from highway vehicles, off-highway vehicles, chemical industry, and solvent use in the United States 1970–2012 (US Department of Commerce, National Emissions Inventory, 2012b).

Concentration, ppb

100

10

1

1960

1970

1980

1990

2000

2010

Year CO Benzene Ethene Acetylene Propane Propene

iso-Butane n-Butane iso-Pentane n-Pentane Benzene Toluene

FIGURE I-G-14.

Volatile organic compound (VOC) concentrations measured in ambient air near Los Angeles in 1960–2010 (Warneke et al., 2012).

55

56

the mechanisms of reactions influencing atmospheric ozone 25

NOx emissions, Mt yr −1

20

15

10

5

0 1900

1920

1940

1960 Year

1980

2000

2020

Emissions of NOx in the United States from 1900 to 2012 (National Air Pollutant Emission Trends, 1900–1998, US EPA http://www.epa.gov/ttnchie1/trends/trends98; US Department of Commerce, National Emissions Inventory, 2012b).

Pollutant emissions

FIGURE I-G-15.

Per capita income FIGURE I-G-16.

Kuznets curve.

terrestrial radiation that cools the Earth. Fourier compared this effect to that of the glass in a greenhouse (hence “greenhouse effect”). It is now known that the main mechanism by which heat is retained in a greenhouse is that the glass prevents loss of heat via convection. However, for historical reasons, the term “greenhouse effect” is still used to describe radiative trapping of heat in the atmosphere. The importance of absorption of infrared radiation by greenhouse gases that are naturally present in the atmosphere has been known for more than

100 years (Tyndall, 1861); the natural greenhouse effect is a well-established scientific fact (IPCC, 2013). Absorption of infrared irradiation by greenhouse gases in the atmosphere is observable by satellites. Figure 1.H.1 shows the infrared radiation reaching a satellite over the Sahara Desert. The dotted lines show emission spectra expected from black bodies at different temperatures. The atmosphere is transparent at wavelengths of 10–12 μm (800–1,000  cm−1), and terrestrial infrared radiation in this “atmospheric window” region escapes

Ozone in the Atmosphere

57

Wavelength (µm) 25

20

10

15

9

8

7

Radiance (mW m−2 sr−1 cm)

320 K 150

H2O

100

IRIS Spectrum (Sahara)

300 K ATM Window CO2

280 K

O3

260 K 50

240 K

CH4

220 K 0 400

H2O

1500

1000 −1)

Wave number (cm FIGURE I-H-1.

Terrestrial infrared radiation spectrum recorded by an orbiting satellite over the Sahara Desert (Hanel et al., 1972; Brasseur et al., 1999). Reproduced with permission from Oxford University Press and the American Geophysical Union.

unhindered into space. As seen from Figure I-H-1 radiation arriving at the satellite in the atmospheric window region has a characteristic temperature of approximately 320 K (47oC), which reflects the surface temperature in the Sahara. It can be seen from Figure I-H-1 that the infrared radiation on either side of the atmospheric window is characteristic of emission from a black body that is significantly cooler than 320 K. Absorption to the left of the atmospheric window shown in Figure I-H-1 is attributed to the presence of CO2 and H2O, whereas absorption in the 9–10 μm region to the right of the atmospheric window in the Figure is attributed to the presence of ozone. The magnitude of the natural greenhouse effect can be estimated as follows. The solar constant is approximately 1,370 W m−2 and is the power of sunlight arriving at the top of the atmosphere. The Earth presents a disc of area πr2 (r = radius of Earth) toward the Sun, and, as the Earth rotates, the sunlight is distributed over a surface area of 4πr2. Hence, the 24 hour average sunlight at the top of the atmosphere has a power of 1,370/4 = 342 W m−2. Reflection from surfaces (e.g., clouds and snow) and scattering by air molecules lead to an average of 107 W m−2 returning to space, leaving 235 W m−2 that heats the Earth. At infrared frequencies, the Earth behaves like a black body. Emission from a black body is given by σ × T4, where σ is the Stefan-Boltzmann constant (5.67 × 10−8 J m−2 K−4 s−1), and T is the temperature of the

black body. In the absence of the greenhouse effect, at radiative equilibrium, the average surface temperature would be approximately 254 K (−19oC). In reality, the average surface temperature of the Earth is 288 K (15oC); the 34oC difference is attributable to trapping of infrared radiation (heat) in the atmosphere by the natural greenhouse effect. Radiative modeling studies have shown that absorption by ozone is responsible for approximately 10% of the total radiative forcing by the radiatively active species H2O, CO2, O3, CH4, and N2O (Kiehl and Trenberth, 1997). In addition to its contribution to the natural greenhouse effect via absorption of infrared radiation, absorption of UV radiation by ozone plays a critical role in defining the structure of the atmosphere. Absorption of solar UV radiation by O3 and O2 provides the heat that warms the air at 20–50 km altitude, which defines the stratosphere as a region that is relatively stable with respect to vertical mixing. Without absorption by ozone, the troposphere would extend to much higher altitudes, leading to atmospheric circulation patterns and a climate very different from the present. Ozone is an important greenhouse gas, helps define the structure of the atmosphere, and plays a major role in defining the Earth’s climate. I-H-2. Contribution of Ozone to Radiative Forcing of Climate Change When the concentrations of greenhouse gases change, the radiative balance of the Earth is altered.

58

the mechanisms of reactions influencing atmospheric ozone

An increase in greenhouse gases causes an increase in the net radiation at the top of the atmosphere. The change in net radiation at the tropopause caused by changes in greenhouse gas or aerosol concentrations is defined as radiative forcing (IPCC, 2007). As discussed in Section I-D-1 historical measurements of ambient ozone levels and modeling studies suggest that, as a result of human activities, there has been a significant increase in the concentration of tropospheric ozone since 1750. The total radiative forcing estimated from modeled ozone changes since 1750 is approximately 0.35 W m–2 (IPCC, 2013). This total value comprises approximately 0.40 W m–2 from increased tropospheric ozone resulting from increase ozone precursor emissions and −0.05 W m–2 from decreased stratospheric ozone resulting from halocarbon emissions (IPCC, 2013). In terms of radiative forcing of climate change as a result of human activities since 1750, changes in ozone represent approximately 15% of the total anthropogenic radiative forcing of 2.29 W m–2 (IPCC, 2013). I-H-3. Impact of Climate Change on Ozone Assessing the likely impact of climate change on future tropospheric ozone levels is complicated by the existence of several counteracting effects (Royal Society, 2008). Modeling studies indicate that in a warmer climate the Brewer-Dobson circulation mixing ozone-rich stratospheric air into the troposphere will be enhanced and lead to increased tropospheric ozone, particularly in the extratropical middle and upper troposphere (Zeng and Pyle, 2003). Counteracting this effect, in a warmer climate, the troposphere will hold more water vapor, and ozone loss via reaction of O(1D) atoms with water vapor will be more effective ( Johnson et al., 1999). A warmer climate will lead to increased convection and mixing of ozone precursors within the troposphere and also to increased lightning and NOx formation. Climate change will lead to changes in vegetation patterns and changes in biogenic emissions. There is no consensus in the scientific community on whether the combination of these effects will lead to increased or decreased total background tropospheric ozone (Royal Society, 2008). Increased temperatures in urban areas will lead to increased evaporation rates and increased VOC emissions. Warmer temperatures will also increase the rate of chemical reactions leading to ozone

formation in urban areas. In particular, increased temperatures decrease the thermal stability of peroxyacylnitrates (RC(O)O2NO2) that would otherwise provide temporary sequestering of NOx in urban areas. It is well established that increased temperatures associated with climate change will, in general, lead to increased formation of ozone in polluted urban areas (Carter et  al., 1979a; Steiner et al., 2006; Ito et al., 2009; Stockwell et al., 2012; Rasmussen et al., 2013). Stockwell et al. (2012) ran simulations of a polluted urban case with temperatures of 296 K, 298 K, or 300 K to isolate the effect of temperature on maximum ozone mixing values. A  linear response of maximum ozone to temperature was found with mixing values increasing at a rate of 4.3 ppb K−1 over the temperature range studied. Rasmussen et al. (2013) considered the effect of temperature on both emissions (particularly biogenic) and chemistry and reported a large range of ozone sensitivity depending on local conditions, with a median sensitivity of 0.8 ppb K−1 and a range of −0.8 to +11.8 ppb K−1 for locations in the South Coast Air Basin in California projected in 2020. Increased CO2 concentrations and ozone loss have led to a cooling of the stratosphere. Stolarski et al. (2010) have estimated that 60–70% of the temperature decrease observed in the upper stratosphere and lower mid-latitude stratosphere over the past 20 years is attributable to the decreased level of stratospheric ozone and hence decreased absorption of UV within the stratosphere. The remaining 20–30% of the temperature decrease is attributable to increased levels of greenhouse gases (principally CO2) leading to increased radiative cooling of the stratosphere. Cooling of the stratosphere leads to changes in the circulation patterns, to a slowing of the gas phase ozone destruction chemistry, and to the potential for additional polar stratospheric clouds that would enhance heterogeneous chemistry leading to ozone loss in the polar stratosphere (WMO, 2011). The overall impact of climate change on the global annually averaged stratospheric ozone is expected to be slightly positive. Model projections indicate that the global average stratospheric ozone levels will return to their 1980 levels before 2050 and earlier than the stratospheric loading of halogens returns to 1980 levels. Beyond approximately 2050, the total column ozone levels are expected to slightly exceed those prior to 1960, reflecting the cooling impact of increased levels of CO2 and the slowing of gas phase ozone destruction chemistry (WMO, 2011).

Ozone in the Atmosphere I - I . A B O U T T H E M AT E R I A L C OV E R E D I N T H I S   B O O K In this chapter, we began the discussions of ozone by considering the properties of the Earth’s atmosphere and the many factors that lead to ozone generation in the troposphere and the stratosphere. A  historical review was given of the measurements of tropospheric ozone that began more than a century ago. The trends with time in tropospheric ozone concentrations as monitored at remote locations and in polluted urban areas were reviewed. Measurements of stratospheric ozone and the chemistry that results in ozone generation and destruction were discussed. An introduction was given to the complex chemistry that leads to ozone generation within the troposphere and stratosphere. Finally, the major sources of the atmospheric trace gases that facilitate ozone generation and ozone formation mechanisms were identified. Trends of ozone concentrations and ozone precursors in various regions of the world were described. It is clear from this discussion that the mechanisms that control the concentrations of ozone at various regions of the world are extremely complicated. In the following chapters, we discuss in detail all the various factors that determine the ozone levels and provide detailed descriptions of the chemistry associated with ozone formation and destruction with an emphasis on tropospheric processes. The detailed discussions begin in Chapter II with a review of the important chemistry involving ozone that leads to the formation of the ubiquitous HO radical in the troposphere. Then, the loss mechanisms for the ozone molecule through its reactions with the various classes of organic compounds are reviewed, and the effects of molecular structure on the rate coefficients of the ozone reactions are identified. Attention is given to the role of the Criegee intermediates that result from ozone reactions with unsaturated organic molecules. Tables are given of the recommended rate coefficients for ozone reactions with the many molecules that have been studied through the years. Structure activity relations (SARs; also often called structure reactivity relations) are reviewed that allow estimation of rate coefficients for the ozone reactions with the many organic molecules. In Chapter III, we review and evaluate the important chemistry of the nitrogen oxides (NO, NO2, N2O5) and their influence on ozone formation and destruction. The mechanisms of the ozone reactions with NO and NO2 to generate NO2 and NO3, respectively, and the important HO2 reaction that

59

oxidizes NO to NO2 are described. The published data related to the primary processes in NO2 photodecomposition to form O(3P) atoms and O3 are reviewed and evaluated. The chemistry of the important reactive intermediate, the NO3 radical, and its many reactions with organic molecules are reviewed and evaluated. Again, extensive Tables of the recommended rate coefficients for the NO3 reactions with organic molecules are given, and SAR-based methods for estimation of these are presented. In Chapter IV, the reactions of the important HO radical and its significant role inozone formation are given. The experimental data related to its mechanism of generation through ozone photodecomposition to form O(1D) atom are reviewed and evaluated. The difficult measurements of the ambient HO radical concentrations are described, and a comparison of measurements with computer simulations using measured precursors is given. The rate coefficients for the reactions of the HO radical with the many organic molecules that have been studied are reviewed, with the focus on identifying the effect of structure on the reactivity of the reactant molecule. Tables are given of the recommended rate coefficients for the HO reactions and SAR estimates of the extent of reaction at the alternative points of HO radical attack, as well as the major products that are observed or expected. In Chapter V, we review the mechanisms and rate coefficients for the peroxy radical reactions (HO2 and RO2, where R is an alkyl radical) that lead in part to the oxidation of NO to NO2 and subsequent ozone generation. These reactions play important roles in defining the extent of ozone generation in the atmosphere. Reviewed in this chapter are the reactions of HO2 with the various reactants: HO2, NO, NO2, NO3, RO2, and the RO2 reactions with NO, RO2, and NO3. In Chapter VI, the reactions of the alkoxy (RO) and acyloxy (RC(O)O) radicals are reviewed. The extent of reactions with oxygen, decomposition, and rearrangement are discussed for reactions involving the many different radical structures that have been studied. The influence of these reactions on the extent of ozone buildup in the atmosphere is discussed. In Chapter VII, a review is given of the tropospheric reactions of some important inorganic trace gases such as CO, CO2, the halogens, and the sulfur compounds, along with their influence on atmospheric ozone levels. Many Cl and Br atom reactions with diverse organic compounds that have been studied are reviewed, and rate coefficients are recommended and listed in Tables that also give the

60

the mechanisms of reactions influencing atmospheric ozone

probable products of atmospheric oxidation. SARs are presented to allow predictions for the Cl atom rate coefficients for its reactions with the various organic compounds that are as yet unstudied. The role of halogen atom chemistry in the polar ozone depletion episodes is discussed. The complex reactions are identified that define the atmospheric chemistry of the reduced sulfur compounds H2S, CH3SH, CH3SCH3, and CH3SSCH3. Atmospheric oxidation of these compounds leads ultimately to sulfuric acid which can influence the extent of ozone development. The mechanisms of sulfuric acid aerosol generation through SO2 reactions in the troposphere are discussed. The mechanism of stratospheric oxidation of COS and CS2 that leads to the Junge sulfuric acid layer in the stratosphere is outlined, and the influence of this layer on ozone development is described. In Chapter VIII, the importance of photochemistry of the light-absorbing oxygenates to ozone generation in the troposphere is reviewed. The methods of determination of photolysis frequencies (j-values), first-order rate coefficients for photochemical decay, are compared. The significance of radical generation and ozone enhancement from the photodecomposition of the aldehydes and ketones is demonstrated. The quantum yields of the photodecomposition pathways and the absorption spectra of the various light-absorbing oxygenates are given; these are used to derive j-values as a function of solar zenith angle. Special emphasis is given to the discussion of formaldehyde photodecomposition and its influence on ozone generation within the troposphere. The photochemistry of acetone and its possible important role in radical generation within the stratosphere are described. The pathways of photodecomposition of the higher aldehydes and ketones are discussed, and the effects of substitution of various functional groups on the carbonyl compounds on the absorption spectra and the extent of photodecomposition are described. The influence of the photochemistry of the alkyl nitrates (RONO2), peroxyacyl nitrates (RCOO2NO2), and HONO on ozone generation is discussed. The recommended j-values for all the light-absorbing trace gases that have been studied are given for conditions of overhead sun and for the diurnal cycles near March 22, June 22, September 22, and December 22.

In Chapter IX, various computer-assisted programs are described that are used today to simulate, predict, and understand the effects of the varied concentrations of NOx, the hydrocarbons, and other trace gases on ozone generation within the troposphere. Computer-generated mechanisms, detailed explicit mechanisms, and condensed mechanisms used today in the simulation of ozone development in the polluted and remote troposphere are described. Process analysis and the sensitivity of ozone formation to VOC and NOx limitation are discussed, and isopeths for O3, PAN, HNO3, H2O2, HO, and HO2 are derived. Methods of sensitivity and process analysis for mechanism assessment are considered. A computer assessment of the effect of [H2O], temperature, and clouds on O3 generation is given. Simulations are made of the effects of CH2O, CO, SO2, NOx, and N2O5 on O3 development. The use of measures of reactivity, maximum ozone incremental reactivity (MOIR), maximum incremental reactivity (MIR), and photochemical ozone creation potential (POCP) are compared. Finally, the potential deficiencies of atmospheric chemistry mechanisms are discussed as implemented in air quality models today, and future needs in the developments in air quality control are described. With the help of the information given in this book, it should be possible to make reasonable predictions of the lifetimes of specific compounds in the troposphere. Also, predictions can be made of the specific products that are expected to form as the sequence of a compound’s degradation steps continue to final products, CO2 and H2O. The information presented should help build rational judgments as to the relative importance of a given organic compound in tropospheric ozone development. NOTES 1. Throughout this book, the formula for the hydoxy radical is written as HO (although commonly written as OH), following the recommendations of IUPAC (1990). The designation of the hydroxyl radical as HO makes consistent the series of formulas used for this and its related species: HO, HO2, HOx. 2. The term “self-reaction” as used in this book and commonly in the literature does not imply reaction of a radical with itself, but rather reaction with another identical species of its kind.

II Mechanisms of Ozone Reactions in the Troposphere

II-A. INTRODUCTION In Chapter I, we identified the origin of stratospheric ozone and its role in limiting the short wavelengths of sunlight reaching the Earth. We also saw the importance of trace impurities of NOx and hydrocarbons in the development of tropospheric ozone. In this chapter, we review and evaluate the chemical reactions of ozone that create the important hydroxyl (HO) radical. It is the photodecomposition of tropospheric ozone that is the major source of the important HO radical, and it is the HO radical that initiates the destruction of most of the reactive trace gases that are emitted into the atmosphere. Ozone also serves as a major reactant for removal of the alkenes and other reactive unsaturated compounds, and, in this chapter, we review and evaluate the rate coefficients and mechanisms of these reactions and the expected products that result from them. II-B. OZONE PHOTODECOMPOSITION AND THE MECHANISM OF HO R A D I C A L F O R M AT I O N The reactions that generate oxygen atoms in their first excited electronic state, O(1D) atoms, and ultimately HO radicals within the atmosphere are initiated through ozone photodecomposition:

O3 (X1A1) + hν → O(1D) + O2(a1Δg)

(I)

→ O(1D) + O2(X3Σ–g) (II) A fraction of the O(1D) atoms formed in the reactions (I)  and (II) react with water molecules to generate HO radicals in reaction (1)  and a larger fraction are deactivated by collisions with N2 and O2 molecules to form ground state O(3P) atoms in reaction (2): O(1D) + H2O → HO + HO O( D) + M (N2, O2) → O( P) + M (N2, O2) 1

3

(1) (2)

The competition between H2O and other air molecules (N2, O2) for reaction with O(1D) atoms results in HO generation being dependent on relative humidity. Rate coefficients for reaction of O(1D) with H2O, N2, and O2 at 298 K (in units of 10−10 cm3 molecule−1 s−1) recommended by the International Union of Pure and Applied Chemistry (IUPAC) panel are 2.14, 0.31, and 0.40, respectively (Atkinson et al., 2004). To better understand the factors that control HO formation, we will review ozone photochemistry, its cross sections, quantum yields of its major photodecomposition modes, and its photolysis frequencies under varied atmospheric conditions.

62

the mechanisms of reactions influencing atmospheric ozone

II-B-1. Absorption Cross Sections of Ozone Ozone absorbs in a strong ultraviolet (UV) band (Figure II-B-1) and in a weak band in the visible region (Figure II-B-2). Within the troposphere, the overlap of the UV band with the available actinic flux occurs only at the long wavelength tail of the absorption. Note the gray curve in Figure II-B-1. However, this small overlap is important in providing the actinic energy to photodissociate ozone in reactions (I)  or (II) and initiate the important chemistry that occurs in the troposphere.

The cross sections (σ) in the long wavelength tail of the UV band of ozone are somewhat sensitive to temperature. For example, in the 342.5–347.5 region, σ (10−20 cm2 molecule−1) decreases from 0.078 at 298 K to 0.044 at 226 K (Molina and Molina, 1986). This feature is generally incorporated into models that simulate ozone photolysis in the atmosphere. The weak visible band of ozone has a nearly perfect match with the wavelengths of light available within the troposphere (see Figure II-B-2). Although absorption of light within this band by

1.4 x 10−17

Cross section, cm2 molecule−1

1.2 x 10

Actinic flux, Z = 0 σ (various enlargements)

σ x 10 σ x 100

σ x 1000

10−17

2.5 x 1014

2.0 x 1014

8.0 x 10−18

1.5 x 1014

6.0 x 10−18 1014 −18

4.0 x 10

5.0 x 1013

2.0 x 10−18 0 180

200

220

240

260

280

300

320

340

Actinic flux, quanta cm−2 s−1nm−1

−17

0

Wavelength, nm

Cross sections for ozone in the ultraviolet band at a temperature of 298 K (Molina and Molina, 1986).

Cross section Actinic flux

6 x 10−21

6 x 1014

4 x 10−21

4 x 1014

2 x 10−21

2 x 1014

0 400

500

600

700

800

Wavelength, nm FIGURE II-B-2.

Cross sections for ozone in the visible (Chappuis) band (Voigt et al., 2001).

0

Actinic flux, quanta cm−2 s−1 nm−1

Cross section, cm2 molecule−1

FIGURE II-B-1.

Mechanisms of Ozone Reactions in the Troposphere ozone always leads to ozone photodecomposition within the atmosphere, no net chemical change results because ground state O(3P) atoms are formed (primary process III), and essentially all of these combine with molecular oxygen to reform ozone under tropospheric conditions. O3 + hν → O(3P) + O2 (3Σ–g)

(III)

It is interesting to note that this absorption band of ozone leads to the light blue color of ozone that is visible when the ozone molecules are concentrated in the liquid state. Absorption of light by ozone at the small concentrations found in the overhead column is very small (~4.9% with an ozone column of 350 DU) even at the 600  nm maximum in the absorption band, and it is not the major source of our blue sky. Our blue sky results from the preferential scattering of sunlight in the short wavelengths in the blue region caused by collisions between visible quanta with N2 and O2 molecules in the atmosphere.

II-B-2. Quantum Yields of O(1D) Formation in Ozone Photodecomposition Many studies of the quantum yield of O(1D) formation as a function of wavelength have been reported since 1977 (see Figure II-B-3). The estimates of ϕ(O1D) scatter at wavelengths in the 300–310 nm range and near the long wavelength cutoff. An accurate assignment of values at the wavelengths of onset of the process is very important in the calculated extent of occurrence of process (1)  within the troposphere. Matsumi and others who have contributed much to the study of ozone photochemistry have derived “best” estimates of ϕ[O(1D)] in the 306–328 nm range, which we adopt here (Matsumi et al., 2002). The adjusted quantum yields and the recommended data describing their dependence on wavelength at 298 K are summarized in Figure II-B-4. The solid black curve is the recommended dependence, whereas the dashed lines represent tentative recommendations. Pioneering studies of

1.2

φ[O(1D)] from O3 + hν −−> Ο(1D) + O2(1∆g)

1.0

0.8

0.6 Arnold et al. (1977) Amimoto et al. (1978) Brock and Watson (1980) Amimoto et al. (1980) Greenblatt and Wiesenfeld (1983) Taherian and Slanger (1985) Trolier and Weisenfeld (1988) Turnipseed et al. (1991) Cooper et al. (1993) Armerding et al. (1995) Silvente et al. (1997) Ball et al. (1997) Smith et al. (2000) Bauer et al. (2000) Nishida et al. (2004b) Qualitative trend line (λ < 300 nm) Matsumi et al. (2002) recommendations

0.4

0.2

0.0

160

180

200

220

240

260

280

300

320

340

Wavelength, nm FIGURE II-B-3.

63

Quantum yields of O(1D) formation as a function of wavelength in ozone photodecomposition.

64

the mechanisms of reactions influencing atmospheric ozone

Φ(Ο1D) from O3 + hν → Ο(1D) +O2

1.0 Talukdar et al. (1998) Takahashi et al. (1998) Ball et al. (1997) Armerding et al. (1995) Bauer et al. (2000) Brock and Watson (1980) Trolier and Wiesenfeld (1988) Smith et al. (2000) Recommended dependence Tentative recommendations

0.8

0.6

0.4

0.2

0.0 290

300

310

320

330

340

350

Wavelength, nm FIGURE II-B-4. Adjusted quantum yields of O(1D) formation in ozone photodissociation as a function of wavelength (298 K) as recommended by Matsumi et al. (2002).

ozone photodissociation suggested ϕ(O1D) = 1.0 for λ of less than 305 nm, but more recent studies give the somewhat lower value of approximately 0.9 that is now recommended. A  wide range of values for ϕ(O1D) for wavelengths of greater than 325 nm have been reported throughout the years. The threshold for the spin-allowed products of primary process (I), O3 + hν → O(1D) + O2(1Δg), is 310 nm, and many early measurements gave ϕ(O1D) near zero at the long wavelengths. Current best estimates suggest that for λ of less than 411 nm, the spin-disallowed products result from primary process (II), O3 + hν → O(1D) + O2(X3Σg–), which is energetically allowed. Quantum yields of O(1D) of 0.08 ± 0.04 are recommended tentatively for the 330–340 nm region (Matsumi et al., 2002). As seen in Figure II-B-3, ϕ(O1D) falls at the short wavelengths, reaching approximately 0.45 at 193 nm (Turnipseed et al., 1991; Nishida et al., 2004b). The measured quantum yields of total O-atom formation are greater than unity in the short wavelength region:  about 1.2 at 193  nm (Turnipseed et al., 1991) and 1.9 ± 0.3 at 157.6 nm (Taherian and Slanger, 1985). From these results, it has been suggested that a new net primary process forming three O(3P) atoms occurs at these shorter wavelengths:

O3 + hν → O + (O2)*→ O(3P) + O(3P) + O(3P)

(IV)

Taherian and Slanger propose that this occurs in two stages, with formation of an initially excited molecule (O2)* which dissociates. The energy threshold for O3 → 3 O(3P) is 197.8 nm. O2 (b1Σ) was observed as a co-product of O(1D) in photolyses at 193 nm, ϕ[O(1D)] = 0.5 ± 0.4, suggesting another primary process (V) (Taherian and Slanger, 1985; Turnipseed et al., 1991): O3 + hν → O(1D) + O2 (b1Σ)

(V)

Quantum yields for O(1D)-atom formation in ozone photolysis are temperature sensitive in the long wavelength region (see Figure II-B-5). Account should be taken of this in calculations of j[O(1D)] in the atmosphere. Two additional ozone photodecomposition modes are possible in the visible regions of ozone absorption. These pathways are spin forbidden, but energetically possible: O(3P) + O2(b1Σg+) products for λ of less than 463  nm, and O(3P) + O2(a1Δg) products for λ of less than 612  nm. For λ of less than 1,180  nm, the spin-allowed process (III) is possible and forms (O3P) + O2(X3Σg–). Processes that form the product O(3P) are “do nothing” processes in the atmosphere because the large excess

Mechanisms of Ozone Reactions in the Troposphere Temperature, 321 K Temperature, 298 K Temperature, 273 K Temperature, 253 K Temperature, 223 K Temperature, 203 K Talukdar et al. (1998), 321 K Talukdar et al. (1998), 298 K Talukdar et al. (1998), 273 K Talukdar et al. (1998), 253 K Talukdar et al. (1998), 223 K Talukdar et al. (1998), 203 K

1.0

φ(Ο1D) from O3 + hν → Ο(1D) +O2

65

0.8

0.6

0.4

0.2

0.0 305

310

315

320

325

330

Wavelength, nm

Data of Talukdar et al. (1998) showing the temperature dependence of the ϕ[O(1D)] for selected wavelengths in the range 306–329 nm; also shown as lines are the recommended values of ϕ[O1D)] as a function of temperature (Matsumi et al., 2002). FIGURE II-B-5.

of O2 present leads O(3P) atoms to reform ozone. The major tropospheric fate of the product O2(1Δg) formed in many of the primary modes of ozone photolysis is deactivation to ground state O2(X3Σg–) through collisions with other atmospheric molecules (largely N2, O2, and H2O). II-B-3. Estimated Photolysis Frequencies j[O(1D)] for Ozone Photodecomposition The temperature-dependent values of σ (Molina and Molina, 1986)  and ϕ[O(1D)] (Matsumi et  al., 2002)  were used together with the appropriate actinic flux values to derive j(O1D) estimates for various temperatures in the troposphere (see Figure II-B-6). The calculated j-values for the lower troposphere at 298 K are shown in Figure II-B-6a as a function of solar zenith angle and extent of overhead ozone column and as a function of temperature for a fixed overhead ozone column (350 DU) in Figure II-B-6b. It is clear that j[O(1D)] is sensitive to both the extent of overhead ozone and the temperature. For 298 K, as the ozone column increases from 300 to 400 DU, the j-value decreases by a factor of 1.5. With the ozone column fixed at 350 DU, the j-value increases by a factor of 1.7 as the temperature is changed from 203 to 321 K. In Figure

II-B-7, the calculated values of j[O(1D)] are shown as a function of altitude (Z = 0°, 350 DU overhead ozone column). Note that the value attained at 60 km has increased by a large factor (~165) from that in the lower troposphere. II-B-4. Comparison of Measured and Calculated j[O(1D)] Values Instrumentation has been developed to measure j-values in real time. Both actinometric and spectrophotometric methods appear to provide reasonable measurements of j[O(1D)] (e.g., see Shetter et  al. [1996] and Hofzumahaus et  al. [2004]). Examples of the measurements taken during a clear day and during a day with extensive cloud cover are shown in Figure II-B-8. Here, the measured values of j[O(1D)] are compared with those that were calculated in an intercomparison of modeling and measurement techniques carried out in Boulder, Colorado. Note that significant attenuation of the photolysis of ozone resulted on June 16 (Figure II-B-8b) as extensive intermittent cloud cover occurred. In Figure II-B-8a, the small attenuation seen near hour 17.5 of June 18 (Figure II-B-8a) resulted from a thin cloud that covered the path of the sun for a moment. Accurate modeling of j[O(1D] during cloudy days is not yet possible, but measurements can be compared in Figure

66

the mechanisms of reactions influencing atmospheric ozone 5 x 10−5

(a)

j (O1D), sec−1

4 x 10−5 O3 column = 300 DU O3 column = 350 DU O3 column = 400 DU

3 x 10−5

2 x 10−5

10−5

0 0

10

20

30

40

50

60

70

80

90

100

Solar Zenith Angle, degrees (b) 5.0 x 10−5

j(O1D), sec−1

4.0 x 10−5

3.0 x 10−5

203 K 223 K 253 K 273 K 298 K 321 K

2.0 x 10−5

10−5

0 0

10

20

30

40

50

60

70

80

90

100

Solar Zenith Angle, degrees

(a) The effect on j[O(1D)] of varied overhead ozone column at 298 K, and (b) the effect of varied temperature with fixed overhead ozone column (350 Dobson units). FIGURE II-B-6.

II-B-8b with the modeling results expected at the site in the absence of clouds. The data of Figure II-B-8 should remind atmospheric chemists that modeling tropospheric chemistry events without an input from j(O1D) measurements can suffer from the ubiquitous presence of clouds because their attenuation of O(1D) and subsequent effects on atmospheric chemistry can be very significant.

Some useful qualitative considerations of the effects of cloud cover on j[O(1D)] have been made (e.g., see Junkermann, 1994; Brasseur et al., 2002; Crawford et al., 2003; Kylling et al., 2005; Kim et  al., 2007), but the presence of clouds remains a substantial problem in the comparison of models of ozone concentrations with tropospheric measurements.

Mechanisms of Ozone Reactions in the Troposphere

67

80

Altitude, km

60

40

20

0 10−5

10−4

10−3 j [O(1D)],

10−2

s−1

Estimated values of j[O(1D)] as a function of altitude as calculated assuming an overhead ozone column of 350 Dobson units and a solar zenith angle of 0°. In the calculations, the temperature-dependent values for σ (Molina and Molina, 1986) and ϕ in the 298–306 nm range (Matsumi et al., 2002) were used. The ϕ(O1D) estimates given by the dashed curve in Figure II-B-3 were used at the shorter wavelengths. FIGURE II-B-7.

II-C. MECHANISMS OF THE O Z O N E R E AC T I O N S W I T H ORGANIC COMPOUNDS Ozone reacts very slowly with saturated compounds, and these reactions are unimportant for tropospheric conditions. For tropospheric temperatures, the alkane-ozone rate coefficients have been reported in the range from 1.4  × 10−24 cm3 molecule−1 s−1 for methane to 2 × 10−23 cm3 molecule−1 s−1 for 2-methyl propane; these values very likely represent upper limits (see review by Calvert et al., 2008). This implies a tropospheric lifetime for reaction with ozone in excess of 1,000 years for the alkanes. Also, the few saturated oxygenates and the aromatic compounds that have been studied are relatively unreactive toward ozone under tropospheric conditions. For example, the rate coefficients at 298 K (cm3 molecule−1 s−1) for some representative compounds reflect this:  toluene (< 10−21), o-cresol (< 2.6 × 10−19), 1-methoxy-2-propanol (< 1  × 10−19), 1,4-benzodioxan (< 1.2  × 10−20), isobutyraldehyde (< 3.9  × 10−20), 3-methylbutanone (6.3 × 10−21), and camphor (< 7 × 10−20). Thus, it can be concluded that the rates of reaction of the saturated organic molecules with ozone at tropospheric concentrations are insignificant compared

with those for HO and NO3. However, introduction of a double or triple bond into the alkane, or the side chain of an aromatic hydrocarbon or other organic molecules, results in a marked increase in reactivity. In this section we review the effects of molecular structure on the rate coefficients of the unsaturated organic compounds. Tables of the rate coefficients for reactions of the various compounds with ozone that are given in this book are based largely on the recommendations of Calvert et al. (2000, 2002, 2008, 2011), where the plots of the original data and the description of the methods used, together with the references to the various studies, may be accessed.

II-C-1. Ozone Reactions with the Acyclic Mono-Alkenes The reactivity of ozone with unsaturated hydrocarbons reflects ozone’s electrophilic nature; ozone seeks reaction at the electron-rich region of the >C=C< bond in the alkene. Ozone, unlike the NO3 and HO radicals, does not extract H atoms from an alkene, as reflected in the unreactive nature of ozone toward the saturated hydrocarbons and oxygenates. The major pathway for reaction of ozone with an

68

the mechanisms of reactions influencing atmospheric ozone and R4C(O)R3], either aldehydes (one or more R’s = H) or ketones (all R’s = alkyl groups). Also

alkene forms an energy-rich ozonide by addition to the >C=CC=CC=C< bond as

additions. Although the NO3 radical is large like the ozone molecule, it requires only a single attachment to the carbon atoms of the >C=C< bond, and structural effects are expected to be less important in determining the rate coefficient (see Chapter III). Only the electrophilic influence of attached groups on the carbon atoms of the C=C group is sufficient to describe the structure–reactivity relations (also often called structure–activity relations [SARs]) for the NO3 reactions (see Section III-E-8.3). The strong influence of structure on the rate coefficient for the ozone-alkene reactions can be noted also with the rate coefficients [1018 × k(298), cm3 molecule−1 s−1] for 3-methyl-1-pentene, 3.8; 2,3-dimethyl-1-butene, 3.9; 3-methyl-2-isopropyl-1-butene, 3.0; or 3,4-diethyl-2-hexene, 4.0. These rate coefficients are very similar even though significant structural and substituent differences are present. The first compound has a single sec-butyl group attached to the CH2=CH group; the second, third, and fourth compounds have two different alkyl group attached to the CH2=C< group. Apparently, the extra hindrance to O3’s approach to the double bond with two alkyl groups attached compensates for its additional enhancement to electrophilic agent attack. The need to incorporate the structural hindrance of alkene reactions with ozone or other factors in structure–reactivity relationships when estimating rate coefficients is evident; this is discussed further in Section II-F. Recommended rate coefficients for the alkenes are summarized in Table II-C-1 where the measured or expected major products are listed. The data shown are taken largely from

Mechanisms of Ozone Reactions in the Troposphere

71

1018 x k(298), cm3 molecule−1 s−1

10000

1000

100

10

1

FIGURE II-C-1.

0

1

2

3

4

Effect on the rate coefficient of the ethylene-ozone reaction of substitution of CH3 groups for H atoms in

ethylene.

the recommendations of Calvert et  al. (2000) and Calvert et al. (2011) where access to the original data and references can be had. The reactivity of ozone with the straight chain 1-alkenes shows a marked increase over that for ethene but little change in k(298) as the number of C-atoms in the alkyl chain increases from 3 to 14 (data points shown as circles in Figure II-C-2). The open circles are the recommended rate coefficients found in Calvert et al. (2000). The gray circles are from McGillen et al. (2011b) who determined the rate coefficients for the larger 1-alkenes at a temperature of 395.9 K (small triangles in Figure II-C2); they normalized these to obtain 298 K values using an activation energy (19.83 kJ mole−1) derived from the k(395.9 K) of decene and the average k(298 K) reported in the literature for decene. The entire dataset for the 1-alkenes suggests that there is little change in k(298) with increasing size of the alkyl chain. A small enhancement of the rate coefficient of the 1-alkenes is seen when a methyl radical substitution for an H atom is made at the 2 position in the 1-alkenes (filled squares in Figure II-C-2). With ozonides formed on reaction of ozone with unsymmetrical alkenes, the rupture of one of the –O–O–O– bonds in the ozonide in either of two positions results in different products. It is interesting to note the preferences between the alternatives as the structure of the

alkene is altered (see Table II-C-2 where data for the 1-alkenes are summarized). If the product yields are a true reflection of the primary aldehydes formed in the reactions, then the sum of the two aldehyde products given in column four is expected to be near unity; this is the case within the experimental error of the analyses. The ratios in column five show that for 1-alkenes larger than pentene, there is almost an equal fragmentation by the two possible modes of the ozonide reactions. For propene and 1-butene, the split to form formaldehyde is favored. Product data for the 1-alkenes of the general formula CH2=C(R1)R2, where the R’s represent alkyl groups, also favor the split of the ozonide to form the formaldehyde product. Note the value of the ratio of CH2O/ketone in the products of the reaction with the 1-alkene with ozone for the following alkenes: CH2=C(CH3)2, 3.1; CH2=C(CH3)CH2CH3, 2.2; CH2=C(CH3)CH2CH2CH3,1.9;CH2=C(C2H5)2,1.3;CH2C(CH3)CH(CH3)2, 1.8; CH2=C[CH(CH3)2]2, 2.3. The mode of rupture of the ozonide favors formation of the larger carbonyl when the branching occurs down-chain from the C=C bonded carbons. For example, the ratio of the smaller carbonyl to larger carbonyl is less than unity for the following alkenes: CH2=CHCH2CH(CH3)2, 0.62; CH2=CHC(CH3)3, 0.48; (CH3)2C=CHC(CH3)3, 0.23; C6H5CH=CH2, 0.53; cyclo-C6H11CH=CH2, 0.76.

72

TABLE II-C-1 . OZONE RE ACTIONS WITH ALKENES: RECOMMENDED R ATE COEFFICIENTS, K(298 K) AND A AND B VALUES (k = A × e −B/T, cm 3 molecule −1 s −1 ). MOST OF THE DATA ARE FROM THE RECOMMENDATIONS OF CALVERT ET AL. (2000). STUDIES MADE IN YE AR S SINCE THIS REVIEW ARE REFERENCED BY FOOTNOTE. THE MA JOR CARBONYL PRODUCTS OF THE RE ACTION THAT HAVE BEEN OBSERVED OR EXPECTED (PRINTED IN ITALICS) ARE SHOWN IN THE FIFTH COLUMN WITH THE YIELD (IF KNOWN) SHOWN IN PARENTHESES

Alkene

a) Mono-alkenes Ethene (CH2=CH2 Propene (CH2=CHCH3) 1-Butene (CH2=CHCH2CH3)

cis-2-Butene (CH3CH=CHCH3) trans-2-Butene (CH3CH=CHCH3) 2-Methylpropene [(CH3)2C=CH2)]

1-Pentene (CH2=CHCH2CH2CH3) cis-2-Pentene (CH3CH=CHCH2CH3) trans-2-Pentene (CH3CH=CHCH2CH3) 2-Methyl-1-butene [CH2=C(CH3)CH2CH3]

3-Methyl-1-butene [CH2=CHC(CH3)2] 2-Methyl-2-butene [(CH3)2C=CHCH3]

1018 × k (298 K) (cm3 molecule−1 s−1)a

1015× A

1.59 ± 0.48 1.45 ± 0.25b 10.1 ± 2.5 9.9u 9.65 ± 2.5 9.7 ± 1.9c 10.2 ± 2.0u 125 ± 31 190 ± 67 187 ± 36u 11.3 ± 3 11.4 ± 2.8r 10.8w? 10.6 ± 3.2 10.0 ± 2.0u 128 ± 38u 315 ± 95 159 ± 32u 13 ± 4.0 (288 K) 13.7 ± 2.7r 14.2 ± 2.8u 9.5 ± 2.9 (293 K) 7.3 ± 2.7r 403 ± 141

9.14

2,580

CH2O (1.0)

5.51

1,878

CH2O (0.62); CH3CHO (0.38)

3.36 3.50c 3.4u 3.22 6.64 5.9u,w 2.70 3.4r 1.6u? 2.13 7.6u 3.7u

1,744 1,756c 1,731u 968 1,059 1,158u,w 1,632 1,697r 2,100u? 1,580 1,978u 1,002u

CH2O (0.64); CH3CH2CHO (0.36)

7.1u

1,132u

6.18r 4.9u

1,822r 1,741u

72.4r 6.51

2,741r 829

B (K) Observed or Predicted (in italics) Major Carbonyl Products Yield of Pathway of Product is Shown in Parenthesesd

CH3CHO (1.0) CH3CHO (1.0) CH2O (0.75); CH3C(O)CH3(0.25)

CH2O (0.51); CH3CH2CH2CHO (0.49) CH3CHO, CH3CH2CHO CH3CHO, CH3CH2CHO CH2O (0.70); CH3C(O)CH2CH3 (0.30)

CH2O (0.50); (CH3)2CHCHO (0.50) CH3C(O)CH3; CH3CHO

1-Hexene (CH2=CHCH2CH2CH2CH3)

cis-3-Hexene (CH3CH2CH=CHCH2CH3) trans-3-Hexene (CH3CH2CH=CHCH2CH3) (CH3CH=CHCH2CH2CH3) 2-Methyl-1-pentene [CH2=C(CH3)CH2CH2CH3] 3-Methyl-1-pentene [CH2=CHCH(CH3)CH2CH3] trans-2-Hexene (CH3CH=CHCH2CH2CH3) cis-2-Hexene (CH3CH=CHCH2CH2CH3) 4-Methyl-1-pentene [CH2=CHCH2CH(CH3)2 cis-3-Methyl-2-pentene [CH3CH=C(CH3)CH2CH3] trans-3-Methyl-2-pentene [CH3CH=C(CH3)CH2CH3] 2,3-Dimethyl-1-butene [CH2=C(CH3)CH(CH3)2] 3,3-Dimethyl-1-butene [CH2=CHC(CH3)2] 2,3-Dimethyl-2-butene [(CH3)2C=C(CH3)2] 2-Ethyl-1-butene [CH2=C(C2H5)CH2CH3] 1-Heptene (CH2=CHCH2CH2CH2CH2CH3)

2,3,3-Timethyl-1-butene [CH2=C(CH3)C (CH3)3] 2-Methyl-1-heptene [CH2=C(CH3)CH2CH2CH2CH2CH3] 1-Octene (CH2=CHCH2CH2CH2CH2CH2CH3)

73

cis-4-Octene (CH3CH2CH2CH=CHCH2CH2CH3) trans-4-Octene (CH3CH2CH2CH=CHCH2CH2CH3) trans-2,5-Dimethyl-3-hexene [(CH3)2CHCH=CHCH(CH3)2] trans-2,2-Dimethyl-3-hexene [(CH3)3CCH=CHCH2CH3]

11.3 ± 2.8 11.5 ± 2.3u 9.0 ± 1x 144 ± 43 (295 K) 157 ± 47 (290 K) 16 ± 5 12.6 ± 1.3x 3.8 ± 1.2 (286 K) 157 ± 47 (290 K) 153 ± 31u 144 ± 43 (295 K) 105 ± 20u 10 ± 3.5 9.4 ± 1.9q 450 ± 135 560 ± 168 10 ± 3 (285 K) 3.9 ± 1.2 (285 K) 3.8 ± 0.8q 1,130 ± 395 13 ± 7 12 (± factor of 1.5) 9.2 ± 0.3u 11.6 ± 2.3u 10.5 ± 1x 7.8 ± 2.3 (294 K) 13.5 ± 1x 14 (± factor of 1.5) 10.1 ± 3l 10.1 ± 0.4x 90 ± 27 (293 K) 131 ± 39 (290 K) 38 ± 11 (290 K) 40 ± 12 (295 K)

CH2O (0.50); CH3CH2CH2CH2CHO (0.50) 2.3u

1,580u CH3CH2CHO (1.0) CH3CH2CHO (1.0) CH2O (0.66); CH3CH2CH2C(O)CH3(0.34) CH2O (0.38); C2H5CH(CH3)CHO (0.62) CH3CHO, CH3CH2CH2CHO

7.6u

1,163

3.2u

1,017u

5.21q

1,884q

CH3CHO, CH3CH2CH2CHO CH2O (0.38); (CH3)2CHCH2CHO (0.62) CH3CHO, CH3C(O)CH2CH2CH3 CH3CHO, CH3C(O)CH2CH2CH3 CH2O (0.65); CH3C(O)CH(CH3)2 (0.35) CH2O(0.32); (CH3)2CHCHO (0.68) 2.68q 3.03

1,956 294

4.2u

1,756u

CH3C(O)CH3 (1.0) CH2O(0.62); C2H5C(O)C2H5 (0.38) CH2O (0.50);, n-C5H11C(O)CH3(0.50)

CH2O (0.65); CH3C(O)C(CH3)CH(CH3)2 (0.35) CH2O, CH3C(O)CH2CH2CH2CH2CH3 CH2O (0.50); n-C6H13CHO (0.50

CH3CH2CH2CHO (1.0) CH3CH2CH2CHO (1.0) (CH3)2CHCHO (1.0) CH3CH2CHO (0.29); (CH3)3CCHO (0.71) (continued)

TABLE II-C-1 . (CONTINUED)

74 Alkene cis/trans-3,4-Dimethyl-3-hexene CH3CH2C(CH3)=C(CH3)CH2CH3 2,4,4-Trimethyl-1-pentene [CH2=C(CH3)CH2C(CH3)3] 2,4,4-Trimethyl-2-pentene [(CH3)2C=CHC (CH3)3] 6-Methyl-5-hepten-2-one [CH3C(O)CH2CH2CH=C(CH3)2] 3-Methyl-2-isopropyl-1-butene [CH2=C{CH(CH3)2}CH(CH3)2] c 1-Nonene CH2=CHCH2CH2CH2CH2CH2CH2CH3 2-Methyl-1-octene [CH2=C(CH3)CH2CH2CH2CH2CH2CH3] 1-Decene (CH2=CHCH2CH2CH2CH2CH2CH2CH2CH3) cis-5-Decene (CH3CH2CH2CH2CH=CHCH2CH2CH2CH3) trans-5-Decene (CH3CH2CH2CH2CH=CHCH2CH2CH2CH3) 3,4-Diethyl-2-hexene [CH3CH=C(C2H5)CH(C2H5)CH2CH3] 1-Undecene [CH2=CH(CH2)8CH3] 2-Methyl-1-decene [CH2=C(CH3)CH2CH2CH2CH2CH2CH2CH2CH3] 1-Dodecene [CH2=CH(CH2)9CH3] 2-Methyl-undecene [CH2=C(CH3)CH2CH2CH2CH2CH2CH2CH2CH2CH3 1-Tridecene [CH2=CH(CH2)10CH3] 1-Tetradecene [CH2=CH(CH2)11CH3] 2-Methyl-1-tridecene [CH2=C(CH3)CH2(CH2)9CH3 b) Dienes 1,2-Propadiene (CH2=C=CH2) 1.3-Butadiene (CH2=CHCH=CH2) 2-Methyl-1,3-butadiene, isoprene (CH2=C(CH3)CH=CH2)

1018 × k (298 K) (cm3 molecule−1 s−1)a > 370 ± 111 15.0 ± 3q 139 ± 42 390 (290–296 K) 3.0 ± 0.9 (294 K)

1015× A 7.31q

B (K)

CH3C(O)CH2CH3 (1.0) 1,844q CH2O, CH3C(O)CH2C(CH3)3 CH3C(O)CH3 (0.82); (CH3)2CHCHO (0.18) CH3C(O)CH3; CH3C(O)CH2CH2CHO CH2O (0.59); (CH3)2CHC(O)CH(CH3)2 (0.41)

9.9 ± 3n 13.8 ± 0.6x 9.3 (± factor of 1.5) 11.1 ± 1x 114 ± 34 (293 K) > 130 ± 39 4.0 ± 1.2 (293 K) 10.3 ± 3.0l 14.8 ± 1x 10.3 ± 3.0 l 13.8 ± 1.4x 14.6 ± 1.1x 9.6 ± 3 l 19.2 ± 1.2x 9.7 ± 3l 24.4 ± 2.4x 28.5 ± 4x 0.185 ± 0.074 6.3 ± 1.9 12.8 ± 3.2 13.3 ± 2.6i 9.6 ± 0.7j (286 K) 11.9 ± 0.9t (293 K) 15.1 ± 2.3u

Observed or Predicted (in italics) Major Carbonyl Products Yield of Pathway of Product is Shown in Parenthesesd

CH2O, CH3(CH2)6CHO CH2O, n-C6H13(C(O)CH3 CH2O (0.52); n-C8H17CHO (0.48) n-C4H9CHO (1.0) n-C4H9CHO (1.0) CH3CHO (0.71); CH3C(O)CH(CH3)CH2CH3 (0.29) CH2O, CH3(CH2)8CHO CH2O, n-C8H17C(O)CH3 CH2O, CH3(CH2)9CHO CH2O, n-C8H17C(O)CH3 CH2O, CH3(CH2)10CHO CH2O, CH3(CH2)11CHO CH2O, n-C11H23C(O)CH3 1.84 13.4 7.86 10.9i

2,690 CO, H2O, CO2, C2H4, O2 2,283 CH2O, CH2=CHCHO 1,913 CH3C(O)CH=CH2(0.17), CH2O; CH2=C(CH3)CHO (0.42) 1,998i

12.1u

1,993u

1,3-Pentadiene (CH2=CHCH=CHCH3) E-1,3-Pentadiene (CH2=CHCH=CHCH3) Z-1,3-Pentadiene (CH2=CHCH=CHCH3) 1,4-Pentadiene (CH2=CHCH2CH=CH2) cis-2,trans-4-Hexadiene (CH3CH=CHCH=CHCH3) trans-2,trans-4-Hexadiene (CH3CH=CHCH=CHCH3) 2-Methyl-1,4-pentadiene [CH2=C(CH3)CH2CH=CH2] 2-Methyl-1,3-pentadiene [CH2=C(CH3)CH=CHCH3] 2,3-Dimethyl-1,3-butadiene CH2=C(CH3)C(CH3)=CH2] E,Z-5-Methyl-1,3-hexadiene [CH2=CHCH=CHCH(CH3)2]

2,5-Dimethyl-1,5-hexadiene [CH2=C(CH3)CH2CH2C(CH3)=CH2 E,Z-5,5-Dimethyl-1,3-hexadiene [CH2=CHCH=CHC(CH3)3] 2,5-Dimethyl-2,4-hexadiene [(CH3)2C=CHCH=C(CH3)2] trans-3,7-Dimethyl-1,6-octadiene [CH2=CHCH(CH3) CH2CH2CH=C(CH3)2] cis-3,7-Dimethyl-1,6-octadiene [CH2=CHCH(CH3)CH2CH2CH=C(CH3)2] c) Trienes 3-Methylene-7-methyl-1,6-octadiene (myrcene) CH2=CHC(=CH2)CH2CH2CH=C(CH3)2

43 ± 15 43 ± 10n 28 ± 4n 14.5 ± 4.4 314 ± 93 374 ± 112 13.2 ± 4.0 80 ± 24 25.6 ± 7.7 23.9 ± 5.5n 24 ± 6 25.3 ± 2.3n 14.2 ± 4.3 25 ± 2n 3,060 ± 880n < 544 ± 83 489 ± 80

474 ± 142

6.9

CH3CHO, CH2=CHCHO, CH2O, CH3CH=CHCHO CH2O, CH(O)CH=CHCH3; CH2=CHCHO, CH3CHO CH2O, CH(O)CH=CHCH3; CH2=CHCHO, CH3CHO CH2O, CH2=CHCH2CHO CH3CHO, CH3CH=CHCHO CH3CHO, CH3CH=CHCHO CH2O, CH3C(O)CH2CH=CH2, CH2=C(CH3)CH2CHO CH2O, CH3C(O)CH=CHCH3, CH3CHO, CH3C(O)CH=CH2 1,668 CH2O, CH3C(O)C(CH3)=CH2 CH2O, CH(O)CH=CHCH(CH3)2 CH(O)CH(CH3)2, CH2=CHCHO CH2O, CH3C(O)CH2CH2C(CH3)=CH2 CH2O, CHOCH=CHC(CH3)3 CH2=CHCHO, (CH3)3CCHO CH3C(O)CH3, CH(O)CH=C((CH3)2 CH2O, CH3)2C=CHCH2CH2CH(CH3)CHO, CH3C(O)CH3, CH2=CHC(CH3)CH2CH2CHO CH2O, CH3C(O)CH3, (CH3)2C=CHCH2CH2CH(CH3)CHO CH2=CHCH(CH3)CH2CH2CHO

3.7-Dimethyl-1,3,6-octatriene (ocimene) [CH2=CHC(CH3)=CHCH2CH=C(CH3)2]

544 ± 163

cis-/trans-1,3,5-Hexatriene [CH2=CHCH=CHCH=CH2 d) Cyclic alkenes

26.2 ± 7.9

CH2O, (CH3)2C=CHCH2CH2C(=CH2)CCHO; CH3C(O)CH3, CH2=CHC(=CH2)CH2CH2CHO; CH2=CHC(O)CH2CH2CH=C(CH3)2 CH2O. (CH3)2C=CHCH2CH=C(CH3)CHO CH3C(O)CH3, CH2=CHC(CH3)=CHCH2CHO CH3C(O)CH=CH2, (CH3)2C=CHCH2CHO CH2O, CH2=CHCH=CHCHO, CH2=CHCHO

2.8 ± 1.1o

CH2O,

Methylenecyclopropane

75

Methylenecyclobutane

O

(?) fragmentation likely

19 ± 8 p CH2O,

O (?)

(continued)

TABLE II-C-1 . (CONTINUED)

76 Alkene

1018 × k (298 K) (cm3 molecule−1 s−1)a

1015× A

Observed or Predicted (in italics) Major Carbonyl Products B (K) Yield of Pathway of Product is Shown in Parenthesesd

90 ± 36 p

Methylenecyclohexane

O (?)

CH2O,

Methylenecyclopentane

28 ± 11 o

CH2O,

O

570 ± 200

CH3CH2CH2CHO, CO2,

673 ± 201

CH3C(O)CH2CH2CH3, CO2 CH3CH2CH(CH3)CHO, CO2

Cyclopentene

1-Methyl-1-cyclopentene

832 ± 24s 334 ± 12s

HC(O)CH(CH3)CH2CH3, CO2 CH3CH2CH(CH3)CHO, CO2 CH(O)CH2CH2CH(CH3)CHO

3-Methyl-1-cyclopentene 81 ± 20

CH3CH2CH2CH2CHO, CO2 CH(O)CH2CH2CH2CH2CHO

Cyclohexene

160 ± 48 1-Methyl-1-cyclohexene

146 ± 10s

5.25

1,040

CH3C(O)CH2CH2CH2CH2CHO CH3C(O)CH2CH2CH2CH3, CO2

55.3 ± 2.6s

CH(O)CH2CH2CH2CH2CH3, + CO2 CH(O)CH(CH3)CH2CH2CH3 + CO2 CH(O)CHCH2CH2CH2CH2CHO CH(O)CH(CH3)CH2CH2CH2CHO

3-Methyl-1-cyclohexene 89 ± 27s

4-Methyl-1-cyclohexene

2.16

952

CH(O)CH2CH(CH3)CH2CH3, CO2 CH(O)CH2CH(CH3) CH2CH2CHO CH(O)CH2CH2CH(CH3)2, CO2 CH(O)CH2CH2CH(CH3)CH2CHO

55.3 ± 11s 1,200 ± 420

CH(O)CH=CHCH2CH3, CO2 CH(O)CH=CHCH2CH2CHO

1,3-Cyclohexadiene

46 ± 16

CHOCH2CH=CHCH3, CO2 CH(O)CH2CH=CHCH2CHO

1,4-Cyclohexadiene

250 ± 63 Cycloheptene

150 ± 53

HC(O)CH2CH2CH2CH=CH2, CO2 HC(O)CH2CH2CH2CH=CHCHO

54 ± 16 (294 K)

1,3,5-Cycloheptatriene

77

a

CH(O)CH2CH2CH2CH2CH3, CO2 CH(O)CH2CH2CH2CH2CH2CHO

CH3C(O)CH2CH2CH2CH2CH3, CO2 CH3C(O)CH2CH2CH2CH2CH2CHO

1,3-Cycloheptadiene

a

494

930 ± 24s 1-Methyl-1-cycloheptene

b

1.31

CH(O)CH=CHCH=CHCH3, CO2 (a > b) CH(O)CH=CHCH=CHCH2CHO CH(O)CH=CHCH2CH=CH2, CO2 CH(O)CH=CHCH2CH=CH2CHO

(continued)

TABLE II-C-1 . (CONTINUED)

78

Alkene

1018 × k (298 K) (cm3 molecule−1 s−1)a 1015× A

B (K)

Observed or Predicted (in italics) Major Carbonyl Products Yield of Pathway of Product is Shown in Parenthesesd OHC

1,600 ± 560

CHO

CHO

Bicyclo[2.2.1]-2-heptene (2-Norbornene)

CO2 OHC

3,600 ± 1,260

CHO

CHO

Bicyclo[2.2.1]-2,5-heptadiene CO2 71 ± 25

OHC

CHO

CHO

Bicyclo[2.2.2]-2-octene

CO2 377 ± 112s

cis-Cyclooctene

0.78

217

CH3CH2CH2CH2CH2CH2CHO, CO2 CHOCH2CH2CH2CH2CH2CH2CHO

386 ± 23s a

270 ± 81

OHC CHO

CHO

CH3

4-Vinylcyclohexene CO2

b

  CHO

(a > b) 1,420 ± 568 1-Methyl-1-cyclooctene 1,420 ± 100s

CO2, CH2O

CH3C(O)CH2CH2CH2CH2CH2CH3, CO2 CH3C(O) CH2CH2CH2CH2CH2CH2CHO

139 ± 56s

CH(O)CH2CH2CH2CH2CH2CH2CH3, CO2CH(O)CH2CH2CH2CH2CH2CH (CHO) CH3CH3CH2CH2CH2CH2CH(CH3)CHO, CO2

20.0 ± 8s

CH(O)CH2CH2CH2CH2CH=CHCHO CH(O)CH2CH2CH2CH2CH=CH2, CO2

152 ± 30s

CH(O)CH2CH2CH=CHCH2CH2CHO CH(O)CH2CH2CH=CHCH2CH3, CO2

2.60 ± 52s

CH(O)CH=CHCH=CHCH=CHCHO CH(O)CH=CHCH=CHCH=CH2, CO2

3-Methyl-1-cyclooctene

cis,cis-1,3-Cyclooctadiene

1,5-Cyclooctadiene

1,3,5,7-Cyclooctatetraene

29 ± 9

CH(O)(CH2)8CHO, CH(O)(CH2)7CH3, CO2

cis-Cyclodecene

O

0.90 ± 0.27n Camphene

(0.04) CH2O,

O O

79 (continued)

TABLE II-C-1 . (CONTINUED)

80

1018 × k (298 K) (cm3 molecule−1 s−1)a 1015× A

Alkene

B (K)

Observed or Predicted (in italics) Major Carbonyl Products Yield of Pathway of Product is Shown in Parenthesesd OHC

230 ± 69

2-Carene

O

O

CO2 37 ± 11

H3C

OHC

3-Carene

O

O

CO2 a

200 ± 50i

Limonene

b O (0.05); CH O, a > b 2

H3C

HCO O

O

CO2 213 ± 42i a

3,000 ± 1,050

α-Phellandrene

2.95i

783i OHC O

b CHO a

β-Phellandrene

b

47 ± 16

OHC

O

 CH2O, CHO

OHC

a>b

a>b

87 ± 26

1.01

732 O OHC

α-Pinene 81 ± 16i

0.48i

530i

15 ± 5

β-Pinene

O

(0.22); CH2O (0.42) 22.2 ± 4.3i

1.74i

1,297i OHC O

< 520

Carvomethene

83 ± 25 Sabinene

O

(0.50); CH2O α-Terpinene

a

21,000 (± factor of 1.5) b b

O

OHC

a~b CHO

140 ± 49 O

γ-Terpinene

a

O

Terpinolene

a

b

1,900 ± 570

OHC

OHC

a~b

O O

81

a > b CH3C(O)CH3 (continued)

TABLE II-C-1 . (CONTINUED)

82

1018 × k (298 K) (cm3 molecule−1 s−1)a

Alkene

1015× A

Observed or Predicted (in italics) Major Carbonyl Products B (K) Yield of Pathway of Product is Shown in Parenthesesd CHO

28 ± 8

O

α-Cedrene

158 ± 47

OHC O

α-Copaene

O

12,000 ± 3,600

b

a

β-Caryophyllene

 CH2O

O

CHO

a>b 11,700 ± 2,920 a

b

CHO OHC

O

CHO

O CHO

α-Humulene c

a~c>b

< 0.5 O

Longifolene

CH2O e) Halogenated alkenes 3-Bromo-1-propene (CH2=CHCH2Br) 1-Chloroethene (CH2=CHCl) 1,1-Dichloroethene (Cl2C=CH2) Z-1,2-Dichloroethene (ClH=CClH) 1,1,2-Trichloroethene (Cl2C=CHCl) 1,1,2,2-Tetrachloroethene (Cl2C=CCl2) 2-Chloro-1-propene (CH2=CClCH3 3-Chloro-1-propene (CH2=CHCH2Cl) 1-Chloro-2-methyl-2-propene [ClCH2C(CH3)=CH2] 3-Chloro-1-butene (CH2=CClCH2CH3) 1-Chloro-3-methyl-2-butene [ClCH2CH=C(CH3)2] 1-Chloro-2-butene (CH3CH2=CHCH2Cl) 3-Iodo-1-propene (CH2=CHCH2I) 3,3,3-Trifluoro-1-propene (CF3CH=CH2) 2,3,3,3-Tetrafluoro-1-propene (CF3CF=CH2) 1,3,3,3-Tetrafluoro-1-propene (CF3CH=CHF) Z-1,2,3,3,3-Pentafluoro-1-propene (CF3CF=CHF) E-1,2,3,3,3-Pentafluoro-1-propene (CF3CF=CHF) Perfluoropropene (CF3CF=CF2) Perfluorobuta-1,3-diene CxF2x+1CH=CH2 (× ≥ 2, 4, 6, 8)

1.9 ± 0.7v (8.1 ± 2.4) × 10−3 u (4.9 ± 2.0) × 10−3 k (4.9 ± 2.0) × 10−3 k (6.9 ± 0.5) × 10−4 k (1.6 ± 0.6) × 10−6 k 0.11 ± 0.4 k 1.79 ± 0.7u 3.7 ± 1.5o 2.4 ± 0.9o 44 ± 18o 23 ± 9o 3.52 ± 0.43v 0.35 ± 0.10e (2.77 ± 0.20) × 10−3 0.028 ± 0.008f 0.0015 ± 0.005g 0.020 ± 0.006g (6.2 ± 1.5) × 10−4 h (6.5 ± 0.2) × 10−3 h 0.20 ± 0.2e

3.47v 46u

2,233v 4,634u

1.0u

1,885u

81.7

2,991

CH2O, BrCH2CHO, CH2O, Cl2CO ClCH(O) ClCH(O) Cl2CO, ClCH(O) Cl2CO CH2O, CH3C(O)Cl CH2O, CH(O)CH2Cl CH2O, ClCH2C(O)CH3 CH2O, CH3CH2C(O)Cl ClCH2C(O)H, CH3C(O)CH3 CH3CHO, CH(O)CH2Cl CH2O. CH(O)CH2I CH2O, CF3C(O)H CF3C(O)F, CH2O FC(O)H, CF3CHO CF3C(OF), HC(O)F CF3C (O)F, HC(O)F CF3C(O)F, CF2O CF2O CxF2x+1CHO, CH2O

a Most recommendations are those of Calvert et al. (2000) where detailed kinetic information is reviewed and evaluated. bAlam et al. (2011). c Shi et al. (2011). dFragmentation products of the Criegee intermediates and other minor products are not shown in the table. eAndersen et al. (2005); fSøndergaard et al. (2007); gHurley et al. (2007); hAcerboni et al. (2001); iKhamaganov and Hites (2001); jKarl et al. (2004); kLeather et al. (2010); l McGillen et al. (2011b); rate coefficients were measured at 396 K and k(298) value was calculated using Ea = 19.83 kJ mole−1; mAlam et al. (2011); nLewin et al. (2001); oJohnson et al. (2000); qLeather et al. (2010); rShi et al. (2011); sCusick and Atkinson (2005); tKlawatsch-Carasco et al. (2004); uAvzianova and Ariya (2002); vGai et al. (2009a); wThese A and B values reported by Avzianova and Ariva (2002) do not yield the k(298) value reported by them and given in this table. xMason et al. (2009).

83

84

the mechanisms of reactions influencing atmospheric ozone

1018 x k(298 K), cm3 molecule−1 s−1

100

10

n -1-Alkenes k(298) 2-Methyl-1-alkenes (k(298)) n -1-Alkenes (k(395.9)), McGillen et al (2011b) n -1-Alkenes k(395.9) data adjusted to 298 K 1

2

4

6

8

10

12

14

Number of C-atoms in 1-Alkene FIGURE II-C-2.

Plot of rate coefficients for the ozone reaction with the 1-alkenes and the 2-methyl alkenes as a function of carbon number. The k(396 K) data of McGillen et al. (2011b) for the 1-alkene-O3 reaction, when adjusted to k(298 K) (using Ea = 19.83 kJ mol−1), show the relative insensitivity of the data to carbon number.

II-C-2. Ozone Reactions with the Cyclic Alkenes The cyclic alkenes react readily with ozone as expected, but it is somewhat surprising at first sight to see that reactivity decreases significantly—although not in a regular progression—with the increasing size of the cyclic alkene (see Figure II-C-3). The  –CH2– group is attached to both ends of the  –CH=CH group in each of the unsubstituted cyclic alkenes, and, if the inductive effect of these substitutions were the only influence on reactivity with ozone, one expects each of these compounds to have a very similar rate coefficient. However, a significant decrease is seen in rate coefficient with increasing size of the cyclo-alkene molecule; k(O3-cyclopentene/ k(O3-cyclodecene) equals 20. In contrast, the reactions of the HO radical with the cyclic alkenes show

little change in rate coefficient with increasing ring size. Values of 1012 × k(298), cm3 molecule−1 s−1 for the HO reactions are cyclopentene, 67; cyclohexene, 68; cycloheptene, 74. Obviously, factors other than inductive effects contribute to the reactivity of the cyclic alkenes with ozone, as indicated in previous discussions. In view of the larger size of the ozone molecule compared to that of HO, it seems probable that steric hindrance and/or differences in ring strain play a role in these ozone reactions. Qualitatively, one can rationalize from the molecular structures of the C5 and C10 cyclic alkenes that the chance for a reactive encounter of the ozone molecule with the C=C bond is expected to be less favorable in progressing from cyclopentene to cyclodecene because unreactive collisions with the larger unreactive  –CH2– framework of the larger molecule become more likely:

TABLE II-C-2 . THE EXTENT OF BOND BRE AKAGE THAT OCCUR S IN THE T WO ALTERNATIVE MODES OF FR AGMENTATION OF THE ORIGINAL OZONIDE FORMED IN OZONE RE ACTIONS WITH THE UNSYMMETRICAL 1-ALKENES; SHOWN ARE THE YIELDS OF FORMALDEHYDE AND OTHER ALDEHYDES THAT RESULT FROM THE ALTERNATIVE BOND BRE AKAGE IN THE OZONIDE, THE SUM OF THE T WO YIELDS, AND THEIR R ATIO. DATA ARE FROM AVER AGES OF YIELDS GIVEN IN TABLE IV-D-1 OF CALVERT ET AL. (2000)

CH2=CHR+O3

O O

O

C H

H2C

(1)

R

(2)

(3)

(4)

(5)

1-Alkene

CH2O Yield

RCHO Yield

Sum: (2) + (3)

Ratio: (2)/(3)

Propene 1-Butene 1-Pentene 1-Hexene 1-Heptene 1-Octene 1-Decene

0.71 0.63 0.55 0.54 0.53 0.50 0.53

0.46 0.35 0.52 0.53 0.58 0.50 0.49

1.17 0.98 1.07 1.07 1.11 1.00 1.02

1.54 1.80 1.06 1.04 0.91 1.00 1.08

1018 x k(298), cm3 molecule−1 s−1

1000

100

Cycloalkenes 1-Methyl-cycloalkene 10 4

5

6

7 8 9 Number of C-atoms in ring

10

11

FIGURE II-C-3.

The variation of the rate coefficients for the unsubstituted, cyclic alkenes as a function of ring size (large open circles) and 1-methyl-substituted cycloalkenes (small, closed circles).

85

86

the mechanisms of reactions influencing atmospheric ozone

As with the acyclic alkenes, substitution of a methyl group on a double-bonded carbon atom increases the rate coefficient of cyclopentene and cyclohexene (see Figure II-C-3). The mechanism of reaction of ozone with the cycloalkenes can be illustrated with cyclohexene as the reactant:

II-C-3. Ozone Reactions with the Dienes Reaction of ozone with the conjugated 1,3-dienes results in addition and subsequent rupture of one of the two double bonds. As with the

O H C

O

HC CH

H2C H2C

O

H2C

+ O3

CH2

C H2

CH

H2C

C H2

CH2

[CHOCH2CH2CH2CH2CHOO] (a)

(b)

CH3CH2CH2CH2CHO + CO2

(c)

(d)

[CHOCH2CH2CH2CH=CHOOH]

CHOCH2CH2CH2CH2CHOO

(M) (e)

HCOCH2CH2CH2CH2CHO + O(3P) Additional products

[CHOCH2CH2CH2CH2COOH]

CHOCH2CH2CH2CHCHO + HO

In the reaction of the cycloalkenes with O3, only one initial transient product is formed because both of the reactive ends of the C=C bond are tied together by the ring atoms. Subsequent rapid decomposition of this intermediate occurs by a variety of reaction channels, as shown in the O3-cyclohexene mechanism outlined above. Pathway (a) involves CO2 elimination and formation of pentanal. In pathway (b) an O(3P) atom is released as a molecule of hexanedial is formed. In (c) the excited initial product is relaxed and stabilized by collisions; it ultimately undergoes a variety of reactions to form stable products. Pathway (d)  is interesting in that it involves an isomerization to form a vibrationally rich, unsaturated hydroperoxide molecule that decays for a large fraction of the time by releasing an HO radical in pathway (e); a portion of the hydroperoxide molecule is stabilized by collisions (see Section II-E-4).

monoalkenes, the rate coefficients for the reactions show a marked influence on alkyl radical substitution (see Figure II-C-4). Addition of CH3 groups to one of the outer C atoms in the –CH=CHCH=CH– group results in a much larger increase in the k(298) than does substitution on the inner C atoms; again, this appears to suggest the importance of steric as well as inductive effects of alkyl group substitutions on ozone-alkene reactions. Substitution of CH3 groups on both inner and outer C atoms in the nonconjugated diene 2,6-dimethyl-2,6-octadien-8-ol gives an increase in k(298) that is intermediate to that seen for single inner and single outer substitutions (triangle in Figure II-C-4). The mechanism for the reaction of 1,3-butadiene with ozone is similar to that given for the monoalkenes:

Mechanisms of Ozone Reactions in the Troposphere

87

1000

1018 x k(298), cm3 molecule−1 s−1

Outer CH3-substitution Inner CH3-substitution Inner and outer CH3-substitution

100

10

1

4

5 Number of C-atoms

6

FIGURE II-C-4.

Effect of CH3-substitution in the  –C=CHCH=C– group on the k(298) for reactions of ozone with 1,3-butadienes; substitution of CH3 on the outer C atoms in the group (squares); substitution on the inner C atoms in the group (filled circles); substitution on both inner and outer C atoms (triangle).

O O O3 + CH2=CHCH=CH2

O CH2

CH2O + O2CHCH=CH2

CHCH=CH2 CH2OO + CH2=CHCHO

Fragmentation products including: CO, CO2, C2H4, C2H2

The ozone reaction with the unsymmetrical 2-methyl1,3-butadiene (isoprene), CH2=CH(CH3)CH=CH2, shows a preference for bond breakage in the ozonide to form CH2=C(CH3)CHO rather than CH3C(O) CH=CH2 (ratio of rate coefficients = 2.5). I I - D. M E C H A N I S M S O F O 3 R E AC T I O N S W I T H U N S AT U R AT E D O X Y G E N AT E S The unsaturated oxygenates react rapidly with ozone by mechanisms that parallel those of the unsaturated alkenes. In this section, we briefly review these reactions for the various families:  alcohols (Section II-D-1); ethers (Section II-D-2), aldehydes (Section II-D-3), ketones (Section II-D-4), acids (Section II-D5), and esters (Section II-D-6). The rate coefficients

and the expected or observed products for these reactions are given in Table II-D-1; these are largely those reviewed and recommended by Calvert et al. (2011). Reference to that study will provide plots, treatment of the data, references to the original work, and more detailed discussions of the mechanism. II-D-1. Ozone Reactions with the Unsaturated Alcohols As one might anticipate from the ozone-alkene reactions, the rate coefficients for the acyclic unsaturated alcohols increase as the number of alkyl groups attached to the C=C bonds (nonconjugated) in the molecule is increased (see Figure II-D-1).

the mechanisms of reactions influencing atmospheric ozone

1017 x k(298), cm3 molecule−1 s−1

88

120

80

40

0

1

2

3

4

5

6

Number of alkyl groups attached to C=C groups in acyclic alcohols (including non-conjugated dienes) FIGURE II-D-1.

Plot of the rate coefficients [k(298)] for acyclic unsaturated alcohols versus the number of alkyl groups attached to the C=C bonds (nonconjugated) in the molecules. Data shown for compounds with 1-group: 2,6-dimethyl-7-octen-2-ol; 3-buten-1-ol; 2-methyl-3-butene-2-ol; 2-propene-1-ol; 1-penten-3-ol; for 2-groups:  cis-3-hexen-1-ol; cis-2-penten-1-ol; 2-buten-1-ol; for 3-groups: 2,7-dimethyl-6-octen-1-ol; for 4-groups: 3,7-dimethyl-1,5-octadien-3-ol; and for 6 groups; 2, 6-dimethy-2,6-octadien-8-ol.

Consider the 2-butene-1-ol molecule as an example of the mechanism for reaction of ozone with the acyclic unsaturated alcohols: CH3CH=CHCH2OH + O3

CH3 CH

C=C group of the ether, have rate coefficients [1017 × k(298), in units of cm3 molecule−1 s−1] that

CHCH2OH O

O O

CH3CHO + OOCHCH2OH

CH3CHOO

+ CHOCH2OH

Fragmentation Products

Unique to these reactions is the formation of an aldehyde or ketone together with a hydroxyaldehyde or hydroxyketone. Rate coefficients for O3 reactions with the aromatic alcohols are relatively small, and loss of these species by ozone attack in the troposphere is probably unimportant. II-D-2. Ozone Reactions with the Unsaturated Ethers The alkyl vinyl ethers of the structure, ROCH=CH2, in which the ether  –O– group is attached to the

are similar:  R  =  C2H5, 19; n-C3H7, 23; n-C4H9, 26. These rate coefficients are all significantly larger than those for the 1-alkenes, which are near 1 × 10−17 cm3 molecule−1 s−1. However, 4,5-dihydro-2-methylfuran with a CH3 group as well as the ether  –O– group attached to a ring C=C bond enhances k(298) by a factor of 350. In contrast, for ethyl-1-propenyl ether, CH3CH2OCH2CH=CH2, and 2,5-dihydrofuran, in which the ether  –O– group is located down-chain

TABLE II-D-1 . OZONE RE ACTIONS WITH UNSATUR ATED OXYGENATES: RECOMMENDED R ATE COEFFICIENTS, 298 ± 2 K (cm 3 molecule −1 s −1 ). MOST RECOMMENDATIONS ARE FROM CALVERT ET AL. (2011). NEWER DATA ARE REFERENCED IN THE FOOTNOTES. ALSO SHOWN IN COLUMN THREE ARE THE OBSERVED OR EXPECTED (SHOWN IN ITALICS) MA JOR CARBONYL PRODUCTS OF THE RE ACTIONS

1017 × k(298)

Unsaturated Oxygenate a) Alcohols 2-Propen-1-ol, allyl alcohol (CH2=CHCH2OH) 2-Buten-1-ol (CH3CH=CHCH2OH) 3-Buten-1-ol (CH2=CHCH2CH2OH) 3-Buten-2-ol [CH2=CHCH(OH)CH3] 2-Methyl-3-buten-2-ol [CH2=CHC(CH3)2OH cis-2-penten-1-ol (CH3CH2CH=CHCH2OH) 1-Penten-3-ol (CH2=CHCH(OH)CH2CH3) cis-3-Hexen-1-ol (CH3CH2CH=CHCH2CH2OH) 3-Methyl-1-penten-3-ol [CH2=CHC(CH3)(OH)CH2CH3] 2,6-Dimethyl-7-octen-2-ol [CH2=CHCH(CH3)CH2CH2CH2C(CH3)2OH] 3,7-Dimethyl-1,6-octadien-3-ol (Linalool) [CH2=CHC(OH)(CH3)CH2CH2CH=C(CH3)2] 2,6-Dimethyl-2,6-octadien-8-ol (Geraniol) [HOCH2CH=C(CH3)CH2CH2CH=C(CH3)2] 2,7-Dimethyl-6-octen-1-ol (Citronellol) [HOCH2CH(CH3)CH2CH2CH2CH=C(CH3)2] OH

α-Terpineol

1.6 ± 0.3 25 ± 10 0.49 ± 0.2 1.6 ± 0.6 0.90 ± 0.23 0.83 ± 0.10a 17 ± 7 1.8 ± 0.7 8.6 ± 4.3 0.52 ± 0.03b ≤ 0.2 44 ± 22 93 ± 47

24 ± 12 30 ± 15

Observed or Expected Major Carbonyl Products CH2O, HOCH2CHO CH3CHO. HOCH2CHO, CH2O CH2O, HOCH2CH2CHO CH3CH(OH)CHO, CH3CHO, CH2O CH2O, (CH3)2C(OH)CHO, CH3C(O)CH3 CH3CH2CHO, HOCH2CHO CH2O, CH3CH2CH(OH)CHO CH3CH2CHO, HOCH2CH2CHO, CH3C(O)CHO, CH3CHO CH2O, CH(O)C(OH)(CH3)CH2CH3 CH2O, OCHCH(CH3)CH2CH2CH2C(CH3)2OH CH3C(O)CH3, CH2O, CH2=CHC(OH)(CH3)CH2CH2CHO CH3C(O)CH3, HOCH2CHO, CH(O)C(O)H, CH3C(O)CHO, CH3C(O)CH2CH2CHO CH3C(O)CH3, HOCH2CH2CH(CH3)CH2CH2CHO CH3C(O)CHO

≤ 0.026 ± 0.0078

Reactions with ozone are unimportant in the troposphere.

≤ 0.019 ± 0.0057

Reactions with ozone are unimportant in the troposphere.

≤ 0.047 ± 0.014

Reactions with ozone are unimportant in the troposphere.

≤ 0.06 ± 0.03

Reactions with ozone are unimportant in the troposphere.

≤ 0.93 ± 0.18

Reactions with ozone are unimportant in the troposphere.

o-Cresol HO

m-Cresol OH

p-Cresol

Benzyl alcohol

OH

CH2OH OH

1,2-Dihydroxybenzene OH

89

TABLE II-D-1 . (CONTINUED)

1017 × k(298)

Unsaturated Oxygenate

1,2-Dihydroxy-3-methylbenzene

Observed or Expected Major Carbonyl Products

≤ 2.8 ± 0.5

Reactions with ozone are unimportant in the troposphere.

≤ 2.6 ± 0.5

Reactions with ozone are unimportant in the troposphere.

OH

OH

1,2-Dihydroxy-4-methylbenzene OH

OH

b) Unsaturated ethers Ethyl vinyl ether (CH3CH2OCH=CH2) 1,1-Dimethoxyethene [(CH3O)2C=CH2] n-Propyl vinyl ether (CH3CH2CH2OCH=CH2) n-Butyl vinyl ether (CH3CH2CH2CH2OCH=CH2) Ethyl 1-propenyl ether (CH3CH2OCH=CHCH3) Ethylene glycol vinyl ether (HOCH2CH2OCH=CH2) Ethylene glycol divinyl ether (CH2=CHOCH2CH2OCH=CH2) Diethylene glycol divinyl ether (CH2=CHOCH2CH2OCH2CH2OCH=CH2) O

19 ± 4 21 ± 4b 48 ± 3d 23 ± 9 23 ± 5b 26 ± 10 26 ± 5b ≥ 50 ± 10

CH2O, CH3OC(O)OCH3 CH2O, CH3CH2CH2OCHO CH2O, CH3CH2CH2CH2OCHO

20 ± 8

CH3CHO, C2H5OCHO, CH2O, CH3CH2OCH2CHO CH2O, HOCH2CH2OCHO

17 ±7

CH2O, HC(O)CH2CH2OCH=CH2

27 ± 11 0.16 ± 0.06

3,4-Epoxy-1-butene

CH2O, CH3CH2OCHO

CH2O, HC(O)OCH2CH2OCH2CH2OCH=CH2 CH2O, O

CHO

O

0.24 ± 0.08

?

Furan a

O

2.0 ± 0.7

Reaction at site a > b

1.6 ± 0.6

HC(O)CH2OCH2CHO?

3-Methylfuran b

2,5-Dihydrofuran

O

O

4,5-Dihydro-2-methylfuran

90

350 ± 140??

CH3C(O)CH2CH2CHO; CH2O, CH3OH, CO, CO2, HC(O)OH; CH2=C=O; CH2=CH2 (continued)

TABLE II-D-1 . (CONTINUED)

1017 × k(298)

Unsaturated Oxygenate

5-Methyl-5-vinyl tetrahydrofuran-2-ol O OH

Observed or Expected Major Carbonyl Products

0.35 ± 0.14

O

(O)HC

OH ?

CH2O, O

2,3-Benzofuran c) Unsaturated aldehydes 2-Propenal, acrolein (CH2=CHCHO) 2-Butenal (CH3CH=CHCHO) 2-Methyl-2-propenal [CH2=C(CH3)CHO] trans-2-Methyl-2-butenal [CH3CH=C(CH3)CHO E-2-Pentenal (CH3CH2CH=CHCHO) 3-Methyl-2-butenal (CH3)2C=CHCHO) 2-Methyl-2-butenal (CH3CH=C(CH3)CHO) E-2-Hexenal (CH3CH2CH2CH=CHCHO) 2,4-Hexadiendial[H(O)CCH=CHCH=CHCHO 4-Methylenehex-5-enal (CH2=CHC(=CH2)CH2CH2CHO) 3(Z)-4-Methylhex-3,5-dienal [CH2=CHC(CH3)=CHCH2CHO] 3(E)-4-Methylhex-3,5-dienal [CH2=CHC(CH3)=CHCH2CHO] 3-Isopropenyl-6-oxo-heptanal CH3C(O)CH2CH2CH[C (CH3)=CH2]CH2CHO 3,7-Dimethyl-6-octenal (CH3)2C=CHCH2CH2CH(CH3)CH2CHO

CHO

0.18 ± 0.06

OH

0.029 ± 0.009 0.14 ± 0.06e 0.11 ± 0.02c 29.5 ± 12 0.16 ± 0.06e 0.18 ± 0.07e 0.53 ± 0.21e 0.21 ± 0.08 ≤ 1.0 1.5 ± 0.6 4.4 ± 1.8 5.7 ± 2.3 0.78 ± 0.23

35 ± 10

CH2O, CH(O)CHO CH3C(O)H, HC(O)CHO CH2O, CH3C(O)CHO CH3CHO, CH3C(O)C(O)H CH3CH2CHO, HC(O)CHO CH3C(O)CH3, HC(O)CHO CH3CHO, CH3C(O)CHO CH3CH2CH2CHO, CH(O)CHO HC(O)CHO, CH(O)CH=CHCHO CH2O, CH(O)C(=CH2)CH2CH2CHO, CH2=CHC(O)CH2CH2CHO CH2O, CH(O)C(CH3)=CHCH2CHO CH2=CHC(O)CH3, CH(O)CH2CHO CH2O, CH(O)C(CH3)=CHCH2CHO CH2=CHC(O)CH3, CH(O)CH2CHO CH3C(O)CH2CH2CH[CH2CHC(O)CH3 CH2O, CH3C(O)CH2CH2CH[C(O)CH3] CH2CHO CH3C(O)CH3, H(O)CCH2CH2CH(CH3)CH2CHO

d) Unsaturated ketones 3-Buten-2-one [CH3C(O)CH=CH2] 1-Penten-3-one [CH2=CHC(O)CH2CH3] 3-Penten-2-one [CH3C(O)CH=CH2CH3] Hex-4-ene-3-one [CH3CH2C(O)CH=CHCH3] Hex-5-ene-2-one [CH2=CHCH2CH2C(O)CH3] 4-Methyl-3-penten-2-one, mesityl oxide [(CH3)2C=CHC(O)CH3] 5-Hexen-2-one (CH2=CHCH2CH2C(O)CH3 6-Methyl-5-hepten-2-one [CH3C(O)CH2CH2CH=C(CH3)2] trans-4-Methoxy-3-buten-2-one [CH3C(O)CH=CHOCH3] cis/trans-4-Oxo-2-pentenal [CH3C(O)CH=CHCHO] cis-Hex-3-ene-2,5-dione [CH3C(O)CH=CHC(O)CH3] trans-Hex-3-ene-2,5-dione [CH3C(O)CH=CHC(O)CH3] 3,4-Dihydroxy-3-hexene-2,5-dione [CH3C(O)C(OH)=C(OH)C(O)CH3]

0.50 ± 0.13i 0.60 ± 0.24 3.2 ± 1.3e 6.4 ± 2.6 5.2 ± 1.3j 0.81 ± 0.28e 9.2 ± 3.7g 39 ± 12 1.3 ± 0.4

CH2O, CH3C(O)CHO CH2O, CH3C(O)CH2CHO CH3C(O)CHO, CH3CHO CH3CHO, CH3C(O)CH2CHO CH2O; CH3C(O)CH2CH2CHO CH3C(O)CH3, CH3C(O)CHO CH2O, CH(O)CH2CH2C(O)CH3 CH3C(O)CH2CH2CHO, CH3C(O)CHO, CH3C(O)CH3 CH3C(O)CHO; CH3OCHO

0.48 ± 0.19

CH3C(O)CHO, HC(O)CHO

0.18 ± 0.07

CH3C(O)CHO,

0.83 ± 0.33

CH3C(O)CHO

0.36 ± 0.14

CH3C(O)C(O)OH (continued)

91

TABLE II-D-1 . (CONTINUED)

1017 × k(298)

Unsaturated Oxygenate e) Ketenes Ketene (CH2=C=O) Methylketene (CH3CH=C=O) Ethylketene (CH3CH2CH=C=O Dimethyhlketene [(CH3)2C=C=O]

< 0.0001 < 0.07 < 0.1 C=C< bond are CH2=CH2, 0.16; CH2=CH–, 1.0; CH2=C 10−1). The reader is referred to Chang et al. for details, but a few general points will be made here. First, the N2O5 uptake process appears to be hindered by the presence of NO3− in the particle. This is likely due to the fact that the incorporation of N2O5 into the aerosol and its subsequent conversion to nitric acid, requires its dissociation to NO2+ (or a hydrated analog) and NO3−, with the NO3− then acting to drive the reverse process (Chang et al., 2011, and references therein) and thus limit N2O5 uptake:

ko = 3.6 × 10−30 (T/300)−4.1 [N2] cm3 molecule−1 s−1

N2O5 (aq) → NO2+ + NO3− (aq)

III-D-5. NO2 + NO3 Reactions: N2O5 Formation and Dissociation Rate coefficients for the reaction pair (and, by definition, the equilibrium constant) describing formation and thermal decomposition of N2O5 are provided by the IUPAC (Atkinson et al., 2004) and JPL (Sander et al., 2011) data panels: NO3 + NO2 + M → N2O5 + M N2O5 + M → NO3 + NO2 + M

k∞ = 1.9 × 10−12 (T/300)0.2 cm3 molecule−1 s−1; Fc = 0.35 For N2O5 + M → NO3 + NO2 + M: ko = 1.3 × 10−3 (T/300)−3.5 exp(−11,000/T) [N2] s−1 k∞ = 9.7 × 1014 (T/300)0.1 exp(−11,080/T) s−1; Fc = 0.35

There is still a great deal of uncertainty regarding the uptake of N2O5 on the array of aerosol compositions found in the atmosphere. However, some general observations (detailed in Chang et  al. [2011]) are that N2O5 uptake is more efficient on liquid rather than crystalline particles and occurs quite efficiently on mineral dust but is generally inefficient on organic substrates or on surfaces with organic coatings.

The Oxides of Nitrogen Another process receiving recent attention is the reaction of N2O5 with halide ions; for example, N2O5(g)+ Cl−(aq) → ClNO2(g) + NO3− (aq) The process is well known in the laboratory (e.g., Finlayson-Pitts et  al., 1989), and recent observations of ambient morning ClNO2 (e.g., Osthoff et al., 2008) confirm the occurrence of this process. Much like HONO, ClNO2 is readily photolyzed and can thus provide an early-morning “bump” to the oxidizing capacity of the troposphere. Surprisingly, this chemistry is not limited to marine/coastal regions (where chloride ion abundances are highest). Thornton et  al. (2010) demonstrated significant production of ClNO2 at an inland site (Boulder, Colorado) in February 2009 and showed that the chemistry may occur to a measurable extent throughout the continental United States. I I I - E . E VA L UAT I O N O F THE MECHANISMS OF NO3 R A D I CA L R E AC T I O N S III-E-1. The Photodecomposition of NO3 [j(NO3] The NO3 radical absorbs strongly in the visible region (see Figure III-E-1). The strong band at 662 nm is often used to detect and follow the concentration of

NO3 in both laboratory and field experiments. The recommended value for σ (662 nm) is (2.25 ± 0.15) × 10−17 cm2 molecule−1 at 298 K (Sander et al., 2011). This cross section appears to be somewhat temperature dependent, increasing in magnitude as the temperature is lowered. Although the data are subject to a rather large uncertainty, the data of Yokelson et al. (1994) suggest that σ(662 nm) increases by a factor of about 50% as the temperature is lowered from 298 K to 200 K. Photodecomposition of NO3 occurs by two pathways: NO3 + hν → NO2 + O(3P) NO3 + hν → NO + O2

6.0 x 1014

1.5

σ (Sander et al., 2011) σ (Yokelson et al.,1994) Actinic flux, Z = 0

1.0

5.0 x 1014

0.5

Actinic flux, quanta cm−2 s−1 nm−1

1017 x σ(NO3), cm2 molecule−1

(II)

7.0 x 1014

2.0

400

(I)

Whereas photolysis to O(3P) and NO2 is neutral with respect to ozone production/loss, photolysis to NO actually leads to net destruction of ozone. As alluded to earlier, the rapid photolysis of NO3 limits its concentration during sunlight hours and thus its impact as a daytime oxidant. In urban areas, the [NO3] can reach rather large values at night. For example, Platt et al. (1980b) measured [NO3] maxima in the range 18–355 ppt in the Riverside, California, area. At these concentrations, one expects significant NO3−initiated VOC oxidation to occur.

2.5

0.0

127

4.0 x 1014 450

500

550

600

650

700

Wavelength, nm FIGURE III-E-1.

Cross sections for the NO3 radical as recommended by Sander et al. (2011), solid gray line. The spectra reported by other workers show reasonable agreement with the Sander et al. data. For example, note that the small dots, data of Yokelson et al. (1994), match the Sander et al. data well. Note the good overlap of the cross sections and the actinic flux (gray curve).

the mechanisms of reactions influencing atmospheric ozone

Quantum Yield: NO3 + hν → NO2 + O(3P)

Quantum yields of photodecomposition by primary processes (I) and (II) have been determined in several studies beginning with the pioneering work of Graham and Johnston (1978). In Figure III-E-2, NO3 photodissociation in process (I)  is seen to increase rapidly in efficiency as one progresses from 650 to 570  nm. Although there is significant scatter in the Magnotta and Johnston (1980) data at the shorter wavelengths, it is probable that ϕ(I) values approach 0.9–1.0 for λ of less than 580 nm. Two recommendations have been made to describe the trend of data: the solid curve from the study of Orlando et al. (1993) and the dashed curve suggested by Johnston et al. (1996). In each case, the recommending group has favored its own measurements in fitting the data to arrive at their recommendation. A  major difference between the solid and the dashed curves is the decrease from the linear decline in ϕ(I) seen in the Orlando et al. data near the strong 1–0 transition at 623  nm. Excitation at 623 nm promotes NO3 from v = 0 in the ground state to v = 1 in the upper state, which lies some 850 cm−1 below the barrier to dissociation; Orlando et  al. (1993) suggested that direct photodissociation may be unlikely at this point, and

fluorescence of the excited molecule may result in lowering of the photodissociation quantum yield for process (I). Davis et  al. (1993) have studied the photochemistry of NO3 under collision-free conditions using molecular beam photofragmentation translational spectroscopy. They concluded that the wavelength threshold for NO3 (0,0,0,0) → NO2 (0,0,0) + O(3P) is 587 ± 3 nm; thus, the observed photodissociation of NO3 at wavelengths of greater than 587 nm must be facilitated by internal energy of the NO3 radical. Quantum yields of the minor primary process (II) have also been determined and are summarized in Figure III-E-3. Here, again, two different descriptions of the scattered data trend have been given; the dashed line from the Johnston et al. (1996) recommendation and the solid line from Orlando et al. (1993). At this writing, it is not clear which (if either) of the two recommendations should be favored. We used both to estimate j-values of processes (I) and (II). In Figure III-E-4, values of j(I) and j(II) are given as a function of solar zenith angle as calculated using the cross sections of Sander et  al. (2011),

1.0

2.0e−17

0.8

1.5e−17

0.6 1.0e−17 0.4 5.0e−18

0.2

0.0 450

0.0 500

550

600

650

Wavelength, nm Magnota and Johnston (1980); O(3P)-detection; low light absorption Magnotta and Johnston (1980); O(3P) detection; high light absorption Magnotta and Johnston (1980); NO2 (NO) detection; low light absorption Orlando et al. (1993); O(3P) detection Recommendation of Orlando et al. (1993) NO3 cross sections (Yokelson et al.,1994) Recommendation of Johnson et al. (1996) FIGURE III-E-2.

Experimental measurements of ϕ(I), NO3 + hν → NO2 + O(3P).

NO3 Cross Sections, cm2 molecule−1

128

2.5 x 103

0.4

2.0 x 103

0.3

1.5 x 103

0.2

103

0.1

5.0 x 103

0.0 580

590

600

610

620

630

640

NO3 Cross Sections, cm2 molecule–1

Quantum Yield: NO3 + h ν → NO + O2

0.5

0

650

Wavelength, nm

Magnotta and Johnston (1980); high light absorption Magnotta and Johnston (1980); low light absorption Orlando et al. (1993) NO3 cross section (Sander et al., 2011) Johnston et al. (1996); recommended φ(λ) relation Orlando et al. (1993); recommended φ(λ) relation FIGURE III-E-3.

Experimental measurements of ϕ(II), NO3 + hν → NO + O2.

0.25

j (NO3), s–1

0.20

0.15

0.10

0.05

0.00

0

10

20

30

40

50

60

70

80

90

100

Solar zenith angle, degrees NO3 + hν → O(3P) + NO2; Orlando et al. (1993) NO3 + hν → O(3P) + NO2; Johnston et al. (1996) NO3 + hν → O2 + NO; Orlando et al. (1993) NO3 + hν → O2 + NO; Johnston et al. (1996) FIGURE III-E-4.

Calculated j-values for the primary photodissociation modes in NO3. Cross sections of Sander et al. (2011), the appropriate solar flux for an overhead ozone column of 350 Dobson units, and the two different recommended trends for ϕ(I) and ϕ(II) vs. λ were used.

129

130

the mechanisms of reactions influencing atmospheric ozone

the two different recommendations for the quantum yield trends, and the appropriate solar flux for clear-sky conditions and a 350 DU overhead ozone column. Both sets of j-values suggest that NO3 has a very short photochemical lifetime, in the range of 4.2–4.3 seconds with overhead sun and cloudless skies. The insensitivity of j(NO2) to the magnitude of the overhead ozone column, which we noted previously, is also a characteristic feature of j(NO3) because most of the action spectra for NO3 is efficiently removed from the region of stratospheric ozone absorption. III-E-2. Mechanism of the Reaction of NO3 with NO Evaluated rate coefficients for the reaction of NO3 with NO, NO3 + NO → 2 NO2, are provided by the JPL (Sander et al., 2011) and IUPAC (Atkinson et al., 2004) panels. Their recommendations agree to better than 10% between 210 and 300 K.  The Arrhenius expression recommended by IUPAC, k  =  1.8 × 10−11 exp(110/T) cm3 molecule−1 s−1, is adopted here. This expression yields k  =  2.6 × 10−11 cm3 molecule−1 s−1 at 298 K, with an estimated uncertainty of ±30%. III-E-3. Rates and Mechanisms for Reactions of NO3 with the Alkanes As mentioned earlier in this chapter, the reactions of NO3 with organic species can be of particular significance in the nighttime tropospheric chemistry either as a loss for the VOC in question and/or as a source of nighttime HOx radicals. These NO3/VOC reactions can occur via abstraction, for example, in the reaction of NO3 with an alkane, NO3 + RH → HONO2 + R•, or via addition, for example in the reaction of NO3 with a simple alkene, CH2=CH2 + NO3 → •CH2CH2ONO2. The abstraction reactions tend to be quite slow because they are often close to thermoneutral and occur through large energy barriers. As will become apparent in Section III-E-7, where structure-reactivity relationships are presented for reaction of NO3 with the entire suite of hydrocarbon species, the addition reactions are generally more rapid and of greater atmospheric significance. The reactions of NO3 with the alkanes have been reviewed in Calvert et al. (2008), and no work has been conducted on these reactions since that time. Thus, the recommendations of Calvert et al.

(2008), as summarized in Table III-E-1, remain unchanged. These rate coefficients tend to fall 4–5 orders of magnitude below the corresponding HO rate coefficients (Calvert et  al., 2008). Because nighttime [NO3] are approximately 109 molecule cm−3 or less, and daytime [HO] approximately 106–107 molecule cm−3, the NO3 reactions generally are not significant alkane loss processes. Note that very little is known about the temperature dependence of these reactions; all available information stems from a single study by Bagley et  al. (1990). Section III-E-8 includes a thorough presentation and discussion of structure-activity relationships (SARs) to predict rate coefficients for reactions of NO3 with organic species, largely based on the work of Kerdouci et al. (2010). At this point, however, a few defining features of the NO3−alkane rate coefficient database are noted. First, for the n-alkanes, the 298 K rate coefficients increase linearly from n-butane to n-decane, with the added reactivity per additional  –CH2– group being 3.56 × 10−17 cm3 molecule−1 s−1 (see Figure III-E-5). Although the individual rate coefficients for most of the NO3 reactions have rather high uncertainties, often greater than ±50%, the plot shows consistent trends, with structural changes that appear to be much better than the estimated uncertainties would suggest. Trends of this sort provide some confidence in the validity of the overall dataset and form the basis for reliable SAR parameterizations. Product studies have not been conducted on NO3-alkane reactions, and thus nothing is known about the actual site of attack. However, a higher reactivity for tertiary versus secondary versus primary C–H bonds is evident in the data (Bagley et al., 1990; Aschmann and Atkinson, 1995; Calvert et al., 2008; Kerdouci et al., 2010). Note, for example, that the reactivity of 2-methylpropane toward NO3 is a factor of 2 greater than that of n-butane, and similar relationships hold for the comparative reactivity of 2-methylbutane versus n-pentane and for 2- and 3-methylpentane versus n-hexane. Table III-E-1 includes predicted sites of attack for NO3 on the alkanes, obtained from the structure-reactivity relationships presented in Section III-E-8. Because the reactions produce simple alkyl radicals, the chemistry occurring subsequent to the initial abstraction reaction parallels exactly that found for the HO-alkane reactions. For

The Oxides of Nitrogen

131

k(298) for NO3 Reaction with n-Alkanes, cm3 molecule–1 s–1

4 x 10–16 Recommended Rate Coefficients Linear Least-Squares Fit

3 x 10–16

2 x 10–16

10–16

0

3

6

9

12

Number of Carbon Atoms in n-Alkane FIGURE III-E-5.

Rate coefficient for reaction of NO3 with the n-alkanes, plotted as a function of chain length.

example, the reaction of NO3 with n-butane would produce almost exclusive the 2-butyl peroxy radical, whose subsequent fate would involve (chiefly) the formation of 2-butanone and acetaldehyde, as shown in the following reaction scheme:

limits in the case of CH3Cl and CH2Cl2. However, recent data from Nakano et al. (2005, 2006) suggest a reasonably rapid reaction between NO3 and both

and O

OOH

OH

RO2

HO2

NO, RO2

NO3 + OO

+ HNO3

O2 O + NO2

O

NO2 CH3CHO + CH3CH2

CH3CHO

OONO2

III-E-4. Rates and Mechanisms for Reactions of NO3 with Haloalkanes Only very limited data are available on the kinetics of reactions of NO3 with haloalkanes. These are summarized in Table III-E-2. Wayne and co-workers (Canosa-Mas et  al., 1989; Boyd et  al., 1991) showed that reaction of NO3 with the chloromethanes is quite slow, obtaining only upper

CH3I and CH2I2, k ≈ 4 × 10−13 cm3 molecule−1 s−1. Given that the thermodynamics for NO3 abstraction from propane, CH3Cl, or CH3I are similar (all three reactions are slightly exothermic), the 4–5 order of magnitude faster rate coefficient for the iodomethanes is surprising. We note that Nakano et al. (2005, 2006) allude to potential complications

TABLE III-E-1 . NO 3 R ADICAL RE ACTIONS WITH THE ALKANES: RECOMMENDED R ATE

COEFFICIENTS, k = A × e −B/T (cm 3 molecule −1  s −1 ) a

Alkane

k(298 K)

Methane, (CH4)

C=C< moieties. Although the Kerdouci et  al. (2010) analysis obviously includes more data and defines more constituent effects, the successes and shortcomings of the procedure remain largely unchanged. For example, the severe underestimation of the NO3 rate coefficient with a number of biogenic species, detailed later, remains unexplained. In general, the rate coefficient database for NO3 is much less developed than that for HO (as will become apparent in Chapter IV). This fact makes the availability of an up-to-date SAR parameterization useful for estimating unknown NO3-VOC rate

162

the mechanisms of reactions influencing atmospheric ozone TABLE III-E-12 . NO 3 + ESTER S: RECOMMENDATIONS FOR R ATE COEFFICIENTS, k = A × e −B/T (cm 3 molecule −1 s −1 ) AT TROPOSPHERIC TEMPER ATURES a

Compound Methyl formate Ethyl formate n-Propyl formate Methyl acetate Ethyl acetate n-Propyl acetate Methyl propionate Ethyl propionate Methyl butyrate 3H-Furan-2-one cis-3-Hexenyl acetate Vinyl acetate Methyl acrylate Ethyl acrylate n-Butyl acrylate Methyl methacrylate Ethyl methacrylate Methyl E-2-butenoate Methyl 3-methyl-2-butenoate Methyl E-2-methyl-2-butenoate

Notes

1016 × k(298 K) 0.036 (± factor of 2)

(1)

(1) (1) (1)

(2) (3) (3) (3)

0.54 (± factor of 2) 0.07 (± factor of 2) 0.13 (± factor of 2) 0.5 (± factor of 2) 0.33 (± factor of 2) 0.33 (± factor of 2) 0.48 (± factor of 2) 1,760 ± 440 2,460 ± 740 90 ± 36 1.1 ± 0.5 1.6 ± 0.6 1.9 ± 0.8 36 ± 11 49 ± 20 18.5 ± 4.6 140 ± 35 490 ± 120

a

Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated. Recommendations that incorporate additional data published since this review are identified by footnotes in this table.

Notes: 1. Value not included in Calvert et al. (2011). Data are from Langer et al. (1993) and were obtained using a discharge flow-visible absorption technique. 2. New value not included in Calvert et al. (2011). Data were obtained by Picquet-Varrault et al. (2010) using both absolute and relative rate techniques. Note that propene was used as reference in the relative rate study; here, we use the International Union of Pure and Applied Chemistry (IUPAC)-recommended NO3/propene rate coefficient, rather than that used by Picquet-Varrault. An uncertainty of ±40% is assigned. 3. New value not included in Calvert et al. (2011). Data were obtained by Wang et al. (2010a) using a relative rate technique and have been adjusted to reflect the reference rate coefficients recommended in this chapter. An uncertainty of ±40% is assigned.

coefficient data, but it also provides limitations on the accuracy with which these rate coefficient estimates can be made. Care should be taken in estimating rate coefficients for species with chemical structures significantly different from those used in deriving the SAR parameters. III-E-8.1 SAR-Based Estimates of Rate Coefficients for NO3 Reactions with Alkanes and Haloalkanes The parameters recommended by Kerdouci et  al. (2010) for estimation of rate coefficients for reaction of NO3 with the alkanes are presented in Table  III-E-14. These parameters reflect expected

tendencies; that is, abstraction from primary, secondary, and tertiary groups are progressively more favorable due to a decrease in the relevant C–H bond energy, and branching at the site adjacent to the reactive site also enhances the rate coefficient. Insufficient data are available on which to base a SAR for the haloalkanes, so these species are not considered further. Comparisons between the measured and estimated rate coefficients are provided in Figure III-E-11. Although agreement is generally very good, significant discrepancies exist in particular for two branched species, namely 2,4-dimethylpentane and 2,2,4-trimethylpentane. These discrepancies for

The Oxides of Nitrogen

163

TABLE III-E-13 . NO 3 R ADICAL RE ACTIONS WITH THE N-ATOM-CONTAINING OXYGENATES; RECOMMENDED R ATE COEFFICIENTS, k = A × e −B/T (cm 3 molecule −1  s −1 )

1014 × k(298 K)

Compound

1012 × A

B (K)

Products

a) Amides Formamideb N-Methylformamideb N,N-Dimethylformamide N,N-Dimethylacetamide N.N-Dimethylpropionamide 1-Methyl-2-pyrrolidinone

C
CHOH –>COR, –>COH –C(=O)R

9.85 32.5 62.2 23.3 352 45.0 52.2 55.1 0.384

them quantitatively, they very clearly show the large enhancement to the rate coefficient imparted by the presence of either an alcohol or ether functionality and indicate that this effect is felt at least as far as the carbon located β to the functional group. III-E-8.3. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with the Alkenes, Haloalkenes, and Alkynes Group rate coefficients and substituent factors proposed by Kerdouci et al. (2010) for reaction of NO3 with the alkenes are summarized in Table III-E-16. Expected trends in reactivity are implied by the parameters. For example, the increase in the group rate coefficients from CH=CH2 to >C=C< reflect the fact that more substitution around the double bond leads to a higher reactivity toward NO3. The increase in reactivity with increasing chain length for both the 1-alkenes and the 2-methyl-1-alkenes

10−14

10−15

10−16

10−17 10−17

10−16

10−15

10−14

Measured Rate Coefficient, k(298), cm3 molecule−1 s−1 k(298) NO3 Reaction with Alcohols k(298) NO3 Reaction with Ethers k(298) NO3 Reaction with Hydroxyketones 1 : 1 Line Factor of 1.5 Disagreement FIGURE III-E-13. Comparison of the estimated and measured rate coefficients for reaction of NO3 with saturated alcohols, ethers, and hydroxycarbonyls. Structure-activity relationship (SAR) parameters are updates to those of Kerdouci et al. (2010), as described in the text and provided in Table III-E-15. The lowest data point shown is for CH3OH, which was not included in the fit.

The Oxides of Nitrogen TABLE III-E-16 . PAR AMETER S

REQUIRED FOR ESTIMATING RE ACTION R ATE COEFFICIENTS FOR RE ACTION OF NO 3 WITH THE ALKENES AND DIENES, TAKEN FROM THE RECENT STUDY OF KERDOUCI ET AL. (2010) Group Rate Coefficients (cm3 molecule−1 s−1) –CH=CH2 –CH=CH–, >C=CH2 –CH=C< >C=C
CH– >C< Chain Length Factor for 1-alkenes a Chain Length Factor for 2-methyl-1-alkenes a −CH=CH2 >C=CH2 –CH=CH– (cis) –CH=CH– (trans) >C=CH– >C=C< 3-membered ring b 4-membered ring b 5-membered ring b 6-membered ring b 7-membered ring b

1.6 1.85 1.46 1.02 −5.95 + 8.70 × (1−e−0.39N) –1.16 + 3.06 × (1-e−0.32N) 4.9 3.4 20 38 8.9 Not defined 0.033 0.73 1.86 0.93 1.35

The parameters shown account for the long-range inductive effects seen in the rate coefficient data for reaction of NO3 with long-chain 1-alkenes and 2-methyl-1-alkenes. “N” in these equations is the number of carbons in the molecule. The factor is applied for all species containing more than four carbon atoms (Kerdouci et al., 2010). b Parameter used to account for effect of a cyclic moiety attached to the double bond, such as in the methylenecycloalkanes. a

is captured by the “chain length factors” presented in the table. Finally, the large substituent factors for double-bonded moieties (–CH=CH2, >C=CH2, etc.) reflect the increased reactivity of conjugated double bonds compared to isolated double bonds. A comparison between estimated rate coefficients (using the parameters given in the table) and those measured is presented in Figure III-E-14. The Kerdouci parameterization does a reasonable job of capturing the large variability in the available data. In all, for the 90 compounds shown on

167

the plot (and listed in Table III-E-3), there is better than ±50% agreement between the measured and predicted rate coefficient for about 75% of the species, whereas 90% of the species show better than a factor of 2 agreement. Some of the more obvious disagreements occur for some methyl-substituted 1-pentene species (where the measured rate coefficients do not seem enhanced in the same way as for other 1-alkenes); some methyl-substituted cyclohexenes, where measured rate coefficients are not consistent with other alkenes with similar amounts of branching at the alkene site; cycloheptatriene, where the measured rate coefficient is considerably lower than that calculated; and a series of biogenics (e.g., β-pinene, sabinene, alloisolongifolene), where measured rate coefficients are considerably higher than expected for what, in many cases, are monounsaturated species. Regarding the haloalkenes, insufficient data are available to construct meaningful SAR parameters. However, it is clear that the presence of a halogen atom (in place of a methyl group) leads to a significant decrease in reactivity. As a rough guide, the inclusion of a substituent factor F(-CH2Cl) ≈ (1–3) × 10−3 with the parameters listed in Table III-E-16 provides a reasonable representation of the rate coefficients for the four species containing this functionality for which data are available. Similarly, insufficient data are available on which to base quantitative SAR parameters for the alkynes. However, it is apparent that a group rate coefficient for the 1-alkynes of k ≈2 × 10−16 cm3 molecule−1 s−1 is reasonable. The rate coefficients for the 1-alkynes increase in roughly linear fashion with number of carbons (between C3 and C6), but clearly this cannot continue indefinitely; work on longer chain species would be required before more quantitative conclusions can be drawn. That the reported rate coefficient for NO3 reaction with 2-butyne exceeds that for 2-hexyne calls the relative accuracy of the data into question, but a very rough group rate coefficient of ≈ 5 × 10−14 cm3 molecule−1 s−1 can be proposed from the two available data points. III-E-8.4. SAR-Based Estimates of Rate Coefficients for NO3 Reactions with Unsaturated Oxygenated Compounds (Alcohols and Ethers) In this and the following section, SAR parameterizations for reaction of NO3 with various classes of unsaturated oxygenated compounds are discussed,

168

the mechanisms of reactions influencing atmospheric ozone

SAR Estimated k(298) for NO3 Reaction, cm3 molecule−1 s−1

10−10

10−11

10−12

10−13

10−14 10−14

10−13

10−12

10−11

10−10

Measured k(298) for NO3 Reaction, cm3 molecule−1 s−1 k(298), NO3 + Acyclic Monoalkenes k(298), NO3 + Acyclic Dienes k(298), NO3 + Cyclic Alkenes, Dienes, Trienes k(298), NO3 + Monoterpenes, Sesquiterpenes 1 : 1 Line Factor of 2 disagreement FIGURE III-E-14.

Comparison of the estimated and measured rate coefficients for reaction of NO3 with the alkenes. Structure-activity relationship (SAR) parameters for the calculations are taken from the recent study of Kerdouci et al. (2010; see Table III-E-16 for parameters used).

thus completing the discussion of NO3 addition reactions. The analysis presented is largely based on the recent study of Kerdouci et al. (2010), although some updates are made. In particular, the parameterization is extended to include the unsaturated aldehydes, which were not dealt with by Kerdouci et al. The SAR parameters derived by Kerdouci et al. (2010) to estimate rate coefficients for reaction of NO3 with the unsaturated alcohols and ethers are adopted here and are listed in Table III-E-17. These substituent factors are used in conjunction with those for the alkenes (Table III-E-16) to derive the estimated rate coefficients that are displayed in Figure III-E-15. These estimated rate coefficients also include the contribution from abstraction and, because some of the abstraction parameters derived in this work are different from those originally used by Kerdouci et  al., slight differences may exist between the current estimates and those of Kerdouci et  al. However, because abstraction

contributes no more than a few percent to the overall rate coefficient in all cases, these differences are not significant. Overall, the parameters quantitatively describe some of the general points made earlier in this chapter. In particular, the presence of an ether functionality directly bonded to the C=C double bond leads to an enhancement in the NO3 rate coefficient by more than 100, whereas the presence of a β-hydroxy group leads to small enhancements of the rate coefficient. The level of agreement between the measured and estimated rate coefficients is generally good. For the 17 species considered in the analysis, agreement is better than ±25% for 8 species, ±50% for 13 species, and within a factor of 2 for all but one species. This large outlier is 3-methyl-2-buten-1-ol, where the measured rate coefficient is considerably smaller than that predicted (Kerdouci et al., 2010). This disagreement is difficult to explain because the

The Oxides of Nitrogen TABLE III-E-17 . PAR AMETER S

REQUIRED FOR ESTIMATING RE ACTION R ATE COEFFICIENTS FOR RE ACTION OF NO 3 WITH UNSATUR ATED OXYGENATES, TAKEN FROM THE RECENT STUDY OF KERDOUCI ET AL. (2010)

Substituent

Substituent Factors

–CH2OH >CH(OH) –>C(OH) –CH2CH2OH –OCH3 –OCH2–, –OCHC(=O)R –C–C(=O)R –C(=O)OR –C–C(=O)OR –OC(=O)R –C–OC(=O)R

1.08 1.33 1.18 1.10 104 176 Not Defined 0.03 0.30 0.014 Not Defined 0.84 0.084

SAR Estimated k(298) for NO3 Reactions with Unsaturated Alcohol or Ether, cm3 molecule−1 s−1

measured rate coefficient is lower than the group rate coefficient k(-CH=C1 0.64 0.40 0.10 0.56 0.49 0.57 1.07 0.48 1.29

[HO] underestimated by a significant amount that which was measured. In the BERLIOZ campaign conducted at a rural site in Germany (Holland et al., 2003; Mihelcic et al., 2003), comparison made for periods of [NOx] of greater than 5 ppb showed reasonable agreement with those calculated using the Master Chemical Mechanism (MCM) model ( Jenkin et al., 1997) and measured trace gas concentrations. However, the model overestimated [HO] by 100% at low [NOx]. The differences between the measured and simulated [HO] may reflect errors in measurements, reactive compounds present but not detected and not included in the simulations, and/or incorrect or incomplete reaction mechanisms. Chen et  al. (2010b) have made an interesting comparison of [HO] as measured using the LIF method at low pressures during the TRAMP-2006 field campaign with those calculated using five different well-known photochemical models that are in common use today. The measurement site was located south of the downtown area of Houston, Texas, at the top of the Moody Tower on the campus of the University of Houston, 60 m above the ground level. Rate coefficients and photolysis frequencies for specific reactions in each mechanism were standardized to the same values to assure that any observed differences of gas phase chemical mechanisms were meaningful. The same measured meteorological parameters, photolysis rates, and concentrations of trace gases were used in each of the models, which were applied in a zero-dimensional

Reference

George et al. (1999) Martinez et al. (2003) Ren et al. (2003) Ren et al. (2006) Emmerson et al. (2005) Emmerson et al. (2005) Shirley et al. (2006) Emmerson et al. (2007) Kanaya et al. (2007) Kanaya et al. (2007)

mode. The models differed mainly in the lumping procedures and the degree of detail used in their construction. The comparison is shown in Figure IV-A-2. The results of the comparison show significant differences between the measured and modeled [HO], although some features of the measured profiles are qualitatively reproduced. For all mechanisms, modeled [HO] was significantly less than measured [HO] during the day (shown in the upper panel of the figure), with the average modeled-tomeasured ratio of 0.69 (CB05 and RACM); 0.67 (LaRC); 0.65 (SAPRC-07); 0.59 (MCM-v3.1); and 0.53 (SAPRC-99). In the lower panel of the figure, all the models overpredicted [HO]. Differences in the structure of the different models are discussed in Chapter IX. In the cleanest regions of the troposphere, it should be possible to make somewhat more rigorous tests of the theories of [HO] generation and loss because, under these conditions, identification and measurement of most of the reactive trace gases present at the site, as well as the [HO] can be made. Such experiments were carried out at the Mauna Loa Observatory in Hawaii during MLOPEX-2 (Eisele et  al., 1996). During periods when free troposphere air masses were present on the mountain there was reasonably good agreement between the measured [HO] and that modeled using the many measured trace gas concentrations (including [H2O], [NO], [NO2], [CO], [CH2O], and the many hydrocarbons and their oxidation products),

176

the mechanisms of reactions influencing atmospheric ozone 0.8

Measured CB05 RACM LaRC SAPRC-99 SAPRC-07 MCM-v3.1

9/2/2006 2, 2006 September

[HO], pptv

0.6

0.4

0.2

0.0

0

6

12

18

24

18

24

Time, hr 0.8 September 26, 2006

[HO], pptv

0.6

0.4

0.2

0.0

0

6

12 Time, hr

FIGURE IV-A-2.

Comparison between [HO] as measured during 2 days of the TRAMP-2006 field campaign in Houston, Texas and [HO] as modeled using the chemical mechanisms from five well-known modeling programs used in simulating atmospheric chemistry. Each model uses a common set of measured meteorological parameters, trace gas concentrations, and photolysis rates. The methods of lumping reactants and other structural differences exist in the models. The figure is reprinted with the permission of Chen et al. (2010b).

as well as measured j(O3) and meteorological quantities. Figure IV-A-3 shows measurements of [HO] made on April 15, 1992, on Mauna Loa, one of the few days that calculated concentrations matched measurements reasonably well. An interesting feature of the measurement on this day is the spike in the [HO] near noon. This reflects the jump in [NO] that resulted from the inadvertent release of NO from some unidentified source on the site (see Figure IV-A-3b). The [HO2] + [RO2] measured at the site at the time of the NO release was about 6 × 108 molecules cm−3, and rapid

generation of HO radicals occurred via the reaction NO + HO2 → HO + NO2. However, even at the very isolated site on Mauna Loa, the influence of unknown trace gases was seen whenever urban pollution was brought to the site by upslope conditions. At these times, the measured [HO] was often lower by a factor of 2 than that calculated from trace gas measurements. Obviously, unknown sinks for HO were present under these conditions. Unambiguous tests of our understanding of HO generation and loss are difficult to design. These demand a complete analysis of

The Hydroxyl Radical and Its Role in Ozone Formation

177

(a) 8 x 106

[HO], molecules cm3

Measured [HO] Calculated [HO] 6 x 106

4 x 106

2 x 106

0

6

8

10

12

14

16

18

20

Time, hr (HST)

(b)

[NO] Measurements

[NO], molecules cm−3

6 x 108

4 x 108

2 x 108

0

6

8

10

12

14

16

18

20

Time, hr (HST) FIGURE IV-A-3.

Panel (a) shows the measured [HO] at Mauna Loa observatory during MLOPEX-2 on April 15, 1992, and that calculated from all measured quantities (trace gas concentrations, j[O3], [H2O], etc.). The spike in the measurements, which occurred shortly after 12:00 reflects the result of the inadvertent NO release that occurred on the site at this time; see panel (b).

all reactive trace gases present and the measurement of the many atmospheric parameters important in O3 generation and destruction (j(O3), [H2O], etc.), an experiment that is almost impossible to achieve. However, within the many uncertainties in the tests that have been made to date, it is clear that the present knowledge of the mechanisms that result in HO radical generation and loss are at least qualitatively correct, although obvious problems remain. Either reactive impurity gases remain unidentified and are not included in the mechanism and/or errors in the specific chemistry of the models exist. The forested regions have been of special interest since

typically measured [HO] levels have been found to be much higher than those modeled (e.g., see Stone et al., 2012). Tropospheric concentrations of HO vary with the season of the year, the time of day, presence of clouds, and the various other factors that control the level of actinic flux, as well as with the concentrations of the various trace gases that generate HO (including O3 and H2O) and those that act as sinks (including the many reactive hydrocarbons and their oxidation products). The measured [HO] within the free troposphere and the relatively clean tropospheric air masses in the mid-latitudes during

178

the mechanisms of reactions influencing atmospheric ozone

the summer months range from values near zero at night and before daybreak to maxima around 107 molecules cm3. In the more polluted air masses, maximum [HO] values are often observed to be much lower (e.g., ~2 × 106 molecules cm3). The global averaged [HO] is of special interest because this is needed in the estimation of the lifetime of reasonably long-lived trace gases in the atmosphere. Prinn et  al. (2001) estimated from ambient 1,1,1-trichloroethane measurements for the 1978–2001 period and emission inventories for CH3CCl3 that the global averaged [HO]  =  (9.4 ± 1.3) × 105 molecules cm3, whereas for the Northern and Southern Hemispheres they estimated that the average [HO] = (8.98 ± 2.02) × 105 and (9.93 ± 2.02) × 105 molecules cm3, respectively. See also Montzka et al. (2011). In estimating the lifetime (τ) of trace gas (X) in the troposphere due to HO reaction {τ = 1/(kX-HO × [HO])}, somewhat different values of k and [HO] are often used, depending on the estimated approximate lifetime of X. For species X with τ of less than 1 day, [HO] = 2.5 × 106 molecules cm−3 and k(298 K) are often chosen; with lifetimes of X between a few days and a year, a diurnally averaged concentration of HO is assumed to be 1.0 × 106 molecules cm3. For species that have lifetimes of greater than 1 year, mixing throughout the whole troposphere is assumed to occur, and more representative values of k at 272 K and [HO] = 106 molecules cm3 are often used in estimating lifetimes. I V- B . T H E M E C H A N I S M O F H O R A D I CA L R E AC T I O N S WITH ALKANES In this chapter, we review the rate coefficients for reaction of HO with organic compounds under atmospheric conditions and identify structural features in the reactive molecules that affect these important quantities. In this section, we discuss the HO radical reactions with the acyclic alkanes. As described in the previous section, the HO radical reacts with all alkanes (RH) by abstraction of an H atom, reaction (1), and in the oxygen-rich atmosphere, the alkyl radical (R)  formed reacts rapidly to form a peroxyalkyl radical (RO2) in reaction (2): RH + HO → (R ....H....OH) → H2O + R R + O2 → RO2

(1) (2)

The reactivity of the HO toward the various types of R–H bonds in the alkanes reflects differences in the strengths of these C‒H bonds:  the rupture of the bond of a primary H atom (as in ethane CH3CH3) requires a relatively large input of energy (ΔHo298 = 465 kJ mol−1); secondary H atoms (as in propane CH3CH2CH3) are broken with somewhat less energy (ΔHo298 = 409 kJ mol−1), whereas the tertiary H atoms (as in iso-butane (CH3)3CH) require even less energy for bond rupture (ΔHo298 = 404 kJ mol−1). The overall H-atom abstraction reactions (1) for all the alkanes are exothermic since the newly formed H–OH bond in the H2O product molecule is large (ΔHo298 = 498 kJ mol−1). An energy barrier to all these H-atom abstraction reactions with the alkanes is reflected in a positive value of the activation temperature (B = Ea/R) in the Arrhenius relation often used to describe the rate coefficient k = A e–B/T. Studies of the rate coefficients for the reactions of the HO radical with many of the alkanes have been reviewed recently by Calvert et al. (2008), and the recommendations from this study are summarized for acyclic alkanes in Table IV-B-1 and with cyclic alkanes in Table IV-B-2. Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. The rate coefficients for the n-alkanes (straight-chain) show a regular increase in rate coefficient with increase in chain length (see Figure IV-B-1). The filled circles are from measurements at 298 K, whereas the open triangles (data of Nolting et al., 1988) have been adjusted to 298 K from the values measured at 312 K using B = 429 K, an average of values determined for n-C5H12 to n-C8H18. The Nolting et al. data were also adjusted to our recommended k values for the reference compounds used. Note that for each additional ‒CH2‒ group added to the alkane chain, an increase in rate coefficient of about 1.4 × 10−12 cm3 molecule−1 s−1 is seen. The limited k(298 K) data for the 2-methylalkanes and the 2,2-dimethylalkanes are also shown in Figure IV-B-1; the rates of increase in k(298 K) with added CH2-groups in the chain (slope in the plots) are about the same as with the n-alkanes. The effect of substitution of CH3 groups for H atoms in butane results in significant changes in the observed HO-alkane rate coefficient in the new compound (see Figure IV-B-2). Substitution at the 2-position increases the rate coefficient by a factor of 1.5; substitution of a second

TABLE IV-B-1 . THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS FOR RE ACTIONS OF THE HO R ADICAL WITH THE ACYCLIC ALKANES, k = A e −B/T (cm 3 molecule −1 s −1 ) FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 5 SHOWS THE APPROXIMATE PERCENTAGES OF H-ATOM ABSTR ACTION FROM SPECIFIED >CH, –CH 2 –, AND –CH 3 SITES A S ESTIMATED USING THE STRUCTURE-ACTIVIT Y REL ATIONSHIPS (SAR) OF TABLES A-IV-3 AND A-IV-4. COLUMN 6 SHOWS THE MA JOR PRODUCTS OBSERVED OR EXPECTED (ITALICS) TO FORM FOLLOWING RE ACTIONS OF THE ALKANE IN NO X-POLLU TED ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN d

179

Acyclic Alkanes

1012 × k(298 K)a

1012 × A

B (K)

Methane (CH4) Ethane (CH3CH3) Propane (CH3CH2CH3) Butane (CH3CH2CH2CH3) 2-Methylpropane [(CH3)2CHCH3] Pentane (CH3CH2CH2CH2CH3)

0.0064 ± 0.00096 0.257 ± 0.038 1.11 ± 0.17 2.45 ± 0.37 2.16 ± 0.32 3.97 ± 0.79

1.85 8.61 10.0 14.0 7.0 18.1

1690 1047 655 520 350 452

2-Methylbutane (CH3)2CHCH2CH3] 2,2-Dimethylpropane [(CH3)4C] Hexane (CH3CH2CH2CH2CH2CH3) 2-Methylpentane [CH3)2CHCH2CH2CH3] 3-Methylpentane [CH3CH2CH(CH3)CH2CH3] 2,2-Dimethylbutane [(CH3)3CCH2CH3] 2,3-Dimethylbutane [(CH3)2CHCH(CH3)2] Heptane (CH3CH2CH2CH2CH2CH2CH3)

3.74 ± 0.75 0.82 ± 0.12 5.28 ± 1.06

10.1 12.0 19.8

296 800 394

(5, 5); 53; 30; 5 (25, 25, 25, 25) 3; 20;27; 27; 20; 3

5.25 ± 1.31

17.7

362

(3, 3); 45; 26, 19; 3

CH2O CH3CHO CH3C(O)CH3 CH3C(O)CH2CH3;CH3CHO CH3C(O)CH3; CH2O (C2H5)2C(O); C2H5C(O)H, CH3CHO; HOCH2CH2C(O)CH2CH3 CH3C(O)CH3; CH3C(O)H CH3C(O)CH3, CH2O C2H5C(O)C3H7; C2H5C(O)H; C6-hydroxycarbonyls CH3C(O)CH3, C2H5C(O)H

5.54 ± 1.39

18.0

351

4; 22; 48; (4); 22; 4

C2H5C(O)CH3; CH3C(O)H

2.23 ± 0.56 6.14 ± 1.53 6.52 ± 1.30

33.7 12.5 27.6

809 212 430

(10, 10, 10); 59; 10 (4, 4); 42; 42; (4, 4) 3; 16; 21; 21; 21; 16; 3

6.57 ± 2.62

15.3

252

(3, 3); 30; 41; (3);17; 3

CH3C(O)H; CH3C(O)CH3 CH3C(O)CH3 C7-hydroxycarbonyls; 3- and 4-heptanone; C3H7C(O)H; C2H5C(O)H C2H5C(O)CH3; CH3C(O)CH3; CH3CHO

5.76 ± 2.31

27

460

(3, 3); 32; 24; 32; (3, 3)

CH3C(O)CH3; (CH3)2CHC(O)H; CH2O

(7, 7, 7); 67; (7, 7)

CH3C(O)CH3; CH2O

2,3-Dimethylpentane [(CH3)2CHCH(CH3)CH2CH3] 2,4-Dimethylpentane [(CH3)2CHCH2CH(CH3)2] 2,2,3-Trimethylbutane [(CH3)3CCH(CH3)2]

3.81 ± 0.95

Estimated % H-Abstraction Observed or Expected Major from Specified Siteb Products (NOx present)c 100 50; 50 16; 68;16 7; 43; 43;7 (10, 10); 70;10 5; 27; 37; 27; 5

(Continued)

180

TABLE IV-B-1 (CONTINUED)

Acyclic Alkanes

1012 × k(298 K)a

Octane (CH3CH2(CH2)4CH2CH3) 2,2,4-Trimethylpentane [(CH3)3CCH2CH(CH3)2] 2,3,4-Trimethylpentane [(CH3)2CHCH(CH3) CH(CH3)2] 2,2,3,3-Tetrametylbutane [(CH3)3CC(CH3)3] Nonane [CH3CH2(CH2)5CH2CH3] Decane [CH3CH2 (CH2)6CH2CH3] Undecane [CH3CH2 (CH2)7 CH2CH3] Dodecane [CH3CH2(CH2)8CH2CH3]

7.81 ± 1.56

Tridecane [CH3CH2 (CH2)9 CH2CH3] Tetradecane [CH3CH2(CH2)10CH2CH3] Pentadecane [CH3CH2(CH2)11CH2CH3] Hexadecane [CH3CH2 (CH2)12CH2CH3]

1012 × A 34.3

B (K) 441

Estimated % H-Abstraction Observed or Expected Major from Specified Siteb Products (NOx present)c 2; 13; (17, 17, 17, 17);13; 2

3.34 ± 0.84

(4, 4, 4); 34; 44; (4, 4)

C8-hydroxycarbonyls; 3- and 4-octanone; C2H5C(O)H; C3H7C(O)H) CH3C(O)CH3; CH2O

6.6 ± 1.7

(3, 3); 26; 35; 26; (3, 3)

CH3C(O)CH(CH3)2; CH3C(O)CH3; CH3CHO

(17, 17, 17); (17, 17, 17) 2; 11; (15,15, 15, 15,15); 11; 2 2; 10; (13, 13, 13, 13, 13, 13); 10; 2 2; 9; (11, 11, 11, 11, 11, 11, 11); 9; 2 1; 8; (10, 10, 10 10, 10, 10, 10, 10); 8; 1 1; 7; (9, 9, 9, 9, 9, 9, 9, 9, 9); 7; 1 1; 6; (8, 8, 8, 8, 8, 8, 8, 8, 8, 8); 6; 1 1; 6; (8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8); 6; 1 1; 5; (7, 7, 7, 7, 7, 7, 7, 7, 7,7 7, 7); 5; 1

CH2O; CH3C(O)CH3 3-, 4-, and 5-nonanones; hydroxy ketones; CnH2n+1C(O)H; n = 2-6; 3-, 4-, and 5-decanones; CnH2n+1C(O)H; n= 2-7; hydroxy ketones 3-, 4-, 5-, and 6-undecanones; CnH2n+1C(O)H; n = 2-8;hydroxy ketones 3-, 4-, 5-, and 6-dodecanones; CnH2n+1C(O)H; n = 2-9; hydroxy ketones 3-, 4-, 5-, 6-, and 7-tridecanones; CnH2n+1C(O)H; n = 2-10; hydroxy ketones 3-, 4, 5-, 6-, and 7-tetradecanones; CnH2n+1C(O)H; n = 2-11; hydroxy ketones 3-, 4-, 5-, 6-, 7-, 8-pentadecanones; CnH2n+1C(O)H; n = 2-12; hydroxy ketones

0.97 ± 0.19 9.75 ± 1.95 11 ± 2.2 11.9 ± 2.4 13.1 ± 2.6 15.1 ± 3.0 16.7 ± 3.3 (19.2 ± 3.8)

(21.6 ± 4.3)

12.0

750

3-, 4-, 5-, 6-, 7-, 8-hexadecanones; CnH2n+1C(O)H; n = 2-13; hydroxy ketones

Recommended rate coefficients are those of Calvert et al. (2008) where detailed kinetic information is reviewed and evaluated for each compound. The abbreviated code in column 5 of the table gives the approximate percentage of H-atom abstraction by HO radical at each successive >CH, –CH2–, or –CH3 site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c Only the major products that have been reported are given; when no product studies have been made, the products shown are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations (shown in italics). d The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V).

a

b

TABLE IV-B-2 . HO RE ACTIONS WITH CYCLIC ALKANES. THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS FOR RE ACTIONS OF THE HO R ADICAL WITH THE CYCLIC ALKANES, k = A e −B/T (CM 3 MOLECULE −1 S −1 ) FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 5 SHOWS THE APPROXIMATE PERCENTAGES OF H-ATOM ABSTR ACTION FROM SPECIFIED >CH, –CH 2 –, AND –CH 3 SITES A S ESTIMATED USING STRUCTURE-ACTIVIT Y REL ATIONSHIPS (SAR) OF TABLES A-IV-3 AND A-IV-4. COLUMN 6 SHOWS THE MA JOR PRODUCTS OBSERVED OR EXPECTED (ITALICS) TO FORM FOLLOWING RE ACTIONS OF THE CYCLIC ALKANES IN NO X-POLLU TED ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN c

1012 × k(298 K)

Cyclic Alkanes

Cyclopropane H2 C H2C

CH2

Cyclobutane H2C

CH2

H2C

CH2

Cyclopentane

1012 B(K) Estimated % Observed or Predicted ×A H-Abstraction Major Productsd from Specified Sites b

0.076 ± 0.023

6.0

1300 Equal reactivity at the three –CH2– sites

OCHCH2CHO?; CO; CO2; oxirane; C2H4

2.0 ± 0.5

11.1

510 Equal reactivity at the four –CH2– sites

HOCH2CH2CHO; CO2

4.9 ± 1.0

22

450 Equal reactivity at the five –CH2– sites

HOCH2CH2CH2CHO; CO2

6.85 ± 1.37

30

440 Equal reactivity at the six –CH2– sites

OCHCH2CH2CH(OH) CH2CHO; cyclo-hexanone

H2 C CH2

H2 C

CH2

H2C

Cyclohexane H2 C CH2

H2C H2C

C H2

CH2

Methylcyclopentane b H2C b H2C

a 30; b 17; c 3

CH3C(O)CH2CH2CHO

9.64 ± 2.89

a 25; b 15; c 2

CH3C(O)CH2CHOH)CH2CHO?; Methyl cyclo-hexanones?

CH-CH3 c a C b H2

Methylcyclohexane b H2 C b H2C b H2C

8.6 ± 2.6

b H2 C

C b H2

a H C

CH3 c

CH2 b

(Continued)

181

TABLE IV-B-2 (CONTINUED)

1012 × k(298 K)

Cyclic Alkanes

Isopropylcyclopropane c H C

d H2C

CH3 b HC a CH b 3

d H2C

Bicyclo[2,2.1]heptane (norbornane)

1012 B(K) Estimated % Observed or Predicted ×A H-Abstraction Major Productsd from Specified Sites b

2.61 ± 0.65

a 80; b 8; c 3; d 1

5.1 ± 1.3

a 10; b 24

?

13.7 ± 3.4

a 22; b 9

?

10.3 ± 2.6

a 22; b 9

?

CH3C(O)CH3; OCHCH2CHO?

a b

a

a b

a

a

Bicyclo[2.2.2]octane b b

a

b

b a

b

b

Bicyclo[3.3.0]octane b

a

b

b a

b

b b

Cycloheptane H2 C

11 ± 2.7 40

388

Equal reactivity at the seven –CH2– sites

OCHCH2CH2CH(OH) CH2CH2CHO;?cyclo-heptanone (minor); HOCH2CH2CHO; HOCH2CH2CH(OH)CH(OH) CH2CHO

13 ± 4.0 52

408

Equal reactivity at the eight –CH2– sites

Ring opening products including hydroxyaldehydes; cyclooctanones?

CH2

H2C

CH2

H2C

CH2

C H2

Cyclooctane H2 C

CH2

H2C

CH2 CH2

H2C C H2

C H2

16.0 ± 4.0

cis-Bicyclo[4.3.0]nonane

a 17; b 11; c 7

?

c c

b b

a

b b

c a

(continued)

182

TABLE IV-B-2 (CONTINUED)

1012 × k(298 K)

Cyclic Alkanes

16.5 ± 4.1

trans-Bicyclo[4.3.0]nonane b

a

b b

b

a 17; b 11; c 7

?

a 18; b 8

?

a 18; b 8

?

10.6 ± 2.7

a 15; b 6

?

21.5 ± 5.4

a 15; b 6

?

12.4 ± 3.1

a 31; b 17; c 12; d 5; e 2

?

2.7 ± 0.7

a 46; b 19; c 4; d 1

?

c

c

a

c

cis-Bicyclo[4.4.0]decane (cis-decalin) 18.6 ± 4.6 b

b b

b

a

b

b b

a

b

trans-Bicyclo[4.4.0]decane (trans-decalin)19.0 ± 4.6 b

b b b

b

b

a

b

1012 B(K) Estimated % Observed or Predicted ×A H-Abstraction Major Productsd from Specified Sites b

b a

Tricyclo[5.2.1.02,6]decane b a

b

b

a a

b

b

a

b

3,7

Tricyclo[3.3.1.1 ]decane (adamantine) a b

b

b

a

a b

b

a b

(1R,2R)2,6,6-Trimethylbicyclo-[3.1.1] heptane(trans-pinane) e

e a

c

e

b

c

b

d

1,7,7-Trimethyltricyclo[2.2.1.02,6] heptane (tricyclene c

c c

d b

d

a b

(Continued)

183

TABLE IV-B-2 (CONTINUED)

1012 × k(298 K)

Cyclic Alkanes

1012 B(K) Estimated % Observed or Predicted ×A H-Abstraction Major Productsd from Specified Sites b

1.7 ± 0.4

Quadricyclo[2.2.1.02,6O3,5]heptane (quadricyclane)

a 69; b 5

?

a b

b b

b

b b

Recommendations are those of Calvert et al. (2008) where detailed kinetic information is reviewed and evaluated. b The abbreviated code in column 5 of the first part of the table gives the approximate percentage of H-atom abstraction by HO radical at >CH, –CH2–, or –CH3 site as designated with lowercase letters shown on the structural formula of the compound in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made, the products shown are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations (shown in italics). a

1012 x k(298 K), cm3 molecule−1 s−1

25

20

15

10

5

0 0

2

4

6

8

10

12

14

16

Number C-atoms in chain

n -Alkanes Y = −2.866 x 10−12 + 1.369 x 10−12 X 2-Methyl-Alkanes 2,2-Di-Methyl-Alkanes n-Alkanes [Nolting et al. (1988) Corrected to 298 K and Reference K FIGURE IV-B-1. Effect of chain length on the HO-alkane rate coefficient for a series of structurally related n-alkanes; the rate coefficients were measured at 298 K or adjusted to this temperature.

184

The Hydroxyl Radical and Its Role in Ozone Formation

185

1012 x k(298 K), cm3 molecule–1 s–1

8

(CH3)2CHCH(CH3)2

6

4

(CH3)2CHCH2CH3

2

0

(CH3)3CCH(CH3)2

CH3CH2CH2CH3

3

4

(CH3)3CCH2CH3

5

6

7

(CH3)3CC(CH3)3

8

9

10

Number C-atoms in molecule FIGURE IV-B-2.

The effect on the HO-alkane rate coefficients (298 K) of CH3 group substitution for H atoms in n-butane.

CH3 group at the 3-position increases k by an additional factor of 1.6. With each of these CH3 group substitutions, a tertiary H atom appears as a secondary H atom is removed and three new primary atoms are introduced. However, if the second CH3 group replaces the tertiary H atom in 2-methylbutane, the k falls below that for butane. When a third CH3 group replaces the tertiary H atom in 2,3-dimethylbutane to form 2,2,3-trimethylbutane, the k is decreased by a factor of 1.6. Finally, if a fourth CH3 group is added at the 3-position in 2,2,3-trimethylbutane a sharp drop by a factor of 3.9 occurs as the compound 2,2,3,3-tetramethylbutane is formed. This compound contains only the less reactive primary H atoms. I V- C . M E C H A N I S M S O F T H E H O R A D I CA L R E AC T I O N S W I T H T H E H A L OA L K A N E S The rate coefficients for the reaction of HO with halogen-atom-substituted alkanes have been reviewed by Calvert et al. (2008). In this section, we draw on that review to summarize the kinetic information on these compounds. As with the unsubstituted alkanes, the mechanism of HO reaction with the halogen-substituted alkanes involves H-atom abstraction. HO abstraction reactions of F, Cl, or Br atoms from the halomethanes (HO + CH3X → HOX + CH3) are endothermic by about 249, 116, and 87 kJ mol−1, respectively, and these pathways are

inaccessible at atmospheric temperatures. However, the abstraction of an I  atom from CH3I has a low endothermicity (26 kJ mol−1), and evidence suggests that, for the case of the CF3I molecule for which the endothermicity is only 14 kJ mol−1, abstraction of the I atom appears to be a viable pathway (Gilles et al., 2000). Listed in Table IV-C-1 are the recommended rate coefficients for the HO radical abstraction of H atoms from the halogen-substituted alkanes (Calvert et  al., 2008). Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. Substitution of halogen atoms for H atoms in the alkanes affects the rate coefficients for HO reaction significantly. A  single halogen-atom substitution for an H atom increases the rate coefficient in CH4. I-atom substitution has the largest increase, whereas that for F, Cl, and Br substitution also produces a significant but somewhat smaller increase (see Figure IV-C-1). Although a single F-atom substitution increases the rate coefficient for HO attack on CH4, multiple F-atom substitutions result in a significant reduction in k(298 K), as shown in Figure IV-C-2. The rate coefficient for HO reaction with CF4 is very near zero. The unusually high stability that is reflected in the low HO reactivity of methane leads to the lower k(298 K) value than that for CH3F. For the higher alkanes, even the monosubstituted fluoride is less reactive

TABLE IV-C-1 . HO RE ACTIONS WITH HALOGEN-ATOM-SUBSTITU TED ALKANES. THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS (k = A T n e ‒B/T, CM 3 MOLECULE −1 S −1 ) FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 6 SHOWS THE APPROXIMATE PERCENTAGE OF H-ATOM ABSTR ACTION FROM SPECIFIED >CH, –CH 2 –, AND –CH 3 SITES A S ESTIMATED USING STRUCTURE-ACTIVIT Y REL ATIONSHIPS (SAR) OF TABLES IV-O-3 AND IV-O-4. b COLUMN 7 SHOWS THE MA JOR PRODUCTS OBSERVED OR EXPECTED (ITALICS) TO FORM FOLLOWING RE ACTIONS OF THE HALOALKANES IN NO X-POLLU TED ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN c

Haloalkane

a) Hydrofluorocarbon CH3F (HFC-41) CH2F2 (HFC-32) CF3H (HFC-23)

1014 × k(298 K)

1012 × A

B (K) n

Estimated % H-abstraction from Specified Sitesb

Observed or Predicted Major Productsd

1.98 ± 0.30 1.07 ± 0.16 0.0283 ± 0.0056

3.14 × 10−6 788 2 2.77 × 10−6 933 2 0.927 × 10−6 1,691 2

100 100 100

CH3CH2F (HFC-161) CH3CHF2 (HFC-152a) CH2FCH2F (HFC-152) CH3CF3 (HFC-143a) CH2FCHF2 (HFC-143) CH2FCF3 (HFC-134a)

21.8 ± 3.3 3.45 ± 0.52 9.96 ± 1.49 0.127 ± 0 019 1.63 ± 0.41 0.415 ± 0.062

4.97 × 10−6 2.10 × 10−6 2.84 × 10−6 1.57 × 10−6 4.11 1.29 × 10−6

210 503 277 1,400 1,648 989

2 2 2 2 0 2

10; 90 28; 72 50; 50 100 78; 22 100

CHF2CHF2 (HFC-134) CHF2CF3 (HFC-125)

0.596 ± 0.149 0.190 ± 0.038

1.27 0.510

1,598 0 1,666 0

50; 50 100

CF3CH2CH3 HFC-263fb)

5.48 ± 1.37

3.94

1,274 0

58; 42

CF3CHFCH2F (HFC-245eb)

1.81 ± 0.36

1.23

1,257 0

60; 40

CHF2CHFCHF2 (HFC-245ea) CF3CH2CHF2 (HFC-245fa) CHF2CF2CH2F (HFC-245ca) CF3CF2CH2F (HFC-236cb)

1.84 ± 0.46

1.93

1,387 0

43; 14; 43

HC(O)F CF2O CF2O via CF3O reactions with NO or CH4c HC(O)F; CH3C(O)F CF2O; CH2O HC(O)F CF3CHO CF2O; HC(O)F; HC(O)F; CF2O via CF3O reactions with NO or CH4; CF3C(O)F CF2O COF2; COF2 via reactions with NO or CH4c CF3C(O)CH3; CF3CHO; CH2O FC(O)CH2F; CF3CF(O); HC(O)F); C(O)F2 CHF2C(O)F; HC(O)F

0.714 ± 0.107

0.671

1,354 0

36; 64

CF3C(O)H; CF2O

0.744 ± 0.186

2.48

1,731 0

22; 78

HC(O)F; CF2O

0.680 ± 0.170

0.261

1,087 0

100

0.0328 ± 0.0066 0.514 ± 0.128

1.28

2,464 0

100

HC(O)F; CF2O;CH3C(O)F COF2 CF3C(O)CF3

1.34

1,658 0

13; 87

0.151 ± 0.030 0.734 ± 0.220

0.655 2.04

1,809 0 1,677 0

CF3CH2CF3 (HFC-236fa) CF3CHFCHF2 (HFC-236ea) CF3CHFCF3 (HFC-227ea) CF3CH2CF2CH3 (HFC-365mfc)

186

100 23; 77

CF2O; HC(O)F; CF3C(O)F CF3C(O)F; COF2 CF3CH2CF2CHO (continued)

TABLE IV-C-1 (CONTINUED)

Haloalkane

1014 × k(298 K)

1012 × A

B (K) n

Estimated % H-abstraction from Specified Sitesb

Observed or Predicted Major Productsd

0.840 ± 0.126

2.53

1,701 0

50; 50

CF3C(O)H

4.18 ± 1.05

0.166

1,097 0

2; 98

HC(O)F; CF3CF2CHO

0.434 ± 0.087

0.782

1,548 0

50; 50

CF2O

0.263 ± 0.066

1.12

1,804 0

50; 50

CF3CHO; CH2O

0.330 ± 0.083

0.568

1,534 0

50; 50

0.83 ± 0.20

1.84

1,610 0

50; 50

CF3C(O)F; CF3CF2C(O)F CF3CF2CHO

0.185 ± 0.037

0.567

1,706 0

100

CF2O

b) Hydrochlorocarbons CH3Cl CH2Cl2 CHCl3 CH3CH2Cl

3.83 ± 0.77 10.4 ± 2.6 9.62 ± 1.92 32.8 ± 8.2

7.28 × 10−6 1.97 2.86 8.80 × 10−6

842 877 1,011 259

2 0 0 2

100 100 100 14; 86

CH2ClCH2Cl

24.2 ± 3.6

10.8 × 10−6

410

2

CH3CHCl2 CH2ClCHCl2

27.4 ± 4.1 22.9 ± 6.9

2.49 × 10−6 3.70 × 10−6

−64 108

2 2

CH3CCl3 CHCl2CHCl2 CHCl2CCl3 CH2ClCH2CH2Cl (CH3)3CCl

1.02 ± 0.15 ~17 ± 8 ~23.3 ± 5.8 ~73 ± 18 ~38.4 ± 9.6

4.07 × 10−6

1,064 2

CH3CH2CH2Cl

86.1 ± 21.5

95.1

1,402 0

CH3CHClCH3

74.7 ± 26.1

18.3

953

0

CH3CH2CH2CH2Cl CH3CH2CH2CH2CH2Cl

148 ± 52 357 ± 125

34.9 14.0

941 407

0 0

CH3CH2CH2CH2CH2CH2Cl 391 ± 78

47.9

747

0

c) Hydrobromocarbons CH3Br CH2Br2 CHBr3 CH3CH2Br

2.39 2.52 1.48 4.76

1,301 900 612 795

0 0 0 0

CF3CH2CH2CF3 (HFC-356mff) CF3CF2CH2CH2F (HFC-356mcf) CHF2CF2CF2CHF2 (HFC-338pcc) CF3CH2CF2CH2CF3 (HFC-456mfcf) CF3CHFCHFCF2CF3 (HFC-43-10mee) CF3CF2CH2CH2CF2CF3 (HFC-10-mcff) CHF2CF2CF2CF2CF2CF3 (HFC-52-13p)

3.04 ± 0.46 12.3 ± 2.5 19.0 ± 4.7 33.0 ± 8.3

HC(O)Cl HC(O)Cl CCl2O CH3C(O)Cl; HCl; CH2O; CO2 50; 50 CH2ClC(O)Cl; HCl; CO2; HC(O)Cl 19; 81 CCl2O; CH2O 58; 42 HC(O)Cl; HCl; CO; CCl2O 100 CCl3CHO 50; 50 CCl2O; HC(O)Cl 100 CCl3C(O)Cl; CCl2O 44; 12; 44 HC(O)Cl; CH2ClCHO 100 CH2O; CH3C(O)CH3; CH3C(O)Cl 20; 33; 47 HC(O)Cl; CH3CH2C(O)Cl; CH3CHO 8; 84; 8 CH3C(O)Cl; CH3C(O) CH3 9; 50; 20; 21 CH3CHO; ClCH2CHO 5; 30; 40; 13; 13 CH3CH2CHO; ClCH2CHO 4; 21; 29; 29; 9; 9 ClCH2CH2CHO; CH3CH2CHO 100 100 100 12; 88

CH2O; (Br) HC(O)Br; (Br) CBr2O; (Br) CH3CHO; (Br) (continued)

187

TABLE IV-C-1 (CONTINUED)

Haloalkane

1014 × k(298 K)

CH2BrCH2Br CH3CH2CH2Br CH3CHBrCH3 CH3CH2CH2CH2Br

22.2 ± 4.4 105 ± 21 84.6 ± 16.9 ~229 ± 57

CH3CH2CH2CH2CH2Br

~371 ± 93

1012 × A

B (K) n

Estimated % H-abstraction from Specified Sitesb

7.69 7.57 10−6 2.90

1,056 0 −134 2 367 0

50; 50 16; 31; 53 7; 86; 7 9; 51; 20; 19

d) Hydroiodocarbons CH3I CH3CH2I CH3CH2CH2I CH3CHICH3

7.86 ± 2.35 77 ± 19 147 ± 37 122 ± 31

4; 21; 28;28; 8; 11

CH2BrCHO; (Br) CH3CH2CHO; (Br) CH3C(O)CH3; (B)r CH3C(O)CH2CH2Br; CH3CHO; BrCH2CHO CH3CH2C(O)CH2CH2Br; CH3CH2CHO; BrCH2CHO C2H5CHO; BrCH2CH2CHO

5; 29; 39; 12; 15

CH3CH2CH2CH2CH2CH2Br ~548 ± 137

Observed or Predicted Major Productsd

2.26

1,001 0

100 10; 90 13; 28; 59 5; 89; 5

CH2O; (I) CH3CHO; (I) CH3CH2CHO; (I) CH3C(O)CH3; (I)

e) Hydrochlorofluorocarbons CH2FCl (HCFC-31) 4.17 ± 0.63 CHF2Cl (HCFC-22) 0.473 ± 0.071 CHFCl2 (HCFC-21) 2.87 ± 0.43 CH3CF2Cl (HCFC-142b) 0.328 ± 0.066 CH3CFCl2 (HCFC-141b) 0.606 ± 0.091 CH2ClCF3 (HCFC-133a) 1.35 ± 0.47

2.80 1.51 10−6 2.05 10−6 4.29 10−6 1.25 2.09 10−6

1,254 997 551 1,417 1,588 780

0 2 2 2 0 2

100 100 100 100 100 100

1.73 ± 0.43 0.894 ± 0.134 1.26 ± 0.32 3.59 ± 0.72 4.99 ± 0.75 1.62 ± 0.41 0.237 ± 0.059

5.31 10−6 1.09 10−6 0.844 1.40 10−6 1.08 0.378 0.792

985 710 1,254 370 916 938 1,732

2 2 0 2 0 0 0

100 100 100 100 100 100 100

HC(O)F; (Cl) COF2; (Cl) FC(O)Cl CF2ClCHO CFCl2CHO CF3C(O)Cl; HCl; CO2; COF2; H(O)Cl; CF3C(O)H ClCF2C(O)Cl CF3C(O)F CF2ClC(O)Cl CF3C(O)Cl; (Cl) Cl2CO; COF2 FC(O)Cl Cl2CFCF2CHO

100

CF3C(O)CFCl2

CH2ClCF2Cl (HCFC-132b) CHFClCF3 (HCFC-124) CHFClCF2Cl (HCFC-123a) CF3CCl2H (HCFC-123) CHCl2CF2Cl (HCFC-122) CHFClCFCl2 (HCFC-122a) CH3CF2CFCl2 (HCFC-243cc) CF3CH2CFCl2 (HCFC-234fb) CF3CF2CCl2H (HCFC-225ca) CF2ClCF2CHFCl (HCFC-225cb)

0.0950 ± 0.030 2.43 ± 0.36

1.47

1,223 0

100

Cl2CO; F2CO,

0.884 ± 0.13

0.543

1,227 0

100

FClCO; F2CO

1.24 1.51 0.784

1,435 0 1,359 0 1,149 0

100 100 100

CF2O; (Br) CF3CHO; (Br) CF3C(O)F; (Br)

f) Hydrobromofluorocarbons CHF2Br (Halon-1201) 1.00 ± 0.15 CF3CH2Br (Halon-2301) 1.58 ± 0.40 CF3CHFBr (Halon-2401) 1.66 ± 0.25

(continued)

188

The Hydroxyl Radical and Its Role in Ozone Formation

189

TABLE IV-C-1 (CONTINUED)

Haloalkane

1014 × k(298 K)

1012 × A

B (K) n

g) Fluoroiodocarbons CF3I 2.44 ± 0.49 h) Hydrobromochlorocarbons CH2ClBr 11.4 ± 1.7 CHCl2Br 12.3 ± 3.1 ClCH2CHBrCH2Br 40.5 ± 10.1

40.5

2,209 0

1.94 2.52 5.7

844 900 215

i) Hydrobromochlorofluoro-carbons CF3CHClBr (Halon-2311) 5.08 ± 1.27 CF2BrCHFCl 1.41 ± 0.35

0.901 0.980

857 0 1,263 0

j) Perhalocarbons CF4 (FC-14) CF3Cl (CFC-13) CF2Cl2 (CFC-12) CFCl3 (CFC-11) CCl4 CF3Br (Halon-1301) CF2Br2 (Halon-1202) CF2ClBr (Halon-1211) CF2BrCF2Br (Halon-2402) CFCl2CFCl2 (CFC-112) CFCl2CF2Cl (CFC-113)

0 0 0

Estimated % H-abstraction from Specified Sitesb

Observed or Predicted Major Productsd

100 100 32; 28; 40

HC(O)Cl; (Br) CBr2ClO; (Br) ClCH2CHBrC(O)H; (Br)

100 100

CF3C(O)Cl; (Br) ClFCO; CF2CO; (Br)

< 0.04 < 0.07 < (0.009–0.1) < (0.004–0.1) < (0.009–0.1) < (0.002–.01) < 0.05 < (0.007–0.1) < 0.015 < 0.03 < 0.03

Recommendations are those of Calvert et al. (2008) where detailed kinetic information is reviewed and evaluated. The abbreviated code in column 6 of the first part of the table gives the approximate percentage of H-atom abstraction by HO radical at each successive >CH, –CH2–, or –CH3 site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text and are necessarily subject to significant uncertainty; they are presented here to provide a qualitative measure of the reactivity at various sites. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made, the products shown are judged to be representative of those expected for reaction at the most reactive sites as suggested by the SAR calculations (shown in italics).

a

b

than the parent hydrocarbon. For example, in Figure IV-C-3 the k(298 K) for F-atom-substituted ethanes show a large decrease in magnitude as the number (n) of F atoms in the C2H6-nFn molecule increases. The F-atom-substituted propanes show a very similar trend:  k(C3H8) > k(CF3CH2CH3) > k(CHF 2 CHFCHF 2 ) > k(CF 3 CHFCH 2 F) > k ( C F 3C H 2C H F 2) > k ( C F 3C F 2C H 2F ) > k(CF3CHFCHF2) > k(CF3CHFCF3). The C3H2F6 compound, CF3CH2CF3, appears to be an exception to the trend cited; its measured k(298 K) value is somewhat lower than the more highly substituted C3HF7 compound.

As seen by comparing Figures IV-C-2 and IV-C4, the impact of Cl-atom substitution on the HO rate coefficient in alkanes differs from that of F-atom substitution. Increased Cl-atom replacement of H atoms continues to increase the rate coefficient for the CH2Cl2 and CHCl3 molecules, but with all H atoms removed in CCl4, the rate coefficient for the HO reaction is near zero. Table IV-C-1 shows that the halocarbons, previously used in many commercial applications, are very unreactive toward HO. As discussed in Chapter I, it is this property that allows them to be transported ultimately to the stratosphere,

190

the mechanisms of reactions influencing atmospheric ozone 0.12

1012 x k(OH), cm3 molecule–1 s–1

0.10 0.08 0.06 0.04 0.02 0.00

CH4

CH3F

CH3Cl

CH3Br

CH3I

Methyl halide FIGURE IV-C-1.

The effect of halogen-atom substitution in methane on the rate coefficient (298 K) for the reactions with HO radical with the methyl halides.

1012 x k(OH), cm3 molecule–1 s–1

0.025

0.020

0.015

0.010

0.005

0.000 CH4

CH3F

CH2F2

CHF3

CF4

Fluorine-atom substituted CH4 FIGURE IV-C-2.

Effect on the rate coefficient (298 K) for the HO reactions with multiple F-atom-substituted methanes.

where photodecomposition and reaction with O(1D) atoms form Cl atoms and lead to ozone destruction. This has led to the ban in the use of chlorofluorocarbons (see Chapter I) and their replacement by compounds such as hydrofluorocarbons, which do not contain chlorine and hence

do not contribute to chlorine-catalyzed ozone destruction cycles. H-atom abstraction by HO in the Br-atom-substituted methanes and ethanes show trends similar to those observed for the Cl-atom-substituted methanes and ethanes.

1012 x k (HO) at 298 K, cm3 molecule–1 s–1

The Hydroxyl Radical and Its Role in Ozone Formation

191

0.30 0.25 0.20 0.15 CH2FCH2F

0.10 0.05

CHF2CH2F

CHF2CH3 CF3CH3

0.00 0

1

2

3

4

5

6

7

Number of F atoms in C2H6-xFx FIGURE IV-C-3.

Effect of multiple F-atom substitutions on the rate coefficient for the HO reaction with F-atom-substituted

ethanes.

1012 x k(OH), cm3 molecule−1 s−1

0.12 0.10 0.08 0.06 0.04 0.02 0.00 CH4

CH3Cl

CH2Cl2

CHCl3

CCl4

Cl-Atom substituted CH4 FIGURE IV-C-4.

Effect of multiple Cl-atom replacements of H atoms in CH4 on the rate coefficient (298 K) for HO reactions with CH4-xClx molecules.

I V- D . M E C H A N I S M S O F H O R A D I CA L R E AC T I O N S W I T H ALKENES IV-D-1. HO Reactions with Mono-Alkenes The alkenes differ from the alkanes in that they contain a carbon-carbon double bond. Ethene

(CH2=CH2) is the simplest member of the alkene family of hydrocarbons. The >C=C< group provides a new and rapid pathway for reactions with the HO radical, namely, addition to the double bond. Abstraction of H atoms attached to the C atoms forming the double bond (i.e., H2C=CH2) is negligible, reflecting the high bond dissociation

192

the mechanisms of reactions influencing atmospheric ozone

energy (ΔHo298  =  465 kJ mol−1). In making all of our structure-activity relationship (SAR) estimates of the percentage of reaction occurring at various H-atom containing sites (given in the tables), we have assumed, as all evidence suggests, that the H atoms attached to the double-bonded C atoms are unreactive toward abstraction by HO radicals. The mechanism for the addition of HO is rationalized in terms of the attack on the electron-rich region of the double bond by the electron-seeking OH radical. Thus, in the following mechanism for HO reaction with ethene, the initial reaction (1)  occurs. Stable products are shown in bold type. The HOCH2CH2 radical formed in the oxygen-rich atmosphere adds O2 in reaction (2); the resulting peroxy radical drives NO to NO2 in reaction (3)  and leads to O3 generation in reactions (7) and (8): CH2=CH2 + OH → HOCH2CH2•

(1)

HOCH2CH2• + O2 → HOCH2CH2O2•

(2)

HOCH2CH2O2• + NO → HOCH2CH2O• + NO2

(3)

HOCH2CH2O• → HOCH2 + CH2O

(4)

HOCH2CH2O• + O2 → HOCH2CH(=O) + HO2

(5)

HOCH2 + O2 → CH2O + HO2

(6)

NO2 + hν → O + NO

(7)

O + O2 + (M) → O3 (+ M)

(8)

The HOCH2CH2O radical either decomposes in (4) with further reaction in (6) to form CH2O, or, if it reacts with O2, hydroxyacetaldehyde is formed in (5). In Table IV-D-1, the recommended rate coefficients for the alkenes are summarized (Calvert et al., 2000). Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived, can be accessed in that work. We discuss briefly in this section the effect on the k(298 K) values of varied structural features in the alkenes. Because of the high reactivity of the alkenes with HO, they are among the most effective generators of ozone among the various classes of hydrocarbons. Although the simplest alkene,

ethene, is very reactive toward HO, it is the least reactive of the alkenes. Substitution of alkyl groups for H atoms on one of the C atoms forming the double bond in CH 2=CH2 enhances the reactivity toward HO. The trend in rate coefficients seen in Figure IV-D-1 shows the effect of substituting an increasing number of CH3 groups for H atoms in CH2=CH 2. With four CH3 groups substituted for H atoms in the ethene structure in 2,3-dimethylbutene [(CH3)2C=C(CH3)2], the rate coefficient with HO radicals is increased by about a factor of 13 over that for ethene. As with the alkanes, the increase in the length of the carbon chain in the alkenes results in an increase in the rate coefficient (see Figure IV-D2). The expected effect of addition of  –CH2– groups in the 1-alkenes and the trans-2-alkenes is shown by the gray stars in Figure IV-D-2. It can be seen that the slope of the lines formed by the expectation of the structure reactivity relations (SARs) is in each case about equal to that for the alkanes, and the SAR expectations fall within error limits given by the experimental points up to 1-nonene. McGillen et al. (2007) measured the rate coefficients for the 1-alkenes C8–C11, and an unexpected rise in the values for C10 and C11 was reported. The lack of an obvious source of error in these measurements suggests that the increased importance of H-atom abstraction reactions may account for this. Such a sudden change in the extent of abstraction after C9 seems highly unlikely. The addition of each – CH2–group for the 1-alkenes appears to show a somewhat greater increase in k(298), 2.3 × 10−12 cm3 molecule −1 s−1 than that for the alkanes: 1.3 × 10−12 cm3 molecule−1 s−1. The increase in k(298) for the 2-alkenes, 1.2 × 10−12 cm 3 molecule−1 s−1, is about the same as that seen for the alkanes. The addition of the HO radical to the >C=C< bond is the dominant mechanism for reaction with the alkenes, whereas abstraction of H atoms on the C atoms attached to a double bond are unimportant, as noted earlier. However, some H-atom abstraction does occur at the weakest C–H bonds in the alkenes, namely, the allylic C–H bonds that are those located on C atoms one C atom removed from the double bond. There are three types of allylic C–H bonds and each has a different C–H bond strength that depends on the number of C groups attached to the C atom. These are illustrated

TABLE IV-D-1 . HO RE ACTIONS WITH THE ALKENES AND ALKYNES. THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS, k = A × e −B/T (CM 3 MOLECULE −1 S −1 ) FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 5 SHOWS STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES OF THE PERCENTAGE OF RE ACTION BY ADDITION AND H-ATOM ABSTR ACTION AT E ACH SITE b (TABLES IV-O-1, IV-O5, IV-O-6 WERE USED FOR THE CYCLIC ALKENES, AND TABLE IV-O-7 FOR THE ALKENES AND DIENES). IT IS A SSUMED, A S EVIDENCE SUGGESTS, THAT ABSTR ACTION OF H ATOMS ATTACHED TO DOUBLE-BONDED C ATOMS (E.G., CH 2 =CH 2 ) IS NEGLIGIBLE. IN COLUMN 6, THE OBSERVED OR PREDICTED (ITALICS) MA JOR PRODUCTS ARE SHOWN. N-CONTAINING PRODUCTS ARE NOT SHOWN g

Alkenes

a) Mono-alkenes Ethened (CH2=CH2) Propenee (CH2=CHCH3) 1-Butene (CH2=CHCH2CH3) cis-2-Butene (CH3CH=CHCH3) trans-2-Butene (CH3CH=CHCH3) 2-Methylpropene [CH2=C(CH3)2] 1-Pentene (CH2=CHCH2CH2CH3)

193

cis-2-Pentene (CH3CH=CHCH2CH3) trans-2-Pentene (CH3CH=CHCH2CH3) 2-Methyl-1-butene [CH2=C(CH3)CH2CH3] 3-Methyl-1-butene [CH2=CHCH(CH3)2]

1012 × k(298)c

1012 × A

8.52 ± 1.3 26.3 ± 3.9

1.96 4.85

−438 −504

A 50; A 50 A 85; A 13; 2

CH2O; HOCH2CHO CH2O; CH3CHO

31.4 ± 6.2

6.55

−467

A 82; A 12; 6; 0

CH3CH2CHO; CH2O

56.4 ± 11.2

11.0

−487

1; A 49; A 49; 1

CH3CHO

64.0 ± 12.8

10.1

−550

1; A 49; A 49; 1

CH3CHO

51.4 ± 10.3

9.47

−504

A 90; A 7; (1, 1)

CH3COCH3; CH2O

31.4 ± 6.3

A 82; A 12; 6; 0; 0

C3H7CHO; CH2O

27.4 ± 3.8i 65 ± 20

1; A 48; A 48; 3; 0

CH3CHO; CH3CH2CHO

67 ± 20

1; A 48; A 48; 3; 0

CH2O; CH3CH2CHO

61 ± 18

A 88; A 7; (1); 4; 0

CH2O; C2H5C(O)CH3

A 80; A 12; 8; (0, 0)

CH2O; iso-C3H7CHO

31.8 ± 6.4

5.32

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

−533

Observed or Predicted Major Productsh

(continued)

194

TABLE IV-D-1 (CONTINUED)

Alkenes

1012 × k(298)c

2-Methyl-2-butene [(CH3)2C=CHCH3] 1-Hexene (CH2=CHCH2CH2CH2CH3) 2-Methyl-1-pentene (CH2=C(CH3)CH2CH2CH3) 2-Methyl-2-pentene [(CH3)2C=CHCH2CH3] trans-4-Methyl-2-pentene [CH3CH=CHCH(CH3)2] 2,3-Dimethyl-2-butene [(CH3)2C=C(CH3)2] 3,3-Dimethyl-1-butene [CH2=CHC(CH3)3] 1-Heptene (CH2=CHCH2CH2CH2CH2CH3) trans-2-Heptene (CH3CH=CHCH2CH2CH2CH3) 2,3-Dimethyl-2-pentene [(CH3)2C=C(CH3)CH2CH3] trans-4,4-Dimethyl-2-pentene [CH3CH=CHC(CH3)3] trans-4-Octene (CH3CH2CH2CH =CHCH2CH2CH3) 1-Octene [CH2=CH(CH2)5CH3] 1-Nonene [CH2=CH(CH2)6CH3]

86.9 ± 17.4

1012 × A

19.2

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb −450

Observed or Predicted Major Productsh

(1,1); A 34; A 63; 1

CH3CHO; CH3C(O)CH3

37 ± 11

A 82; A12; 6; 0; 0; 0

C4H9CHO; CH2O

63 ± 19

A 88; A 7; (1); 4; 0; 0

CH2O; C3H7C(O)CH3

89 ± 22

(1, 1); A 34; A 62; 2; 0

CH3C(O)CH3; C2H5CHO

61 ± 18

1; A 47; A 47; 5; (0, 0)

CH3CHO; iso-C3H7CHO

110 ± 22

(1, 1); A 48; A 48; (1, 1)

CH3C(O)CH3

29 ± 9

A 87; A 13; 0; (0, 0)

CH2O; (CH3)3CCHO

40 ± 12

A 82; A 12; 6; 0; 0; 0; 0

CH2O; C5H11CHO

68 ± 17

1; A 48; A 48; 3; 0; 0; 0; 0

CH3CHO; C4H9CHO

103 ± 31

(1, 1)A 48; A 48; (1); 2; 0

CH3C(O)CH3; C2H5C(O)CH3

55 ± 17

1; A 49; A 49; 0, (0, 0, 0)

CH3CHO; (CH3)3CCHO

69 ± 17

0; 0; 4; A 46; A 46; 4; 0; 0

C3H7CHO

36 ± 7i 42 ± 4i

A 82; A 12; 6; 0; 0; 0; 0; 0; A 82; A; 12; 6; 0; 0; 0; 0; 0;0

CH2O; CH3(CH2)5CHO CH2O; CH3(CH2)6CHO

Alkenes

1012 × k(298)c

1-Decene 70 ± 10i [CH2=CH(CH2)7CH3] 124 ± 16i 1-Undecene [CH2=CH(CH2)8CH3] b) Dienes Propadiene (CH2=C=CH2) 9.82 ± 2.0 1,2-Butadiene (CH2=C=CHCH3) 26 ± 8 1,3-Butadiene (CH2=CHCH=CH2) 66.6 ± 13.3 1,2-Pentadiene (CH2=C=CHCH2CH3) 36 ± 11 cis-1,3-Pentadiene (CH2=CHCH=CHCH3) 101 ± 30 1,4-Pentadiene 53 ± 16 (CH2=CHCH2CH=CH2) 3-Methyl-1,2-butadiene [(CH2=C=C(CH3)2] 57 ± 17 2-Methyl-1,3-butadiene 100 ± 20 [CH2=C(CH3)CH=CH2] (isoprene) trans-1.3-Hexadiene 112 ± 34 (CH2=CHCH=CHCH2CH3) trans-1,4-Hexadiene 91 ± 27 (CH2=CHCH2CH=CHCH3) 1,5-Hexadiene (CH2=CHCH2CH2CH=CH2) 62 ± 19 cis/trans-2,4-Hexadiene 134 ± 40 (CH3CH=CHCH=CHCH3) 2-Methyl-1,4-pentadiene 79 ± 24 [CH2=C(CH3)CH2CH=CH2] 2-Methyl-1,3-pentadiene 136 ± 41 [CH2=C(CH3)CH=CHCH3 4-Methyl-1,3-pentadiene 131 ± 39 [CH2=CHCH=C(CH3)2]

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

7.66

−74

14.8

−448

25.4

−410

Observed or Predicted Major Productsh

A 82; A 12; 6; 0; 0; 0; 0; 0; 0; 0

CH2O; CH3(CH2)7CHO

A 82; A 12; 6; 0; 0; 0; 0; 0; 0; 0; 0

CH2O; CH3(CH2)8CHO

No estimates No estimates A44; A6; A6; A44 No estimates A43; A7; A7; A43; 1 A43; A7; 1; A7; A43

? ? CH2=CHCHO; CH2O ? CH2=CHCHO; CH3CHO; CH2O; CH3CH=CHCHO CH2O; CH2=CHCH2CHO

No estimates A59; A4; (1); A4; A31

CH2O; CH3COCH3 CH2O; CH2=CHC(O)CH3; CH2=C(CH3)CHO

A29 A4; A29; A35; 2; 1

CH2O; C2H5CH=CHCHO; CH2=CHCHO; C2H5CHO

A31; A4; 2; A31; A31; 1

CH2O; CH3CH=CHCH2CHO; CH3CHO; CH2=CHCH2CHO

A41; A6; 3; 3; A6; A41 0.6; A27; A22; A22; A27; 0.6

CH2O; CH2=CHCH2CH2CHO CH3CHO; CH3CH=CHCHO

A57; A5;(1); 2; A5; A31

CH2O; CH3C(O)CH2CH=CH2; CH2=C(CH3)CH2CHO

A56; A5; (1); A20; A20; 1

CH2O; CH3C(O)CH=CHCH3; CH3CHO; CH2=C(CH3)CHO

A21; A3; A39;A36; (1,1)

CH2O; (CH3)2C=CHCHO; CH3C(O)CH3; CH2=CHCHO

(continued)

195

196

TABLE IV-D-1 (CONTINUED)

Alkenes

1012 × k(298)c

2.3-Dimethyl-1,3-butadiene [CH2=C(CH3)C(CH3)=CH2] 2-Methyl-1,5-hexadiene [CH2=C(CH3)CH2CH2CH=CH2] 2,5-Dimethyl-1,5-hexadiene [(CH2=C(CH3)CH2CH2C(CH3)=CH2 2,5-Dimethyl-2,4-hexadiene [CH3)2C=CHCH=C(CH3)2] cis-1,3,5-Hexatriene (CH2=CHCH=CHCH=CH2) trans-1,3,5-Hexatriene (CH2=CHCH=CHCH=CH2) Myrcene [CH2=CHC(=CH2) CH2CH2CH=C(CH3)2] cis/trans-Ocimene [CH2=CHC(CH3)=CHCH2CH=C(CH3)2] c) Cyclic alkenes Cyclopentene

122 ± 37

A46; A4; (0.6), A4; (0.6); A46

CH2O; CH3C(O)C(CH3)=CH2

96 ± 29

A55; A5; (1); 2; 2; A5; A30

CH2O; CH3C(O)CH2CH2CH=CH2; CH2=C(CH3)CH2CH2CHO

120 ± 36

A47; (0.7); 3; 3; (0.7); A47

CH2O; CH3C(O)CH2CH2C(CH3)=CH2

210 ± 63

CH3C(O)CH3; (CH3)2C=CHCHO

110 ± 33

(0.4;0.4); A23; A26; A23; (0.4; 0.4) A29; A4; A4; A:29; A4; A29?

111 ± 33

A29; A4; A4; A:29; A4; A29?

CH2O; CH2=CHCH=CHCHO; CH2=CHCHO

215 ± 65

A 16; A 2; A 16; (A 31); 1; 1; A 29; 2; (0.4, 0.4) A 17; A 2; A 14; (1); A 26; 1; A 25; A 14; (0, 0)

CH2O; CH3C(O)CH3; CH2=CHC(O)CH3CH2CH=C(CH3)2; CH2=CHC(=CH2)CH2CH2CHO CH2O; CH3C(O)CH3; CH2=CHC(O)CH3; (CH3)2C=CHCH2CHO

67 ± 23

a A 90; b 4; c 2

HC(O)CH2CH2CH2C (O)H

67.7 ± 17

a A 88; b 4; c 2

HC(O)CH2CH2CH2CH2C(O)H

252 ± 76

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

Observed or Predicted Major Productsh

CH2O; CH2=CHCH=CHCHO; CH2=CHCHO

a b b

c

Cyclohexene a b c

b c

1012 × k(298)c

Alkenes

1,3-Cyclohexadiene

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

Observed or Predicted Major Productsh

164 ± 49

a A 48; b 1

HC(O)CH2CH2CH=CHCHO

99.5 ± 25

a A 48; b 2

HC(O)CH2CH=CHCH2CHO

74 ± 22

a A 86; b 4; c 2

HC(O)(CH2)5CHO

139 ± 42

a A48; b 2; c 1

HC(O)CH2CH2CH2CH=CHCHO

97 ± 29

Equal rates of addition reaction at the two a-sites > addition at the b-site >> c

HC(O)CH2CH=CHCH=CHCHO; HC(O) CH=CHCH2CH=CHCHO

a a b b

1,4-Cyclohexadiene a b b

a

Cycloheptene a

b c

b c

c

1,3-Cycloheptadiene a

a

b

b c

1,3,5-Cycloheptatriene a

b

c a

(continued)

197

198

TABLE IV-D-1 (CONTINUED)

1012 × k(298)c

Alkenes

1-Methyl-1-cyclohexene c

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

94 ± 28

a A 91; b 2; c 2; d 1

49 ± 15

a A 85; b 5; c 2; d 1

Observed or Predicted Major Productsh CH3C(O)CH2CH2CH2CH2CHO

b

c

d a b

Bicyclo[2.2.1]-2-heptene (2-Norbornene) d

CHO b

c

CHO a

b

c

Bicyclo[2.2.1]-2,5-heptadiene

120 ± 36

a A 47; b 3; c 1

c

CHO CHO

b a a

b

Bicyclo[2.2.2]-2-octene

41 ± 12

a A 83; b 5; c 2

53 ± 16

a A 84; b5; c 2; d 0.3

b c c

c

OHC

CHO

a

c b

Camphene b c

d

CH2O;

d

e a

c b

O

1012 × k(298)c

Alkenes

2-Carene

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

80 ± 24

a A 95; b 3; c 1; d 0.2; e 0; f 2

88 ± 26

a A 94; b 3; c 1; d 0; e 0.2

c

a

e

1012 × A

Observed or Predicted Major Productsh CHO

O

d d

b

e f

3-Carene c

O CHO

a b

b d

d

e

e

170 ± 51

Limonene

O HC

a A 58; b A 34; c 2; d 1; e 0.5

c a

O

b

e

d e

d

313 ± 94

α-Phellandrene c f

O

c

a A 57; b A 37; c 2; d 2; e 0.5; f 0.1

a

d c

CHO O

e

f

CHO

CHO

b

β-Phellandrene

168 ± 50 b

g

f

c

O

a

g d

a A 43; b 48; c 3; d 2; e 1; f 3; g 0.1

d

(continued)

199

200

TABLE IV-D-1 (CONTINUED)

1012 × k(298)c

Alkenes

53.7 ± 13.4

α-Pinene

1012 × A

12.1

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb −444

a A 93; b 2; c 3; d 2; e 0.1; f 0.4

Observed or Predicted Major Productsh Acetone;

f O

a c e e

CHO

d e

b

β-Pinene

78.9 ± 19.7

23.8

−357

a A 88; b 4; c 2; d 2

a c

O

b

d d

c

Acetone; CH2O;

c c

Sabinene

117 ± 35

a A 86; b 4; c 2; d 0; e 0.2

363 ± 109

a A 48; b 1; c 1; d 0.3; e 0

O

b c

a

b

e

d

e

d

α-Terpinene c e e

c

b

d a

a

CHOCH2CH=C[CH(CH3)2]CH2CH2C(O)CH3; (CH3)2CHC(O)CH2CH2C(CH3)=CHCHO

1012 × k(298)c

Alkenes

γ-Terpinene

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb

177 ± 53

a A 47; b 2; c 1; d 0.4

225 ± 68

a A 42; b A54; c 1; d 0.4

c d

Observed or Predicted Major Productsh CHOCH2C[CH(CH3)2]CH=CH2C(O)CH3; (CH3)2CHC(O)CH2CH=C(CH3)CH2CHO

a

b

d

d

a c

Terpinolene

CH3C(O)CH3;

CHO

c d

a

b c

O

O

d

d c

α-Cedrene a c

67 ± 20

a A 86; b 3; c 2; d 1; e 0.2

90 ± 27

a A 89; b 2; c 1; d 0.4; e 0.2

d

OHC

O

d b

d

e

e

d d

e

d

α-Copaene c

a b

e d c

O

CHO

d

d c

d

e

c e

201

(continued)

202

TABLE IV-D-1 (CONTINUED)

1012 × k(298)c

Alkenes

β-Caryophyllene f

197 ± 59 f

f

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb a A 58; b A 34; c 2; d 1; e 0.5; f 0.1

Observed or Predicted Major Productsh CH2O; O

e b

e d

c

c

O

c

CHO

a e

293 ± 88

α-Humulene

a A 36; b 23; c 1; d 0.3; e 0

CHO CHO

d a

O

b c c

OHC

e e

c c

O

a d

47 ± 14

Longifolene

a A 83; b 3; c 2; d 0.2

CH2O

c

b d

c

d

b

a c

O d

c

c

CHO

Alkenes

Alkynes Acetylene (ethyne, HC≡CH) Methyl acetylene (propyne, CH3C≡CH) 1-Butyne (HC≡CCH2CH3) 2-Butyne (CH3C≡CCH3)

1012 × k(298)c

0.78 ± 0.16 5.4 ± 1.1 7.5 ± 1.5 27 ± 5

1012 × A

B (K) % Addition (A) or H-atom Abstraction at Each C-atom Siteb A 100 2; A 98 A 82; 16; 2 1; A 99; 1

Observed or Predicted Major Productsh

HC(O)OH; CO; HCOCHOf HC(O)OH; CH3C(O)CHOf HC(O)OH; CH3CH2C(O)CHO? CH3C(O)OH; CH3C(O)C(O)CH3f

Recommendations are those of Calvert et al. (2000) where detailed kinetic information is reviewed and evaluated. The abbreviated code in column 5 gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive C-atom site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the Peeters et al. SAR (2007) method of estimation plus allylic H-atom contributions. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c Except for ethene, propene, and propadiene, these are essentially the high-pressure rate coefficients. d Uses the Troe relation for P < 760 Torr: k∞ = 9.1 × 10−12 (T/298)−0.9 cm3 molecule−1 s−1; ko = 6 × 10−29 cm6 molecule−2 s−1; F = 0.70. e Uses the Troe relation for P < 760 Torr: k∞ = 2.8 × 10−13 (T/298)−1.2 cm3 molecule−1 s−1; ko = 3 × 10−27 (T/298)−4 cm6 molecule−2 s−1; F = 0.5. f From Hatakeyama et al. (1986b). g The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). h Only the major products that have been reported are given; when no product studies have been made, the products shown are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. i McGillen et al. (2007). j For the cyclic alkenes, the Atkinson et al. method of estimation (Table IV-0-5) was used to determine the % HO addition (A) at the double-bonded carbon site or H-atom abstraction at the site indicated by the lowercase letters on the structure. a

b

203

2,3-Dimethy-2-Butene

1012 x k(298 K), cm3 molecule−1 s−1

140 120

2-Methyl-2-Butene

100 trans-2-Butene

80 60 Propene

40 20 0

cis-2-Butene

Ethene

0

1

2

3

4

Number of methyl-groups substituted for H-atoms in ethylene FIGURE IV-D-1.

Rate coefficients (298 K) for the reactions of the HO radical with ethene-like molecules with varied numbers of CH3-group substitutions for H atoms.

FIGURE IV-D-2.

Effect of chain length on the rate coefficients (298 K) for HO reaction with the straight chain 1-alkenes and 2-alkenes. Data for the C10 and C11 alkenes are probably in error. Data for the n-alkanes are shown as a reference.

204

205

The Hydroxyl Radical and Its Role in Ozone Formation here for the compound 3-ethyl-4-methyl-2-pentene that contains all three types of allylic C–H bonds:

The three light gray colored H atoms attached to the C atom at the top left of the figure are primary allylic atoms; the medium gray colored atoms on the lower left are C are secondary allylic H atoms; the black atom is a tertiary allylic H atom. Although the extent of abstraction is relatively small in the alkene-HO reactions, there is significant interest in the extent of these reactions because the products expected following allylic H-atom abstraction are α,β-unsaturated aldehydes and ketones (for example, CH2=CHCHO), particularly objectionable products that are especially active eye irritants. The abstraction of allylic H atoms is very important for HO reactions at temperatures above 500 K, and

these kinetic data have been extrapolated to 298 K to derive approximate rate coefficients for the three types of allylic H atoms at 298 K (Calvert et  al., 2000). The estimated values per C–H allylic bond are primary, 2.5 × 10−13 and secondary, 11.6 × 10−13; two different estimates of the tertiary allylic bond are 37 × 10−13 or (24 ± 8) × 10−13, with an average of 31 × 10−13 cm3 molecule−1 s−1. The percentage of the total HO reaction that is expected to occur via allylic H-atom abstraction at 298 K for some typical alkenes in urban air can be estimated at about 2.9% from propene, about 5.0% for 2-methyl-1-butene, about 7.4% for 1-butene, and about 9.8% for 3-methyl-1-butene. IV-D-2. Mechanism of HO Addition to Alkenes The addition reactions of the HO radical to the alkenes, dienes, or trienes are all stimulated by transfer of charge from a π bond to the approaching electron-seeking HO group. This transient complex ultimately relaxes as attachment of the HO occurs on one of the two carbons of the original double bond. Although both C atoms of the double bond are involved to some extent in the formation of the new bond, the final position of the addition of the HO group favors the formation of the radical product, with the odd electron localized on the carbon atom containing the greater number of alkyl groups. The reaction is illustrated for the case of propene as follows:

•

206

the mechanisms of reactions influencing atmospheric ozone

In this case, HO addition at the terminal C atom is favored (~87%; Peeters et  al., 2007). Addition at either site leads ultimately to the formation of CH2O and CH3CHO in equal amounts. IV-D-3. HO Reactions with Dienes The dienes are an important class of alkenes that are highly reactive with HO radicals. The conjugated, 1,3 dienes (>C=CHCH=CC=C< or >C≡C< groups are present in addition to the functional group characteristic of the oxygenate), then addition of HO to the multibonded carbon atoms becomes the most important mode of reaction. IV-G-1. HO Reactions with Acylic Alcohols The recommended rate coefficients for the reactions of the alcohols with the HO radical were recently reviewed (Calvert et al., 2011) and are summarized in Table IV-G-1. Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. The acyclic alcohols of structure CH3(CH2)nOH show a regular increase in the measured rate coefficients as the length of the chain increases from n = 0 to 6; see Figure IV-G-1. The slope of the line fitted to these data indicates a significant increase in rate

212

the mechanisms of reactions influencing atmospheric ozone CH3

*

CH3 H

CH3

H

(O2)

O

OH

OH

H

O2 H

(O2)

+ HO O

(M) CH3

CH3

CH3 H

OH

H

OH

+ HO2

O

O2 H

CH3

O

H

H

H

OH

HO + O O

NO2

CH3

CH3 O O2

O2

NO

NO2

NO

OH O

O

O H

HO2

OH O H HO

H

HO

H CH3

CH3

H H O

H

O

O

O

O

O

H

O O

O2

OH H

H

CH3 HO2 O

CH3 O

FIGURE IV-F-3.

A simplified mechanism of reaction of the HO adduct of the toluene molecule (ortho-addition). Similar reactions are expected from the addition of HO radicals to the m- and p-positions in toluene.

coefficient per CH2 group added, about 2.5 × 10−12 cm3 molecule−1 s−1, a factor of 1.8 larger than that seen with the alkanes (dotted line in Figure IV-G-1). Although the enhancement of the rate coefficient in the alcohols appears to continue several atoms down-chain from the OH group, the precision of the data is insufficient to confirm the extent of long range enhancement. SAR estimates are shown as gray stars and the dotted dark gray curve. The SAR expectations fall within the limits of the error bars of the experimental data and suggest that enhancement continues to a chain length of only three or

four. The slope of the SAR curve approaches that of the alkane series at about carbon number four (see discussion in Section IV-O). The data for the family of alcohols of formula (CH3)2CH(CH2)nOH, given by the dashed line in Figure IV-G-1, show enhancement in HO reactivity by the OH groups (4.4  × 10−12 cm3 molecule−1 s−1 per CH2 group added). This is even higher than that seen in straight-chain alcohols. In this case, the SAR estimates fit the experimental data poorly for the n  =  0 and 3, and the large slope observed in the experimental data is not reproduced.

TABLE IV-F-1 . HO RE ACTIONS WITH AROMATIC HYDROCARBONS. THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS, k = A e −B/T (CM −3 MOLECULE −1 S −1 ) FOR TROPOSPHERE TEMPER ATURES. a ESTIMATES OF THE EXTENT OF RE ACTION AT GIVEN SITES ARE GIVEN IN COLUMN 5, WHERE AVAIL ABLE; ADDITION TO AROMATIC RING IS THE MA JOR PATHWAY FOR RE ACTION. IT IS A SSUMED, A S EVIDENCE SUGGESTS, THAT ABSTR ACTIONS OF H ATOMS ATTACHED TO DOUBLE-BONDED C ATOMS (E.G., CH 2 =CH 2 ) AND AROMATIC C ATOMS (E.G., C 6 H 6 ) ARE NEGLIGIBLE. COLUMN 6 SHOWS THE MA JOR PRODUCTS OBSERVED OR EXPECTED (ITALICS) TO FORM FOLLOWING RE ACTIONS OF THE AROMATIC HYDROCARBONS IN NO X-POLLU TED ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN e

213

Aromatic Hydrocarbon

1012× k(298 K)

1012 × A

B (K)

Reaction Site

Observed or Predicted Major Products f

a) Monocyclic aromatic hydrocarbons Benzeneb (C6H6) Toluene (CH3–C6H5)

1.22 ± 0.24 5.63 ± 1.13

2.33 1.81

193 −338

Phenol; glyoxal; carbonyls (unidentified); Benzaldehyde; o-, m-, and p-cresol; glyoxal; methylglyoxal; unsaturated dicarbonyls

Ethylbenzene (CH3CH2–C6H5)

7.0 ± 1.8

o-Xylene

13.6 ± 3.4

13.6

0

Addition to ring ~94% addition to ring; ~6% H-abstraction from CH3-group Addition to ring largely; probably > 6% H-abstraction from C2H5-group ~90% addition to ring;~10% H-abstraction from CH3-groups

m-Xylene

23.1 ± 3.5

23.1

0

~94% addition to ring;~4% H-abstraction from CH3-groups

Methylglyoxal; glyoxal; m-tolualdehyde; dimethylphenols; 4-oxo-2-pentenal

p-Xylene

14.3 ± 3.6

14.3

0

~ 93-94% addition to ring 7-8% H-abstraction from CH3-Groups

Methylglyoxal; dimethyl-phenols; p-tolualdehyde; 3-hexene-2,5-dione; 2-methylbutenedial

n-Propylbenzene (CH3CH2CH2C6H5)

5.8 ± 1.5

Addition to ring largely; probably >6% H-abstraction from C3H7-group

Ring fragmentation products may dominate.

Biacetyl; methylglyoxal; o-tolualdehyde; dimethylphenols; unsaturated dicarbonyls

(continued)

214

TABLE IV-F-1 (CONTINUED)

Aromatic Hydrocarbon

1012× k(298 K)

iso-Propylbenzene [(CH3)2CHC6H5)] o-Ethyltoluene

6.3 ± 1.9

m-Ethyltoluene

1012 × A

Reaction Site

Observed or Predicted Major Products f

Addition to ring largely; probably >6% H-abstraction from C3H7-group Addition to ring largely; probably >10% H-abstraction from alkyl-groups

Ring fragmentation products may dominate.

18.6 ± 6.5

Addition to ring largely; probably >4% H-abstraction from alkyl-groups

Ring fragmentation products may dominate.

p-Ethyltoluene

11.8 ± 4.1

Addition to ring largely; probably >7-8% H-abstraction from alkyl-groups

Ring fragmentation products may dominate.

1,2,3-Trimethylbenzene

32.7 ± 8.1

32.7

0

~93–94% addition to ring;~6–7% H-abstraction from CH3-groups

Biacetyl; methylglyoxal; glyoxal; unsaturated dicarbonyls

1,2,4-Trimethylbenzene

32.5 ± 8.1

32.5

0

~93–94% addition to ring;; ~6–7% H-abstraction from CH3-groups

Methylglyoxal, glyoxal; biacetyl; unsaturated dicarbonyls; 3-hexene-2,5-dione; dimethylbenzaldehydes; 3-methyl-3-hexene-2,5-dione

B (K)

11.9 ± 4.2

Ring fragmentation products may dominate.

Aromatic Hydrocarbon

1012× k(298 K)

1012 × A

B (K)

Reaction Site

Observed or Predicted Major Products f

1,3,5-Trimethylbenzene

56.7 ± 11.3

56.7

0

~96–97% addition to ring; ~3–4% H-abstraction from CH3-groups

Methylglyoxal; dimethylfuranones; 2-methyl-4oxo-2-pentanal; 3,5-dimethylbenzaldehyde

tert-Butylbenzene [(CH3)3CC6H5] p-Cymene

4.5 ± 1.6 14.5 ± 4.4b

Largely addition to aromatic ring Addition to ring largely; probably >7–8% H-abstraction from alkyl-groups

? Ring fragmentation products may dominate.

Indane

19 ± 8

Probably largely addition to the aromatic ring

Ring fragmentation products may dominate.

Indene

78 ± 31

Largely addition at the –C=C– group CHO CHO

Tetralin

34 ± 10b

Both addition to aromatic ring and reaction at CH2-groups of cyclohexane ring

Styrene

58 ± 12

Largely addition of HO to –CH=CH2 group

?

Benzaldehyde; CH2O

215

(continued)

216

TABLE IV-F-1 (CONTINUED)

Aromatic Hydrocarbon

1012× k(298 K)

α-Methylstyrene

1012 × A

Reaction Site

Observed or Predicted Major Products f

51 ± 15b

Largely addition of HO to the >CH=CH2 group

Methyl phenyl ketone; CH2O

β-Methylstyrene

57 ± 17b

Largely addition of HO to the –CH=CH– group

Benzaldehyde; CH3CHO

β,β-Dimethylstyrene

32 ± 10b

Largely addition of HO to >CH=CH– group

Benzaldehyde; CH3C(O)CH3

Addition to aromatic rings

2-Formylcinnamaldehyde; 1-, and 2-naphthols; naphthoquinone

Largely addition to aromatic rings

Ring fragmentation products may dominate.

B (K)

b) Polycyclic aromatic hydrocarbons Naphthalene

23.0 ± 5.8

1-Methylnaphthalene

53.0 ± 16

15.53

−117

Aromatic Hydrocarbon

1012× k(298 K)

2-Methylnaphthalene

1012 × A

Reaction Site

Observed or Predicted Major Products f

52.3 ± 16

Largely addition to aromatic rings

Ring fragmentation products may dominate.

2,3-Dimethylnaphthalene

76.8 ± 23

Largely addition to aromatic rings

Ring fragmentation products may dominate.

Biphenyl

7.1 ± 2.1

Largely addition to aromatic rings

2-Hydroxybiphenyl; ring fragmentation products may dominate

Fluoranthene

14 (± factor of 1.5)

Largely addition to aromatic rings

3-Hydroxyfluoranthene; ring fragmentation products may dominate

Acenaphthene

80 (± factor of 1.5)

Largely addition to aromatic rings

Ring fragmentation products may dominate

Acenaphthalene

109 ± 33

Largely addition to C=C bond

B (K)

CHO

CHO

(continued)

217

218

Aromatic Hydrocarbon

1012× k(298 K)

Phenanthrene

1012 × A

Reaction Site

Observed or Predicted Major Products f

18 (± factor of 2)

Addition to aromatic rings

Ring fragmentation products may dominate

Anthracene

~ 18

Largely addition to aromatic rings

Ring fragmentation products may dominate

Fluoranthene

~18c

Largely addition to aromatic rings

Ring fragmentation products may dominate

a

B (K)

Most recommendations are those of Calvert et al. (2002) where detailed kinetic information is reviewed and evaluated. The values given in the table for benzene are those for atmospheric pressure; the high-pressure limiting value for the benzene rate coefficient is k = 1.39 × 10−12 cm3 molecule−1 s−1. Average value at 356–386 K. d Not shown are nitrogen-containing products, which can be important for high-NOx conditions. e The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors. f Only the major products that have been reported are given; when no product studies have been made, the products shown are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations (shown in italics). b

c

TABLE IV-G-1 . HO RE ACTIONS WITH THE ALCOHOLS. THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS, k = A × e −B/T (CM 3 MOLECULE −1 S −1 ), FOR HO RE ACTION WITH THE ALCOHOLS AT TROPOSPHERIC TEMPER ATURES. a COLUMN 5 ALSO SHOWS STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES (USING TABLES IV-O-1, IV-O-2, IV-O-5, AND IV-O-6) OF THE PERCENTAGE OF RE ACTION BY ADDITION AND H-ATOM ABSTR ACTION AT E ACH SITE. b COLUMN 6 SHOWS THE MA JOR PRODUCTS OBSERVED OR EXPECTED (ITALICS) TO FORM FOLLOWING RE ACTIONS OF THE ALCOHOL IN NO X-POLLU TED ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN. c PREFIX “A” DENOTES HO ADDITION AT THE SITE

Alcohol a) Acyclic alcohols Methanol (CH3OH) Ethanol (CH3CH2OH) 1-Propanol (CH3CH2CH2OH) 2-Propanol [CH3CH(OH)CH3]

219

1-Butanol (CH3CH2CH2CH2OH) 2-Butanol [CH3CH(OH)CH2CH3] 2-Methyl-1-propanol [(CH3)2CHCH2OH] 2-Methyl-2-propanol [(CH3)2C(OH)CH3] 2,2-Dimethyl-1-propanol [(CH3)3CCH2OH] 1-Pentanol (CH3CH2CH2CH2CH2OH) 2-Pentanol [CH3CH(OH)CH2CH2CH3) 3-Pentanol [CH3CH2CH(OH)CH2CH3) 3-Methyl-1-butanol [(CH3)2CHCHCH2OH] 2-Methyl-2-butanol [CH3)2C(OH)CH2CH3] 3-Methyl-2-butanol [CH3CH(OH)CH(CH3)2] 1-Hexanol [CH3CH2CH2CH2CH2CH2OH 2-Hexanol [CH3CH(OH)CH2CH2CH2CH3 2-Methyl-2-pentanol [(CH3)2C(OH)CH2CH2CH3] 4-Methyl-2-pentanol [CH3CH(OH)CH2CH(CH3)2]

1012 × k(298 1012 × A K)

B (K)

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted Major Products (NOx present)d

0.90 ± 0.13 3.2 ± 0.3 5.70 ± 0.85 5.10 ± 0.51 5.27 ± 0.5 8.3 ± 1.2 8.14 ± 1.22 9.55 ± 1.43 1.1 ± 0.2 5.49 ± 1.10 11.0 ± 1.7 11.0 ± 2.2 13.0 ± 2.0 13.9 ± 2.1 3.42 ± 1.02 11.9 ± 1.8 13.0 ± 3.3 12.0 ± 3.0 7.1 ± 1.4

2.85 3.0 6.3 2.7

345 −20 30 −190

78; 22 9; 87; 4 3; 36; 60; 2 4; 91; (2); 4

CH2O CH3CHO CH3CH2CHO; CH3CHO; CH2O CH3C(O)CH3

5.1 2.7 2.75 1.6 5.0

−146 −329 −371 124 −28

1.5 2.0 2.5

−664 −160 −464

2; 13; 35; 48; 2 3; 74; (1); 21; 1 (2, 2); 52; 43; 2 (30, 30, 30); 11 (3, 3, 3); 87; 3 2; 12;14; 30; 41; 1 3; 59; (1); 25; 10;1 1; 15; 66; (1); 15; 1 (2, 2); 24; 30; 41; (1) (12, 12); (4); 68; 5 2; 59; (1); 35;(1,1) 2; 10; 12: 12: 26; 36: 1 2; 58; (1); 20; 10; 8; 1 (7,7); (3); 58; 22; 3

C3H7CHO; C2H5CHO; CH3CHO; CH2O CH3C(O)C2H5; CH3CHO CH3COCH3; CH2O; (CH3)2CHCHO CH3C(O)CH3; CH2O (CH3)3CCHO C4H9CHO; CH2O; C3H7CHO; CH3CHO; C2H5CHO, CH3C(O)C3H7; CH3CHO; C2H5CHO C2H5C(O)C2H5; C2H5CHO; CH3CHO iso–C4H9CHO; iso-C3H7CHO; CH3C(O)CH3; CH2O CH3C(O)CH3; CH3CHO; CH3C(O)C2H5 iso-C3H7C(O)CH3; CH3C(O)CH3; CH3CHO C5H11CHO; C4H9CHO; CH2O C4H9C(O)CH3; C3H7CHO; CH3CHO CH3C(O)CH3; CH3CHO; C2H5CHO; C3H7C(O)CH3

1; 59; (1); 20; 16; (1, 1)

CH3CHO; CH3C(O)CH3; iso-C4H9C(O)CH3; iso-C3H7CHO

17.0 ± 3.4

(continued)

220

TABLE IV-G-1 (CONTINUED)

Alcohol

1012 × k(298 1012 × A K)

B (K)

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted Major Products (NOx present)d

2,3-Dimethyl-2-butanol [(CH3)2CHC(OH)(CH3)2] 1-Heptanol (CH3CH2CH2CH2CH2CH2CH2OH) 4-Heptanol [CH3CH2CH2CH(OH)CH2CH2CH3] 2,4-Dimethyl-2-Pentanol [(CH3)2C(OH)CH2CH(CH3)2]

9.3 ± 1.9

−554

(3, 3); 81; (2) (6,6)

CH3C(O)CH3

13.0 ± 2.0

1; 9; 11; 11; 11; 22; 32; 1

C6H13CHO; C5H11CHO; CH2O; hydroxycarbonyls

17 ± 5

1; 6; 15; 55; (1); 15; 6; 1

C3H7C(O)C3H7; C3H7CHO; C2H5CHO

11.0 ± 3.3

(3,3); (2); 48; 39, (3, 3)

13.0 ± 3.9

1: 8; 10; 10; 10; 10; 21; 29; 1

CH3C(O)CH3; iso-C3H7CHO; C2H5C(CH3)(OH) CH2C(O)CH3; CH3C(OH)(CH3)CH2C(O)CH3; iso-C4H9C(O)CH3 C7H15CHO; C6H13CHO; CH2O; hydroxycarbonyls

13.0 ± 3.9

2: 28; (4); (2); 34; 27; (2, 2)

11 ± 2

a 54; b 15; c 7; 1

1-Octanol (CH3CH2CH2CH2CH2CH2CH2CH2OH) 3,5-Dimethyl-3-hexanol CH3CH2C(CH3)(OH)CH2CH(CH3)2 b) Cyclic alcohols cyclo-Pentanol

b H2C c H2 C

1.45

(CH3)2CHCH2C(O)CH3; (CH3)2C(OH)CH2C(O)CH3; C2H5C(O)CH3; iso-C3H7CHO; CH3C(O)CH3

OH d

H2C

CH a CH2 b

H2C

CH2 c

17 ± 5

cyclo-Hexanol

a 51; b 14; c 7; d 1

b H2C

CH2 b

c H2C

CH2 c

C H2 c

C=O C H2

H(O)C(CH2)4CHO; 3-and 4-hydroxy-cyclohexanone; H2 C

OH d CH a

H2 C

H2C H2C

C H2

O CH2

Alcohol c) Diols 1,2-Ethanediol (HOCH2CH2OH) 1,2-Propanediol [HOCH2CH(OH)CH3] 1,2-Butanediol [HOCH2CH(OH)CH2CH3] 1,3-Butanediol [HOCH2CH2CH(OH)CH3] 2,3-Butanediol [CH3CH(OH)CH(OH)CH3] 2-Methyl-2,4-pentanediol [CH3)2C (OH) CH2CH(OH)CH3] d) Unsaturated alcohols 2-Propen-1-ol [HOCH2CH=CH2) 2-Buten-1-ol [HOCH2CH=CHCH3\ 3-Buten-1-ol (HOCH2CH2CH=CH2) 3-Buten-2-ol [CH3CH(OH)CH=CH2] cis-2-Penten-1-ol (HOCH2CH=CHCH2CH3) 1-Penten-3-ol [CH2=CHCH(OH)CH2CH3 2-Methyl-2-propen-1-ol [HOCH2C(CH3)=CH2] 3-Methyl-2-buten-1-ol [HOCH2C=C(CH3)2] 3-Methyl-3-buten-1-ol [HOCH2CH2C(CH3)=CH2] 2-Methyl-3-buten-2-ol [CH3)2C(OH)CH=CH2] 3-Methyl-1-penten-3-ol [CH2=CHC(OH)(CH3)CH2CH3] cis-3-Hexen-1-ol (HOCH2CH2CH=CHCH2CH3) 6-Methyl-5-hepten-2-ol [(CH3)2C=CHCH2CH2CH(OH)CH3] 2,6-Dimethyl-7-octen-2-ol (CH3)2C(OH)CH2CH2CH2CH(CH3)CH=CH2 3,7-Dimethyl-1,6-octadien-3-ol (linalool) CH2=CHC(OH)(CH3)CH2CH2CH=C(CH3)2

1012 × k(298 1012 × A K)

B (K)

14.6 ± 4.3 21.0 ± 6.3 24.3 ± 7.3 29.9 ± 9.0 21.2 ± 6.4 13.6 ± 4.1

50 ± 8 87 ± 17 55 ± 8 59 ± 9 117 ± 18 74 ± 15 90 ± 13 160 ± 40 94 ± 14 63 ± 13

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted Major Products (NOx present)d

1; 49; 49; 1 1: 32; 66; 1; 1 1; 29; 61; (1); 8; 1 1; 21; 32; 44; (1); 2 1; 49; (0.04); 49; (0.04); 1 (3, 3): (1); 23; 67; (1); 3

HOCH2CHO HOCH2C(O)CH3; H(O)CCH(OH)CH3 HOCH2C(O)CH2CH3; H(O)CCH(OH)CH2CH3 HOCH2CH2C(O)CH3; H(O)CCH2CH(OH)CH3 CH3C(O)CH(OH)CH3 (CH3)2C(OH)CH2C(O)CH3

CH2O; HOCH2CHO HOCH2CHO; CH2O; CH3CHO CH2O; HOCH2CH2CHO; HOCH2CHO CH2O; CH3CHO; HOCH2CHO; CH3CH(OH)CHO HOCH2CHO; CH3CH2CHO; CH2O HOCH2CHO; CH2O CH2O; HOCH2C(O)CH3 CH3C(O)CH3; HOCH2CHO CH2O; HOCH2CH2C(O)CH3; HOCH2C(O)CH3; HOCH2CHO CH3C(O)CH3; CH2O; HOCH2CHO; (CH3)2C(OH) CHO CH2O; CH(O)C(OH)(CH3)CH2CH3

5.7 7.4 3.8

−650 −734 −797

8.1

−716

6.1

−814

0.3; 7; A 92; 0.1; 3; A 94; 2 0.4; 7; 12; A 80 0.3; 14; (0.2); A 85 0.1; 4; A 95; 1; 0.1 A 78; 16; (0.3); 6; 0.3 0.2; 5; (1); A 94 0.1; 2; A 97; (0.5, 0.5) 0.2; 7; 5; (1); A 87

8.1

−610

(4, 4); (0.3); A 91

62 ± 1.8j

A 90; (1); (3); 3; 1

107 ± 32 100 ± 30j

0.2; 6; 5; 4; A 85; 1 (0.1; 0.1); A 86; 2; 3; 8; (0.1); 0.3 (1, 1); (0.4); 8; 4; 4; 9; (0.05); A 73 A 31; (0.1); (0.3); 2; 1; A 64; (1, 1)

38 ± 11 190 ± 100 130 ± 30jj 170 ± 30j

HOCH2CH2CHO; CH3CH2CHO CH3C(O)CH3; CH(O)CH2CH2CH(OH)CH3 CH3C(O)CH3; iso-C3H7CHO; CH3CH2CH(CH3)CHO; CH(O)CHO; CH3C(O)CHO CH3C(O)CH3; CH3C(O)CH2CH2CH=C(CH3)2; CH(O)CH2CH2C(CH3)(OH)CH=CH2

221

(continued)

222

TABLE IV-G-1 (CONTINUED)

1012 × k(298 1012 × A K)

Alcohol

B (K)

2,6-Dimethyl-2,6-octadien-8-ol (geraniol) 230 ± 120 (CH3)2C=CHCH2CH2C(CH3)=CHCH2OH 3,7-Dimethyl-6-octen-1-ol (citronellol) 170 ± 90 HOCH2CH2CH(CH3)CH2CH2CH=C(CH3)2 2-(4-Methylcyclohex-3-en-1-yl)propan-2-ol 190 ± 100 (α-terpineol) c H2C eH3C

C a

d CH2 CH b

HC

CH2 c

Observed or Predicted Major Products (NOx present)d

(0.3, 0.3); A 37; 1; 1; (0.3); A 59; CH3C(O)CH3; HOCH2CHO; CH(O)CHO; CH3C(O) 1; 0 CHO CH3C(O)CHO;CH3C(O)CH2CH2CHO 0.1; 4; 3; 3; (0.2);1; 1; A 86; CH3C(O)CH3; CH(O)CHO; CH3C(O)CHO; (1, 1) HOCH2CH2CH(CH3)CH2CH2CHO a A 85; b 8; c 2; d 1; e 0.7; f 0.3; CH3C(O)CH3; CH(O)CHO; CH3C(O)CHO g 0.1

CH3f OHg CH3f

2-Propyn-1-ol (HOCH2C≡CH) 3,5-Dimethyl-1-hexyn-3-ol [HC≡CC(OH)(CH3)CH2CH(CH3)2] d) Aromatic alcohols Phenol

Estimated % H-atom Abstraction at Each Siteb

9.2 ± 4.6 29 ± 15

28 ± 6

1; 23; A 76 A 65; (1); (1); 17; 14; (1, 1)

0.45

−1,229

OH

Addition to the aromatic ring ~ 88%; H-abstraction from OH-group ~12%

HC(O)C(O)CH2OH; HC≡CCHO HC(O)OH CH3C(O)CH3; HC≡CC(O)CH3; (CH3)2CHCHO; CH(O)CHO; (CH3)2CHCH2C(O)CH3; CH3C(O)CHO; CH3C(O)C(O)CH3 OH OH O

46 ± 14

o-Cresol OH

CH3

1.6

−999

Addition to the aromatic ring ~ 95%; H-abstraction, ~5%

OH

O

H3C OH O CH3

O

Alcohol

1012 × k(298 1012 × A K)

B (K)

Estimated % H-atom Abstraction at Each Siteb

m-Cresol

62 ± 19

−969

Addition to the aromatic ring ~64-97%; H-abstraction from HO- and CH3-groups, ~3–36%

2.4

OH

H3C

Observed or Predicted Major Products (NOx present)d OH OH

50 ± 15

H3C

OH

4.5

−720

Addition to the aromatic ring, ~83–93%; H-abstraction from HO-groups, ~7–17%

CH3

OH OH

CH3

p–Cresol

O

CH3

O

OH OH

CH3

27 ± 5

Benzyl alcohol

Mainly addition to the aromatic ring

CH2OH

2,3,Dimethylphenol

HC(O)CHO; CH3C(O)CH2CH2CH(O); CHO

82 ± 16

Mainly addition to the aromatic ring

Dihydroxy-dimethylbenzenes

73 ± 15

Mainly addition to the aromatic ring

Dihydroxy-dimethylbenzenes

OH CH3 CH3

2,4-Dimethylphenol OH

H3C

CH3

223

(continued)

TABLE IV-G-1 (CONTINUED)

224

Alcohol

1012 × k(298 1012 × A K)

2,5-Dimethylphenol

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted Major Products (NOx present)d

85 ± 17

Mainly addition to the aromatic ring

Dihydroxy-dimethylbenzenes

67 ± 13

Mainly addition to the aromatic ring

Dihydroxy-dimethylbenzenes

83 ± 17

Mainly addition to the aromatic ring

Dihydroxy-dimethylbenzenes

114 ± 23

Mainly addition to the aromatic ring

Dihydroxy-dimethylbenzenes

125 ± 38

Mainly addition to the aromatic ring

Trimethyl-dihydroxybenzenes

OH H3C

B (K)

CH3

2,6-Dimethylphenol OH CH3

H3C

3,4-Dimethylphenol CH3 CH3

HO

3,5-Dimethylphenol OH

H3C

CH3

2,3,5-Trimethylphenol H3C CH3

H3C HO

1012 × k(298 1012 × A K)

Alcohol 2,3,6-Trimethylphenol

B (K)

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted Major Products (NOx present)d

118 ± 35

Mainly addition to the aromatic ring

Trimethyl-dihydroxybenzenes

104 ± 21

Mainly addition to the aromatic ring

Trihydroxybenzenes

198 ± 40

Mainly addition to the aromatic ring

Methyl-trihydroxybenzenes

156 ± 31

Mainly addition to the aromatic ring

Methyl-trihydroxybenzenes

5; 88; 8 3; 69; 27 58; 42 2; 94; 4 58; 42 17; (83) 30; 70

CH2FCHO CHF2CHO CF3CHO CxF2x+1CH2CHO CxF2x+1CHO CF3C(O)CF3 CF3C(O)CF3

CH3

HO

CH3

H3C

1,2-Dihydoxybenzene OH

OH

1,2-Dihydroxy-3-methylbenzene OH OH

CH3

1,2-Dihydroxy-4-methylbenzene HO HO

CH3

e) Halogenated alcohols CH2FCH2OH CHF2CH2OH CF3CH2OH CxF2x+1CH2CH2OH (x = 1, 4, 6, 8) CxF2x+1CH2OH (x = 2, 3, 4, 6) CF3CH(OH)CF3 (CF3)2CHOH

1.56 ± 0.31 0.49 ± 0.15 0.105 ± 0.015 0.94 ± 0.14 0.102 ± 0.020 0.025 ± 0.006 0.025 ± 0.003f

4.72

330

1.90

863

1.68 0.69

835 987

225

(continued)

226

TABLE IV-G-1 (CONTINUED)

Alcohol

1012 × k(298 1012 × A K)

(CF3)2C(OH)CH3

0.0078 ± 0.0008g 0.126 ± 0.031 0.102 ± 0.015 0.185 ± 0.07i 0.119 ± 0.03i (8.4 ± 0.8) × 10−4 h 1.28 ± 0.38 0.245 ± 0.074 0.116 ± 0.034

CF3CHFCF2CH2OH CHF2CF2CH2OH CF3CF2CH2OH (CF3)3COH CH2ClCH2OH CCl3CH2OH CxF2x+1CH(OH)2 (x =1, 3, 4) a

1.91 1.68 1.35 1.36

B (K)

811 835 605 730

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted Major Products (NOx present)d

(29); 71

CH2O, CF3C(O)CF3

3; 56; 41 7; 30; 69

CF3CHFCF2CHO CHF2CF2CHO

30; 70 100

CF3CF2CHO CF3C(O)CF3; COF2; CF3OH

24; 66; 10 57; 43 Reaction mostly at CH group

CH2ClCHO CCl3CHO CxF2x+1C(O)OH

Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated. The abbreviated code in column 5 of the table gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive >CH, –CH2–,–CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made the products shown are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations (shown in italics). e Orkin et al. (2012); temperature dependence represented best by 1.46 × 10−11 exp(−883/T) + 1.30 × 10−12 exp(371/T) cm3 molecule−1 s−1. f Orkin et al. (2012); temperature dependence represented best by 1.19 × 10−12 exp(−1,207/T) + 7.85 × 10−16 exp(502/T) cm3 molecule−1 s−1. g Orkin et al. (2012); temperature dependence represented best by 1.68 × 10−12 exp(−1,718/T) + 7.32 × 10−16 exp(371/T) cm3 molecule−1 s−1. h Orkin et al. (2012); temperature dependence represented best by 3.0 × 10−20 (T/298)11.3 exp(3,060/T) cm3 molecule−1 s−1. i Antiñolo et al. (2012). j Bernard et al. (2012). b

The Hydroxyl Radical and Its Role in Ozone Formation

227

Effect of chain length on the rate coefficient for the reaction of HO radical with alcohols of structure CH3(CH2)nOH and (CH3)2CH(CH2)nOH. For comparison, the alkane-HO rate coefficients are plotted with n = carbon number − 1.

1012 x k(298 K) for HO + alcohol, cm3 molecule–1 s–1

FIGURE IV-G-1.

FIGURE IV-G-2.

35

HOCH2CH(OH)CH2CH3

30

HOCH2CH(OH)CH3

25

HOCH2CH2OH

20 15 10

C2H5OH 5 0 Alcohol

Rate coefficients for the reactions of HO with a series of structurally related alcohols and diols.

IV-G-2. HO Reactions with Diols Substitution of a second HO group for an H atom in the ethanol molecule strongly enhances

the reactivity toward HO (see Figure IV-G-2). Lengthening the chain of carbon atoms in the ethane diol molecule increases the reactivity further.

228

the mechanisms of reactions influencing atmospheric ozone

IV-G-3. Mechanism of HO Reactions with Saturated Alcohols The mechanism of reaction of the HO radical with saturated alcohols is similar to that seen with the alkanes. For example, consider the reactions of the propanol molecule, where four different positions of attack by HO can occur. Product yields and other evidence (Calvert et al., 2011) suggest that the HO reaction proceeds largely through reaction (3), at the CH2 group attached to the –OH group: HO + CH3CH2CH2OH → H2O + •CH2CH2CH2OH (~3%)

(1)

→ H2O + CH3CH(•)CH2OH (~36%)

(2)

→ H2O + CH3CH2C(•)HOH (~60%)

(3)

→ H2O + CH3CH2CH2O• (~2%)

(4)

Reaction at the CH2 group one position down-chain from the  –OH group is also important, accounting for about 36% of the reaction. The subsequent reactions of the free radical formed in reactions (3)  and (2)  lead to propanal, ethanal, hydroxyacetone, hydroxyethanal, and other products shown in bold type:

CH3CH2CHOH + O2

CH3CH2C(O)H + HO2

CH3CHCH2OH + O2

CH3CH(O2)CH3OH2 (NO) CH3CH(O )CH2OH + NO2

(O2)

Major HOCH2C(O)H + CH3

CH3C(O)CH2OH + HO2

CH3C(O)H + CH2OH (O2) CH2O + HO2

IV-G-4. HO Reactions with Aromatic Alcohols The cresols (CH3C6H4OH), a type of aromatic alcohol, show a significantly enhanced reactivity for HO over that for the structurally related aromatic hydrocarbon toluene (CH3C6H5), with factors of 11, 9, and 8 for the m-, p-, and o-cresol, respectively (see Figure IV-G-3). As additional CH3 groups are added to replace ring H atoms in the cresols, a significant further enhancement in the HO rate coefficient occurs, almost a linear increase with the number of CH3 groups added. The mechanism of the HO reaction with the aromatic alcohols proceeds by three pathways:  (1)  addition to the aromatic ring, (2)  abstraction of H atoms from the HO group, and (3) abstraction of H atoms from alkyl groups attached to the ring. With o-cresol, the reactions of HO occur largely through pathways (1)  and (2). As noted previously, H abstraction from the tightly

bound H atoms attached to the aromatic ring is unimportant. IV-G-5. HO Reactions with Unsaturated Alcohols The unsaturated alcohols are very reactive with HO radicals as one expects in view of the high reactivity of HO with alkenes. Insertion of an HO group for an H atom in the CH3 group in propene to form HOCH2CH=CH2 increases k(298 K) by a factor of 9 (see Figure IV-G-4). As CH3 groups replace H atoms on the terminal CH2 group, further significant increases in k(298 K) occur. The unsaturated alcohols are some of the most reactive compounds toward HO, and their atmospheric oxidation can give a significant boost to ozone formation in the NOx/RH-polluted troposphere. The mechanism of the HO reaction with the simplest unsaturated alcohol, CH2=CHCH2OH,

The Hydroxyl Radical and Its Role in Ozone Formation

229

102 x k(298 K) for HO-aromatic alcohol reactions, cm3 molecule–1 s–1

160 2,3,5-Trimethylphenol 2,3,6-Trimethylphenol

140 120 m-Cresol p-Cresol o-Cresol

100 80 60

2,5-Dimethylphenol 2,3-Dimethylphenol 2,4-Dimethylphenol 2,6-Dimethylphenol

40 20 0

Toluene

0

1

2

3

Number of CH3-groups attached to aromatic ring FIGURE IV-G-3.

Comparison of the HO rate coefficients (298 K) for toluene and several structurally related aromatic alcohols with added CH3 groups.

1012 x k(298 K) for HO-unsaturated alcohol reactions, cm3 molecule–1 s–1

200

150

(CH3)2C=CHCH2OH

100

CH3CH=CHCH2OH

50

CH2=CHCH2OH CH2=CHCH3 0 Propene

FIGURE IV-G-4.

----------------------Unsaturated alcohol----------------------

Comparison of the k(298 K) for HO reaction with propene and some structurally related unsaturated

alcohols.

involves largely addition to the >C=C< bond (about 94 ± 12%) and with a smaller fraction of H-atom abstraction from the  –CH2OH group (5 ± 2%).

IV-G-6. HO Reactions with the Halogen-Atom-Substituted Alcohols The substitution of F atoms for H atoms in the alcohols results in a significant lowering of the k(298 K)

230

the mechanisms of reactions influencing atmospheric ozone 4

1012 x k(298 K) for HO reaction with F-atom-substituted ethanols

CH3CH2OH 3

CH2FCH2OH

2

1

CHF2CH2OH CF3CH2OH

0

0

1

2

3

Number of F-atom substituted for H-atoms in C2H5OH FIGURE IV-G-5.

The rate coefficients for HO reactions with F-atom-substituted ethanols at 298 K.

for HO reaction, as is seen with the hydrocarbons. This is illustrated in Figure IV-G-5. With one, two, and three F atom substitutions for H atoms in the CH3 group of ethanol, the effect on the rate coefficient is a nonlinear decrease from that of ethanol by factors of 0.49, 0.15, and 0.03, respectively. In summary, other than the halogen-atomsubstituted alcohols, the relatively large rate coefficients for the HO-alcohol reactions point to the potential importance of these compounds in influencing ozone generation in the polluted troposphere. I V- H . M E C H A N I S M S O F R E AC T I O N S O F H O WITH ETHERS The ethers are a family of compounds that have a characteristic structure containing the R3C–O–CR3 linkage. They have rate coefficients for the HO reactions that are somewhat greater than those of the alkanes and consequently have a significant potential to enhance ozone formation. The recommended rate coefficients for the HO-ether reactions (from Calvert et al., 2011) are given in Table IV-H-1. Plots of the complete datasets and evaluation of the individual measurements used in deriving these recommendations can be accessed in that work. IV-H-1. HO Reactions with Acyclic Ethers The rate coefficients measured experimentally for the HO reactions with the dialkyl ethers

(R–O–R) show a regular increase in magnitude as one progresses from the simplest, dimethyl ether, to di-n-pentyl ether (see Figure IV-H-1). The presence of the O atom in the ethers enhances the reactivity with HO from that observed for the alkanes. There is a regular increase in k(298 K) by about 3.8 × 10−12 cm3 molecule−1 s−1 with the addition of each new pair of CH2 groups. In the alkanes, the introduction of two CH2 groups in the chain creates an increase of about 2.7  × 10−12 cm3 molecule−1 s−1. The SAR expected values of the rate coefficients, shown by the gray stars, do not show the same magnitude of increase in k(298 K) with carbon number, and the slope of the curve approaches that observed for the alkanes as the carbon number reaches about 6.  Note that, as with the alcohols, with the exception of the data point at C4, the SAR estimates fall within the limits set by the error bars on the experimental values, and evidence for a long-range enhancement resulting from the  –O– group in the ethers is not convincing. With diethyl ether (CH3CH2OCH2CH3) and most of the larger dialkyl ethers, a small decrease in the rate coefficient occurs with increase in temperature. The B factor (Ea/R) in the relation describing the rate coefficient (k = A e−B/T) is negative. The mechanism of HO reactions with the dialkyl ethers can be illustrated for diethyl ether (Calvert et  al., 2011), as shown in Figure IV-H-2. HO radical abstraction of an H atom from diethyl ether occurs largely at the  –CH2– groups. As seen

TABLE IV-H-1 . HO RE ACTIONS WITH THE ETHER S. THIS TABLE GIVES THE RECOMMENDED R ATE COEFFICIENTS, k = A × T n × e −B/T 3

(CM MOLECULE −1 S −1 ) FOR RE ACTIONS AT TROPOSPHERIC TEMPER ATURES. a COLUMN 6 SHOWS STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES (USING TABLES IV-O-1; IV-O-2, IV-O-5, AND IV-O-6) OF THE PERCENTAGE OF RE ACTION BY ADDITION (PREFIX “A”) AND H-ATOM ABSTR ACTION AT E ACH SITE. b COLUMN 7 GIVES THE MA JOR PRODUCTS THAT ARE OBSERVED OR PREDICTED (ITALICS) FOR NO X-CONTAINING ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN c

Ethers

1012 × 1012 × A B (K) k(298 K)

231

a) Acyclic ethers Dimethyl ether (CH3OCH3) Methyl ethyl ether (CH3OCH2CH3)

2.7 ± 0.4 6.3 ± 3.2

Diethyl ether (CH3CH2OCH2CH3) Methyl n-butyl ether (CH3OCH2CH2CH2CH3) Methyl tert-butyl ether [CH3OC(CH3)3] Di-n-propyl ether (CH3CH2CH2OCH2CH2CH3) Di-iso-propyl ether [(CH3)2CHOCH(CH3)2] Ethyl n-butyl ether (CH3CH2OCH2CH2CH2CH3) Ethyl tert-butyl ether [CH3CH2OC(CH3)3] Methyl tert-amyl ether [CH3OCH2C(CH3)3] Isopropyl isobutyl ether [(CH3)2CHOCH2CH(CH3)2] Di-n-butyl ether (CH3CH2CH2CH2OCH2CH2CH2CH3) Di-isobutyl ether [(CH3)2CHCH2OCH2CH(CH3)2] Di-tert-butyl ether [(CH3)3COC(CH3)3] Di-n-pentyl ether (CH3CH2CH2CH2CH2OCH2CH2CH2CH2CH3)

5.7

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

222

0 50; 50 12; 86; 2

12.2 ± 1.8 4.62 14.5 ± 3.6 10.1

−289 −107

0 1: 49; 49; 1 0 8; 71; 10; 8; 1

3.1 ± 0.5 20 ± 3

4.0 10.3

80 −197

0 73; (9, 9, 9) 0 1; 5; 44; 44; 5; 1

CH3OC(O)H CH3OC(O)H; CH3OCOCH3; CH2O CH2O C2H5OC(O)H; CH2O; C2H5OC(O)CH3; CH3CHO CH3OC(O)H; C2H5CHO; CH3C(O)CH2CH2OCH3 and/or CH3CH2C(O)CH2OCH3; HOCH2OCH2C(O)CH2CH3? O=CHOC(CH3)3; CH3OC(O)CH3; CH2O C3H7OC(O)H; CH3CHO; C2H5CHO; C3H7OC(O)C2H5

10.0 ± 1.5 2.97 22 ± 3 6.6

−361 −362

0 (0.5, 0.5); 49; 49; (0.5, 0.5) 0 1; 38; 47; 6; 1

iso-C3H7OC(O)CH3; CH2O C4H9OC(O)H; C2H5OC(O)H; CH3CHO; C3H7CHO

8.7 ± 1.7 5.6 ± 0.8 21 ± 7

3.4 3.13

−280 −172

28 ± 4

4.5

−550

0 2; 92; (2, 2, 2) 0 10; 85; (2, 2, 2) (0.5, 0.5); 61; 29; 9; (0.5, 0.5) 0 1; 5; 6; 38; 38; 6; 5; 1

tert-C4H9OC(O)H; CH2O; tert-C4H9OC(O)CH3; CH3CHO C2H5C(CH3)2OC(O)H; CH2O; CH3OC(O)CH3; CH3CHO CH3C(O)CH3; iso-C3H7OCHO; iso-C4H9OC(O)CH3; CH2O; (CH3)2C(OH)CH2OC(O)CH3 C4H9OC(O)H; C2H5CHO; C3H7CHO; C4H9OC(O)C3H7

(0.7, 07); 10; 39; 39; 10; (0.7; 0.7) (17, 17, 17); (17; 17, 17) 0 0.1; 4; 5; 5; 35; 35; 5; 4; 0.1

CH3COCH3; (CH3)2CHCH2OCHO

26 ± 8 3.7 ± 0.9 33 ± 5

7.8

−429

CH2O; (CH3)3COC(O)CH3 C5H9OC(O)H; HOCH2CH2CH2CHO (continued)

232

TABLE IV-H-1 (CONTINUED)

Ethers

b) Multi-functional ethers Dimethoxymethane (CH3OCH2OCH3) 2-Methoxyethanol (CH3OCH2CH2OH) 2-Ethoxyethanol (CH3CH2OCH2CH2OH)

1012 × 1012 × A B (K) k(298 K)

4.6 ± 0.7 12 ± 2 19 ± 5

2.7 × 10−6 −879 6.0 × 10−6 −938

n

Estimated % H-atom Abstraction at Each Siteb

2 11; 78; 11 2 8; 64; 27; 1 0.7; 36; 44; 19; 0.6

1,2-Dimethoxyethane (CH3OCH2CH2OCH3) 1-Methoxy-2-propanol [CH3OCH(CH3)CH2OH] 1,1-Dimethoxyethane [(CH3O)2CHCH3] 2-Propoxyethanol (CH3CH2CH2OCH2CH2OH) Diethoxymethane (CH3CH2OCH2OCH2CH3) 1,2-Dimethoxypropane [CH3OCH2CH(OCH3)CH3] 2,2-Dimethoxypropane [CH3C(OCH3)2CH3] 2-Isopropoxyethanol [(CH3)2CHOCH2CH2OH]

28 ± 6 5.3 × 10−6 −1,220 20 ± 4 8.9 ± 3.1 17.3 ± 3.5 19.7 ± 3.0 3.8 × 10−6 −1,212 ~ 31 ± 16

2 5; 45; 45; 5 7; 49; (1); 42; 1 (6, 6); 87; 1 0.7; 5; 42; 34; 18; 0.6 2 0.6; 33; 33; 33; 0.6 4; 34; 57; (4); 0.6

4.1 ± 1.0 21 ± 5

0 7; (43, 43); 7 (0.5, 0.5) 57; 27; 14; 0.5

1-Ethoxy-2-methoxyethane [CH3CH2OCH2CH2OCH3) 3-Methoxy-1-butanol [HOCH2CH2CH(OCH3)CH3] 3-Ethoxy-1-propanol (HOCH2CH2CH2OCH2CH3) 2-Isopropoxyethanol HOCH2CH2OCH(CH3)2] 1.3-Dimethoxypropane (CH3OCH2CH2CH2OCH3)

17.5 ± 6.1

1; 31; 31; 31; 6

23.6 ± 8.3

0.4; 14; 10;70; (4); 0.4

22.0 ± 7.7

0.5; 17; 12; 39; 31; 0.7

2-n-Butoxyethanol (CH3CH2CH2CH2OCH2CH2OH) 1,2-Diethoxyethane (CH3CH2OCH2CH2OCH2CH3)

3.73

−30

20.6 ± 7.2 49.3 ± 17.2 30 ± 15 57 ± 14

5.8 × 10−6 −1,400

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

CH3OCH2OC(O)H; CH3OC(O)OCH3 CH3OC(O)H; CH2O; CH3OCH2C(O)H HC(O)OCH2CH2OH; C2H5OC(O)H; C2H5OCH2CHO; CH3C(O)OCH2CH2OH CH3OC(O)H CH3OC(O)H; CH3CHO; CH3OCH2C(O)CH3 CH3OC(O)OCH3; CH2O C3H7OC(O)H; CH2O; C3H7OCH2CHO; 2-ethyl-1,3-dioxolane C2H5OC(O)OC2H5; CH2O; C2H5OCH2OC(O)H CH3OC(O)CH3; CH3OC(O)H; CH2O (CH3)2C(OCH3)OCHO; CH2O iso-C3H7OC(O)H; HOCH2CH2OC(O)CH3; CH2O; HOCH2CH2OC(O)CH3 CH3CHO; HC(O)OC2H5; CH3C(O)OCH2OCH2OCH3; CH3CH2OC(O)CH2OCH3; CH3CH2OCH2C(O)OCH3 HOCH2CH2C(O)OCH3; CH2O; CH3C(O)OCH3; HOCH2CHO HOCH2CH2CH2OC(O)CH3; CH2O; HOCH2CH2C(O)OCH2CH3

0.5; 13; 32; 53; (0.5, 0.5) CH3C(O)CH3; HOCH2CHO 5; 42; 6; 42; 5 CH3OC(O)CH2CH2OCH3; CH3OCH2CHO; CH3OCH2CHO; HC(O)OCH3; CH3OCH2CHO 0.6; 4; 4; 37; 37; C4H9OC(O)H; C2H5CHO; HOCH2CH2OC(O)H; 16; 0.5 HC(O)OCH2CH2CH(OH)CH3; C4H9OCH2CHO 2 0.5; 22; 27; 27; 22; 0.5 C2H5OC(O)H; CH2O; HC(O)OCH2CH2OCH2CH3

Ethers

1012 × 1012 × A B (K) k(298 K)

1,4-Dimethoxybutane 30 ± 6 (CH3OCH2CH2CH2CH2OCH3) Di-n-propoxymethane 27 ± 4 (CH3CH2CH2OCH2OCH2CH2CH3) 2,2-Diethoxypropane 11.1 ± 2.8 [H3CC(OCH2CH3)2CH3] Di-isopropoxymethane 38 ± 6 [(CH3)2CHOCH2OCH(CH3)2] 1-Butoxy-2-propanol 38 ± 10 [CH3CH2CH2CH2OCH(CH3)CH2OH] Di-n-butoxymethane 36 ± 5 [CH3CH2CH2CH2OCH2OCH2CH2CH2CH3) Di-sec-butoxymethane 43 ± 6 [CH3CH2CH(CH3)OCH2OCH(CH3)CH2CH3] Di-iso-butoxymethane 36 ± 9 [(CH3)2CHCH2OCH2OCH2CH(CH3)2] 1,1,3-Trimethoxypropane [(CH3O)2CHCH2CH2 16.7 ± 5.8 OCH3] Trimethoxymethane [(CH3O)3CH] 6.0 ± 2.1 Diethylene glycol ethyl ether 57.2 ± 20.0 (CH3CH2OCH2CH2OCH2CH2OH) Diethylene glycol n-butyl ether 74.4 ± 26.0 (CH3CH2CH2CH2OCH2CH2OCH2CH2OH) c) Alkoxy-substituted ethers Methoxymethyl formate [CH3OCH2OC(O)H] Methoxymethyl acetate [CH3OCH2OC(O)CH3] Ethoxymethyl formate [CH3CH2OCH2OC(O)H] Ethoxymethyl acetate [CH3CH2OCH2OC(O)CH3]

1.7 ± 0.5 2.8 ± 0.8 4.6 ± 1.4 6.8 ± 2.0

n

Estimated % H-atom Abstraction at Each Siteb 5; 40; 6; 6; 40; 5

4.3

−546

10.6

−15

4.4

−646

5.2

−580

8.7

−477

0 0.5; 4; 30; 30; 30; 4; 0.5 0 1; (48; 1); (48; 1); 1

−56 −134 −317 −380

CH3OC(O)H; HOC(O)CH2CH2OCH3; CH3OC(O)CH2CH(OH)OCH3 C3H7OC(O)OC3H7; C3H7OCH2OCHO; CH3CHO CH3CH2OC(CH3)2OCHO; CH2O

0 (0.4, 0.4); 40; 19; 40; (0.4, CH3C(O)OCH2OCH(CH3)2; (CH3)2CHOC(O)CH(CH3)2 0.4) 0.5; 3; 4; 23; 57; (0.5);12; CH3CHO; C4H9OC(O)H; C2H5CHO; C4H9OCH2C(O)CH3 0.4 0 0.4; 3; 4; 28; 28; 28; 4; C4H9OCH2OC(O)H; C2H5CHO; C4H9OC(O)OC4H9 3; 0.4 0 0.3; 2; 38; (0.3); 18; CH3CHO; sec-C4H9OCH2OC(O)CH3; sec-C4H9OC(O)O-sec-C4H9 38; (0.3); 2; 0.3 (0.5, 0.5); 7; 28; 28; iso-C4H9OCH2OC(O)H; CH3C(O)CH3 28; 7; (0.5, 0.5) (3,3); 61; 4; 24; 3 CH3OC(O)OCH3; CH3OCH2CHO (6, 6, 6); 82 0.4; 19; 23; 23; 23; 10; 0.3 0.3; 3; 3; 22; 22; 22; 18; 9; 0.3

1.4 1.8 1.6 1.9

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

0 0 0 0

19; 79; 2 8; 91; 0.2 1; 61; 37; 0.7 1; 38; 61; 0.2

CH3OC(O)OCH3; CH2O CH2O; HOCH2CH2OC(O)H; C2H5OC(O)H; CH3CH2OCH2CH2CHO; HOCH2CH2OCH2CH2OCHO CH2O; CH3CH2CHO; HOCH2CH2OCHO; n-C4H9OCHO; n-C4H9OCH2CH2OCHO ? ? ? ?

(continued)

233

234

TABLE IV-H-1 (CONTINUED)

Ethers

2-Methoxyethyl formate [CH3OCH2CH2OC(O)H] 2-Methoxyethyl acetate [CH3OCH2CH2OC(O)CH3] 2-Ethoxyethyl formate [CH3CH2OCH2CH2OC(O)H] 2-Ethoxyethyl acetate [CH3CH2OCH2CH2OC(O)CH3] 2-Ethoxyethyl isobutyrate [CH3CH2OCH2CH2OC(O)CH(CH3)2] Ethyl 3-ethoxypropionate CH3CH2OC(O) CHCH2OCH2CH3] d) Unsaturated ethers Methyl vinyl ether (CH3OCH=CH2) Ethyl vinyl ether (CH2CH2OCH=CH2) n-Propyl vinyl ether (CH3CH2CH2OCH=CH2) n-Butyl vinyl ether (CH3CH2CH2CH2OCH=CH2) Isobutyl vinyl ether (CH3)2CHCH2OCH=CH2] tert-Butyl vinyl ether [(CH3)3COCH=CH2] Ethylene glycol monovinyl ether (HOCH2CH2OCH=CH2) Ethylene glycol divinyl ether (CH2=CHOCH2CH2OCH=CH2) Diethylene glycol divinyl ether (CH2=CHOCH2CH2OCH2CH2OCH=CH2) e) Cyclic ethers

1012 × 1012 × A B (K) k(298 K)

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

6.5 ± 2.0

2.2

−324

0 12; 80; 7; 1

?

8.2 ± 2.5

2.9

−309

0 10; 83; 6; 1

?

11.3 ± 3.4 2.9

−405

0 1; 43; 52; 4; 0.4

?

12.1 ± 3.6 1.4

−642

0 1; 45; 45; 9; 0.5

14.8 ± 4.4

0.8; 38; 47; 9; 3; (1, 1)

22 ± 7

CH3C(O)CH2CH2OCHO; C2H5OCHO; CO; CH3(CO)OH; HC(O)OC(O)CH3 ?

1; 8; 2; 49; 40; 1

C2H5OCHO; CH3CHO; C2H5OC(O)CH2OC(O)C2H5; CH3CH2OC(O)CH2CH2OCHO; HC(O)CH2C(O)OC2H5

3; A 97 0.4; 19; A 81 0.3; 3; 22; A 76 0.3; 3; 3; 21; A 73

CH3OCHO; CH2O C2H5OCHO; CH2O C3H7OCHO; CH2O C4H9OCHO; CH2O

34 ± 9.5 73 ± 11 100 ± 15 109 ± 16

6.1 16.4 9.3 16

−511 −445 −708 −572

0 0 0 0

107 ± 16 107 ± 16 104 ± 31

16 17

−567 −549

0 (0.4, 0.4); 5; 21; A 73 0 (0.5, 0.5, 0.5); A 99 0.2; 7; 35; A 58

iso-C4H9OCHO; CH2O tert-C4H9OCHO; CH2O HOCH2CH2OCHO; CH2O

127 ± 38

A 39; 11; 11; A 39

CH2=CHOCH2CH2OCHO; CH2O

144 ± 43

A 32; 9; 9; 9; 9; A 32

CH2O; HC(O)OCH2CH2OCH2CH2OCH=CH2

1012 × 1012 × A B (K) k(298 K)

Ethers

0.091 ± 0.023

Ethylene oxide O H2C a

~ 0.50 ± 0.25 0.30 ± 0.10’

O

HC a

HC(O)O(O)CH; CO; HC(O)OH; CH2O

a 53; b 25; c 22

CH3C(O)OH; CO; HC(O)O(O)CCH3

1.9 ± 0.7

a 57; b 25; c 10; d 9

CH3CH2C(O)OH; CO; C2H5C(O)OC(O)H

10.3 ± 2.6

a 47; b 7

O

H3C d H2C a

HC b

Oxetane b H2 C

CH2 a

a H2 C

O

Tetrahydrofuran O

b H2C

2 a 50

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

CH2 b

Ethyl oxirane

a H2C

856

Estimated % H-atom Abstraction at Each Siteb

CH2 a

Methyl oxirane

H3C c

1.81 × 10−5

n

CH2 c

17.0 ± 2.6 9.4

−177

0 a 44; b; 6

?

HC(O)OCH2CH2CHO

CH2 a CH2 b

(continued)

235

236

TABLE IV-H-1 (CONTINUED)

1012 × 1012 × A B (K) k(298 K)

Ethers

2-Methyl-tetrahydrofuran

22.3± 4.5 2.20

n

Estimated % H-atom Abstraction at Each Siteb

−690

0 a 61; b 29; c 4; d 1

CH3C(O)OCH2CH2CHO

O

d H3C a

CH2 b

HC

CH2 c

c H2C

Tetrahydropyran

12 ± 2

7.8

−136

0 a 41; b 6

HC(O)OCH(OH)CH2CH2CHO

18 ± 5

9.8

−179

0 a 39; b 6

HC(O)OCH2CH(OH)CH2CH2CHO

O CH2 a

a H2C b H2C C H2 b

CH2 b

Oxepane O

a H2C

CH2 a

b H2C

CH2b

b H2 C

CH2 b

10 ± 4

1,3-Dioxolane

a 29; b 36

O

O

HC(O)OCH2OCHO; H2C

CH2 a

b H2C

H2C

O

b H2C

11.5 ± 2.3 6.8

1,4-Dioxane O CH2 a

a H2C a H2C

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

O

CH2 a

−159

0 a 25

HC(O)OCH2CH2OCHO

C O

O

1012 × 1012 × A B (K) k(298 K)

Ethers

O

1,3-Dioxane

10.0 ± 1.5 8.6

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd HC(O)OCH2OCHO; CH2O; HC(O)OCH(OH)OCH2CHO; HC(O)OC(O)OCH2CH2OH; HC(O)OH; HOCH2CHO; CH2O

C H2 b

11.3 ± 2.8

4-Methyl-1,3-dioxane

b H2C

0 a 37; b 34; c 5

O

c H2C

O

Estimated % H-atom Abstraction at Each Siteb

CH2 a

b H2C

b H2 C

−45

n

a 51; b 25; c 4; d 0.4

?

O a HC

CH3 d

CH2 c

1,3-Dioxepane b H2C

O

14 ± 3

6.0

−250

0 a 26; b 32; c 5

5.4 ± 0.8

15

302

0 a 33

?

CH2 a

c H2C

O

c H2C

1,3,5-Trioxane

CH2 b O

HC(O)OCH2OCHO

CH2 a

a H2C O

O C H2 a

Glycidaldehyde

O cH C 2

C H b

a CHO

16 ± 6

40 ± 8

Furan

a 97; b 2; c 1

13

−333

0 a A 50

HC(O)OCH(O); HC(O)OH; CH2O; CO2

HC(O)CH=CHC(O)H

O HC a HC

CH a CH

(continued)

237

238

TABLE IV-H-1 (CONTINUED)

1012 × 1012 × A B (K) k(298 K)

Ethers

2-Methylfuran O HC a HC

C a CH

CH3 b

2-Ethylfuran C a CH CH a CH3 c

C

HC b

O C a CH

C a HC

2,5-Dihydrofuran O b H2C HC

67 ± 23 73 ± 4g

a A 30; b 39; c 1

116 ± 41

a A 59; b A; 38; c 3; d 0.2

?

93 ± 33 113 ± 22e 110 ± 20f 87 ± 2g

a A 60; b A 39; c 1

?

132 ± 46 120 ± 4g

a A 49; b 1

?

218 ± 65

a A 78; b 11

?

CH3C(O)CH=CHC(O)H

C H

2,5-Dimethylfuran b H3C

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

CH3d

3-Methylfuran O

Estimated % H-atom Abstraction at Each Siteb

H2 c C

O HC b HC

n

CH2b a

CH

CH3 b

1012 × 1012 × A B (K) k(298 K)

Ethers

2-Furfural C a CH

3-Furfural H C a C

O HC

b

a A 51; b A 33; c 16

?

49 ± 15

a A 51; b A 33; c 16

?

51 ± 15

a A 43; b A 43; c 13; d 0.1

?

O C a CH

74 ± 22

a A 54; b A 39; c 7; d 0.2; e 0.2

c CHO

5-Methyl-5-vinyl-tetrahydrofuran-2-ol H2C

35 ± 11

CH

C b HC b

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

CHO c

5-Methyl-2-furfural d H3C

Estimated % H-atom Abstraction at Each Siteb

c CHO

O HC b HC

n

CH

d H3C

O C

c H C

CH3C(O)CH2CH2CHO; HOCH2CHO From isomer: CH2=CHC(OH)(CH3)CH2CH2CHO

OH e

HC a CH

(continued)

239

240

TABLE IV-H-1 (CONTINUED)

1012 × 1012 × A B (K) k(298 K)

Ethers

Benzene oxide H a C HC

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

100 ± 30

a A 49; b 0.3

HC(O)CH=CHCH=CHCHO

210 ± 63

a A 49; b 0.3

CH3C(O)C=CHCH=CHCHO

10.4 ± 3.6

a 32; b 15; c 2

?

a A 95; b 5

?

b CH O CH b

HC a C H

Toluene oxide d H C

c H3 C a

C

O CH d

C C b H

CH

1,8-Cineole a H C

b H2C b H2C C

c H3C

CH3 c

b CH2 C b CH2 O

f) Aromatic ethers Methoxybenzene H C HC

CH3 c

a

HC

C CH

C H

O

CH3 b

18 ±5

5.2

−370

0

1012 × 1012 × A B (K) k(298 K)

Ethers

36 ± 11

1,2-Dmethoxybenzene H C HC

a

HC

C H

O

C C

O

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

a A 99; b 0.4

?

a A 50

?

40 ± 12

a A 83; b 15; c 2

?

28 ± 8

a A 65; b 17

?

37 ± 13

a A 40; b 28; c 3

?

CH3 b CH3 b

Diphenyl ether a

n

~ 8.2

O

a

2,3-Dihydrobenzofuran O a

CH2 b C c H2

1,4-Benzodioxan O CH2 b

a O

CH2 b

Isochroman H2 b C O

a C H2 c

CH2 b

(continued)

241

242

TABLE IV-H-1 (CONTINUED)

1012 × 1012 × A B (K) k(298 K)

Ethers

1,3,4,6,7,8-Hexahydro-4,6,6,7,8,8hexamethylcyclopenta[γ]-2-benzopyran CH3 e

H3C e

c HC b H 2C

C a

O

n

26 ± 9

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

a A 55; b 17; c 5; d 3; e 0.3

?

CH3 e d CH CH3 e

C CH3 e e H3C

C b H2

~ 3.8

Dibenzofuran

~ 13.5 ~ 374

0

a A 50

?

45 ± 23

1.7

−979

0

a A 50

?

0.035 ± 0.009 0.06 (± factor of 2) 0.012 ± 0.002 0.0023 ± 0.0005 0.00041 ± 0.00008

6.0

1,530

0

14; 86 50; 50

CHF2OCHO HC(O)F; CH2FOC(O)F

2.1 1.17

1,540 1,855

0 0

100 50; 50

CF3OCHO COF2

0.47

2,100

0

100

COF2

O a

a

Dibenzo-p-dioxin O a

a O

g) Halogen atom-substituted ethers CH3OCHF2 (HFE-152a) CH2FOCH2F (HFE-152) CH3OCF3 (HFE-143a) CHF2OCHF2 (HFE-134) CF3OCHF2 (HFE-125)

1012 × 1012 × A B (K) k(298 K)

Ethers

CH3CH2OCF3 CF3CH2OCH3 CH3OCF2CHF2 CF3CHFOCH3 CF3CH2OCHF2 CH2FCF2OCHF2 CH3OCF2CF3 CHF2CHFOCF3 CF3CHFOCHF2 CHF2OCF2CHF2 CF3CHFOCF3 CF3OCF2CF2H Perfluorooxetane 2,2,3,4,4-Pentafluorooxetane O F2C

0.15 ±0.05 0.59 ± 0.12 0.0222 ± 0.0044 0.16 ± 0.04 0.011 ± 0.003 0.0054 ± 0.0011 0.012 ± 0.002 0.0067 ± 0.0033 0.0057 ± 0.0029 0.0025 ± 0.0005 0.0011 ± 0.0003 0.0021 ± 0.0004 < 0.0002 ≤ 0.0025

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

2.6

1,420

0

51; 49 90; 10 89; 11

2.0 2.5 1.11

738 1,610 1,588

0 0 0

79; 21 60; 40 84; 16

HC(O)CHFCF3; FC(O)OCH3; COF2 HC(O)CHF2; CF3C(O)OCHF2; COF2 COF2; HC(O)F

1.90

1,510

0

100 47; 53

HC(O)OCF2CF3 COF2; FC(O)OCF3

43; 57

CF3C(O)F, CF2O

CH3C(O)OCF3; CH2O, HC(O)OCF3 CF3CH2OCHO; CF2O HC(O)OCF2CHF2

1.2

1,837

0

90; 10

COF2

0.439

1,780

0

100

CF3O(O)CF, COF2

0.53

1,655

0

100

COF2

0 100

None CF2O

51; 48; 1 0; 11; 89 11; 89 42; 58

HC(O)OCF2CF2H; CH2O; CH3C(O)OCF2CF2H CHF2CF2C(O)OCH3 CF3CF2C(O)OCH3O HC(O)OCH(CF3)2; CF3C(O)OCH3; COF2

CF2 CFH

CH3CH2OCF2CF2H CHF2CF2CH2OCH3 CF3CF2CH2OCH3 CH3OCH(CF3)2

0.22 ± 0.04 0.93 ± 0.33 0.68 ± 0.24 0.12 (± factor of 2)

2.57

730

0

(continued)

243

244

TABLE IV-H-1 (CONTINUED)

Ethers

CF3CHFCF2OCH3 CF3CH2OCH2CF3 CHF2OCH2CF2CHF2 CF3CH2OCF2CF2H CH3OCF(CF3)2 CH3OCF2CF2CF3 CHF2OCH2CF2CF3 CHF2CF2OCHFCF3 CF3CFHCF2OCF2H (CF3)2CHOCHF2 CF3CFHCF2OCF3 CF3OCH(CF3)2 CF3CHFCF2OCH2CH3 (CF3)2CFCF2OCH3 (iso-HFE-7100) CF3CF2CF2CF2OCH3 (n-HFE-7100) CF3CHFCF2OCH2CF3 CF3CF2CF2OCHFCF3 CH3OCF2CF2CF2CF2CF3

1012 × 1012 × A B (K) k(298 K) 0.021 (± factor of 2) 0.137 ± 0.027 0.0162 ± 0.0032 0.0093 ± 0.0013 0.015 ± 0.003 0.0117 ± 0.0023 0.011 ± 0.002 0.0041 ± 0.0012 0.0016 ± 0.0008 0.0028 ± 0.0007 0.00155 ± 0.00034 0.00030 ± 0.00009 0.14 ±0.06 0.012 ± 0.004 0.012 ± 0.004 0.0087 ± 0.0013 < 0.001 0.012 ± 0.006

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

9; 91

HC(O)OCF2CHFCF3

1.7 2.49

751 1,500

0 0

50; 50 70; 27; 3

HC(O)OCH2CF3; CF3C(O)OCH2CF3; COF2 CHF2OC(O)CF2CHF2; CHF2OCHO; COF2

1.0

1,394

0

50; 50

HC(O)OCF2CF2H; COF2

1.94 2.06

1,450 1,540

0 0

100 100

HC(O)OCF(CF3)2 HC(O)OCF2CF2CF3

2.14 0.55

1,580 1,462

0 0

72; 28 12; 88

HC(O)OCHF2; COF2 FC(O)CF3; COF2

43; 57

CF3C(O)F; COF2

0.96

1,738

0

18; 82

(CF3)2CHO; COF2

0.41

1,662

0

100

CF3C(O)F; COF2

100

CF3OC(O)CF3; COF2

1; 54; 46 100 100 36; 64

HC(O)OCF2CHFCF3; CH2O; CH3C(O)OCF2CHFCF3 iso-C4F9OCHO C4F9OCHO CF3CHFCF2OC(O)CF3

100 100

FC(O)OCF2CF2CF3; COF2; CF3CF2CF2OC(O)CF3 HC(O)OCF2CF2CF2CF2CF3

1.2

1,469

0

Ethers

CF3CHFCF2OCH2CF2CF3 CF3CHFCF2OCH2CF2CHF2 CF3CF2CF2CF2OCH2CH3 (n-HFE-7200) (CF3)2CFCF2OCH2CH3 (i-HFE-7200) CH3OCH2CH2Cl CH3OCH2CHCl2 CH3OCH2CH2Br CH3OCF2CHFCl CHF2OCF2CHFCl (Enflurane) CHF2OCHClCF3 (Isoflurane) CH2FOCH(CF3)2 (Sevoflurane) CF3CHFOCHF2 (Desflurane) CF3CH2OCClF2 CF3CHClOCH2CH3 n-C3F7CF(OCH2CH3)CF(CF3)2 (HFE-7500) CHF2OCF2OCHF2 CHF2OCF2CF2OCHF2 CHF2OCF2CF2OCF2OCHF2 CH3O(CF2CF2O)mCH3 CF3OCF(CF3)CF2OCF2OCF3

1012 × 1012 × A B (K) k(298 K) 0.0063 ± 0.0016 0.0129 ± 0.0032 0.065 ± 0.020 0.079 ± 0.024 4.3 ± 1.5 2.1 ± 0.7 6.3 ± 1.9 0.038 ± 0.008 0.012 ± 0.002 0.015 ± 0.003 0.015 ± 0.015f 0.027 ± 0.014 0.039 ± 0.003f 0.0039 ± 0.0003f 0.040 ± 0.012 < 0.30 0.022 ± 0.008 0.0023 ± 0.0012 0.0045 ± 0.0023 0.0044 ± 0.0022 0.027 ± 0.007 < 0.00063

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

0.846

1,460

0

44; 56

CF3CHFCF2OC(O)H; COF2; CF3CHFCF2OC(O)CF2CF3

2.44

1,563

0

41; 53; 6

CF3CHFCF2OC(O)H; COF2; CF3CHFCF2OC(O)CF2CHF2

49; 51 49; 51 26; 64; 10 27; 65; 8 23; 71; 6 66; 34 20; 80 46; 54

CH3C(O)OCF2CF2CF2CF3; HC(O)OCF2CF2CF2CF3 CH3C(O)OCF(CF3)CF2CF3; HC(O)OCF(CF3)CF2CF3 HC(O)OCH2CH2Cl; HC(O)Cl; CH3OC(O)CH2Cl; CH3COCHO HC(O)OCH2CHCl2; COCl2 CH3OC(O)CHCl2; CH3OCHO HC(O)OCH2CH2Br; CH3OCHO; CH3OC(O)CH2Br HC(O)OCF2CHFCl FC(O)Cl; CF2O; COF2 CF3C(O)OCHF2

96; 4

FC(O)OCH(CF3)2

60; 40

CF3C(O)F; CF2O

100 43; 40; 16 49; 51 50; 50

HC(O)OCClF2; COF2; CF3C(O)OCClF2 HC(O)OCHClCF3; CH2O; CH3C(O)OCHClCF3 C3F7CF[OC(O)CH3]CF(CF3)2; C3F7CF[OC(O)H]CF(CF3)2 COF2

50; 50

COF2

50; 50

COF2

50; 50 0

HC(O)O(CF2CF2O)mCH3 No reaction

2.59 0.746 1.12

1,260 1,230 1,280

0 0 0

1.00 0.705

969 1,551

0 0

1.6

1,100

0

(continued)

245

246

TABLE IV-H-1 (CONTINUED)

1012 × 1012 × A B (K) k(298 K)

Ethers

n-C4F9O(CH2)3O-n-C4F9 CF3CFHCF2O(CH2)3OCF2CFHCF3 2-Ethoxy-3,3,4,4,5-pentfluoro-tetrah ydro-2,5-bis[1,2,2,2-tetrafluoro-1(trifluoromethyl)ethyl]-furan

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

11; 78; 11 0.1; 11; 78; 11; 0.1 a 49; b 51

C4F9OCHO; C4F9OC(O)CH2CH2OC4F9 CF3CFHCF2OCHO; CF3CFHCF2OC(O)CH2CH2OCF2CFHCF3

Addition to double bond Addition to both rings with reaction with ring b somewhat favored

COF2; CF3OCOF; FC(O)OF

F3C

F3C C F FC

F3C

0.13 ± 0.05 0.23 ± 0.08 0.055 ± 0.017

n

CF2`

O

FC C

CF3 H2 a C CH3 b

O

CF2

CF3OCF=CF2

~ 2.8

1-Chlorodibenzodioxin

~ 4.4

Cl O a

?

b O

2-Chlorodibenzodioxin

~ 39

O

Cl a

b O

Addition to both rings with reaction with ring b somewhat favored

?

1012 × 1012 × A B (K) k(298 K)

Ethers

2,3-Dichlorodibenzodioxin a Cl

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

~ 22

Addition to both rings with reaction with ring b somewhat favored

?

~ 7.4

Addition to both rings

?

~ 7.1

Addition to both rings

?

~ 18

Addition to ring b is favored

?

~ 3.4

OH addition with Cl-atom elimination; this mechanism is also possible with highly Cl-atom-substituted congeners

O

Cl

n

b O

2,7-Dichlorodibenzodioxin O

Cl a

a Cl

O

2,8-Dichlorodibenzodioxin O

Cl

Cl

O

1,2,3,4-Tetrachlorodibenzodioxin Cl O

Cl a Cl

b O

Cl

Octachlorodibenzodioxin Cl

Cl O

Cl

Cl

Cl

O

Cl

247

Cl

?

Cl

(continued)

248

TABLE IV-H-1 (CONTINUED)

1012 × 1012 × A B (K) k(298 K)

Ethers

2-Bromodiphenyl ether

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

~ 4.7

Addition to both rings with reaction with ring b somewhat favored

?

~ 4.6

Addition to both rings with reaction with ring b somewhat favored

?

~ 5.8

Addition to both rings with reaction with ring b somewhat favored

?

~ 1.3

Addition to both rings

?

~ 3.9

Addition to both rings with reaction with ring b somewhat favored

?

Br O a

n

b

3-Bromodiphenyl ether O

Br

b

a

Bromodiphenyl ether O a

b

Br

2,2’-Dibromodiphenyl ether Br

Br O

a

a

2,4-Dibromodiphenyl ether Br O a Br

b

1012 × 1012 × A B (K) k(298 K)

Ethers

3,3’-Dibromodiphenyl ether O

Br a

n

Estimated % H-atom Abstraction at Each Siteb

Observed or Predicted (italics) Major Products for NOx-containing Atmospheresd

~ 3.2

Addition to both rings

?

~ 4.9

Addition to both rings

?

Br a

4,4’-Dibromodiphenyl ether O a Br a

a Br

Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated. The abbreviated code in column 6)of the first part of the table gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive >CH, –CH2–,–CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made the products shown (italics) are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. e Tapia et al. (2011). f Andersen et al. (2012a). g Aschmann et al. (2011).

b

249

250

the mechanisms of reactions influencing atmospheric ozone 40

1012 x k(OH), cm3 molecule–1 s–1

Dialkyl Ether (ROR) Y = −3.74 x 10−12 + 3.82 x10−12 X n-Alkanes (Reference) SAR Expectation

30

20

10

0 2

4

6

8

10

Number of C-atoms in dialkyl ether (ROR) or reference n-alkane FIGURE IV-H-1.

Effect of chain length on the rate coefficient (298 K) for HO reactions with structurally related dialkyl ethers (ROR); n-alkane data are shown for comparison.

HO + CH3CH2OCH2CH3

CH3CHOCH2CH3 + H2O (O2)

CH3CH(O2)OCH2CH3 (NO) [CH3CH(O)OCH2CH3 ] (a)

(M)

(b)

H + CH3C(=O)OCH2CH3

CH3CH(O)OCH2CH3

(O2)

(O2) CH3 + O=CHOCH2CH3

HO2 (O2)

(O2)

CH3C(=O)OCH2CH3 + HO2

CH3 + O=CHOCH2CH3

CH3O2 (NO) CH3O + NO2

(O2) CH2O+HO2 FIGURE IV-H-2.

A simplified mechanism for HO initiated oxidation of diethyl ether.

with the other similar radical reactions with O2, a peroxy radical is formed. However, in this case, there is a subtle difference; subsequent reaction of this peroxy radical with NO creates an energy-rich

alkoxy radical, [CH3CH(O)OCH2CH3]‡, which may either spontaneously decompose (as shown in the Figure IV-H-2 above) or be deactivated by collisions. Note that this “activated” alkoxy

The Hydroxyl Radical and Its Role in Ozone Formation radical decomposition occurs in other cases as well (see Chapter VI for details). At pressures near 1 atm, decomposition of the energy-rich alkoxy radical occurs by pathway (a) about 10–15% of the time and by pathway (b)  about 40% of the time; the remaining excited radicals are thermalized by collisions and subsequently either react with O2 or decompose by eliminating a CH3 radical. The major products formed are ethyl formate, ethyl acetate, and formaldehyde. IV-H-2. HO Reactions with Difunctional Ethers The difunctional ethers are commonly used today in a variety of applications. In the troposphere, they react with HO radicals by H-atom abstraction, as do the alkanes. However, their rate coefficients are significantly larger than those of the alkanes of similar carbon number (see Figure IV-H-3). As the size of the R groups in ROCH2OR is increased by successive addition of pairs of CH2 groups, k(298 K) value increases by about 10  × 10−12 cm3 molecule−1 s−1, whereas for the alkanes a similar addition of two CH2 groups results in an increase of about 2.7 × 10−12 cm3 molecule−1 s−1 in the rate coefficient. The presence of the two O atoms in these difunctional ethers activates the HO reaction for the C3, C5, C7, and C9 ether by factors of 3.5, 5.1, 4.l, and 3.5, respectively. IV-H-3. HO Reactions with Vinyl Ethers The vinyl ethers have not only the presence of an activating O atom in the chain, but also the reactive C=C bond as well, so, as one might expect, the rate coefficients for HO reactions with the vinyl ethers are large (see Table IV-H-1 [from Calvert et  al.,  2011]). Figure IV-H-4 gives a comparison of the rate coefficients for the vinyl ethers (ROCH=CH2) and the alkenes of the same C number. The presence of the O atom in the vinyl ethers enhances the rate coefficient by factors of 1.3, 2.3, 3.2, and 3.0, respectively, for the C3–C6 compounds. The major products formed, methyl formate and formaldehyde, suggest that addition to the C=C bond is the major pathway for reaction of HO with methyl vinyl ether. The reaction sequence in this case is as follows: HO + CH3OCH=CH2 → CH3OCH(•)CH2OH CH3OCH(•)CH2OH + O2 → CH3OCH(O2•) CH2OH

251

CH3OCH(O2•)CH2OH + NO → CH3OCH(O•)CH2OH + NO2 CH3OCH(O•)CH2OH → CH3OCH(O) + •CH2OH •CH2OH +O2 → CH2O + HO2 IV-H-4. HO Reactions with Cyclic Polyethers The cyclic polyethers show some interesting effects of reactivity with increasing ether functional groups. In Figure IV-H-5, compare the rate coefficients for four six-membered ring compounds with an increasing number of ether groups. One CH2 group in compound (a)  has been replaced with an O atom in compound (b). If the O atom had no enhancement of the rate coefficient, then kb would be about 5/6 of ka ≈ 5.7 × 10−12 cm3 molecule−1 s−1. Instead kb  =  12  × 10−12 cm3 molecule−1 s−1:  the reactivity of each of the remaining five CH2 groups, on average, increased by about a factor of 2.1. Structure (c) has four of six of the CH2 groups in (a), and, without enhancement of reactivity, kc would be 4/6 of k a or about 4.67 × 10−12 cm3 molecule−1 s−1; with the measured kc  =  10  × 10−12 cm3 molecule−1 s−1, the reactivity of the remaining carbon atoms is increased again, on the average by about a factor of 2.1. With half of the CH2 groups removed in (d), one expects kd ≈ 3.4 × 10−12 if there were no enhancement; with the measured kd = 5.4 × 10−12, an enhancement of a factor of 1.6 is seen. Regardless of the number of O atoms replacing CH2 groups in cyclohexane, an enhancement of roughly a factor of 2 at each remaining CH2 groups occurs. These results suggest that in SAR calculations the total rate coefficient for these multiple O-atom-substituted species can be obtained using a single enhancement factor on the CH2 group (see Section IV-O). Note, however, that in compounds (b)  and (c), the reactivity of each individual CH2 group is not known and may not be accurately determined using this approach. More refined treatments await more information regarding the site of HO attack in these multifunctional species. IV-H-5. HO Radical Reactions with Halogen-Atom-Substituted Ethers Kinetic studies of a large number of F-atomsubstituted ethers have been made (see Table

252

the mechanisms of reactions influencing atmospheric ozone

1012 x k(298 K) for HO reactions with ROCH2OR or RCH2R, cm3 molecule–1 s–1

ROCH2OR Y = –8.6 x 10–12 + 5.08 x 10–12 X

40

RCH2R Y = –3.2 x 10–12 + 1.42 x 10–12 X

30

20

10

0 2

3

4

5

6

7

8

9

10

Number of C-atoms in the molecule

Comparison of the k(298 K) for HO radical reactions with some difunctional ethers (ROCH2OR) and n-alkane of the same carbon number (RCH2R).

1012 x k(298 K) for HO reaction with vinyl ethers and alkenes of the same C-number, cm3 molecule–1 s–1

FIGURE IV-H-3.

Vinyl Ethers (ROCH=CH2) Alkene of Same C-Number (RCH=CH2)

120 100 80 60 40 20 0

3

4

5

6

Carbon number FIGURE IV-H-4. Comparison of the k(298 K) for HO radical reactions with some vinyl ethers (ROCH=CH2) and 1-alkenes of the same carbon number (RCH=CH2).

IV-H-1). As with the other classes of organic compounds, the F-atom-substituted ethers show a significant decrease in k(298 K) from that of the unsubstituted compound (see Figure IV-H6). A  decrease in the k(298 K) is seen as the number of F atoms substituted for H atoms in CH3CH2OCH3 is increased. With seven F atoms substituted, the k(298 K) is decreased by about

a factor of 2.5  × 10−4. F atoms withdraw electrons from the H atoms in the fluorinated compounds and increase the barrier for attack by the electron-seeking HO radical. An extended Hückel calculation shows that the effective charge on the H atoms in the CH2 group in CH3CH2OCH3 decreases from +0.0126 to –0.0085 for the H atom in CF3CFHOCF3.

The Hydroxyl Radical and Its Role in Ozone Formation H2 C H2C H2C

C H2

CH2

H2C

CH2

H2C

(a)

CH2

CH2 H2C

CH2 H2C C H2

O

O

O

O

O

CH2 H2C

(b)

253

O

C H2

C H2

(c)

(d)

1012 x k(298 K) for HO-cyclic ether reactions, cm3 molecule–1 s–1

15

10

5

0

(a)

(d)

(c)

(b) Compound

FIGURE IV-H-5.

The effect on k(298 K) of O-atom substitution in cyclohexane.

1012 x k(298 K) of fluorinated methyl ethyl ether, cm3molecule–1 s–1

10

1

CF3CH2OCH3 CH3CH2OCF3

CH3CH2OCH3

0.1 CF3CF2OCH3 CF3CHFOCF3 CHF2OCF2CHF2

CF3CHFOCH3 CH3CHFOCH3

0.01

CF3CF2OCH3 CF3CH2OCHF2 CH2FCF2OCHF2

0.001

0

1

2

3

4

5

CF2HCF2OCF3 CF3CHFOCF3

6

7

Number of F-atoms substituted for H-atoms in CH3OCH2CH3 FIGURE IV-H-6.

Effect on k(298 K) for the HO reactions with F-atom-substituted methyl ethyl ether molecules.

I V- I . M E C H A N I S M S O F R E AC T I O N O F H O R A D I CA L WITH ALDEHYDES The aldehydes contain a relatively weak RC(O)–H bond, ΔD°298 (CH3C(=O)–H) = 374 kJ mol−1, and

this is the most reactive bond in the molecule; by comparison, dissociation of the CH3CH2–H bond in ethane requires 423 kJ mol−1. The recommended rate coefficients for the HO reactions with the aldehydes are given in Table IV-I-1 (from Calvert et al.,

254

TABLE IV-I-1 . HO RE ACTIONS WITH THE ALDEHYDES. THIS TABLE GIVES THE RECOMMENDATIONS FOR R ATE COEFFICIENTS, K = A T N E −B/T (CM 3 MOLECULE −1 S −1 ) AT TROPOSPHERIC TEMPER ATURES. a COLUMN 6 SHOWS STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES (USING TABLES IV-O-1, IV-O-2, IV-O-5, AND IV-O-6) OF THE PERCENTAGE OF RE ACTION BY ADDITION AND H-ATOM ABSTR ACTION AT E ACH SITE. B COLUMN 7 SHOWS THE MA JOR PRODUCTS OBSERVED OR PREDICTED (ITALICS) FOR NO X-CONTAINING ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN. c PHOTODECOMPOSITION OF THE ALDEHYDES IS AN IMPORTANT ALTERNATE ROU TE OF RE ACTION (SEE CHAPTER VIII)

Aldehyde a) Acyclic aldehydes Formaldehyde (HCHO) Acetaldehyde, ethanal (CH3CHO) Propanal (CH3C H2CHO) Butanal (CH3CH2CH2CHO) 2-Methylpropanal [(CH3)2CHCHO] Pentanal (CH3CH2CH2CH2CHO) 2-Methylbutanal [CH3CH2CH(CH3)CHO] 3-Methylbutanal [(CH3)2CHCH2CHO] 2,2-Dimethylpropanal [(CH3)3CCHO] Hexanal (CH3CH2CH2CH2CH2CHO) 2-Methylpentanal [CH3CH2CH2CH(CH3)CHO] 3-Methylpentanal [CH3CH2CH(CH3)CH2CHO] 4-Methylpentanal [(CH3)2CHCH2CH2CHO] 3,3-Dimethylbutanal [(CH3)3CCH2CHO] 2-Ethylbutanal [CH3CH2CH(CH2CH3)CHO] Heptanal (CH3CH2CH2CH2CH2CH2CHO)

1012 × k(298 K) 8.8 ± 1.3 15.3 ± 2.3 18.7 ± 2.8 23.7 ± 4.7 25.7 ± 5.1 26.6 ± 4.0 33.3 ± 13.3 25.9 ± 5.2 27.8 ± 5.6 28.5 ± 4.3

1012 × A

B (K)

n

1.26 × 10−5 5.2 4.8 5.97 6.7 8.6

−613 −322 −405 −411 −400 −336

2 0 0 0 0 0

5.3 4.28

−494 −565

Estimated % HO Reaction at Each Siteb 50; 50 1; 99 3; 4; 93 1: 17; 4; 78 (5, 5); 7; 82 0.7: 5; 19; 4; 72 0.7; 19; 8; (2); 71 (0.7, 0.7); 29; 3; 66 (3, 3, 3); 90 0.7; 5; 6; 18; 3; 68

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

33 ± 13

0.7; 5; 18; 7; (2); 68

29.1 ± 11.6

0.8; 5; 18; (0.8); 4; 71

26.3 ± 10.5

(0.7, 0.7); 5; 19; 4; 72

21.4 ± 8.4

(1, 1, 1); 5; 92

CO CH2O; CO2 CH3CHO; CO2 C2H5CHO; CO2 CH3C(O)CH3; CO2 HOCH2CH2CH2CHO; CO2; CH3CH2CH3CHO CH3C(O)CH3; CO2 CH2O; CH3C(O)CH3; (CH3)2CCHO; CO2 CH3C(O)CH3; CH2O; CO2 CH3CH(OH)CH2CH2CHO; CO2; CH3CH2CH2CH2CHO HOCH2CH2CH2C(O)CH3; CO2; CH3CH2CH2C(O)CH3 CO2; CH2O; C2H5C(O)CH3; CH3CHO; HOCH2CH2CH(CH3)CHO CH2O; (CH3)2CHCHO; CO2; HOCH2C(CH3) CH2CHO CO2: CH2O; CH3C(O)CH3

40.2 ±16.1

0.6; 15; 8; (15; 0.6); 61

C2H5C(O)C2H5; CO2; CH3CHO; CH3CH2CHO

29.6 ± 11.8

0; 4; 5; 5; 7; 4; 63

CH3CH2CH2CH2CH2CHO; CO2; CH3CH2CH(OH)CH2CH2CHO

1012 × k(298 K)

Aldehyde

1012 × A

B (K)

n

Estimated % HO Reaction at Each Siteb

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

2,3-Dimethylpentanal [CH3CH2CH(CH3)CH(CH3)CHO]

42 ± 17

0.5; 4; 31; (0.5); 6; (2);56

Nonanal (CH3CH2CH2CH2CH2CH2CH2CH2CHO)

32 ± 13

1; 4; 5; 5; 5;5; 15; 4; 57

21.0 ± 8.4

a 99; b 0.2; c 0.5

26.3 ± 10.5

a 83; b 3; c 6; d 2

?

31.3 ± 12.5

a 65; b 5; c 11; d 3

?

37.0 ± 14.8

a 52; b 7; c 14; d 4

?

b) Carbaldehydes Cyclopropanecarbaldehyde b H C

c H 2C

C2H5CH(CH3)C(O)CH3; CO2; CH3CHO; C2H5C(O)CH3; HOCH2CH2CH(CH3)C(O)CH3 CH3CH2CH2CH2CH2CH2CH2CHO; CO2; CH3CH2CH2CH(OH)CH2CH2CHO HC(O)CH2CHO; CO2

a CHO

c H 2C

Cyclobutanecarbaldehde CH2 c

d H2C

a C CHO H b

c H2C

Cyclopentanecarbaldehyde c H2 C

b H C

d H2C d H2C

a CHO

CH2 c

Cyclohexanecarbaldehyde c H2 C d H2C d H2C

d C H2

a b H CHO C CH2 c

(continued)

255

256

TABLE IV-I-1 (CONTINUED)

Aldehyde c) Hydroxyaldehydes Hydroxyacetaldeyde (HOCH2CHO) 2-Hydroxypropanal [CH3CH(OH)CHO] 3-Hydroxypropanal (HOCH2CH2CHO) 2-Hydroxybutanal [CH3CH2CH(OH)CHO] 3-Hydroxybutanal [CH3CH(OH)CH2CHO] 2-Hydroxy-2-methypropanal [(CH3)2C(OH)CHO]

1012 × k(298 K)

1012 × A

8.3 ± 1.7 17.0 ± 6.8 19.9 ± 8.0 23.7 ± 9.5 29.5 ± 11.8 14.0 ± 5.6

B (K)

n

~0

Estimated % HO Reaction at Each Siteb

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

1; 13; 87 0.7; 23; (0.6); 75 0.7; 16; 4; 80 0.6; 9; 24; (0.5); 65 0.6; 31; (0.5); 7; 61 (1,1); (1); 97

CO2; CH2O; HC(O)CHO CO2; CH3CHO CO2; CH2O; HOCH2CHO CO2; CH3CH2CHO CO2; CH3CHO; CH2O CO2; CH3C(O)CH3

d) Multifunctional aldehydes Glyoxal (HCOCHO) Methylglyoxal (CH3COCHO)

9.0 ± 1.8 13.0 ± 3.9

3.1 1.82

−318 −585

0 0

50; 50 1; 99

CO2; CO CH2O; CO; CO2

e) Unsaturated aldehydes 2-Propenal (CH2=CHCHO) 2-Butenal (CH3CH=CHCHO) 2-Methyl-2-propenal [CH2=C(CH3)CHO] E-2-Pentenal (CH3CH2CH=CHCHO)

21.7 ± 4.3 36 ± 7 32 ± 10 44 ± 13

7.1 6.0 9.0 7.9

−333 −533 −380 −510

0 0 0 0

A 35; 65 0.5; A 53; 47 A 51; (0.5); 49 0.4; 3; A 41; 45

CO2; CO; CH2O; HOCH2CHO CO; CO2; CH(O)CHO; CH3CHO; CH2O CO; CO2; CH2O; HOCH2C(O)CH3 C2H5CHO; CH(O)CHO; C2H5C(O)CHO; C2H5CH(OH)CHO HC(O)CHO; CH3C(O)CHO;

E,Z-4-Oxo-2-pentenal [CH3C(O)CH=CHCHO]

61 ± 24

0.2; A 50; 49

O

O

C CH

O

E-2-Hexenal (CH3CH2CH2CH=CHCHO)

43 ± 13

7.5

−520

0

0.4; 3; 6; A 48; 43

E-2-Heptanal (CH3CH2CH2CH2CH=CHCHO)

44 ± 13

9.7

−450

0

0.4; 3; 3; 6; A 47; 41

C

C H

C3H7CHO; CH(O)CHO; C3H7C(O)CHO; C3H7CH(OH)CHO C4H9CHO; CH(O)CHO; C4H9C(O)CHO; C4H9CHOH)CHO

1012 × k(298 K)

Aldehyde f) Ene-dial aldehydes E-Butenedial (OCHCH=CHCHO)

1012 × A

B (K)

n

≥ 26

Estimated % HO Reaction at Each Siteb 42; A 16; 42

Observed or Predicted Major Products for NOx-containing Atmospheresc,d HC(O)CHO; CH2O; CO; O O

C CH

O

Z-Butenedial (OCHCH=CHCHO) E,E-2,4-Hexadienedial (OCHCH=CHCH=CHCHO) E,Z-2,4-Hexadienedial (OCHCH=CHCH=CHCHO) E,E-2-Methyl-2,4-hexadienaedial [HC(O)C(CH3=CHCH=CHCHO] 4-Methylenehex-5-enal [CH2=CHC(=CH2) CH2CH2CHO] 3(Z)4-Methylhex-3,5-dienal [CH2=CHC(CH3)=CHCH2CHO] 3(E)4-Methylhex-3,5-dienal [CH2=CHC(CH3)=CHCH2CHO] g) Aromatic aldehydes Benzaldehyde

C

C H

57 ± 23 82 ± 33 76 ± 8f 91 ± 37 74 ± 19f 118 ± 47

42; A 16; 42 21; A 29; A 29; 21

Same as E-butendial HC(O)CH=CHCHO; HC(O)CHO

21; A 29; A 29; 21

Same as E,Z-isomer

20; (0.2); A 36; A 23; 20

CH3COCHO; HC(O)CH=CHCHO

155 ± 62

A 27; (A 53); 2; 1; 17

CH2O; CH2=CHC(O)CH2CH2CHO

161 ± 64

A16;0.5;A60;0.5;21

CH2=CHC(O)CH3; HC(O)CH2CHO

252 ± 101

A16;0.5;A60;0.5;21

CH2=CHC(O)CH3; HC(O)CH2CHO

12.6 ± 2.0

6.8

−185

0

a > A b

?

a > c > A b

?

a CHO b

o-Tolualdehyde

18.9 ± 2.0

a CHO b

257

CH3 c

(continued)

258

TABLE IV-I-1 (CONTINUED)

1012 × k(298 K)

Aldehyde m-Tolualdehyde

1012 × A

B (K)

n

Estimated % HO Reaction at Each Siteb

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

16.8 ± 4.0

a > c > A b

?

16.8 ± 4.0

a > c > A b

?

25.5 ± 5.0

a > c > A b

?

31.2 ± 10.0

a > c > A b

?

31.5 ± 10.0

a > c > A b

?

b CHO a

c H3C

p-Tolualdehyde c H3 C b a CHO

2,3-Dimethylbenzaldehyde a CHO b CH3 c c H3C

2,4-Dimethylbenzaldehyde a CHO b c H3 C

CH3 c

2,5-Dimethylbenzaldehyde a CHO

c H3 C b

CH3 c

1012 × k(298 K)

Aldehyde 2.6-Dimethylbenzaldehyde CH3 c

1012 × A

B (K)

n

Estimated % HO Reaction at Each Siteb

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

30.3 ± 8.0

a > c > A b

?

23.6 ± 4.0

a > c > A b

?

27.8 ± 6.0

a > c > A b

?

48 ± 12

A a > b > A c

?

a CHO

b CH3 c

3,4-Dimethylbenzaldehyde a CHO b c H3 C CH3 c

3,5-Dimethylbenzaldehyde a CHO

c H3 C b

CH3 c

3-Phenyl-2-propenal c

CH

a

CH

b CHO

259

(continued)

260

TABLE IV-I-1 (CONTINUED)

1012 × k(298 K)

Aldehyde 2-Formylbenzaldehyde

1012 × A

B (K)

n

Estimated % HO Reaction at Each Siteb

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

23 ± 6

a > A b

?

17 ± 4

a > c > A b

?

a CHO a CHO

b

2-Acetylbenzaldehyde a CHO b

C(O)CH3 c

h) Aldehydes from biogenic hydrocarbons Pinonaldehyde

42 ±13

5.6

−600

0

a 84; b 4; c 5; d 0.8; e 3; f 2; g 0.5

g CH3

CH3C(O)CH3; CH2O; unidentified carbonyl products

O

a CHO CH 2 f CH2 b d H3C C C d H3C Hc e HC

50 ± 20

Caronaldehyde CH3 d b H2C e HC c H 3C

C

O

a CHO

CH2b CH e CH3 c

a 88; b 4; c 1; d 0.5; e 0.4

?

1012 × k(298 K)

Aldehyde

1012 × A

B (K)

n

Estimated % HO Reaction at Each Siteb

Observed or Predicted Major Products for NOx-containing Atmospheresc,d

110 ± 44

a A 67; b 22; c 5; d 2; e 1; f 0.1

2-Isopropenyl-5-oxo-hexanal

151 ± 45

a A 78; b 15; c 3; d 1; e 0.2

HC(O)CHCH(CH3)(CH2)2CHO; CH3C(O)CH3

i) Halogenated aldehydes HC(O)F

< 5 × 10−3

HF; HC(O)OH (aqueous phase)

CHF2CHO

1.5 ± 0.4

CxF2x+1CHO (x = 1, 2, 3, 4) CxF2x+1CH2CHO (x = 1, 4, 8) CH(O)Cl CH2ClC(O)H CHCl2C(O)H CCl3C(O)H CHClFC(O)H CF2ClC(O)H

0.58 ± 0.9 2.8 ± 1.4 < 0.32 3.15 ± 0.63 2.52 ± 0.50 0.84 ± 0.21 2.05 ± 0.51 1.15 ± 0.29

Loss by hydolysis in cloud, rain, or sea water probably dominates Photolysis probably major loss mechanism with some HO attack at the CHO-group Similar to CHF2CHO Similar to CHF2CHO Similar to HC(O)F Similar to CHF2CHO Similar to CHF2CHO Similar to CHF2CHO Similar to CHF2CHO Similar to CHF2CHO

Isopropenyl-6-oxo-heptanal b OHC CH2 e O f H 3C CH c CH2 d e H2C

CH2 C a CH3 f

3,7-Dimethyl-6-octen-1-al CH3 e e H3C

C a C H

H2 d C

CH3 e

b CHO CH c C C H2 d H2 d

2.05

378

0

1.85

235

0

CO2; F2CO

CO2; F2CO CO2; CH2O; F2CO CO2; (Cl) CO2;HC(O)Cl; HCl; CH2O CO2; HC(O)Cl CO2; Cl2CO CO2; HC(O)F; Cl CO2; F2CO; Cl (continued)

261

262

TABLE IV-I-1 (CONTINUED)

Aldehyde CFCl2C(O)H CHClFC(O)H CF2ClC(O)H CFCl2C(O)H

1012 × k(298 K) 0.82 ± 0.21 2.05 ± 0.51 1.15 ± 0.29 0.82 ± 0.21

1012 × A

B (K)

n

Estimated % HO Reaction at Each Siteb Similar to CHF2CHO Similar to CHF2CHO Similar to CHF2CHO Similar to CHF2CHO

Observed or Predicted Major Products for NOx-containing Atmospheresc,d CO2; FC(O)Cl; Cl CO2; HC(O)F; Cl CO2; F2CO; Cl CO2; FC(O)Cl; Cl

Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated and references to the original data are given. The abbreviated code in column 6 of the first part of the table gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive >CH, –CH2–, –CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the points of attack of the HO radical. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the R(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made, the products shown (italics) are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. e Antiñolo et al. (2010). f Klotz et al. (1999). a

b

The Hydroxyl Radical and Its Role in Ozone Formation

263

the dotted gray line; the SAR predictions lie below the experimental points, and the slope of the line beyond the C4 aldehyde is close to that seen for the alkanes. It appears that the activating effect of the formyl group extends only two or three atoms down-chain, as the SAR formulations suggest (see Section IV-O).

IV-I-1. HO Radical Reactions with Acyclic Aldehydes As with the rate coefficients of the other families of organic compounds, the straight chain aldehydes (CnH2n+1CHO) show an increase with increasing carbon number (see Figure IV-I-1). Experimental rate coefficients for the five smallest aldehydes follow reasonably well a linear relationship with increasing carbon number, with k(298 K) increasing by about 4.4 × 10−12 cm3 molecule−1 s−1 for each additional CH2 group added to the alkyl chain. The results show a significant enhancement of k(298 K) by the presence of the –CH(=O) group over that shown by the n-alkanes (~1.4  × 10−12 cm3 molecule−1 s−1) and the n-alcohols (about 2.5 × 10−12 cm3 molecule−1 s−1) but about the same as that for the straight chain methyl ethers (3.8  ×10−12 cm3 molecule−1 s−1). Within the large uncertainties, the enhancement of the rate coefficients for the aldehydes appears to lessen beyond the C4 aldehydes. This is in accord with the prediction from SAR calculations shown by the gray stars and

IV-I-2. HO Radical Reactions with Unsaturated Aldehydes The rate coefficients for the unsaturated aldehydes [RCH=CHCH(=O)] are significantly greater than the saturated aldehydes, thus reflecting the importance of the presence of the reactive >C=C< group (see Figure IV-I-2). As with the HO reactions with the alkenes, enhancement of the reactivity is seen as CH3 or C2H5 groups are added to the terminal CH2 group in CH2=CHCH2CHO. The mechanism of reaction of HO with the simplest unsaturated aldehyde, CH2=CHCHO, involves both addition at the double bond and abstraction of H atoms from the carbonyl carbon atom (see Figure IV-I-3). Abstraction by pathway (a) accounts for about 68 ± 8% of the reaction, and pathway (b) occurs about 32% of the time. As noted previously, abstraction of the strongly bonded H atoms on the double-bonded C atoms (–HC=CH–) is unimportant.

1012 x k(298 K) for HO + aldehyde, cm3 molecule–1 s–1

2011). Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. Removal of the aldehydes in the troposphere occurs by both HO radical attack and by photolysis (see Chapter VIII).

RCHO; Data Not Used in Linear Fit 2nd Order Regression All Data RCHO; Data Used in Linear Fit Y = 5.4 x 10–12 + 4.4 x 10–12 X n-Alkanes (Reference) SAR Expectation

50

40

30

20

10

0 1

2

3

4

5

6

7

8

Carbon number of the straight chain aldehyde (CnH2n-1CHO) FIGURE IV-I-1.

Plot of k(298 K) for HO reactions with a series of straight-chain aldehydes.

9

1012 x k(298 K) of HO + RCH=CHCH(=O), cm3 molecule–1 s–1

60

50

40

30

20

10

0

1

2

3

4

Number of C-atoms in the R-group of the aldehydes: R-CH=CHCH(=O) FIGURE IV-I-2. Plot of k(298 K) for HO reaction with a series of structurally related, unsaturated aldehydes [RCH=CHCH(=O)].

H

H2 C

(a)

O + HO

C CH

C H2C

(b)

O + H2O

CH (O2)

O2

H C H2 C HO

C

O H2C

CH

CH (NO)

(O2)

O

H C

O

H2 C H HO

C

O

H2C

CH

H2C

CH + _CO2

O2 (NO)

(O2)

H H2 C HC HO

O

C

O

CH2

CHOO

CO2 + CH3 (O2)

O

HOCH2CH(=O) + HCO

CH2O + HCO (O2)

(O2) CO + HO2

CH3O2 (NO) CH3O + NO2 (O2) CH2O + HO2

FIGURE IV-I-3.

264

Mechanism of reaction of HO with CH2=CHC(=O)H.

The Hydroxyl Radical and Its Role in Ozone Formation IV-I-3. HO Radical Reactions with Aromatic Aldehydes The aromatic aldehydes (benzaldehyde and methylbenzaldehydes) show increased reactivity with increasing number of CH3 groups added to the benzaldehyde molecule. Abstraction of the acyl-H is the dominant pathway for reaction of HO radicals with benzaldehyde and leads to peroxybenzoyl nitrate and nitrophenol products. The addition of benzaldehyde to an irradiated air-NOx-RH mixture shows a very unusual result; it decreases the extent of ozone generation. This unusual behavior is explained by the NO2 scavenging effect of nitrophenols and peroxybenzoyl nitrate products (Derwent et al., 1998; Calvert et al., 2011). The product, peroxybenzoylnitrate (C6H5C(O)O2NO2) is a powerful eye irritant. IV-I-4. HO Radical Reactions with Halogen-Atom-Substituted Aldehydes Fluorine substitution in aldehydes results in a strong inhibition of the reactions with the HO radical, as is also seen in the other classes of organic molecules (see Figure IV-I-4). For molecules with a given carbon number, we see a dramatic lowering of the k(298 K) values for

265

an unsubstituted n-aldehyde and the two series of aldehydes with F-atom substitution. In each case, the fluorinated alkyl group is unreactive, and the reaction with HO must occur at the  –CHO or  – CH2CHO end positions. As expected, the length of the fluorinated alkyl group in a given series of F-substituted aldehydes does not affect the reactivity of the molecules. I V- J . M E C H A N I S M S O F H O R A D I CA L R E AC T I O N S W I T H KETONES The ketones are formed in the atmosphere by H-atom abstraction reactions from the alkanes by HO. They are also emitted directly into the atmosphere from the combustion of fuels, solvent use, and from vegetation. Calvert et  al. (2011) have reviewed in detail the inventory of these sources. Acetone and 2-butanone are the major ketones emitted into the atmosphere from anthropogenic sources. A  major loss process for ketones in the atmosphere is their reaction with the HO radical. As with the aldehydes, the ketones are also removed from the troposphere by photolysis (see Chapter VIII). In this section, we review the recommended rate coefficients that control the rate of

1012 x k(298 K) for HO + aldehyde, cm3 molecule–1 s–1

100

10

1

CnH2n+1CHO CnF2n+1CHO CnF2n+1CH2CHO

0.1

2

3

4

5

6

7

8

9

10

Number of C-atoms in the molecule FIGURE IV-I-4.

Comparison of the k(298 K) values for the HO reaction with the unsubstituted n-aldehydes (CnH2n+1CHO) and those with F-atom substitution for H atoms at all carbon atoms but the acyl-carbon (CnF2n+1CHO) or the acyl and the 2-carbon atom positions (CnF2n+1CH2CHO).

266

the mechanisms of reactions influencing atmospheric ozone

the HO removal reactions that lead to subsequent ozone generation. IV-J-1. HO Radical Reactions with the Acyclic Ketones Table IV-J-1 summarizes the recommended rate coefficients for the reactions of the ketones with the HO radical (Calvert et al., 2011). Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. Like the alkanes, the acyclic ketones [RC(O)R′] react with HO by H-atom abstraction. In Figure IV-J-1, the rate coefficients for reaction of HO with a series of structurally related ketones, CH3(CO)R, are plotted CH3(CO)R versus the carbon number minus one. For comparison, the HO-alkane k(298 K) data are also plotted but versus carbon number. This forces a match of the number of reactive C positions in the corresponding ketone; for example, CH3CH3 is compared with a ketone with equivalent reactive groups, CH3C(=O)CH3. The rate coefficients for the

first two pairs plotted are similar: ethane and acetone, k(298 K)  =  0.26  × 10−12 cm3 molecule−1 s−1 and 0.18  × 10−12 cm3 molecule−1 s−1; and propane and 2-butanone, 1.11 × 10−12 cm3 molecule−1 s−1 and 1.11 × 10−12 cm3 molecule−1 s−1, respectively. However, as one proceeds to the larger compounds, it is obvious that the k(298 K) for the ketone is significantly larger than that of the alkane with equivalent reactive CH2 groups. The experimental rate coefficients for the 4-, 5-, and 6-carbon ketones show a nearly constant increase of about 2.8 × 10−12 cm3 molecule−1 s−1 for each CH2 group added. Note that from the 7-carbon ketone to the larger ketones, the slope of the curve approaches that expected from the SAR estimates and is about equal to that seen for the alkanes. The increase in magnitude of the enhancement of k(298 K) induced in the ketones with increasing C number is greater than that observed with the alkanes (1.4 × 10−12); it is about the same as that seen with the alcohols (2.5  × 10−12) but is somewhat less than that of the ethers (3.8  × 10−12) or the aldehydes (4.4  × 10−12). Note that the gray stars and

All CH3C(O)R Data (Plotted as C-Number-1) Fit Omitting CH3C(O)CH3 Point Only Fit Including C-Numbers 3, 4, 5, and 6 Only n-Alkanes (Reference)

1012 x k(OH), cm3 molecule−1 s−1

20

SAR Expectations

15

10

5

0

2

3

4

5

6

7

8

9

10

Carbon number –1 for methyl ketones [CH3C(O)R] Carbon number for alkanes FIGURE IV-J-1.

Plot of the k(298 K) values for the HO reaction with methyl ketones [CH3C(=O)R] and structurally related alkanes. The methyl ketones are plotted at their carbon number − 1 to match the number of reactive C atoms in the reference alkane.

TABLE IV-J-1 . HO RE ACTIONS WITH KETONES AND KETENES. THIS TABLE GIVES THE RECOMMENDATIONS FOR R ATE COEFFICIENTS, k = A × T n × e −B/T (CM 3 MOLECULE −1 S −1 ) FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 6 SHOWS STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES (USING TABLES IV-O-1, IV-O-2, IV-O-5, AND IV-O-6) OF THE PERCENTAGE OF RE ACTION BY ADDITION (“A” PREFIX) AND H-ATOM ABSTR ACTION AT E ACH SITE. b COLUMN 7 GIVES THE MA JOR PRODUCTS OBSERVED OR PREDICTED (ITALICS) FOR HO ATTACK IN AN NO X-CONTAINING ATMOSPHERE. N-CONTAINING PRODUCTS ARE NOT SHOWN. e PHOTOLYSIS IS AN IMPORTANT ALTERNATIVE RE ACTION PATHWAY FOR THE KETONES (SEE CHAPTER VIII)

Ketone

1012 ×k(298 K)

1012 × A

B (K)

n

a) Acyclic ketones Acetonec [CH3C(O)CH3] 2-Butanone [CH3C(O)CH2CH3] 2-Pentanone [CH3C(O)CH2CH2CH3] 3-Pentanone [CH3CH2C(O)CH2CH3]

0.18 ± 0.03 1.1 ± 0.22 4.1 ± 1.2 2.0 ± 0.6

(c) 3.35 × 10−6 0.38

−391 −705

2 0

50; 50 8; 53; 40 2; 18; 76; 4 22; 28; 28; 22

3-Methyl-2-butanone [CH3C(O)CH(CH3) 2 2-Hexanone [CH3C(O)CH2CH2CH2CH3]

3.0 ± 1.2 7.5 ± 2.3

1.45 0.84

−219 −654

0 0

7; 76; (9,9) 1; 12; 65; 20; 2

3-Hexanone [CH3CH2C(O)CH2CH2CH3

6.4 ± 2.6

9; 12; 15; 62; 3

3-Methyl-2-pentanone [CH3C(O)CH(CH3)CH2CH3] 4-Methyl-2-pentanone [CH3C(O)CH2CH(CH3)2] 3,3-Dimethyl-2-butanone [CH3C(O)C(CH3)3] 2-Heptanone [CH3C(O)CH2CH2CH2CH2CH3]

6.2 ± 2.5

2; 31; (3); 62; 3

12.8 ± 2.6 1.21 ± 0.48 9.4 ± 2.8

267

5-Methyl-2-hexanone 12.0 ± 4.8 [CH3C(O)CH2CH2CH(CH3)2] 2,4-Dimethyl-3-pentanone 4.9 ± 2.0 [(CH3)2CHC(O)CH(CH3)2] 2-Octanone [CH3C(O)CH2CH2CH2CH2CH2CH3] 11.0 ± 4.4

Estimated % H-atom Abstraction at Each Site

0.794

−828

0

1; 10; 85; (2, 2) 6; (31, 31, 31) 1; 11; 55; 17; 14; 2

1.25

−674

0

1; 11; 56; 30; (2,2) (10,10); 29; 29; (10,10) 1; 9; 47; 15; 15 12; 2

Observed or predicted Major Products in NOx-containing Atmospherese

CH2O; CO2; CH3CHO; CH2O: CO2 CH3C(O)CHO; CH3CHO CH3CHO; CO2; CH3CHO; CH3C(O)C(O)CH2CH3 CH3C(O)CH3; CH2O; CO2 HOCH2C(O)CH2CH(O)CH2CH3; H3CC(O)CHO; C2H5CHO C2H5CHO; CH3CHO; CO2; CH3C(OH)C(O)CH2CH(O)CH3 CH3C(O)C(O)CH3; CO2; CH3CHO; CH3C(O)C2H5; CH3C(O)CH3; CH3C(O)CHO CH2O; CH3C(O)CH3; CO2; CH3C(O)CHO; C3H7CHO; CH3C(O)C(O)CH2CH2CH(OH)CH3 CH3C(O)CH3; CH3C(O)CH2CHO; CH3C(O)CHO; HC(O)CH(CH3)2 CH3C(O)CH3; CO2 CH3C(O)CH2CHO; C3H7CHO; CH3C(O)CH2C(O)(CH2)2CH(OH)CH3 (continued)

268

TABLE IV-J-1 (CONTINUED)

Ketone

1012 ×k(298 K)

2-Nonanone [CH3C(O)CH2CH2CH2CH2CH2CH2CH3] 2-Decanone [CH3C(O)CH2CH2CH2CH2CH2CH2CH2CH3]

12.2 ± 4.9

1; 8; 41; 13; 13; 13; 10; 2

13.2 ± 5.3

0.8; 7; 39; 11; l1; 11; 11; 9; 1

CH3C(O)CH2CHO; C4H9CHO; CH3C(O)CH2C(O)(CH2)2CH(OH)C2H5 CH3C(O)CH2CHO; C5H11CHO; CH3C(O)CH2C(O)(CH2)2CH(OH)C3H7

5; 91; 4 4; 71; 20; 5 2; 90; (2); 6 2; 29; 67; 2 0.7; 5; 65; 28; 0.9

CH3C(O)CHO; HC(O)C(O)C2H5 CH3C(O)C(O)CH3 CH3C(O)CH2CHO CH3C(O)CH2CH2CHO

14.6 ± 5.8

0.6; 22; (0.9); 76; 0.8

0.85 ± 0.34

11; (15)(37, 37)

22.0 ± 8.8

1; 59; (1); 27; 11; 1

CH3C(O)CH(CH3)C(O)CH3; CH3C(O)C(O)CH3; CH3CHO CH2O; CH3C(O)C(O)CH3; CH3C(O)CH3; CO2 CH3C(O)C(O)C3H7

13.6 ± 5.4

2; 7; 64; (1); 24; 2

C2H5C(O)C(O)C2H5

17.0 ± 6.8

2; 8, 10; 34; 44; 2

C2H5C(O)CH2CH2CHO

3.8 ± 1.7

4; 62; (5); (15, 15)

CH3C(O)CH(O); CH3C(O)CH3; CH2O

19.0 ± 7.6

0.6; 5; 17; 61; (1); 14; 1

CH3C(O)CH2CH2C(O)C2H5

16.0 ± 6.4

4; 5; 12; 20; 58; (1); 1

C2H5C(O)CH2CH2C(O)CH3

10.0 ± 4.0

1; 22; 30; 6; 6; 30; 1

HC(O)CH2CH2C(O)C3H7

b) Hydroxyketones Hydoxyacetone [HOCH2C(O)CH3] 1-Hydroxy-2-butanone [HOCH2C(O)CH2CH3] 3-Hydroxy-2-butanone [CH3C(O)CH(OH)CH3] 4-Hydroxy-2-butanone [CH3C(O)CH2CH2OH] 5-Hydroxy-2-pentanone [CH3C(O)CH2CH2CH2OH] 4-Hydroxy-3-methyl-2-butanone [CH3C(O)CH(CH3)CH2OH] 3-Hydroxy-3-methyl-2-butanone [CH3C(O)C(OH)(CH3)2] 3-Hydroxy-2-hexanone [CH3C(O)CH(OH)CH2CH2CH3] 4-Hydroxy-3-hexanone [CH3CH2C(O)CH(OH)CH2CH3] 6-Hydroxy-3-hexanone [CH3CH2C(O)CH2CH2CH2OH] 4-Hydroxy-4-methyl-2-pentanone [CH3C(O)CH2C(OH)(CH3)2] 5-Hydroxy-2-heptanone [CH3C(O)CH2CH2CH(OH)CH2CH3] 6-Hydroxy-3-heptanone [CH3CH2C(O)CH2CH2CH(OH)CH3] 1-Hydroxy-4-heptanone [HOCH2CH2CH2C(O)CH2CH2CH3]

4.2 ± 1.7 7.6 ± 2.0 9.3 ± 3.3 10.4 ± 3.0 15.5 ± 4.0

1012 × A

1.5

B (K)

−305

n

0

Estimated % H-atom Abstraction at Each Site

Observed or predicted Major Products in NOx-containing Atmospherese

Ketone

1012 ×k(298 K)

5-Hydroxy-2-octanone [CH3C(O)CH2CH2CH(OH)CH2CH2CH3] 6-Hydroxy-3-octanone [CH3CH2C(O)CH2CH2CH(OH)CH2CH3] 7-Hydroxy-4-octanone [CH3CH2CH2C(O)CH2CH2CH(OH) CH3]

23.0 ± 9.2

c) Diones 2,3-Butanedione [CH3C(O)C(O)CH3] 2,4-Pentanedione [CH3C(O)CH2C(O)CH3]; Largely in enol form: [CH3C(OH)=CHC(O)CH3] 2,5-Hexanedione [CH3C(O)CH2CH2C(O)CH3] 3-Methyl-2,4-pentanedione (keto) [CH3C(O)CH(CH3)C(O)CH3] 3-Methyl-2,4-pentanedione (enol) CH3C(OH)=C(CH3)C(O)CH3 d) Unsaturated ketones 3-Buten-2-one [CH3C(O)CH=CH2] 1-Penten-3-one [CH2=CHC(O)CH2CH3] 3-Penten-2-one [CH3C(O)CH=CH2CH3] 3-Methyl-3-buten-2-one [CH3C(O)CH(CH3)=CH2] 4-Hexen-2-one [CH3C(O)CH2CH=CHCH3] 4-Hexen-3-one [CH3CH2C(O)CH=CH2CH3] 5-Hexen-2-one [CH3C(O)CH2CH2CH=CH2] 4-Methyl-3-penten-2-one CH3C(O)CH=C(CH3)2]

1012 × A

B (K)

n

Observed or predicted Major Products in NOx-containing Atmospherese

0.4; 3; 36; 41; (0.5); 14; CH3C(O)CH2CH2C(O)C3H7; 4; 0.6 dihydroxy-dicarbonyls 0.7; 3; 4; 34; 43; (0.6); 10; C2H5C(O)CH2C(O)C3H7; dihydroxy-dicarbonyls 0.7 0.7; 15; 3; 4; 40; 36; C3H7C(O)CH2C(O)C2H5; dihydroxy-dicarbonyls (0.6); 1

15.0 ± 6.0 19.0 ± 7.6

0.235 ± 0.070 90.0 ± 18.0

Estimated % H-atom Abstraction at Each Site

50; 50 0.2; (0.2); A 99; 0.1

CH2O; CO; CO2 CH3C(O)OH; CH3C(O)CHO; CH3C(O)C(O)C(O)CH3

7.13 ± 2.85 11.6 ± 3.5

1; 49; 49; 1 5; 56; (34); 5

CH3C(O)CHO CH3C(O)C(O)CH3; CO2; CH2O

61.0 ± 18.3

0.1; (0.1); A 99; (0.1); 0.1 CH3C(O)OH; CH3C(O)C(O)CH3

20.3 ± 3.0 25.0 ± 10.0 29.0 ± 7.9f 60 ± 24 72 ± 17g 50 ± 20 60 ± 24 90 ± 21h 52 ± 13h 80 ± 32

1.065 3.3

2.6

450 −985

−612

0 0

0

0.4; A 99; A 97; 3; 1

CH3C(O)CHO; CH2O; HC(O)CH2OH; CO2 C2H5C(O)CHO; CH2O

0.2; A 99; 0.2

CH3C(O)CHO; CH3CHO

0.2; A 98; (1)

CH3C(O)CH2OH; CO2; CH2O

0.2; 2; A 98; 0.3

CH3C(O)CH2CHO; CH3CHO

3; 3; 14; A 80 0.1; A 98; (1,1)

CH2O, CH3C(O)CH2CH2CHO CH3C(O)CH3; CH3C(O)CHO (continued)

269

270

TABLE IV-J-1 (CONTINUED)

Ketone

1012 ×k(298 K)

E-3-Methyl-3-penten-2-one [CH3C(O)C(CH3)=CHCH3] 6-Methyl-5-hepten-2-one [CH3C(O)CH2CH2CH=C(CH3)2] trans-4-Methoxy-3-buten-2-one [CH3C(O)CH=C(OCH3)CH3] 4-Chloro-1-buten-3-one [CH2=CHC(O)CH2Cl] cis/trans-4-Oxo-2-pentenal [CH3CH=CHC(O)CH(O)] cis-3-Hexene-2,5-dione [CH3C(O)CH=CHC(O)CH3] trans-3-Hexene-2,5-dione [CH3C(O)CH=CHC(O)CH3] 3,4-Dihydroxy-3-hexene-2,5-dione CH3C(O)C(OH)=C(OH)C(O)CH3] Cyclobutanone

80 ± 32

0.1; 2; (1); A 97; 1

CH3C(O)C(O)CH3; CH3CHO

157 ± 55

0.1; 1; 3; A 95; (1, 1)

70 ± 28

0.1; A 98; (1); 1

CH3C(O)CH2CH2CHO; CH3C(O)CH3; CH3C(O)CH3; CH2O CH3C(O)CHO; CH3OCHO

190 ± 76 61.8 ± 24.7

A 99; 1 0.1; A 75; 25

CH(O)C(O)CH2Cl; CH2O CH3C(O)CHO; CH(O)CHO

72.0 ± 28.8

0.2; A 99; 0.2

CH3C(O)CH(OH)CHO

51.0 ± 25.5

0.2; A 99; 0.2

CH3C(O)CH(OH)CHO

270 ± 108 0.87 ± 0.30

0.1; (0.1); A 99; (0.1); 0.1 CH3C(O)C(OH)2C(O)C(O)CH3; CH3C(O)C(O)C(O)C(O)CH3? a 25; b 50 CHOC(O)CH2CHO

2.9 ± 1.0

a 19; b 31

C

b H2C

CH2 a

Cyclopentanone

b H2C

B (K)

n

Estimated % H-atom Abstraction at Each Site

Observed or predicted Major Products in NOx-containing Atmospherese

O

a H2C

b H2 C

1012 × A

H2 a C C O C H2 a

CHOCH2C(O)CH2CHO

Ketone

1012 ×k(298 K)

Cyclohexanone

6.4 ± 2.2

a 15; b 24;

CHOCH2C(O)CH2CH2CHO

31.0 ± 9.3

a A 84; b 7; c 4; d 3; e 2

CH3C(O)CH2CH2CH2C(O)CHO

24.0 ± 7.2

a A 87; b 7; c 3; d 1

CH3C(O)CH2C(CH3)2CH2C(O)CHO

2.9 ± 0.9

Reaction is largely by addition at bond a

a H2 C

B (K)

n

Estimated % H-atom Abstraction at Each Site

Observed or predicted Major Products in NOx-containing Atmospherese

O

b H2C b H2C

1012 × A

C H2 b

CH2 a

3-Methyl-2-cyclohexen-1-one H2 c C CH2 d

b H2C e H3C

C

O

a C H

3,5,5-Trimethyl-2-cyclohexene-1-one CH3 d

d H3C C

CH2 c

b H2 C c H3C

C a C H

C

O

f) Aromatic ketones 1,4-Naphthoqinone O

b

?

a

O

(continued)

271

272

TABLE IV-J-1 (CONTINUED)

1012 ×k(298 K)

Ketone

g) Ketones derived from biogenic hydrocarbons Nopinone 15.2 ± 3.0 e H3C CH3 e

1012 × A

B (K)

n

Estimated % H-atom Abstraction at Each Site

Observed or predicted Major Products in NOx-containing Atmospherese

a 47; b 20; c 12; d 7; e 1

CH3C(O)CH3?

6.0 ± 2.4

a 37; b 22; c 14; d 9; e 2

CH3C(O)CH3; CH2O?

Camphenilone

48.0 ± 19.2

a 35; b 21; c 14; d 2

?

4-Methylcyclohex-3-en-1-one

110 ± 44

A a 95; b 3; c 1; d 1

CH3C(O)CH2CH2C(O)CH2CHO?

C

c CH2 d C H 2 C C H b O

a HC c H2C

Sabinaketone e H3C

CH3 e c CH2

HC a C c H2C

CH b

c H2 C b H2C d H3C

C a C H

C

CH2 d C O

O

CH2 c

Ketone

1012 ×k(298 K)

Camphor

4.1 ± 1.6

h) Ketones used as fragrance compounds β-Ionone d H3C O

C

H b C C C H a d H3C

121 ± 36

CH3 d

d H3C

d H2C d H2C

C c H2

H C a C H

n

Estimated % H-atom Abstraction at Each Site

Observed or predicted Major Products in NOx-containing Atmospherese

a 47; b 18; c 11; d 2

CH3C(O)CH3; C10-dicarbonyl; hydroxydicarbonyl

A a 67; A b 29; c 1; d 0.1

CH3C(O)CHO; 2,6,6-trimethylcyclohex-1-ene-carbaldehyde?

CH2 d

C

CH3 e C

B (K)

C

C H2 c

CH2 d

1-(1,2,3,4,5,6,7,8,-Octahydro-2,3,8,8-tetramethyl-2- 99 ± 40 naphthalene)ethanone e H3C

1012 × A

e H3C C C c H2

A a 90; b 24; c 2; d 1; e 0.1

?

O C

CH3 e

CH b CH3 e

(continued)

273

274

TABLE IV-J-1 (CONTINUED)

1012 ×k(298 K)

Ketone

1012 × A

B (K)

n

Estimated % H-atom Abstraction at Each Site

Observed or predicted Major Products in NOx-containing Atmospherese

77.0 ± 30.8

A a 90; b 3; c 2; d 1; e 0.1

i) Halogen-substituted ketones CH2FC(O)CH3 CF3C(O)CH3 CH2ClC(O)CH3 CHCl2C(O)CH3 CCl3C(O)CH3

0.215 ± 0.053 (1.1 ± 0.6)×10−3 0.438 ± 0.110 0.402 ± 0.100 0.0154 ± 0.0039

39; 51 100 72; 28 68; 32 100

HCOF: CO2 COF2; CO2 HC(O)Cl, CO2, CH2O, HCl, CO HC(O)Cl, Cl, CO2, CH2O COCl2, Cl, CH2O, CO2

j) Ketenes Ketene (CH2=C=O) Methyl ketene (CH3CH=C=O) Ethyl ketene (CH3CH2CH=C=O) Dimethyl ketene [(CH3)2CH=C=O]

14.0 ± 4.9 72 ± 36 130 ± 52 120 ± 48

No estimate No estimate No estimate No estimate

CO2, CH2O? CO2, CH3CHO,? C2H5CHO, CO2,? CH3C(O)CH3, CO2?

Acetyl cedrene eH C 3 c H2C c CH C

e H3C d H2C

d H2 C

C H c

C

O

C a

C

?

CH3 e

d CH b C H2 C CH3 e CH3 e

a Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated. b The abbreviated code in column 6 of the first part of the table gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive >CH, –CH2–, –CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c Data are best represented by a sum of two exponential terms: 8.8 × 10−12 e−1320/T + 1.7 × 10−14 × e423/T. d Products of the photodecomposition of the ketones, often the major loss mechanism for ketones in the troposphere, are not given here (see Chapter VIII). e The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors. Only the major products that have been reported are given; when no product studies have been made the products shown (italics) are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. f Blanco and Teruel (2011). g Blanco et al. (2012).

The Hydroxyl Radical and Its Role in Ozone Formation the dotted gray curve from the SAR calculations in Figure IV-J-1 lie within the error bars of the experimental points. So, as with the alcohols, ethers, and aldehydes, evidence for a long-range enhancement of rate coefficients by the presence of a ketone group is not confirmed. Realistic tests for the existence of such enhancement in reactivity of the HO radical with the ketones, alcohols, ethers, and aldehydes will require experimental measurements of greater precision than now exist. The mechanism for HO reaction with ketones can be illustrated using methyl ethyl ketone as the reactant, as in Figure IV-J-2. Reaction by pathway

FIGURE IV-J-2.

275

(a), abstraction from the CH2 group, ultimately leads to fragmentation of the molecule that generates acetaldehyde and formaldehyde. Reaction by pathway (b)  can lead ultimately to fragmentation of the •OCH2CH2C(=O)CH3 radical to form formaldehyde and methylglyoxal by pathway (c), or, by rearrangement, the hydroxydicarbonyl O=CHCH2C(=O)CH2OH can be formed in a series of steps. It is estimated that reaction by pathway (a) is favored. Presumably, reaction at the activated CH3 group in pathway (b)  contributes most of the remaining reaction. It has been suggested that the fraction of the alkoxy radical that reacts by

A simplified mechanism of the reaction of HO with methyl ethyl ketone.

276

the mechanisms of reactions influencing atmospheric ozone

pathway (e) is larger than that by pathways (c) and (d) (Calvert et al., 2011; Ferenac et al., 2003).

IV-J-3. HO Radical Reactions with Unsaturated Ketones Substitution of a >C=C< bond in a ketone molecule enhances the k(298 K) significantly, as one might expect in view of the reactivity of the alkenes. For example, k(298 K) for CH2=CHC(O)CH3 is 18 times larger than that for the corresponding saturated ketone, CH3CH2C(O)CH3, with essentially all the reactivity expected to occur via HO addition to the double bond.

IV-J-2. HO Radical Reactions with Hydroxyketones The k(298 K) values for the reaction of HO with the hydroxy-2-alkanones and the corresponding ketone of the same carbon number are plotted versus C number in Figure IV-J-3. All the available data for the hydroxy-2-alkanones are plotted without regard to the position of HO-group substitution. Data include 1-, 3-, 4-, and 5-HO-substituted ketones. Except for the data for 1-hydroxy-2-heptanone (plotted as a star), the hydroxy-substituted ketone show a significant enhancement in k(298 K) over that of the unsubstituted equivalent ketone. The k(298 K) for acetone shows the greatest change on HO group replacement of an H atom, a factor of 23. The other hydroxyketones show factors of 2–9 enhancement in k(298 K), which is similar to that seen for the HO substitution in the alkanes. Insertion of an HO group in a molecule causes an increase in reactivity, not because of added HO radical attack on the H atom of the alcohol group (–HO), but by alteration of the electron distribution in the molecule with increased reactivity at the C‒H bonds, particularly those adjacent to the HO group.

I V- K . M E C H A N I S M S O F R E AC T I O N O F H O R A D I CA L W I T H O R G A N I C AC I D S A N D AC I D A N H Y D R I D E S The simple organic acids (RCO2H) react with HO radicals by abstraction of H atoms. The measured rate coefficients for reaction of HO with HC(=O)OH, D(C=O)OH, and DC(=O)OD at 298 K are

2-Alkanones Hydroxy-2-Alkanones Regression Excluding 1-Hydroxy-2-Heptanone 1-Hydroxy-2-Heptanone

25 1012 x k(298 K) for hydroxy-2-alkanones and 2-alkanones, cm3 molecule−1 s−1

IV-J-4. HO Radical Reactions with Halogen-Atom-Substituted Ketones As observed with other families of compounds, F-atom substitution in ketones results in a significant lowering in the k(298 K) for reaction with HO. Thus, k(298 K) is lowered by about a factor of 4.0 × 10−3 when three F atoms are substituted for H atoms in CH3C(O)CH3 to form CF3C(O)CH3.

20

15

10

5

0

3

4

5

6

7

8

Carbon number FIGURE IV-J-3.

ketones.

Comparison of the k(298 K) for HO reactions with hydroxyketones and structurally similar unsubstituted

The Hydroxyl Radical and Its Role in Ozone Formation 4.6 × 10−13, ≤ 1.5 × 10−13, and 0.64 × 10−13 cm3 molecule−1 s−1, respectively (Wine et al., 1985; Singleton et  al., 1988). These rate data and the fact that the dimer of formic acid is less reactive with HO than the monomer, all suggest the intuitively unexpected result that the major pathway for reaction is abstraction of the acidic hydrogen in reaction (1): HO + HC(=O)OH → H2O + HC(=O)O• (1) → H2O + •C(=O)OH

(2

Reaction is expected to proceed through an initial formation of a prereactive complex, HO∙∙∙∙H∙∙∙∙OC(=O)H. Both the HC(=O)O• and •C(=O)OH radicals are expected to react rapidly with O2 to form CO2 and HO2• Reaction of HO at the acidic H atom is also the favored pathway for acetic acid, but abstraction of H atoms from the alkyl chain is expected also to occur in the larger acids. Tables IV-K-1 and IV-K-2 summarize the recommended rate coefficients for the HO reactions with the simple organic acids and SAR estimates for the HO reaction with the acid anhydrides (Calvert et al., 2011). Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. In Figure IV-K-1, the effects of the alkyl chain length on the rate coefficients (298 K) for the

277

reactions of HO with the simple organic acids are compared with those for the hydrocarbons of the same carbon number. Note that the three simplest organic acids have rate coefficients that are greater than those of the alkanes of equal carbon number even though the number of C–H bonds is less. I V- L . M E C H A N I S M S O F R E AC T I O N O F H O R A D I CA L S WITH ESTERS IV-L-1. HO Radical Reactions with Acyclic Esters The HO rate coefficients for a large number of esters have been determined, and the recommended values for these are summarized in Table IV-L-1 (Calvert et al., 2011). Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. In Figure IV-L-1, the HO rate coefficients, k(298 K), for reactions with the formate esters [HC(O)OR], the acetate esters [CH3C(O)OR], and the methyl esters [CH3(O)OR] are plotted versus the number of reactive groups in the molecule; R is an n-alkyl group. The formate and acetate esters show a consistent trend of increasing reactivity with increasing size of the molecules. Least squares fits to the data suggest that each additional CH2 group added to the acetate esters [CH3C(O)OR] or the methyl esters [CH3OC(O)R] increases k(298 K) by about 1.9 ×

1012 x k(298 K) for HO reactions with acids and n-alkanes, cm3 molecule−1 s−1

3.0 RC(=O)OH n-Alkane (RH)

2.5 2.0 1.5 1.0 0.5 0.0 1

2

3

4

Carbon number of acid or n-alkane FIGURE IV-K-1.

Comparison of the rate coefficients (298 K) for HO reactions with the simple organic acids with those of the alkanes of the same carbon number.

TABLE IV-K-1 HO RE ACTIONS WITH THE ORG ANIC ACIDS. THIS TABLE GIVES THE RECOMMENDATIONS FOR R ATE COEFFICIENTS, k = A × e −B/T (CM 3 MOLECULE −1 S −1 ), FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 5 SHOWS STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES (USING TABLES IV-O-1, IV-O-2, IV-O-5, AND IV-O6) OF THE PERCENTAGE OF RE ACTION BY ADDITION AND H-ATOM ABSTR ACTION AT E ACH SITE. b COLUMN 6 SHOWS THE MA JOR PRODUCTS THAT ARE OBSERVED OR PREDICTED FOR NO X-CONTAINING ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN c

1013 × 1013 × A B (K) k(298 K)

Estimated % H-atom Abstraction at Each Site; Preceding “A” Indicates Addition to C=C bondb

4.3 ± 0.9

3.66

−50

Acetic acid [CH3C(O)OH]

7.0 ± 1.4

0.53

−767

Experimental evidence shows reaction at the C(O)O‒H site is favored 42; 58

Propionic acid [CH3CH2C(O)OH]

12 ± 3

~0

14; 73, 12

Butanoic acid [CH3CH2CH2C(O)OH]

21 ± 6

70

7, 50; 37; 6

Acid

a) Acyclic acids Formic acid [HC(O)OH]

b) Unsaturated acids Acrylic acid [CH2=CHC(O)OH]

170 (± factor of 2)

c) Keto-acids Pyruvic acid CH3C(O)C(O)OH]

1.2 ± 0.4

d) Aromatic acids Benzoic acid [C6H5C(O)OH]

~11

e) Halogen-substituted acids CF3C(O)OH 1.24 ± 0.62 CxF2x+1C(O)OH (x =2,3,4)

26

1.55 ± 0.47

0.49

−276

H2O; CO2

H2O; CH2O; CO2; HOCH2C(O)OH (minor) CH3CHO; CH3C(O)C(O)OH; CO2 CH3CHO; HC(O)C(O)OH; CH3C(O)CH2C(O)OH

A 98; 2

CH3C(O)C(O)OH; CH2O; CO2

42; 58

Photolysis dominant loss

A 79; 21

~0

Observed or Predicted Major Products for NOx-Containing Atmospheresd

100 100

?

Dry and wet deposition largely, CO2, COF2 Dry and wet deposition largely, CO2, COF2

Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated. The abbreviated code in column 5 of the table gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive >CH, –CH2–, –CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made the products shown in italics are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. a

b

278

The Hydroxyl Radical and Its Role in Ozone Formation

279

TABLE IV-K-2 . HO + ACID ANHYDRIDE RE ACTIONS: THE STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR)-ESTIMATED R ATE COEFFICIENTS k (CM 3 MOLECULE −1 S −1 ) AT 298 K ARE GIVEN HERE. THESE WERE NOT DETERMINED EXPERIMENTALLY, BU T THEY WERE ESTIMATED FROM SAR METHODS AND HAVE A HIGH UNCERTAINT Y

1013 × k(298K)

Acid anhydride Formic acid anhydride Formic acetic anhydride Acetic acid anhydride

~12 ~6.5 ~0.9

ROC(O)H

10

1012 x k(OH), cm3 molecule−1 s−1

ROC(O)CH3 Y = −4.17 x 10−12 + 1.94 x 10−12 X RC(O)OCH3

8

Y = −5.40 x 10−12 + 2.14 x 10−12 X Straight Chain Alkanes (Reference) 6

4

2

0

2

3

4 5 Number of reactive C-atoms

6

FIGURE IV-L-1.

Plot of k(298 K) for HO reaction with three series of structurally related esters (HC(O)OR, CH3C(O)OR, and RC(O)OCH3) versus the number of reactive C atoms in the molecule; in this plot, the carbon atoms in the –C(=O)O– group of all the esters are considered to be unreactive. Only the esters with straight carbon chains are plotted.

10−12 and 2.1 × 10−12 cm3 molecule−1 s−1, respectively. These values are somewhat greater than, but within the error limits equal to, those seen for the n-alkanes (1.4 × 10−12). IV-L-2. HO Radical Reactions with Unsaturated Esters The HO reactions with the unsaturated esters have large k(298 K) values that reflect the presence of the >C=C< group. The data for the unsaturated acetates, the alkyl acrylates, and the alkyl methacrylates are plotted in Figure IV-L-2. Each of the k(298 K) values are much larger than the alkanes with the same number of reactive C atoms. Both

the unsaturated acetates (closed inverted triangles) and the alkyl acrylates (open squares) have one alkyl group attached to the double bond, and k(298 K) values are significantly smaller than those for compounds with two groups attached to the double-bonded carbon atoms (filled circles and open triangles, respectively). IV-L-3. HO Radical Reactions with Halogen-Atom-Substituted Esters F-atom substitution for H atoms in the esters lowers k(298 K) significantly for the HO reactions with the substituted ester, as seen with the other organic compounds. For example, two F atoms

280

the mechanisms of reactions influencing atmospheric ozone Unsaturated Acetates (1 Group) Unsaturated Acetates (2 Groups) Alkyl Acrylates (1 Group) Alkyl Methacrylates (2 Groups) n-Alkanes

1012 x k(298 K) for HO-unsaturated ester reactions, cm3 molecule−1 s−1

100

80

60

40

20

0

3

4

5

6

7

Number of reactive C-atoms FIGURE IV-L-2.

Plot of the k(298 K) values for the HO reaction with some unsaturated esters. Three of the unsaturated acetates contain one group attached to the double-bonded carbon atoms (plotted as inverted filled triangles); two contain two groups (filled circles); the alkyl acrylates (open squares) contain one group, whereas the alkyl methacrylates (open triangles) all contain two groups. Data for the n-alkanes are also shown for comparison.

substituted for H atoms in CH3C(O)OCH3 to form CF2HC(O)OCH3 lowers k(298 K) by a factor of 0.38, whereas substitution of a third F atom to form CF3C(O)OCH3 lowers k(298 K) by an additional factor of 0.37. I V- M . M E C H A N I S M S O F R E AC T I O N S O F H O R A D I CA L W I T H N - C O N TA I N I N G O X Y G E N AT E S The atmospheric chemistry of the several families of N-containing oxygenates has been reviewed and the rate coefficient data evaluated (Calvert et al., 2011). Table IV-M-1 summarizes the recommended rate coefficients. Plots of the complete datasets and evaluation of the various individual measurements from which these recommendations were derived can be accessed in that work. In this section, we discuss the HO reactions of one very important family of N-containing compounds, the alkyl nitrates (RONO2). The alkyl nitrates are common products of the atmospheric oxidation of the hydrocarbons. A  small fraction of the reactions of NO with the

alkyl peroxy radicals (RO2) form alkyl nitrates, and this fraction increases somewhat as the R group becomes larger (see Chapter V). The alkyl nitrates have somewhat longer lifetimes in the atmosphere than the alkanes and contribute to the transport of NOx throughout the troposphere. Rate coefficients for the simple alkyl nitrates are compared to those for the alkanes in Figure IV-M-1. Each of the nitrates shown has rate coefficients that are below those of the alkane of the same carbon number. The extent of the suppression of k(298 K) in the nitrates depends on the position of the nitrate group in the alkyl chain of the atoms. Although the data for the alkyl nitrates are of limited accuracy, they suggest that the deactivation of the nitrates is least important for substitution of the ‒ONO2 group at the terminal or 1-position, whereas substitution at the 2- and 3-positions in the carbon chain results in somewhat greater reductions even though, in this case, a tertiary H atom is created with the  –ONO2 group substitution for a secondary H atom. Perhaps the greater overall effect results from the more central location of the –ONO2 group.

TABLE IV-L-1 . HO RE ACTIONS WITH THE ESTER S. THIS TABLE GIVES THE RECOMMENDATIONS FOR R ATE COEFFICIENTS, k = A × T n × e −B/T (CM 3 MOLECULE −1 S −1 ) FOR TROPOSPHERIC TEMPER ATURES. a COLUMN 6 SHOWS SAR ESTIMATES (USING TABLES IV-O-1, IV-O2, IV-O-5, AND IV-O-6) OF THE PERCENTAGE OF RE ACTION BY ADDITION (PREFIX “A”) AND H-ATOM ABSTR ACTION AT E ACH SITE. B COLUMN 7 SHOWS THE MA JOR PRODUCTS THAT ARE OBSERVED OR PREDICTED (ITALICS) FOR NO X-CONTAINING ATMOSPHERES. N-CONTAINING PRODUCTS ARE NOT SHOWN c

Ester

1012 × k(298 K)

a) Acyclic esters Methyl formate [CH3OC(O)H

0.179 ± 0.053

Ethyl formate [CH3CH2OC(O)H]

0.875 ± 0.219

n-Propyl formate [CH3CH2CH2OC(O)H]

1.85 ± 0.46

iso-Propyl formate [(CH3)2CHOC(O)H] n-Butyl formate [CH3CH2CH2CH2OC(O)H]

2.29 ± 0.46

tert-Butyl formate [(CH3)3COC(O)H] Methyl acetate [CH3OC(O)CH3] Ethyl acetate [CH3CH2OC(O)CH3]

Methyl propionate [CH3OC(O)CH2CH3]

3.68 ± 0.74

0.782 ± 0.195 0.346 ± 0.069 1.67 ± 0.25 1.54 ± 0.22g 0.93 ± 0.13g

1018 × A

1.28 0.900 2.41

B (K)

n

Estimated % H-Atom Abstraction at Each Site;b

−136

2

47; 52

−713

2

20; 69; 11

−643

2

8; 55; 33; 4 (11, 11); 73; 6

4.95

5.41 0.669 1.69 × 105

−633

2

5; 33; 40; 20; 3

−145

2

(27, 27, 27); 18

−525

2

69; 32

−683

0

9; 85; 6

83; 11; 6

Observed or Predicted Major Products for NOx-containing Atmospheresd

HC(O)OH; HC(O)O(O)CH; CO2; CH2O HC(O)O(O)CH; CH3C(O)O(O)CH; HC(O)OH; CH3CHO; CH2O; CO2 CO2; C2H5CHO; C2H5OC(O)H; CH3C(O)CH2OC(O)H CH3C(O)O(O)CH; CH3C(O)CH3; HC(O)OH; CO2 CH3CH2C(O)CH2OC(O)H; C3H7C(O)OCHO; HC(O)OH; CH3C(O)CH2CH2CHOC(O)H CH(O)C(CH3)2OCHO; CO2; CH3C(O)CH3; CH2O CH3C(O)OH; CH3C(O)OC(O)H; CO; CH3C(O)O(O)CH CH3C(O)OH; CH2O; CH3C(O)OCH2CHO; HC(O)CH2OC(O)CH3 CH2O; CH3CHO; CO2; CH(O)OC(O)C2H5

281

(continued)

282

TABLE IV-L-1 (CONTINUED)

Ester

1012 × k(298 K)

Ethyl propionate [CH3CH2OC(O)CH2CH3]

2.1 ± 0.2i 2.1 ± 0.3j

n-Propyl acetate [CH3CH2CH2OC(O)CH3] iso-Propyl acetate [(CH3)2CHOC(O)CH3] n-Butyl acetate [CH3CH2CH2CH2OC(O)CH3] iso-Butyl acetate [(CH3)2CHCH2OC(O)CH3] sec-Butyl acetate [CH3CH2CH(CH3)OC(O)CH3 tert-Butyl acetate [(CH3)3COC(O)CH3] n-Pentyl acetate [CH3CH2CH2CH2CH2OC(O)CH3 Methyl propionate [CH3OC(O)CH2CH3] Ethyl propionate [CH3CH2OC(O)CH2CH3] n-Propyl propionate [CH3CH2CH2OC(O)CH2CH3] Methyl n-butyrate [CH3OC(O)CH2CH2CH3]

1018 × A

B (K)

n

Estimated % H-Atom Abstraction at Each Site;b

Observed or Predicted Major Products for NOx-containing Atmospheresd

8; 71; 13; 8

CH3(O)OC(O)CH2CH3; CH3CHO; CO2

2.1 ± 0.2k 3.45 ± 0.86

1.46

−978

2

5; 45; 47; 3

3.79 ± 0.76

0.455

−1,353

2

(5, 5); 88; 3

5.66 ± 1.13

3.03

−908

2

4; 25; 30; 39; 2

6.47 ± 0.97

1.50

−1,157

2

(4,4); 51; 39; 2

6.1 ± 1.2

1.07

−1,240

2

3; 21; 71; (3); 2

HC(O)O(O)CCH3; CH3CHO; CH3C(O)OCH2CHO; CH3C(O) OH; CH3C(O)OCH2CH2CHO CH3C(O)CH3; CH3C(O)OH; CH3C(O)O(O)CCH3 CH3CH2C(O)CH2OC(O)CH3; C3H7C(O)O(O)CCH3 CH3C(O)CH3; CH3C(O)O(O)CH; CH3C(O)OCH2C(O)H; CH3C(O)OH C2H5C(O)O(O)CCH3; C2H5C(O)CH3

0.575 ± 0.144 7.39 ± 1.85

3.19 3.71

−211 −927

2 2

(28, 28, 28); 17 3; 19; 23; 23; 30; 2

CH3C(O)O(O)CCH3; CH3C(O)CH3 C4H9C(O)O(O)CCH3

0.874 ± 0.175

2.00

−475

2

20; 64; 20

C2H5C(O)O(O)CH; C2H5C(O)OH; CO; CH3C(O)C(O)OCH3; CH3CHO CH3C(O)O(O)CC2H5; CH3C(O)OCH2CHO C2H5(C(O)O(O)CC2H5; CH3C(O)CH2OC(O)C2H5 CH3CH2OC(O)O(O)CH3; CH3C(O)OCH2CHO

2.1 ± 0.6i

7; 59; 27; 7

4.0 ± 1.2

4; 29; 46; 17; 4

3.36 ± 0.67

1.35

−993

2

8; 32; 53; 6

Ester

1012 × k(298 K)

Ethyl n-butyrate [CH3CH2OC(O)CH2CH2CH3] n-Propyl n-butyrate [CH3CH2CH2OC(O)CH2CH2CH3]

4.5 ± 1.6

4; 37; 27; 35; 4

4.7 ± 1.4 7.3 ± 1.9f

3; 21; 33; 15; 25; 3

n-Butyl propionate 6.0 ± 1.6f [CH3CH2CH2CH2OC(O)CH2CH3] n-Butyl n-butyrate 10.6 ± 3.1 [CH3CH2CH2CH2OC(O)CH2CH2CH3] 9.8 ± 6.1f Methyl iso-butyrate [CH3OC(O) 1.7 ± 0.4 CH(CH3)2] iso-Propyl iso-butyrate 7.0 ± 2.1 (CH3)2CHOC(O)CH(CH3)2] Methyl n-pentanoate 5.26 ± 1.31 [CH3OC(O)CH2CH2CH2CH3] Methyl pivalatec 1.19 ± 0.30 [CH3OC(O)C(CH3)3] Methyl n-hexanoate 7.36 ± 1.84 [CH3OC(O)CH2CH2CH2CH2CH3] Bornyl acetate 12.5 ± 3.8

1018 × A

1.92 2.98

B (K)

n

−400 −209

Estimated % H-Atom Abstraction at Each Site;b

3; 23; 28; 37; 6; 3 2; 16; 20; 26; 12; 20; 2

5.35

1.88

3.33

Observed or Predicted Major Products for NOx-containing Atmospheresd CH3C(O)O(O)C3H7; C3H7CHO; CO2; CH3CHO C2H5CHO; CO2; C3H7C(O)O(O)CC2H5 n-C3H7C(O)OC(O)C2H5; CO2; n-C3H7CHO, n-C3H7C(O)O(O)CC3H7; C2H5CHO; CO2; n-C3H7CHO

180

−1,028

−958

11; 72; (8, 8)

CH3COCH3; CO2; CH2O

(3,3); 60; 28; (3,3)

CH3C(O)CH3; CO2;

2

5; 21; 35:35; 4

b

30; (23, 23, 23)

2

4; 16; 26; 26; 26; 3 a 28; b 36; c 11; d 1

CH3OC(O)CH2C(O)CH2CH3; C3H7CHO; CO2; CH2O HC(O)O(O)C(CH3)3; CH2O; CO2; CH3C(O)CH3; (CH3)3C(O)OH CH3OC(O)CH2C(O)C3H7; C5H11CHO; CO2; CH2O 1,7,7-Trimethyl-6-acetyloxybicyclo[2.2.1]-heptan-2,3-dione

(continued)

283

284

TABLE IV-L-1 (CONTINUED)

1012 × k(298 K)

Ester

b) Difunctional esters Ethylene glycol diformate 0.484 ± 0.12l [HC(O)OCH2CH2OC(O)H] Ethylene glycol diacetate 2.16 ± 0.54 [CH3C(O)OCH2CH2O(O)CCH3] Dimethyl succinate 1.36 ± 0.34 [CH3O(O)CCH2CH2C(O)OCH3] Dimethyl glutarate 3.23 ± 0.81 [CH3O(O)CCH2CH2CH2C(O)OCH3] Dimethyl adipate 8.15 ± 2.04 [CH3O(O)CCH2CH2CH2CH2C(O)OCH3] c) Cyclic esters (lactones) 3H-Furan-2-one 44.2 ± 11.1 b H2C O

C

H C

O

C

B (K)

n

Estimated % H-Atom Abstraction at Each Site;b

Observed or Predicted Major Products for NOx-containing Atmospheresd

5; 45; 45; 5

HC(O)O(O)CH; HC(O)OH; CO

1; 49; 49; 1

CH3C(O)O(O)H; CH2O; CO2

19; 31; 31; 19

HC(O)CH2C(O)OCH3; CH2O; CO2; HC(O)O(O)CCH2CH2C(O)OCH3 CH3OC(O)CH2CH2CH2C(O)OH; CH3OC(O)CH2C(O)CH2C(O)OCH3 CH3OC(O)CH2C(O)CH2CH2C(O) OCH3; HC(O)CH2C(O)CH3; CH(O)CH2CH2C(O)OCH3

8; 14; 55; 14; 8 5; 9; 36; 36; 9; 5

A a 98; b 2

HC(O)CH2C(O)OCHO?

A a 98; b 1; c 1

HC(O)CH2C(O)OC(O)CH3?

a CH

O

5-Methyl-3H-furan-2-one b H2C

1018 × A

H C

a C

O

CH3 c

67.8 ± 17.0

Ester

d) Unsaturated esters Vinyl acetate [CH2=CHOC(O)CH3] Allyl acetate [CH2=CHCH2OC(O)CH3] iso-Propenyl acetate CH2=CH(CH3) OC(O)CH3] Vinyl propionate [CH2=CHOC(O)CH2CH3] 4-Pentenyl acetate [CH2=CHCH2CH2CH2OC(O)CH3] cis-3-Hexenyl acetate [CH3CH2CH=CHCH2CH2OC(O)CH3] Methyl acrylate [CH3OC(O)CH=CH2] Ethyl acrylate [CH3CH2OC(O)CH=CH2] Methyl crotonate [CH3OC(O)CH=CHCH3] Ethyl crotonate [CH3CH2OC(O)CH=CHCH3] n-Butyl acrylate [CH3CH2CH2CH2OC(O)CH=CH2 Methyl methacrylate [CH3OC(O)C(CH3)=CH2] Ethyl methacrylate [CH3CH2OC(O)C(CH3)=CH2] n-Butyl methacrylate [CH3CH2CH2CH2OC(O)C(CH3)=CH2]

1012 × k(298 K)

24.9 ± 5.0 26.6 ± 5.3 67.2 ± 13.4

1018 × A

4.77 × 106 2.29 × 106 4.36 × 106

B (K)

n

−493 −731 −815

0 0 0

Estimated % H-Atom Abstraction at Each Site;b

Observed or Predicted Major Products for NOx-containing Atmospheresd

A 99; 1 A 91; 8; 0.1 A 99; (1); 0.1

CH2O; HC(O)O(O)CCH3 CH2O; HC(O)CH2OC(O)CH3 CH2O; CH3C(O)O(O)CCH3

24.6 ± 2.8e

A 98; 1, 1

CH2O; CH(O)OC(O)C2H5

43 ± 11

A 84; 7; 5; 5; 0.5

CH2O; CH(O)CH2CH2CH2OCCH3

77.4 ± 15

0.3; 4; A 90; 4; 2; 0.1

C2H5CHO; CH(O)CH2CH2OC(O)CH3

12.9 ± 2.6 15.9 ± 3.2

2.00 × 106 2.50 × 106

−556 −552

0 0

3; A 97 2; 17; A 81

HCOC(O)OCH3; CH2O CH2O; HC(O)C(O)OC2H5

47 ± 7h

1: A 98; 1

CH3C(O)CHO; CH3CHO

50 ± 6h

0; 3; 97; 0

CH3CH2OC(O)CHO; CH3CHO

19.4 ± 6.8

3.74 × 105

−1,177

0

2; 11; 13; 13; A 61

HC(O)C(O)OC4H9; CH2O

40.7 ± 8.1

2.68 × 106

−811

0

1; (5); A 94

CH3C(O)CO)OCH3; CH2O

1; 12; (5); A 82

CH3C(O)C(O)OC2H5; CH2O

1; 7; 8; 8; (4); A 76

CH3C(O)C(O)OC4H9; CH2O

43 ± 11 65.8 ± 13

2.44 × 106

−982

0

(continued)

285

286

TABLE IV-L-1 (CONTINUED)

1012 × k(298 K)

Ester

e) Aromatic esters Methyl salicylate

1018 × A

B (K)

n

~11

Estimated % H-Atom Abstraction at Each Site;b

Observed or Predicted Major Products for NOx-containing Atmospheresd

A a 96; b 2; c 1

?

50; 50

HC(O)OC(O)OCH3; CH2O; CO2;

2.59 ± 0.78

6; 80; (5); 8

CH3C(O)C(O)OCH3

3.66 ± 1.09

4; 53; (3); 36; 4

C2H5C(O)C(O)OCH3

O C a

O

CH3 b

OH c

f) Carbonates Dimethyl carbonate [CH3OC(O)CH3] g) Lactates Methyl lactate [CH3CH(OH)C(O)OCH3] Ethyl lactate [CH3CH(OH)C(O)OCH2CH3]

0.326 ± 0.082

h) Halogen-substituted esters CF3OC(O)H 0.0163 ± 0.0041

CF3CFHOC(O)H C2F5OC(O)H n-C3F7OC(O)H FC(O)OCH3 CHF2C(O)OCH3 CF3C(O)OCH3

0.0168 ± 0.0042 0.0120 ± 0.0030 0.0202 ± 0.0050 0.0255 ± 0.0064 0.133 ± 0033 0.0498 ± 0.0125

0.631

−525

2

1.17 × 106

1,274

0

8.84 × 105 1.91 × 106 1.45 × 106 2.12 × 105

1,181 1,511 1,274 631

0 0 0 0

3.23 × 105

557

0

Probably dissolution and hydrolysis in oceans most significant fate; 100 (as above); 29; 71 (as above); 100 (as above); 100 (as above); 100 14; 85 100

CO2; CF3OH; COF2

CO2; CF3C(O)F; HC(O)F CO2; CF2O; CF3OH; COF2 CO2; C3F7O →? CH2O; CO2; HF? CH2O; CO2; COF2 CH2O; CO2; CF3OH; COF2

Ester

1012 × k(298 K)

CF3C(O)OCH2CH3 0.236 ± 0.059 CF3C(O)OCH2CF3 0.094 ± 0.024 CH2=CHC(O)11.5 ± 2.3 OCH2CH2CF2CF2CF2CF3

1018 × A

B (K)

n

Estimated % H-Atom Abstraction at Each Site;b

63; 37 100 A 82; 18; 0.2

Observed or Predicted Major Products for NOx-containing Atmospheresd CH3CHO; CO2; CF3OH; COF2 CF3CHO; CO2; CF3OH; COF2 CH2O; CH(O)C(O)OCH2CH2C4F9

Most recommendations are those of Calvert et al. (2011) where detailed kinetic information is reviewed and evaluated. The abbreviated code in column 6 of the table gives the approximate percentage of H-atom abstraction or addition (labeled “A”) by HO radical at each successive >CH, –CH2–, –CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column 1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c The two major families of N-containing compounds expected are the RONO2 species, formed in a fraction of RO2-NO encounters, and the RC(O)O2NO2 species formed in the RC(O)O2 + NO2 reactions. The fraction of the products consisting of these N-containing compounds varies with the nature of the R group, [NOx], temperature, and other factors (see Chapter V). d Only the major products that have been reported are given; when no product studies have been made, the products shown (in italics) are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. e Blanco and Teruel (2011). f Liang et al. (2010). g Andersen et al. (2011). h Teruel et al. (2012). i Andersen et al. (2012b). j Wallington et al. (1988a). k Cometto et al. (2009). a

b

287

288

TABLE IV-M-1 . RECOMMENDED R ATE COEFFICIENTS (k = A e −B/T CM 3 MOLECULE −1 S −1 ) FOR THE HO RE ACTIONS WITH

N-CONTAINING OXYGENATES; WHEN SUFFICIENT DATA ARE AVAIL ABLE, STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES OF THE PERCENTAGE OF RE ACTION AT E ACH SITE ARE GIVEN IN COLUMN 5. B ALSO SHOWN IN COLUMN 6 ARE THE MA JOR PRODUCTS OBSERVED OR PREDICTED (ITALICS) FOR NO X-CONTAINING ATMOSPHERES. PHOTODECOMPOSITION PROVIDES ANOTHER ROU TE TO PRODUCTS FOR SOME OF THE COMPOUNDS LISTED IN THE TABLE (SEE CHAPTER VIII)

N-Atom-Containing Oxygenate

1012 × k(298 K) 1012 × A

B, K

Position of OH Attack

Observed or Predicted Major Products

a) Amides N-Methylformamide (HC(O)NHCH3] N,N-Dimethylformamide [HC(O)N(CH3)2] N-Methylacetamide [CH3C(O)NHCH3] N,N-Dimethylacetamide [CH3C(O)N(CH3)2] N-Methylpropionamide [CH3CH2C(O) NHCH3 N,N-Dimethylpropionamide [CH3CH2C(O)N(CH3)2] 1-Methyl-2-pyrrolidinone

8.0 ± 2.8 14 ± 4 5.2 ± 2.6 16 ± 5

No estimates No estimates No estimates No estimates

? ? ? ?

7.6 ± 3.8

No estimates

?

20 ± 10

No estimates

?

23 ± 5

No estimates

CH3 O

N C

O

CH2

H2C

N

N

O

CH2 CH3 N

C

C

H2C

CH2

O

CH2O

N-Methyl succinimide

O

CHO

CH3

O

13 ± 4

No estimates

?

N-Atom-Containing Oxygenate

1012 × k(298 K) 1012 × A

N-Formyl pyrrolidinone

6±3

No estimates

?

110 ± 27d 128 ± 10e 103 ± 22f 132 ± 33g

Addition to ring favorede

?

No estimates

?

No estimates

?

B, K

Position of OH Attack

Observed or Predicted Major Products

O

HC N

O

C

CH2

H2 C

CH2

Pyrrole

H N

HC HC

CH CH

2-(Dimethylamino)ethanol [(CH3)2NCH2CH2OH] 2-Amino-2-methyl-1-propanol [(CH3)2C(NH2)CH2OH] c) Alkyl nitrates Methyl nitrate (CH3ONO2)

85 ± 60

185

230

28 ± 14

0.023 (± factor of 2) Ethyl nitrate (CH3CH2ONO2) 0.18 ± 0.09 n-Propyl nitrate (CH3CH2CH2ONO2) 0.58 ± 0.17 iso-Propyl nitrate [(CH3)2CHONO2] 0.29 (± factor of 1.5) n-Butyl nitrate [CH3CH2CH2CH2ONO2) 1.6 ± 0.3 sec-Butyl nitrate [CH3CH2CH(CH3)ONO2] 0.86 ± 0.26 iso-Butyl nitrate [(CH3)2CHCH2ONO2] 1.5 ± 0.6 n-Pentyl nitrate (CH3CH2CH2CH2CH2ONO2) 3.6 ± 1.8 2-Pentyl nitrate 1.7 ± 0.5 [CH3CH(ONO2)CH2CH2CH3]

40

845

100

CH2O

65 1.2 62

387 168 230

42; 58 42; 47; 12 (45,45); 10

CH3CHO CH3C(O)CH2ONO2; CH3CHO; CH2O? CH3COCH3

11; 72; 15;3 27; 30; 15; (27) (20, 20); 56; 5 6; 38; 47; 8; 2 8; 5; 11; 68; 8

CH3C(O)CH2CH2ONO2 C2H5COCH3 CH3COCH3; CH2O C2H5C(O)CH2CH2ONO2 CH3CH(ONO2)CH2C(O)CH3

289

(continued)

290

TABLE IV-M-1 (CONTINUED)

N-Atom-Containing Oxygenate 3-Pentyl nitrate [CH3CH2CH(ONO2)CH2CH3] 2-Methyl-1-butyl nitrate [CH3CH2CH(CH3)CH2ONO2] 3-Methyl-1-butyl nitrate [(CH3)2CHCH2CH2ONO2] 3-Methyl-2-butyl nitrate [(CH3)2CHCH(ONO2)CH3] 2,2-Dimethyl-1-propyl nitrate [(CH3)3CCH2ONO2] 2-Hexyl nitrate [CH3CH(ONO2)CH2CH2CH2CH3] 3-Hexyl nitrate [CH3CH2CH(ONO2)CH2CH2CH3] cyclo-Hexyl nitrate

1012 × k(298 K) 1012 × A

B, K

Position of OH Attack

Observed or Predicted Major Products

1.04 ± 0.5

20; 23; 14; 23; 20

CH3CHO; C2H5CHO

2.3 ± 1.2

8; 57; 24; (8); 2

CH3CHO; CH3C(O)CH2ONO2

2.4 ± 1.2

(6, 6) 78; 7; 3

CH3C(O)CH3; CHOCH2ONO2

1.7 ± 0.3

(16.16); 45; 8; 16

CH3C(O)CH3; CH3CHO

0.79 ± 0.3

(31, 31, 31); 8

CH2O; CH3C(O)CH3

2.94 ± 0.88

1; 3; 7; 46; 37; 5

C3H7CHO; CH3CHO CH3CH(ONO2)CH2C(O)C2H5

2.50 ± 0.75

8; 9; 6; 11; 57; 8

3.06 ± 0.92

a 2; b 5; c 29

CH3CH2CH(ONO2)CH2C(O)CH3; CH3CHO; CH(O)CH(ONO2)C2H5 HCOCH2CH(ONO2)CH2CH2CHO?

1.59 ±0.48

(8, 8); 12; 58; 8

2.80 ± 0.84

7; 4; 5; (7); 49; 7

3.42 ± 1.0

5; 6; 3; 7; 41; 34; 5

3.60 ± 1.l

4; 3; 2; 5; 29; 23; 4

b H2 a C H ONO2 C c H2C c H2 C

C c H2

CH2 b

2-Methyl-2-pentyl nitrate [(CH3)2C (ONO2)CH2CH2CH3] 3-Methyl-2-pentyl nitrate [CH3CH(ONO2)CH(CH3)CH2CH3] 3-Heptyl nitrate [CH3CH2CH(ONO2)CH2CH2CH2CH3] 3-Octyl nitrate [CH3CH2CH(ONO2)CH2CH2CH2CH2CH3]

(CH3)2C(ONO2)CHO; CH3CHO; (CH3)2C(ONO2)CH2C(O)CH3 CH3CHONO2CH(CH3)C(O)CH3; CH3CH(ONO2)C(O)CH3; CH3CHO C2H5C(O)CH2CH(ONO2)C2H5; C2H5CHO; CHOCH(ONO2)C2H5 C2H5CH(ONO2)CH2C(O)C3H7; C2H5CH(ONO2)CHO; CHOC3H7

N-Atom-Containing Oxygenate d) Saturated dinitrates 1,2-Dinitrooxypropane [CH3CH(ONO2)CH2ONO2] 1,2-Dinitrooxybutane [CH3CH2CH(ONO2)CH2ONO2] 2,3-Dinitrooxybutane [CH3CH(ONO2)CH(ONO2)CH3] 1,4-Dinitrooxybutane (O2NOCH2CH2CH2CH2ONO2) trans-1-Methyl-1.2-dinitrooxycyclohexane b H2 C

c H2C

C b H2

B, K

Position of OH Attack

Observed or Predicted Major Products

< 0.3

19; 55; 26

CH3C(O)ONO2; CH2O; NO2

1.63 ± 0.49

34; 39; 19; 8

1.02 ± 0.30

34; 16; 16; 34

6.3 ± 1.9

2; 48; 48; 2

1.66 ± 0.92

a 1; b 7; c 40; d 3

CH3C(O)CH(ONO2)CH2ONO2; CH3CHO; CHOCH2ONO2; NO2 CH2O; CHOCH(ONO2)CH3; NO2; CHOCH(ONO2)CH(ONO2)CH3 O2NOCH2C(O)CH2CH2ONO2; CH2O; CHOCH(ONO2)CH3 CH(O)CH2CH2C(ONO2)(CH3)CH-(ONO2)CH2CHO?

14 ± 6

0.3; A 99; 0.3

O2NOCH2CHO

9.3 ± 2.8

A 99; 0.6; 0.3

CH2O; CHOC(ONO2)CH2ONO2; CHOC(O)C(O)CH2ONO2

6.7 ± 2.0

3; 91; 2; 4

HCOCH(ONO2)CH3

5.1 ± 1.5

2; 89; 5; 5

O2NOCH2C(O)CH3

7.4 ± 2.2

7; 65; 10; 9; 8

CHOCH(ONO2)CH2CH3

7.0 ± 2.1

2; 37; (3); 54; 4

O2NOCH2CH(OH)C(O)CH3; O2NOCH2C(O) CH2CH3 (continued)

CH3 a C

c H2C

1012 × k(298 K) 1012 × A

ONO2

CH2 d ONO2

e) Unsaturated dinitrates cis-1,4-Dinitrooxy-2-butene [O2NOCH2CH=CHCH2ONO2 3,4-Dinitrooxy-1-butene [CH2=CHCH(ONO2)CH2ONO2] f) Hydroxyalkyl nitrates 2-Nitrooxy-1-propanol [HOCH2CH(ONO2)CH3] 1-Nitrooxy-2-propanol [O2NOCH2CH(OH)CH3 2-Nitrooxy-1-butanol [HOCH2CH(ONO2)CH2CH3] 1-Nitrooxy-2-butanol [O2NOCH2CH(OH)CH2CH3]

291

292

TABLE IV-M-1 (CONTINUED)

N-Atom-Containing Oxygenate 4-Nitrooxy-2-butanol [CH3CH(OH)CH2CH2ONO2] 3-Nitrooxy-1-butanol [HOCH2CH2CH(ONO2)CH3] 4-Nitrooxy-1-butanol [HOCH2CH2CH2CH2ONO2] 1-Nitrooxy-2-pentanol [O2NOCH2CH(OH)CH2CH2CH3] 5-Nitrooxy-2-pentanol [CH3CH(OH)CH2CH2CH2ONO2] 4-Nitrooxy-1-pentanol [HOCH2CH2CH2CH(ONO2)CH3] 6-Nitrooxy-1-hexanol (HOCH2CH2CH2CH2CH2CH2ONO2) 1-Nitrooxy-2-butanol-3-ene [O2NOCH2CH(OH)C=CH2] 4-Nitrooxy-1-butanol-2-ene (HOCH2CH=CHCH2ONO2) 2-Nitrooxy-1-cyclopentanol b H2C c H2C C e H2

a H OH C CH d ONO2

1012 × k(298 K) 1012 × A

B, K

Position of OH Attack

Observed or Predicted Major Products

10.4 ± 3.1

2; 91; (2); 5; 0.5

CH3C(O)CH2CH2ONO2

11.9 ± 3.6

3; 85; 10; 2; 0.6

CHOCH2CH(ONO2)CH3

12.6 ± 3.8

2; 54; 40; 3; 0.6

CHOCH2CH2CH2ONO2

9.8 ± 2.9

2; 13; (3); 57; 22; 3

C2H5CHO; O2NOCH2CHO

32.1 ± 9.6

1; 70; (1); 25; 2; 0.4

CH3C(O)CH2CH2CH2ONO2

28.6 ± 8.6

2; 46; 34; 16; 3; 0.5

HC(O)CH2CH2CH(ONO2)CH3

30.9 ± 9.3

1; 39; 29; 14; 2; 0.5

CH(O)CH2CH2CH2CH2CH2ONO2

36.2 ± 11

0.2; 3; (0.3); A 96

CH2O; O2NOCH2CH(OH)CHO

22.0 ± 6.6

0.3; 7; A 93; 0.1

HOCH2CHO; CH(O)CH2ONO2

3.53 ± 1.1

a 25; b 45; c 21; d 2; e 4

CHOCH2CH2CH(ONO2)CHO

N-Atom-Containing Oxygenate g) Carbonyl nitrates α-Nitrooxyacetone (O2NOCH2C(O)CH3) 1-Nitrooxy-2-butanone [O2NOCH2C(O)CH2CH3] 3-Nitrooxy-2-butanone [CH3C(O)CH(ONO2)CH3] 2-Oxo-cyclohexyl-1-nitrate

1012 × k(298 K) 1012 × A

B, K

Position of OH Attack

Observed or Predicted Major Products

b

?

5.0 ± 1.5

Probably b A > a A

?

5.2 ± 1.5

Probably b A > a A

?

CH

C H

m-Nitrotoluene H C

b H3C

1.0 ± 0.4

B, K

NO2

C

HC

1012 × k(298 K) 1012 × A

C

a

NO2

C CH

HC C H

1-Nitronaphthalene NO2

a

b

2-Nitronaphthalene O2N a

b

N-Atom-Containing Oxygenate 2-Methyl-1-nitronaphthalene

1012 × k(298 K) 1012 × A

B, K

Position of OH Attack

< 8.3

Probably b A > a A > c

0.0185 ± 0.0034 29 ± 9

100 Addition to >C=C< bond is dominant

2.3 ± 0.7 3.5 ± 1.1

No estimates No estimates

Observed or Predicted Major Products ?

NO2 c H3 C a

b

l) Peroxyacyl nitrates Peroxyacetyl nitrate (CH3C(O)O2NO2) Peroxymethacryoyl nitrate [CH2=C(CH3)C(O)O2NO2] m) Nitrosamines and nitramines N,N-Dimethylnitrosamine [(CH3)2NNO] N,N-Dimethylnitramine [(CH3)2NNO2] a

CH2O; CO2; NO2? CH2O; CH3C(O)C(O)O2NO2?

? ?

Data are from Calvert et al. (2011), in which the individual estimates of the kinetic parameters are evaluated. When data are available, the abbreviated code in column 5 of the table gives the approximate percentage of H-atom abstraction or addition (preceded by “A”) by HO radical at each successive >CH, –CH2–, –CH3, –OH, or C=C site in the same order (left to right) as given in the structural formula in column (1. These numbers were derived using the procedures outlined in the SAR section in the text. They are necessarily subject to significant uncertainty and are presented here to provide a qualitative measure of the reactivity at various sites. c Only the major products that have been reported are given; when no product studies have been made the products shown (in italics) are judged to be representative of those expected for reaction at the most reactive sites, as suggested by the SAR calculations. d Wallington (1986); data point for P = 100 Torr Ar and 298 K. e Dillon et al. (2012); k is pressure- and temperature-dependent; value given for 760 Torr, 298 K, PLP-LIF method. f Dillon et al. (2012); relative rate data using isoprene reference, 298 K. g Atkinson et al. (1984a); relative rate data using propene as reference; 295 K, 750 Torr air.

b

295

296

the mechanisms of reactions influencing atmospheric ozone

1012 x k(298 K) for HO reactions with alkyl nitrates, cm3 molecule−1 s−1

10

Alkyl-1-Nitrates Alkyl-2-Nitrates Alkyl-3-Nitrates n-Alkanes (Reference)

8

6

4

2

0 1

2

3

4

5

6

7

8

Carbon number of the organic nitrate FIGURE IV-M-1.

Comparison of the k(298 K) values for some alkyl nitrates with those for the alkanes of same carbon

number.

I V- N . S U M M A R Y O F H O R A D I CA L R E AC T I O N S W I T H ORGANIC COMPOUNDS In Figure IV-N-1, a comparison is given of the rate coefficient data [k(298 K)] for the reactions of HO with the 3-carbon representatives of the various families of organic compounds that we have discussed in this chapter plus the amides and hydroxy nitrates reviewed in Calvert et al. (2011). With propane as a reference point, eight of the compounds listed show inhibition of the k(298 K), whereas 10 show enhanced k(298 K) values. The rate coefficients for the reactions of these C3 compounds with the HO radical span a factor of 500 from the least reactive, the fluorinated hydrocarbon (CF3CH2CH3), to the most reactive, the alkene (CH2=CHCH3). Other compounds near the top of the HO reactivity chart of the representative C3 compounds are the aldehyde (CH3CH2CH(=O), dimethyl formamide (CH3)2NCH(=O), the hydroxynitrate CH3CH(ONO2)CH2OH, the ether (CH3CH2OCH3), methyl acetamide (CH3NHC(=O)CH3), and the alcohol (CH3CH2CH2OH). Of course, the ozone-generating ability of a given compound depends not only on the rate coefficient of its HO reactions, but also on its concentration in the atmosphere and the subsequent reactions of the initial products and the number of RO2 and HO2 radicals that are subsequently formed and oxidize NO to NO2. Some of the most

reactive compounds shown in Figure IV-N-1 are present in negligible quantities in the atmosphere and offer little in the way of drive to create ozone in the real world. Since the complex mixture of compounds found in an urban atmosphere contains not only the C3 species that we have compared in Figure IV-N-1, but also many thousands of others, reactivities toward HO can be expected to exceed those presented here. However, it is interesting to recognize the classes of compounds that are most reactive toward HO, a very important factor contributing to a compound’s ozone-generating ability. I V- O . S T R U C T U R E - A C T I V I T YR E L AT I O N S ( S A R S ) F O R E S T I M AT I N G H O R AT E COEFFICIENTS The database for reactions of the HO radical with organic compounds is now quite extensive, and for those compounds not yet studied, reasonably reliable methods of estimating the rate coefficients are now available (typically, within a factor of 2). In our discussions in this chapter, we have seen how structural properties of a given family of organic compounds are related to their reactivity with the HO radical in a very systematic fashion. Here, we summarize the often-used method of rate coefficient estimation developed and tested largely by Atkinson and colleagues (Atkinson, 1986, 1987; Kwok and Atkinson, 1995; Kwok et al., 1996b). This method

The Hydroxyl Radical and Its Role in Ozone Formation

297

CF3CH2CH3 CH3C(O)CH3 CH3C(O)OCH3 CH3CH2CH2NO2 CH3CH2CH2ONO2 CH3CH2CH2Cl CH3CH2CH2Br CH3CH2CH2ONO CH3CH2CH3 CH3CH2C(O)O2NO2 CH3CH2C(O)OH CH3CH2CH2I CH3CH2CH2OH CH3C(O)NHCH3 CH3CH2OCH3 CH3CH(ONO2)CH2OH (CH3)2NC(O)H CH3CH2C(O)H CH2=CHCH3

0

5

10

15

20

25

30

1012 x k(OH), cm3 molecule−1 s−1 FIGURE IV-N-1.

Comparison of the magnitude of k(298 K) for HO reactions with C3 compounds that represent the various families of organic compounds considered in this work.

depends on the use of SARs; estimation of the rate coefficients for the abstraction of H atoms from ‒CH3, ‒CH2‒, >CH‒, and ‒OH groups are made using group values that are adjusted to account for the particular groups that are attached to the given group, as was outlined in Chapter III. For primary H-atom abstraction, k(CH3‒X) = kprim × F(X); for secondary H-atom abstraction, k(X‒CH2‒Y)  =  ksec × F(X) × F(Y); whereas for tertiary H-atom abstraction:

k X

H C

Y

= ktert x F(X) x F(Y) x F(Z)

Z

where kprim, ksec, and ktert are the group rate constants for H-atom abstraction from –CH3, –CH2‒, and >CH–, –O–H;  –C(=O)–H, –OC(O)–H groups. The F(X), F(Y), and F(Z) are substituent factors for the groups X, Y, and Z, respectively. If the H atom is located on a carbon atom in a cyclic compound with rings smaller than six carbon atoms, then an additional factor that corrects for ring strain is added to the product of terms in estimating the reactivity

of the particular H-atom site. The estimated total rate coefficient is derived from the sum of those estimated for all the H-atom-containing sites. The suggested values for the group and substituent factors given by Kwok and Atkinson (1995) for several different families of compounds are listed in Tables IV-O-1 and IV-O-2. These tables were used in this work to estimate the rate coefficients and the extent of attack by OH radicals at specific sites for all the oxygenates. A newer set of derived SAR parameters is used in considering the reactivities of the alkanes and haloalkanes with HO radicals. This was derived by Calvert et  al. (2008) in their treatment of more extensive data on the HO-haloalkane reactions available in 2008 (see Tables IV-O-3 and IV-O-4). In illustration of the use of the SAR calculations, consider the molecule sec-butyl acetate, [CH3CH2CH(CH3)OC(O)CH3]. Using Tables IV-O-1 and IV-A-2, the successive terms for estimation of the k(298) in units of cm3 molecule−1 s−1 are 1012 × k(298) ≈ (0.136 × 1.23) + (0.934 × 1 ×

TABLE IV-O-1 . GROUP FACTOR S DERIVED BY KWOK AND ATKINSON (1995) FOR ESTIMATING K(298 K) VALUES FOR HO ABSTR ACTION OF H ATOMS FROM ORGANIC COMPOUNDS. ALSO GIVEN ARE TEMPER ATURE-DEPENDENT PAR AMETER S C AND D IN k = CT 2 e −D/T FOR E ACH GROUP. THE VALUES IN THIS TABLE AND THOSE IN TABLE IV-O-2 WERE USED IN THIS WORK IN MAKING THE STRUCTURE-ACTIVIT Y REL ATIONSHIP (SAR) ESTIMATES OF PERCENTAGE OF ABSTR ACTION AT VARIOUS SITES FOR ALL GROUPS OF COMPOUNDS OTHER THAN THE ALKANES AND HALOALKANES; FOR THE L ATTER COMPOUNDS, THE REVISED VALUES IN TABLES IV-O-3 AND IV-O-4 WERE USED A S DERIVED FROM THE MORE EXTENSIVE DATA AVAIL ABLE IN 2008

Group

1018 × C

D, K

1012 × k(298 K), cm3 molecule−1 s−1

–CH3 –CH2– >CH– Formyl-H kabst (–OH) kabst (–OC(O)H

4.49 4.50 2.12

320 −253 696

2.1

85

0.136 0.934 1.94 1.94 0.14 0.09b

Allylic H-atomsa ‒CH3– (Primary) –CH2– (Secondary) >CH– (Tertiary)

0.75/CH3-group 2.32/CH2-group 3.1/CH-group

Allylic H-atoms are located on carbon atoms that are next to a C=C group. Value assigned by Le Calvé et al. (1997).

a

b

TABLE IV-O-2 . SUBSTITUENT FACTOR S F(X) AT 298

K (KWOK AND ATKINSON, 1995)

X –CH3 –CH2– >CH– >C< –F –Cl –Br –I –CH2Cl –CHCl2 –CHCl– –>CCl– –CH2Br –CHBr– –CCl3 –CF3 –CHF2 –CH2F –CHF– –CF2–

298

F(X) at 298 K 1.00 1.23 1.23 1.23 0.094 0.38 0.28 0.53 0.36 0.36 0.36 0.36 0.46 0.46 0.069 0.071 0.13 0.61 0.21 0.018 (continued)

TABLE IV-O-2 (CONTINUED)

X –CF2Cl –CFCl2 –OCF3 –OCF2– –OCHF2 –OCH2F –OCH2CF3 –OCH(CF3)2 –OCHClCF3 –C(=O)CF3 =O –CH(=O) >C=O –CH2C(=O)– >CHC(=O)– –>CC(=O)– –C6H5 >C=C< –C≡C– –OH –CH2OH >CHOH –>COH –OR (R = alkyl group) –C(=O)Cl –C(=O)OR (R = alkyl group) –OC(=O)R (R = alkyl group) –OC(=O)H –C(=O)OH –CH2ONO2 >CCHONO2 –>CONO2 –ONO2 –CN –CH2CN –NO2 –CH2NO2 –NHC(O)OR –OC(O)NHR –NH2 –NHR –NR2 –N(R)NO –N(R)NO2 3-member ring 4-member ring 5-member ring 6-,7-, or 8-member ring

F(X) at 298 K 0.031 0.044 0.17 0.17 0.17 0.17 0.44 0.44 0.44 0.11 8.7 0.75 0.75 3.9 3.9 3.9 1.0 1.0 1.0 3.6 2.6 2.6 2.6 8.4 0.067 0.31b 1.6 0.6a 0.74 0.20 0.20 0.20 0.04 0.19 0.12 0 0.14 7.5b 4.8b 9.3 9.3 9.3 9.3 9.3 0.02 0.28 0.64 1.0

a

Value assigned by Le Calvé et al. (1997). Value assigned by Kwok et al. (1996b).

b

299

TABLE IV-O-3 . THE k(298 K) VALUES OF THE GROUP R ATE COEFFICIENTS FOR HO ABSTR ACTION OF H ATOMS DERIVED FROM BEST FITS OF THE ALKANE AND HALOALKANE DATA BY CALVERT ET AL. (2008). THESE VALUES WERE USED IN THIS WORK TO ESTIMATE R ATE COEFFICIENTS FOR H-ATOM ABSTR ACTION IN THE ALKANES AND HALOALKANES

Group –CH3 –CH2– >CH–

1012 × k(298 K), cm3 molecule−1 s−1 0.136 0.78 1.37

TABLE IV-O-4 . SUBSTITUENT FACTOR S F(X) AT 298 K FOR H-ATOM ABSTR ACTION BY HO R ADICALS DERIVED FROM ALKANE AND HALOALKANE DATA FITS BY CALVERT ET AL. (2008)

X ‒CH3 ‒CH2– >CH– >C< –F –Cl –Br –I –CH2Cl –CHCl2 –CCl3 –CHCl– >CCl– –CH2Br –CHBr– –CF3 –CHF2 –CH2F –CHF– –CF2– –CF2Cl –CFCl2 –CF2Br –CH2I –CHI– 3-membered ring 4-membered ring 5-membered ring 7-membered ring 8-membered ring a

F(X) at 298 K 1.00 1.35 1.35 1.35 0.164 0.419 0.514 0.792 0.403 0.403 0.086 0.403 0.403 0.40 0.40 0.052 0.106 0.106 0.216 0.045 0.110 0.045 0.15 0.50 0.50 0.02a 0.28a 0.64a 1.0a 1.0a

Additional F(X) factor that corrects reactivity of a given group for ring strain in cyclic alkanes (Kwok and Atkinson, 1995).

300

The Hydroxyl Radical and Its Role in Ozone Formation 1.23) + (1.94  × 1  × 1.23  × 1.6) + (0.136  × 1.23) + (0.136  × 0.31)  =  5.3; with the measured value of 1012 × k(298)  =  5.7. Agreement between the SAR estimate and measurement is usually within a factor of 2. A similar SAR method of estimation of the rate coefficients for the HO radical addition to >C=C< and  –C≡C– bonds was developed by Atkinson (1986, 1987)  and Kwok and Atkinson (1995). In the calculation, the k value for a given structural unit is modified by the product of the C(X) substituent factors for the groups attached to the structural unit. For example, k(CH2=CHX)  =  k(CH2=CH–) × C(X). Table IV-O-5 gives the group rate coefficients for HO radical addition (298 K), and Table IV-O-6 gives the group substituent factors C(X) that modify the group rate coefficients for the addition reactions. Peeters et  al. (2007) developed a SAR method for estimation of HO radical rate coefficients for reactions with the alkenes and polyenes that allows site-specific rate-constant estimates at each C atom in the >C=C< bond and in the conjugated diene bonds >C=CHCH=CC=C< CH2=CHCH=CH– CH2=CHC(–)=CH2 CH2=CHCH=C< CH2=CHC(–)=CH– –CH=CHCH=CH– CH2=C(–)C(–)=CH2 CH2=CHC(–)=C< CH2=C(–)CH=C< –CH=CHCH=C< CH2=C(–)C(–)=CH– –CH=C(–)CH=CH– >C=CHCH=C< CH2=C(–)C(–)=C< –CH=C(–)C(–)=CH– –CH=CHC(–)=C< –CH=C(–)CH=C< –C≡CH –C≡C–

26.3 51.4 56.4 64.0 86.9 110 105 105 142 142 142 142 190 190 190 190 190 260 260 260 260 260 7.0 27

TABLE IV-O-6 . GROUP SUBSTITUENT FACTOR S [C(X)] AT 298 K FOR HO R ADICAL

ADDITION TO >C=C< AND –C≡C– BONDS (KWOK AND ATKINSON, 1995)

Substituent group –F –Cl –Br –CH2Cl –CH(=O) –C(=O)CH3 –CH2ONO2 >CHONO2 –C(=O)OR (R = alkyl group) –OR (R = alkyl group) –CN –CH2OH

302

C(X) at 298 K 0.21 0.21 0.26 0.76 0.34 0.90 0.47 0.47 0.25 1.3 0.16 1.6

303

The Hydroxyl Radical and Its Role in Ozone Formation

TABLE IV-0-7 . VALUES OF THE PAR AMETER S USED IN ESTIMATING HO R ATE COEFFICIENTS FOR ALKENE AND CONJUGATED DIENE COMPOUNDS BY THE METHOD OF PEETER S ET AL. (2007)

10−11 × Value (cm3 molecule−1 s−1)

Parameter Monoalkenes And non-conjugated polyalkenes Conjugated alkenes

0.45 3.0 5.5 3.0 3.7 5.0 5.7 8.3 9.9

kprim ksec ktert ksec/prim ksec/sec ksec/tert ktert/prim ktert/sec ktert/tert

dienes. These extra terms account for the possible resonance stabilization of the radical formed after addition of the HO radical. Such resonance stabilization can occur only when addition occurs at one of the outer C atoms of the conjugated diene. The specific rate coefficient for HO attack on the inner two carbon atoms in the conjugated group are assigned values similar to the monoalkenes

because no resonance structures are possible with these positions. In illustration of the assignment terms used in estimating rate coefficients, consider the isoprene molecule (2-methyl-1,3-butadiene, CH2=C(CH3)CH=CH2). Addition of HO at the two terminal C atoms creates a radical that can have two resonance forms, and these define the specific site k.

HO H

CH3

HO

Ca

CH3

H Ca

H

Cb Cc

H

Cd

H

Cb

H

Cd

Cc

ktert/prim

H

H

HO

H

H

CH3

H Ca

H

Cb

H

Cc

Cd

H

CH3

H Ca

H

Cb

H

Cc H

CH3

H

HO

Cd

ksec/prim

H

Ca

HO H

Cb

H

Cc

Cd

H H

H

CH3 Ca

HO

H

Cb

H

Cc H

H

Cd H

304

the mechanisms of reactions influencing atmospheric ozone

In the first case shown, the two radicals that form the resonance hybrid have tertiary and primary designations by the rules given for the alkenes, and the site ktert/prim is assigned to the Ca atom. In a similar fashion, the HO addition to the Cd atom generates resonance hybrids that combine secondary and primary radicals; hence, a ksec/prim rate coefficient is assigned to this atom site. Atom Cb and Cc both generate primary radicals. Thus, the total estimated rate coefficient is given by ktotal = ksec/tert + kprim+ kprim + ksec/  = (5.7 + 0.45+ 0.45 + 3.0) × 10−11 = 0.96 × 10−10 prim. cm3 molecule−1 s−1. The experimental value is 1.0 × 10−10 cm3 molecule−1 s−1. In estimating the extent of HO attack (addition or abstraction) shown in the table of alkene rate coefficients (Table IV-D-1), the smaller contributions from H-atom abstraction from allylic H atoms were added using the data of Table IV-O-1, and the sum of the percentages of reaction at each site was then normalized to 100%. Estimation of the rate coefficients for the gas phase reactions of HO with aromatic hydrocarbons has received limited attention. The HO radical is considered to be an electrophilic reactant, and a method based on the σ+ values used by chemists (Brown and Okamoto, 1958) in the study of electrophilic reactions has been suggested (Zetzsch, 1982). The various substituent groups used by Zetzsch and derived by Brown and Okamoto are given in Table IV-O-8. As suggested by Zetzsch, the rate coefficient at 298 K is calculated from the equation:

log10[k(298 K)] = ‒11.71 ‒ 1.34 × Σni=1 (σ+)i. It should be noted that the method does not apply to compounds such as benzaldehyde and its methyl-substituted derivatives because H-atom abstraction from the aldehyde group is the dominant mode of reaction of these compounds and addition to the ring, the reactions to which the equation applies, is less important. The SAR methods employed here consider the effects of activation and deactivation of substituents on H atoms on the same C atom and those in the α-position relative to a specific group. Thus, although use of the substituent factor F(–ONO2) = 0.04 decreases the rate coefficient for the H atoms on a given ONO2-substituted carbon atom, the substituent factor F(–CH2ONO2) = 0.20 decreases the rate constant for H atoms on the carbon atom next to the ONO2-substituted carbon. Similarly, action of enhancement of the effects of OH and OR groups is extended down-chain by F(–CH2OH), F(>CHOH), F(–CR2COR), and so on. Thus, in the Atkinson et  al. SAR method, the influence of a given group is extended down-chain by one carbon atom. Some workers (e.g., see Mellouki et  al., 2003)  support the thesis that deactivating and activating group substituents effect the H-atom reactivity well down-chain, well beyond the range of influence suggested by the SAR formulations of Atkinson and co-workers (Kwok and Atkinson,

TABLE IV-O-8 . ELECTROPHILIC SUBSTITUENT CONSTANTS (Σ + ) FOR USE IN ESTIMATING THE R ATE COEFFICIENTS [k(298 K)] FOR HO ADDITION TO THE AROMATIC RING IN COMPOUNDS (ZETZSCH, 1982)

Group Methyl (–CH3) Ethyl (–CH2CH3) iso-Propyl [–CH(CH3)2] tert-Butyl [–C(CH3)3 Phenyl (–C6H5) Methoxy (–OCH3) Phenoxy (–OC6H5) Hydroxy (–OH) Carboxy [–C(=O)OH] Carbomethoxy [–C(=O)OCH3] Carboethoxy [–C(=O)OCH2CH3] Nitro (–NO2)

σ+ (ortho−, para−positions −0.331 −0.295 −0.280 −0.256 −0.179 −0.778 −0.5 −0.92 0.421 0.489 0.482 0.79

σ+ (meta-position) −0.066 −0.064 −0.060 −0.59 0.109 0.047

0.322 0.368 0.366 0.674

The Hydroxyl Radical and Its Role in Ozone Formation 1995). However, the evidence for this is limited and inconclusive. For example, Mellouki et  al. (2003) suggest that rate coefficients in the alkyl ethers (units of 10−12 cm3 molecule−1 s−1) vary from 6.2, 4.3, 2.9, to 1.2 for CH2 groups that are, respectively, in the α, β, γ, and δ positions with respect to the activating –O– group of the ether. From the discussion in this chapter, we have seen that the experimental rate coefficients for the first few members of a series of straight-chain alcohols, ethers, aldehydes, ketones, or esters increase in an almost linear fashion as more  –CH2– groups are added to the alkyl chain. However, we noted that the use of the more limited SAR terms of the Atkinson group can rationalize the observed data within the error limits of the experimental points; see Figures IV-G-1 (alcohols), IV-H-1 (ethers), IV-I-1 (aldehydes), IV-J-1 (ketones), and IV-I-1 (esters). With the SAR terms derived by Atkinson and co-workers and used here, one sees a near linear increase in the rate coefficient for the first two or three CH2 groups added, followed by a transition to a linear region of slope equal to that observed with the alkanes. Within the error limits of the existing data, this trend describes the current experimental data. Demonstration of the existence of long-range enhancement of a given substituent group will require rate data of much higher accuracy than now exists. The use of more terms to tune the SAR fit to the observed rate data available at this time provides very limited improvement in the agreement from that achieved using the original SAR schemes derived by Atkinson and his co-workers. Atmospheric chemists probably will continue to use the less detailed SAR treatment of Atkinson and co-workers until such time that more accurate rate coefficients have been determined, more detailed SAR schemes have been devised and tested, and the expanded SAR terms have been published. In the SAR treatment used here for the alkanes and haloalkanes, some new group rate coefficients were derived using the more extensive dataset available in 2008 (see Calvert et  al., 2008, and Tables IV-O-3 and IV-O-4). In the study of Calvert et  al. (2008), an account was taken of a deactivating influence of the CF3–,  –CF2–, and –CF2Cl group substituents when they occur in a position β to the H atom that is being abstracted. Without adding more specific substituent factors into the scheme, Calvert et al. (2011) treated their influence as that of an F-atom substituent on the C atom undergoing

305

abstraction. The SAR substituent terms in Table IV-O-4 for the halocarbons were optimized assuming that multiple substitutions on a given >CH– or –CH2– group apply, and multiple enhancement or deactivation terms are used in all calculations of the SARs of the halocarbons. The effect on k(298 K) of multiple O-atom insertions for CH2 groups in the cyclo-hexane ring, given in Section IV-H-2, introduces a note of caution in the application of enhancement factors on the  –OCH2O– groups in a molecule. It appears that application of only a single enhancement factor on the CH2 group [F(–OR)] is appropriate, as Kwok and Atkinson (1995) noted in their original SAR publications. This is also apparent in the estimation of the rate coefficients for the difunctional compounds CH3OCH2OCH3, C2H5OCH2OC2H5, C3H7OCH2OC3H7, and C4H9OCH2OC4H9. If one applies the rules for effects of adjacent groups, as outlined earlier in this section, namely, taking (8.42 × 0.934) for the group of atoms –OCH2O–, then the estimated rate coefficient (10−12 units) for the series of difunctional ethers are 10.1, 81.9, 87.8, and 90.2, respectively. The measured k(298) values are very different:  4.6, 19.7, 27, and 36, respectively. However, if the rate coefficient for the –OCH2O– group is calculated as 8.4 × 0.934, using the enhancement factor only once, then the rate coefficients obtained for the series of compounds is much closer to those measured: 6.8, 23.4, 31.8, and 34.4, respectively. In the present work, only a single enhancement factor was applied in estimating reactivity of a CH2 group that is adjacent to two –O– groups. In other cases where multiple groups of enhancement or deactivation are present on a given group, the SAR calculations made in this work applied both factors with better fits to the experimental rate coefficients than did the use of only one. Application of the described procedure of SAR estimation of rate coefficients for  –CH2– or >CH– groups attached to two or three different functional groups requires some additional study. Rather large errors in the estimated rate coefficient can be anticipated when both enhancing and deactivating groups are attached to a given –CH2– or >CH– group. For example, consider the SAR for HO reaction with CF3CH2OH:  k(298 K) = (0.934 × 0.071 × 3.6 + 0.14) × 10−12 = 0.38 × 10−12; the experimental value is 0.11  × 10−12 cm3 molecule−1 s−1, which is different from the SAR estimate by a factor of 3.5.

306

the mechanisms of reactions influencing atmospheric ozone

In the following sections of this chapter, we use the information given in the SAR tables to derive estimates of the HO radical rate coefficients with the various classes of compounds. Since in the SAR method estimates the rate coefficients from the sum of the estimated reactivity at each H-atom site in a molecule, the percentage of attack by HO radicals at each site can be estimated; these data are given in each table of recommended HO radical rate coefficients. Although SAR estimates as they are currently made are of limited and uncertain accuracy, they are useful in making qualitative judgments of the extent of attack on each H-atom position. The current SAR data presented in the tables in this book and that identify points of attack of HO radicals on a given molecule can be used to identify the appropriate RO2 radicals (and corresponding RO radicals) that are expected in a given reaction. Using this information, together with the mechanism considerations

FIGURE IV-O-1.

given in Chapters V and VI, one can develop a reasonably detailed reaction scheme that predicts the products that will be formed in the atmospheric reactions of a given compound. In Figure IV-O-1, the rate coefficients for the alkane molecules, as estimated using the parameters in Tables IV-O-3 and IV-O-4, are plotted versus those measured. Reasonable agreement is seen for both the acyclic and cyclic alkanes. As seen from Figure IV-O-1, in all cases, the SAR-based estimations are well within a factor of 2 from the measured data. A  similar plot is shown for haloalkanes in Figure IV-O-2. Again, most of the points of disagreement lie within a factor of 2, as shown by dashed lines. Some of the least reactive of the hydrofluorocarbons and the hydrochlorofluorocarbons show significant scatter, likely the result of both limitations on the SAR methods and the difficulties in measuring the rate coefficients of these very unreactive compounds.

Plot of the measured and structure-activity relationship (SAR)-estimated rate coefficients for the HO radical abstraction of H atoms from the acyclic and cyclic alkanes (using the parameters of Tables IV-O-3 and IV-O-4).

Measured: 1012 x k(298 K), HO + halocarbons, cm3 molecule−1 s−1

The Hydroxyl Radical and Its Role in Ozone Formation

1000

100

307

Hydrofluorocarbons Hydrochlorocarbons Hydrobromocarbons Hydroiodocarbons Hydrochlorofluorocarbons Hydrobromofluorocarbons Hydrobromochlorocarbons Hydrobromochlorofluorocarbons 1 : 1 Reference Line Disagreement by Factor of 2

10

1

0.1

0.01 0.01

0.1 SAR estimated:

1 1012

10

x k(298 K), HO + halocarbons,

100 cm3

molecule−1

1000 s−1

FIGURE IV-O-2.

Plot of the measured and structure-activity relationship (SAR)-estimated rate coefficients for HO radical abstraction (using Tables IV-O-3 and IV-O-4) of H atoms from haloalkanes. The range of k(298 K) values is larger than that seen of the alkanes (Figure IV-O-1), and although most of the data lie within the lines marking disagreement by a factor of 2, scatter of these data is greater than that seen for the alkanes.

In Figures IV-O-3 through IV-O-11, the SAR estimates of the rate coefficients for the HO reactions with the alkenes, aromatic hydrocarbons, alcohols, ethers, aldehydes, ketones, acids, esters, and N-containing oxygenates are compared with the measured values. In each case, the predictions

are seen to be reasonably good, within a factor of 2 for most of the compounds. Certainly, the SAR methods appear to offer reasonable results, and the predictions of reactivity of these compounds at specific sites should provide useful—albeit qualitative—information for atmospheric chemists.

Measured: 1012 x k(298 K) HO + alkenes, cm3 molecule−1 s−1

Acyclic Monoalkenes Cyclic and Bicyclic Alkenes 1:1 line Diagreement by Factor of 2 Acyclic Dienes

100

10 10

100 12

SAR estimated: 10

x k(298 K), HO + alkenes, cm3 molecule-1 s-1

FIGURE IV-O-3. Plot of the measured and estimated total rate coefficients for HO radical-alkene reactions of abstraction of H atoms and addition to the C=C bonds and (using the parameters of Table IV-O-5 and IV-O-6). Most of the data lie within the lines marking disagreement by a factor of 2. Use of the Peeters et al. structure-activity relationship (SAR) method gives a somewhat improved match between experimental and estimated rate coefficients.

308

Measured: 1012 x k(298 K), HO + aromatics, cm3 molecule−1 s−1

102

101

100 Aromatic Hydrocarbons Other Aromatic Compounds 1: 1 Reference Line Disagreement by a Factor of 2

10−1 10−1

100

101

102

SAR estimated: 1012 x k(298 K), HO + aromatics, cm3 molecule−1 s−1 FIGURE IV-O-4.

Plot of the measured and estimated total rate coefficients for HO radical addition to the ring of benzene and its derivatives using the structure-activity relationship (SAR) parameters in Table IV-O-8. A small fraction of the experimental rate coefficients results from abstraction of H atoms from H-containing groups attached to the ring. Much of the data lie within the lines marking disagreement by factor of 2.

309

Measured:1012 x k(298 K) for HO-alcohol reactions, cm3 molecule−1 s−1

Acyclic alcohols Cyclic alkanols Diols Alkeneols Alkyneols Phenols Halogenated alcohols 1 : 1 Reference line Disagreement by factor of 2

1000

100

10

1

0.1

0.01 0.01

0.1

1

10

100

1000

SAR estimated: 1012 x k(298 K) for HO-alcohol reactions, cm3 molecule−1 s−1 FIGURE IV-O-5.

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for HO radical-alcohol reactions of abstraction of H atoms and addition (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data lie within the lines marking disagreement by factor of 2.

Acyclic Ethers Multifunctional Ethers Ether-Substituted Esters Unsaturated Ethers Cyclic Ethers Aromatic Ethers Halogen Substituted Ethers 1 :1 Reference Line Disagreement by Factor of 2

Measured: 1012 x k(298 K) HO + ethers, cm3 molecule−1 s−1

1000

100

10

1

0.1

0.01

0.001

0.0001 0.0001

0.001

0.01

SAR estimated: FIGURE IV-O-6.

1012

0.1

1

x k(298 K) HO + ethers,

10 cm3

100 molecule−1

1000 s−1

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for HO radical-ether reactions of abstraction of H atoms and addition (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data for the various families of ethers lie within the lines marking disagreement by factor of 2. However, the estimates for the relatively unreactive halogen-substituted ethers show more scatter.

310

Measured: 1012 x k(298 K) HO + aldehydes, cm3 molecule−1 s−1

n-Alkanals 2-Methylalkanals 1 :1 Reference Line Disagreement by Factor of 2 Carboaldehydes Hydroxyaldehydes Multifunctional Aldehydes Unsaturated Aldehdes Aromatic Aldehydes Aldehydes from Biogenic Hydrocarbons

100

10

10

100

SAR estimated: 1012 x k(298 K), HO + aldehydes, cm3 molecule−1 s−1 FIGURE IV-O-7.

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for HO radical-aldehyde reactions of abstraction of H atoms and addition (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data lie within the lines marking disagreement by factor of 2.

311

Measured: 1012 x k(298 K) for HO + ketone, cm3 molecule–1 s–1

Acyclic Ketones 1 : 1 Reference Line Disagreement by Factor of 2 Hydroxy Ketones Diones Unsaturated Ketones Cyclic Ketones Ketones of Biogenic Origin Fragrence Ketones Halogenated Ketones

100

10

1

0.1

0.01 0.01

0.1

SAR estimated:

1012

1

10

x k(298 K) for HO + ketone,

100 cm3

molecule–1 s–1

FIGURE IV-O-8.

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for HO radical-ketone reactions of abstraction of H-atoms and addition (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data lie within the lines marking disagreement by factor of 2.

Measured: 1012 x k(298 K) for HO + organic acid reactions, cm3 molecule–1 s–1

100

10

Acylic Organic Acids Keto Acid Unsaturated Acid Halogenated Acids 1 :1 Reference Line Disagreement By Factor of 2

1

0.1

0.01 0.01

0.1

1

10

100

SAR estimated: 1012 x k(298 K) for HO + organic acid reactions, cm3 molecule–1 s–1 FIGURE IV-O-9.

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for HO radical-organic acid reactions of abstraction of H atoms and addition (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data lie within the lines marking disagreement by factor of 2.

312

Measured: 1012 x k(298 K) for HO + ester reactions, cm3 molecule−1 s−1

100

10

Acyclic Esters 1 : 1 Reference Line Disagreement By Factor of 2 Difunctional Esters Cyclic Unsaturated Esters Unsaturated Esters Aromatic Esters Carbonates Lactates Fluoroesters

1

0.1

0.01

0.001 0.001

0.01 SAR estimated:

0.1

1

10

100

1012

x k(298 K) for HO + ester reactions, cm3 molecule–1 s–1

FIGURE IV-O-10.

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for HO radical-ester reactions of abstraction of H atoms and addition (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data lie within the lines marking disagreement by factor of 2.

313

Measured: 1012 x k(298 K) for HO + organic nitrates, cm3 molecule–1 s–1

100

10

Alkyl nitrates 1 :1 Line Disagreement By factor of 2 Saturated dinitrates Unsatured nitrates Hydroxyalkyl nitrates Carbonyl nitrates Nitroalkanes

1

0.1

0.01

0.001 0.001

0.01 SAR estimated: 10

FIGURE IV-O-11.

0.1

1

10

100

12

x k(298 K) for HO + organic nitrates, cm3 molecule–1 s–1

Plot of the measured and structure-activity relationship (SAR)-estimated total rate coefficients for the abstraction of H atoms and addition reactions of HO radical with N-atom-containing oxygenates (using the parameters of Tables IV-O-1 and IV-O-2). Most of the data lie within the lines marking disagreement by factor of 2.

314

V Mechanisms of Reactions of HO2 and RO2 Radicals

V- A . I N T R O D U C T I O N The peroxy radicals are an important link in the reaction chain that develops ozone in the atmosphere through their reactions with NO. This chapter explores the kinetics and mechanisms of these RO2 reactions. In Chapters III and IV, the kinetics and mechanisms of the reactions of organic compounds with the major atmospheric oxidants [HO, NO3, and O3] were discussed. The organic radicals formed in these reactions add O2 to form organic peroxy radicals (RO2). The rate coefficient for these reaction is typically of the order of (10−12–10−11) cm3 molecule−1 s−1 under tropospheric conditions. One atmosphere (1 atm) of air contains 5  × 1018 molecule cm−3 of O2, and the lifetime of organic radicals with respect to addition of O2 to give peroxy radicals is 10–100 nanoseconds. Addition of O2 is essentially the sole atmospheric fate of the organic radicals formed during the oxidation of organic compounds. As examples, consider the HO-initiated oxidation of ethane and acetone (M is a third body, such as N2, which collisionally deactivates the nascent peroxy radical): HO + CH3CH3 → CH3CH2 + H2O CH3CH2 + O2 + M → CH3CH2O2 + M HO + CH3C(O)CH3 → CH3C(O)CH2 + H2O CH3C(O)CH2 + O2 + M → CH3C(O)CH2O2 + M

Because of the rapidity and exclusivity of the O2 addition to alkyl radicals, the organic peroxy radicals (CH3CH2O2 and CH3C(O)CH2O2) can be thought of as the primary products of the initial oxidation step. HO2 radicals are formed in reactions of O2 with alkoxy radicals (e.g., CH3O) and by the association reaction of H atoms with O2: CH3O + O2 → CH2O + HO2 H + O2 + M → HO2 + M Peroxy radicals (HO2 and RO2) have a rich atmospheric chemistry and undergo reactions with NO, NO2, HO2, and other peroxy radicals (R′O2). Unimolecular isomerization is also an important fate for larger organic peroxy radicals where the peroxy radical can abstract a hydrogen atom from another part of the organic moiety (the peroxy radical bites its own tail). Reactions of peroxy radicals with NO3 radicals at night, and ClO and BrO radicals in maritime environments, can also be of importance on local scales. The atmospheric lifetime of peroxy radicals depends on the chemical environment but is typically of the order of a minute. HO2 radicals form an adduct with water (HO2–H2O) which is weakly bound by approximately 30 kJ mol−1 (Aloisio and Francisco, 1998;

316

the mechanisms of reactions influencing atmospheric ozone

Suma et al., 2006). In 1 atm of air at 297 K at a relative humidity of 50%, the ratio of [HO2–H2O]/ [HO2] is approximately 0.19 (Kanno et al., 2005). The presence of water vapor, and hence the adduct, enhances the rate of the HO2 self-reaction and needs to be accounted for in atmospheric models of ozone chemistry and H2O2 formation (Atkinson et al., 2004). The presence of water does not lead to any discernible enhancement of the reactions of other reactions of atmospheric importance involving HO2 radicals. There is a much weaker interaction between water and organic peroxy radicals, and there is no observed effect of water on RO2 reaction kinetics (Kan and Calvert, 1979; English et al., 2008). The central theme of this book is ozone chemistry. The critical role of peroxy radical chemistry in promoting ozone formation in the troposphere can be appreciated by reference to Figure I-F-7. Reactions of peroxy radicals with NO are of paramount importance in ozone chemistry because they provide the photochemical source of ozone in the troposphere via photolysis of the major product of the reaction, NO2. As an example, consider the initial step of the HO-radical initiated oxidation of propane (C3H8) at the 2-position: HO + C3H8 → H2O + CH3CHCH3 CH3CHCH3 + O2 + M → CH3CH(OO)CH3 + M CH3CH(OO)CH3 + NO → CH3CH(O)CH3 + NO2 CH3CH(O)CH3 + O2 → CH3C(O)CH3 + HO2 HO2 + NO → HO + NO2 2 × (NO2 + hν (λ < 420 nm) → NO + O(3P)) 2 × (O + O2 + M → O3 + M) net:C3H8 + 4O2 → H2O + 2O3 + CH3C(O)CH3 The reactions of HO2 and C3H7O2 with NO generate NO2 and hence ozone and reform the HO radical, which can then initiate another oxidation cycle. Limitations to this reaction cycle are described in detail in Chapter I  and illustrated schematically in Figure I-F-7. The formation of organic nitrates as a minor but significant pathway in the RO2 + NO reaction converts the reactive peroxy radicals and NO into relatively unreactive organic nitrates (RONO2) and limits the formation of ozone:

CH3CH(OO)CH3 + NO → CH3CH(O)CH3 + NO2 CH3CH(OO)CH3 + NO + M → CH3CH(ONO2)CH3 + M In more pristine environments, loss of HO2 and RO2 radicals via self- and cross-reactions compete with loss of HO2 and RO2 via reaction with NO and limits the formation of ozone: HO2 + HO2 → H2O2 + O2 HO2 + RO2 → ROOH + O2 RO2 + RO2 → products On a timescale of minutes, the atmospheric chemistry of HO2 and RO2 radicals leads to the first-generation end-products of the oxidation process and dictates the efficiency of photochemical ozone formation. The chemistry of these radical species and the end-products that result are the topics of this chapter and build on previous reviews by Wallington et  al. (1992b), Lightfoot et  al. (1992), Tyndall et  al. (2001), Calvert et  al. (2008), and Orlando and Tyndall (2012). Reactions of peroxy radicals with NO2 are of minimal importance in the lower troposphere because the peroxynitrate products of these reactions are thermally unstable and dissociate rapidly back to reactants under these conditions (typically within a few seconds), for example: CH3CH2O2 + NO2 + M ↔ CH3CH2O2NO2 + M The peroxyacyl nitrates (RC(O)ONO2) such as peroxyacetyl nitrate (PAN; CH3C(O)O2NO2) are, however, sufficiently stable to play a role as radical reservoir species, particularly in the colder, upper troposphere. In addition to stable end-products like the organic nitrates, peroxynitrates, hydroperoxides, alcohols, and carbonyls, the major by-products of peroxy radical chemistry are the alkoxy radicals, such as CH3CH2O in the example just presented. The alkoxy radicals are very reactive under atmospheric conditions, with lifetimes usually less than 100 microseconds at the Earth’s surface. The chemistry of alkoxy radicals is the subject of Chapter VI.

Mechanisms of Reactions of HO2 and RO2 Radicals V- B . R E A C T I O N S O F H O 2 W I T H NO AND RO2 WITH NO V-B-1. Kinetics of the HO2 + NO and the RO2 + NO Reactions The reaction of peroxy radicals with NO plays a central role in tropospheric ozone formation. Recognition of the importance of this reaction has led to a large number of experimental and computational studies that have provided a wealth of information. The kinetics of the reactions of simple alkyl peroxy radicals (e.g., HO2, CH3O2, C2H5O2, C3H7O2, CH3C(O)O2) with NO are well established (Wallington et al., 1992b; Tyndall et al., 2001; Atkinson et  al., 2006, 2008b; Calvert et  al., 2008; Sander et al., 2011). Figure V-B-1 shows Arrhenius plots of the rate coefficients for reactions of RO2 radicals (including HO2) with NO determined in experimental studies as recommended by the International Union of Pure and Applied Chemistry (IUPAC) data evaluation panel (Atkinson et al., 2006, 2008b). As indicated from Figure V-B-1, for the temperature range of 200–400 K over which the recommendations are available, the reactions of NO with HO2 and the three smallest alkyl peroxy radicals (CH3O2, C2H5O2, n-C3H7O2, iso-C3H7O2) have rate coefficients that are indistinguishable within the

317

experimental uncertainties. At 298 K, the average of the rate coefficients for these reactions is 8.8 × 10−12 cm3 molecule−1 s−1. Given the relatively large change in the size of the radical going from HO2 to C3H7O2, it is quite striking that the kinetics of the reactions of these peroxy radicals with NO are so similar. There is no discernible influence of structure or the size of the peroxy radicals on the kinetics of their reactions with NO. Reports in the literature of lower rate coefficient for reactions of larger peroxy radicals with NO obtained by monitoring the kinetics of the formation of NO2 following radical formation by pulse radiolysis (Sehested et al., 1993) are now believed to be in error due to complications from recycling and delayed NO2 formation (Orlando and Tyndall, 2012). The kinetic database for reactions of substituted peroxy radicals with NO is rather limited. However, from the existing database, it appears that peroxy radicals with substituents that are one or more C–C bonds separated from the –O-O• radical group react with kinetics that are indistinguishable from the simple unsubstituted cases. Thus, as indicated in Table V-B-1, the recommended 298 K rate coefficients for reactions of CH3C(O)CH2O2, HOCH2CH2O2, and CH3OCH2O2 radicals with NO are 8 × 10−12, 9 × 10−12, and 9.1× 10−12 cm3

3 x 10–11

k, cm3 molecule–1 s–1

2 x 10–11

10–11 9 x 10–12 8 x 10–12 7 x 10–12 6 x 10–12 5 x 10–12 4 x 10–12 3 x 10–12

2.5

3.0

3.5

4.0

4.5

5.0

1000/T, K–1 FIGURE V-B-1.

Rate coefficients for reactions of selected peroxy (RO2) and acyl peroxy (RC(O)O2) radicals with NO as recommended by International Union of Pure and Applied Chemistry (IUPAC) (Atkinson et al., 2006, 2008b). The thick dashed lines are the recommendations for k(RO2+NO) and k(RC(O)O2+NO) from the present work.

318

the mechanisms of reactions influencing atmospheric ozone TABLE V-B-1 . R ATE COEFFICIENTS FOR RE ACTIONS OF PEROXY

R ADICALS WITH NO (ADAPTED FROM ORL ANDO AND T YNDALL, 2012)

Peroxy Radical

1012 × k 1012× A 3 (298 K), cm molecule−1 s−1

HO2 CH3O2 C2H5O2 n-C3H7O2 i-C3H7O2 t-C4H9O2 2-C5H11O2 cyclo-C5H9O2 CH3C(O)O2 C2H5C(O)O2 CH3C(O)CH2O2 HOCH2CH2O2 HO-isoprene-O2 CH3OCH2O2 CH3CH(OH)CH(O2)CH3 CF3O2 CF2ClO2 CFCl2O2 CCl3O2 ClCH2CH2O2 CH3SCH2O2

OH

O O

B (K) Reference

a) Unsubstituted 8.5 3.45 −270 7.6 1.96 −403 9.2 2.62 −373 9.4 2.9 −350 9.0 2.7 −360 7.9 8.0 10.9 b) Oxygenated 20 7.5 −290 21 6.7 −340 8 9 8.8 9.1 9.3 c) Halogenated radicals 16.1 15 15 18 9.7 d) Miscellaneous 12 4.9 −260 7.7

Atkinson et al. (2004) Calvert et al. (2008) Calvert et al. (2008) Calvert et al. (2008) Calvert et al. (2008) Calvert et al. (2008) Calvert et al. (2008) Calvert et al. (2008) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Langer et al. (1995b) Atkinson et al. (2006) Atkinson et al. (2008b) Atkinson et al. (2008b) Atkinson et al. (2008b) Atkinson et al. (2008b) Atkinson et al. (2008b) Atkinson et al. (2004) Elrod (2011)

O

O

molecule−1 s−1. Elrod and co-workers studied the kinetics of the reactions of the peroxy radicals formed following addition of chlorine (Cl) atoms or HO radicals to a series of C2–C5 alkenes. The average of the rate coefficients determined for the reactions of the 2-chloro peroxy radicals with NO was 9.2 × 10−12 (Patchen et al., 2005), whereas the average of the 2-hydroxy peroxy reactions was 10.3 × 10−12 cm3 molecule−1 s−1 (Miller et  al., 2004).

Within the ±30% estimated experimental accuracy, the rate coefficients measured at 298 K for the reactions of NO with the chloro- and hydroxy-peroxy radicals derived from ethene, propene, 1-butene, 2-butene, 2-methyl propene, 1,3-butadiene, and isoprene were indistinguishable from the value of 8.8 × 10−12 cm3 molecule−1 s−1 derived from the average for NO reactions with HO2, CH3O2, C2H5O2, n-C3H7O2, and iso-C3H7O2.

Mechanisms of Reactions of HO2 and RO2 Radicals The reaction of HO with isoprene is of particular importance and has been the subject of several studies. The addition of HO and O2 to isoprene gives six structurally isomeric peroxy radicals (shown below) and two geometric isomers, depending on the relative orientation about the double bond to give E- and Z-variants of the 1,4- and 4,1 isomers; the 1,4- and 4,1 isomers shown on the right below are the E-variants (the Z-variants are not shown).

319

RC(O)O2 are approximately twice as reactive toward NO as the simple peroxy radicals. They have a similar negative temperature dependence, again consistent with a reaction mechanism proceeding via the formation of an adduct. The average of the rate coefficients for the reactions of CH3C(O) O2 and C2H5C(O)O2 radicals is 2.0 × 10−11 cm3 molecule−1 s−1 at 298 K.  The rate coefficients for OO

OO HO

OH OO

HO

OO

HO

OH

OH

OO

OO

Experiments by Zhang et  al. (2003), Miller et  al. (2004), and Park et al. (2004) provide rate coefficients of (9 ± 3)  × 10−12, (8.8 ± 1.2) × 10−12, and (9 ± 3) × 10−12 cm3 molecule−1 s−1, respectively, for the mixture of different HO-isoprene-O2 radicals formed in the system. Ghosh et al. (2010) employed photolysis of 2-iodo-2-methylbut-3-en-1-ol to generate the HOC5H8 isomer, which corresponds to the major channel for HO addition to isoprene and subsequently two of the HO-isoprene-O2 isomers. Ghosh et  al. (2010) measured a rate coefficient of (8.1+3.4−2.3) × 10−12 cm3 molecule−1 s−1 at 298 K, which is indistinguishable from the results reported for the mixture of six isomers. The available evidence suggests that there are no significant reactivity differences between the various HO-isoprene-O2 isomers, and their reactivities are indistinguishable from those of the simplest peroxy radicals. Elrod (2011) has measured the reactivity of the bicyclic peroxy radical derived from HO-initiated oxidation of 1,3,5-trimethylbenzene. The structure of this peroxy radical is shown in the bottom row of Table V-B-1. Its reported reactivity toward NO, k = (7.7 ± 0.7) × 10−12 cm3 molecule−1 s−1, is again indistinguishable from that of the simple peroxy radicals just described. As evident from Figure V-B-1 and Table V-B1, the acylperoxy radicals of the general formula

reactions of halogenated alkyl peroxy radicals with NO tend to be larger than those for unsubstituted peroxy radicals when the halogen substituents are in the 1-position (Atkinson et al., 2008b), but not when the halogen is in the 2-position (Patchen et al., 2005). The value reported for the HOCH2O2 + NO reaction shown in Figure V-B-1 was derived in an indirect fashion, has substantial uncertainties, and is not considered further. Taking an average of the exponential factors for the reactions of HO2, CH3O2, C2H5O2, n-C3H7O2, and iso-C3H7O2 with NO and adjusting the A factor to give the average k(298K) value gives k = 2.5 × 10−12 exp(380/T) cm3 molecule−1 s−1. We recommend this for all peroxy radicals except those with halogen substituents at the 1-position. Although limited, the available data for acylperoxy radicals shown in Figure V-B-1 indicate that the nature of the alkyl group, R, does not have a large impact on the kinetics of the RC(O)O2 + NO reaction. Taking an average of the exponential factors recommended for the reactions of CH3C(O)O2 and C2H5C(O)O2 with NO and adjusting the A factor to give the average k(298K) value gives k = 7.1 × 10−12 exp(310/T) cm3 molecule−1 s−1, which we recommend for all acyl peroxy radical + NO reactions.

320

the mechanisms of reactions influencing atmospheric ozone

V-B-2. Products and Mechanisms of the HO2 + NO and RO2 + NO Reactions It is well established, mainly due to the work of Atkinson and co-workers (e.g., Arey et  al., 2001), that the reaction of peroxy radicals proceeds via two pathways to give either an alkoxy radical and NO2 or an alkyl nitrate: RO2 + NO → RO + NO2 RO2 + NO + M → RONO2 + M One cannot overstate the importance of an accurate description of the branching ratio of this reaction in atmospheric models. The channel giving an alkoxy radical and NO2 leads to radical propagation and promotes photochemical ozone formation. The channel giving alkyl nitrate removes both radicals and NOx from the system and hence hinders photochemical ozone formation. It has been shown that the nitrate yield increases with total pressure, size of the peroxy radical, and with decreasing temperature (Atkinson et  al., 1983, 1987b; Carter and Atkinson, 1989; Harris and Kerr, 1989). Nitrate yields are reported to be highest for secondary radicals (RCH(OO)R′) and approximately one-half to one-third those for primary (RCH2OO) and tertiary RR′R″COO) radicals. However, recent experimental studies by Espada et al. (2005) and Cassanelli et al. (2007) report approximately equal yields from secondary, primary, and tertiary radicals of the same size. Furthermore, as discussed in Calvert et  al. (2008), unpublished data from one of our laboratories (NCAR) indicates that tertiary alkyl nitrates are thermally unstable at gas chromatography (GC) injection temperatures. Hence, the previous studies that used GC analysis may have underestimated the yields of tertiary nitrates. Clearly, given the importance of well-established branching ratios for these reactions to atmospheric models of ozone formation, there is an urgent need for further work in this area. Arey et  al. (2001) have provided an empirical parameterization of the nitrate yields for secondary radicals (RCH(OO)R′) accounting for the impact of total pressure, temperature, and size of the radical. The ratio between the rate coefficient for nitrate formation (kRONO2) and that for NO2 formation (kNO2) is given by kRONO2/ kNO2 = [A/(1 + (A/B))] × Fz, where A = 2 × 10−22 × exp(n) × [M] × (T/300) and B  =  0.43 × (T/300)−8, F  =  0.41, z  =  (1 + (log(A/B))2)−1 and n is the number of carbon atoms

in the radical. Evaluation of the nitrate yields in the reactions of secondary peroxy radicals with NO using this parameterization in 1 atm of air at 298 K and for the conditions of temperature and pressure for the US Standard Atmosphere at 0, 5, and 10 km are given in Figure V-B-2. As seen from the figure, the nitrate yield at 298 K in 1 atm of air increase from a few percent for the smallest radicals (4.2 % for CH3CH(OO)CH3) to approximately 30% for radicals containing 10 carbon atoms. Figure V-B-2 shows the large range of nitrate yields depending on radical size and altitude (and hence temperature and pressure). For small radicals, nitrate formation is a rather minor channel. For large radicals at the low temperatures in the upper troposphere, the nitrate formation channel is the dominant reaction channel. Figure V-B-2 conveys the richness of this atmospheric chemistry, the precise details of which still require further work to place models of ozone chemistry on a more solid foundation. The trend from the parameterization for secondary radicals described here indicates that the nitrate yields from the smallest radicals CH3O2, C2H5O2, and C3H7O2 are minor. Experimental determination of such small yields is challenging. However, as noted by Orlando and Tyndall (2012), the recent experimental studies by Elrod and co-workers (Scholtens et  al., 1999; Ranschaert et  al., 2000; Chow et  al., 2003)  and Butkovskaya et  al. (2010a, 2010b, 2012)  of nitrate yields in the reactions of methyl, ethyl, and propyl peroxy radicals are broadly consistent with those predicted using the parameterization from Arey et al. (2001). For the reaction of ethyl peroxy radicals with NO, the results from Butkovskaya et  al. (2010b) when extrapolated to atmospheric pressure give a nitrate yield of 3%, which is larger than the value of 2.1% predicted by the parameterization. The nitrate formation observed in the reaction of methyl peroxy radicals with NO by Butkovskaya et al. (2012) corresponds to a yield of approximately 1% at 1 atm at 298 K. This yield is consistent with the value expected by extrapolation of the parameterization derived for secondary radicals by Arey et al. (2001). However, this yield is inconsistent (by a factor of 100!) with the yield estimated based on the concentrations of methyl nitrate observed in the atmosphere in field measurements (Flocke et  al., 1998). Further work is needed to better define the nitrate yields from methyl and ethyl peroxy radicals. The available data suggest that the nitrate yields for substituted peroxy radicals can be lower

Mechanisms of Reactions of HO2 and RO2 Radicals

321

298 K, 760 Torr 0 km US Standard Atmosphere 5 km US Standard Atmosphere 10 km US Standard Atmosphere

1.0

Nitrate yield

0.8

0.6

0.4

0.2

0.0

2

4

6

8 10 12 Number of carbon atoms

14

16

18

FIGURE V-B-2.

The fraction of the NO-RCH(OO•)R′ reactions that leads to the nitrate product plotted versus the number of carbon atoms in the molecule. Data were calculated using the parameterization from Arey et al. (2001) for 298 K and 1,013 mbar (760 Torr) total pressure (filled circles) and for the temperature and pressures in US Standard Atmosphere at 0 km (288.2 K, 1,013 mbar [760 Torr]), 5 km (255.7 K, 540 mbar [405 Torr]), and 10 km (223.2 K, 265 mbar [199 Torr]).

than those for unsubstituted alkylperoxy radicals of a similar size, particularly when the substituent is located close to the peroxy moiety. Nishida et  al. (2004a) reported a nitrate yield of 1.7% for the reaction of CF3OO• radicals with NO in 700 Torr of air at 296 K.  Espada et  al. (2005) report that the nitrate yield from BrCH2CH(OO•)CH3 radicals is approximately a factor of 2 less than for CH3CH(OO•)CH3 radicals, but the yield from BrCH2CH2CH2OO• radicals is indistinguishable from that from CH3CH2CH2OO• radicals. Espada and Shepson (2005) showed that when compared to alkyl peroxy radicals, the radicals produced during the oxidation of glycol ethers have decreased nitrate yields when the ether linkage is adjacent to the peroxy moiety and increased yields when the ether linkage is separated from the peroxy radical. Ziemann and co-workers (Matsunaga and Ziemann, 2009, 2010; Matsunaga et al., 2009) have studied the nitrate yields from the peroxy radicals formed in the HO-radical initiated oxidation of a series of linear C8–C17 1-alkenes and C14–C17 internal alkenes. It was found that the nitrate yields from reactions of the β-hydroxy-peroxy radicals with NO was dependent on the structure of the radical with tertiary > secondary > primary. The nitrate yields increased with number of carbons in the radicals

and plateaued at limiting values of 25% (tertiary), 15% (secondary), and 12% (primary), which are significantly lower than observed for unsubstituted alkyl peroxy radicals (see Figure V-B-2). The oxidation of linear alkanes leads to the formation of alkoxy radicals that can isomerize via a 1,5-hydrogen shift (alkoxy radical bites its tail), which leads to the formation of a hydroxyperoxy radical, for example: CH3CH2CH2CH2CH(O•)CH3 → CH3CH(•)CH2CH2CH(OH)CH3 CH3CH(•)CH2CH2CH(OH)CH3 + O2 + M → CH3CH(OO•)CH2CH2CH(OH)CH3 + M Studies by Arey et al. (2001) and Lim and Ziemann (2005) have shown that the nitrate yields for these substituted peroxy radicals have a limiting value of approximately 15% for large radicals. This yield is approximately a factor of 2 lower than observed for unsubstituted alkyl peroxy radicals. The nitrate yield for peroxy radicals produced in the HO-initiated oxidation of alkenes is of major importance in understanding ozone chemistry because of the very large emissions of isoprene and monoterpenes on regional and global scales. Based on a nitrate yield of 8%, it has been estimated that

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the mechanisms of reactions influencing atmospheric ozone

approximately 10% of NOx emitted in the eastern United States is converted into isoprene nitrate (Orlando and Tyndall, 2012). The HO-initiated oxidation of isoprene gives six structurally isomeric peroxy radicals (and two stereoisomers). Measurements of the nitrate yield lie in the range of 4–15% (Chen et al., 1998; Tuazon and Atkinson, 1990; Chuong and Stevens, 2002; Sprengnether et al., 2002; Patchen et al., 2007; Paulot et  al., 2009a; Lockwood et  al., 2010). The IUPAC data evaluation panel recommends a nitrate yield of 8 ± 3% based on the results from the more recent studies (Atkinson et  al., 2006). Lockwood et  al. (2010) identified and quantified each of the eight isomers of hydroxyl-isoprene nitrate formed in the HO-initiated oxidation and reported a total yield of (7 +2.5−1.5) %. Three isomers, representing nitrates resulting from OH addition to a terminal carbon, represent 90% of the total nitrate yield with the 4,3-, 1,2- and Z-4,1-hydroxy isoprene nitrate isomers accounting for 50%, 31%, and 9%, respectively (Lockwood et al., 2010). The structures of these isomers are shown in Figure V-B-3. Orlando and Tyndall (2012) have pointed out that the results from the study by Lockwood et  al. (2010) imply a very large range of approximately 1–15% in the nitrate yields from the individual RO2 + NO reactions. It is interesting and somewhat surprising that there should be such a large range of nitrate yields for these peroxy radicals, which are structurally very similar. Paulot et  al. (2009a) identified a wide range of products in an experimental study of the HO-initiated oxidation of isoprene. The yields of 4,3-, 1,2-, and Z-4,1-hydroxy isoprene nitrate

isomers inferred by Paulot et al. (2009a) accounted for 13%, 23%, and 25% of the total nitrate, respectively. The absolute and relative importance of the nitrate isomers reported by Paulot et al. (2009a) differ significantly from those reported by Lockwood et al. (2010). Although there has been major progress during the writing of this book, given the central importance of isoprene in atmospheric chemistry, further work is needed to refine our understanding of the yields of the hydroxynitrates formed in the HO-initiated oxidation in the presence of NOx. The HO-initiated oxidation of isoprene produces methyl vinyl ketone and methacrolein as the major products. The subsequent oxidation of these primary products produces a variety of secondary products including hydroxynitrates. From an analysis of the products formed in this complex chemical system, Paulot et al. (2009a) provided estimates of 15 ± 3% and 11 ± 3% for the overall nitrate yield from reactions of NO with the peroxy radicals produced in the HO-initiated oxidation of methyl vinyl ketone and methacrolein (structures shown in Figure V-B-4), respectively. As noted by Orlando and Tyndall (2012), our understanding of the nitrate yields for the atmospherically relevant large biogenic organics is extremely limited for the monoterpenes (containing two isoprene units) and sesquiterpenes (containing three isoprene units). As an example, the nitrate yield following the HO-initiated oxidation of α-pinene has been reported as 18 ± 9% by Nozière et  al. (1999) using Fourier transform infrared (FTIR) spectroscopy and 1% by Aschmann et  al. (2002) using atmospheric pressure

OH

ONO2 OH FIGURE V-B-3.

OH

Structures of 4,3-, 1,2- and Z-4,1-hydroxy isoprene nitrate isomers.

OO

OH

OO

O

OO

OH CH3

CH3

FIGURE V-B-4.

ONO2

ONO2

OH

O

CHO

CHO

OO

CH3

OH

CH3

Structures of the hydroperoxy radicals formed from methyl vinyl ketone (left) and methacrolein (right).

Mechanisms of Reactions of HO2 and RO2 Radicals ionization mass spectrometry. The determination of Nozière et al. (1999) may be an overestimate because of interferences with other nitrates, and the determination by Aschmann et al. may be an underestimation because of sampling losses or fragmentation of the parent ion upon protonation. Lee et al. (2006) conducted a study of the products of the HO-initiated oxidation of a series of terpenes and oxygenated terpenes and used proton-transfer mass spectrometry to identify organic nitrates in yields of 0.1–1.9% for 10 compounds (longifolene, limonene, myrcene, methyl chavicol, α-pinene, β-pinene, γ-terpinene, α-terpinene, verbenone, and linalool). Lee et al. (2006) noted the possibility of sampling losses and/or fragmentation of the parent ion leading to potential underestimation of the nitrate yields. The yields reported by Lee et al. (2006) seem implausibly low when considering the size of the peroxy radicals formed, and it seems likely that sampling and/or instrumental artifacts led to an underestimation of the nitrate yields. Browne and Cohen (2012) conducted a modeling study and have recognized the sensitivity of global and regional ozone, NOx, and HOx budgets on the organic nitrate yield in the reactions of NO with peroxy radicals produced in the oxidation of biogenic organics. As highlighted by Browne and Cohen (2012), an accurate representation of organic nitrate chemistry is required to provide accurate assessments of past, present, and future air quality and climate. Further laboratory studies on the formation of RONO2 oxidation, physical loss, and oxidation products are needed. The mechanism of the RO2 + NO reaction can be represented as (* denotes nascent species formed with chemical activation): RO2 + NO ⟷ ROONO* ROONO* → RO* + NO2 ROONO* → RONO2* RONO2* +M → RONO2 + M* RO* + M → RO + M* RO* → decomposition and/or isomerization RO → decomposition, isomerization, and/or reaction with O2 The RO2 + NO reaction has a substantial exothermicity, and chemical activation in the nascent RO* radicals can be an important factor in determining their subsequent atmospheric reactivity, as will be

323

discussed in Chapter VI. Numerous theoretical treatments of the RO2 + NO reaction mechanism have been published. These include quantum chemical calculations (Sumathy and Peyerimhoff, 1997; Lohr et  al., 2003)  of the structures and relative energies of the intermediates, and master equation (Barker and Golden, 2003; Golden et al., 2003; Zhang et al., 2004a; Zhang and Donahue, 2006) and classical trajectory simulations (Stimac and Barker, 2008; Chen et al., 2009a). The ROONO* → RONO2 isomerization step has posed particular theoretical challenges (Butkovskaya et al., 2009). The general mechanism for the homologous RO2 + NO reactions is currently believed to be analogous to that for HO2 + NO. The initial reaction produces vibrationally excited cis,cis-HOONO, which can rapidly re-dissociate or pass through a transition structure on the way to forming HO + NO2 or HONO2. Quantum chemistry calculations for this reaction have proved very challenging, and the details of this reaction are not fully understood. Dynamics calculations indicate that slow intramolecular vibrational energy redistribution may play a role (Stimac and Barker, 2008). High-level quantum chemistry methods (i.e., UCCSD(T)/CBS(T,Q)//UB97-1/6-311++G**) generally support the calculations of Butovskaya et al., which were carried out at lower levels of theory (Butkovskaya et  al., 2009). The results suggest that after passing through a transition state the reaction encounters a very shallow well on the potential energy surface, where the reactive flux bifurcates: more than 99% of the reaction proceeds to form HO + NO2, but a small fraction may instead proceed into the HONO2 potential energy well, possibly explaining the reported experimental yields of HONO2 (Butkovskaya et  al., 2005, 2007, 2009). As discussed in Chapter III, extrapolation of the Butkovskaya et al. data to zero pressure yield significant (nonzero) intercepts, perhaps indicative of the occurrence of wall reactions (Sander et al., 2011). Further work is needed to better define the yield of HONO2 in this reaction. Multiwell, multichannel master equation simulations of RO2 + NO reactions have been successful in fitting the experimental data on organic nitrate formation (Barker et al., 2003; Zhang et al., 2004a), but the parameters needed in the simulations were recognized as unphysical. Stimic and Barker (2008) showed the importance of the highly excited chemically activated CH3OONO* complex in the reaction and the difficulties in theoretical treatment of this complex. RO2 + NO reactions have yet to yield to theoretical treatments (Vereecken and Francisco,

324

the mechanisms of reactions influencing atmospheric ozone

2012). Further computational studies of RO2 + NO reactions are needed to provide insight into the factors impacting the nitrate yields, which would enable an improved description of organic nitrate formation in atmospheric models. V- C . R E A C T I O N S O F H O 2 + N O 2 A N D R O 2  +   N O 2 Reactions of alkylperoxy radicals with NO2 occur via addition and lead to the formation of thermally unstable peroxynitrates: RO2 + NO2 + M ⟷ RO2NO2 + M The reactions are typical three-body processes, and their rate coefficients vary with pressure and temperature in the manner described by Troe (1983). The effective second-order rate coefficient can be expressed as a function of the limiting low- (ko) and high- (k∞) pressure rate coefficients and the broadening factor Fc, which determines the shape of the fall-off curve using the expressions (Atkinson et al., 2004): k=

log10 F ≅

k0 k∞ F k0 + k∞

log10 Fc 1 + [ log10 (k0 / k∞ )/ N ]

2

N = [ 0.75 − 1.27 logFc ]

The rate coefficients at the low- and high-pressure limits vary with temperature ko  =  ko (T/300)−m and k∞ = k∞ (T/300)−n. The reactions of NO2 with HO2, CH3O2, and CH3C(O)O2 are in the fall-off region at atmospheric pressure, as illustrated in Figure V-C-1. Kinetic parameters for the reactions of selected peroxy radicals with NO2 are listed in Table V-C-1. The high-pressure limits for the reactions are of the order of 10−11 cm3 molecule−1 s−1. As the size of the peroxy radical increases, the density of states increases and the lifetime of the initially formed RO2NO2 adduct increases. Hence, for a given pressure of diluent gas, the effective second-order rate constant tends toward the high-pressure limit as the size of the peroxy radical increases. At 298 K in 1 atm of air, the reactions of peroxy radicals larger than C2H5O2 are close (within a factor of 2) to the high-pressure limit. The effects of decreased temperature (which increases the rate) and decreased pressure (which decreases the rate) offset each other, and hence the rate coefficients of the reactions of peroxy radicals with NO2 do not change substantially with altitude in the atmosphere. The alkylperoxynitrates, RO2NO2, formed in the reaction of peroxy radicals with NO2 are bound by approximately 80–90 kJ mol−1 and are thermally unstable in the lower troposphere, with lifetimes of the order of seconds at 298 K (see Table V-C-2). The rate of thermal decomposition of RO2NO2 is very sensitive to temperature. For example, the decomposition

1012 x k, cm3 molecule–1 s–1

8 HO2 6

CH3C(O)O2

4

2

0

FIGURE V-C-1.

CH3O2

0

200

400 Pressure, Torr

600

800

Effective second-order rate coefficients for reactions of NO2 with HO2, CH3O2, and CH3C(O)O2 versus pressure in air at 298 K (Atkinson et al., 2004, 2006).

Mechanisms of Reactions of HO2 and RO2 Radicals

325

TABLE V-C-1 . KINETIC DATA FOR SELECTED RO 2 + NO 2 RE ACTIONS (ATKINSON ET AL., 2004, 2006; ORL ANDO AND T YNDALL, 2012) a

Peroxy Radical

HO2 CH3O2 C2H5O2 CH3C(O)CH2O2 CH3OCH2O2 CF3O2 CH3C(O)O2 CF3C(O)O2

1030 × ko, cm6 m 1012 × k , cm3 ∞ molecule−2 s−1 molecule−1 s−1 0.14 2.5 13

56 270

n

Fc

3.1 5.5 6.2

4.0 18 8.8

0 0 0

0.4 0.36 0.31

9 7.1

7.7 12

0.67 0.9

0.31 0.3

1012 × k298K, 1atm, cm3 molecule−1 s−1 4.0 6.1 6.4 7.9 5.5 11 6.6

a

The Troe parameters ko, k∞, m, n, and Fc are as defined near the beginning of this section.

TABLE V-C-2 . KINETIC DATA FOR THERMAL DECOMPOSITION OF SELECTED RO 2 NO 2 (ATKINSON ET AL., 2004, 2006; ORL ANDO AND T YNDALL, 2012) a

RO2NO2 HO2NO2 CH3O2NO2 C2H5O2 NO2 CH3C(O)CH2O2 CH3OCH2O2 CF2ClO2 CH3C(O)O2 C2H5C(O)O2

ko, cm3 molecule−1 s−1

k∞, s−1

Fc k(298K and 1 atm), s−1

4.1 × 10−5 exp(−10,650/T) 4.8 × 1015 exp(−11,170/T) 0.6 9 × 10−5 exp(−9,690/T) 1.1 × 1016 exp(−10,560/T) 0.4 4.8 × 10−4 exp(−9,285/T) 8.8 × 1015 exp(−10,440/T) 0.31 1.4 × 1016 exp(−10,730/T) 1.8 × 10−3 exp(−10,500/T) 1.6 × 1015 exp(−11,990/T) 0.3 4.9 × 10−3 exp(−12,100/T) 5.4 × 1016 exp(−13,830/T) 0.30 1.7 × 10−3 exp(−11,280/T) 8.3 × 1016 exp(−13,940/T) 0.36

0.25 1.8 5.4 ≈3 2.6 0.05 3.3 × 10−4 4.0 × 10−4

The Troe parameters ko, k∞, m, n, and Fc are as defined near the beginning of this section.

a

of CH3O2NO2 proceeds with an activation energy of 88 kJ mol−1 and slows by a factor of 3 million over the temperature range 298–210 K.  Hence, for the colder temperatures of the upper troposphere, the lifetimes of alkylperoxynitrates with respect to thermal decomposition is of the order of days, and other loss mechanisms such as photolysis and/or reaction with HO radicals can become important. The acylperoxynitrates, RC(O)O2NO2, formed in the reaction of acylperoxy radicals with NO2 are bound by approximately 110–120 kJ mol−1. Acylperoxynitrates have a lifetime with respect to decomposition at 298 K in 1 atm of air of the order of 1 hour and centuries in the upper troposphere. Acylperoxynitrates such as PAN (CH3C(O)O2NO2) play an important role in

ozone chemistry because they act as reservoir species for NOx. Aldehydes are abundant in the atmosphere, and their oxidation (other than CH2O) leads to the formation of acyl and hence acylperoxy radicals. Acyl peroxy radicals add NO2 to form acylperoxynitrates that, when lofted to altitude, are stable with respect to thermal decomposition before descending in air circulation to lower altitudes where they undergo decomposition to release NOx, as illustrated for the formation and decomposition of peroxyacylnitrate (CH3C(O)O2NO2) here: CH3CHO + HO → CH3CO + H2O CH3CO + O2 + M → CH3C(O)O2 + M

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the mechanisms of reactions influencing atmospheric ozone

CH3C(O)O2 + NO2 + M → CH3C(O)O2NO2 + M CH3C(O)O2NO2 + M → CH3C(O)O2 + NO2 + M The preceding discussion implies that peroxynitrates are the sole products of the reaction of RO2 radicals with NO2. However, there is evidence that an additional reaction channel is available for some peroxy radicals and that it leads to the formation of NO3 radicals: RO2 + NO2 + M → RO2NO2 + M RO2 + NO2 → RO + NO3 For simple alkylperoxy radicals of the general formula CxH2x+1O2, the reaction channel forming NO3 radicals is endothermic by approximately 50 kJ mol−1 and is not important. Some substituted peroxy radicals react with NO2 to give NO3 radicals. For example, Orlando and Tyndall (2001) provided evidence that the peroxy radical formed in the oxidation of glyoxal reacts with NO2 to give NO3 radicals: HO + HC(O)CHO → HC(O)CO + H2O HC(O)CO + O2 + M → HC(O)C(O)O2 + H2O + M HC(O)C(O)O2 + NO2 → HC(O)C(O)O + NO3 HC(O)C(O)O + M → HC(O) + CO2 + M Further work is needed to establish whether the formation of NO3 radicals is a general feature or an isolated occurrence in the reactions of oxygenated peroxy radicals with NO2 V- D . R E A C T I O N S O F H O 2 + H O 2 A N D R O 2  +   H O 2 Reactions of peroxy radicals with HO2 radicals are radical sinks and hence slow down the chain reactions that lead to photochemical ozone formation in the troposphere (see Figure 1.E.7). In large areas of the troposphere, particularly those with NOx concentrations below about 20 ppt, reaction with HO2 radicals is the dominant atmospheric loss mechanism for HO2 and RO2 radicals (see Section I-E-5). To understand the chemistry associated with tropospheric ozone formation in nonpolluted environments and on a global scale, it is important to consider the HO2 + HO2 and RO2 + HO2 reactions. The rate coefficients for all reactions of peroxy radicals with HO2 radicals have significant negative temperature dependencies; that is, the reaction

rate increases with decreasing temperature. This negative temperature dependence indicates that the reaction proceeds via the formation of a short-lived intermediate. The HO2 self-reaction is interesting because it alone among this class of reactions has a rate that is dependent on total pressure and water vapor concentration. The reaction has both bimolecular and termolecular channels, both of which produce H2O2 in a yield of 100% under atmospheric conditions (Atkinson et al., 2004): HO2 + HO2 → H2O2 + O2 HO2 + HO2 + M → H2O2 + O2 + M In the presence of water vapor, the rates of both channels increase by a factor of (1 + 1.4 × 10−21 [H2O] exp(2,200/T), where [H2O] is in units of molecule cm−3. The impact of water on the reaction rate is substantial. In 1 atm of air at 298 K, the rate coefficient for the overall reaction increases by a factor of 2.7 as the humidity increases from 0% to 100%. Figure V-D-1 shows the rate coefficient for the HO2 + HO2 reaction calculated as a function of altitude for the US Standard Atmosphere, assuming 0% and 100% relative humidity (Murphy and Koop, 2005). Mozurkewich and Benson (1985) considered the HO2 + HO2 reaction theoretically and concluded that the negative temperature dependence, pressure dependence, and observed isotope effects can most reasonably be explained in terms of the reaction proceeding on the triplet surface (i.e., with the two unpaired spins preserved) via a cyclic hydrogen-bonded (HO2)2 intermediate. In contrast to the case for the HO2 + HO2 reaction, no discernable effect of either total pressure or water vapor concentration has been reported in experimental studies of the kinetics of RO2 + HO2 reactions. As seen from Table V-D-1, the RO2 + HO2 reactions proceed rapidly, with rate coefficients in the range (5–20) × 10−12 cm3 molecule−1 s−1 at 298 K. It is well established that the rate coefficients for reactions of alkylperoxy radicals with HO2 increase with the size of the alkyl fragment (Boyd et al., 2003; Calvert et al., 2008; Orlando and Tyndall, 2012; see Figure V-D-2). Boyd et al. (2003) have shown that a function of the form k = A × (1 − exp(Bn)), where A and B are parameters and n is the number of carbons in the alkyl fragment, can be fitted to the rate coefficients. Calvert et al. (2008) performed a least squares fit (see Figure IV-D-2) to give k = 21.3 × [1 − exp(−0.256n)], with k in units

Mechanisms of Reactions of HO2 and RO2 Radicals

327

10

Altitude, km

8

6 CH3O2 + HO2 C2H5O2 + HO2 cyclo-C6H11O2 + HO2

4

CH3C(O)O2 + HO2 HO2 + HO2 (0% RH)

2

HO2 + HO2 (100% RH) 0

10–11

10–10 k,

FIGURE V-D-1.

cm3

molecule–1

s–1

Rate coefficients for HO2 + HO2 and RO2 + HO2 reactions as a function of altitude.

TABLE V-D-1 . KINETIC DATA FOR

SELECTED RO 2 + HO 2 RE ACTIONS (ATKINSON ET AL., 2006; CALVERT ET AL., 2008; ORL ANDO AND T YNDALL, 2012)

Peroxy Radical

1013 × A, cm3 Ea/R, 1012 × molecule−1 s−1 K k(298K), cm3 molecule−1 s−1

HO2 a

2.2 1.9 × 10−33 [N2] 3.8 7.4

CH3O2 C2H5O2 CH3C(O)CH2OO HOCH2CH2O2 (CH3)2C(OH) CH2O2 CH2FO2 CF2ClO2 CF3CF2O2 CF2ClCH2O2 (CH3)3CCH2O2 cyclo-C5H9O2 cyclo-C6H11O2 C10H21O2 C14H29O2 HO-isoprene-O2 HO-α-pinene-O2 CH3C(O)O2

0.56

−600 −980 −780 −700

1.6 5.2 × 10−32 [N2] 5.2 7.8 9 13 −1,650 14

3.2

−820

2.39 1.37 2.66

−1,228 −1,427 −1,240

5.2

−980

5 3.4 1.2 6.8 14.7 15.6 17 19.5 21 17 20.9 14

a

The rate of this reaction is dependent on pressure, temperature, and water vapor concentration, see text for details.

of 10−12 cm3 molecule−1 s−1. All RO2 + HO2 reactions have large negative activation energies (−E/R typically

in the range 700–1,700 K), suggesting they proceed via the formation of a short-lived complex. Product studies of the reactions of alkylperoxy radicals with HO2 have been conducted in a few specific cases. As seen from Table V-D-2, under ambient conditions, the simple alkylperoxy radicals (CH3O2, C2H5O2, C3H7O2,…) react with HO2 radicals to give alkylhydroperoxides in yields that are, in general, indistinguishable from 100%: RO2 + HO2 → ROOH + O2 This channel is typically assumed to occur exclusively (Tyndall et  al., 2001; Calvert et  al., 2008). However, it should also be noted that a second channel in the reaction of CH3O2 with HO2, one involving the production of water, has been discussed in the literature ( Jenkin et  al., 1988; Moortgat et  al., 1989; Elrod et al., 2001): CH3O2 + HO2→ CH3OOH + O2 → CH2O + H2O + O2 Elrod et al. (2001) have presented evidence for the occurrence of the H2O channel with a yield of 11% at 298 K and 31% at 218 K. These results need corroboration. Further studies of the branching ratios in the reaction of larger alkylperoxy radicals with HO2, particularly at low temperature, are required to clarify the importance of the H2O channel. The early work by Niki et  al. (1985) showed that the reaction of CH3C(O)O2 with HO2 radicals at atmospheric pressure and 298 K proceeded

328

the mechanisms of reactions influencing atmospheric ozone 24

1012 x k, cm3 molecule–1 s–1

20

16

12 HO2 Data Fit to HO2 Data

8

4

0 0

2

4

6

8

10

12

14

16

Number of carbon atoms in RO2 FIGURE V-D-2. Rate coefficients for reaction of alkylperoxy radicals with HO2 as a function of the number of carbon atoms in the peroxy radical (Calvert et al., 2008).

via at least two channels, giving peracetic acid and acetic acid. Ozone is formed as a co-product with acetic acid. The magnitude of ozone formation in this reaction is negligible when considering the atmospheric budget of ozone, but this observation is of interest on two accounts. First, it is one of the few chemical reactions in the atmosphere that gives ozone as a product. Second, it is a clear example of an RO2 + HO2 reaction that proceeds by more than one channel under atmospheric conditions: CH3C(O)O2 + HO2 → CH3C(O)OOH + O2 CH3C(O)O2 + HO2 → CH3C(O)OH + O3 Hurley et al. (2006) investigated the reactions of the perfluorinated acyl peroxy radicals CxF2x+1C(O)O2 (x = 1, 2, 3, 4) with HO2 and showed that they proceed via three channels: CxF2x+1C(O)O2 + HO2 → CxF2x+1C(O)OOH + O2 CxF2x+1C(O)O2 + HO2 → CxF2x+1C(O)OH + O3 CxF2x+1C(O)O2 + HO2 → CxF2x+1C(O)O + O2 + HO Recent interest in the product channels available in RO2 + HO2 reactions has been focused not on the possibility that H2O is formed, but on the possibility that HO radicals are formed. Hasson et al. (2004) showed that HO radicals are formed in the reactions

of HO2 with the CH3C(O)O2 and CH3C(O)CH2O2 radicals. Jenkin et al. (2007, 2010) and Dillon and Crowley (2008) confirmed the formation of HO radicals in these reactions. As evident from inspection of Table V-D-2, over the past several years experimental evidence has been reported for the formation of HO radicals in several reactions of substituted alkyl peroxy radicals with HO2 radicals; for example, in the reaction of acetonyl peroxy radicals: CH3C(O)CH2O2 + HO2 → CH3C(O) CH2OOH + O2 CH3C(O)CH2O2 + HO2 → CH3C(O) CH2O + HO + O2 Theoretical studies have provided insight into the mechanism of RO2 + HO2 reactions and have explained why some reactions appear to have a simple mechanism proceeding via a single reaction channel to give a hydroperoxide product, whereas others proceed via multiple channels and give a variety of products (Hou and Wang, 2005; Hasson et  al., 2005; Hou et  al., 2005a, 2005b). The formation of the hydroperoxide product occurs on a triplet surface via a hydrogen-bonded ROO••HOO complex that decomposes via hydrogen transfer to give ROOH and O2 as products. The formation of HO radicals occurs via the formation of the singlet ROO••OOH tetroxide

TABLE V-D-2 . PRODUCTS OF THE RE ACTION OF HO 2 WITH SELECTED PEROXY

R ADICALS IN 1 ATM OF AIR AT 298 K (ORL ANDO AND T YNDALL, 2012)

Peroxy Radical HO2 CH3O2

C2H5O2

CF3C(O)O2

C2F5C(O)O2

C3F7C(O)O2

C4F9C(O)O2

CH3OCH2O2

CH3C(O)O2

C2H5C(O)O2

C3H7C(O)O2

C6H5C(O)O2 CH3C(O)CH2OO

CH3C(O)CH(OO)CH3 CH2FO2

Products (% Yield) H2O2 + O2 (100%) CH3OOH (92 ± 8%) CH3OH + O3 (93 ± 10%) C2H5OH + O3 ( (CH3)3CO2. As seen in Figure V-H2, there is a trend in temperature dependence with reactivity of the RO2 radical. The least reactive radical shows the most positive temperature dependence, and the most reactive radicals show substantial negative temperature dependencies. As the reactivity of the radical increases along the series tert-C4H9O2, iso-C3H7O2, C2H5O2, to CH3O2, the temperature dependence becomes less pronounced. CH3C(O)O2 has the most rapid self-reaction of any peroxy radical studied to date, and this reaction has a substantial negative temperature dependence. As seen from Figure V-H-3, the cross-reactions of peroxy radicals generally proceed rapidly. Villenave and Lesclaux (1996) performed a systematic study of the kinetics of peroxy-peroxy cross-reactions of atmospheric importance and found that, in most cases, the rate coefficient for the cross-reactions were between those of the two self-reactions. Typically, the rate of the cross-reaction was close to that of the fastest of the self-reactions. Madronich and Calvert (1990b) suggested that the rate coefficient for cross-reactions could be estimated as the square root of the sum of the squares of the rate coefficients for the self-reactions:

k ( RO 2 + R ′O 2 ) =

Kinetic data for self- and cross-reactions of selected peroxy radicals are given in Table V-H-1 and are plotted in Figures V-H-2 and V-H-3. As seen from the table and figures, the rate coefficients for peroxy-peroxy reactions span a very wide range. The self-reactivity of unsubstituted alkyl peroxy radicals decreases with increasing size and decreases from primary to secondary to tertiary. At 298 K, the rate coefficients for the self-reactions of CH3O2, C2H5O2, iso-C3H7O2, and tert-C4H9O2 are 3.5 × 10−13, 7.6 × 10−14, 1.0 × 10−15, and 2.0 × 10−17 cm3 molecule−1 s−1, respectively. In all cases studied to date, the rate coefficients for the self-reactions of substituted peroxy are greater than those of

2

2

(k ( RO2 + RO 2 ) + k ( R ′O 2 + R ′O 2 )

The experimental work by Villenave and Lesclaux (1996) showed that this suggestion provides a reasonable and convenient description of the kinetics of peroxy radical cross-reactions at 298 K. Based on their experimental observations, Villenave and Lesclaux (1996) noted that the temperature dependence of peroxy radical cross-reactions is usually similar to that measured for the peroxy self-radical, with a similar k(298K) value to the cross-reaction. We adopt the root of the sum of the squares expression just presented as our recommendation for estimating k(298K) values for peroxy radical cross-reactions for atmospheric models, with the temperature dependence estimated as suggested by Villenave and Lesclaux (1996).

Mechanisms of Reactions of HO2 and RO2 Radicals

339

TABLE V-H-1 . KINETIC DATA FOR SELECTED ORGANIC PEROXY R ADICAL SELF-

AND CROSS-RE ACTIONS: k = A × e −B/T

k(298K), cm3 A, cm3 −1 −1 molecule s molecule−1 s−1

Action CH3O2 + CH3O2 C2H5O2 + C2H5O2 n-C3H7O2 + n-C3H7O2 i-C3H7O2 + i-C3H7O2 tert-C4H9O2 + tert-C4H9O2 HOCH2CH2O2 + HOCH2CH2O2 2 CH3CH(OH)CH(CH3)O2 2 (CH3)2C(OH)C(CH3)2O2 CH3C(O)O2 + CH3C(O)O2 CH3C(O)CH2O2 + CH3C(O)CH2O2 CH2ClO2 + CH2ClO2 CH2ClCH2O2 + CH2ClCH2O2 CF3CFHO2 + CF3CFHO2 CH3O2 + C2H5O2 CH3O2 + CH3C(O)O2 CH3O2 + CH3C(O)CH2O2 CH3O2 + CH2ClO2 C2H5O2 + CH3C(O)O2 C2H5O2 + C2H5C(O)O2

3.5 × 10−13 7.6 × 10−14 3.0 × 10−13 1.0 × 10−15 2.0 × 10−17 2.2 × 10−12 6.7 × 10−13 4.8 × 10−12 1.6 × 10−11 8.0 × 10−12 3.5 × 10−12 3.3 × 10−12 4.7 × 10−12 2.0 × 10−13 1.1 × 10−11 4.0 × 10−12 2.5 × 10−12 1.6 × 10−11 1.2 × 10−11

B, K

Reference

1.03 × 10−13 7.6 × 10−14

−365 0

1.6 × 10−12 9.5 × 10−12 7.8 × 10−14 7.7 × 10−15 1.4 × 10−14 2.9 × 10−12

2,200 3,894 −1,000 −1,330 −1,740 −500

3.1 × 10−14 4.2 × 10−14 6.2 × 10−13

−735 −1,300 −605

2.0 × 10−12 7.5 × 10−13

−500 −500

4.4 × 10−13

−1,070

Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Wallington et al. (1992b) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Bridier et al. (1993) Dagaut et al. (1988) Atkinson et al. (2006) Atkinson et al. (2006) Villenave and Lesclaux (1996) Atkinson et al. (2006) Tyndall et al. (2001) Dagaut et al. (1988) Atkinson et al. (2006) Le Crâne et al. (2005)

10–11

k, cm3 molecule–1 s–1

10–12 10–13 CH3O2 + CH3O2

10–14

C2H5O2 + C2H5O2 iso-C3H7O2 + iso-C3H7O2

10–15

tert-C4H9O2 + tert-C4H9O2 HOCH2CH2O2 + HOCH2CH2O2

10–16 10

2.0 FIGURE V-H-2.

CH3C(O)O2 + CH3C(O)O2 2 (CH3)2C(OH)C(CH3)2O2

–17

2.5

3.0

3.5 4.0 1000/T, K−1

4.5

5.0

Kinetic data for selected peroxy radical self-reactions.

Finally, we note that Shallcross et al. (2005) have shown that the rate coefficients at 298 K for peroxy radical self- and cross-reactions are correlated with the stabilization energy of the tetroxide intermediate. The rate coefficient at 298 K can be estimated from the

heats of formation of the peroxy radicals and the tetroxide (in units of kJ mol−1) using the empirical equation log10(k) = −{ΔHf(RO4R′) − ΔHf(RO2) − ΔHf(R′O2)}/13.22.

340

the mechanisms of reactions influencing atmospheric ozone

k, cm3 molecule–1 s–1

10–10

10–11

CH3O2 + C2H5O2 CH3O2 + CH3C(O)O2

10–12

CH3O2 + CH3C(O)CH2O2 C2H5O2 + CH3C(O)O2 C2H5O2 + C2H5C(O)O2 10–13 2.0

2.5

3.0

3.5 1000/T,

FIGURE V-H-3.

4.0

4.5

5.0

K–1

Kinetic data for selected peroxy radical cross-reactions.

V-H-3. Products of RO2 Reaction with RO2 and RO2 with R′O2 The majority of data available for the products of the RO2 + RO2, RO2 + R′O2 reactions come from static chamber experiments. Typically, the peroxy radicals are generated by the Cl atom or HO radical-initiated oxidation of a suitable organic compound, and the resulting products are identified and quantified using gas chromatography and/or FTIR spectroscopy. As an example, the products of the CH3O2 self-reaction can be studied by UV irradiation of CH4/Cl2/O2 mixtures: Cl2 + hν → 2 Cl Cl + CH4 → CH3 + HCl CH3 + O2 + M → CH3O2 + M CH3O2 + CH3O2 → CH3O + CH3O + O2

(a)

CH3O2 + CH3O2 → HCHO + CH3OH + O2

(b)

As with the kinetic experiments, the formation of HO2 radicals, and hence hydroperoxides, in the system is a complicating factor: CH3O + O2 → CH2O + HO2 CH3O2 + HO2 → CH3OOH + O2 Provided that HO2 radicals react exclusively with CH3O2 to give CH3OOH, and that the products are

not lost to secondary reactions, then the branching ratio for channels (a)  and (b)  can be determined from the ratio of the yields of CH2O, CH3OH, and CH3OOH. [CH2O]

=

[CH3OH] [CH3OOH] [CH3OH]

2ka + kb kb

=

2ka kb

The products from selected peroxy radical self- and cross-reactions are given in Table V-H-2. As indicated in Table V-H-2 and as discussed earlier, the observed products following the self- and cross-reactions of peroxy radicals have been interpreted in terms of two reactions channels, one giving alkoxy radicals and the other giving alcohol + carbonyl products. Typically, but not always, the alkoxy radical channel is the dominant reaction pathway. Interestingly, the simplest and the most atmospherically relevant self-reaction, that of CH3O2 radicals, has an unusually low yield of 37% alkoxy radicals at 298K (Atkinson et al., 2006). This can be contrasted with the more typical alkoxy radical yields of 50–90% in alkylperoxy radical self- and cross-reactions at 298 K (see Table V-H-2). The data concerning the temperature dependence of the self-reaction branching ratio are limited to CH3O2, C2H5O2, and iso-C3H7O2 (Wallington et  al., 1992b). It is well established that increased temperature favors the alkoxy radical

Mechanisms of Reactions of HO2 and RO2 Radicals

341

TABLE V-H-2 . PRODUCTS FORMED IN SELF- AND CROSS-RE ACTIONS OF

SELECTED PEROXY R ADICALS IN 1 ATM OF AIR AT 298 K

Reaction CH3O2 + CH3O2 a CH3CH2O2 + CH3CH2O2 a HOCH2CH2O2 + HOCH2CH2O2 a CH3O2 + CH3C(O)O2 a CH3C(O)O2 + CH3C(O)O2 t-C4H9O2 + t-C4H9O2 CF3O2 + CF3O2 CCl3O2 + CCl3O2 CHF2CF2O2 + CHF2CF2O2 i-C3H7O2 + i-C3H7O2 b CH3C(O)CH2O2 + CH3C(O)CH2O2 b CH3O2 + CH3C(O)CH2O2 b

CH2FO2 + CH2FO2 b CHF2O2 + CHF2O2 b CH2ClO2 + CH2ClO2 b CHCl2O2 + CHCl2O2 b CF3CHFO2 + CF3CHFO2 b ClCH2CH2O2 + ClCH2CH2O2 b

Product Yields CH3O + CH3O + O2 (37%) CH3OH + HCHO + O2 (63%) C2H5O + C2H5O + O2 (60%) C2H5OH + CH3CHO + O2 (40%) HOCH2CH2O + HOCH2CH2O + O2 (50%) HOCH2CH2OH + HOCH2CHO + O2 (50%) CH3O + CH3CO2 + O2 (90%) HCHO + CH3COOH + O2 (10%) CH3CO2 + CH3CO2 + O2 (no other channels possible) 2 tert-C4H10O + O2 (no other channels possible) CF3O + CF3O + O2 (no other channels possible) CCl3O + CCl3O + O2 (no other channels possible) CHF2CF2O + CHF2CF2O + O2 (no other channels possible) iso-C3H7O + iso-C3H7O + O2 (56%) iso-C3H7OH + (CH3)2CO + O2 (44%) CH3C(O)CH2O + CH3C(O)CH2O + O2 (75%) CH3C(O)CH2OH + CH3C(O)CHO + O2 (25%) CH3O + CH3C(O)CH2O + O2 (30%) HCHO + CH3C(O)CH2OH + O2 (20%) CH3OH + CH3C(O)CHO + O2 (50%) CH2FO + CH2FO + O2 (100%) CHF2O + CHF2O + O2 (100%) CH2ClO + CH2ClO + O2 (100%) CHCl2O + CHCl2O + O2 (100%) CF3CHFO + CF3CHFO + O2 (93%) CF3CHFOH + CF3CFO + O2 (7%) ClCH2CH2O + ClCH2CH2O + O2 (63%) ClCH2CH2OH + ClCH2CHO + O2 (37%)

Reference Atkinson et al. (2006) Atkinson et al. (2006) Barnes et al. (1993b) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Atkinson et al. (2006) Catoire et al. (1996) Nielsen et al. (1992b) Atkinson et al. (2006) Bridier et al. (1993) Bridier et al. (1993)

Dagaut et al. (1988) Nielsen et al. (1992a) Dagaut et al. (1988) Catoire et al. (1996) Atkinson et al. (2006) Atkinson et al. (2006)

Yield data are estimated by Orlando and Tyndall (2012) to have uncertainties of 10% (absolute). Yield data are estimated by Orlando and Tyndall (2012) to have typical uncertainties of 20% (absolute).

a

b

channel at the expense of the molecular channel (Horie et al., 1990; Lightfoot et al., 1990a), as illustrated in Figure V-H-4 for the CH3O2 self-reaction. The IUPAC recommended expression for the temperature dependence of the branching ratio ka/(ka + kb) for the CH3O2 self-reaction is based on the analysis of ka/(ka + kb) at 298 K by Tyndall et  al. (2001) and the temperature dependence reported by Horie et al. (1990) and is ka/(ka + kb) = α = 1/ (1 + exp(1,131/T)/19) over the temperature range 200–330 K (Atkinson et  al., 2006). As seen from Figure V-H-4, there is significant data scatter in the

reported temperature dependence of α, and further studies are warranted. There are four studies of the branching ratio for the C2H5O2 self-reaction. As seen from Figure V-H-5, there is a substantial discrepancy between the finding from the study of Noell et al. (2010) of a temperature-independent value of α = 0.26 ± 0.06 at 221–296 K and the results from the work performed by Niki et al. (1982), Anastasi et al. (1983), and Wallington et  al. (1989a). The result from Noell et  al. (2010) was derived from monitoring the time-resolved formation of HO2 radicals by diode laser spectroscopy, whereas the previous

342

the mechanisms of reactions influencing atmospheric ozone Atkinson et al. (2006) Parkes (1974) Alcock and Mile (1975) Weaver et al. (1975) Selby and Waddington (1979) Kan et al. (1980) Niki et al. (1981a) Simon et al.(1990) Lightfoot et al. (1990a) Horie et al. (1990)

1.0

0.8

ka/k

0.6

0.4

0.2

0.0 1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1000/T, K−1 FIGURE V-H-4.

Temperature dependence of the branching ratio for the methoxy radical formation channel of CH3O2 self-reaction.

1.0

0.8

ka/k

0.6

0.4

Niki et al. (1982) Anastasi et al. (1983) Wallington et al. (1989a) Noell et al. (2010)

0.2

0.0

1.5

2.0

2.5

3.0

3.5

1000/T, FIGURE V-H-5.

4.0

4.5

5.0

K–1

Temperature dependence of the branching ratio for the ethoxy radical formation channel of C2H5O2 self-reaction.

studies derived values for α from the ratios of products formed in static chamber experiments. The origin of the discrepancy between the studies is unclear, and further work is needed. Temperature-dependent data are available for the self-reaction of iso-C3H7O2 radicals over the range 302–373 K from studies by Kirsch et  al. (1979) and Cowley et  al. (1982). The branching ratio is described by the expression α  =  1.42–255/T, which gives α  =  0.56 at 298

K. Given the paucity of data concerning the temperature dependence of the branching ratios for peroxy radical self- and cross-reactions and the discrepancies in the literature even for relatively simple systems such as the C2H5O2 self-reaction, it is evident that further work is needed to improve our understanding of these reactions. The products from the self- and cross-reactions of peroxy radicals have been interpreted in terms of

Mechanisms of Reactions of HO2 and RO2 Radicals two reaction channels giving alkoxy radicals, alcohols, and carbonyl compounds. Using the C2H5O2 self-reaction as an example: C2H5O2 + C2H5O2 → C2H5O + C2H5O + O2 C2H5O2 + C2H5O2 → C2H5OH + CH3CHO + O2 As described previously, the mechanism proposed by Russell (1957) to explain the observed products involves the formation of a tetroxide intermediate that decomposes either via a channel leading to alkoxy radicals or via a six-membered intermediate (see Figure V-H-1) to molecular products. Computational studies of the mechanism of the CH3O2 and the C2H5O2 self-reactions have been reported by Ghigo et  al. (2003), Liang et  al. (2011b), and Zhang et  al. (2012). Ghigo et  al. (2003) studied the CH3O2 self-reaction and reported that the formation of CH3O radicals is best described as a concerted process in which two O–O bonds break, rather than a step-wise process involving the CH3O3 intermediate. Ghigo et  al. (2003) did not find a concerted synchronous transition structure connecting the tetroxide with the molecular products formaldehyde, methanol, and molecular oxygen. Liang et al. (2011b) studied the CH3O2 self-reaction and found channels connecting the tetroxide to four sets of products: 2CH3O + O2, CH3OH + CH2O + O2, CH3OOOH + CH2O, and CH3O + CH2O + HO2. Liang et al. (2011b) noted that the pathway leading to CH3OOOH + CH2O has the lowest reaction barrier. Zhang et al. (2012) studied the C2H5O2 self-reaction and found that the reaction proceeds via the formation of the tetroxide C2H5OOOOC2H5. However, in contrast to the Russell mechanism, it was found that the tetroxide decomposes to give C2H5O + HO2 + CH3CHO as dominant products. The formation of C2H5OH, which is a well-established product from chamber systems, was attributed by Zhang et al. (2012) to a secondary reaction of C2H5O + HO2 radicals C2H5O + HO2 → C2H5OH + O2 This latter suggestion by Zhang et al. (2012) is incorrect because in all of the chamber work there is a large excess of O2, and, essentially, the sole fate of C2H5O radicals will be reaction with O2 to give CH3CHO and HO2. The mechanism proposed by Zhang et  al. (2012) is incompatible with the experimental observations of a substantial yield of C2H5OH. The theoretical studies reported to

343

date do not provide an entirely satisfactory quantitative explanation of the experimentally observed products. V- I . R O 2 U N I M O L E C U L A R R E AC T I O N S V-I-1. RO2 → QOOH Isomerization The importance of peroxy radical isomerization in low-temperature combustion and autoignition has been recognized since the 1960s (Cox and Cole, 1985; Glowacki and Pilling, 2010; Zádor et al., 2011). The classic isomerization, which plays a central role in low-temperature combustion and cool flames (T < 1,000 K), is known as the RO2 → QOOH reaction. In this isomerization, the peroxy radical bites its own tail with the end oxygen of the peroxy radical abstracting a hydrogen atom from the alkyl chain. The resulting QOOH radical can undergo thermal decomposition, giving either a reactive HO radical (and a cyclic ether) that propagates the oxidation chain or a less reactive HO2 radical (and an alkene) that slows down the oxidation and contributes to the “negative temperature coefficient” region of hydrocarbon oxidation (Zádor et  al., 2011). The QOOH radicals can also add O2 to give the OOQOOH hydroperoxide peroxy radical, which can in turn isomerize to give a species with two hydroperoxide groups, U(OOH)2. Decomposition of the U(OOH)2 gives two HO radicals and is an important chain-branching process in low-temperature combustion (Cox and Cole, 1985; Battin-Leclerc et al., 2010; Zádor et al., 2011). The formation of QOOH, OOQOOH, and U(OOH)2 species are illustrated for n-pentane oxidation here: CH3CH2CH2CH2CH3 + O2 → CH3CH2CH2CHCH3 + HO2 CH3CH2CH2CHCH3 + O2 → CH3CH2CH2CH(OO)CH3 CH3CH2CH2CH(OO)CH3 → CH3CHCH2CH(OOH)CH3 CH3CHCH2CH(OOH)CH3 + O2 → CH3CH(OO)CH2CH(OOH)CH3 CH3CH(OO)CH2CH(OOH)CH3 → CH3CH(OOH)CH2C(OOH)CH3 The temperatures relevant for atmospheric oxidation processes (220–300 K) are obviously quite different from those prevailing in cool flames (600–800 K) and low-temperature combustion

344

the mechanisms of reactions influencing atmospheric ozone

(< 1,000 K). The RO2 to QOOH isomerizations for peroxy radicals derived from alkanes typically have activation barriers in the range 80–170 kJ mol−1 but can, in certain cases (e.g., 1,7 shift from tertiary C–H bond) be as low as 60 kJ mol−1 (Zádor et al., 2011). Assuming an A factor of 1012 s−1 for the isomerization gives lifetimes typically ranging from minutes to millennia but can be less than a second at 298 K.  Given that the atmospheric lifetimes of RO2 radicals with respect to bimolecular reactions with NO, HO2, and R′O2 radicals is of the order of seconds to minutes, it can be concluded that there may be situations where the RO2 to QOOH isomerization is an important fate of RO2 radicals. The HO-initiated oxidation of acetylene provides an example in which there is little or no activation barrier to the RO2 → QOOH isomerization, and the reaction gives glyoxal and regenerates HO radicals (Hatakeyama et al., 1986b; Yeung et al., 2005): HO +HC≡CH + M → HOCH=CH + M

stronger R–OO bonds. Hence, the nascent RO2 radical contains a considerable amount of internal excitation that, in many cases, exceeds the barrier for isomerization to QOOH (as illustrated in Figure V-I-1). In most cases, the atmospheric fate of the chemically activated RO2* radical is collisional deactivation. However, at low total pressures, the collisional deactivation process slows down and isomerization can become important. An example of this behavior is the case of the peroxy radical formed from dimethylether whose behavior can be represented as: CH3OCH2 + O2 + M → CH3OCH2O2* + M CH3OCH2O2* + M → CH3OCH2O2 + M* CH3OCH2O2* → CH2OCH2OOH CH2OCH2OOH → CH2O + CH2O + HO CH3OCH2O2 + CH3OCH2O2 → CH3OCH2O + CH3OCH2O + O2

HOCH=CH + O2 → HOCH=CHOO

CH3OCH2O2 + CH3OCH2O2 → CH3OCH2OH + CH3OCHO + O2

HOCH=CHOO → CHO=CHOOH

CH3OCH2O + O2 → CH3OCHO + HO2

CHO=CHOOH → CHOCHO + HO

CH3OCH2O2 + HO2 → CH3OCH2OOH + O2

It has been shown recently by Glowacki et  al. (2012) that vibrational excitation of the initially formed HOCH=CH radical plays an important role in determining the products of its reaction with O2. An additional factor that is important in dictating whether QOOH isomerization is important is the chemical activation of the nascent RO2 radicals. The R–OO bond strength is 125–150 kJ mol−1 and depends on the nature of the alkyl group with more substituted alkyl groups forming somewhat

At low total pressures (< 5 Torr), the lifetime of the chemically activated CH3OCH2O2* radical is sufficient that isomerization to CH2OCH2OOH and decomposition to CH2O is the dominant reaction path. At high pressures (> 100 Torr), the CH3OCH2O2* radical is rapidly deactivated, isomerization is not significant, and CH3OCHO and CH3OCH2OOH are the dominant oxidation products (see Figure V-I-2). There is evidence for similar RO2 → QOOH isomerization chemistry in the oxidation of diethyl ether

R + O2 Products

RO2

QOOH

FIGURE V-I-1. Simplified potential energy surface for RO2 → QOOH isomerization. There is no barrier to the formation of the RO2 radical, which sits in a well that is typically 125–150 kJ mole−1 below that of the R + O2 reactants. The activation energy barrier for isomerization to QOOH is typically 80–170 kJ mol−1. The products of decomposition are typically either HO2 and an alkene or HO and a cyclic ether.

Mechanisms of Reactions of HO2 and RO2 Radicals

345

100

Product yield, percent

80

60

CH2O CH3OCH2OOH + CH3OCHO 40

20

0

1

10

100

1000

Pressure, torr FIGURE V-I-2.

Yields of CH2O and CH3OCH2OOH + CH3OCHO following the chlorine atom-initiated oxidation of CH3OCH3 in N2/O2 diluent versus total pressure (Sehested et al., 1996). The lines are fits of a sigmoid function to aid in visual inspection of the data trends.

(Orlando, 2007)  and di-isopropyl ether (Collins et al., 2005; Orlando, 2007). Another example of RO2 → QOOH isomerization occurring under ambient conditions is the formation of 1,4-dicarbonyls following the HO-initiated oxidation of >C5 alkanes in low-NOx environments (Atkinson et  al., 2008a). The 1,4-dicarbonyls are presumed to result from dehydration of 1,4-carbonyl-hydroperoxides. The sequence of reactions leading to 1,4-carbonyl-hydroperoxide formation in the oxidation of n-heptane is illustrated here: CH3CH2CH2CH2CH2CH2CH3 + HO → CH3CHCH2CH2CH2CH2CH3 + H2O CH3CHCH2CH2CH2CH2CH3 + O2 → CH3CH(OO)CH2CH2CH2CH2CH3 CH3CH(OO)CH2CH2CH2CH2CH3 + RO2 → CH3CH(O)CH2CH2CH2CH2CH3 + RO + O2 CH3CH(O)CH2CH2CH2CH2CH3 → CH3CH(OH)CH2CH2CHCH2CH3 CH3CH(OH)CH2CH2CHCH2CH3 + O2 → CH3CH(OH)CH2CH2CH(OO)CH2CH3

CH3CH(OH)CH2CH2CH(OO)CH2CH3 → CH3C(OH)CH2CH2CH(OOH)CH2CH3 CH3C(OH)CH2CH2CH(OOH)CH2CH3 + O2 → CH3C(O)CH2CH2CH(OOH)CH2CH3 + H2O On the basis of computational work, Peeters et  al. (2009) proposed that the RO2 → QOOH isomerization via 1,5-H- and 1,6-H shifts was important for the hydroxyl-peroxy radicals formed during the oxidation of isoprene (see Section V-B-1 for structures of these radicals). Peeters et al. estimated that the 1,5-H shift could occur on a timescale of a few minutes and that the resulting QOOH species would decompose to form HO radicals: HOCH2C(CH3)(OO)CH=CH2 → OCH2C(CH3)(OOH)CH=CH2 OCH2C(CH3)(OOH)CH=CH2 → CH2O + HO + CH3C(O)CH=CH2 It was estimated that the 1,6-H shift could occur on a timescale of approximately 1 second, and the

346

the mechanisms of reactions influencing atmospheric ozone

resulting hydroperoxide species would undergo photolysis to generate HO radicals. The mechanism of the 1,6-H shift is illustrated here: OH

The realization that HO radicals can be regenerated following isomerizations of hydroxyl-peroxy

O

OH

+ O2 – HO2

OO

OOH

Given the importance of isoprene and HO radicals in global atmospheric chemistry, the findings by Peeters et  al. (2009) that the isomerization of hydroxyl-peroxy radicals formed during the oxidation of isoprene could reform HO radicals generated a considerable amount of interest and has led to a substantial research effort to unravel the complexities of this chemistry. Crounse et al. (2011) conducted an experimental study and showed that the 1,6-H shifts were somewhat slower than estimated by Peeters et al. (2009) and occur at a rate of approximately 0.002 s−1. Crounse et al. (2011) estimated that the fraction of isoprene peroxy radicals reacting by 1,6-H-shift isomerization is 8–11% globally, with values up to 20% in tropical regions. Computational studies by da Silva and co-workers (Asatryan et  al., 2010; da Silva, 2010a, 2010b) have identified isomerizations of oxygenated peroxy radicals derived from biogenic volatile organic compounds (VOCs) that may be of atmospheric relevance. The barriers to isomerization in peroxy radicals derived from organic acids and α-carbonyls were estimated to be 77 kJ mol−1 and 67 kJ mole−1, respectively (da Silva, 2010a, 2010b). In an experimental study, Crounse et  al. (2012) inferred a rate constant of 0.5 s−1 for the 1,4-H-shift isomerization of the peroxy radical derived from addition of HO to methacrolein. Using simulations with a global chemical transport model, Crounse et al. (2012) suggest that most of the methacrolein hydroxy peroxy radicals formed in the atmosphere undergo isomerization and decomposition to generate HO radicals: HO + CH2=C(CH3)CHO → HOCH2C(CH3)CHO HOCH2C(CH3)CHO + O2 → HOCH2C(OO)(CH3)CHO HOCH2C(OO)(CH3)CHO → HOCH2C(OOH)(CH3)C(O) HOCH2C(OOH)(CH3)C(O) → HOCH2C(O)CH3 + HO + CO

OOH

radicals derived from biogenic VOC emissions is a recent development in our understanding of peroxy radical atmospheric chemistry. Further work is needed in this area to quantify the importance of this process for the variety of important biogenic VOCs, such as the terpenes and sesquiterpenes. V-I-2. Decomposition Reactions of α-Hydroxy-Peroxy Radicals It is well established that α-hydroxy radicals such as CH2OH and CH3CHOH react rapidly with O2 and lead to the formation of an aldehyde and HO2 radical under ambient conditions (Atkinson et al., 2006). Indeed, the reaction of CH2OH radicals with O2 is often used in smog chamber studies to generate HO2 radicals: CH2OH + O2 → CH2O + HO2 Until relatively recently, it was believed that the reaction was concerted and did not proceed via the formation of a peroxy radical. However, it is now clear that an α-hydroxy-peroxy is formed, but it is short lived and decomposes to an aldehyde and HO2 radical (Orlando et al., 2000b; Peeters et al., 2001; Aschmann et al., 2010). In smog chamber experiments employing sufficiently high concentrations of NO, the α-hydroxy-peroxy can be made to react to give an alkoxy radical that is then converted into an organic acid: CH3CHOH + O2 → CH3CH(OO)OH CH3CH(OO)OH → CH3CHO + HO2 CH3CH(OO)OH + NO → CH3CH(O)OH + NO2 CH3CH(O)OH + O2 → CH3C(O)OH + HO2 At the low temperatures found in the upper troposphere, the lifetime of α-hydroxy-peroxy radicals may be sufficient for these species to undergo bimolecular reactions.

VI Mechanisms of Reactions of the RO Radicals

VI-A. INTRODUCTION The atmospheric chemistry of alkoxy radicals determines the first-generation oxidation products of organic compounds in the atmosphere. There are three competing fates for alkoxy radicals:  reaction with molecular oxygen (O2), isomerization, and decomposition (Atkinson and Arey, 2003b; Devolder, 2003; Orlando et al., 2003b; Calvert et al., 2008). Reaction with O2 preserves the carbon chain of the parent alkane and results in the production of a carbonyl compound and HO2. Unimolecular decomposition usually results in the formation of an alkyl radical and a carbonyl compound with a shortening of the carbon chain. Unimolecular isomerization usually leads to multifunctional oxidation products (e.g., 1,4-hydroxycarbonyls and 1,4-hydroxynitrates) and a preservation of the carbon chain. These potentially competing pathways are illustrated in Figure VI-A-1 for the 2-pentoxy radical: Absolute rate coefficients for these processes have been obtained for only a few of the smaller alkoxy radicals. For example, rate coefficients have been firmly established only over a range of temperatures for reaction of a subset of the C1–C6 alkoxy radicals with O2; dissociation rate coefficients have only been directly measured for ethoxy, 2-propoxy, 2-butoxy, and tert-butoxy radicals (Balla et al., 1985; Blitz et al., 1999; Caralp et al., 1999; Devolder et al., 1999; Fittschen et  al., 1999, 2000; Falgayrac et  al.,

2004); and no direct measurement of isomerization rates have been reported to date. A  large portion of the database describing the atmospheric behavior of alkoxy radicals has been built up primarily from two sources:  (1)  environmental chamber experiments, where end-product distributions observed under atmospheric conditions have been used to infer relative rates of competing alkoxy radical reactions (e.g., Carter et al., 1976; Cox et al., 1981; Niki et al., 1981a; Eberhard et al., 1995; Aschmann et al., 1997; Orlando et al., 2000a; Cassanelli et al., 2006); and (2) from theoretical methodologies that lend themselves well to the study of unimolecular processes (e.g., Somnitz and Zellner, 2000a, 2000b, 2000c; Méreau et  al., 2000a, 2000b; Fittschen et  al., 2000; Lin and Ho, 2002; Méreau et al., 2003; Davis and Francisco, 2011). An overview of these three classes of competing alkoxy radical reactions (reaction with O2, unimolecular decomposition, and isomerization) is given in this section. This is followed by a summary of the available information regarding the reaction rate coefficients for selected individual alkoxy radicals derived from hydrocarbons (Sections VI-C-1–VI-C-26) and oxygenated compounds (Sections VI-C-27–VI-C-30). We note also that alkoxy radicals can react with both NO and NO2: RO + NOx → RONOx → R′=O + HNOx

(1a) (1b)

348

the mechanisms of reactions influencing atmospheric ozone O H2 C H3C

Unimolecular dissociation

CH C H2

CH3 Isomerization

Unimolecular dissociation H2 C H3C

H C C H2

O

H3C

H3C

OH

CH3

O C H + H2 C

+ CH3

FIGURE VI-A-1.

+ O2

H2 C H3C

CH

C C H2 + HO2

CH2

O

H3 C

C H2

CH2

CH2

Reaction pathways for the 2-pentoxy radical.

These reactions proceed with rate coefficients of about (2.0–4.5) × 10−11 cm3 molecule−1 s−1 at 298 K and give the alkyl nitrite or nitrate (channel 1a) as the major product. Although these reactions will not be dealt with here, they can play a role in very polluted atmospheres (i.e., with NOx of ≥ 5 ppm or so) and in environmental chamber experiments, and thus cannot be totally ignored. VI-B. MODES OF ALKOXY R A D I CA L R E AC T I O N I N T H E AT M O S P H E R E VI-B-1. Alkoxy Radical Reactions with O2 Reaction with O2 is an important atmospheric fate for many alkoxy radicals: RR′CHO + O2→ RR′C=O + HO2 Formally, the reaction occurs via O2 abstraction of an α-hydrogen, resulting in the production of a carbonyl compound and an HO2 radical (Zellner, 1987; Hartmann et  al., 1990). Thus, the reaction is common to all primary and secondary alkoxy radicals but cannot occur for tertiary radicals such as 2-methyl-2-propoxy radical [tert-butoxy, (CH3)3CO•] and 2-methyl-2-butoxy radical [CH3CH2C(CH3)2O•]. The reaction, however, does not appear to involve a simple abstraction process. Recent theoretical studies on the methoxy radical reaction with O2 suggest that the reaction occurs instead via the formation of an alkoxy radical−O2 complex (RR′CHO⋅⋅⋅O2) that is held together by a noncovalent bond and that subsequently

rearranges to products via H-atom transfer (Bofill et al., 1999; Setokuchi and Sato, 2002). The theoretical study by Setokuchi and Sato (2002) suggests that tunneling plays a major role in the H-atom transfer process, at least in the case of methoxy. However, in the experimental study by Hu et al. (2012) of the relative importance of abstraction of H and D from CH2DO radicals, there was no evidence of non-Arrhenius behavior over the range 250–333 K, which runs counter to expectations if tunneling were of major importance. The precise role of tunneling in RO + O2 reactions remains unclear. Absolute rate coefficients for alkoxy radical reactions with O2 have been well-established over a range of temperatures for only the simplest of the alkoxy radicals (e.g., methoxy, ethoxy, 1- and 2-propoxy radicals). Direct studies of a few C4–C7 radicals exist, although insufficient data are available to make firm recommendations in most cases. Nonetheless, some general observations can be made and some trends established. As a result of the complexity of the mechanism involved, A-factors and reaction rate coefficients are quite small. With the exception of methoxy + O2 (k298K = 2 ×10−15 cm3 molecule−1 s−1), rate coefficients near 298 K for those radicals studied fall within a factor of 2 of 10−14 cm3 molecule−1 s−1. A-factors fall within an order of magnitude of 10−14 cm3 molecule−1 s−1 in all cases studied, and, as illustrated in Figure VI-B-1, activation energies tend to decrease with increasing carbon number (from about 10 kJ mole−1 for the methoxy radical reaction to −4 kJ mole−1 for the corresponding 2-butoxy radical reaction). A notable exception is the cyclohexoxy

Mechanisms of Reactions of the RO Radicals

349

20

Methoxy Radical Acyclic Primary Radicals Acyclic Secondary Radicals Cyclic Secondary Radicals

Activation energy, kJ mole−1

15

10

5

0

−5

−10

0

1

2

3 4 5 Number of carbon atoms

6

7

8

Plot of activation energy (kJ mole−1) versus number of carbon atoms for reaction of alkoxy radicals with O2. Data for methoxy, ethoxy, 1-propoxy, and 2-propoxy are from Calvert et al. (2008); data for larger alkoxy radicals are individual determinations. FIGURE VI-B-1.

radical, k  =  4.7  ×10−12 exp(−1,720/T) cm3 molecule−1 s−1 (Zhang et al., 2004b), for which an anomalously high A-factor and activation energy have been reported (see Figure VI-B-1). Despite the relatively small rate coefficients for the alkoxy radical reactions with O2, the large atmospheric abundance of O2 results in a very short lifetime for the alkoxy radicals throughout the troposphere. For example, assuming for illustrative purposes kRO+O2  =  1.5 × 10−14 cm3 molecule−1 s−1 independent of temperature (Atkinson, 2007), RO lifetimes of 10 microseconds (μs) at the Earth’s surface and 40 μs at 10 km are obtained. Obviously, only those unimolecular processes that occur on these timescales will be able to compete with the O2 reaction and be of any atmospheric relevance. Again, the one exception to this general discussion is the tertiary alkoxy radicals for which no O2 reaction can occur, thus implying longer atmospheric lifetimes and the importance of slower unimolecular processes. VI-B-2. Unimolecular Decomposition Reactions of the Alkoxy Radicals VI-B-2.1. Unimolecular Decomposition of RR′R″CO• Radicals In general, alkoxy radicals can undergo unimolecular decomposition via three separate bond rupture processes, as illustrated here for the generic alkoxy radical RR′R″CO•:

RR′R″CO• → R• + R′R″C=O → R′• + RR″C=O → R″• + R′RC=O where R, R′, and R″ represent alkyl groups or H atoms for alkoxy radicals derived from the alkanes. Carbon-carbon bond scissions are more favorable than carbon-hydrogen scissions, and so a shortening of the carbon chain almost always results. As a general rule of thumb, a more reactive set of products is generated from the bond dissociation processes than is generated from the O2 reaction; that is, more aldehydes from bond dissociation versus more ketones from the O2 reactions. Table VI-B-1 summarizes some of the more atmospherically relevant alkoxy radical dissociation processes (Calvert et al., 2008). A more comprehensive summary is provided in table 1 of Orlando et al. (2003b). Rate coefficients for alkoxy radical decomposition processes have been measured directly (via laser-induced fluorescence [LIF] detection of the alkoxy radical) in only a few cases, namely for ethoxy (Caralp et al., 1999), 2-propoxy (Balla et  al., 1985; Devolder et  al., 1999), tert-butoxy (Blitz et  al., 1999; Fittschen et al., 2000), and 2-butoxy (Falgayrac et al., 2004). Theoretical studies provided a large body of internally consistent rate data for most alkoxy radicals containing 2–5 carbon atoms (Méreau et al., 2000b;

350

the mechanisms of reactions influencing atmospheric ozone TABLE VI-B-1 . MA JOR ALKOXY R ADICAL UNIMOLECUL AR DECOMPOSITION RE ACTIONS ENCOUNTERED IN THE ATMOSPHERIC CHEMISTRY OF THE ALKANES (CALVERT ET AL., 2008)

Radical

Decomposition Products

Approximate Rate Ea, kJ Competing (298 K, 1 atm) mole−1 Reactions

2-Butoxy tert-Butoxy Isobutoxy 3-Pentoxy Neopentoxy 2-Methyl-2-butoxy

CH3CH2• + CH3CHO CH3C(=O)CH3 + •CH3 CH3CH(•)CH3 + CH2O CH3CH2• + CH3CH2CHO (CH3)3C• + CH2O CH3CH2• + CH3C(=O)CH3

2.5 × 104 s−1 3,000 s−1 5.7 × 104 s−1 3 × 104 s−1 2 × 106 s−1 9.4 × 105 s−1

2-Methyl-1-butoxya 3-Methyl-2-butoxya Cyclopentoxy Cyclohexoxy

CH3CH2CH(•)CH3 + CH2O CH3CH(•)CH3 + CH3CHO •CH2CH2CH2CH2CHO •CH2CH2CH2CH2CH2CHO

≈ 105 s−1 ≈ 107 s−1 > 107 s−1 5 × 104 s−1

53 60 52 54 40 44 ≈ 47 ≈ 38 < 38 48

Reaction with O2 None Reaction with O2 Reaction with O2 Reaction with O2 b None Isomerization (major) Reaction with O2 Reaction with O2 b Reaction with O2 b Reaction with O2

a

Rate coefficient and Ea estimated using parameterizations given in Orlando et al. (2003b). Minor process at 298 K.

b

Somnitz and Zellner, 2000a, 2000b; Fittschen et al., 2000; Vereecken and Peeters, 2009). Relative rate data have been obtained for a number of systems from end-product studies, usually in static environmental chambers, but also in slow-flow systems. A classic example is the 2-butoxy radical, for which decomposition and reaction with O2 are competitive in 1 atm air at 298 K (Carter et al., 1979b; Cox et al., 1981; Drew et al., 1985; Libuda et al., 2002; Meunier et al., 2003; Cassanelli et al., 2005): CH3CH2CH(O•)CH3 → CH3CH2• + CH3CH=O CH3CH2CH(O•)CH3 + O2 → CH3CH2C(=O) CH3 + HO2 Here, ratios of acetaldehyde and 2-butanone yields provide a measure of the rate coefficient ratio for the competing processes. Given the mechanistic similarities of all the alkoxy radical decomposition reactions, it is not surprising to find that they are all characterized by similar A-factors. The body of recent theoretical and experimental data points to A-factors at the high-pressure limit that fall between about 1013 and 1014 s−1. Because the decomposition reactions are typically in the fall-off region at atmospheric pressure, effective A-factors at atmospheric pressures are somewhat lower, with estimates in the 1012 or 1013 s−1 range (e.g., Somnitz and Zellner, 2000b; Falgayrac et al., 2004; Vereecken and Peeters, 2009).

Alkoxy radical decomposition reactions are almost always endothermic and possess energy barriers that exceed the endothermicity. Note that in the remainder of this chapter, we will distinguish as best as possible between the energy barrier (activation energy at 0 K, designated Eo or Eb) and the Arrhenius activation energy (Ea) for a given process. For typical atmospheric temperatures, the activation energy at infinite pressure exceeds the energy barrier by 3–4 kJ mole−1, whereas effective activation energies near 1 atm are often intermediate between these two extremes. As first discussed by Baldwin et al. (1977), the endothermicity for an alkoxy radical decomposition and the activation energy for the reaction are correlated, Ea ≈ A + B × (ΔHr), so that a reasonable estimate of the rate coefficient for a given decomposition reaction can be obtained from a knowledge of the endothermicity. Choo and Benson (1981) later showed that the intercept of such plots was dependent on the nature of the alkyl radical leaving group and could, in fact, be correlated with the ionization potential (IP) of the leaving group. Thus, structure-reactivity estimates of the following form are now found in the literature (Atkinson, 1997b; Méreau et al., 2000b; Orlando et al., 2003b): Ea = (A × IP − B) + C × (ΔHr)

(1)

Orlando et  al. (2003b) suggested Ea (kJ mole−1)  = (10.0  ×IP(ev) −49.4) + 0.58  × ΔHr (kJ mole−1)

Mechanisms of Reactions of the RO Radicals on the basis of available data involving CH3 radical elimination. Other structure−reactivity approaches have appeared in the recent literature. For example, Johnson et al. (2004b) showed that both the 298 K rate coefficient, k298K, and the activation energy for alkoxy radical decomposition reactions correlated well with the mean of the ionization potential (IPm, in eV) of the two reaction products. Their analysis for unsubstituted (i.e., alkane-derived) alkoxy radicals leads to the following expressions: ln (k298K , cm3 molecule−1 s−1) = −2.57(IPm)2 + 43.76(IPm) −176.00 Ea (kJ mole−1) = 14.62(IPm) −82.58 where an A-factor per available reaction pathway of 1013 s−1 has been assumed at atmospheric pressure for all reactions. Méreau et al. (2000b) demonstrated that a reasonable estimate of the activation energy for alkoxy radical decomposition reactions could be obtained using two parameters—the reaction enthalpy and the number of hydrogen atoms (nH) on the carbon bonded to the alkoxy oxygen: Ea (kJ mole−1) = (45.81 − 22.34 × nH) + 1.2 × ΔHr(kJ mole−1)

The SAR is based on a barrier to decomposition of ethoxy radicals (Eb  =  74.9 kJ mol−1) with the high-pressure rate coefficient for decomposition estimated using the expression: kdiss∞ = L × 1.8 × 1013 exp (−Eb/RT) s−1 where L is the reaction path degeneracy. For the ethoxy radical kdiss∞ = 1 × 1.8 × 1013 exp (−74,900/ RT), which gives kdiss∞ = 1 s−1 at 298 K. The SAR expresses the barrier to decomposition as a linear function of the effect of substituents, Eb  =  74.9 kJ mole−1 + Σ Ns × Fs, where Ns is the number of substituents, and Fs is the effect of the substituent. The substituents have different effects depending on whether they are in α- or β-position with respect to the alkoxy group in the ethoxy radical framework. Values of Fs are listed in Table VI-B-2. As an example of an estimation using this SAR, we consider the radicals CH2(OH)CH(O•)OCH3 and CH2(NO2)CH(O•)OCH3, which have Eb values of 74.9 – 38.5 – 31.4 = 5 and 74.9 – 38.5 + 1.7 = 38.1 kJ mole−1 and would decompose via C–C bond scission at rates of approximately 2 × 1012 and 4 × 106 s−1, respectively.

(2) TABLE VI-B-2 . ALKOXY DECOMPOSITION STRUCTUREACTIVIT Y REL ATIONSHIP (SAR) FACTOR S, F S, FOR SUBSTITUENTS ON THE Α- OR Β-CARBON IN CH 3 CH 2 O (VEREECKEN AND PEETER S, 2009)

They further combined equation (1) with their own version of (2) to obtain an equation based only on the readily available leaving-group IP and nH parameters: Ea(kJ mole−1) = 10.46 × IP(ev) + 8.79 × nH − 43.51

351

(3)

Somnitz and Zellner (2000c) pointed out that decompositions of linear alkoxy radicals could be divided into three categories:  those for which both products (the alkyl radical and the carbonyl species) contain one carbon atom (ethoxy radical, Eb = 72.4 kJ mole−1), those for which one product contains two or more carbon atoms and the other one carbon (e.g., 1-propoxy, 1-butoxy, 2-propoxy, Eb ~ 62 kJ mole−1), and those for which each product contains two or more carbon atoms (e.g., 2-butoxy, 3-pentoxy, Eb = 50.2 kJ mole−1). Peeters et al. (2004) and Vereecken and Peeters (2009) used quantum mechanical calculations to develop a structure-activity relationship (SAR) to estimate the rates of decomposition (via C–C bond scission) of a large number of alkoxy radicals.

Substituent Fs, kJ Substituent Fs, kJ mole−1 mole−1 α-alkyl α=O α-OH α-OR α-OOH α-OOR α-NO α-NO2 α-ONO α-ONO2 α=C α-C=C a

−9.6 −53.1 −37.2 −38.5 −37.2 −26.8 a

−9.2 −17.6 −15.9 90.0 −20.5

β-alkyl β =O β-OH β-OR β-OOH β OOR β NO β-NO2 β-ONO β-ONO2 β=C β-C=C

−14.2 −35.6 −31.4 −38.1 −38.9 −30.1 −66.9 1.7 −25.1 −11.7 20.9 −40.2

Alkoxy radicals of structure >C(O)NO spontaneously decompose to give >C=O + NO.

352

the mechanisms of reactions influencing atmospheric ozone

Vereecken and Peeters (2009) suggest that the barrier heights estimated using their SAR method are accurate to within approximately 2 kJ mol−1 and that the rates of decomposition estimated at 298 K have an uncertainty factor of approximately 5–10. The SAR developed by Vereecken and Peeters (2009) provides the most convenient method available to date to estimate the rates of decomposition via C–C bond scission for a large variety of alkoxy radicals. The accuracy of this method is typically sufficient to assess whether decomposition is a dominant or negligible fate compared to other atmospheric processes for alkoxy radicals. VI-B-2.2. Unimolecular Decomposition of Acyloxy Radicals [RC(O)O] The acyloxy radicals are somewhat of a special case because they can decompose via elimination of CO2. This is a highly thermodynamically favorable process. Theoretical and experimental studies have shown that decomposition of CH3C(O)O radicals proceeds at a rate of the order of 107–109 s−1 at atmospherically relevant temperatures of 200–300 K (Herk et al., 1961; Braun et al., 1962; Zhou et al., 2008). Using the SAR developed by Vereecken and Peeters (2009), it can be estimated that decomposition of RCH2C(O)O radicals proceeds with a rate of the order of 1011–1012 s−1 at 200–300 K. Elimination of CO2 is the sole atmospheric fate of RC(O)O radicals: RC(O)O → R + CO2 VI-B-2.3. Unimolecular Isomerization Reactions of the Alkoxy Radicals Another important reaction that alkoxy radicals undergo is unimolecular isomerization via a 1,5-hydrogen shift, as shown here for the 1-butoxy radical (Carter et  al., 1976; Baldwin et  al., 1977; Atkinson, 1997b): CH3CH2CH2CH2O• → •CH2CH2CH2CH2OH

(1)

For alkoxy radicals derived from alkanes, these reactions are roughly thermoneutral or slightly exothermic (depending on the nature of the hydrogen being abstracted). They have reduced A-factors relative to the simple decomposition reactions just discussed, and they typically occur with barriers of less

than 40 kJ mole−1 (Vereecken and Peeters, 2010). Although other isomerization processes (e.g., 1,4or 1,6-hydrogen shifts) are also possible, the 1,5-H shifts via a six-membered ring transition state are the most favored (Carter et al., 1976; Baldwin et al., 1977; Vereecken and Peeters, 2010; Davis and Francisco, 2011)  and are typically the only isomerizations sufficiently rapid to be important under atmospheric conditions. Isomerization reactions need to be considered for alkanes possessing carbon chains of four atoms or more. No direct experimental measurement of an isomerization reaction rate coefficient has been made to date (2013). However, rate data have been derived from either theoretical studies ( Jungkamp et al., 1997; Lendvay and Viskolcz, 1998; Somnitz and Zellner, 2000a, 2000b; Hack et  al., 2001; Lin and Ho, 2002; Ferenac et al., 2003; Vereecken and Peeters, 2003; Méreau et  al., 2003; Vereecken and Peeters, 2010)  or, in a relative sense, either from end-product data analysis (Carter et al., 1976; Cox et al., 1981; Niki et al., 1981a; Eberhard et al., 1995; Atkinson et al., 1995; Heiss and Sahetchian, 1996; Geiger et al., 2002; Johnson et al., 2004a; Cassanelli et  al., 2005, 2006), or most recently from detection of the relative yield of the HOROOH radical as a function of [O2] (Sprague et al., 2012). A summary of available data (updated from Calvert et al., 2008) is presented in Table VI-B-3. The earliest experimental data on the rates of the isomerization processes involved studies of the competition between isomerization of the 2-pentoxy (Carter et  al., 1976)  and 1-butoxy radical (Carter et al., 1979b; Cox et al., 1981; Niki et al., 1981c) and their reaction with O2; for example: CH3CH2CH2CH2O• → •CH2CH2CH2CH2OH CH3CH2CH2CH2O• + O2 → CH3CH2CH2CH=O + HO2 Carter et  al. (1976) noted a lower yield of 2-pentanone than 3-pentanone in the OH-initiated oxidation of n-pentane, consistent with a significant occurrence of isomerization for the 2-pentoxy radical. Note that 3-pentoxy cannot undergo isomerization via a 1,5-H shift. In the case of 1-butoxy, from observed yields of butanal versus O2 partial pressure, kisom/kO2 values of about (1.5–2) ×1019 molecule cm−3 were established (Carter et al., 1979b; Cox et al., 1981; Niki et al., 1981c). Similar studies were

TABLE VI-B-3 . SUMMARY OF AVAIL ABLE DATA FOR THE ISOMERIZ ATION REACTIONS OF ALKANE-DERIVED ALKOXY R ADICALS. FOR THEORETICAL STUDIES OF ACTIVATION ENERGIES−BARRIER HEIGHTS, DATA HAVE BEEN REPORTED AS EITHER (A) BARRIER HEIGHTS (ACTIVATION ENERGIES AT 0 K); (B) ACTIVATION ENERGIES (kJ mole −1 ) AT 1 ATM; OR (C) ACTIVATION ENERGIES AT INFINITE PRESSURE. LEVEL OF THEORY USED IS GIVEN IN PARENTHESES (SAR = STRUCTURE-ACTIVIT Y REL ATIONSHIP)

Radical

Isomerization Product

Activation Approximate Energy or Barrier Rate at 298 K, Height(kJ 1 atm (s−1) mole−1)

Reference

1-Butoxy

•CH2CH2CH2CH2OH

1.6 × 105 1.5 × 105 1.9 × 105 1 × 105 2 × 105 1.3 × 105 1.2 × 105 1.1 × 105 1.7 × 105 1.4 × 105 1.3 × 105 2.0 × 105 2.0 × 105 3.2 × 105 2.4 × 105

40.6 35.1 (SAR) 39.3 (b3lyp) A 38.5 (bac4) A 41.4 (g2) B 36.8 (dft) B 41.4 (b3lyp) A 41.0 (b3lyp) A 40.2 (g2/mp2) A 34.3 (b3lyp) B 27.2 a 29.3 a 35.8 (CBS-QB3) B

Carter et al. (1979b) Cox et al. (1981) Niki et al. (1981c) Heiss and Sahetchian (1996) Atkinson (1997b) Jungkamp et al. (1997) Lendvay and Viskolcz (1998) Somnitz and Zellner (2000b) Méreau et al. (2003) Ferenac et al. (2003) Lin and Ho (2002) Hack et al. (2001) Vereecken and Peeters (2003) Geiger et al. (2002) Cassanelli et al. (2005) Cassanelli et al. (2006) Vereecken and Peeters (2010) Sprague et al. (2012)

2-Pentoxy

•CH2CH2CH2CH(OH)CH3

2.5 × 105 2 × 105 3.3 × 105 5.0 × 105 4.1 × 105 ≈3 × 105

35.1 (SAR) 38.9 36.8 (g2) 36.1 (b3lyp) 30.5 b

Atkinson et al. (1995) Atkinson (1997b) Méreau et al. (2003) Somnitz and Zellner (2000b) Lin and Ho (2002) Johnson et al. (2004a) Sprague et al. (2012)

2 × 105 2.3 × 105

35.1 (SAR) 40.6

2-Methyl1-butoxy

•CH2CH2CH(CH3)CH2OH

3-Hexoxy

CH3CH2CH(OH)CH2CH2CH2•

2 × 105 35.1 (SAR) 1.7 × 105 40.2 (1.8–4.3) × 105

C B A

C C

Atkinson (1997b) Méreau et al. (2003) Atkinson (1997b) Méreau et al. (2003) Eberhard et al. (1995)

2-Methyl•CH2CH2CH2C(OH)(CH3)2 2-pentoxy

2 × 105 5.3 × 105

35.1 (SAR) 37.7 33.6 (b3lyp)

C A

Atkinson (1997b) Méreau et al. (2003) Lin and Ho (2002)

2-Methyl•CH2CH2CH2C(OH)(CH3)2 2-pentoxy

2 × 105 5.3 × 105

35.1 (SAR) 37.7 33.6 (b3lyp)

C A

Atkinson (1997b) Méreau et al. (2003) Lin and Ho (2002)

CH3CH(•)CH2CH2CH2OH

2 × 106 2.0 × 106 2.2 × 106 3.4 × 106 3.3 × 106

28.5 (SAR) 32.2 (g2) B 33.1 C 24.3 a 25.9 (CBS-QB3) B

1-Pentoxy

Atkinson (1997b) Somnitz and Zellner (2000b) Méreau et al. (2003) Johnson (2004a) Vereecken and Peeters (2010)

(continued)

353

354

the mechanisms of reactions influencing atmospheric ozone TABLE VI-B-3 . (CONTINUED)

Radical

Isomerization Product

1-Hexoxy

CH3CH2CH(•)CH2CH2CH2OH

2-Hexoxy

CH3CH(•)CH2CH2CH(OH) CH3

Activation Approximate Energy or Barrier Rate at 298 K, Height(kJ 1 atm (s−1) mole−1) 2 × 106 2.9 × 106 3.3 × 106

28.5 (SAR) 32.2 C 25.9 (CBS-QB3) B

2 × 106 28.5 (SAR) (1.4–4.7) × 106 31.0 5 × 106 25.0 (b3lyp)

C A

Reference Atkinson (1997b) Méreau et al. (2003) Vereecken and Peeters (2010) Atkinson (1997b) Eberhard et al. (1995) Méreau et al. (2003) Lin and Ho (2002)

CH3CH(•)CH2CH2C(OH) (CH3)2

2 × 106 1.1 × 107

28.5 (SAR) 29.3

C

Atkinson (1997b) Méreau et al. (2003)

4-Methyl(CH3)2C•CH2CH2CH2OH 1-pentoxy

5 × 106 1.1 × 108

23.0 (SAR) 21.0

C

Atkinson (1997b) Méreau et al. (2003)

5 × 106 9.5 × 106 5 × 108

23.0 (SAR) 19.7 b 15.5 15.8 (b3lyp)

C A

Atkinson (1997b) Johnson (2004a) Méreau et al. (2003) Lin and Ho (2002)

5 × 106 4.4 × 108

23.0 (SAR) 16.3

C

Atkinson (1997b) Méreau et al. (2003)

2-Methyl2-hexoxy

5-Methyl2-hexoxy

(CH3)2C•CH2CH2CH(OH)CH3

2,5-Dimethyl- (CH3)2C•CH2CH2C(OH)(CH3)2 2-Hexoxy

−14

a

3

−1 −1

Determined from measurements of kisom / kO2, using kO2 = 6 × 10 exp(−550/T) cm molecule  s . Determined from measurements of kisom / kO2, using kO2 = 1.5 × 10−14 exp(−200/T) cm3 molecule−1 s−1.

b

later conducted on 2-pentoxy and 2- and 3-hexoxy (Dóbé et al., 1986; Atkinson et al., 1995; Eberhard et  al., 1995). From a combination of these room temperature data and estimated A-factors (Baldwin et  al., 1977), the following set of parameters was derived to describe the rate coefficients for the isomerization reactions at atmospheric pressure (Atkinson, 1997b; Atkinson, 2007): k(primary) = 3.2 ×105 s−1 k(tert.) = 1.1 ×107 s−1

k(sec.) = 3.3 ×106 s−1

A(primary) = 1.2 ×1011 s−1 A(sec.) = 8.0 ×1010 s−1 A(tert.) = 4.0 ×1010 s−1 Ea/R(primary) = 3,825 K Ea/R(sec.) = 3,010 K Ea/R(tert.) = 2,440 K Here, the k, A, and Ea/R values are, respectively, the 298 K rate coefficients, A-factors, and activation temperatures for isomerization involving the 1,5-transfer of a hydrogen from a –CH3 (primary), – CH2– (sec.), or >CH– (tert.) group. It has been firmly established from experimental studies (Atkinson et al., 1995; Kwok et al., 1996a;

Arey et  al., 2001; Reisen et  al., 2005; Aschmann et  al., 2012)  that alkoxy radical isomerization results in the formation of 1,4-hydroxycarbonyl and 1,4-hydroxynitrate compounds via the reaction sequence outlined here for 1-butoxy (stable end-products are bold font for clarity): CH3CH2CH2CH2O• → •CH2CH2CH2CH2OH •CH2CH2CH2CH2OH + O2 → •O2CH2CH2CH2CH2OH •O2CH2CH2CH2CH2OH + NO → •OCH2CH2CH2CH2OH + NO2 O2CH2CH2CH2CH2OH + NO→ O2NOCH2CH2CH2CH2OH •OCH2CH2CH2CH2OH → HOCH2CH2CH2CH(•)OH HOCH2CH2CH2CH(•)OH+ O2 → HOCH2CH2CH2CH=O + HO2 The multifunctional nature of the end-products makes their detection and quantification difficult, a fact that has complicated the characterization

Mechanisms of Reactions of the RO Radicals of the isomerization reactions. In a series of experiments, Atkinson and co-workers (Atkinson et  al., 1995; Kwok et  al., 1996a; Arey et  al., 2001; Reisen et  al., 2005; Aschmann et  al., 2012)  have employed an array of gas chromatographic and mass spectrometric techniques to first identify and later quantify (and in some cases speciate) the 1,4-hydroxycarbonyl and 1,4-hydroxynitrate compounds formed in the HO-initiated oxidation of a number of C4–C8 straight- and branched-chain alkanes. These reactions are extremely important. In fact 1,4-hydroxycarbonyl formation accounts for more than 50% of the total first-generation products for the HO-initiated oxidation of C5–C8 n-alkanes in the presence of NOx (Kwok et  al., 1996a; Arey et  al., 2001; Reisen et  al., 2005; Aschmann et  al., 2012). More discussion of these reactions is given in Section VI-C. A number of theoretical and experimental studies of the isomerization reactions have appeared since 1996 (Heiss and Sahetchian, 1996; Jungkamp et  al., 1997; Lendvay and Viskolcz, 1998; Méreau et  al., 2000a; Somnitz and Zellner, 2000a, 2000b; Hack et  al., 2001; Lin and Ho, 2002; Ferenac et  al.,  2003; Méreau et  al., 2003; Vereecken and Peeters, 2003; Johnson et  al., 2004a; Cassanelli et al., 2005, 2006; Davis and Francisco, 2011). Most of these studies address the temperature dependence of the rate parameters (see Table VI-B-3). For the 1-butoxy isomerization, activation energies or energy barriers derived from theory lie in the range 33–42 kJ mole−1 ( Jungkamp et  al., 1997; Lendvay and Viskolcz, 1998; Méreau et al., 2000a; Somnitz and Zellner, 2000a, 2000b; Lin and Ho, 2002; Ferenac et al., 2003; Vereecken and Peeters, 2003), whereas estimated A-factors at 1 atm pressure fall in the range of approximately 1011–1012 s−1. However, as discussed in detail by Vereecken and Peeters (2003), these studies neglect the contribution of different rotamers of both the 1-butoxy reactant and the transition state to the isomerization and often neglect the effects of tunneling; thus, they may be subject to significant but potentially cancelling errors. The treatment of Vereecken and Peeters (2003) gives kisom  =  1.4  ×1011 exp(−4,100/T), in good agreement with the Atkinson (2007) recommendation. The end-product data of Cox and co-workers (Cassanelli et al., 2005, 2006), obtained using either a slow-flow system or an environmental chamber, yield parameters of A ≈ 2 ×1010 s−1 and Ea/R ≈ 3,520 K.

355

There have been a number of studies of the isomerizations of larger alkoxy radicals since 2000 (Somnitz and Zellner, 2000a, 2000b; Lin and Ho, 2002; Méreau et al., 2003; Johnson et al., 2004a; Vereecken et al., 2010). The data show the expected decrease in activation energy (or barrier heights) for isomerization involving the H transfer from a primary, secondary, or tertiary group. For abstraction from a secondary site (e.g., 1-pentoxy, 1-hexoxy, and 2-hexoxy radical isomerizations), the data center on activation energies of about 25–33 kJ mole−1 and 298 K rate coefficients of (2–5) ×106 s−1, in fairly good agreement with the Atkinson parameterization (Somnitz and Zellner, 2000b; Lin and Ho, 2002; Méreau et  al., 2003; Johnson et al., 2004a). For isomerizations involving transfer of a tertiary hydrogen, the theoretical studies of Méreau et al. (2003), Lin and Ho (2002), and Vereecken and Peeters (2010) find activation energies (or barrier heights) of approximately 15–20 kJ mol−1, consistent with the Atkinson (2007) parameterization. The end-product study of Johnson et  al. (2004a) for 5-methyl-2-hexoxy isomerization gives Ea  =  21 kJ mole−1 with k  =  1  ×107 s−1 near 300 K. Johnson et  al. (2004a) noted a correlation between the isomerization rate coefficient and the bond energy of the hydrogen being abstracted; although this was based on only three isomerization data points, data for the analogous reaction of alkoxy radicals with alkanes were also included in the development of the correlation. Finally, as first noted by Somnitz and Zellner (2000b), decreases in activation energy due to substitution on the carbon atom at the C–O• site are seen in some cases (Somnitz and Zellner, 2000b; Lin and Ho, 2002; Méreau et al., 2003). Calvert et al. (2008) used the data in Table VI-B-3 to derive a SAR for activation energies of isomerization for radicals of structure HC(R)(R)CH2CH2C(R′) (R′)O•: Ea (kJ mole−1) = 37.7 − 8.8 × Nalk,abs − 2.0 × Nalk,oxy, where Nalk,abs refers to the number of alkyl substituents (R-groups) at the abstraction site and Nalk,oxy refers to the number of alkyl substituents (R′-groups) at the oxy radical site. Figure VI-B-2 shows the activation energies calculated using this SAR plotted versus the experimental values. Most of the scatter is from the wide range of 1-butoxy

356

the mechanisms of reactions influencing atmospheric ozone Calculated activation energy/barrier height, kJ mole−1

45 40 35 30 25 20 15 15

20

25

30

35

40

45

Measured activation energy/barrier height, kJ mole−1 FIGURE VI-B-2.

Comparison of calculated (using the structure-additivity method described in the text) versus measured activation energies (or energy barriers) for isomerization of alkoxy radicals. Reprinted with permission from Calvert et al. (2008). Copyright 2008, Oxford University Press.

data points (calculated Ea  =  36 kJ mole−1, “measured” values between 27 and 41 kJ mole−1). Some of the scatter is due also to the interchangeable use of calculated barrier heights and activation energies at infinite pressure and Ea at 1 atm. In any event, it is clear that there is a separation of the data into abstraction from CH3 groups (activation energy 29–42 kJ mole−1), CH2 groups (25–33 kJ mole−1), and CH groups (17–21 kJ mole−1). Vereecken and Peeters (2010) have used quantum mechanical calculations to develop a SAR built on the parameterization from Atkinson (2007) to estimate the rates of isomerization of a large number of alkoxy radicals at 1 bar and 250–350 K. The predicted rate coefficients for isomerization at 298 K for different migration spans, C–H bonds (primary, secondary, tertiary), and substitution of the alkoxy radical are given in Table VI-B-4. The values for 1,5-H shifts in Table VI-B-4 for alkyl-only alkoxy radicals from primary, secondary, and tertiary sites are taken from the Atkinson (2007) parameterization. The other values in Table VI-B-4 were calculated using theoretical results to derive correction factors to account for the effect of different spans (1,4-, 1,6-, 1,7- etc.) for substituents. Inspection of Table VI-B-4 shows that 1,5-H shifts are preferred, and the rate of these shifts is of the order of 106–107 s−1 for shifts from –CH2– and –CH(OH)– groups.

TABLE VI-B-4 . ISOMERIZ ATION R ATE COEFFICIENTS k(298 K) (sec −1 ) FOR SELECTED ALKOXY R ADICALS (VEREECKEN AND PEETER S, 2010)

Substitution Span Primary Secondary Tertiary Alkyl only

Acyloxy Aldehyde H

α-OH

1,4 2.1 × 10−2 1,5 3.2 × 105 1,6 6.0 × 104 1,7 6.7 × 102 1,8 21 1,5 a 9.3 × 103 1,6 7.9 × 103 b 1,5 1,6 1,7 1,5 c 1,6

0.23 3.3 × 106 5.7 × 105 6.7 × 103 2.3 × 102 3.1 × 104 5.1 × 103

2.4 × 107 1.8 × 107

0.82 1.1 × 107 2.0 × 106 2.4 × 104 8.5 × 102 1.1 × 105 1.9 × 104 1.3 × 107 5.9 × 105 3.0 × 104 7.5 × 107 5.9 × 107

e.g., OC(O)CH2CH2CH3. e.g., OCH2CH2CH2CH3. e.g., OCH2CH2CH2CH2OH.

a

b c

Combining a typical value of kRO+O2  =  1.5 × 10−14 cm3 molecule−1 s−1 (Atkinson, 2007) with the [O2] in 1 atm of air gives a pseudo-first-order loss of 105 s−1 with respect to reaction with O2. Decomposition of RCH2CH(O•)R radicals via C–C bond scission

Mechanisms of Reactions of the RO Radicals occurs at a rate of the order of 104 s−1 (Vereecken and Peeters, 2009). Clearly, isomerization via 1,5-H shifts is a very important fate for the alkoxy radicals formed during the atmospheric oxidation of alkanes larger than butane. VI-B-2.4. Unimolecular Decomposition of RCHClO• Radicals Alkoxy radicals containing hydrogen and chlorine atoms in the α-position (e.g., CH2ClO• and CH3CHClO•) can decompose via intramolecular elimination of HCl. For some radicals (e.g., CH3CHClO•) the elimination of HCl is faster than the O2 reaction and is the dominant loss process under tropospheric conditions (Shi et al., 1993): CH3CHClO• + M → CH3C•O + HCl + M CH3CHClO•+ O2 → CH3C(O)Cl + HO2 Maricq et  al. (1993) measured a lower limit of 5 × 105 s−1 for the rate of HCl elimination from CH3CHClO• in 1 atm of N2 diluent at 295 K. The effects of temperature and chemical activation were investigated by Orlando and Tyndall (2002). In the absence of NOx, reaction of CH3CHClO• with O2 and decomposition via HCl elimination were shown to be competing fates of the CH3CHClO• radical, with kHCl/kO2 = 3.3 × 1023 exp(−2,230/T) molecule cm−3. The CH3CHClO• radical displays a chemical activation effect. When produced in the CH3CHClO2 + NO reaction, about 50% of the nascent CH3CHClO• radicals decompose promptly via HCl elimination (Orlando and Tyndall, 2002). Wallington et  al. (1995) studied the competition between HCl elimination and reaction with O2 as fates for the chloromethoxy radical over the temperature range 264–336 K. At 700 Torr, a rate coefficient ratio of kHCl/kO2 = 5.6 × 10−23 exp(3,300/T) cm3 molecule−1 was reported; this gives 3.6 × 10−18 cm3 molecule−1 at 298 K.  Assuming an activation energy of 1.5 kcal mol−1 for the reaction with O2 gives an activation energy of 8.6 kcal mol−1 for HCl elimination: CH2ClO• + M → HC•O + HCl + M CH2ClO• + O2 → HC(O)Cl + HO2 Bilde et  al. (1999) observed a reduced yield of HC(O)Cl in the presence of NO due to enhanced

357

decomposition of the CH2ClO• radicals formed chemically activated. This finding supports the low activation energy found by Wallington et  al. (1995). The elimination of HCl from CH2ClO• radicals was studied by Wu and Carr (1999, 2001)  using flash photolysis and time-resolved photoionization mass spectrometry of the products HCl and HC(O)Cl. They worked at pressures of up to 35 Torr and temperatures in the range 265–306 K and confirmed the dependence on pressure, with an activation energy of 9.5 ± 1.4 kcal mol−1 at 10 Torr. Wu and Carr used the Rice-Ramsperger-Kassel-Marcus (RRKM) theory to extrapolate their data to 700 Torr. When combined with their measured rate coefficient for kO2  =  2.0  ×10−12 exp(−934/T), a rate coefficient ratio at 700 Torr of kHCl/kO2 = 2 × 10−18 molecule cm−3 at 296 K is obtained, in reasonable agreement with that from Wallington et  al. (1995). Elimination of HCl has also been observed in the chemistry of the CH2ClCHClO• (Wallington et  al., 1996a), CCl3CHClO (Møgelberg et  al., 1996a), and CF3CHClO• radicals (Møgelberg et al., 1995a). Hou et al. (1999) calculated that the elimination of HCl from CHCl2O• is exothermic by 96 kJ, with a barrier of 33 kJ. However, loss of chlorine is in this case exothermic, with a barrier of only 8 kJ mole−1. Thus, loss of a chlorine atom is the fate of CHCl2O• radicals at all atmospherically relevant temperatures. In the case of bromine and fluorine substituents, the energetics probably preclude the occurrence of the HX elimination reaction for any of the radicals. For CHF2O•, although the overall reaction to eliminate HF is 50 kJ mole−1 exothermic, the barrier is approximately 140 kJ mole−1 (Hou et  al., 1999). In the case of Br, the elimination of HBr is approximately 29 kJl mole−1 exothermic. However, the C–Br bond is very weak, loss of Br is very rapid, and Orlando et al. (1996) concluded that HBr elimination from CH2BrO• probably does not occur ( ϕ(total) > 1.0 (Burkholder et al., 2002). The approximate j-values for overhead Sun are given in Figure VII-D-10. Photochemical lifetimes for HOCH2C(O)CH3, ClCH2C(O)CH3, and BrCH2C(O)CH3 for overhead Sun in the lower troposphere are approximately 7 days, 4 hours, and 3 hours, respectively. The fluorinated ketones, CF3C(O)CH3 and CF3C(O)CF3, photodecompose in processes that mirror those in acetone, and the quantum yields appear to be similar (Calvert et  al., 2011). Approximate j-values are plotted in Figure VIII-D-10. Photodecomposition of iso-C3F7C(O)C2F5 is also rather inefficient, with an average ϕ(total) = 0.043 for tropospheric sunlight. The approximate j-values calculated for overhead Sun (Figure VIII-D-10) suggest a photochemical lifetime of approximately 1.3 days. VIII-D-5. Photodecomposition Pathways for Some Difunctional Ketones The absorption spectra of acetone [CH3C(O) CH3], biacetyl [CH3C(O)C(O)CH3], pyruvic acid

[CH3C(O)C(O)OH], and methyl vinyl ketone [CH3C(O)CH=CH2] are very different, reflecting the very different chromophores, –C(=O)–, –C(=O)C(=O)–, –C(=O)C(=O)OH, and CH2=CHC(=O)–, respectively (see Figure VIII-D-11). The three difunctional ketones have new bands that extend to the longer wavelengths, with a good overlap with the actinic flux. The three compounds show very different photodecomposition patterns and efficiencies (Calvert et al., 2011). Biacetyl photodecomposes by rupture of the central bond with a fraction of the CH3CO radicals formed in a vibrationally excited state that decomposes spontaneously (process (II)): CH3C(O)C(O)CH3 + hν → CH3CO + CH3CO

(I)

→CH3CO + (CH3CO‡) → CH3CO + CH3 + CO

(II)

Pyruvic acid photodecomposition occurs with a competition between several modes (Berges and Warneck, 1992; Mellouki and Mu, 2003; Calvert et al., 2011): CH3C(O)C(O)OH + hν → CO2 + (CH3COH) → CH3CHO + CO2

(I)

450

the mechanisms of reactions influencing atmospheric ozone 10−3

Photolysis frequency (j-value), s−1

10−4

10−5

10−6

10−7

HOCH2C(O)CH3 ClCH2C(O)CH3

10−8

BrCH2C(O)CH3 CF3C(O)CH3 CF3C(O)CF3

10−9

10−10

iso-C3F7C(O)C2F5

0

10

20

30

40

50

60

70

80

90

Solar zenith angle, degrees FIGURE VIII-D-10.

Photolysis frequencies (j-values) for a series of HO- and halogen-atom-substituted acetone molecules and perfluoro-2-methyl-3-pentanone (altitude 0.5 km, clear sky, ozone column = 350 Dobson units). CH3C(O)CH3 CH3C(O)C(O)CH3 CH3C(O)C(O)OH CH2=CHC(O)CH3 Actinic Flux (Z = 0°; Lower Troposphere)

8 x 1014

6 x 10–20

6 x 1014

4 x 10–20

4 x 1014

2 x 10–20

2 x 1014

0 200

Actinic flux, quanta cm–2 s–1 nm–1

Cross section, cm2 molecule–1

8 x 10–20

0 250

300

350

400

450

Wavelength, nm FIGURE VIII-D-11. Cross sections for some difunctional ketones compared to that of acetone; data for CH3C(O)CH3 are from Martinez et al. (1992); CH3C(O)C(O)CH3, Horowitz et al. (2001); CH3C(O)C(O)OH, Mellouki and Mu (2003); CH2=CHC(O)CH3, Gierczak et al. (1997).

CH3C(O)C(O)OH → CO2 + CH3CHO

(II)

→ CH3CO + C(O)OH (III)

→CH3C(O)C(O) + OH

(IV)

Processes (I)  and (II) appear to be dominant and lead to the same final products, CH3CHO and CO2.

Photodecomposition of Light-Absorbing Oxygenates and Influence on Ozone Levels Process IV is relatively minor for tropospheric conditions but becomes important at short wavelengths of absorbed light available in the upper atmosphere. ϕ(total) is near unity. Photodecomposition of methyl vinyl ketone occurs by bond rupture at the carbonyl group, as with other ketones (Gierczak et al., 1997), but the primary quantum yields of photodecomposition are small at all wavelengths available in the troposphere (

70°. So, the effect of high morning levels of HONO will be a burst of HO, NO to NO2 conversion, and then ozone-forming activity that may be short-lived. However, the effect is sufficient to enhance significantly the total ozone formation that can occur during that day. However, it has been shown that in some heavily polluted urban areas such as Santiago, Chile, the sources are sufficiently strong to maintain significant levels of HONO (~2 ppb) throughout the day (Elshorbany et  al., 2009). It is clear that HONO can be an important trace gas in enhancing O3 generation in the urban atmosphere. VIII-E-4. Photodecomposition of Other N-Atom-Containing Oxygenates and Other Photochemically Active Trace Gases The photodecomposition of several N-atomcontaining oxygenates and other trace gases that

10–17

1016

10–18

1015

10–19

1014

10–20

1013

10–21

1012

CH3C(O)OONO2

10–22

Actinic flux, quanta cm–2 s–1 nm–1

the mechanisms of reactions influencing atmospheric ozone

Cross section, cm2 molecule–1

456

1011

CH3CH2C(O)OONO2 Actinic Flux (Z = 0)

10–23

1010

200

220

240

260

280

300

320

340

Wavelength, nm

Absorption cross sections for CH3C(O)O2NO2 (Talukdar et al., 1995) and C2H5C(O)O2NO2 (Harwood et al., 2003). Actinic flux is shown for clear skies, overhead Sun, 0.5 km altitude, and 350 Dobson units ozone column.

Photolysis frequency, j(total), s–1 RC(O)OONO2 + hν → RC(O)O2 + NO2 (I) → RCO2 + NO3 (II)

FIGURE VIII-E-3.

10–6

10–7

10–8

CH3C(O)O2NO2 C3H5C(O)O2NO2 10–9

0

10

20 30 40 50 60 70 Solar zenith angle, degrees

80

90

FIGURE VIII-E-4. Photolysis frequencies [j(total)] for CH3C(O)O2NO2 and C2H5C(O)O2NO2 as a function of solar zenith angle as calculated for clear skies (altitude = 0.5 km; ozone column = 350 Dobson units).

are less abundant and/or seeming less important in ozone generation are not discussed here but have been reviewed by Calvert et al. (2011). Additional information can be found in that reference on the photochemistry of aldehydes and ketones, cyclic

ketones, nitrites, nitrates, di-methylnitrosamine, dimethylnitramide, nitroalkanes, nitrosoalkanes, bifunctional organic nitrates, hydrogen peroxide, organic peroxides, nitric acid, and some aromatic oxygenates. Although discussion of the quantum

Photodecomposition of Light-Absorbing Oxygenates and Influence on Ozone Levels

457

FIGURE VIII-E-5.

Comparison of the wavelength dependence of the cross sections for HONO and the actinic flux available in the lower troposphere with overhead Sun (altitude, 0.5 km; ozone column = 350 Dobson units).

yields and cross sections for these compounds are not given here, their lifetimes are shown in Table VIII-F-1 for overhead Sun and several diurnal cycles (March 22, September 22, June 22, and December 22). The limited recommended quantum yields and cross section data for the peroxides and PANs given in Calvert et al. (2011) have been used to calculate photochemical lifetimes for these compounds for four different periods of the year; these important data are summarized in Table VIII-L-1. V I I I - F. S U M M A R Y O F PHOTOCHEMICAL PROCESSES IN THE TROPOSPHERE The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons are significant in defining the chemistry of the urban, rural, and remote atmospheres. These processes provide sources of radicals and other reactive products that help drive the chemistry that leads to ozone generation and other important chemistry in the troposphere. In this chapter, we have reviewed the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrates, and peroxyacyl nitrates. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies or j-values.

These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere and the rates at which various reactive products are formed through photolysis. In Table VIII-F-1 (updated from Calvert et al., 2011), we have summarized the photolysis frequencies (j-values) for each of the molecules for which sufficient data exist to allow reasonable estimations to be made (298 K; latitude 40° N; 500 m altitude; ozone column 350 DU). In the second and third columns of this table, the j-values for an overhead Sun and the photochemical lifetime, 1/(j-value), respectively, are given. Of course, the Sun is rarely at the zenith, and more representative photochemical lifetimes are the diurnally averaged values. These lifetimes were calculated for clear skies at 40° N latitude (ozone column  =  350 D), for representative periods during the year: in the fourth column, Spring equinox (near March 22)  and Fall equinox (near September 23); in the fifth column, a period of extended sunshine (near June 22); and in column six, a period of limited daylight (near December 22). The calculations of these quantities were made by averaging the j-values over each half hour of the day and night for the 24 hour period. As one expects, diurnal averaged lifetimes are much longer than the lifetimes at solar zenith. On the average, at 40° N latitude they are about three times those for the periods near June 22 (τJune 21/τZ = 0 = 3.1 ± 0.4).

458

the mechanisms of reactions influencing atmospheric ozone

FIGURE VIII-E-6.

Calculated photolysis frequencies for HONO as a function of solar zenith angle for clear sky conditions (altitude, 0.5 km; ozone column = 350 Dobson units).

FIGURE VIII-E-7.

The minimum [HONO] necessary to provide a rate of HO generation equal to that provided by ozone (assumed present at 50 ppb) versus solar zenith angle; altitude, 0.5 km; 298 K, clear skies; results are shown for 50% and 80% relative humidity and 300 and 400 Dobson units overhead ozone column.

For the periods near December 22, the lifetimes are very much longer than those for an overhead Sun, and the variation within families of oxygenates becomes more evident. For example, for the compounds shown in Table VIII-F-1, the range of ratios for τZ  =  0/τDec. 22 for the acyclic aldehydes are 17 ± 2; dicarbonyls, 9 ± 2; acyclic ketones, 29 ± 6;

hydroxycarbonyls, 21  ± 3; alkyl nitrites, 8.5  ± 0.5; and mononitrate alkanes, 22 ± 2. A more detailed picture of the yearly variation of the diurnal photochemical lifetimes can be seen in Figure VIII-F-1. Here, the diurnal lifetimes have been calculated for locations at latitudes of 30°, 40°, and 50° N for a representative compound, formaldehyde,

TABLE VIII-F-1 . SUMMARY OF THE ESTIMATED PHOTOCHEMICAL LIFETIMES (Τ) FOR THE LIGHT-ABSORBING (a) INORGANIC AND (b) ORGANIC OXYGENATES FOR THE CLOUDLESS, LOWER TROPOSPHERE (L ATITUDE 40°; 298 K ; 500 M ALTITUDE; VERTICAL OZONE COLUMN = 350 DOBSON UNITS); EXPANDED FROM CALVERT ET AL. (2011)

Compound

a) Inorganic compounds Br2 Cl2 I2 HONO HONO2 HOONO2 HOOH NO2 NO3 O3, j(O1D) channel only OCS (altitude = 0.5 km) OCS (altitude = 25 km) b) Organic compounds CH2O CH3CHO C2H5CHO n-C3H7CHO iso-C3H7CHO (Assuming maximum j) iso-C3H7CHO (Assuming minimum j) n-C4H9CHO (CH3)3CCHO (Assuming maximum j) (CH3)3CCHO (Assuming minimum j) (CH3)2CHCH2CHO n-C5H11CHO n-C6H13CHO (Assuming maximum j) n-C6H13CHO (Assuming minimum j) Glycidaldehyde HOCH2CHO CHOCHO (Assuming maximum j) CHOCHO, (Assuming minimum j) CH3C(O)CHO Cl3CCHO CF3CHO CHF2CHO CH2=CHCHO CH2=C(CH3)CHO cis-CH3C(O)CH=CHCHO trans-CH3C(O)CH=CHCHO CHOCH=CHCHO C6H5CHO (OH product only) C6H5CHO (total) Pinonaldehyde

j (Total), s−1 τ, Overhead Overhead Sun Sun

τ, Diurnal Cycle Mar. 22 & Sept. 22

June 22

Dec. 22

3.67 × 10−2 2.64 × 10−3 1.71 × 10−1 1.74 × 10−3 7.20 × 10−7 7.71 × 10−6 8.81 × 10−6 9.57 × 10−3 2.39 × 10−1 3.88 × 10−5 1.0 × 10−12 2.81× 10−7

27 s 6.3 min 6s 9.6 min 16 d 1.5 d 1.3 d 1.7 min 4.2 s 7.2 h 3.1× 104 y 41d

1.2 min 20 min 15 s 29 min 2.3 mon 5.8 d 4.7 d 5.2 min 10.1 s 1.5 d

1.0 min 17 min 13 s 25 min 1.8 mon 4.8 d 3.9 d 4.5 min 9.4 s 1.2 d

2.8 min 59 min 32 s 1.4 h 1.0 y 24.7 d 17.5 d 15 min 21 s 13 d

334 d

228 d

28 y

9.02 × 10−5 7.31 × 10−6 1.18 × 10−5 1.98 × 10−5 6.24 × 10−5 5.58 × 10−5 1.80 × 10−5 3.36 × 10−5 1.44 × 10−5 1.81 × 10−5 2.41 × 10−5 1.31 × 10−5 9.49 × 10−6 6.85 × 10−5 1.48 × 10−5 1.55 × 10−4 1.24 × 10−4 1.45 × 10−4 8.64 × 10−5 1.45 × 10−5 4.60 × 10−5 1.42 × 10−6 6.61 × 10−6 7.65 × 10−4 9.95 × 10−4 1.20 × 10−3 3.10 × 10−6 3.12 × 10−4 1.41 × 10−5

3.1 h 1.6 d 1.0 d 14 h 4.5 h 5.0 h 15 h 8.3 h 19 h 15 h 12 h 21 h 1.2 d 4.1 h 19 h 1.8 h 2.2 h 1.9 h 3.2 h 19 h 6.0 h 8.2 d 1.8 d 22 min 17 min 14 min 3.7 d 53 min 20 h

11 h 6.9 d 3.9 d 2.3 d 17 h 19 h 2.5 d 1.4 d 3.1 d 2.4 d 1.9 d 3.4 d 4.7 d 15 h 3.4 d 5.7 h 7.3 h 5.7 h 13 h 2.9 d 21 h 27 d 5.9 d 1.1 h 49 min 41 min 1.0 mon 4.6 h 3.2 d

9.1 h 5.5 d 3.2 d 1.9 d 14 h 15 h 2.0 d 1.1 d 2.6 d 2.0 d 1.5 d 2.8 d 3.8 d 12 h 2.7 d 4.8 h 6.1 h 4.9 h 10 h 2.4 d 18 h 23 d 5.0 d 56 min 42 min 35 min 22 d 3.9 h 2.6 d

1.7 d 1.3 mon 19 d 10 d 3.1 d 3.3 d 11 d 6.4 d 14 d 11 d 8.1 d 15 d 20 d 2.5 d 19 d 17 h 23 h 16 h 2.3 d 11 d 3.2 d 3.0 mon 20 d 3.1 h 2.2 h 1.9 h 1.2 yr 15.h 15 d (continued)

459

TABLE VIII-F-1 . (CONTINUED)

Compound

CH2=C=O CH3C(O)CH3 (Assuming maximum j) CH3C(O)CH3 (Assuming minimum j) C2H5C(O)CH3 n-C3H7C(O)CH3, (Assuming maximum j) n-C3H7C(O)CH3 (Assuming minimum j) n-C4H9C(O)CH3 (Assuming maximum j)d n-C4H9C(O)CH3 (Assuming minimum j)d cyclo-Propanone cyclo-Butanone HOCH2C(O)CH3 (Assuming maximum j) HOCH2C(O)CH3 (Assuming minimum j) ClCH2C(O)CH3 (Assuming maximum j) ClCH2C(O)CH3 (Assuming minimum j) BrCH2C(O)CH3 (Assuming maximum j) BrCH2C(O)CH3 (Assuming minimum j) CF3C(O)CH3 (Assuming maximum j) CF3C(O)CH3 (Assuming minimum j) CF3C(O)CF3 CF3CF2C(O)CF(CF3)2 CH3C(O)C(O)OH, Case Aa CH3C(O)C(O)OH, Case Ba CH3C(O)C(O)OH, Case Ca CH3C(O)CH=CH2 CH3C(O)C(O)CH3, Case Ab CH3C(O)C(O)CH3, Case Bb CH3C(O)C(O)CH3, Case Cb trans-3-Hexene-2,5-dione cis-3-Hexene-2,5-dione HC(O)Br HC(O)Cl BrC(O)Br CH3C(O)Cl ClCH2C(O)Cl Cl2CHC(O)Cl Cl3CC(O)Cl CF3C(O)Cl CH3ONO (Assuming maximum j) CH3ONO, (Assuming minimum j) C2H5ONO n-C3H7ONO iso-C3H7ONO n-C4H9ONO iso-C4H9ONO tert-C4H9ONO (CH3)2NN=O CH3ONO2 C2H5ONO2 n-C3H7ONO2

460

j (Total), s−1 τ, Overhead Overhead Sun Sun

2.88 × 10−4 8.62 × 10−7 5.20 × 10−7 4.68 × 10−6 2.67 × 10−6 1.57 × 10−6 8.04 × 10−6 1.39 × 10−6 3.70 × 10−4 2.63 × 10−5 2.09 × 10−6 1.05 × 10−6 1.01 × 10−4 5.05 × 10−5 2.21 × 10−4 1.11 × 10−4 1.06 × 10−6 8.66 × 10−7 1.87 × 10−6 9.10 × 10−6 4.21 × 10−4 3.59 × 10−4 1.18 × 10−4 4.73 × 10−6 3.54 × 10−4 1.96 × 10−4 7.01 × 10−5 4.73 × 10−5 4.43 × 10−5 1.76 × 10−5 1.61 × 10−7 2.92 × 10−6 3.58 × 10−9 3.62 × 10−7 1.44 × 10−6 2.63 × 10−6 5.08 × 10−7 2.29 × 10−3 1.79 × 10−3 5.84 × 10−4 7.67 × 10−4 7.40 × 10−4 4.37 × 10−4 5.09 × 10−4 3.81 × 10−3 2.75 × 10−3 1.16 × 10−6 1.37 × 10−6 1.79 × 10−6

0.96 h 14 d 22 d 2.5 d 4.3 d 7.4 d 1.4 d 8.3 d 45 min 9.5 h 5.5 d 11 d 2.8 h 5.5 h 1.3 h 2.5 h 11 d 13 d 6.2 d 1.3 d 40 min 46 min 2.4 h 2.5 d 47 min 1.4 h 4.0 h 5.9 h 6.3 h 16 h 2.4 mon 4.0 d 8.8 y 1.1 mon 8.0 d 4.4 d 23 d 7.3 min 9.3 min 29 mine 22 mine 23 mine 38 mine 33 mine 4.4 min 6.1 min 10 d 8.5 d 6.5 d

τ, Diurnal Cycle Mar. 22 & Sept. 22

June 22

Dec. 22

3.2 h 1.9 mon 2.6 mon 11 d 20 d 1.2 mon 6.0 d 1.3 mon 2.4 h 1.7 d 22 d 1.5 mon 10 h 21 h 4.6 h 9.2 h 1.6 mon 2.0 mon 27 d 4.4 d 2.1 h 2.4 h 7.3 h 8.5 d 2.3 h 3.9 h 11 h 19 h 22 h 2.7 d 11 mon 16 d 86 y 5.3 mon 1.3 mon 19 d 3.7 mon 22 min 29 min 1.4 he 1.1 he 1.1 he 1.9 he 1.6 he 13 min 18 min 1.4 mon 1.2 mon 28 d

2.7 h 1.5 mon 2.0 mon 8.5 d 16 d 28 d 4.8 d 1.0 mon 2.1 h 1.3 d 18 d 1.2 mon 8.6 h 17 h 3.8 h 7.6 h 1.3 mon 1.6 mon 21 d 3.7 d 1.8 h 2.1 h 6.3 h 7.1 d 2.0 h 3.4 h 9.9 h 16 h 19 h 2.2 d 8.5 mon 13 d 53 y 4.1 mon 1.0 mon 15 d 2.9 mon 19 min 25 min 1.2 he 56 mine 58 mine 1.6 he 1.4 he 11 min 15 min 1.1 mon 29 d 22 d

10.4 h 12 mon 1.6 y 1.8 mon 4.4 mon 8.2 mon 1.0 mon 8.5 mon 8.2 h 9.0 d 3.5 mon 6.9 mon 1.9 d 3.8 d 18 h 1.5 d 9.4 mon 1.0 y 4.6 mon 16 d 6.2 h 7.5 h 22 h 1.0 mon 6.5 h 9.8 h 1.3 d 2.8 d 3.6 d 13 d 5.4 y 2.5 mon ~7 × 103 y 3.7 y 9.6 mon 3.1 mon 2.2 y 1.1 h 1.4 h 4.2 he 3.1 he 3.2 he 5.5 he 4.8 he 33 min 45 min 7.0 mon 6.8 mon 4.9 mon (continued)

Photodecomposition of Light-Absorbing Oxygenates and Influence on Ozone Levels

461

TABLE VIII-F-1 . (CONTINUED)

Compound

iso-C3H7ONO2 n-C4H9ONO2 tert-C4H9ONO2 2-C5H11ONO2 3-C5H11ONO2 cyclo-C5H9ONO2 CH3C(O)CH(ONO2)CH3 CH3C(O)CH2ONO2 2-Oxo-cyclohexyl-1-nitrate CH3CH2C(O)CH2ONO2 CH3CH(ONO2)CH(ONO2)CH3 CH3CH(ONO2)CH2ONO2 CH3CH2CH(ONO2)CH2ONO2 CH2(ONO2)CH=CHCH2ONO2 CH2=CHCH(ONO2)CH2ONO2 1-Methyl-cyclohexyl-1,2-dinitrate HOCH2CH2ONO2 CH3C(O)O2NO2 C2H5C(O)O2NO2 CH3OOH HOCH2OOH Benzene oxide / Oxepin Toluene oxide / Methyloxepin o-Nitrobenzaldehyde Nitrobenzene 2-Methyl-1-nitronaphthalene Phthaldialdehyde 2-Acetylbenzaldehyde

j (Total), s−1 τ, Overhead Overhead Sun Sun

3.00 × 10−6 1.59 × 10−6 8.34 × 10−6 2.58 × 10−6 2.52 × 10−6 9.01 × 10−7 5.82 × 10−5 3.81 × 10−5 2.19 × 10−5 2.09 × 10−5 1.19 × 10−5 1.13 × 10−5 1.54 × 10−5 6.76 × 10−6 6.53 × 10−6 5.32 × 10−6 9.64 × 10−8 8.93 × 10−7 1.56 × 10−6 5.98 × 10−6 5.74 × 10−6 4.33 × 10−4 3.91 × 10−4 7.34 × 10−3 2.06 × 10−5 1.66 × 10−4 8.23 × 10−4 8.87 × 10−4

3.9 d 7.3 d 1.4 d 4.5 d 4.6 d 13 d 4.8 hc 7.3 hc 13 hc 13 hc 23 h 25 h 18 h 1.7 d 1.8 d 2.2 d 4.0 mon 13 d 7.4 d 1.9 d 2.0 d 39 min 43 min 2.3 min 14 h 1.7 h 20 min 19 min

τ, Diurnal Cycle Mar. 22 & Sept. 22

June 22

Dec. 22

16 d 1.0 mon 5.5 d 19 d 20 d 2.0 mon 18 hc 28 hc 2.2 dc 2.2 dc 3.6 d 3.8 d 2.7 d 7.0 d 6.8 d 10 d 1.7 y 1.7 mon 29 d 6.7 d 7.1 d 1.9 h 2.1 h 7.3 min 1.8 d 5.2 h 1.2 h 1.0 h

13 d 25 d 4.5 d 15 d 16 d 1.5 mon 15 hc 23 hc 1.8 dc 1.8 dc 3.0 d 3.2 d 2.3 d 5.7 d 5.6 d 7.9 d 1.3 y 1.4 mon 24 d 5.6 d 5.9 d 1.6 h 1.8 h 6.2 min 1.5 d 4.4 h 1.0 h 0.87 h

2.6 mon 5.6 mon 25 d 3.3 mon 3.5 mon 1.0 y 3.2 dc 5.2 dc 12 dc 11 dc 15 d 15 d 11 d 1.2 mon 1.0 mon 2.2 mon 13.6 y 7.2 mon 4.2 mon 23 d 26 d 5.2 h 5.8 h 23 min 5.5 d 16 h 2.0 h 3.3 h

Case A: assumed ϕtotal = 1.0, independent of λ; case B: ϕtotal = 1 for λ < 370 nm, and 0 for λ >370 nm; case C: assumed ϕtotal = 0.28 at all λ to fit EUPHORE estimates. b Case A:  simulation made to match EUPHORE estimates; case B:  assumed cutoff of photodissociation occurs for λ > 492  nm; case C: assumed cutoff of photodissociation occurs for λ > 410 nm. c Lifetimes are minimum values since the quantum yields of photodecomposition may be less than the unity that has been assumed in the calculations for these compounds. d Value for process (IV) only; n-C4H9C(O)CH3 + hν → CH3CH=CH2 + CH2=C(OH)CH3. e Values are maximum because the measured quantum yields accepted for the calculations in this work are less than the unity that has been observed for the other alkyl nitrates and HONO. a

during a continuous period of 18  months. During the period from March 22 to September 22, formaldehyde’s diurnal photochemical lifetime is relatively constant:  9.1–10.2 hours at 30° N, 9.1–10.7 hours at 40° N, and 9.2–12.0 h at 50° N. Its rate of photodecomposition is competitive with its reaction with HO; for an averaged [OH] = 106 molecule cm−3, its lifetime for reaction with OH is about 33 hours. Note that the nearly constant values of the lifetimes seen during the period from March 21 to September 22

rise dramatically as the winter months arrive, and a strong peak in the values is seen at the shortest day in the year, December 22. As one proceeds from 30° N to the higher latitudes, the diurnal lifetimes during the winter months show a dramatic increase. For the short-lived molecules that photodissociate relatively near the point of their entry into the atmosphere and for which migration of its air parcel to different latitudes is unlikely, estimates such as those given in Table VIII-F-1 are useful. However, it must be remembered

462

the mechanisms of reactions influencing atmospheric ozone

FIGURE VIII-F-1.

The diurnal photochemical lifetime for CH2O as calculated for clear-sky conditions in the lower troposphere with an ozone column of 350 Dobson units. Values are shown for locations at latitudes of 30°, 40°, and 50° over an 18 month period.

that these estimates are based on the assumption of cloudless skies, an uncommon situation. Hence, the values obtained are minimum values. Long-lived molecules survive to travel long distances and to latitudes where the diurnal pattern of Z, and hence the photolysis frequencies, can be very different from those at the emission source. It can be seen in Figure VIII-F-2a that for the period near January 1 an air parcel that travels north or south encounters a significant variation in the pattern of diurnal Z; a midday Z value of 23º at the equator increases to a value greater of than 90º, with the complete absence of sunlight at latitudes above about 70º N.  The changing pattern of Z variation with latitude also occurs during the spring and fall periods; see Figure VIII-F-2b for March 20. If migration of the air parcel occurs to the north or south, it encounters a midday Sun that varies from near overhead (Z = 0º) at the equator to a value of Z = 90º at the North pole. Here, the Sun circles at the horizon during the usual daytime and nighttime periods. An air parcel that migrates during the summer period can also encounter significant changes in the diurnal behavior of Z and the available actinic flux; see Figure VIII-F-2c for June 30. As an air parcel moves from the lower latitudes to those greater than 65º N, the near overhead Sun at 12:00 local time at the equator continues to fall until, at about 65º N, the Sun stays above the horizon during the entire 24

hour cycle, at Z ~ 66º in Figure VIII-F-2c for June 20. The detailed knowledge of the movement of the air parcel containing the long-lived species that is necessary to derive an accurate estimate of its photochemical lifetime is uncommon, and one usually settles for lifetime estimates for long-lived species based on an assumed transport at constant latitude. For some long-lived organic nitrates, representative “globally averaged lifetimes (24-hour cycles)” have been estimated using the relation:  τ (global)  =  1/jZ  =  60º (e.g., see Luke and Dickerson [1988] and Barnes et al. [1993a]). The rationale for the use of this relation is unclear, and the results do not have the quantitative value that the designation of “τ (global)” seems to imply. There are a large range of lifetimes listed in Table VIII-F-1. The most photochemically active class of organic molecules of those shown are the alkyl nitrites, which have lifetimes of 4 minutes (tert-C4H9ONO) to 38 minutes (n-C4H9ONO) with an overhead Sun and diurnal photochemical lifetimes of 11 minutes and 1.6 hours during the June 22 period. The least reactive photochemical species shown is CH3C(O)Cl, which has a photochemical lifetime in the lower troposphere of hundreds of years during periods of low solar flux. Obviously, the losses of these species by other than photodecomposition, such as by hydrolysis in cloud, rain, or ocean water, will be important.

Photodecomposition of Light-Absorbing Oxygenates and Influence on Ozone Levels

463

FIGURE VIII-F-2. Diurnal variation of the solar zenith angle (degrees) at different latitudes (degrees N) and dates: (a) January 1, (b) March 20, and (c) June 20.

The most important class of these molecules in promoting ozone generation within the troposphere is the aldehydes, and formaldehyde leads this list. Photodecomposition of the other higher aldehydes is somewhat less competitive with the HO reaction, but still important. This is especially true at the higher altitudes of the troposphere where the photodissociation is often enhanced by the lowered collisional quenching of the exited aldehyde molecules and the increased solar flux. The ketones also share their decay pathways between HO attack and photodecomposition. With acetone in the lower troposphere during the summer months, photochemical lifetimes between 1.5 and 2.0  months are expected, whereas the lifetime for HO attack is about 2.1  months. The acceleration of the photodecomposition of acetone that is expected at the higher altitudes results from lowered

collisional deactivation of the excited acetone molecules at the low pressure and higher actinic flux levels. This is somewhat attenuated by the lower quantum yields that are expected at the lower temperatures of the upper troposphere. With these results, together with the suppression of the reaction with HO as a result of the activation energy barrier, the photodecomposition pathway stays competitive. With the higher ketones, the preference for photodecomposition over HO reaction is somewhat less. The lifetime for photodecomposition of CH3C(O)C2H5 during summer months is about 9  days, whereas that for reaction with HO is about 10 days in the lower troposphere. With 2-hexanone, the photodecomposition lifetime in the summer months is about 5 days, whereas that for reaction with HO is about 2 days. The alkyl nitrates have somewhat longer photochemical lifetimes, but these also are competitive

464

the mechanisms of reactions influencing atmospheric ozone

with the lifetimes for their reactions with HO in the lower troposphere. CH3ONO2, C2H5ONO2, CH3CH2CH2ONO2, (CH3)2CHONO2, and CH3CH2CH2CH2ONO2 have diurnal lifetimes of photodissociation during the summer months (40° latitude) in the lower troposphere of about 34, 29, 22, 13, and 25 days, respectively, whereas those for HO attack on these compounds lead to a lifetime of about 16 months, 2 months, 20 days, and 7 days, respectively. The peroxyacyl nitrates show a similar behavior:  PAN has a photochemical lifetime in the lower troposphere during the summer months of about 41  days, whereas that for HO attack is 85–154  days. For the peroxyacyl nitrates, thermal decomposition is very important, especially in the warm lower troposphere where lifetimes of hours can be expected. As we have seen in our discussions in this chapter, photochemistry provides an important part of the atmospheric chemistry that occurs within the troposphere. The direct photochemistry of the alkanes and alkenes is unimportant within the troposphere because they do not absorb available sunlight, and photodecomposition of those aromatic hydrocarbons that absorb tropospheric sunlight (largely the polycyclic hydrocarbons) appears to be unimportant (Calvert et al., 2002). However, photochemistry is indirectly responsible for the decay of all of the hydrocarbons by the dominant pathway of HO attack. Photodecomposition of ozone, a

major source of the HO radicals in the atmosphere, accounts for the majority of the observed destruction of the hydrocarbons (RH) and the subsequent generation of oxidation products, including ozone buildup in the troposphere: O3 + hν → O(1D) + O2 O(1D) + H2O → HO + HO HO + RH → H2O + R R + O2 → RO2 RO2 + R′O2, NO, NO2, HO2 → RO, RONO2, RO2NO2, RO2H, etc. → Other Products Through the oxidation of NO to NO2 via HO2 and RO2 radicals, and the regeneration of HO radicals, the subsequent buildup of ozone can result in the lower atmosphere: NO + HO2 (RO2)→ NO2 + HO (RO) NO2 + hν → O(3P) + NO O(3P) + O2 (+ N2/O2) → O3 (+ N2/O2) We have seen in the discussions given throughout this book that, as a consequence of the cycle of reactions that drive NO to NO2 and the rapid photolysis of NO2, photochemistry leads to the formation of ozone within the troposphere.

IX Chemical Mechanisms for Air Quality Modeling and Their Applications

IX-A. DEVELOPMENT OF MECHANISMS FOR AIR Q UA L I T Y M O D E L I N G IX-A-1. Requirements for Mechanisms for Air Quality Modeling, Their Development, and an Overview of the Types of Mechanisms Available A chemical mechanism is a critical component of an air quality model. Tropospheric gas phase chemical mechanisms for air quality modeling are designed to simulate the production of ozone, acids, and aerosol precursors. Therefore, their focus is on the oxidation chemistry of ozone, nitrogen oxides, sulfur compounds, and organic compounds. Figure IX-A-1 is an overview of the most important cycles of radicals that must be represented in a chemical mechanism for air quality modeling. The processes shown schematically on one level may appear to be relatively simple, but, in reality, the chemical mechanism is extremely complicated due to the very large number of organic compounds present in the atmosphere. Atmospheric chemistry mechanisms are based on laboratory data and tested against environmental experiments and field measurements (Stockwell et al., 2012). Usually, the mechanism is considered to consist of chemical species and their reactions and rate coefficients, along with the photochemical data (used to calculate photolysis frequencies). An

atmospheric chemical mechanism employed in an air quality model could be considered to include the rules for aggregating emissions and initial concentrations into species (Middleton et al., 1990). There are many thousands of volatile organic compounds (VOCs) emitted into the atmosphere, and each has its own decomposition mechanism that determines the effect of the VOC on ozone production. It is critical for a chemical mechanism to characterize the chemistry of the VOCs and their differences in chemical reactivity as accurately as possible. Middleton et al. (1990) pointed out that air quality models have only a limited number of species compared to emission inventories. An emissions aggregation scheme is the process of mapping a detailed emissions inventory into the limited number of species used in an air quality model. The scheme is an important component of any model chemical mechanism. Middleton et  al. published their aggregation process for the mechanism used in the Regional Acid Deposition Model (RADM2, Stockwell et al., 1990), but, too often, the emissions aggregation scheme for a given chemical mechanism is in the gray literature and difficult to access. There is a tension between the desire to include as much chemical detail in the mechanism and the desire for an efficient air quality model that has high spatial and vertical resolution and adequate treatment of other processes, including transport and

466

the mechanisms of reactions influencing atmospheric ozone HCHO, RCHO

O3 hν

NO2 PAN

CO, VOC RO2

HO HO2 HNO3

from HO2

H2O2 ROOH

from RO2

NO2

NO

hν

O3 FIGURE IX-A-1. The photolysis of ozone is a major source of HO radicals. CO and VOC convert HO to HO2 and organic peroxy radicals (RO2). The HO2 and RO2 radicals convert NO to NO2; these and other subsequent reactions recycle HO and HO2 while the photolysis of NO2 produces more ozone. The reaction of HO with NO2 to produce HNO3 and peroxy radical-peroxy radical reactions that produce H2O2 and organic peroxides terminate the free radical chains. The formation and decomposition of peroxyacetyl nitrate (PAN) serves as a reservoir of NO2 and acetyl peroxy radicals.

deposition. A complete explicit mechanism for the oxidation of even a few initial VOCs requires such a large number of reactions and intermediate species that it would not be practical to write by hand (Aumont et al., 2005). However, the simulation of very large chemical mechanisms is computationally expensive: every model species to be simulated prognostically requires the solution of one ordinary differential equation (ODE) and sufficient memory to store its concentration per model grid per time step ( Jacobson, 1999). Thus, a detailed mechanism with 5,000 species would require the solution of 5,000 ODEs and the storage of time series of 5,000 floating-point numbers per grid box. Figure IX-A-2 shows a range of atmospheric chemistry mechanisms used for air quality modeling. For example, Aumont and co-workers have developed atmospheric chemical mechanisms with millions of reactions and simulated them in single-grid box models (Aumont, et  al., 2005). Three-dimensional regional or global Eulerian atmospheric chemistry models pose a much more difficult computational problem. These models consist of tens of thousands of grid boxes in which the concentrations of model chemical species are calculated. Widely used regional air quality models

include the Community Multi-Scale Air Quality Model (CMAQ; Byun and Ching, 1999; Byun and Schere, 2006), the Weather Research and Forecasting Model with Chemistry (WRF/Chem; Grell et al., 2005), the Comprehensive Air-quality Model with extensions (CAMx; Environ, 2011), and the global atmospheric chemistry and transport model GEOS–Chem (Bey et  al., 2001). Typically, these types of models are run with configurations that consist of 105 or more grid boxes, and this complexity adds at least the same factor of 105 to the computational cost compared to a model consisting of a single box. The addition of any species stable enough to undergo chemical transport adds one more differential equation for chemical transformation that must be solved for each grid box of the model. The real computational cost would be much greater because transport, deposition, and other calculations would be required for the stable model chemical species, in addition to their chemistry. The combination of large numbers of grid boxes and the need to accommodate the calculation of transport and other processes, along with currently available computational resources, places severe limits on the number of chemical species that can be simulated. Although it is possible

Chemical Mechanisms for Air Quality Modeling and Their Applications Completely explicit

467

Computer generated explicit mechanisms e.g. Aumont et al. (2005)

Level of detail

Explicit mechanisms e.g. MCMV3.2 (University of Leeds, 2013)

Detailed mechanisms e.g. Madronich and Calvert (1990) SAPRC reactivity (Carter, 2010)

Mechanisms aggregated by species e.g. RACM2 (Goliff et al., 2013) SAPRC (Carter, 2010)

Highly condensed

Mechanisms aggregated by constituent molecular groups e.g. Carbon bond Yarwood et al, (2005)

Surrogate mechanisms e.g. EKMA (Dimitriades and Dodge, 1983)

FIGURE IX-A-2.

The range of chemical mechanisms used for air quality modeling. The arrow indicates that information flows between the developers of highly explicit and condensed mechanisms in both directions.

to use a chemical mechanism with 5,000 species or more in an Eularian model, the solution of 5 × 108 ODEs and the large data storage requirements are not practical for most applications, especially for those involving simulations for long time periods such as an ozone season or a complete year. However, as computational resources increase, the complexity of the chemical mechanisms that can be used in three-dimensional Eulerian air quality models will increase. The chemical detail included in atmospheric chemical mechanisms ranges from the extreme, produced by computer-generated mechanisms with millions of reactions (Aumont et al., 2005) and the Master Chemical Mechanism (MCM) with thousands of reactions ( Jenkin et  al., 1997, 2003)  to the more typical condensed mechanisms with about one hundred model chemical species in a few hundred reactions (Makar et al., 1996) that are

used in most configurations of air quality models (Figure IX-A-2). The limited number of inorganic species emitted and their limited number of reaction products make their mechanisms manageable to treat explicitly. In contrast, there are thousands of organic compounds emitted into the atmosphere from biological and anthropogenic sources (Fuentes et  al., 2000; Simon et al., 2010). Each organic compound has potentially many decomposition products. Millions of reactions would be necessary for an explicit chemical mechanism that included every known emitted compound (Szopa et al., 2005), but the available laboratory kinetics data do not support this level of detail. There are significant gaps in laboratory kinetics data that prevent the development of explicit chemical mechanisms based only on laboratory measurements (Crutzen, 1995a,b). Structure-activity relationships (SARs), estimation,

468

the mechanisms of reactions influencing atmospheric ozone

TABLE IX-A-1 . SOME COMMON MECHANISMS USED FOR AIR QUALIT Y MODELING.

Mechanism

Number of Species

a) Explicit Mechanisms MCMv3.2 Variable MCMv3.1 4,647 NCAR Master Mech. 800 b) Mechanisms Aggregated by Chemical Moiety CB04.1 33 CB05 54 CB06 92 c) Mechanisms Aggregated by Molecule SAPRC-99 76 SAPRC07C 118 RADM1 42 RADM2 63 RACM1 77 RACM2 119

Number of Reactions References Variable 13,568 2,200

University of Leeds (2013) Jenkin et al. (2003); Saunders et al. (2003b) Madronich and Calvert (1990a)

82 156 218

Gery et al. (1989); Milford et al. (1992) Yarwood et al. (2005) Yarwood et al. (2010)

214 599 80 157 237 363

Carter (2000) Carter (2010) Stockwell (1986) Stockwell et al. (1990) Stockwell et al. (1997) Goliff et al. (2013)

and other forms of analogy are used to fill in measurement gaps and to construct explicit mechanisms ( Jenkin et al., 1997; 2003; Aumont et al., 2005). Chen et  al. (2010b) performed a comparison of several widely used chemical mechanisms for air quality modeling. They found that there were few differences in their inorganic schemes. The simplest organic compounds, CH4, C2H4, and HCOOH, were treated explicitly. Isoprene was one organic compound with a complicated decomposition mechanism that was treated explicitly, presumably because of its importance and its use as a surrogate for large numbers of organic compounds from biogenic sources. The mechanisms used in the three-dimensional Eulerian atmospheric chemistry models are reduced in size according to an aggregation scheme. Aggregation not only makes the mechanisms more computationally tractable, but it also facilitates the mechanisms’ comparability with the available emissions inventories. The organic aggregation schemes differ between different families of chemical mechanisms, thus providing different emitted model species, reactive intermediates, and their products. Mechanism developers have made different choices on the treatment of organic peroxy radicals. Along with differences in rate constants and product yields, especially for organic compounds, they have used differing schemes to fill large gaps in laboratory knowledge of the chemistry of aromatic compounds and biogenic VOC compounds

(BVOC). Table IX-A-1 lists several widely used chemical mechanisms, and a number of these are discussed in the following sections. IX-A-2. Computer-Generated and Detailed Explicit Mechanisms Explicit mechanisms consist of specific chemical reactions for real individual chemical compounds believed to exist in the atmosphere, and the most detailed atmospheric chemistry mechanisms have been developed through automated mechanism generation (Aumont et  al., 2005; Khan et  al., 2009; Khan and Broadbelt, 2009). Automatic mechanism-generating methods are computer-based expert systems that rely on laboratory measurements of rate constants, product yields, and the like that are implemented as decomposition schemes and SARs that allow them to estimate very complex mechanisms for VOCs ( Jenkin et al., 1997, 2003). Atmospheric chemical mechanism generators generally follow similar protocols ( Jenkin et  al., 1997, 2003; Saunders et al., 2003a, 2003b; Aumont et al., 2005). A major assumption used in the construction of mechanism generators is that reaction sequences are very similar within classes of organic compounds. Automatic mechanism generators mimic the approach used in generating mechanisms by hand. First, a set of primary emitted VOC species are identified. The mechanism generator

Chemical Mechanisms for Air Quality Modeling and Their Applications determines all of the initial reactions for these initial VOCs based on their possible reaction with HO, O3, NO3, and photolysis. The generator does this by examining the molecular structure of every chemical species in each reaction tier to determine the reactive sites and possible reactions (Aumont et  al., 2005). Extensive laboratory-based databases are searched to find rate constants and products, but if they are not available, then SARs are used to estimate them. The majority of the reactions in the first tier produce species that immediately produce organic peroxy radicals (RO2) and HO2 radicals that initiate free radical chain reactions. The peroxy radicals react with NO, NO2, NO3, HO2, or RO2 radicals. These reactions produce species that are stable products or organic alkoxy radicals (RO). The organic alkoxy radicals react with O2, isomerize, or decompose to produce additional peroxy radicals or new stable species. The stable species produced in these steps constitute the first set of secondary organic compounds. The complete reaction along with its rate constant is saved to a mechanism file, and the product species are checked to find if they are new to the mechanism. If the species are new to the mechanism, the species are stored and then further reactions are determined for every new species. The process continues until there are no remaining species and all of the VOCs have been decomposed to CO2 and H2O. Many believe that the future development of atmospheric chemistry mechanisms must include automated generation of a highly detailed mechanism, combined with testing the explicit mechanism against both field and chamber data, followed by its objective condensation to a size that can be used in three-dimensional air quality models (Carter, 2012; Evans, 2012). Automatic mechanism generation has the advantages of speed, accuracy, and ease of updating to accommodate new data into large explicit mechanisms (Aumont et  al., 2005). For extremely large mechanisms, automatic mechanism generators will be more accurate in expressing an oxidation protocol, and a mechanism can be updated relatively easily if there is a change in the SARs due to new data. A mechanism generated by an automatic system has the potential to calculate more accurately the changing composition of VOC in an aging air parcel and its effect on HOx, VOC reactivity, and the nitrogen budget. A  highly detailed representation of soluble and low vapor pressure compounds

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allows a potentially better representation of the linkages between the gas-, aqueous-, and aerosol-phase chemistries. A potential weakness of automatic mechanism generation is the false sense of security that can be associated with the explicit mechanisms. The assumed protocols for the unmeasured compounds may be wrong. Explicit mechanisms and condensed, aggregated mechanisms are derived from the same laboratory database. Therefore, the difference in the actual amount of real information represented by the two types may not be as large as the difference in the number of reactions appears to suggest. Very detailed explicit mechanisms have been developed without very large-scale automation, including the NCAR Master Mechanism (Madronich and Calvert, 1990a) and the MCM ( Jenkin et  al., 1997, 2003; Saunders et  al., 1997). These are very large mechanisms. The National Center for Atmospheric Research (NCAR) Master Mechanism has nearly 800 organic species and 2,200 reactions (Madronich and Calvert, 1990a). Various versions of the MCM mechanism exist and range in size from about 5,000 to a few tens of thousands of reactions that treat the reaction of 10 to more than 100 emitted organic compounds, their reactive intermediates, and their products. The most recent version of the MCM as of this writing is MCMv3.2 (University of Leeds, 2013); it was updated to include better aromatic chemistry (Bloss et al., 2005). The website (University of Leeds, 2013) lists 143 primary VOCs that may be selected, thus number of reactions and species in an MCMv3.2 mechanism may be regarded as variable depending on the number of selected VOCs. The MCM has been tested successfully against environmental chamber and field measurements ( Jenkin et al., 2000; Hynes et al., 2005; Zádor et al., 2005; Evtyugina et  al., 2007; Pinho et  al., 2009; Cheng et al., 2010), and, although it is too large for routine use, it has been implemented in a three-dimensional Eulerian air quality model ( Jenkin et  al., 2008b; Watson et al., 2008). IX-A-3. Condensed Mechanisms (EKMA, Carbon Bond, SAPRC, RADM/ RACM) Simplified tropospheric gas phase mechanisms for air quality include the inorganic chemistry of ozone, NO, NO2, H2O2, HO, HO2, and SO2 explicitly, with about 20 species reacting in about 45

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inorganic reactions. Most of the effort in developing an air quality mechanism is to produce mechanisms for organic chemistry that are faithful to the chemistry but highly simplified. There are two major approaches to simplifying the organic chemistry:  surrogate mechanisms and aggregated mechanisms. A  surrogate chemical mechanism uses an explicit treatment of a very limited number of organic species to represent the contribution of all VOCs to ozone formation. Aggregated mechanisms group similar compounds into model species whose chemical reactions represent the chemistry of the group. Aggregation has been accomplished by mechanisms that group organic compounds by chemical moiety and by mechanisms that group organic compounds by chemical species (Stockwell et al., 2012). One of the first and most widely used surrogate chemical mechanisms was the Empirical Kinetics Modeling Approach (EKMA; Dimitriades and Dodge, 1983). The EKMA mechanism used butane to represent the slower reacting organic species and propene to represent the more reactive organic species. The EKMA mechanism was implemented in a box model and used to generate ozone isopleths for a particular site. An ozone isopleth is a contour plot of the daily maximum O3 concentration as a function of the initial NOx and VOC concentrations and/or their emissions. For EKMA, the site’s emission inventory was used to determine the relative proportions of butane and propene. For this VOC mixture, a series of simulations was made with differing initial concentrations or emissions of NOx and VOC to produce an ozone isopleth. It is a drastic oversimplification to aggregate all organic emissions into two surrogate species, so the EKMA approach by itself is not widely used today. However, it is not unusual to use a single-model species to represent a range of organic chemicals. The application of ozone isopleths to the development of ozone control strategies is further discussed in Section IX-B-4. The Carbon Bond series of mechanisms began with a highly innovative approach toward the aggregation of organic compounds. The Carbon Bond approach grouped organic compounds according to chemical moiety. Organic compounds are “broken up” into their constituent chemical groups, and then the chemical groups are aggregated into model species (Whitten et  al., 1980; Gery et  al., 1989, Whitten and Hogo, 1999). The Carbon Bond species represent the concentrations of reactive organic

groups, without regard to their attachment to other groups in the organic molecule that comprise a gas mixture. This method of aggregating the organic composition of the atmosphere into model species leads to a relatively smaller mechanism because fewer species are required than for the molecular approach (Chen et  al., 2010b). The original Carbon Bond mechanisms included the species PAR (alkane carbon atoms), OLE (a pair of double bonded carbon atoms), ARO (six carbon atoms in an aromatic ring), and CAR (a carbon atom in a carbonyl group; Gery et al., 1989). As an example of the aggregation approach, consider a mixture of 1.0 ppbV of propane (CH3CH2CH3), 1.0 ppbV of 1-butene (CH3CH2CH=CH2), and 1.0 ppbV of 2-butene (trans-2-butene and/or cis-2-butene; CH3CH=CHCH3). These chemical species would be grouped as 3.0 ppbV PAR from the propane, 2.0 ppbV PAR and 1.0 ppbV OLE from the 1-butene, and another 2.0 ppbV PAR and 1.0 ppbV OLE from the 2-butene for a total of 7.0 ppbV PAR and 2.0 ppbV OLE in the mixture. Notice that there is no distinction between the double bonds at the 1- and 2-positions in the butenes that affects the aggregation into the Carbon Bond species. Although some information was lost, the Carbon Bond approach provided important advantages in that it provided a relatively easy way of aggregating initial concentrations and emissions into the model species; carbon atoms were conserved, and it required a relatively low number of chemical species to represent organic chemistry compared to aggregated molecule approaches. Nitrogen is conserved in the Carbon Bond series of mechanisms, as it is in most condensed mechanisms. The Carbon Bond IV (or CB04; Gery et  al., 1989)  was one of the most widely used and longest used mechanisms for regulatory air quality modeling. Comparisons of the Carbon Bond series of mechanisms’ simulations with environmental chamber data were an integral part of their development (Environ, 2011). The reliance on the comparisons was so great that the decomposition reaction was adjusted to make peroxyacyl nitrate (PAN) more stable to match better the CB4 simulations with University of North Carolina (UNC) chamber data. When applied in three-dimensional air quality models, the overly high stability of PAN shut down ozone production at lower atmospheric temperatures. The rate constants for PAN formation and decomposition were readjusted to

Chemical Mechanisms for Air Quality Modeling and Their Applications laboratory measurements (Dodge, 1991; Milford et  al., 1992)  to create Carbon Bond 4.1. Carbon Bond 4.1 became the EPA standard regulatory mechanism in part because it was a condensed mechanism with fewer species compared to the Statewide Air Pollution Research Center (SAPRC) and RADM/Regional Atmospheric Chemistry Mechanism (RACM) series of air quality mechanisms. The aggregation scheme of the mechanism made it somewhat more straightforward to prepare emission inventories for air quality models as well. Development of the Carbon Bond mechanism has continued, and, more recently, peroxy radical–peroxy radical termination reactions and the isoprene chemistry were updated in the Carbon Bond mechanism (Carter, 1996; Whitten et  al., 1996; Environ, 2011). Its chemistry was further extended to improve the simulation of ozone formation for regional and rural regions and for the simulation of secondary organic aerosols (SOAs), mercury, and toxics (Yarwood et al., 2005; Environ, 2011). However, more recent versions of the Carbon Bond mechanism aggregate organic compounds less by chemical moiety, and more recent versions use the aggregation-by-molecule approach to a much greater extent. This change was made because a compound’s overall molecular structure and its total molecular weight does affect its atmospheric chemistry, especially its contribution to the formation of secondary aerosol (Gery et al., 1989; Yarwood et al., 2005; Stockwell et al., 2012). On the other hand, it is surprising that the Carbon Bond mechanism, version CB5, can be adapted to estimate SOA concentrations that are very similar to the more comprehensive RACM2 (Kim et al., 2011). Rate coefficients and product yields in the CB04a mechanism were updated according to the International Union of Pure and Applied Chemistry (IUPAC) and NASA evaluated chemical kinetics data available from 2003 to 2005 to create a new version of the Carbon Bond mechanism, CB05 (Yarwood et  al., 2005; Environ, 2011). The number of inorganic reactions was increased, and the set of NOx recycling reactions improved to simulate more accurately conditions that range from remote to highly polluted urban environments (Chen et al., 2010b). A  number of organic species were treated explicitly: methane, ethane, methyl hydroperoxide, formic acid, and the methylperoxy radical. New species were added: an alkene species with an internal double bond, a higher aldehyde species, a terpene

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species, species to represent higher organic peroxides, organic acids, peroxyacids, and a higher PAN species. Introduction of the higher aldehyde species allows the original aldehyde species to represent acetaldehyde (Environ, 2011). The CB05 mechanism was heavily evaluated against environmental data from UNC and the University of California, Riverside. The revisions to the mechanism improved its performance in the production of oxidants at low VOC-to-NOx ratios and its overall performance under the lower NOx conditions typical of rural and regional conditions. The most recent version of the Carbon Bond mechanism is version 6 (CB06) that was derived from CB05 (Environ, 2011). The inorganic chemistry was updated according to the available evaluated databases (Atkinson et al., 2004, 2006; Sander et al., 2011), and one of the more significant revisions was reduction of the gas phase rate constant for the reaction of dinitrogen pentoxide (N2O5) with H2O (Environ, 2011). Propane, ethyne, benzene, and acetone are treated explicitly in CB06. The aromatic oxidation mechanism was revised based on Whitten et al. (2010), and the isoprene mechanism was revised based on Paulot et  al. (2009a, 2009b) and Peeters and Müller (2010). A  number of α-dicarbonyl compounds (glyoxal and analogues) were added because of their potential to improve the modeling of the formation of SOAs (Carlton et  al., 2007). CB06 was tested extensively against 339 environmental chamber experiments from chambers at the University of California at Riverside and the Tennessee Valley Authority, and the CB06 simulations more closely reproduced the data than the CB05 simulations (Environ, 2011). The typical bias of CB05 and CB06 for the simulation of maximum ozone in the chamber experiments containing alkanes, alkenes, alcohols, and aldehydes was ±20% or less (Environ, 2011). CB06 performed considerably better than CB05 in simulating the maximum ozone for chambers containing ethyne, benzene, and acetone. Both mechanisms underpredict the maximum ozone for chamber experiments containing aromatics; the bias was near 30% for CB05 and 20% for CB06. CB06 is a larger mechanism with a greater number of species so it is computationally more expensive than CB05. The aggregation-by-molecule approach places organic compounds with similar molecular structure and HO rate constants into grouped model species. This is a more obvious approach to mechanism

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condensation than the system used by the Carbon Bond mechanisms (Stockwell et  al., 2012). For example a model species ALK could be used to represent the chemistry of a range of alkanes, whereas OLE could represent a range of alkenes. Weighting factors that account for variations in reactivity or to improve carbon mass conservation may be applied in the aggregation scheme (e.g., Stockwell et  al., 1997). However, most mechanisms that use the aggregation-by-molecule approach usually do not strictly conserve carbon (Stockwell et  al., 2012). The series of mechanisms named after the SAPRC (Carter, 1990, 2000, 2010)  at the University of California, Riverside, and the series of the RADM mechanism (Stockwell et  al., 1990, 1997)  and the RACM (Stockwell et  al., 1997; Goliff et  al., 2013) are examples of the aggregation-by-molecule approach that are widely used for air quality modeling. The approach used to create these mechanisms aggregates organic species based on similarity of molecular structure and reactivity toward the HO radical. SAPRC was originally developed for modeling polluted urban atmospheres such as Los Angeles, but the recent versions have been developed extensively to treat regional chemistry, too. For example, although CB05 and CB06 use operators to represent reactions of organic peroxy radicals, the current SAPRC treatment is more extensive. Major versions of SAPRC include SAPRC-90 (Carter, 1990), SAPRC-99 (Carter, 2000), and SAPRC07 (Carter, 2010). Each version was the result of extensive updates to improve the representation of the chemistry. Some versions of SAPRC have an adjustable number of species to represent organic species, and these can become very explicit. The most recent updates to SAPRC07 include the rate constants for reactions involving NO2, HO, HO2, HNO3, CH2O, and PAN (Cai et  al., 2011). SAPRC07 has revised mechanisms for aromatic chemistry and more explicit representation of peroxy-peroxy reactions and hydroperoxide formation so that effects of changes in nitrogen oxide (NOx) concentration on organic product formation can be represented more accurately. The number of VOC species increased from the first to last versions of SAPRC. SAPRC-99 includes 76 species reacting in 214 reactions, whereas SAPRC07C includes 118 species reacting in 599 reactions. Air quality models with larger chemical mechanisms require more computational resources. A CMAQ simulation with

SAPRC07C requires twice the simulation time as a CMAQ simulation with SAPRC99 with present-day computers (Cai et al., 2011). The SAPRC series of mechanisms have been developed and tested against environmental chamber data (e.g., Carter and Lurmann, 1991; Carter, 1995). Simulations made with air quality models using SAPRC have been compared with field data (Lin et  al., 2005; Arnold and Dennis, 2006), and it has been intercompared with other mechanisms ( Jimenez et  al., 2003; Faraji et  al., 2008; Luecken et  al., 2008). The revisions to the organic chemistry of SAPRC-07 have resulted in an increase of more than 20% to the estimated reactivity for many organic compounds (Chen et  al., 2010b). This increase may have significance for the development of public policy because EPA regulatory guidance for air quality modeling requires that simulations be used in a relative rather than an absolute sense (EPA, 2005; Cai et al., 2011). The Carbon Bond and the SAPRC series of mechanisms have their developmental roots in the urban airshed modeling of Los Angeles dating back to the 1970s and 1980s, when pollutant concentrations were very high (Chen et al., 2010b). For example, in early versions of SAPRC the reaction of HO with organic species produced the products of the peroxy radicals with NO and an operator species to convert NO to NO2 directly in the primary reaction. This is a valid assumption under the highly polluted conditions of Los Angeles but fails under NO levels typical of more regional locations. Although the more recent versions of Carbon Bond and SAPRC no longer make this particular assumption, and they have been highly updated and revised to be valid for a wider range of urban and regional conditions, it is possible that their conceptual formulation remains focused on more polluted conditions. Regional-scale mechanisms were developed originally for the modeling of acid deposition during the 1980s. The modeling of atmospheric deposition required that the mechanisms apply to a wider range of conditions, from highly polluted urban areas to more remote regions to which the original urban airshed mechanisms had not been applied. On the regional scale, lower NOx conditions, organic compounds from biogenic sources, and less reactive organic compounds become more important than for urban conditions. The mechanisms for the RADM (Chang et al., 1987) are examples of regional-scale mechanisms that aggregate organic

Chemical Mechanisms for Air Quality Modeling and Their Applications species according to molecular structure and reactivity toward HO radicals (RADM1, Stockwell, 1986; RADM2, Stockwell et  al., 1990). For example, RADM2 aggregates alkanes into five different model species:  methane, ethane, a model species with three carbon atoms, a model species with five carbon atoms, and a model species representing alkanes with more than five carbon atoms. RADM2 included model species for alkenes, ethene, terminal alkenes, internal alkenes, and dienes, along with model species for the treatment of the biologically emitted isoprene and two primary groups for aromatics, less reactive (TOL) and more reactive (XYL). Some simple organic species were treated explicitly, including methane and formaldehyde. To better represent the chemistry of lower NOx conditions in more remote regions, the RADM2 used a more explicit treatment of the reactions of all organic peroxy radicals with NO, NO3, HO2, CH3O2, and CH3CO3 radicals than did the urban airshed mechanisms during the late 1980s. The RADM2 mechanism was used in several models, and it impacted further research in atmospheric chemical mechanisms (Goliff et  al., 2013). In addition to the RADM (Chang et  al., 1987), the RADM2 mechanism was incorporated into other models including the CMAQ (O’Neill and Lamb, 2005), the three-dimensional Meteorology-Climate-Chemistry Model (MCCM; Grell et  al., 2000), and the Weather Research and Forecasting Model (WRF; Grell et al., 2005). The RADM2 mechanism was updated in view of its wide use and the availability of new laboratory data to create the RACM1 (Stockwell et al., 1997). Much of the new data came from the German Tropospheric Research Program (TFS) and other European tropospheric chemical research programs (Geiger et  al., 2002). Important new additions to the RACM1 included chemistry for the biologically emitted α-pinene and d-limonene and a partially explicit oxidation scheme for isoprene. RACM1 was updated recently to create the RACM2 (Golliff et  al., 2013). RACM2 currently has a total of 119 model chemical species (17 stable inorganic species, 4 inorganic intermediates, 55 stable organic species and 43 organic intermediates) in a total of 363 reactions. The intended domain of the RACM2 mechanism ranges from the Earth’s surface to the upper troposphere under conditions that vary from remote to highly polluted. The RACM2 aromatic chemistry includes species for benzene,

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toluene, and all three xylenes, with a new reaction scheme based on Calvert et al. (2002). Acetaldehyde was separated from the generalized aldehyde species to allow a more accurate treatment of PAN, and acetone was separated from the generalized ketone so that its role in upper tropospheric chemistry could be simulated. The isoprene treatment was improved by treating its product, methyl vinyl ketone, more explicitly. The RACM2 mechanism was tested against environmental chamber data and compared with previous RACM1 scenario simulations. RAC+M2 has been implemented in a beta-test version of CMAQ by the US Environmental Protection Agency (EPA). They have made regional-scale simulations to evaluate the performance of RACM2 (Sarwar et  al., 2012, 2013). The evaluation shows that RACM2-CMAQ-simulated HO is greater than that simulated by the Carbon Bond 5-CMAQ model, and the RACM2-CMAQ-simulated HO is in better agreement with observational data. RACM2-CMAQ better simulated PAN and NOx observations than did the Carbon Bond 5-CMAQ model, whereas the Carbon Bond 5-CMAQ model simulated HNO3 slightly better. IX-B. METHODS OF ASSESSING THE INFLUENCE O F VO C S A N D N O X O N O Z O N E G E N E R AT I O N U S I N G COMPUTER MODELS IX-B-1. Process Analysis and the Sensitivity of Ozone Formation to VOC and NOx A series of one-day box model simulations were made with varying initial NOx (NOx = NO + NO2) and VOC mixing ratios for surface conditions with the RACM2 mechanism (Goliff et  al., 2013)  to illustrate the overall behavior of a current chemical mechanism. Figure IX-B-1A shows mixing ratios produced with an initial total VOC of mixing ratio of 780 ppb total carbon with varying initial NOx mixing ratios. Figure IX-B-1B shows mixing ratios generated with an initial NOx mixing ratio of 35 ppb, with varying initial total carbon concentrations. For these simulations, the relative molar composition of the VOC mixture was taken from Stockwell et al. (2012). The simulations show that the maximum ozone mixing ratio increases as the initial NOx increases from 0 to about 100 ppb (Figure IX-B-1A). This is consistent with the role NOx plays in the formation of tropospheric ozone. The

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FIGURE IX-B-1.

Plot A shows simulated mixing ratios of O3, peroxyacyl nitrate (PAN), H2O2, HNO3, HO, and HO2 as a function of the initial NOx with the initial volatile organic compounds (VOC) equal to 780 ppbC for all runs in the series. The maximum mixing ratios for all species are plotted, and the mixing ratios of O3, PAN (×25). H2O2 (×25) and HNO3 (×2) are given in ppb, whereas the mixing ratios of HO (×500) and HO2 (×5) are given in ppt. Plot B is similar to Plot A, but the simulations were made with an initial NOx mixing ratio of 35 ppb, and the initial VOC was varied.

photolysis of nitrogen dioxide (NO2) is the source of ozone (Blacet, 1952): NO2 + hν (+O2) → O3 + NO However, the NO produced through the NO2 photolysis reaction reacts with ozone to reform NO2:

O3 + NO → NO2 + O2 A photostationary steady state can be assumed for NO, which leads to an expression for the ozone concentration known as the Leighton relationship, where j is the photolysis rate constant of NO2 (which depends on the solar radiation actinic flux), k is the

Chemical Mechanisms for Air Quality Modeling and Their Applications rate constant for the reaction of ozone with NO, and [O3], [NO2] and [NO] are the concentrations of O3, NO2, and NO respectively; see also Chapter III. j [NO2 ] [O3 ] ≈ k [NO] Ozone concentrations are directly related through j to solar radiation and to the [NO2]/[NO] ratio. This is consistent with expectations; higher concentrations of NO2 lead to a faster ozone production rate, whereas lower NO concentrations lead to a lower destruction rate. Reactions that convert NO to NO2 produce ozone because the ozone concentration depends on the [NO2]/[NO] ratio. The reactions of HO2 and organic peroxy radicals with NO convert it to NO2. This role of organic peroxy radicals couples VOC chemistry with the inorganic NOx chemistry. Note that Figure IX-B-1B shows that the ozone mixing ratio increases with increasing initial VOC between 0 and 780 ppbC for simulations made with constant initial NOx. The link between atmospheric inorganic and organic chemistries begins with the photolysis reaction of ozone that produces an excited oxygen atom, O(1D): O3 + hν → O(1D) + O2 O(1D) + M → O(3P) + M O(1D) + H2O → 2HO A small but significant fraction of the O(1D) atoms react with water vapor to produce HO. Figure IX-B-1A shows that the peak HO mixing ratio closely follows the ozone mixing ratio because the initial NOx is varied to reflect the increased production of HO via ozone photolysis and efficiency of radical recycling. However, Figure IX-B-1B shows that although the mixing ratio of HO increases with the rise in the maximum ozone mixing ratio for the simulations made with constant initial NOx and increasing VOC, it decreases for initial VOC mixing ratios greater than 250 ppb due to increasing loss of the HO through its reaction with VOC. This suggests that the synergetic behavior of NOx and VOCs for ozone formation can be understood in terms of competition between reactions involving HO, HO2, and organic peroxy radicals (Stockwell et al., 2012). Peroxy radicals are produced through the reactions of HO with CO and VOC. The following

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reaction shows that, in the presence of O2, the reaction of HO with CO produces the HO2 radical: HO + CO (+O2) → HO2 + CO2 The reactions of HO with VOC produce both organic peroxy radicals (RO2) and HO2. For example, the reactions for HO with an arbitrary alkane, RCH3, and the subsequent conversion of NO to NO2 by the peroxy radicals are shown here (only the abstraction of a primary hydrogen atom is shown for clarity): HO + RCH3 → RCH2 + H2O RCH2 + O2 → RCH2O2 RCH2O2 + NO → RCH2O + NO2 RCH2O + O2 → RCHO + HO2 HO2 + NO → HO + NO2 The consequences of these reactions can be seen in Figure IX-B-1. Some NOx is required to support the free radical chains that produce ozone. In Figure IX-B-1A, initially the HO2 mixing ratio increases with increasing ozone due to the increasing production rate of HO and its subsequent peroxy radical-producing reactions. The maximum ozone produced reaches a peak, but, as the available NOx rises further, an increasing fraction of the peroxy radicals reacts with NO, and their concentration drops along with the production of H2O2 for this series with constant initial VOC. This enhanced loss of HO2 leads to lower rates of conversion of NO to NO2 and therefore to less ozone production. It is possible to consider ozone formation to be a strong function of the relative radical termination rates of reactions between peroxy radicals and the reaction of HO with NO2. The extent of termination of HOx and organic free radical chains depends very strongly on the available NOx. The reactions that convert NO to NO2 constitute a chain reaction mechanism, and, at some point, these reactions must terminate. In a polluted atmosphere, there is usually sufficient NOx such that the formation of nitric acid, HNO3, will be the major sink for radicals: HO + NO2 → HNO3 Figure IX-B-1A shows that the maximum HNO3 and PAN mixing ratio increases sharply with increasing initial NOx. The maximum HNO3 increases until

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the initial NOx reaches about 245 ppb and then it decreases. If NOx concentrations are low, then many of the peroxy radicals are lost through peroxy-peroxy radical reactions, as shown here, where RO2 and ROOH represents an arbitrary organic peroxy radical and arbitrary organic peroxide. Note that the rate of the HO2 + HO2 reaction is dependent on pressure, temperature, and water vapor concentration, and its rate constant has been misrepresented often in air quality models (Stockwell, 1995): HO2 + HO2 → H2O2 + O2 RO2 + HO2 → ROOH + O2 RO2 + RO2 → Products (including alcohols and carbonyls) On the other hand, at very high initial levels of NOx, the mixing ratios of PAN decrease due to the titration of the acetyl peroxy radical by NO. The maximum PAN mixing ratio decreases rapidly for initial NOx concentrations greater than 195 ppb. These sets of chemical reactions make the production of ozone a very nonlinear function of initial VOC and NOx. Figure IX-B-1A shows that for a series of simulations with constant initial VOC there is almost an anticorrelation between the mixing ratios of H2O2 and HNO3; that is, the mixing ratio of H2O2 falls as HNO3 increases with increasing initial NOx. Note that the maximum ozone and HO mixing ratios occur after there is sufficient initial NOx to greatly reduce the production of H2O2. Figure IX-B-1 is consistent with this chemical picture, but increasing the initial VOC while keeping the initial NOx constant leads to greater and greater production of peroxy radicals. The maximum HO mixing ratios occur at an initial VOC of 250 ppbC, and then the HO mixing ratios fall as the higher initial VOC-to-NOx ratio leads to a higher HO2/NO ratio. The higher levels produce more and more H2O2 at some expense to the production of HNO3. The production of ozone levels off with increasing initial VOC because the available NOx limits its formation. For this series of simulations, the maximum PAN, H2O2, and HO2 mixing ratios increase with increasing initial VOC. The plots in Figure IX-B-1 show that NOx and VOC are required for ozone production, but increases in NOx or VOC beyond certain levels do not lead to greater ozone concentrations. If there is a very high concentration of NOx in the atmosphere,

further increases in the initial NOx (for a fixed initial concentration of VOC) lead to lower HO concentrations due to increases in the rate of the HO + NO2 reaction. The lack of HO radicals will reduce ozone formation, and this condition is known as “VOC sensitive.” Similarly, if the initial NOx concentrations are low, then there will be insufficient NO2 to produce ozone. Many of the peroxy radicals will be lost through the peroxy-peroxy radical reactions. This condition is known as “NOx sensitive.” Hydrocarbon and NOx sensitivity have important implications for the development of air pollution control strategies, as discussed in the next section. IX-B-2. O3, PAN, HNO3, H2O2, HO, and HO2 Isopleths: VOC and NOx Sensitivity It is necessary to estimate the effectiveness of proposed NOx or VOC controls because the vast majority of air quality abatement strategies require reductions in their emissions. The well-known ozone isopleth diagram that shows the relationship between VOC and NOx is a traditional plot for expressing the relationship between VOC and NOx limitation and ozone concentrations (Figure IX-B-2). The ridgeline connects the turning points of the ozone isopleths, and it represents the optimum initial NOx and VOC concentrations for ozone production. The points A  and B are two arbitrarily chosen points above and below the ridgeline. The horizontal arrows represent reductions in the initial VOC mixing ratios, and the vertical arrows represent reductions in the initial NOx. The response of the maximum ozone is very different for these two points. For the chemical environment represented by point A, reduction in initial NOx leads to an increase in the maximum ozone, whereas a reduction in initial VOC leads to a decrease in the maximum ozone. This condition is known as VOC sensitive (or NOx-inhibited). In contrast, at point B, reductions in VOC have only a very small effect on the maximum ozone mixing ratios, whereas ozone is reduced by reductions in the initial NOx. This condition is known as NOx sensitive. Isopleths remain a useful tool for understanding air pollution and have been recently used to discuss Los Angeles air quality (Fujita et al., 2003, 2013). In addition, isopleth diagrams further illustrate the atmospheric chemistry of ozone formation. Figure IX-B-3 shows a series of isopleths for O3,

Chemical Mechanisms for Air Quality Modeling and Their Applications

477

300

250 100

50

A

NOx, ppb

200

200

250

300

350

150

400 350

150

200 250 300

150 100

400

50

100

250 200

150

50

Ridge Line 250

300

100 200

50

0

200

B

150 100

150 100

0

350

400 600 VOC, ppbC

800

250 200

1000

FIGURE IX-B-2.

Simulated isopleths of the maximum mixing ratios of O3 with the ridgeline. The points A and B are two arbitrarily chosen points, and the horizontal arrows represent reductions in the initial volatile organic compound (VOC) mixing ratios, and the vertical arrows represent reductions in the initial NOx. The simulations were made with the RACM2 mechanism using the VOC mixture discussed in Stockwell et al. (2012).

PAN, HNO3, H2O2, HO, and HO2. As discussed previously, the formation of ozone and PAN depend strongly on both the available NOx and VOC, and therefore the shape of the O3 and PAN isopleths is very similar. At a given level of VOC, there is a NOx mixing ratio at which a maximum amount of ozone is produced; an optimum VOC/NOx ratio. This optimum ratio, expressed as a molar ratio of VOC in ppbC to NOx in ppbN, is about 10 and corresponds to the ridgeline in an isopleth plot of maximum ozone as a function of the initial VOC and NOx mixing ratios. The relatively high production of HNO3 at low VOC/NOx ratios is shown in the HNO3 isopleth. At low VOC/NOx ratios, HO reacts predominantly with NO2 to form HNO3, and this process removes radicals and slows down ozone formation. Under these conditions (VOC sensitive), a decrease in the available NOx increases the rate and efficiency of ozone formation. At very high VOC/NOx ratios (NOx-sensitive), a decrease in NOx favors HO2 +

HO2, RO2 + HO2, and RO2 + RO2 reactions, which reduces ozone formation by removing HO2 and RO2 radicals from the system, and favors the formation of H2O2 (in the HO2 + HO2 reaction), as shown in the H2O2 isopleth. A  similar behavior is found in the formation of organic peroxides (in the RO2 + HO2, RO2 + CH3O2, and RO2 + RC(O)O2 reactions).The HO2 isopleth mixing ratio falls very sharply with increasing NOx, while the maximum HO mixing ratio reaches a maximum before falling again (see also Figure IX-B-1). The maximum HO concentration is a function of NOx and VOC; it tends to reach a maximum in the region were the maximum HNO3 and H2O2 mixing ratios are both at their lowest levels along the ridgeline. IX-B-3. Methods of Sensitivity and Process Analysis for Mechanism Assessment Sensitivity and process analyses are used to investigate the role that individual reaction parameters

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FIGURE IX-B-3. Simulated isopleths of the maximum mixing ratios of O3, peroxyacyl nitrate (PAN), HNO3, H2O2, HO, and HO2. The simulations were made with the RACM2 mechanism using the volatile organic compound (VOC) mixture discussed in Stockwell et al. (2012).

play in determining the overall behavior of a chemical mechanism. One measure of the relative importance of a reaction is the sensitivity of a simulated chemical concentration to incremental variations in the reaction’s rate parameter or product yields. Other methods used to assess reaction importance include comparisons of relative reaction rates at specific times (Calvert and Stockwell, 1983a) or comparisons of reaction rates integrated over the

simulation period (Leone and Seinfeld, 1985). These analyses can help set priorities for chemical mechanism development, including laboratory, environmental chamber, and field measurements. A sensitivity coefficient for a chemical mechanism is the change in a chemical concentration that results from a variation of any parameter in the mechanism. A local sensitivity coefficient is defined as the change in concentration, Ci, that results from

Chemical Mechanisms for Air Quality Modeling and Their Applications an incremental variation of a parameter, pj, (e.g., a rate coefficient; Rabitz et  al., 1983; Saltelli et  al., 2005). Local sensitivity coefficients for atmospheric chemical mechanism are functions of time. The local sensitivity coefficients may be represented as a matrix s(t) with elements  Ci (t) : s ij (t) =

∂Ci (t ) . ∂ pj

Reaction rate coefficients in an atmospheric chemical mechanism differ by several orders of magnitude, and this makes it difficult to compare local sensitivity coefficients for rate constants. Therefore, the sensitivity coefficients are weighted by the nominal values of the parameters, p j , and concentrations,  Ci (t) : sij* (t ) =

p j ∂Ci (t ) Ci (t ) ∂ p j

In general, local sensitivity coefficients as just defined are calculated from the system of differential equations that consists of chemical rate equations, f (f  =  dC/dt), and sensitivity equations. Sensitivity equations are obtained by differentiating the chemical rate equations with respect to the parameters pi (Gao, 1995), as in the following equation:

∂ (dC(t )/ dt ) ∂C(t ) = J[C(t ), t ; p(t )] × + ∂ pj ∂ pj Fj [C(t ), t ; p(t )]

where Fj = ∂ f / ∂ p j and the Jacobian matrix is defined by  J = ∂ f / ∂C . Direct methods integrate the combined system of differential equations to obtain the chemical concentrations and sensitivity coefficients as functions of time. One direct method is the adjoint method based on Green’s functions (Hwang et al., 1978; Dougherty et al., 1979; Rabitz et al., 1983). It has been applied to combustion (Dougherty and Rabitz, 1980)  and to a simple ethane pyrolysis chemical mechanism (Kramer et  al., 1981). A potential advantage of the adjoint method based on Green’s functions is that it can be applied to systems with time-dependent parameters through a functional approach (Demiralp and Rabitz, 1981a,

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1981b; Rabitz et  al., 1983). However, the chemical rate equations derived from an atmospheric chemical mechanism are extremely stiff (difficult to solve by numerical methods) due to vast differences in the lifetimes of chemical species. The coupled system of differential equations consisting of chemical rate equations and sensitivity equations is even stiffer. This stiffness makes the Green’s function method numerically unstable for atmospheric chemical mechanism simulations, and it can fail completely for them (Koda et  al., 1979; Dunker, 1981; Yang, 1995). The decoupled direct method (DDM) is an alternative method of local sensitivity analysis (Dunker, 1984; McCroskey and McRae, 1987). DDM has significant advantages over the Green’s function method in that it is much easier to use, it is numerically stable, and it has low computational costs (Yang, 1995). DDM has been used much more successfully for the modeling of atmospheric chemistry than the Green’s function method (McCroskey and McRae, 1987; Milford et  al., 1992; Gao et  al., 1995; Russell et  al., 1995; Yang et  al., 1995b, 1996). The method has been applied to the evaluation of the sensitivity of ozone concentrations to NOx and VOC emissions (Yang et  al., 1997). DDM splits the solution between the rate equations and the sensitivity equations. The numerical integration of the rate equations alternates with the sensitivity equations to find the concentrations and the sensitivity coefficients (Dunker, 1984). Both sets of differential equations are integrated with an implicit numerical solver for highly stiff equations such as a Gear-type ODE integration routine (Gear, 1971). Gao et al. (1995) used DDM to make a detailed analysis of the RADM chemical mechanism. DDM was used to calculate sensitivity coefficients for the response of ozone to the mechanism’s rate constants and product yields. They performed the analysis for several scenarios representing summertime surface conditions that ranged from rural to polluted-urban. Although the analysis of Gao et al. was performed some years ago, it remains very consistent with more recent studies. Table IX-B-1 shows the 20 most significant parameters that affect ozone concentrations, although the ranking of the reactions depended somewhat on the particular scenario. Table IX-B-1 shows that seven of the top 20 most significant parameters are the rate constants for the inorganic reactions from the standard ozone

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production mechanism: the photolysis of NO2 and O3 (to produce O(1D)), the back reaction of O3 with NO, the production of HO from O(1D) via reaction with H2O, the conversion of HO to HO2 by reaction with CO, the NO to NO2 conversion by reaction with HO2, and the termination reaction of HO with NO2 to give HNO3. Ozone concentrations are sensitive to the photolysis frequency for the formaldehyde reaction that produces HO2 radicals. Ozone formation is sensitive to the aromatic oxidation scheme, and, in the analysis of Gao et al. (1995), xylene was particularly important. Ozone formation was sensitive to the rate coefficient and the yield of peroxy radicals in the reaction of HO with xylene. Ozone was also highly sensitive to the yields of HO2 and higher dicarbonyl species or other highly reactive products of aromatic oxidation, and it was very sensitive to the production of peroxy radicals from the photolysis of aromatic oxidation products.

PAN plays a very important role in the chemistry leading to ozone formation. PAN is a reservoir for both acetyl peroxy radicals and NO2. Acetyl peroxy radicals convert NO to NO2, and photolysis of NO2 leads to ozone formation. Ozone concentrations are sensitive to reactions involving PAN (Table IX-B-1). Ozone concentrations are also sensitive to the yields of HO2 from alkenes. Finally, ozone is sensitive to the relative fraction of NO to NO2 conversion versus organic nitrate formation in the reactions of alkyl peroxy radicals with NO. IX-B-4. NOx and VOC Sensitivity and Indicator Ratios for NOx and VOC Three-dimensional air quality models are used to estimate ozone reduction isopleths from emissions inventories. However, it is very useful to have empirical indicators for VOC or NOx sensitivity that are based on measured ambient concentrations to corroborate model estimates.

TABLE IX-B-1 . T WENT Y HIGHLY SIGNIFICANT PAR AMETER S THAT AFFECT OZONE CONCENTR ATIONS, A S DETERMINED BY GAO ET AL. (1995) ACROSS SEVER AL SCENARIOS. THE CHECKED BOXES FOR “R ATE COEFFICIENTS” INDICATE THAT THE SIGNIFICANT PAR AMETER IS THE R ATE COEFFICIENT, BU T IF THERE IS A SPECIES UNDER “PRODUCT YIELD,” THEN THE SIGNIFICANT PAR AMETER IS THE YIELD OF THE LISTED SPECIES.

Parameter 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Xylene + HO → RO2 (organic peroxy radicals) HO + NO2 (+ O2) → HNO3 NO2 + hν → O3 + NO O3 + NO → NO2 + O2 CH2O + hν (+ 2 O2) → 2 HO2 + CO RO2 from primary alkenes + NO → NO2 + products RO2 from xylene + NO → NO2 + Dicarbonyl species from aromatics (DCB) Xylene + HO → RO2 + products RO2 from xylene + NO → NO2 + products CH3CO3 + NO → NO2 + CH3O2 + CO2 CH3CO3 + NO2 → PAN RO2 from ethene + NO → NO2 + products DCB + hν → RO2 PAN → CH3CO3 + NO2 HO + CO (+ O2) → HO2 + CO2 HO2 + NO → HO + NO2 O3 + hν → O(1D) + O2 O(1D) + H2O → 2 HO RO2 from C5–C6 alkanes + NO → NO2 + products RO2 from C3–C4 alkanes + NO → NO2 + products

Rate Coefficients

Product Yield RO2

X X X X X X X X X

HO2 DCB HO2

HO2 RO2 X X X X X NO2 NO2

Chemical Mechanisms for Air Quality Modeling and Their Applications Two indicator ratios were originally used as markers for simulated chemical activity and used to compare chemical mechanisms (Stockwell, 1986). The final [HNO3] /[H2O2] ratio for a simulated episode was compared with the initial [VOC]/ [NOx] ratio. The simulations showed that there was a strong positive relationship between the two ratios. Therefore, measurements of HNO3 and H2O2 concentrations have the potential to be used to estimate the initial [VOC]/[NOx] ratio of an air mass. Higher [HNO3]/[H2O2] ratios are associated with conditions above the ozone isopleth ridgeline where the conditions are VOC sensitive. However, it is not easy to measure H2O2 accurately. The correlation between the simulated final [H2O2] and final [CH2O] was examined as well. These two species were well correlated according to the three mechanisms evaluated. Greater levels of radical activity produced more H2O2 and CH2O, suggesting that measurements of these species could be incorporated into indicators for VOC or NOx sensitivity. Sillman (1995) proposed four indicators: [NOy], [O3]/([NOy]  – [NOx]), [CH2O]/[NOy], and [H2O2]/[HNO3], where NOy]  =  [NO2] + [HNO3] + 2[N2O5] + [NO3] + [organic nitrates] + [particulate nitrate]. These were based on air quality simulations made for a range of emission rates of anthropogenic and biogenic compounds. Sillman (1999) found that ozone formation was VOC sensitive when NOy was greater than 20 ppbV, the [O3]/ ([NOy]  – [NOx]) ratio was less than 7, [CH2O]/ [NOy] was less than 0.28, and [H2O2]/[HNO3] was less than 0.4. The ratios proposed by Sillman and Stockwell have been widely used and, along with other analysis, they can show limitations of conclusions derived from air quality models alone. For example, recently, Fujita et  al. (2013) applied these ratios within an analysis of ozone over Los Angeles. They examined how changes in precursor concentrations affect maximum ozone mixing ratios and formation rates in the South Coast Air Basin (SoCAB). They found that the rate of ozone formation over the SoCAB is greater than air quality model-derived estimates based on the current emissions inventory. Their measurement-based analysis indicated that the SoCAB may be close to the ozone isopleths ridgeline, and concurrent reductions in both NOx and VOC emissions may be required to further reduce ozone over Los Angeles.

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IX-C. COMPUTER ASSESSMENT OF THE EFFECTS O F [ H 2 O ] , T E M P E R AT U R E , A N D C L O U D S O N O 3 G E N E R AT I O N A series of simulations were made with the RACM2 mechanism (Goliff et  al., 2013)  over the temperature range 290–310 K and relative humidity range 0–100% (see Figures IX-B-4 and IX-B-5). These figures are plotted as isopleths with the axes linear in terms of the water vapor concentration and the temperature. The plots are somewhat fan shaped because the water vapor concentration at a given relative humidity is a function of temperature. This is due to the fact that the vapor pressure of water is a function of temperature. A  relative humidity of 100% at 310 K corresponds to a water vapor concentration that is more than three times that for 290 K. The heavier curved vertical lines show the water vapor concentration at constant relative humidity as a function of temperature. Figure IX-B-4A shows that the warmest and driest conditions produce the highest ozone mixing ratios. Maximum ozone mixing ratios increase with temperature for fixed water vapor and relative humidity, but the ozone mixing ratios decrease with increasing water vapor. This behavior is in contrast to HNO3, which has its highest maximum ratios for the warmest and wettest conditions (Figure IX-B-4B). The maximum HNO3 mixing ratios increase with temperature and water vapor. This behavior is somewhat surprising at first sight because increasing production of HO radicals is associated with higher water vapor concentrations due to the HO producing reaction of O(1D) with H2O. Figure IX-B-5A shows that, in general, this expectation is correct, and the highest maximum HO mixing ratios occur for the warmest and wettest conditions. The effect of temperature on the maximum HO2 mixing ratio is strong, whereas the effect of the H2O mixing ratio on HO2 at a given temperature is somewhat less than for HO. The net effect is that the ratio of HO2 to HO reaches a maximum at the coolest and driest conditions, and the ratio reaches its minimum at the warmest and wettest conditions simulated. This behavior is at least partially due to the enhancement of the HO2 + HO2 reaction by water vapor (Stockwell, 1995). A low HO2-to-HO ratio favors radical termination through the HO + NO2 reaction, and therefore the maximum HNO3 mixing ratio will peak under the warmest conditions with the greatest HOx

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the mechanisms of reactions influencing atmospheric ozone Water vapor (ppb) 0.00 1E+7 2E+7 3E+7 4E+7 5E+7 6E+7 310 150

170 160

Temperature (K)

305 150 160 150

300

150

50%

Ozone (ppb)

140

25% 140

295

100%

75% 130

130

120

120 110

290

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Temperature (K)

305

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25% 50% 300

4.5

4.5 4.0

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3.5 3.0

5.5

75% 100%

5.0

HNO3 (ppb)

4.0 3.5 3.0

290 FIGURE IX-B-4.

Figure IX-B-4. Maximum ozone (plot A) and maximum HNO3 (plot B) isopleths. The isopleths are plotted as functions of water vapor concentration and temperature. The heavier curved vertical lines represent constant relative humidity values of 25%, 50%, 75%, and 100%.

concentrations and under the wettest conditions with the lowest HO2-to-HO ratio, as seen in Figure IX-B-5B. Greater ozone mixing ratios are favored by high conversion rates of NO of NO2, so greater ozone mixing ratios will occur under the drier conditions with a high HO2-to-HO ratio (Figure IX-B-5C), but the overall HOx concentrations are important also. Thus, the maximum ozone concentrations are greatest at the warmest and driest conditions simulated, as shown in Figure IX-B-4A. The overall effect of clouds on tropospheric ozone is more difficult to assess. Clouds strongly affect ozone concentrations because they modify the distribution of actinic flux by scattering and absorbing solar radiation and by their aqueous-phase

chemistry, vertical redistribution of trace gases, NOx production by lightning, trace gas scavenging, and other processes (Barth et al., 2007). In this section, the focus is on the effect of clouds on actinic flux because this is the most direct influence that clouds have on ozone. Cloud-induced increases in actinic flux lead to increases in the photolysis frequencies, whereas cloud-induced decreases have the opposite effect. Increased photolysis frequency of NO2 increases the rate of ozone production, whereas decreased photolysis frequency of NO2 decreases the rate of ozone production. Therefore, the scattering solar radiation by clouds changes the distribution of actinic flux, and this affects tropospheric ozone production. Clouds may increase ozone production in some regions of the atmosphere and decrease it in others. Measurements made at the Earth’s surface show that actinic flux can be reduced under clouds, and this leads to lower photolysis frequencies. For example, Kim et al. (2007) measured spectrally resolved flux over a 4-day episode during the wintertime at Storm Peak Laboratory, Colorado. The first 2 days were cloudy, and the second 2 days were clear. They used the measured flux to derive rates of photolysis of NO2, photolysis of ozone to produce O(1D), and photolysis of CH2O (both the radical- and the molecular-producing reactions). They found that, for their conditions, clouds reduced the photolysis frequency of NO2 and the CH2O radical-producing reaction by approximately a factor of 2 when the cloudy and clear days were compared. Clouds reduced the photolysis frequency of ozone by a factor of 3. The photolysis frequency of the CH2O molecular-producing reaction was reduced by nearly a factor of 3 on the first cloudy day and by a factor of 2 on the second. The situation is much more complicated because clouds have a strong effect on the vertical distribution of actinic flux. Junkermann (1994) made measurements of the photolysis frequency of ozone producing O(1D) reaction with an instrumented hang glider. Junkermann flew within and above stratiform clouds when the ground was covered by snow. He found that the photolysis frequency above the cloud was a factor of 2 greater than it would have been without the cloud. Kylling et  al. (2005) made irradiance and down-welling actinic flux measurements at four sites located northeast of Norwich International Airport, in the United Kingdom. Simultaneous

Chemical Mechanisms for Air Quality Modeling and Their Applications

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FIGURE IX-B-5.

Maximum HO (plot A) and maximum HO2 (plot B) molar mixing ratios and their ratio (plot C) isopleths. The isopleths are plotted as functions of water vapor concentration and temperature. The heavier curved vertical lines represent constant relative humidity values of 25%, 50%, 75%, and 100%.

solar radiation measurements were made from aircraft over the same area. These measurements showed the expected reduction in actinic flux below and enhancement above the cloud, with the maximum down-welling flux occurring just below the cloud top. Greater cloud optical depth increased the reduction in actinic flux below the cloud and increased the actinic flux enhancement above the cloud. The scattering effects of clouds are reasonably well understood, and their radiative transfer model was able to simulate the total actinic flux to within about 10% for cloudless conditions. However, the spatial variability of the actinic flux was surprisingly high for conditions when the clouds appeared to be horizontally homogeneous. The downward actinic flux could vary by up to 40% between the ground stations that occupied an area of about 12 × 12 km2, whereas, over the same region, the total actinic flux above the clouds varied by about 11%. The three-dimensional distribution of spectral actinic flux is extremely variable in the presence of realistic cloud systems (Brasseur et  al., 2002). Brasseur et  al. used a radiative transfer model to examine the effects of convective clouds on spectrally resolved actinic flux and photolysis frequencies. They found that the distribution of atmospheric actinic flux is very inhomogeneous in and near clouds due to the distribution of water droplets and ice crystals. Radiation scattering by ice

crystals can make a significant contribution to the enhancement of the actinic flux. Variations in cloud microphysics, especially for ice crystals (shape, concentration, distribution, and phase function), lead to inhomogeneity in actinic flux. The level of enhancement depends on the extinction coefficient derived from the microphysics, and it depends on the solar zenith angle (Z). Optically thick portions of a cloud produce local maxima, and the maximum actinic flux occurs within a cloud near its top. The enhancement of actinic flux at the sides of a cloud, its top edge, and above ranged by factors between 2 and 5 compared to clear air. Measurements made by Thiel et al. (2008) confirm that three-dimensional cloud fields produce highly inhomogeneous distributions of actinic flux. Brasseur et  al. (2002) calculated photolysis frequencies of NO2, O3, and CH2O from the simulated actinic flux and found that their distribution is very inhomogeneous within and around a cloud. The inhomogeneous nature of clouds means that simple methods for regional or global models that apply a factor for cloud cover to account for the effect of clouds on photolysis frequencies are not very reliable. Brasseur et al. (2002) found that above clouds the increases in actinic flux could increase HO concentrations in the upper troposphere by 120–200%. The ozone production rates increase by 15% under these conditions. It is clear that on-site continuous measurement of the

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the mechanisms of reactions influencing atmospheric ozone

spectrally resolved actinic flux is important for air quality modeling due to the need for accurate photolysis frequencies (e.g., j(NO2), j[O(1D)]). I X - D . S I M U L AT I O N O F T H E E F F E C T S O F C H 2O, C O, S O 2, N O 3, A N D N 2O 5 O N O 3 G E N E R AT I O N Many atmospheric trace gases in addition to NOx and VOCs impact the formation of ozone, including CH2O, CO, NO3, N2O5, and SO2. The photolysis of ozone and formaldehyde (CH2O) are the two most important sources of HOx in the troposphere. One of the photolysis reactions of ozone produces an excited oxygen atom, O(1D), and a small fraction of these react to produce HO, as discussed previously. The production of HO through the photolysis of CH2O is a more complex process. CH2O has two photolysis reactions (see Chapter VIII):  one that produces molecular products and one that produces radical products. The radical products react to produce HO2 and CO: CH2O+ hν → H2 + CO CH2O + hν → H + HCO H + O2 (+ M) → HO2 (+ M) HCO + O2 → HO2 + CO Total: CH2O + hν → 2 HO2 + CO The HO2 radicals produce HO when they react with NO: HO2 + NO → HO + NO2 Although the photolysis of ozone is the major source of HO under clean to remote conditions, the photolysis of CH2O can be the major source under polluted conditions. Simulations were made using the RACM2 mechanism with 35 ppb initial NOx and 780 ppbC initial VOC with the same VOC mixture as before (Stockwell et  al., 2012)  except that an initial 1 ppb of HONO was added. These simulations illustrate the relative importance of the O3, CH2O, and HONO sources of HO for a polluted urban case. There were no emissions included in the simulations. Figure IX-B-6 shows the absolute and relative fraction of HO production from O3, CH2O, and HONO. Note that the production rate of HO from CH2O was calculated by multiplying

the production rate of HO2 from CH2O photolysis by the fraction of HO2 radicals that react with NO as a function of time. Notice that the figure shows HONO photolysis is the major source directly after sunrise (Zhang et al., 2012), and it is non-negligible during the first 2 hours of daytime (see Chapter III). The relative importance of HONO as an HO source will be directly proportional to its concentration produced through heterogeneous processes. Figure IX-B-6A shows that the photolysis of CH2O can produce HO at a greater rate than ozone photolysis during the morning and late afternoon hours. Ozone is the major HO source during the midday, and CH2O returns to being the major HO source during the late afternoon. This is due to the differences in the action spectra between ozone and CH2O for the photolysis reactions. Figure IX-B-6B shows the fraction of HO produced between O3, CH2O and HONO photolysis. The figure shows that the photolysis of CH2O produces a significant fraction of the HO during the day for polluted conditions. CH2O is very reactive for ozone production, with very high maximum incremental reactivity (MIR) and maximum ozone incremental reactivity (MOIR) values (Fuentes et  al., 2000; see Section IX-E. Valid characterization of the formation of CH2O through the oxidation of organic compounds in atmospheric chemical mechanisms is critical for their accuracy (see also Section VII-C-1). Figure IX-B-7 shows the fraction of HO reacting with RACM2 species for the polluted urban case simulation discussed earlier. For this polluted urban case, the fraction of HO that reacted with inorganic species (0.17) was much lower than the fraction that reacted with the organic species (0.83; see Figure IX-B-7). A significant fraction of the HO reacted with NO2 (0.07), and another significant fraction reacted with CO (0.05) to produce HO2: HO + CO (+O2) → HO2 + CO2 CO has some measure of control on [HO2]/ [HO], and higher ratios will lead to greater ozone production. The relative importance of the CO background increases under cleaner conditions where the fraction of HO reacting with CO will be even greater. Under cleaner conditions, the fraction of HO reacting with CO may approach 0.35 or more (Stockwell et al., 2012). However, the MIR and MOIR ozone reactivity of CO is much lower than that of most organic compounds because the

Chemical Mechanisms for Air Quality Modeling and Their Applications

485

FIGURE IX-B-6.

Plot A: Daytime production rates of HO initiated through the photolysis of O3 (dotted solid line), CH2O (dashed line), and the photolysis of HONO (solid line). Plot B: Relative fraction of HO initiated through the photolysis of O3 (dotted solid line), the photolysis of CH2O (dashed line), and the photolysis of HONO (solid line).

organic species decompose to produce additional organic peroxy radicals along with HO2, whereas CO has a relatively low rate coefficient for its reaction with HO. The organic oxidation products were very reactive, consisting of CH2O, higher aldehydes, dicarbonyl species, and the like. In principle, SO2 can increase ozone production through its conversion of HO to HO2 (Stockwell and Calvert, 1983a): HO + SO2 (+M) → HOSO2 (+M) HOSO2 + O2 → HO2 + SO3

The SO3 makes atmospheric aerosol through its reaction with H2O to produce sulfuric acid: SO3 + H2O → H2SO4 The production of HO2 radicals from SO2 oxidation results in NO-to-NO2 conversions that can lead to the production of some ozone. There have been few field measurement programs to examine this possibility. Klemm et al. (2000) made aircraft measurements of SO2, NOx, and VOC in a power plant plume over the former East Germany. Klemm et al. measured an SO2 mixing ratio of 114.5 ppb in

486

the mechanisms of reactions influencing atmospheric ozone CH2O CH3CHO Alkenes Alkanes Higher aldehydes Other Organics Organic peroxides Aromatics CO NO2 Other Inorganics Ketones MACR and MVK CH4 0.00

0.02

0.04 0.06 0.08 0.10 0.12 Fraction of HO reacting with species

0.14

0.16

FIGURE IX-B-7.

The gray bars show the total fraction of HO reacting with the compound indicated for the polluted urban scenario discussed in the text.

a power plant plume that was located to the south of the city of Leipzig. The power plant plume was a special case due to its extremely high SO2 mixing ratios, but simulations made with the RACM mechanism showed the SO2 increased the production of ozone by 4.6 ppbV (+21%) compared with a simulation without SO2. For the more typical cases modeled by Klemm et  al. with SO2 mixing ratios 30 ppbV or less, SO2 increased the peak ozone mixing ratios by only 1 ppb or less. Therefore, ozone will be affected significantly by SO2 only in locations where SO2 emissions are extremely high. Nighttime chemistry has the potential to affect ozone production primarily through its effect on the reactive nitrogen budget. One major difference between day- and nighttime chemistry is the nitrate radical, NO3. NO3 is produced through the reaction of NO2 with O3 (see Chapter III): O3 + NO2 → O2 + NO3 But NO3 has very high photolysis frequencies (much greater than NO2) that lead to very low daytime NO3 concentrations: NO3 + hν → O2 + NO NO3 + hν → O(3P) + NO2 However, at sunset, when the photolysis of NO3 stops, its concentrations increase in the absence of

NO. NO reacts rapidly with NO3 to titrate it back to NO2: NO3 + NO → 2 NO2 The NO3 radical reacts rapidly with alkenes and aldehydes (Atkinson, 2000; see also Chapter III). Its reaction with CH2O and other aldehydes is a source of nighttime peroxy radicals (Stockwell and Calvert, 1983b). NO3 reacts with acetaldehyde, CH3CHO, to produce acetyl peroxy radicals, and the acetyl peroxy radicals can react with NO2 to produce PAN (CH3C(O)O2NO2) (Cantrell et al., 1985): NO3 + CH3CHO (+O2) → HNO3 + CH3C(O)O2 CH3C(O)O2 + NO2 → CH3C(O)O2NO2 CH3C(O)O2NO2 → CH3C(O)O2 + NO2 The significance for ozone formation is that PAN is a reservoir for NO2 and peroxy radicals that can affect the daytime production of ozone. The production of dinitrogen pentoxide, N2O5, from NO3 has potentially a more significant effect on the reactive nitrogen budget. N2O5 exists in equilibrium with NO3 and NO2: NO3 + NO2 → N2O5 N2O5 → NO3 + NO2

Chemical Mechanisms for Air Quality Modeling and Their Applications Although the gas phase reaction of N2O5 with water vapor is very slow (k < 2  × 10−21 cm3 molecule−1 s−1), it reacts with liquid water on aerosols, cloud water droplets, and the like to produce HNO3 (Calvert and Stockwell, 1983a; Mozurkewich and Calvert, 1988): N2O5(g) + H2O (aq) → 2 HNO3(aq) The fraction of NOx lost could be in the range of several tens of percent for humid regional conditions (Mozurkewich and Calvert, 1988). Significant conversion of reactive nitrogen to HNO3 through the nighttime chemistry of N2O5 may reduce the formation of O3 on subsequent days. IX-E. MEASURES OF OZONE F O R M AT I O N R E A C T I V I T Y IX-E-1. Incremental Reactivity: MIR and MOIR Urban ozone control strategies involving VOC emission reductions based on ozone reactivity have the potential to be more effective and less expensive than control strategies based on the reduction of VOC by mass alone (Russell et  al., 1995). Ozone is produced by organic compounds through their production of organic peroxy radicals and HO2 that convert NO to NO2. Given the differences in the oxidation mechanisms of VOC, different organic compounds have differing potentials to produce ozone. Incremental reactivity (IR) is a measure of a compound’s ozone formation potential; it is the change in mass of ozone produced due to an incremental change in the mass of an organic compound for a defined scenario (Carter, 1994), IR =

mO3 − mO′ 3 mVOC − mVOC ′

where mO3 is the maximum mass of ozone per unit volume reached during the base simulation with a nominal mass of an organic compound, mVOC, and m′O3 is the maximum mass of ozone per unit volume reached during a simulation with an incremental change in an organic compound with a total of mass m′VOC. To calculate the aggregate incremental reactivity, mVOC and m′VOC may represent the total mass of the nonmethane organic compounds. IR is calculated for defined scenarios of NOx and organic

487

compounds usually by using chemical box models (Carter, 1994), although three-dimensional air quality models have been used (Russell et al., 1995; Martien et al., 2003; Hakami et al., 2004). Martien et  al. and Hakami et  al. implemented a version of direct sensitivity analysis in the CIT air quality model and used it to calculate the reactivity of 32 explicit and nine lumped compounds. They found that the absolute reactivity varied greatly over the model grid, but the calculated reactivity was relatively robust and consistent between different meteorological episodes and emission scenarios. On a relative basis, the relative VOC reactivities calculated with the CIT model and with those produced by Carter using a box model were similar. Two types of IR are used commonly to represent the reactivity of organic compounds; the MOIR and the MIR. MOIR and MIR calculations differ in that different NOx concentrations are used. IX-E-1.1. Maximum Ozone Incremental Reactivity: MOIR MOIR values are calculated at the NOx emission rate that maximizes the ozone concentration for a defined scenario of initial organic compound concentrations (Carter, 1994). First, the NOx emission rate for MOIR calculations is found. To find the MOIR NOx emission rate, a series of box model simulations are made and the maximum ozone mixing ratios are plotted (figure IX-E-1A). At the lower NOx levels, there is insufficient NOx to produce high O3 concentrations; that is, O3 production is NOx sensitive. The maximum O3 increases as the NOx emission rates are increased until the daily peak O3 mixing ratio reaches a maximum value, and, at some level, the daily peak O3 decreases because there are insufficient organic compounds to produce O3. This is a VOC-sensitive condition. Once the MOIR point is found, a series of simulations is made with that level of NOx; a slight increments of ±5–10% are made to each non methane organic compound concentration, and the MOIR incremental reactivities are calculated. IX-E-1.2. Maximum Incremental Reactivity: MIR To find the MIR point, the aggregate VOC incremental reactivity is calculated. At each NOx emission rate, the total initial organic compound concentrations are varied by a small increment (±5–10%), and the aggregate incremental reactivity is calculated as for the

488

the mechanisms of reactions influencing atmospheric ozone

FIGURE IX-E-1.

Plot A: Peak ozone concentrations as a function of the NOx emission rate for a base volatile organic compound (VOC) scenario. The vertical line marks the NOx emission rate that is used to calculate maximum ozone incremental reactivity (MOIR) values for this scenario. Plot B: Aggregate VOC incremental reactivity as a function of the NOx emission rate for a base VOC scenario. The vertical line marks the NOx emission rate that is used to calculate maximum incremental reactivity (MIR) values for this scenario.

MOIR. The MIR point is where the aggregate incremental reactivity for the defined scenario reaches its maximum (Figure IX-E-1B). The MIR point is found under very VOC-sensitive conditions (high NOx emission rates) where increases in organic compound levels more strongly affect O3 production. For the MIR point, a series of simulations are made with MIR NOx emission rate and with a slight increments of ±5–10% to each nonmethane organic compound concentration, and the MIR incremental reactivities are calculated. IX-E-1.3. Comparison of MOIR and MIR Values Figure IX-E-2 shows a comparison of MOIR and MIR values that were calculated with the

RACM2 mechanism using the VOC mixture described earlier. In general, MIR values are greater than MOIR values. This difference is due to the fact that MIR is calculated under very high NOx emission rates where O3 production is very limited by the available VOC. The MIR point is where the aggregate VOC incremental reactivity is the greatest. Compounds such as ethane, acetone, acetylene, alkanes, and ketones have low reactivity, whereas alkenes, aldehydes, aromatics, dicarbonyl compounds, and CH2O have higher MIR and MOIR values because they are very potent sources of HOx and organic peroxy radicals. The incremental reactivity values for the ketones and alkanes are on the low end of the

Chemical Mechanisms for Air Quality Modeling and Their Applications

489

Species: Ethane Acetone Acetylene Higher Alcohols Benzene Low Molecular Wt Alkanes Middle Molecular Wt Alkanes Methyl Ethyl Ketone Methanol High Molecular Wt Alkanes a-Pinene Ethanol Toluene Glyoxal Methacrolein Ketone Terminal Alkenes Internal Alkenes Formaldehyde Methylglyoxal Higher Aldehydes Dienes m,p-Xylene Isoprene d-Limonene Acetaldehyde o-Xylene Ethene Methyl Vinyl Ketone

MIR MOIR

0.0

0.5

1.0

1.5

2.0

Molar incremental reactivity, ∆O3, ppb/∆VOC, ppbC FIGURE IX-E-2.

Calculated incremental reactivities for selected organic species in the RACM2 mechanism. Note that in this figure the incremental reactivity values are presented as molar quantities; for regulatory purposes, incremental reactivity values are calculated in terms of mass.

scale due to their relatively lower rate coefficients for their reactions with HO. A  complete listing of MOIR and MIR values calculated from the SAPRC chemical mechanism is available from WPL Carter, University of California, Riverside (http://www.cert.ucr.edu/~carter/SAPRC/). Usually, MOIR and MIR reactivity estimates are based on single-day or short episode simulations but these can be misleading. Stockwell et al. (2001) found that VOCs estimated to be unreactive for a day-long simulation have a significant effect on ozone concentrations over several days due to the production of more reactive product species. They also found that, for scenarios ranging from 1 to 6 days, the estimated incremental reactivities increased as the length of the simulation period increased. IX-E-2. Photochemical Ozone Creation Potential (POCP) There are alternative methods to access the reactivity of organic compounds. One is the POCP index that was developed for use in Europe (Derwent et al.,

1996). The POCP of an organic compound is its contribution by unit mass to ozone formation relative to the contribution of ethene (Derwent and Jenkin, 1991; Andersson-Skold et al., 1992). A one-dimensional air quality model is used to make a base case simulation of ozone production within an air parcel as it is transported over a trajectory that may continue for several days. A simulation must be made for ethene and for each compound to be evaluated. A  series of simulations are made, with the emissions of the organic compound incremented by a small amount by mass one compound at a time. The amount may be arbitrary, but Derwent et al. (1996) used an amount that was equal to 4% of the total integrated organic compound emission of the base case simulation. The additional organic compound emissions may produce more ozone in comparison with the base case. The increase in ozone resulting from increased ethene emissions is compared with the increase in ozone from each organic compound. The comparison can be made at multiple points along the trajectory or for the ozone integrated over the entire trajectory. The POCP for a

490

the mechanisms of reactions influencing atmospheric ozone

given compound can be calculated according to the following equation: POCPi =

O3Test _ Case − O3 Base _ Case O3 ethene − O3 Base _ Case

× 100

Saunders et al. (1997) found that toluene, n-butane, ethene, and the xylenes together account for more than 33% of the POCP of European emissions. POCP values have been calculated with several mechanisms including the MCM MCMv3.1 and SAPRC-07 (Saunders et  al., 1997; Derwent et  al., 2007a, 2010). Derwent et al. (2010) compared POCP values calculated with MCMv3.1 and SAPRC-07 for 116 organic compounds. In their 2010 study, Derwent et al. compared the POCP values for alkanes, alkenes, aldehydes, ketones, aromatics, oxygenates, and halocarbons and found that the values between the two mechanisms compared very well for most of the compounds. A  comparison of MIR values calculated by Carter (2009) and POCP values derived by Derwent et al. (2010) is shown in Figure IX-E-3. The POCP values have been multiplied by 0.0876 to normalize the POCP value for ethylene (100) to the MIR value for ethene (8.76). It is seen that the ozone creation potentials as estimated by the two methods show a correlation, as one might expect. Both methods estimate highest ozone formation potential for aromatic

hydrocarbons and alkenes and lowest potential for the chloroalkanes. However, the POCP values show a somewhat lower sensitivity to change in structure than the MIR values, as evidenced by the fact that the slope of the least squares fit to the data in the figure is less than unity. This difference is not unexpected since very different pollution scenarios are used in the two methods (i.e., a 3-day transport across Europe at relatively low [NOx] in the POCP method vs. the MIR with reactions at urban-like pollution levels). Both measures of ozone formation potential have proved valuable in air quality planning. I X - F. M O D E L I N G O F S E C O N DA R Y I N O R G A N I C AEROSOL AND ORGANIC A E R O S O L F O R M AT I O N Widely used air quality models such as CMAQ, WRF/Chem, PMCAMX, EMEP, CHIMERE, and GEOS-Chem all have modules that simulate aerosol generation (Byun and Ching, 1999; Bey et al., 2001; Grell et al., 2005; Byun and Schere, 2006; Hodzic et al., 2010a, 2010b; Environ, 2011). High concentrations of aerosol particles may cause serious human health problems (Gurjar et al., 2008; Hodzic et al., 2010a, 2010b; Zhang et al., 2013), so the modeling of aerosol concentrations is important. The effects of atmospheric aerosol range from the urban scale, such as the degradation of air quality over megacities (Molina et al., 2004) to

POCP × 0.0876

10

1

Alkanes Chloroalkanes Alkenes Aromatic Hydrocarbons Alcohols Ethers Aldehydes Ketones Organic Acids Esters Least Squares Fit 1:1 Line

0.1

0.01 0.01

0.1

1

10

MIR, gm O3 gm–1 compound FIGURE IX-E-3.

Plot of the Photochemical Ozone Creation Potential (POCP; Derwent et al., 2010) versus maximum incremental reactivity (MIR) values (Carter, 2009) as calculated for the various families of organic compounds. The POCP values have been multiplied by 0.0876 to normalize the ethene value (100) to the MIR value of 8.76.

Chemical Mechanisms for Air Quality Modeling and Their Applications the global scale (IPCC, 2007). Aerosol optical properties and their concentrations have a major impact on climate through their effects on the balance and distribution of atmospheric radiation (see Section I-H). The need to represent aerosol processes is great in air quality, climate, and other atmospheric models. Figure IX-F-1 shows an overall scheme for the formation of aerosol. There are emission sources of primary aerosols and gas phase NOx, NH3, SO2, and VOC. The gas phase chemistry of these compounds produces some secondary products with low equilibrium vapor pressures that can partition into the aerosol phase. Because secondary aerosols are produced through the photooxidation of NOx, SO2, and VOC, there is a great amount of overlap between the chemical mechanisms that affect ozone and atmospheric aerosols. Common atmospheric aerosol components include sulfate, nitrate, soot, and organic compounds. Atmospheric measurements show that the mass of organic aerosols varies from less than 1  µg m−3 in the remote troposphere to greater than 70 µg m−3 in highly polluted regions (Coe et  al., 2006; Sun et al., 2010). The mass of organic aerosols can often be greater than the total aerosol mass of sulfate, nitrate, and soot (Zhang et al., 2007; Jimenez et al., 2009; Lee-Taylor et al., 2011). However, chemical and physical modules for the modeling of inorganic and organic aerosol concentrations are in a relatively primitive state of development compared with the ozone modules. Secondary inorganic aerosol concentrations depend on the gas phase production of particulate precursors, mainly nitric acid (HNO3) and sulfuric acid (H2SO4). The oxidation of NO2 to HNO3 and SO2 to H2SO4 by the HO radical were discussed previously in this book. These homogeneous gas phase reactions are (Stockwell and Calvert, 1983a):

SO2 (g) + HO (g) → HOSO2 (g) HOSO2 (g) + O2 (g) → SO3 (g) + HO2 (g) SO3 (g) + H2O (g) → H2SO4 (l) NO2 + HO → HNO3 The gas phase oxidation rate of SO2 has a maximum rate of a few percent per hour (Calvert and Stockwell, 1983a). The gas phase oxidation rate of NO2 is usually faster than the oxidation of SO2 because the rate coefficient for the HO + NO2 reaction is about 10 times greater than the rate coefficient for the HO + SO2 reaction (Sander et  al., 2011; Atkinson et  al., 2007). Another important difference between the gas phase oxidation mechanisms is that the oxidation of sulfur dioxide does not terminate the HOx radical chain (Stockwell and Calvert, 1983a). Gas phase chemistry produces new aerosol particles. The aqueous-phase oxidation rates of SO2 due to its reaction with H2O2 can be extremely fast, and this adds mass to the existing aerosol particles (Seinfeld and Pandis, 1998). SO2 dissolves in the aqueous phase to produce bisulfate, HO(SO)O−, and the bisulfate is oxidized by hydrogen peroxide (Jacobson, 1999): SO2 (aq) + H2O (aq) → HO(SO)OH HO(SO)OH (aq) → HO(SO)O− + H+

Gas-phase Emissions

Gas-Phase aerosol precursor production

(aq)

HO(SO)O− (aq) + H2O2 (aq) + H+

(aq)

+ → SO 2− 4 (aq) + H2O (aq) + 2H

(aq)

N2O5 is present at nighttime and undergoes heterogeneous reaction with H2O on the

Low equilibrium Vapor pressure products

Condensation to aerosol phase

Meteorology humidity temperature FIGURE IX-F-1.

(aq)

(aq)

Direct particle emission Emissions

491

Overview of the formation of inorganic and organic secondary aerosols.

Particle growth, condensation, etc.

492

the mechanisms of reactions influencing atmospheric ozone

surfaces of aerosol particles and water droplets to give HNO3: NO3 (g) + NO2 (g) ⟷ N2O5 (g) N2O5 (g) + H2O (l) → 2 HNO3 (l) Aerosol particles are formed from the gas phase when the precursors form low vapor products. The vapor pressure of sulfuric acid is sufficiently low that it condenses to form particles very rapidly after it is produced. However, the vapor pressure of HNO3 is too high to form new particles in the absence of ammonia (NH3). Ammonia reacts with HNO3 and H2SO4 to produce ammonium nitrate, ammonium bisulfate, and ammonium sulfate: NH3 (g) + HNO3 (g) ⟷ NH4NO3 (s) NH3 (g) + H2SO4 (l) → NH4HSO4 (s) NH3 (g) + NH4HSO4 (s) → (NH4)2SO4 (s) The partitioning of these inorganic compounds and the relative fractions of the ions in the aerosol have been modeled using equilibrium models (Kim et al., 1993a,b; Kim and Seinfeld, 1995; Kuhns et al., 2003). For example, little of the ammonia reacts with nitric acid until most of any sulfuric acid present has reacted. Furthermore, the formation of solid ammonium nitrate aerosol is a strong function of temperature and relative humidity. Measurements of atmospheric organic aerosols find that these aerosols are composed of minimally oxygenated organic compounds and highly oxygenated organic compounds (Zhang et  al., 2007; Lee-Taylor et al., 2011). Primary organic aerosols are emitted directly from sources or formed from unreacted emissions of compounds with low volatility, and these primary organic aerosols are more likely to consist of minimally oxygenated organic compounds (Donahue et al., 2006; Zhang et al., 2007; Lee-Taylor et al., 2011). SOA is produced through the oxidation of organic compounds from both anthropogenic and biogenic sources (Heisler and Friedlander, 1977; Pandis et al., 1991; Turpin et al., 1991; Pandis et al., 1992; Griffin et al., 1999; Aumont et al., 2000; Claeys et al., 2004; Goldstein and Galbally, 2007; de Gouw and Jimenez, 2009; Lee-Taylor et  al, 2011). The oxidation of organic compounds may produce products with lowequilibrium vapor pressures that condense to form SOAs. SOAs consist primarily of highly oxygenated organic compounds.

In general, in organic aerosols there is greater mass of highly oxygenated organic compounds than minimally oxygenated organic compounds (Zhang et  al., 2007; Lee-Taylor et  al., 2011). This is true of organic aerosols found in urban source regions, and the relative mass fraction of highly oxygenated organic compounds increases relative to the minimally oxygenated organic compounds downwind of source regions. It follows that SOAs consist of highly oxygenated organic compounds that are formed through photochemistry and other chemical processes as an air mass ages (Zhang et al., 2007; de Gouw et al., 2008; Lee-Taylor et al., 2011). A very large number of organic compounds have been measured in organic aerosol (Ketseridis et al., 1976; Middlebrook et  al., 1998; Hamilton et  al., 2004; Lee-Taylor et al, 2011). The organic chemical mechanisms for the formation of the SOA components are congruous with the mechanisms that focus primarily on ozone (Lee-Taylor et al., 2011). There is high uncertainty in representation of the chemistry of secondary organic formation because much of the aerosol is formed from organic compounds with more that six carbon atoms (Volkamer et al., 2006; Hallquist et al., 2009; Hodzic et al., 2010a, 2010b; Lee-Taylor et al, 2011; Zhang et al., 2013). Air quality models have used several approaches to estimate the production of SOA. One of the first methods developed was based on inserting aerosol yields derived from chamber studies into gas phase chemical mechanisms (Odum et al., 1996; Griffin et al., 1999; Pankow et  al., 2001; Jang and Kamens, 1999; Kroll et  al., 2005; Lee-Taylor et  al., 2011). However, the measured yields could be dependent on the chamber conditions. Models with this simple parameterization severely underestimated organic aerosol for conditions that ranged from remote to polluted urban atmospheres (Heald et al., 2005; Volkamer et al., 2006). The agreement between model simulations and measurements has been improved by more explicit consideration of the chemistry of organic compounds with greater numbers of carbon atoms (C11–C25) and the vapor pressure of their products (Lee-Taylor et al., 2011). The formation of SOA may be more strongly affected by the range of physical and chemical properties from the hundreds of emitted organic species than by ozone (Goodwin et al., 2001; Johnson et al., 2004c). Derwent and co-workers developed a photochemical trajectory model that included larger chemical mechanisms for Europe to examine the impact of including a more complete chemical mechanism on air quality model simulations that includes 124

Chemical Mechanisms for Air Quality Modeling and Their Applications primary nonmethane anthropogenic VOC and three biogenically emitted organic compounds (Derwent et  al., 1996, 1998; Jenkin et  al., 2002). The model’s chemical mechanism was derived from the Master Chemical Mechanism, version MCM v3.1 (Johnson et  al., 2004c). Johnson et  al. (2004c) modified the trajectory model to include the formation of SOA. Gas-to-particle transfer reactions were added to the model to represent the partitioning between the gas and aerosol phases for about 2,000 oxygenated organic compounds. The gas-to-particle transfer reactions were defined for those chemical species with an estimated boiling point of greater than 450 K according to previous studies (Jenkin, 2004; Johnson et al., 2005). The absorptive partitioning theory of Pankow (1994) was used to derive the gas-to-particle transfer reactions. In Pankow’s theory, a species’ partitioning is defined by its thermodynamic equilibrium between the gas phase and its absorption in a condensed organic phase, with an associated equilibrium coefficient that is known as a partitioning coefficient. The analysis of Johnson et al. (2004c) showed that the regional contribution of VOC oxidation was an important source of organic aerosol mass; however, the model did not account for the magnitude of the mass of SOA found in field measurements. Robinson et  al. (2007) developed a volatility basis set (VBS) approach that improves on the Pankow approach. The model compounds are grouped by vapor pressure in decadal intervals. The initial volatility distribution was derived from laboratory dilution experiments conducted on diesel exhaust (Robinson et al., 2007). The total initial aerosol mass is scaled to either the observed primary aerosol concentrations or the primary emissions inventory. Every time an organic molecule of one of these compounds is oxidized (e.g., by HO), the vapor pressure of the product is assumed to drop by a decadal factor. Additional adjustments to the VBS scheme to account for variations in conditions have been developed by Grieshop et al. (2009). The VBS approach has been implemented in a number of models ranging from box models (Dzepina et al., 2009) to three-dimensional air quality models including WRF-Chem, PMCAMx, CHIMERE, and EMEP. These models have been used to simulate organic aerosol over the United States (Shrivastava et al., 2008; Murphy and Pandis, 2009), over Mexico City (e.g., Hodzic et al., 2010a, 2010b; Tsimpidi et al., 2010), and over Europe (Fountoukis et  al., 2011; Bergström et  al., 2012). Overall, it was found that simulations made with the VBS approach produced

493

greater aerosol mass concentrations that were more consistent with observations than did the earlier organic aerosol parameterization methods. For example, Lee-Taylor et  al. (2011) developed a highly explicit gas phase photochemistry box model, the Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A), for the simulation of SOA. GECKO-A follows the VBS approach in which the chemical mechanism accounts for the oxidation of precursor hydrocarbons to produce product species. Vapor pressures are computed for the products and used to estimate the partitioning of the products between the gas and particles phases. They found for Mexico City that GECKO-A could simulate the magnitude and diurnal time series for both primary organic aerosol and SOA. They found that most of the SOA, more than 75%, was produced from alkanes with higher carbon numbers. Most of the remaining SOA was produced from aromatic compounds. More chemical research is needed to better characterize the chemical composition of SOAs and to quantify aerosol growth as air masses move downwind of urban regions. For example, the simulations of Lee-Taylor et  al. (2011) show that the plume-integrated mass of aerosol particles continues to grow for several days as the particles are transported from source regions. These model simulations showed that growth continued even as the plume was diluted, in contrast to previous studies (Pankow, 1994; Odum et al., 1996; Donahue et al., 2006). The aerosol mass increase was due to gas phase production of low-volatility products. Major factors in the rate of the simulated growth are the SARs that characterize the rate of alkoxy radical fragmentation relative to alkoxy radical isomerization for the larger alkoxy radicals; however, there is insufficient available laboratory data to accurately determine these ratios for the larger compounds. IX-G. POTENTIAL DEFICIENCIES OF AT M O S P H E R I C C H E M I S T R Y MECHANISMS AS IMPLEMENTED IN AIR Q UA L I T Y   M O D E L S The chemistry of HO and HO2 radicals is the basis of daytime photochemical oxidation of organic compounds and ozone production, and the scientific understanding of this chemistry is generally considered to be strong. However, Chen et al. (2010b) made comparisons of HO and HO2 measurements

494

the mechanisms of reactions influencing atmospheric ozone

and zero-dimensional box model simulations that suggest there may be gaps in the understanding of this HOx chemistry. Chen et al. compared HO and HO2 mixing ratios with measurements made during the TexAQS II Radical and Aerosol Measurement Project (TRAMP-2006) that occurred in 2006. Simulations were made with the model using six chemical mechanisms (RACM, CB05, LaRC, SAPRC-99, SAPRC-07, and MCMv3.1). The zero-dimensional box model used the same simultaneously measured data to calculate the HO and HO2 mixing ratios diagnostically from each mechanism. They found that the models underestimated the HO and HO2 mixing ratios by an average mean of between 20% (CB05) and 40% (SAPRC-99). The response of the HO2/HO ratio to changes in NO mixing ratios is very different between the measurements and simulations (see Figure IX-G-1). All the mechanisms predict similar trends, but the model predicts a much higher HO2/HO ratio at low NO mixing ratios than do the measurements, and the model predicts a much lower ratio at the high NO mixing ratios for all six mechanisms. The modeled and measured ratios are similar only when NO mixing ratios are between about 0.1 and 1 ppbV. Although errors in measurements are one potential explanation, it is possible that there are unmeasured

species and omissions or errors in the HOx chemistry in the mechanisms that account for these differences (see also the discussion in Section IV-A-1). Stone et al. (2012) made a detailed comparison of HO and HO2 concentrations simulated by models and those measured by several methods employed in a number of field measurement campaigns. They found that improvements are needed both in the measurement methods for HOx and the chemical mechanisms. Measurement methods are subject to possibly unknown interferences for environments that range from the open ocean, through remote forests, to highly polluted urban sites. There remain serious questions on the accuracy of HOx measurements. For example, Stone et al. (2012) found that in forested areas with high biogenic VOC concentrations and low NOx, the agreement between simulations and measurement for HO2 varied between campaigns. They attributed the variability to possible interferences in HO2 measurements. However, they recommend new studies with currently available instrumentation to make long-term measurements of HO2. Stone et  al. reported that field measurements show that there may be important unknown HO sources in regions with high biogenic VOC concentrations and low NOx. They showed that even

1000

Modeled Ratio Measured Ratio

[HO2]/[HO]

100

10

1 0.1

1

10

100

[NO], ppbV FIGURE IX-G-1.

The figure shows measured and modeled HO2/HO ratio as a function of the ambient NO concentration. The observed ratio is an average of averaged 10-minute data points. The modeled ratio is an average obtained by averaging results from a model using six mechanisms: CB05, RACM1, LaRC, SAPRC-99, SAPRC-07, and MCMv3.1. Reproduced with permission from Chen et al. (2010b). Copyright 2010, Elsevier.

Chemical Mechanisms for Air Quality Modeling and Their Applications models that are constrained with measured VOC underpredicted HO. In contrast, polluted urban regions have high NOx and VOC concentrations. In these regions, there appear to be missing HO sinks. In this case, the mechanisms may lack sufficient detail to describe the oxidation of higher molecular weight VOCs and their (often) multifunctional products. Aromatic chemistry is a major example of chemistry that contributes to this uncertainty. Relatively few air quality chemical mechanisms include halogen chemistry, but in coastal areas and over the open ocean, simulations show that bromine, iodine, and their products (e.g., IO and BrO) have significant effects on HOx chemistry (Stone et al., 2012). Faxon and Allen (2013) have recently reviewed the chlorine chemistry that may be necessary to model urban regions. A better understanding of the heterogeneous chemistry of halogens, the HO2 uptake coefficients for aerosols, and the heterogeneous rates of HO and HO2 loss is needed. It seems clear that this chemistry needs to be added to more air quality chemical mechanisms. This addition will require improvements in the inventories of the oxygenated VOCs and halogen species. Future field studies made in coastal urban regions will need to measure halogens and oxygenated VOCs species to test model simulations. Another problem with the ozone simulated by three-dimensional air quality models is that, in general, the models underestimate the response of ozone concentrations to changes in emissions or meteorology (Gilliland et al., 2008). This mismatch between simulations and observations makes the development of emission control strategies much more uncertain. Usually, emission control strategies are developed by the simulation of a base case derived from a field study. In principle, NOx and VOC emissions are reduced in the model, ozone reduction isopleths are plotted, and the necessary reductions determined. Of course, the process is much more complicated and involves balancing the concerns of many stakeholders, but the point is that the process depends on the correlation between the modeled and actual response of ozone concentrations to emission changes. Gilliland et  al. showed that the CMAQ model underestimated O3 reductions that were observed after implementation of NOx emission reductions across the eastern half of the United States. Although modeling problems are often blamed on emission inventories, it appears in this case that the models underestimated long-range transport of O3 and precursors. The transport

495

underestimates differed between CMAQ with CB04 and SAPRC99 mechanisms, and this indicates especially that there may be problems representing tropospheric chemistry. A related problem with three-dimensional air quality models is that simulated ozone mixing ratios do not have the same variability as measurements (Stockwell et al., 2013). Lower sensitivity to conditions and lower ozone variability appear to be a common problem in air quality modeling. For a given episode, simulated ozone mixing ratios usually show lower variability than measurements. The variability in ozone measurements and three-dimensional air quality model forecasts were compared (Stockwell et  al., 2013). Ozone measurements were made in the Paso del Norte Region of Texas and Mexico for a January day, and the Comprehensive Air Quality Model with Extensions (CAMX; Environ, 2011) was used to make the simulations. The CB04 and CB05 mechanisms were used for this study (Stockwell et al., 2012). It was found that although the simulated ozone mixing ratios were higher than the measurements, the simulations tracked the measurements reasonably well. A more significant difference between the measurements and the simulations was that low or zero measured ozone mixing ratios occurred much more frequently than in the simulations (Figure IX-G-2). Some of the greater ozone variability in the ozone measurements may be due to the comparison of point measurements with the model’s grid average. Highly localized sources of NO could titrate ozone near the measurement site. The model displays a bias toward the prediction of a much narrower range of ozone mixing ratios than the observations. It is possible that this bias is partially traceable to the mechanism development and testing process (Stockwell et  al., 2013). Atmospheric chemical mechanisms that are used for air quality simulations are heavily tested against environmental chamber data, where chemical concentrations are very high, and for intensive field studies that are conducted during high ozone episodes. There is very little testing of the mechanisms for more average day-to-day conditions and for the concentrations found in rural-suburban regions (where about 100 million people live in the United States). The US National Atmospheric and Oceanic Administration’s program on air quality forecasting provides one new source of air quality model evaluation for a much wider range of atmospheric conditions (NOAA Air Quality Forecast Website).

496

the mechanisms of reactions influencing atmospheric ozone 30 Observations CB4 CB5

Probability, percent

25 20 15 10 5 0 0–5

5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45 Ozone interval, ppbV

FIGURE IX-G-2.

Histogram expressed as the probability of measuring or modeling the plotted range of ozone mixing ratios. The ozone measurements were made in the Paso del Norte Region of Texas and Mexico for a January day, and the air quality modeling was performed with the CAM-X model using either the CB04 or the CB05 mechanisms. The simulations and data are from Stockwell et al. (2013).

IX-H. FUTURE DEVELOPMENT O F AT M O S P H E R I C C H E M I C A L MECHANISMS FOR AIR Q UA L I T Y M O D E L I N G There has been very great progress made in the development of atmospheric chemistry mechanisms over the past 30 years, but there still remain serious gaps in our chemical knowledge. The oxidation mechanisms of aromatics and most biogenically emitted compounds are critical research needs. Part of the problem is the very large number of products formed by these compounds. The peroxy radical-peroxy radical reactions are important under low NOx conditions, but they are difficult to study. For remote regions, peroxy radical unimolecular chemistry may also play a role, but data on these processes under atmospheric conditions are extremely limited (e.g., Peeters et  al., 2009; Crounse et  al., 2011). More generally, there is a need for better data on the pressure and temperature dependence of rate coefficients, especially for the organic compounds, but also for key inorganic reactions as discussed in Section IX-B-3. Better

data for the calculation of photolysis frequencies are needed as well. As the mechanisms are extended to model secondary aerosol formation, the simulation of aerosol precursors adds a new set of considerations for mechanism developers. Better data for heterogeneous reactions, vapor pressure, Henry’s law coefficients, and similar physical data will be required to model the formation of secondary aerosol formation from the fundamental physics and chemistry. Modelers will also need to explore how to better condense explicit chemical mechanisms to develop sufficiently compact mechanisms for three-dimensional air quality models. There now exists a highly qualified scientific community that is capable of making much progress in improving the mechanisms and models. Nonetheless, there is a question on how accurate the mechanisms need to be for regulatory purposes. Many high-level managers in air quality agencies worldwide are asking this question. Further research is needed to assure that the best science is applied to the continuing and future problems that require deeper understanding of atmospheric chemistry.

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AU T H O R   I N D E X

van Aardenne, J., 18, 31, 412 Aardenne, J.V., 49 Abbatt, J.P.D., 116, 117, 120, 121, 126, 127, 128, 129, 130, 317, 323, 378, 383, 439, 457, 471, 491 Abd el Aal, Y., 454 Abe, K., 174 Acerboni, G., 83, 142, 143 Achad, M., 395, 398, 405 Acker, K., 108, 435 Ackerman, A.S., 482 Ackermann, R., 454 Adachi, H., 122 Adams, A., 175 Adams, J., 175 Aguilar, E., 6, 7 Ahammed, Y.N., 30 Aiken, A.C., 491 Aikin, K., 21, 320 Aird, R.W.S., 143 AIRPARIF, 18 Akagi, K., 23 Akimoto, H., 31, 60, 104, 175, 203, 208, 209, 344, 371, 414 Alaghmand, M., 322 Alam, M.S., 83, 101 Albaladejo, J., 226, 262, 384, 385, 393, 396, 398, 400, 403 Albert-Lévy, 16, 17, 18 Alcock, W.G., 341, 361 Alconcel, L.S., 338, 341, 342 Alecu, I.M., 384, 385 Alexander, B., 127, 377, 412, 413 Alexander, M., 175 Alfarra, M.R., 491, 492 Alicke, B., 108, 175, 454 Allan, J.D., 491, 492 Allen, D., 472 Allen, D.T., 471, 495 Aloisio, S., 315 Alpert, J.C., 5, 39 Altieri, K.E., 471 Alvarado, A., 102, 141 Alvarez-Idaboy, J.R., 154, 155, 163

Alyea, F., 173 Amann, M., 44 Amedro, D., 119 Amimoto, S.T., 63 Ammann, M., 125 Ammannato, L., 66, 482, 483 Ammar, R., 125 Anastasi, C., 341, 342 Andersen, M.P.S., 42, 43, 83, 249, 287, 328, 329, 358, 391, 399, 405 Andersen, S.O., 43, 44 Andersen, V.F., 358, 398 Anderson, J.G., 5, 39, 40, 98, 101, 102, 113, 173, 322, 492 Anderson, L.G., 486 Anderson, P., 48, 409, 411 Anderson, R., 385 Anderson, W.P., 490 Andersson-Skold, Y., 489 Andino, J.M., 368 Andreae, M.O., 492 Andreasen, Ø., 154, 155, 157 Andreoni, J.F., 330 Andres, R.J., 18, 412 Anfossi, D., 16 Anglada, J.M., 107, 348 Angove, D.E., 469 Anlauf, K., 23 Antiñolo, M., 226, 262, 393 Anwyl, P., 173 Apel, E.C., 334, 384, 491, 492, 493 Appel, B.R., 454 Appel, K.W., 119, 473 Aranda, A., 136, 142, 386, 387, 388, 392, 417 Archibald, A.T., 109, 110 Arey, J., 69, 99, 102, 103, 105, 111, 112, 113, 133, 136, 141, 146, 147, 150, 151, 154, 157, 158, 159, 165, 320, 321, 345, 346, 347, 352, 353, 354, 355, 366, 367, 371 Argüello, G.A., 393, 437 Ariya, P.A., 48, 83, 107, 384, 408, 409, 411 Armerding, W., 63, 64 Arnold, F., 5

550

Author Index

Arnold, I., 63 Arnold, J.R., 472 Arnold, S.R., 444, 445, 446 Arroyo, M.C., 328 Artaxo, P.E., 490, 492 Arya, B.C., 30 Asatryan, R., 104, 346 Aschmann, S.M., 69, 99, 100, 101, 102, 103, 105, 111, 112, 113, 130, 133, 136, 141, 143, 145, 146, 147, 148, 150, 151, 153, 157, 158, 159, 160, 161, 163, 165, 208, 296, 299, 320, 321, 345, 346, 347, 352, 353, 354, 355, 357, 358, 366, 367, 371, 385 Aschmutat, U., 174 Ase, P., 336 Asher, W.E., 492 Ashfold, M.N.R., 64 Astolos, R.J., 277 Atherton, C.S., 44 Atkinson, D.B., 338 Atkinson, R., 46, 47, 61, 67, 69, 71, 72, 83, 85, 96, 97, 98, 99, 100, 101, 102, 103, 109, 110, 111, 112, 113, 116, 117, 119, 120, 121, 122, 123, 126, 130, 133, 136, 138, 141, 143, 145, 146, 147, 148, 150, 151, 153, 154, 157, 158, 159, 160, 161, 163, 165, 192, 203, 205, 208, 211, 218, 295, 296, 297, 298, 299, 300, 301, 302, 304, 305, 316, 317, 318, 319, 320, 321, 322, 324, 325, 326, 327, 329, 332, 333, 334, 335, 337, 339, 340, 341, 342, 345, 346, 347, 349, 350, 352, 353, 354, 355, 357, 358, 360, 366, 367, 371, 376, 378, 379, 380, 383, 384, 385, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 425, 454, 464, 467, 471, 473, 484, 486, 491 Atlas, E.L., 48, 52, 55, 320, 492 Aucott, M.L., 377 Aumont, B., 466, 467, 468, 469, 491, 492, 493 Aunan, K., 31 Avery, M.A., 21 Averyt, K.B., 114 Avise, J.C., 472 Avzianova, E.V., 83 Ayers, G.P., 377 Azzi, M., 469 Bacak, A., 339 Bächmann, K., 108, 435 Badia, A., 119 Baeza, M.T., 153, 155, 391 Bagley, J.A., 130 Bahreini, R., 491, 492 Bais, A., 39, 44, 65, 66, 119, 482, 483 Bajaj, P.N.B., 384 Baker, A., 491, 492, 493 Baker, J., 157 Baldasano, J.M., 119, 467, 472, 484 Baldocchi, D., 467, 484 Baldwin, R.R., 350, 352, 354 Bale, C.S.E., 136, 333, 334 Balkanski, Y., 125 Ball, J.C., 83, 328, 329, 339, 341, 342, 358, 372, 387, 399, 437

Ball, S.M., 63 Balla, R.J., 247, 341, 349, 362, 363, 372 Ballabrera-Poy, J., 412 Ballesteros, B., 226, 384, 385 Baltaretu, C.O., 211 Baltensperger, U., 490, 491, 492 Bandow, H., 371 Bandy, B., 483 Bardwell, M.W., 339 Bares, S.R., 101, 110 Barker, J.R., 116, 117, 120, 121, 126, 127, 128, 129, 130, 323, 350, 352, 354, 360, 361, 378, 383, 439, 457, 471, 491 Barnaba, F., 66, 482 Barnes, I., 93, 108, 122, 123, 141, 161, 163, 262, 274, 322, 323, 329, 341, 352, 353, 358, 359, 364, 365, 366, 367, 372, 384, 393, 394, 395, 397, 398, 399, 405, 406, 407, 408, 409, 410, 418, 419, 420, 421, 436, 437, 454, 473 Barone, S.B., 436, 450, 451 Barrick, J.D., 119 Barrie, L., 48 Barrie, L.A., 377, 409, 410 Barth, M., 66, 385, 483 Barth, M.C., 482 Barthe, Ch., 482 Bartkiewicz, E., 341 Bartoe, J.-D.F., 427 Basco, N., 122 Bassis, J., 175 Bastian, V., 406, 408 Bates, D.R., 4, 35 Bates, T.S., 127, 492 Batt, L., 360 Battin-Leclerc, F., 343 Bauer, D., 63, 64 Bauerle, S., 442, 443, 444, 445, 446 Baughcum, S.L., 360, 361 Baulch, D.L., 46, 47, 61, 99, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 208, 316, 317, 318, 322, 324, 325, 326, 327, 329, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 376, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 471, 491 Bayes, K.D., 118 Bazhin, N.M., 60 Bean, B.D., 352, 353, 364 Bechara, J., 103, 107 Beck, L., 467 Beck, M., 103 Becker, E., 145 Becker, K.H., 67, 99, 101, 102, 103, 107, 108, 122, 123, 146, 147, 161, 163, 211, 218, 262, 322, 323, 329, 341, 352, 353, 358, 359, 364, 365, 366, 367, 372, 380, 384, 406, 407, 408, 409, 410, 436, 437, 464, 473, 489 Beddows, D.C.S., 491 Bedjanian, Y., 406 Beekmann, M., 490, 492 Behnke, W., 178, 184 Beig, G., 30 Beine, H.J., 48, 410 Bejan, I., 93, 141, 384, 397, 398, 399, 400, 405, 454

Author Index Bell, N., 44 Bell, T.G., 412 Benesch, J.A., 44 Benitez-Billalba, J., 287, 398 Benkelberg, H.-J., 366, 367 Benkovitz, C.M., 377, 412 Benson, S.W., 327, 350, 360, 361, 409 Benter, Th., 93, 125, 141, 145, 398, 400, 405, 454 Bérces, T., 354, 366, 367, 410, 443, 445, 446 Berg, M., 44 Berges, M.G.M., 449 Bergin, M.S., 479, 487 Bergmann, D.J., 44 Bergström, R., 499 Berhanu, T.A., 287, 358, 398 Berho, F., 384, 385 Bernard, F., 226 Berndt, T., 136, 151, 153, 106, 108, 414 Bernhard, M.J., 385 Berntsen, T.K., 7, 31 Berresheim, H., 174 Bertman, S.B., 123, 124 Bessagnet, B., 31, 49, 490, 492 Beukes, J.A., 83, 142, 143 Bey, I., 44, 466, 490, 492 Bhartia, P.K., 22 Bhave, P.V., 126 Bias, A.F., 427 Bierbach, A., 384, 407, 408, 409, 410, 436, 437 Biermann, H.W., 454 Biessing, A.M.B., 122, 357 Bilde, M., 122, 357, 358, 372, 384 Bingham, P., 44 Birdsall, A.W., 211, 330 Blacet, F.E., 5, 25, 118, 446, 474 Blackadar, A.K., 15 Blake, D.R., 48, 52, 55, 469 Blake, N.J., 173 Blanco, M.B., 93, 274, 287, 393, 394, 395, 397, 398, 399, 405 Blank, D.A., 443 Blaszczak-Boxe, C., 119 Blitz, M.A., 119, 344, 347, 349, 365, 442, 443, 444, 445, 446 Bloss, C., 352, 353, 364, 469 Bloss, W.J., 83, 101, 175, 469 Blumthaler, M., 66, 482, 483 Bock, W., 336 Boddenberg, A., 108, 435 Bodeker, G., 23 Bofill, J.M., 348 Boge, O., 136, 151, 153, 366, 367 Bogumil, K., 10, 62 Bohdan, V., 492 Bohn, B., 65, 352, 353, 358, 359, 364, 365, 366, 367, 454, 473, 483 Bojkov, R.D., 18 Boldi, R., 173 Bolzacchini, E., 150 Bond, T., 31, 49 Bopp, L., 412

551

Borbon, A., 490, 492 Borén, H.B., 44 Borrmann, S., 491, 492 Bortoli, D., 30 Bottenheim, J.W., 48, 103, 107, 334, 409, 410, 411, 467, 484 Boullart, W., 203, 206, 301, 303, 308 Boutonnet, J.C., 44 Bouvier-Brown, N.C., 124 Bower, K., 491, 492 Bower, K.N., 491 Bowman, F., 323, 492, 493 Bowman, J.M., 323 Boyd, A.A., 131, 133, 326 Boynton, G., 175 Bozzelli, J.W., 104, 346 Braams, B.J., 323 Bradley, C.E., 5, 25, 44 Braers, T., 101, 110 Brandenburger, U., 174 Brasseur, A-L., 66, 483 Brasseur, G.P., 9, 13, 14, 15, 18, 39, 40, 48, 57, 121, 123, 412, 413 Brauers, T., 83, 141, 174, 331, 352, 353, 358, 359, 364, 365, 366, 367, 473 Brault, J.W., 174 Braun, W., 352 Bravo, I., 386, 387 Bravo-Perez, G., 154, 155, 163 Breitenbach, L.P., 100, 102, 105, 111, 327, 341, 342, 347, 352, 353, 364, 372 Brewer, N.W., 4 Brice, K.A., 6 Bridier, I., 339, 341 Brietbeil, F.W., 124 Brigante, M., 48 Brimblecombe, P., 25 Broadbelt, L.J., 468 Brock, C.A., 63, 64, 492 Brockmann, K.J., 101, 102, 103, 107, 141 Brost, R.A., 472, 473 Brown, H.C., 304 Brown, R.H., 60 Brown, S.S., 126, 127, 141, 331, 377 Browne, E.C., 124, 323 Bruce, T., 401 Brueckner, G.E., 427 Brühl, C., 412 Brune, W.H., 5, 39, 40, 124, 174, 175, 468, 470, 471, 472, 493, 494 Brunke, E., 19, 22, 23, 25 Bruschi, M., 150 Buck, R.C., 83, 437 Buehler, B., 384 Buehler, R., 6, 17, 19 Bugarin, A., 319 Bui, T.P., 320 Buisson, H., 3 Buitrage, A.A., 430, 441, 449, 450 Buja, L., 18

552

Author Index

Bullister, J.L., 38, 39, 44 Burkholder, J.B., 42, 101, 116, 117, 120, 121, 123, 124, 127, 128, 317, 323, 378, 383, 436, 437, 439, 442, 443, 444, 445, 446, 449, 450, 451, 453, 454, 456, 457, 471, 491 Burnett, C.R., 173 Burnett, E.B., 173 Burrows, J.P., 10, 48, 62, 118, 119, 327, 409, 411 Burton, C.S., 471 Busarow, K.L., 486 Buszek, R.J., 360 Butcher, G., 173 Buth, R., 387 Butkovskaya, N., 117, 320, 323 Butler, J.E., 360, 361, 362 Butler, T.M., 44, 208, 331, 490 Byun, D., 466, 490 Byun, D.W., 490 Caballero, J.N., 287, 398 Cabañas, B., 136, 142, 153, 155, 157, 170, 249, 391 Cabañas-Galan, B., 398 Cofala, J., 44 Cafmeyer, J., 492 Cahill, T.M., 44 Cai, C., 175, 472 Cainey, J.M., 377 Calamari, D., 44, 377 Callahan, K.M., 371, 372 Callies, J., 174 Calogirou, A., 158 Calvert, J.G., 37, 44, 47, 60, 65, 66, 67, 71, 72, 83, 85, 87, 89, 93, 94, 96, 97, 98, 99, 103, 105, 109, 111, 118, 119, 121, 122, 124, 125, 128, 129, 130, 132, 133, 136, 138, 141, 146, 147, 148, 150, 151, 153, 154, 155, 157, 158, 159, 160, 161, 162, 163, 172, 173, 175, 178, 180, 184, 185, 189, 192, 203, 205, 297, 211, 218, 226, 228, 230, 249, 251, 253, 262, 265, 266, 274, 276, 277, 278, 280, 287, 295, 296, 297, 300, 305, 316, 317, 318, 320, 326, 327, 328, 336, 338, 342, 347, 349, 350, 352, 355, 356, 360, 361, 362, 363, 364, 372, 373, 374, 380, 381, 383, 384, 385, 386, 387, 389, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 403, 405, 412, 413, 425, 427, 428, 429, 431, 432, 433, 434, 435, 437, 438, 439, 440, 442, 446, 447, 448, 449, 451, 452, 453, 454, 456, 457, 459, 464, 467, 468, 469, 473, 478, 485, 486, 487, 491 Calvert, P., 412 Cammas, J.P., 21 Campbell, D.E., 476, 481 Campbell, M.J., 174 Campbell, S., 329, 330, 331, 391 Campolongo, F., 479 Campos, T., 21 Campuzano-Jost, P., 380 Camredon, M., 83, 101, 491, 492, 493 Canagaratna, M.R., 490, 491, 492, 493 Canosa-Mas, C.E., 109, 130, 131, 133, 136, 143, 145, 150, 155, 157, 159, 163, 170, 332, 333, 334, 384 Cantrell, C.A., 65, 66, 99, 118, 119, 175, 410, 412, 427, 428, 486

Cao, J.M., 479 Cao, R., 175 Cape, J.N., 454 Cappa, C.D., 320, 493 Caralp, F., 347, 349, 350, 351, 352, 353, 354, 355, 358, 359, 361, 363, 364, 365, 366, 367, 368, 369, 370 CARB, 6, 27, 28 Cardenas, B., 175 Carey, T., 109, 110 Carl, S.A., 119 Carlier, P., 83, 93, 332 Carlson, M.J., 107 Carlton, A.G., 471, 473 Carpenter, L.J., 48, 175 Carslaw, D.C., 175 Carslaw, N., 175 Carlson, M.J., 107 Carlton, A.G., 119 Carpenter, L.J., 48, 175, 377, 409, 411 Carr, R.W., 330, 333, 357 Carr, S., 119, 155, 159, 368, 369, 493 Carr, T., 83, 101 Carslaw, N., 175 Carter, W.P.L., 58, 109, 110, 147, 154, 208, 320, 347, 350, 352, 357, 358, 359, 360, 364, 365, 369, 454, 465, 467, 468, 469, 471, 472, 487, 490 Carver, G.D., 377 Cass, G.R., 322, 492 Cassanelli, P., 320, 347, 350, 351, 352, 353, 354, 355, 358, 359, 364, 365, 366, 370 Catoire, V., 330, 341, 408 Cautenet, S., 482 Ceacero, A.A., 385 Ceacero-Vega, A.A., 384 Cee, V.J., 102 Chagnon, C.W., 421 Chambers, R.M., 133 Chameides, W., 6, 44 Chameides, W.L., 31, 208 Chan, C.Y., 174 Chan, E., 6, 19, 20, 21, 22, 24 Chan, L.Y., 174 Chan, S.I., 118, 119 Chandra, S., 22, 23 Chandru, S., 472 Chang, J.S., 41, 465, 468, 472, 473 Chang, W., 48 Chang, W.L., 126 Chant, S.A., 347, 350, 352, 353, 359, 365 Chapman, S., 4, 33 Charalampidis, P.E., 493 Cheema, S.A., 372 Chen, C., 323 Chen, J., 31, 330 Chen, L., 142, 151, 153, 343 Chen, S., 174, 468, 470, 471, 472, 493, 494 Chen, W., 124 Chen, X.H., 322 Chen, Y., 174

Author Index Chen, Z., 108, 114, 468, 470, 471, 472, 493, 494 Cheng, H.R., 469 Cheung, A.S.-C., 9 Chevallier, E., 350, 359, 360, 362, 363, 365, 367 Chew, A.A., 100, 101, 102, 103, 143, 148, 150, 153, 347, 371 Chiappero, M.S., 437 Chin, M., 421, 423 Ching, J.K.S., 466, 490 Chiou, P., 472 Chipperfield, M.P., 444, 445, 446 Choo, K.Y., 350 Chow, J.M., 320, 492 Chowdhury, P.K., 448 Christensen, L.K., 122, 358, 385, 399 Christoph, E.H., 44 Chu, H.W., 472 Chung, M.Y., 101, 102, 103 Chuong, B., 322 Ciccioli, P., 467, 484 Cicerone, R.J., 5, 36 CITEPA, 50, 51, 52, 53 Claeys, M., 492 Clain, G., 29 Clapp, L.J., 47 Clark, J.H., 431 Clark, M., 36 Clarke, J.S., 98, 492 Claude, H., 21, 22, 23 Clemitshaw, K.C., 173 Clifford, D., 48 Clifford, G.M., 48, 157 Cocker, D.R., 492, 493 Coe, H., 491, 492 Cofala, J., 44 Coffman, D., 127 Cohen, R.C., 58, 122, 139, 141, 323, 331 Cole, J.A., 343 Coleman, P.J., 492 Collins, D.R., 491 Collins, D., 492, 493 Collins, E.M., 345 Collins, W.J., 44, 58 Colomb, A., 490, 492 Comes, F.J., 63, 64, 65, 67 Cometto, P.M., 287, 398 Comrie, A.C., 15 Concepción Parrondo, M., 33 Conley, A.J., 48 Connell, B.T ., 319 Converse, A.D., 101 Conway, T.J., 412 Cook, D.J., 143 Cook, D.R., 124 Cooper, I.A., 63 Cooper, O.R., 6, 20, 21, 22, 24 Coquet, S., 377 Corchnoy, S.B., 105, 150, 157, 352, 353, 354, 355, 366, 367 Cornu, A., 3 Costa, C.S., 469

553

Cottrell, L., 491, 492 Couch, T., 320, 321 Cowley, L.T., 342 Cowling, E.B., 208 Cox, R.A., 46, 47, 60, 61, 99, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 208, 316, 317, 318, 319, 320, 322, 324, 325, 326, 329, 332, 333, 334, 335, 337, 338, 339, 340, 341, 342, 343, 346, 350, 351, 352, 353, 354, 355, 358, 359, 364, 365, 366, 367, 368, 369, 370, 376, 377, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 471, 491, 493 Cozic, J., 127, 377 Cramer, G.R., 44 Crawford, A., 173 Crawford, J., 66, 427 Crawford, J.H., 119, 174, 468, 470, 471, 472, 493, 494 Cremer, D., 100, 101, 102 Crippa, M., 490, 492 Croes, B., 479, 487 Crosley, D.R., 173 Crounse, J.D., 124, 208, 322, 331, 346, 496, 471 Crowley, J.N., 46, 47, 61, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 208, 295, 316, 317, 318, 319, 322, 324, 325, 326, 327, 329, 331, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 376, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 443, 445, 471, 491 Croxatto, G., 454 Crutzen, P.J., 5, 6, 35, 44, 48, 377, 409, 412, 421, 467 Cruz-Torres, A., 163 Cubison, M.J., 491 Cuevas, C.A., 385, 396, 398, 400, 403 Cuevas, E., 22, 23 Cundall, R.B., 442, 443 Cunnold, D., 173, 178 Curley, M., 101, 102 Cusickk, R.D., 83 Dabdub, D., 48, 119, 126, 472 Daby, E.E., 28 Dachs, J., 412 Dacol, D., 479 Daële, V., 226, 287, 387, 398 Dagaut, P., 287, 316, 317, 339, 340, 341 Dallmann, G., 108, 435 Dall’Osto, M., 491, 493 Danilin, M.Y., 37 D’Angiola, A., 31, 49 D’Anna, B., 48, 125, 154, 155, 157, 332 Darnall, K.R., 58, 347, 352, 364, 369 Dasgupta, P.K., 454 Dassau, T.M., 334 Daugey, N., 326 Daum, P.H., 108 Davidson, J.A., 119, 428 Davies, A.S., 442, 443 Davies, J., 22, 23, 33 Davies, T.J., 493 Davis, A.C., 347, 352, 355

554

Author Index

Davis, A.J., 276, 352, 353, 355, 362, 363, 364, 367 Davis, D.D., 421, 423, 454 Davis, H.F., 128, 129 Davy, P., 492 De Baar, H., 412 De Backer, H., 21 De Carlo, P.F., 490, 491, 492, 493 Dearth, M.A., 385 DeBruyn, W., 44 Delannoy, A., 66, 483 Delgado-Arias, S., 18, 412 Demerjian, K.L., 44, 98, 101, 102, 113, 175, 322, 491, 492 Demeter, A., 443, 445, 446 Demiralp, M., 479 Deng, D.Q., 174 Deng, L., 328, 330 Deng, W., 349, 362, 363, 364, 367, 371, 372 Deng, Z.Z., 31 Denier van der Gon, H.A.C., 490, 492, 493 Dennis, R.L., 472 Dentener, F.J., 44 Derbyshire, D., 371, 372 Derwent, R.G., 6, 17, 21, 22, 24, 29, 37, 42, 44, 58, 67, 121, 130, 132, 161, 163, 172, 173, 178, 180, 184, 185, 189, 265, 297, 300, 305, 316, 317, 320, 326, 327, 328, 347, 349, 350, 352, 355, 356, 360, 361, 362, 363, 364, 372, 373, 374, 380, 381, 383, 400, 401, 468, 469, 489, 490, 493 DeSain, J.D., 385 Destriau, M., 316, 341 Deuel, H.P., 471 Devolder, P., 347, 349, 350, 361, 362, 363, 364, 365 Dhanya, S., 384 Di Carlo, P., 174 Diaz-de-Mera, Y., 386, 387, 388, 392 Dibble, T.S., 276, 349, 352, 353, 355, 358, 361, 362, 363, 364, 367, 371, 372, 399, 405 Dibbs, J.E., 127 Dick, A.L., 377 Dickerson, R.R., 119 van Dijk, C.A., 414, 417 Dillon, T.J., 208, 295, 328, 329, 331, 333, 334 Dimitriades, B., 467, 470 Ding, A., 31 Dinis, C.M.F., 352, 353, 358, 359, 364, 365, 366, 367, 473 Divita, F., 467 Dlugokencky, E., 178 Dobson, G.M.B., 3, 31 Dóbé, S., 354, 366, 367, 410, 443, 445, 446 Docherty, K., 491, 492 Docherty, K.S., 321, 491 Dodge, M.C., 467, 468, 470, 471 Doenig, G., 36 Dogru, A.H., 479 Doherty, R.M., 44 Dommen, J., 492 Donahue, C.J., 377 Donahue, N.M., 98, 100, 101, 102, 103, 105, 113, 322, 323, 491, 492, 493

Donaldson, D.J., 48 Donders, C., 125 Donner, B., 123, 352, 353, 358, 359, 364, 365, 366, 367, 473 Dore, C.J., 492 Dorn, H.-P., 83, 141, 173, 174, 331, 352, 353, 358, 359, 364, 365, 366, 367, 473 Dorokhov, V., 33 Doskey, P.V., 124 D'Ottone, L., 63, 64 Dougherty, E.P., 479 Douglass, A., 32 Douglass, A.R., 58 Doussin, J.F., 83, 93, 130, 132, 133, 148, 151, 155, 157, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 171, 332, 350, 358, 359, 360, 362, 363, 365, 367, 490, 492 Dransfield, T., 323 Drevet, J., 44 Drew, R.M., 350, 359, 360, 364 Drewnick, F., 490, 491, 492 Drougas, E., 333 Drozd, G.T., 98, 105 Du, Y., 343 Dubé, W., 127, 141, 377 Dubé, W.P., 331 Dulitz, K., 295 Dunk, R.M., 377 Dunker, A.M., 454, 479 Dunlea, E., 491, 492 Duplissy, J., 491 Dupont, J.-C., 490, 492 Dupuis, M., 439 Durand-Jolibois, R., 151, 155, 157, 332, 350, 358, 359, 360, 362, 363, 365, 367 Dyke, J.M., 104 Dzepina, K., 491, 492, 493 Eberhard, J., 347, 352, 353, 354, 360, 369, 374 Eddingsaas, N.C., 331 Edelbuttel-Einhaus, J., 385 Eder, B., 466, 473, 490 Edwards, G.D., 65, 119, 427 EEA, 49, 50, 51 Ehhalt, D.H., 174 Ehn, M., 491 Eickel, K.H., 60 Eirich, F.R., 352 Eisele, F.L., 174, 175 Elend, M., 352, 353, 358, 359, 364, 365, 366, 367, 473 Elkins, J.W., 412 Ellermann, T., 122, 318, 341, 372 Ellingsen, K., 44 Ellis, D.A., 328, 329, 330 Elrod, M.J., 211, 318, 329, 330 Elshorbany, Y.F., 454, 455 Elste, T., 174 Elterman, L., 427 Emeis, S., 473 Emmerson, K., 175 Emmons, L., 18

Author Index Emrich, M., 442, 443 Enami, S., 333, 334 Endo, Y., 316 Engel, A., 423 Engelsen, O., 483 English, A.M., 316 Ennoble, J., 427 Environ, 466, 470, 471, 490 EPA, 6, 28, 29, 30, 52, 56 Erdakos, G.B., 492 Erickson, D., 49 Erickson, D.J., 377 Ervens, B., 471 Eskes, H.J., 44 Eskola, A.J., 104 Esmond, J.R., 9 Espada, C., 320, 321 Esponosa Ruiz, J.L., 398 Estes, E., 468 Estupiñan, E.G., 449 Etyemezian, V., 492 Evans, M.J., 48, 469 Evtyugina, M.G., 469 Ezell, A.A., 384 Ezell, M.J., 384 Fabry, C., 3 Fajer, R.W., 453 Falbe-Hansen, H., 417, 420 Falgayrac, G., 347, 349, 350, 364, 365 Fall, R., 49 Faloona, I.C., 124, 174 Faluvegi, G., 6, 7 Fan, S.-M., 48, 410 Fantechi, G., 150, 384, 385 Faraji, M., 472 Faria, B.V.E., 48 Farley, R.D., 482 Farman, J.C., 4, 5, 32 Farmer, D.K., 122, 124 Fast, J.D., 454, 490, 492, 493 Faxon, C.B., 495 Fehsenfeld, F.C., 28, 127, 174, 208, 492 Feilberg, A., 122, 357 Feister, U., 17, 18, 19 Feld, M., 352 Fels, M., 108 Felton, C.C., 174, 175 Felton, H.D., 174 Fenske, J.D., 99, 101, 102, 104, 106, 108 Ferenac, M.A., 276, 349, 352, 353, 355, 364, 371, 372 Ferm, M., 454 Fernandes, R.X., 343, 344 Fernández-Gómez, M., 385 Ferronato, C., 358, 372, 384, 388 Fida, M., 443, 445 Fiddler, M.N., 322 Field, B., 466, 490 Fink, E.H., 99

555

Finlayson-Pitts, B.J., 48, 125, 384 Fioletev, V., 32 Fiore, A.M., 44, 466, 490 Fischer, C.A., 65 Fischer, H., 208, 331 Fisher, D.A., 41 Fishman, J., 6, 23 Fittschen, Ch., 119, 347, 349, 350, 361, 362, 363, 364, 365, 366, 368 Fitzgerald, R.M., 495, 496 Flad, J.E., 445 Flagan, R.C., 323, 492, 493 Flaud, P.-M., 326 Fleming, Z.L., 29 Flittner, D., 427 Flocke, F.M., 48, 63, 64, 124, 320, 334, 410 Flocke, S.J., 65, 427 Flugge, M.L., 136 Flynn, L., 36 Flynn, J., 174, 468, 470, 471, 472, 493, 494 Flynn, M.J., 491 Fockenberg, Ch., 362, 363 Foley, K.L., 495 Force, A.P., 63 Ford, H.W., 118 Ford, K.M., 320, 321, 324 Forkel, R., 473 Forst, W., 358, 359 Fost, G.J., 175 Fountoukis, C., 493 Fourier, J., 54, 56 Fournet, R., 343 Fowler, A., 3 Fox, D.J., 320 Fox, M.M., 5, 25, 44 Fox, R.W., 127, 128 Fracheboud, J.-M., 358, 449 Fraire, J.C., 393 Frame, C.L., 174 Francisco, J.S., 315, 323, 347, 352, 355, 360 Frank, H., 44 Franklin, J., 44 Fraser, P., 173, 178 Freeman, D.E., 9 Frenzel, A., 347, 349, 361, 362, 363 Freutel, F., 490, 492 Fridlind, A.M., 482 Fried, A., 174, 175, 334 Friedl, R.R., 116, 117, 120, 121, 126, 127, 128, 129, 130, 378, 383, 317, 323, 439, 454, 457, 471, 491 Friedlander, S.K., 492 Frie b, U., 48, 409, 410, 411 Frith, S.M., 32 Fritz, B., 360 Fröhlich, M., 21 Froidevaux, L., 22, 23, 33, 36 Frost, G., 65, 427, 466, 473, 490 Frost, G.J., 31, 49, 101, 119, 454, 456 Fry, J.L., 141, 331

556

Author Index

Fuchs, H., 141, 174, 331 Fuentes, J.D., 467, 484 Fujimoto, T., 23 Fujiwara, S., 174 Fujita, E.M., 476, 481 Fukuda, M., 175 Fukui, Y., 124 Gäb, S., 108, 435 Gaedtke, H., 118 Gaffney, J.K., 108 Gai, Y., 83, 93, 387 Gaimoz, C., 155, 157 Galan, B.C., 143 Galano, A., 154, 155 Galbally, I.E., 22, 23, 492 Gallego-Iniesta Garcia, M.P., 398 Galvez, O., 490 Gamborg, E., 122 Gandini, A., 443 Ganzeveld, L., 208, 331 Gao, D., 468, 471, 479 Gao, D.F., 479, 480 Gao, J., 31 Gao, R., 31 Gao, R.-S., 20 Gao, W., 31 Gao, Y.D., 384, 385 Garcia, R.R., 40 Gardiner, B.G., 4, 5, 32, 427 Gardiner, E.P., 118 Gariner, B., 119 Garland, E.R., 352, 353, 364 Garrick, J., 427 Garzón, A., 385, 386, 387, 388 von der Gathen, P., 33 Gauss, M., 44 Gavriloaiei, T., 125, 454 Gawinski, G.R., 363, 363, 364 Gbor, P.K., 490 Ge, M., 83, 93 Ge, M.F., 388 Gear, C.W., 479 Gefen, Z., 154, 155 Geiger, H., 101, 102, 103, 352, 353, 358, 359, 364, 365, 366, 367, 473, 489 Geng, F., 31 Geng, F.H., 31 George, C., 48, 125, 492 George, L.A., 174, 175 Gernandt, H., 33 Geron, C., 49, 331, 467, 484 Gertler, A.W., 490 Gery, M.W., 468, 470, 471 Geyer, A., 108, 454 Gezelter, J.D., 443 Ghalaieny, M., 71, 83, 84, 93 Ghigo, G., 343 Ghosh, B., 319

Ghude, S.D., 30 Giebel, B., 108 Gierczak, C.A., 341, 342 Gierczak, T., 123, 436, 442, 443, 444, 445, 446, 449, 450, 451 Gieskes, W., 412 Gilge, S., 6, 21, 22, 24 Gilles, M.K., 64, 65, 185, 437, 449 Gillett, R.W., 377 Gilliland, A.B., 495 Gilman, J.B., 124 Gilpin, T.M., 65 Giri, B.R., 388, 391, 407, 409, 410 Giroir-Fendler, A., 125 von Glasow, R., 48, 377, 409, 411 Glasson, W.A., 360, 361 Glasius, M., 158 Glaude, P.-A., 343 Gleason, J.F., 413 Glowacki, D.R., 343, 344 Gobbi, G.P., 66, 482, 483 Goddard, A., 103 Godin-Beekmann, S., 32 Godowitch, J.M., 473, 495 Goldan, P.D., 492 Golden, D.M., 116, 117, 120, 121, 126, 127, 128, 129, 130, 317, 323, 350, 352, 354, 360, 361, 378, 383, 409, 439, 457, 471, 491 Goldstein, A.H., 7, 17, 58, 124, 322, 492 Goliff, W.S., 58, 66, 465, 467, 468, 470, 471, 472, 473, 475, 477, 478, 481, 482, 484, 495 Golombek, A., 423 Gomez, N., 347, 349, 361 van der Gon, D., 31, 49 Gonçalves, M., 119 Gondwe, M., 412 Gong, H., 141 Gong, S., 413 Gong, S.L., 377 González, J., 107 González, S., 226 Gonzalez-Garcia, N., 391 Goodman, M.A., 143, 145, 157 Goodsite, M.E., 48, 409, 411 Goodwin, J.W.L., 492 Goody, R., 7, 11 Götz, F.W.P., 4 Gouget, H., 18 de Gouw, J.A., 52, 55, 124, 492 Graber, W., 6, 17, 19 Graedel, T.E., 49, 377 Graham, B., 492 Graham, R.A., 128 Grahek, F.E., 28 Gramsch, E., 454, 455, 490 Granier, C., 18, 31, 49, 316, 317, 326, 327, 338, 339, 341 Graus, M., 139, 141 Gravestock, T., 175 Greater London Authority, 18

Author Index Green, M., 492 Greenblatt, G.D., 63 Grell, G.A., 466, 473, 490 Grennfelt, P., 489 Grieshop, A.P., 491, 493 Griffin, R., 491 Griffin, R.J., 491, 492 Griffioen, E., 65, 119, 427 Grimvall, A.B., 44 Gröbner, J., 483 Gros, V., 490, 492 Grosjean, D., 100, 111, 123 Grosjean, E., 100, 111, 123 Gross, M., 36 Grosse, G-.J., 60 Grossenbacher, J., 320, 321 Grossmann, D., 108, 352, 353, 358, 359, 364, 365, 366, 367, 435, 473 Gu, L., 467, 484 Gu, Y., 357 Guenther, A., 31, 49, 63, 64, 124, 331, 467, 484 Guillemot, C.J., 8 Guillet, J.E., 447 Guo, H., 469 Gurjar, B.R., 490 Guschin, A., 122 Gustin, M.S., 44 Gutbrod, R., 100, 101, 102 Gutman, D., 360, 361, 362 Gutzler, D., 173 Guyon, P., 492 Haagen-Smit, A.J., 5, 25, 26, 44 Haberkorn, S., 449 Hack, W., 352, 353, 355, 364 Hackett, P.A., 443, 449 Hadler, A.B., 211 Haeffelin, M., 490, 492 Haffner, D.P., 33 Hahn, J., 492 Hakami, A., 487 Hakola, H., 112, 113 Hales, C.H., 41 Hall, B.D., 178, 412 Hall, S.R., 65, 119, 175, 410, 427 Hall, T.C., 118 Halla, J.D., 125 Hallquist, M., 148, 150, 439, 492 Ham, J.E., 150, 158 Hamer, P.D., 329 Hamilton, J.F., 492 Hammer, M.-U., 108 Hampson, J., 4 Hampson, R.F., 46, 47, 61, 99, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 208, 316, 317, 318, 319, 322, 324, 325, 326, 327, 329, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 376, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 471, 491

557

Han, M., 31 Han, S.Q., 31 Hancock, G., 63, 64, 65, 67 Hanel, R.A., 57 Haney, J.L., 471 Hansel, A., 101, 110, 139, 141, 157 Hansen, J., 6, 7 Hansen, J.C., 316 Hanson, D., 39, 371 Hao, J., 31 Hard, T.M., 174, 175 Harder, H., 106, 108, 174, 175, 208, 331 Harder, J., 174 Harder, J.W., 39 Harker, A.G., 118 Harley, P., 49, 331 Harley, R.A., 58, 487, 492 Harper, D.B., 377 Harris, G.W., 127, 408, 454 Harris, J.M., 22, 23 Harris, S.J., 320 Harrison, D.N., 3, 31 Harrison, J.C., 150, 153, 158 Harrison, R.M., 491 Harrold, S.A., 124 Harth, C., 173, 178 Hartley, D., 173 Hartley, G.H., 447 Hartley, W.N., 3 Hartmann, D., 348, 361, 362 Harvey, J.N., 193, 203, 469 Harwood, M.H., 454, 456 Háseler, R., 174 Hashimoto, S., 333, 334, 335 Hass, H., 352, 353, 358, 359, 364, 365, 366, 367, 473 Hasson, A., 391 Hasson, A.S., 99, 101, 102, 103, 104, 106, 107, 108, 328, 329, 330, 331 Hatakeyama, S., 101, 105, 203, 208, 209, 344, 414, 491, 492 Hauglustaine, D.A., 44 Hausmann, M., 48, 174, 410 Haverd, V., 469 Hayasaka, T., 31 Hayden, K., 23 Hayman, G.D., 42, 316, 327, 341, 372 Hazen, N.L., 174 He, Y., 175 Heald, C.L., 492 Heaps, W.S., 173 Heard, A.C., 133 Heard, D.E., 46, 48, 103, 119, 173, 174, 175, 177, 409, 411, 442, 443, 444, 445, 446, 469, 494, 495 Heβling, M., 174 Heicklen, J., 335, 342, 360, 361, 362, 364, 365, 366 Heidt, L.E., 41 Heikes, B.G., 48, 99, 410 Heil, A., 31, 49 Heimann, G., 366, 367 Hein, H., 364, 367, 473

558

Author Index

Heisler, S.L., 492 Heiss, A., 352, 353, 364 Heitlinger, M., 101, 175 Helgaker, T., 157 Hellman, T.M., 360, 361 Hellpointner, E., 108 Helsdon, J.H., 482 Henderson, B., 473 Henderson, G.S., 48, 410 Hendry, D.G., 350, 352, 354 Henne, S., 29 Heo, G., 468, 471 Herbinet, O., 343 Herk, L., 352 Herlocker, D., 492 Hermes, M.R., 107 Hern, C.H., 430, 441, 449, 450 Hernandez, S.Q., 329, 330, 331 Herndon, S.C., 491 Herron, J.T., 97, 99 Herrmann, H., 103, 106, 108, 414, 492 Hess, P., 18 Hewitt, C.N., 49, 108 Hidalgo, D., 490 Higgs, J., 63, 64, 124 Hillery, B., 108 Hills, A.J., 334 Hippler, H., 347, 349, 350, 361, 362, 363, 364, 365, 366, 368 Hirota, M., 174 Hisham, M.W.M., 360 Hites, R.A., 83 Hjorth, J., 83, 141, 142, 143, 150, 157, 158, 384, 385, 417, 418, 419, 420, 421 Ho, A.W., 99, 104, 105, 106, 107, 108 Ho, J.-J., 347, 352, 353, 354, 355, 364, 367, 369, 370 Ho, T.C., 472 Ho, W., 118 Hobson, M.M., 492 Hodzic, A., 490, 491, 492, 493 Hoffmann, A., 352, 353, 358, 359, 364, 365, 366, 367, 473 Hoffmann, T., 492, 493 Hofmann, D.J., 36, 39, 422 Hofzumahaus, A., 63, 64, 65, 67, 101, 102, 103, 108, 119, 125, 173, 174, 175, 427, 454 Hogo, H., 470 Hogrefe, C., 495 Holand, F., 125 Holbrook, K.A., 127, 372, 377 Holland, F., 83, 101, 102, 103, 108, 174, 175, 454 Holloway, A.S., 276, 352, 353, 355, 364 Holloway, J.S., 52, 127, 155, 377, 492 Hollwedel, J., 48 Holm, C., 154 Holm-Hansen, O., 44 Honoré, C., 490, 492 Hooker, W.J., 18 Hopkins, J.R., 48, 175, 492 Hoppe, L., 352, 353, 358, 359, 364, 365, 366, 367, 473 Hopper, J.R., 472

Horie, O., 99, 101, 103, 105, 107, 108, 341, 342 Horii, N., 31 Hornsby, K.E., 83, 101, 377 Horowitz, A., 295, 431, 437, 439, 442, 443, 450 Horowitz, L.W., 44 Hou, H., 328, 330, 357 Hou, J., 175 Houk, K.N., 101, 102 Hov, Ø., 60 Howard, C.J., 413 Howell, N.W., 430, 441, 449, 450 Hoyermann, K., 352, 353, 355, 364, 385, 386 Hoyle, C., 7 Hsie, E.Y., 173 Hsieh, P.C., 384, 385 Hsu, Y., 467 Hu, H., 361 Hu, J., 58 Hu, M., 490 Huang, J., 173, 178 Huang, L., 385 Huebert, B.J., 492 Hueglin, C., 491 Huey, G., 108, 491 Huey, L.G., 63, 64, 124, 334, 410 Huffman, J.A., 491 Huie, R.E., 97, 99, 116, 117, 120, 121, 126, 127, 128, 129, 130, 317, 323, 358, 378, 383, 439, 457, 471, 491 Huisman, A., 124 Hulbert, D., 322 Hülsemann, F., 352, 353, 358, 359, 364, 365, 366, 367, 473 Hunt, B.G., 5, 35 Hunt, S.W., 48 Huntzicker, J.J., 492 Hurley, M.D., 42, 83, 100, 102, 105, 111, 122, 328, 329, 330, 339, 357, 358, 371, 372, 384, 385, 386, 387, 388, 395, 399, 401, 403, 405, 437 Husainy, S., 136, 148, 150, 155, 157, 169 Hutterli, M., 48, 409, 411 Hutton-Squire, H.R., 170, 384 Hutzell, W.T., 43, 44 Hwang, J.-T., 479 Hynes, A.J., 63, 64, 65, 67 Hynes, R.G., 46, 47, 61, 63, 64, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 208, 316, 317, 318, 319, 322, 324, 325, 326, 327, 329, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 376, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 469, 471, 491 Iannone, R., 385 Ibarra, Y., 329, 330, 331 Idir, M., 287, 398 Igarashi, S., 174 Iinuma, Y., 492 Imamura, T., 101 Imrik, K., 347, 361, 362, 363 Ingham, T., 175 IPCC, 5, 6, 7, 41, 43, 56, 58, 375, 376, 491

Author Index IPCC/TEAP, 44 Iraci, L.T., 347, 371, 374 Irwin, R.S., 277 Isaksen, I.S.A., 7, 44, 472, 473 Ishiwata, T., 131, 133, 333, 334 Ito, A., 58 IUPAC, 61, 431 Jacob, D.J., 18, 35, 46, 48, 49, 410, 412, 413, 466, 490, 492 Jacob, M., 175 Jacobi, H.W., 48, 409, 411 Jacobson, M.Z, 412, 466 Jaenicke, R., 492 Jaffe, D., 21 Jaffe, S., 118 Jain, S.L., 30 Jäkel, E., 66, 482, 483 Jakoubek, R.O., 174 Jalbout, A.F., 333 Jammoul, A., 48 Jang, C., 472 Jang, M., 492 Janjic, Z., 119 Japar, S.M., 329, 341, 342, 410 Javadi, M.S., 42 Jayne, J.T., 491, 492 Jefferson, A., 174 Jeffries, H., 6, 44 Jenkin, M.E., 29, 42, 46, 47, 61, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 175, 208, 265, 316, 317, 318, 319, 322, 324, 325, 326, 327, 329, 330, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 372, 376, 378, 379, 380, 383, 384, 387, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 467, 468, 469, 471, 490, 489, 491, 492, 493 Jensen, N.R., 83, 141, 142, 143, 150, 157, 158, 384, 385, 417, 420 de Jesus-Medeiros, D., 125 Jia, L., 83 Jiang, F., 469 Jiménez, E., 123, 226, 262, 385, 393 Jimenez, J.L., 490, 491, 492, 493 Jimenez, P., 472 Jobson, B.T., 175 Jobson, T., 175 Jöckel, P., 178 Joens, J.A., 430, 441, 449, 450 Johnson, A.P., 468 Johnson, B., 33, 36 Johnson, B.J., 21, 22, 23 Johnson, C.E., 16, 18, 58 Johnson, D., 83, 98, 101, 102, 103, 105, 110, 113, 350, 351, 352, 353, 354, 355, 358, 359, 364, 365, 366, 367, 370 Johnson, J.E., 412 Johnson, M.S., 136, 287, 358, 398 Johnston, H.S., 34, 35, 128, 129, 486 Johnston, P., 65, 119, 427 Johnston, P.V., 48, 174, 427 Jokinen, T., 106, 108, 414

559

Jolly, G.S., 277 Jones, B.T., 150 Jones, C.E., 377, 492 Jones, I.T.N., 118 Jongsma, C.G., 319 Jorba, O., 119 Jørgensen, S., 106, 124, 208, 287, 346, 358, 398 Jung, G.J., 468 Jung, J., 468 Junge, C.E., 6, 44, 421, 422 Jungkamp, T.P.W., 352, 353, 355, 364, 365 Junkermann, W., 65, 66, 108, 119, 482, 483 Junninen, H., 106, 108, 414 Juranić, I., 430 Ka, O., 125 Kaduwela, A.P., 29, 472, 490 Kaiser, E.W., 331, 357, 384, 385, 395, 403 Kaiser, J.W., 31, 49 Kajii, Y., 174 Kaleschke, L., 48, 409, 411 Kalthoff, N., 125, 454 Kambanis, K.G., 417 Kamens, R.M., 67, 146, 147, 211, 218, 380, 464, 473, 492 Kan, C.S., 104, 316, 342 Kanaya, Y., 174, 175 Kanno, N., 316 Karcher, J., 18 Karl, M., 83 Karl, T., 331 Karpichev, B., 249, 391 Karthäuser, J., 348, 349, 361, 362 Karunanandan, R., 443, 445 Karydis, V.A., 493 Katz, D.R., 362, 363, 364, 367 Kaus, A., 119 Kawano, T., 44 Kawasaki, M., 63, 64, 65, 67, 333, 334, 335, 358 Kawasato, T., 22 Kazadzis, S., 66, 482, 483 Kearsey, S.V., 361 Keene, W.C., 377 Keislar, R.E., 476 Keller, R.A., 439 Keller-Rudek, H., 426, 438 Kelly, J.T., 472 Keoshian, C.J., 384 Kercher, J.P., 127, 377 Kerdouci, J., 130, 132, 133, 148, 151, 155, 157, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 171 Kerminen, V.-M., 106, 414 Kerr, J.A., 44, 60, 67, 71, 72, 83, 85, 96, 97, 98, 99, 109, 111, 133, 136, 138, 141, 150, 151, 161, 192, 203, 205, 320, 347, 350, 352, 353, 354, 359, 360, 364, 369, 372, 374, 380, 425 Kersten, C., 352, 353, 355, 364 Kesselmeier, J., 421 Ketseridis, G., 492 Kettle, A.J., 410

560

Author Index

Keutsch, F.N., 124 Khalil, M.A.K., 377 Khamaganov, V.G., 83, 226, 443, 445 Khan, S.S., 468 Kiang, C.S., 31, 208 Kibler, M., 108, 435 Kiehl, J.T., 57, 412 Kiendler-Scharr, A., 141, 492 Kiep, A.C., 318, 319, 322 Kift, R., 66, 482, 483 Killus, J.P., 468, 470, 471 Kim, B., 128 Kim, D., 66, 482 Kim, S., 106, 414 Kim, S.-W., 482 Kim, Y., 471 Kimmel, J.R., 492, 491 Kimura, Y., 491, 471, 472 Kind, I., 136 Kindler-Scharr, A., 331 King, A.D., 131, 133 King, K.D., 409 King, K.R., 492 King, M.D., 136, 155, 157, 159, 163, 170, 384 Kinne, S., 31, 49 Kinnison, D.E., 40, 48 Kirchner, F., 123, 468, 472, 473 Kirk, A.D., 443 Kirsch, L.J., 342, 361 Kirschbaum, S., 398, 400, 405 Kirwan, S.P., 442, 443 Kitney, K.A., 349, 371, 372 Kivi, R., 33 Kjaergaard, H.G., 106, 124, 208, 322, 331, 346, 496, 471 Kjeldstad, B., 66, 482, 483 Kjellström, E., 423 Klaaseen, W., 412 Klasinc, L., 60 Klawatsch-Carasco, N., 83, 93 Kleeman, M.J., 58 Kleffmann, J., 125, 454, 455 Kleiman, G., 377 Kleinman, L., 108, 490, 492, 493 Klemm, O., 485 Kley, D., 17, 18, 101, 175, 352, 353, 358, 359, 364, 365, 366, 367, 473 Klimont, Z., 18, 31, 49, 412 Klinger, L., 49 Klippenstein, S.J., 344 Klockow, D., 60 Kloster, S., 31, 49 Klotz, B., 93, 262, 352, 353, 358, 359, 364, 365, 366, 367, 473 Knap, H.C., 124, 208, 346 Knapp, D., 48 Knapp, D.J., 410 Kniffka, A., 66, 482 Knipping, E.M., 48 Knoche, R., 473

Ko, M.K.W., 37, 41, 44 Kobayashi, H., 104, 414 Kockarts, G., 427 Koda, M., 479 Koepke, P., 119, 427 Kohl, S., 492 Koike, M., 175 Kok, G.L., 99, 108, 412 Kolb, C.E., 116, 117, 120, 121, 126, 127, 128, 129, 130, 317, 323, 378, 383, 439, 457, 471, 490, 491 Komazake, Y., 175 Komenda, M., 83 Kondo, Y., 175, 491, 492 Konig-Langlo, G., 48 Konovalov, I.B., 29 Konrad, S., 175 Koo, J.-H., 400 Koop, T., 327 Koppenkastrop, D., 123 Koppmann, R., 101, 110, 125, 454 Koropalov, V., 377 Kosciuch, E., 371 Koshi, M., 316 Kosmas, A.M., 333 Kotamarthi, V.R., 44, 124 Kouremeti, N., 483 Kovacs, G., 443, 445, 446 Kovács, T.A., 175 Kraka, E., 101, 102 Kramer, M., 479 Kramer, M.A., 479 Kramm, G., 468, 472, 473 Kramp, F., 102 Kraus, A., 65, 175, 427 Krautstrunk, M., 485 Kreher, K., 48 Kreutter, K.D., 414, 417 Krishnarajanagar, N., 472 Krol, M., 65, 119, 178, 412, 427 Krol, M.C., 44 Kroll, J.H., 98, 101, 102, 105, 113, 322, 323, 331, 471, 491, 492 Krohn, D., 101 Krotgkov, N., 427 Krueger, A.J., 5, 39 Kubistin, D., 174 Kuhn, M., 468, 472, 473 Kuhns, H., 492 Kukui, A., 117, 320, 323 Kulkarni, P.S., 30 Kulmala, M., 106, 414, 491 Kumar, A., 30 Kunde, V.G., 57 Kurokawa, J., 31 Kurten, A., 208 Kurtén, T., 106, 414, 471 Kurtenbach, R., 125, 454, 455 Kurylo, M.J., 116, 117, 120, 121, 126, 127, 128, 129, 130, 226, 287, 316, 317, 323, 339, 340, 341, 358, 378, 383, 439, 457, 471, 491

Author Index Kuster, W.C., 124, 492 Kutsuna, S., 142, 151, 153 Kuwata, K.T., 101, 102, 105, 107, 108, 328 Kuznets, S., 53 Kvalevag, M., 7 Kwok, E.S.C., 147, 150, 154, 160, 163, 296, 297, 298, 299, 300, 301, 302, 304, 305, 320, 321, 354, 355 Kylling, A., 65, 66, 119, 427, 482, 483 Kyro, E., 33 Laaksonen, A., 491 Labow, G.J., 22 Labs, D., 427 Lacis, A., 6, 7 LaFranchi, B.W., 124 Laine, P.L., 406, 408 Lam, S.H.M., 469 Lamarque, J.-F., 18, 31, 44, 48, 49 Lamb, B., 175, 467, 484 Lamb, B.K., 473 Lammel, G., 454 Lana, A., 412 Lane, J.R., 106 Lane, S.I., 287, 398 Lane, T.E., 493 Langer, S., 122, 136, 143, 148, 150, 153, 154, 157, 162, 318, 322, 384, 405 Langford, C.H., 48, 410 Laniewski, K., 44 Lantz, K.O., 65, 427 Lanz, V.A., 491 Lapson, L.B., 173 Larsen, J.C., 23 Larsen, N., 33 Larson, S.M., 492 Laufer, A.H., 439 Law, K.S., 6, 21, 22, 24 Lawrence, J., 3, 31 Lawrence, M.G., 44, 208, 331 Lawson, C.V., 58, 465, 467, 468, 470, 471, 472, 473, 475, 477, 478, 481, 484, 495 Lawson, D.R., 476, 481 Lazarou, Y.G., 417 Lazrus, A., 99, 412 Le Bras, G., 117, 299, 304, 305, 320, 323, 345, 396, 398, 400, 401, 403, 406, 436, 437 Le Calve, S., 299, 345 Le Crâne, J.-P., 339 Leather, K.E., 83, 109, 110 Leblanc, T., 20, 21 Lee, A., 323 Lee, C.C.W., 412, 413 Lee, E.K.C., 117, 118 Lee, E.P.F., 104 Lee, J., 175 Lee, J.D., 48, 175, 469 Lee, L.C., 103 Lee, M., 48 Lee, Y., 108

561

Lee, Y.T., 128, 129, 443 Lee-Taylor, J., 491, 492, 493 Leeds Master Mechanism, 420, 421 Lefer, B., 65, 66, 174, 468, 470, 471, 472, 493, 494 Leffer, B.L., 119, 427 Lefohn, A.S., 22, 23 Legates, D.R., 44 Lehmann, R., 33 Lehrer, E., 48, 410 Lei, W., 136, 493 Leichnitz, K., 60 Leighton, P.A., 47, 118 Lelieveld, J., 178, 208, 331, 412, 490 Lemos, L.T., 469 Lendvay, G., 352, 353, 355, 364 Leone, J.A., 478 Lerdau, M., 49, 467, 484 Leriche, M., 482 Lerner, B.M., 127, 492 Lesclaux, R., 316, 317, 326, 327, 330, 338, 341, 342, 371, 384, 385 Lesher, R., 174, 175 Lester, Jr., W.A., 439 Leue, C., 411 Levelt, P.F., 33 Lewin, A.G., 83, 102, 103 Lewis, A.C., 48, 492 Li, C., 343 Li, J., 328, 330, 343 Li, Q., 466, 490 Li, Q.B., 412 Li, Q.S., 352 Li, S., 119, 352 Li, S.-M., 467 Li, X., 343 Li, Y., 175 Li, Y.F., 377 Liang, J., 412 Liang, P., 287, 387, 398 Liang, Q., 48 Liang, Y., 343 Liao, H., 492 Liao, J., 48, 334 Libre, J.M., 44 Libuda, H.G., 350, 352, 353, 358, 359, 364, 365, 366, 367, 473 Lichtman, E.I., 211 Lightfoot, P.D., 316, 341, 342, 371, 442, 443 Lightman, P., 173 Ligon, A.P., 108, 435 Lim, H.J., 471 Lim, Y.B., 321 Lin, C.J., 472 Lin, C.-Y., 347, 352, 353, 354, 355, 364, 367, 369, 370 Lin, W., 18 Lin, W.L., 31 Lin, Z.Y., 414 Lind, J., 99, 412 Lindqvist, J.-E.O., 60

562

Author Index

Linville, D.E., 18 Liousse, C., 31, 49 Liss, P.S., 412 Lissi, E., 454, 455 Little, M.R., 130 Liu, H., 466, 490 Liu, R., 287 Liu, S.C., 6, 173 Liu, X., 18, 36 Livesey, N.J., 33 Ljungström, E., 122, 143, 148, 150, 153, 154, 157, 162, 318, 332, 384, 405, 439 Lloyd, A.C., 347, 350, 352, 359, 360, 364, 365, 369 Lloyd, S., 427 Lloyd, S.A., 65, 119 Lo, J.M.H., 391 Lobert, J.M., 384 Lockhart, J.M., 344 Lockwood, A.L., 139, 141, 322 Logan, J.A., 5, 19, 21, 23, 466, 490 Logue, M., 493 Lohmann, U., 412 Lohr, L.L., 323 Lohse, C., 141 Loirat, H., 332 Longfellow, C.A., 64, 65, 433 Lorenz, K., 360 Lörzer, J.C., 125, 454 Lotz, Ch., 364 Loughner, C., 66, 482 Louie, P.K.K., 469 Lovejoy, E.R., 335 Lovelock, J.E., 38, 39 Loza, C.L., 331 Lu, D., 495, 496 Lü, J., 343 Lu, X.Q., 31 Luecken, D.J., 43, 44, 472 Luke, W.T., 462 Luo, D., 358 Lurmann, F.W., 472 Ma, N., 31 Ma, Y., 101 Ma, Z., 18 Mabury, S.A., 44, 328, 329 MacKay, M., 360 Madronich, S., 29, 44, 65, 66, 67, 71, 72, 83, 85, 96, 97, 98, 99, 109, 111, 119, 133, 136, 138, 141, 150, 151, 161, 192, 203, 205, 320, 336, 338, 372, 380, 425, 427, 428, 466, 467, 468, 469, 472, 473, 490, 491, 492, 493 Maenhaut, W., 492 Maggs, R.J., 38, 39 Magneron, I., 436, 437 Magnotta, F., 128, 129 Mahajan, A.S., 48 Mahmud, A., 58 Makar, P.A., 467

Maker, P.D., 100, 102, 105, 111, 327, 341, 342, 347, 352, 353, 364, 372 Makshtas, A., 33 Malanca, F.E., 393, 437 Malinverno, G., 44 Malkin, T.L., 103 Malkina, I.L., 358 Manney, G., 33 Manning, A.J., 17 Manning, M., 114 Manson, J.E., 421, 422 Manzanares, E.R., 103 Mao, J., 124, 174, 175, 468, 470, 471, 472, 493, 494 Maranzana, A., 343 Marenco, A., 18 Margitan, J.J., 320 Mari, C., 482 Maricq, M.M., 316, 357 Marley, N., 108 Marovich, E., 174 Marquis, M., 114 Marriott, R.A., 391 Marsh, D.R., 40 Marsh, W.R.W., 173 Marshall, P., 358, 384, 385, 399, 405 Marston, G., 83, 98, 101, 102, 103, 105, 110, 113, 143 Márta, F., 354, 366, 367, 443, 445, 446 Martien, P.T., 487 Martin, G.R., 446 Martin, J.W., 328, 329 Martin, P., 136, 142, 151, 153, 155, 157, 170, 249, 391 Martin, R.S., 109, 136, 150 Martin, S.E., 63, 64 Martin, T., 119 Martin, T.J., 427 Martinez, E., 136, 142, 153, 155, 249, 391, 392, 393, 396, 398, 400, 403, 417 Martinez, M., 174, 175, 208, 331 Martinez, R.D., 430, 441, 449, 450 Martini, F.S., 492 Martin-Porrero, M.P., 398 Martin-Reviejo, M., 469, 492, 493 Mashino, M., 358 dal Maso, M., 141 Mason, S., 133, 136 Masui, T., 31, 49 Mather, J.H., 174 Mathur, R., 119 Matross, D.M., 124 Matsumi, Y., 63, 64, 65, 67, 83 Matsunaga, A., 141, 321 Matthew, B.M., 492 Matthews, J., 119 Mauersberger, K., 39 Mauldin, III, R.L., 106, 108, 277, 414 Maurer, T., 352, 353, 358, 359, 364, 365, 366, 367, 473 Mayer, B., 65, 66, 119, 427, 482, 483 Mazely, T.L., 454 Mazri, L., 454

Author Index McBride, S., 479, 487 McConnell, J.C., 48, 410 McCroskey, P.S., 479 McCulloch, A., 44, 173, 178, 377 McCulloch, R.D., 360 McDaniel, A.H., 119, 428 McDermid, I.S., 21 McDermid, S., 36 McDonald, J.R., 347, 349, 360, 361, 362, 363, 372 McDonald-Buller, E., 471, 472 McDonnell, L., 136, 157 McElroy, C.T., 33 McElroy, M.B., 5 McFiggans, G.B., 175, 377, 491, 492 McGee, T.J., 36, 173 McGill, C.D., 83, 98, 101, 102, 103, 105, 110, 113 McGillen, M.R., 71, 83, 84, 93, 109, 110, 193, 203, 329 McGown, M., 492 McKay, M., 124 McKay, W.A., 49 McKee, M.L., 406, 408, 411 McKeen, S.A., 173, 466, 473, 490 McKenney, D.J., 277 McKenzie, R., 65, 119 McKenzie, R.L., 427 McLaren, R., 125 McLeod, A., 384, 385 MCM, 467, 468, 469, 490, 493, 494 McMillan, G.R., 434, 447 McNair, L., 479, 487 McPeters, R.D., 5, 36, 39 McQuaid, J.B., 48 McRae, G.J., 468, 471, 479 Meagher, J., 127, 342 Meagher, J.F., 108, 208 Medtronic, S., 427 Meetham, A.R., 4 Megretskaia, I.A., 21 Mehrabzadeh, A.A., 174 Meinardi, S., 150 Meleux, F., 31, 49 Mellberg, J., 468 Meller, R., 430, 432, 439, 450 Mellouki, A., 47, 67, 71, 87, 89, 93, 94, 122, 125, 146, 148, 151, 153, 154, 155, 157, 158, 159, 160, 161, 162, 163, 226, 228, 230, 249, 251, 253, 262, 265, 266, 274, 276, 277, 278, 280, 287, 295, 296, 297, 299, 304, 305, 345, 372, 373, 374, 380, 384, 385, 386, 387, 389, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 403, 405, 427, 428, 429, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 442, 446, 447, 448, 449, 450, 451, 452, 453, 456, 457, 459 Meloni, G., 104 Mendenhall, G.D., 360, 361 Mendes, L., 48 Meng, F., 490 Mensah, A., 141, 331 Mentel, Th.F., 126, 331, 492 Menut, L., 29

563

de Mera, Y.D., 417 Méreau, R., 29, 347, 349, 350, 351, 352, 353, 354, 355, 359, 361, 363, 364, 365, 366, 367, 368, 369, 370 Mertens, L.A., 352, 353, 364 Messer, B.M., 320 Metcalf, A.R., 175 Meunier, N., 350, 359, 360, 362, 365, 367 Meyer, C., 23 Meyer, S., 100, 101 Meyrahn, H.J., 442, 443 Michael, J.V., 385 Michalakes, J., 473 Michoud, V., 490, 492 Mickley, L.J., 18, 466, 490 Middala, S., 391 Middlebrook, A.M., 127, 377, 491, 492 Middleton, P., 465, 468, 472, 473 Midgley, P., 377 Mielke, L.H., 139, 141 Mieville, A., 31, 49 Mihalopoulos, N., 417, 418, 419, 420, 421 Mihelcic, D., 101, 102, 103, 108, 174, 352, 353, 358, 359, 364, 365, 366, 367, 473 Mile, B., 341, 361 Milford, J., 479, 487 Milford, J.B., 468, 487, 471, 479, 480 Miller, A.M., 318, 319, 320, 344 Miller, B., 48 Miller, B.R., 173, 178 Miller, H.L., 114 Millet, D.B., 7, 17 Mills, M., 41 Mills, R., 407, 410 Milne, R.T., 360 Min, K.-E., 124 Minschwaner, K.R., 8, 173 Mitchell, M.J., 175 Mittal, J.P., 448 Miyamote, K., 174 Miyano, S., 333, 334, 335 Miyoshi, T.S., 491, 492 Mizoguchi, I., 436 Mobley, J.D., 467 Mochida, M., 125 Møgelberg, T.E., 357, 358 Mohnen, V.A., 472, 473 Mok, D.W.K., 104 Molina, L.T., 29, 39, 62, 63, 64, 65, 67, 175, 427, 431, 490, 492, 493 Molina, M.J., 5, 36, 38, 39, 62, 63, 64, 65, 67, 175, 427, 431, 490, 492 Moller, S.J., 48 Mollner, A.K., 352, 353, 364 Monedero, E., 155, 157, 170 Monge, M.E., 125 Monks, P.S., 29, 65, 83, 101, 119, 143, 175, 427, 483 Monod, A., 492 Montanaro, V., 44 Montreal Protocol, 5, 40, 41

564

Author Index

Montzka, S.A., 178, 412 Moore, C.B., 431 Moore, R.M., 377 Moortgat, G.K., 63, 67, 71, 72, 83, 85, 96, 97, 98, 99, 101, 103, 105, 107, 108, 109, 111, 116, 117, 118, 119, 120, 121, 126, 127, 128, 129, 130, 133, 136, 138, 141, 150, 151, 161, 192, 203, 205, 316, 317, 323, 326, 327, 338, 339, 341, 342, 352, 353, 358, 359, 364, 365, 366, 367, 372, 378, 380, 383, 425, 426, 430, 431, 432, 435, 436, 437, 438, 439, 450, 457, 467, 471, 473, 491 Morabito, P., 364 Mount, G.H., 174 Mozurkewich, M., 326, 487 Mu, Y.J., 287, 387, 398, 449 Mueller, J.-F., 44 Mueller, M., 427 Mühlemann, J., 44 Muir, D.C.G., 44 Müller, C., 347, 352, 353, 354, 360, 369, 374 Müller, J.-F., 44, 207, 471 Müller, M., 65, 119 Müller, R., 32 Müller, S.R., 44 Mund, Ch., 362, 363 Munoz, A., 398 Murcray, F., 36 Murphy, B.N., 493 Murphy, D.M., 326, 492 Murphy, S.M., 492 Murrells, T.P., 492 Müsgen, P., 101, 175 Myhre, G., 7, 83, 142, 143 Mylona, S., 18 Nádasdi, R, 443, 445, 446 Nagar, N.S., 431 Naik, P.D., 384, 448 Nakajima, H., 33 Nakano, T., 22 Nakano, Y., 131, 133, 333, 334 Nakayama, T., 83, 333, 334, 335 Nalty, A., 109, 136 Naoe, H., 22 Nardi, B., 48 NASA, 32, 39 Nash, E.R., 33 Nava, D.F., 385 Ndour, M., 125 Neckel, H., 427 Nédélec, P., 18, 21 Neeb, P., 99, 101, 103, 105, 107, 108, 352, 353, 358, 359, 364, 365, 366, 367, 473 Neff, B., 48, 409, 411 Neill, P.J., 63 Nelson, H.H., 347, 349, 362, 363, 372 Nemitz, E., 63, 64, 124 Neuman, J.A., 48, 410 Neuroth, R., 173 Newchurch, M.J., 36

Newman, P.A., 5, 39 Ng, N.L., 323 Nguyen, T.L., 206, 207, 208, 308, 345, 346, 496 Nichol, S., 22, 23 Nicol, C.H., 447 Nicolet, M., 4, 35 Nicovich, J.M., 406, 408, 411, 414, 417, 420 Nielsen, A.T., 44 Nielsen, C.J., 83, 142, 143, 145, 146, 154, 155, 157, 158, 332 Nielsen, O.J., 42, 43, 44, 83, 122, 249, 287, 317, 318, 328, 329, 341, 357, 358, 368, 371, 372, 384, 385, 387, 391, 398 Niki, H., 28, 48, 100, 102, 105, 111, 327, 330, 341, 342, 347, 352, 353, 364, 372, 384, 408, 410 Nilsson, E.J.K., 287, 358, 398 Ninomiya, Y., 358 Nishida, S., 63, 64, 174 NIST Chemical Kinetics Database, 406 NOAA, 9, 10, 11, 19, 20, 21, 22, 23, 24, 26, 27, 40, 427 Noda, J., 136, 148, 150, 154 Noell, A.C., 338, 341, 342 van Noije, T.P.C., 44 Nolting, F., 178, 184 Nordmeyer, T., 384 Noremsaune, I.M.W., 143, 145, 146 Norrish, R.G.W., 117, 118 North, S.W., 319, 443 Notario, A., 136, 385, 386, 387, 392, 393, 396, 398, 400, 401, 403 Novelli, A., 106, 108 Nowak, J.B., 48, 410 Nozière, B., 322, 323, 346, 371, 385 Nunes, T., 469 Nunes, T.V., 469 Nunnermacker, L.J., 108 Nyman, G., 136, 154 O’Brien, R.J., 174, 175 O’Connor, F.M., 377 O’Connor, M.P., 436 O’Doherty, S., 173, 178 Odom, J.M., 44, 492, 493 O’Dowd, C.D., 491, 493 Oetjen, H., 48 Ohara, T., 31, 49 Ohta, T., 436 Okamoto, H., 151, 153 Okamoto, Y., 304 Okuda, M., 371 Okumura, M., 338, 341, 342, 352, 353, 364 Olariu, R., 352, 353, 358, 359, 364, 365, 366, 367, 473 Oldenborg, R.C., 360, 361 Oldershaw, G.A., 372 Olea, C., 391 Oliger, A., 175 Olive, J., 350, 359, 360, 364 Olivella, S., 348 Olson, B., 18 Olson, J., 174, 468, 470, 471, 472, 493, 494

Author Index Olszyna, K.L., 108 Oltmans, S.J., 21, 22, 23, 36 Olzmann, M., 352, 353, 355, 364, 391 Onasch, T.B., 491, 492 O’Neill, I., 60 O’Neill, S.M., 473 Onoe, A., 174 Ordóñez, C., 48 Orkin, V.L., 116, 117, 120, 121, 126, 127, 128, 129, 130, 226, 317, 323, 358, 378, 383, 439, 457, 471, 491 Orlandi, M., 150 Orlando, J.J., 9, 13, 14, 15, 18, 37, 39, 40, 47, 48, 57, 65, 67, 71, 87, 89, 93, 94, 118, 119, 121, 122, 123, 124, 125, 128, 129, 130, 132, 146, 148, 151, 153, 154, 155, 157, 158, 159, 160, 161, 162, 163, 172, 178, 180, 184, 185, 189, 226, 228, 230, 249, 251, 253, 262, 265, 266, 274, 276, 277, 278, 280, 287, 295, 296, 297, 300, 305, 316, 317, 318, 320, 321, 325, 326, 327, 328, 329, 331, 332, 334, 336, 345, 346, 347, 349, 350, 352, 353, 355, 356, 357, 358, 360, 361, 362, 363, 364, 365, 369, 371, 372, 373, 374, 380, 381, 383, 384, 385, 386, 387, 388, 389, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 403, 405, 406, 408, 410, 411, 412, 413, 427, 428, 429, 431, 432, 433, 434, 435, 437, 438, 439, 440, 442, 446, 447, 448, 449, 451, 452, 453, 456, 457, 459 Ørnsø, K.B., 124, 208, 287, 346, 398 Orphal, J., 10, 62 Ors, A., 384, 385 Orzechowska, G., 101, 102, 103, 105, 107, 108, 346 Osamura, Y., 439 Osborn, D.L., 99, 104, 106, 107, 108 Osthoff, H.D., 127 Ott, L.E., 482 Ottobrini, G., 143 Overath, R., 406, 408 Owen, P.S., 136 Oyola, P., 490 Ȍzer, U., 60 Paasonen, P., 106, 108, 414 Pages, J.-P., 18 Pagnotti, V., 15 Pala, I.R., 331, 384 Palmer, P.I., 331 Pandis, S.N., 40, 490, 491, 492, 493 Pankow, J.F., 492, 493 Papadimitriou, V.C., 42, 395 Papagiannakopoulos, P., 395, 417 Papagni, C., 154 Papanastasiou, D.K., 395 Papasavva, S., 43, 44 Paraskevopoulos, G., 277 Park, J, 319 Park, J.-H., 124 Park, R.J., 412, 413, 492 Parker, A.E., 119 Parkes, D.A., 342, 361

565

Parkinson, W.H., 9 Parr, A.D., 130 Parrella, J.P., 48 Parrish, A., 36 Parrish, D.D., 6, 7, 17, 21, 22, 24, 28, 29, 52, 55, 174 Pascoe, S., 468 Pashynska, V., 492 Passant, N.R., 265, 490, 492 Pasternack, L.R., 360, 361 Patchen, A.K., 318, 319, 322 Patrick, K.F., 347, 350, 352, 353, 359, 360, 364, 365 Patrick, K.G., 361 Pätz, H.W., 174, 175 Paulot, F., 124, 208, 322, 331, 346, 496, 471 Paulson, S.E., 99, 101, 102, 103, 104, 105, 106, 107, 108, 346, 384, 492 Pauly, H.J., 442, 443 Pavelin, E.G., 16, 18 Pawson, S., 58 Payne, W.A., 385 Peckham, S.E., 466, 473, 490 Pedersen, T., 417, 420 Peeters, J., 138, 141, 142, 148, 150, 203, 206, 207, 208, 301, 303, 308, 345, 346, 350, 351, 352, 353, 354, 355, 356, 357, 358, 362, 364, 366, 367, 368, 369, 372, 384, 385, 471, 496 Peischl, J., 52, 55 Peltier, R.E., 492 Peng, L., 31 Peng, N., 384, 410 Penkett, S.A., 6, 60, 173 Penner, J.E., 58 Pennino, M.J., 318, 319, 322, 344 Percival, C.J., 71, 83, 84, 93, 99, 104, 106, 107, 108, 109, 110, 193, 203, 329, 339 Perea, R., 495, 496 Pérez, C., 119 Perner, D., 127, 174, 454, 486 Perring, A.E., 139, 141 Perrussel, O., 490, 492 Perry, J., 175 Petäjä, T., 106, 109, 414 Petersen, E.B., 328 Petetin, H., 490, 492 Peyerimhoff, S.D., 323 Pfeilsticker, K., 411 Pfister, G., 65, 119, 427 Pfrang, C., 109, 136, 150, 332 Phillips, D., 449 Phillips, L.F., 60 Phillips, S., 472 Pickering, K.E., 482 Picquet-Varrault, B., 130, 132, 133, 148, 151, 155, 157, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 171, 332, 350, 358, 359, 360, 362, 363, 365, 367 Pierce, J.R., 493 Pierce, T., 49 Piety, C.A., 411 Pilinis, C., 493

566

Author Index

Pilling, M.J., 29, 42, 47, 48, 67, 71, 87, 89, 93, 94, 122, 124, 125, 146, 148, 151, 153, 154, 155, 157, 158, 159, 160, 162, 163, 175, 226, 228, 230, 249, 251, 253, 262, 265, 266, 274, 276, 277, 278, 280, 287, 295, 296, 305, 316, 317, 327, 338, 339, 341, 343, 344, 347, 349, 365, 372, 373, 374, 380, 384, 385, 386, 387, 389, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 403, 405, 427, 428, 429, 431, 432, 433, 434, 435, 437, 438, 439, 440, 442, 443, 444, 445, 446, 447, 448, 449, 451, 452, 453, 454, 455, 456, 457, 459, 467, 468, 469, 490, 493 Pimentel, A.S., 125, 358, 399, 405 Pinder, R.W., 119, 473, 495 Pinho, P.G., 469 Pinot et Moira, J.C., 63, 64 Pinty, J.-P., 482 Pio, C., 469 Pitari, G., 44 Pitts, J.N., Jr., 58, 118, 119, 120, 121, 127, 154, 347, 350, 352, 359, 360, 364, 365, 369, 434, 446, 447, 454 Pitts, M.C., 33 Plane, J., 48, 377, 409, 411 Plane, J.M.C., 48 Plass-Dülmer, C., 174 Plastridge, R.A., 384 Platt, U., 48, 108, 127, 174, 454, 409, 410, 411 Platz, J., 122, 371, 384, 385, 387 Pleijel, K., 489 Plum, C.N., 154 Pollock, W.H., 41 Polymeneas, P., 493 Poole, L.R., 33 Poppe, D., 83, 352, 353, 358, 359, 364, 365, 366, 367, 473 Porter, G.B., 443 Porter, L., 173, 178 Portmann, R.W., 42 Poskrebyshev, G., 347, 349, 362 Potts, A.R., 368 Poulain, L., 490, 492 Poulet, G., 406 Pouliot, G.A., 473 Pouvesle, N., 117, 323 Prabhakara, C., 57 Prather, M.J., 37, 44 Presto, A.A., 102, 103 Prévôt, A.S.H., 490, 491, 492, 493 Price, D.W., 83, 102, 103 Prinn, R.G., 173, 178, 422, 423 Prinz, K.D., 427 Proffitt, M.H., 5, 39, 40 Protoschill-Krebs, G., 421 Pruppacher, H.R., 60 Pultau, V., 203, 206, 301, 303, 308 Purnell, C.J., 60 Pusede, S.E., 124 Pushpa, K.K., 384 Pye, S.T., 492 Pyle, J.A., 44, 48, 58, 384

Qi, B., 101 Qi, F., 343 Qin, D., 114 Quam, D., 400 Quinn, P.K., 127, 377, 492 Raatikainen, T., 491 Rabitz, H., 479 Rabitz, S., 479 Racherla, P.N., 493 Ragains, M.L., 384 Rahman, M.M., 100, 101 Rajakumar, B., 445 Rajbenbach, L., 352 Ramacher, B., 406, 408, 410 Ramanathan, V., 6 Ramaroson, R., 66, 483 Ramazan, K.A., 125, 384 Ramsdell, J., 492 Ran, L., 31 Ranschaert, D.L., 320, 327 Rao, S.T., 495, 467, 468, 471 Rappenglück, B., 174, 454, 468, 471, 472, 493, 494 Rasch, P.J., 412 Rasmussen, R.A., 48, 58, 173, 409 Rast, S., 44 Ratajczak, E., 104 Ratto, J.J., 118 Ratto, M., 479 Raut, J.-C., 31, 49 Rautiainen, J., 491, 492 Raventos-Duran, M.T., 329, 339 Ravishankara, A.R., 42, 63, 64, 65, 67, 101, 123, 124, 127, 128, 185, 316, 317, 335, 436, 437, 442, 443, 444, 445, 446, 449, 450, 451, 453, 454, 456 Rayez, J.C., 117, 323, 347, 349, 350, 351, 352, 353, 354, 355, 359, 361, 363, 364, 365, 366, 367, 368, 369, 370 Rayez, M.-T., 117, 323, 347, 349, 350, 351, 352, 353, 354, 355, 359, 361, 363, 364, 365, 366, 367, 368, 369, 370, 384, 385 Rea, G., 469 Read, K.A., 48 Reading, S., 323 Redondas, A., 22, 23 Reeser, D.I., 48 Reeves, C.E., 483 Reff, A., 471 Regener, E., 6, 44 Reisen, F., 354, 355 Rem, X., 174, 468, 470, 471, 472, 493, 494 Ren, X., 174, 175 Restelli, G., 141 Revah, I., 57 Rex, M., 33 Rhäsa, D., 360 Riahi, K., 31, 49 Richter, A., 48, 409, 411 Richter, R.C., 63 Rickard, A.R., 83, 98, 101, 102, 103, 105, 110, 113, 454, 454

Author Index Ridley, B.A., 28 Riedel, K., 48 Riedel, T.P., 127, 377, 409, 411 Riemer, D.D., 384 Riemer, N., 126 Rimai, L., 357 Rind, D., 18 Rindone, B., 150 Ripperton, L.A., 6, 44 Roberts, J., 174 Roberts, J.M., 101, 123, 124, 127, 175, 453, 454, 456 Robertson, S.H., 344, 347, 349, 365 Robichaud, D.J., 338, 341, 342 Robinson, A.L., 491, 492, 493 Robinson, G.N., 360, 361 Rodgers, M.O., 454 Rodriguez, A., 386, 387, 388, 392, 443, 445 Rodriguez, D., 386, 387, 388, 392, 417 Rodriguez, J.M., 37, 44 Roehl, C.M., 118, 119 Roeselová, M., 48 Roeth, E.-P., 427 Rohrer, F., 83, 101, 110, 331 Rolf, S.R., 39 Rolle, W., 151, 153 Rollins, A.W., 141, 331 Romanius, M.N., 395 de Rooij, C., 44 Roscoe, H., 48, 409, 411 Roscoe, J.M., 388, 391, 407, 409, 410 Rosen, J.M., 39 Rosen, R., 173 Rosenbohm, E., 150 Rossi, M.J., 46, 47, 61, 99, 116, 117, 119, 120, 121, 122, 123, 126, 136, 145, 208, 316, 317, 318, 319, 322, 324, 325, 326, 327, 329, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 376, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 471, 491 Röth, E.P., 65, 119, 174 Rotstayn, L.D., 412 Roussel, P., 367 Rowland, F.S., 5, 36, 38 Rowley, D.M., 371 Royal Society, 6, 24, 25, 44, 58 Rubin, M.B., 3 Rubio, M., 454, 455 Rudich, Y., 346, 492 Rudolf, M., 174 Rudolph, J., 385 Ruedy, R., 6, 7 Ruiz, M.E., 154, 155, 163 Ruggaber, A., 65, 119, 427 Rugh, J.P., 43, 44 Rughooputh, S., 16, 18 Rupp, L., 83, 125, 454 Ruppert, L., 352, 353, 358, 359, 364, 365, 366, 367, 372, 473 Rusch, G.M., 44 Russell, A., 479, 487 Russell, A.G., 468, 471, 479, 487

567

Russell, G.A., 336, 343 Russell, L.M., 492 Ryerson, T., 52, 55 Ryerson, T.B., 21, 52, 55 Ryzhkov, A.B., 107 Saastad, O., 384 Sage, A.M., 493 Sahay, S.R., 98 Sahetchian, K., 352, 353, 364 Saiz-Lopez, A., 48, 377 Sakamoto, U., 333, 334, 335 Sakata, T., 31 Salameh, P., 173, 178 Salawitch, R.J., 5 Salberg, E.A., 8 Salce, J., 151 Salcedo, D., 491, 492 Salgado, M.S., 155, 157, 170, 249, 391 Salgado, S., 136, 142, 153, 155, 391, 417 Salgado-Muñoz, M.S., 398 Saliba, N.A., 125 Salomonson, V.V., 57 Saltelli, A., 479 Saltzman, E.S., 63, 412 Salway, A.J., 492 Samson, P.J., 15 Sanchez, O., 490, 492 Sand, J.P., 384, 385 Sander, R., 48, 208, 377, 409, 410, 411 Sander, S.P., 48, 116, 117, 120, 121, 126, 127, 128, 129, 130, 249, 352, 353, 364, 378, 383, 391, 439, 454, 457, 471, 491 Sanders, N., 360, 361, 362 Sanderson, M.G., 44 Sandroni, S., 16 Santee, M.L., 33 Sartelet, K., 471 Sarwar, G., 119, 472, 473 Sato, K., 93, 101 Sato, M., 348, 361 Sauer, C.G., 108, 122, 352, 353, 358, 359, 364, 365, 366, 367, 408, 409, 410, 473 Sauer, F., 105, 107, 108 Saunders, E., 58, 465, 470, 471, 472, 473, 475, 477, 478, 484 Saunders, S.M., 42, 175, 467, 468, 469, 489, 490, 493 Sauté, M., 60 Savage, C.M., 100, 102, 105, 111, 327, 341, 342, 347, 352, 353, 364, 372 Savage, N.H., 44, 377 Savarino, J., 412, 413 Savee, J.D., 99, 104, 106, 107, 108 Sawerysyn, J.P., 348, 361, 362 Saxena, P., 492 Scarfogliero, M., 151, 160, 162 Scarnato, B., 31 Scarr, P.J., 136 Schaefer, III, H.F., 439 Schafer, C., 105 Schäfer, H.-J., 175

568

Author Index

Schäfer, J., 175 Schallhart, B., 66, 482, 483 Schaub, A., 83 Scharzenbach, R.P., 44 Schauffler, S.M., 48, 320 Scheel, H.-E., 6, 21, 22, 23, 24 Scheirer, R., 66, 482, 483 Schere, K.L., 466, 490 Schiff, H.I., 60 Schill, H., 31 Schindler, R.N., 100, 101, 102, 145 Schlager, H., 485 Schlomski, S., 108, 435 Schlosser, E., 125, 174, 454 Schmailzl, U., 421 Schmeltekopf, A., 486 Schmidlin, F., 22, 23 Schmidt, S., 66, 482, 483 Schmidt, U., 423 Schmitt, R., 65, 66, 119, 427, 482, 483 Schmitz, T., 175 Schmitz, R., 466, 473, 490 Schmoltner, A.M., 124, 456 Schneider, J., 490, 491, 492 Schneider, N.J., 320, 327 Schneider, W., 118, 342, 387, 401, 442, 443, 467 Schneider, W.F., 8, 44, 329, 330, 341 Schnell, R., 36 Schnell, R.C., 48, 409 Schoeberl, M.R., 5, 33, 39 Schoemaecker, C., 119 Schoenemeyer, T., 473 Scholes, B., 49 Scholtens, K.W., 320 Schönbein, C.F., 3 Schreder, J., 483 Schtlike, B., 63, 64 Schultz, M., 466, 490 Schultz, M.G., 31, 44, 49 Schurath, U., 174 Schuster, G., 327 Schütze, N., 398, 400, 405 Schwab, J.J., 175 Schwander, H., 119, 427 Schwartz, S.E., 412 Sciare, J., 490, 492 Scott, B.F., 44 Scruggs, A., 391 Seakins, P.W., 103, 344, 347, 349, 365 Seeba, J., 387 Seefeld, S., 468, 472, 473 Sehested, J., 44, 122, 317, 318, 329, 341, 357, 358, 371, 387 Seidl, W., 473 Seigneur, C., 471 Seiler, W., 431 Seinbrecher, R., 49 Seinfeld, J.H., 40, 67, 105, 123, 146, 147, 208, 211, 218, 322, 323, 331, 352, 353, 355, 364, 365, 380, 464, 471, 473, 478, 479, 491, 492, 493

Seip, H.M., 31 Seitzinger, S., 471 Sekiya, A., 142, 151, 153 Seland, J.G., 145, `46 Selby, T.M., 104 Selby, K., 342 Selden, T.M., 53, 104 Sellevag, S.R., 157 Sen, A.D., 103 Senff, C.J., 492 Senkan, S.M., 400 Setokuchi, O., 348, 361 Seuzaret, C., 377 Shadwick, D., 22, 23 Shallcross, D.E., 99, 104, 106, 107, 108, 109, 110, 155, 157, 159, 163, 193, 203, 329, 333, 334, 339, 469 Shanklin, J.D., 4, 5, 32 Shao, M., 31, 490 Sharkey, T.D., 467, 484 Sharma, A., 384 Sharp, J.H., 118, 119 Shaw, J.H., 99, 103, 105, 342 Shepler, B.C., 323 Sheppard, J.C., 174 Shepson, P.B., 48, 139, 141, 320, 321, 322, 334, 409, 410, 411 Shestakov, O., 350, 352, 353, 358, 359, 364, 365, 366, 367, 473 Shetter, R.E., 65, 66, 118, 119, 174, 175, 427, 428 Shi, J., 357, 385 Shi, Y., 83 Shimono, A., 491, 492 Shindell, D., 6, 7 Shindell, D.T., 44 Shirinzadeh, B., 174 Shirley, T., 175 Shirley, T.R., 175 Shores, B., 112 Shorees, B., 99, 103 Shorter, J.A., 44 Shortridge, R., 342 Shrivastava, M.K., 493 Shroll, R.M., 323 Shu, Y., 103 Shu, Y.H., 102, 150, 157, 352, 353, 354, 355, 366, 367 Sidebottom, H., 101, 102, 226, 304, 305, 345 Siegl, W.O., 385, 410 Siese, M., 101, 102, 103, 125, 454 Sihler, H., 48, 410 Silbernagl, R., 66, 482 Sillman, S., 481 da Silva, G., 346 Silvente, E., 63 Simmonds, P., 173 Simmonds, P.G., 17, 23, 173, 178 Simó, R., 412 Simon, F., 342, 467 Simon, H., 467 Simonaitis, R., 108, 335

Author Index Simpas, J.B., 175 Simpas, J.G., 174 Simpson, D., 492, 493 Simpson, I.J., 469 Simpson, W.R., 48, 409 Singh, H.B., 29 Singh, O.N., 60 Singh, R., 42, 83 Singh, R.B., 490 Singh, S., 329, 330, 331 Singh, U., 36 Singleton, D.L., 277 Sinha, A., 119, 360, 413 Siour, G., 490, 492 Sipilä, M., 106, 108, 414 Sistema de Monitoreo Atmosférico, 30 Sjödin, A., 454 Sjostedt, S.J., 48, 334, 410 Skamarock, W.C., 66, 466, 473, 483, 490 Skeie, R., 7 Skewes, L.M., 385, 410 Skov, H., 141 Slanger, T.G., 63, 64 Slanina, J., 60 Slemr, F., 108, 435 Sloan, J.J., 490 Slott, R.S., 490 Slusher, D.L., 63, 64, 124 Smith, D., 431 Smith, G.D., 63, 64, 136, 148, 150, 155, 157, 169 Smith, J., 385 Smith, J.D., 491 Smith, J.N., 352, 353, 355, 364, 365, 410 Smith, S., 492 Smith, S.C., 175 Smith, S.J., 18, 31, 49, 130, 131, 133, 145, 412 Smythe, K., 44 Snelson, A., 336 Sobolev, I., 44 Sodeau, J., 48, 409 Sohn, M., 126 Sohn, Y.S., 406, 408 Sokolov, O., 384, 385 Sokolowski-Gomez, N., 347, 349, 350, 364, 365 Solé, A., 348 Solignac, G., 161, 163 Solman, Z., 339 Solomon, K.R., 44 Solomon, S., 39, 41, 114, 453 Sommariva, R., 127, 377 Somnitz, H., 347, 350, 351, 352, 353, 358, 359, 361, 362, 363, 364, 365, 366, 367, 443, 445, 473 Søndergaard, F., 83 Song, D., 53 Song, X.L., 328 Sorensen, S., 417, 420, 436 Soret, J.L., 3 Soskin, G., 384 Soto, A., 386, 387, 388

569

Sousa, Santos, G., 48 Søvde, O., 7 Spada, M., 119 Spain, T.G., 17, 19, 24 Sparks, L.P., 63, 64, 124 Spencer, C., 44 Sperry, P.D., 118 Spicer, C.W., 454 Spillman, J.L., 338 Spiridonov, V., 482 Spittler, M., 141, 329 Sprague, M.K., 352, 353, 364 Sprengnether, M., 322 Springston, S.R., 108 Sprung, J.L., 350, 352, 359, 360, 364, 365 Stabel, J.R., 136 Staebler, R.M., 48, 410 Staehelin, J., 6, 17, 19, 21, 22, 24, 31 Staffelbach, T.A., 108 Stange, G., 174 Stanier, C.O., 492, 493 Stark, H., 123, 127 Starkey, D.P., 372 Stavrakou, T., 207 St. Clair, J.M., 208, 471 Stedman, D.H., 28 Stedman, J.R., 493 Stefanopoulos, V.G., 395 Stefels, J., 412 Steffen, A., 409, 411 Stein, T.N.N., 122 Steinbacher, M., 6, 17, 19, 21, 22, 24, 26 Steiner, A.L., 58 Stemmler, K., 125 Stenchikov, G., 482 Stenstrøm, Y., 157 Stephens, C.R., 410 Stevens, P., 322 Stevens, P.S., 174 Stevenson, D.S., 44, 58 Stewart, D.J., 170, 384 Stickel, R., 63, 64, 124 Stief, L.J., 385 Stimac, P.J., 323 Stimpfle, R.M., 173 Stith, J., 21 Stocker, D.W., 347, 352, 353, 354, 360, 369, 374 Stockwell, W., 473 Stockwell, W.R., 47, 58, 66, 99, 352, 353, 358, 359, 364, 365, 366, 367, 412, 413, 465, 467, 468, 489, 470, 471, 472, 473, 475, 476, 477, 478, 479, 480, 481, 482, 484, 485, 486, 487, 491, 492, 495, 496 Stohl, A., 21, 492 Stolarski, R.S., 3, 5, 32, 39 Stone, D., 119, 494, 495 Stone, D.L., 46, 173, 174, 177 Straccia, A.M., 387 Strahan, S.E., 44 Stratmann, F., 106, 414

570

Author Index

Strausz, O.P., 103, 107 Striebel, F., 347, 349, 361, 362 Stroud, C.A., 175 Strum, M., 467 Strutt, R.J., 3 Stübi, R., 21, 31 Stuhl, F., 28 Sturges, W.T., 377 Stutz, J., 126, 175, 384, 454 Su, F., 99, 103, 105, 107 Sudo, K., 44 Sueper, D., 491 Suh, I., 136, 384 Sullivan, A.P., 492 Suma, K., 316 Sumathy, R., 323 Sumiyoshi, Y., 316 Sumner, A.L., 125, 384 Sun, J.Y., 491, 492 Sun, Y.L., 491 Sundet, J.K., 83, 142, 143 Surratt, J.D., 492 Suto, M., 103 Sutton, H.C., 445 Swartz, W., 65 Swartz, W.H., 119, 427 Sweeney, C., 20, 21, 412 Syomin, D., 125, 384 Sze, N., 37 Sze, N.D., 41, 44 Szente, J.J., 316, 357 Szidat, S., 490, 492, 493 Szilágyi, I., 410, 443, 445, 446 Szmigielski, R., 492 Szopa, S., 29, 44, 466, 467, 468, 469 Szwarc, M., 352 Taatjes, C.A., 99, 104, 106, 107, 108, 343, 344, 385 Taccone, R., 153, 385 Taddonio, K., 43, 44 Tadić, J., 430 Taherian, M.R., 63, 64 Takagi, H., 371, 414 Takahashi, K., 63, 64, 333, 334, 335, 386, 387, 388 Takami, A., 101, 491, 492 Takayanagi, T., 93 Takegawa, N., 175, 491, 492 Taketani, F., 63, 64 Taketsugu, T., 93 Tallamraju, R., 49, 173 Talukdar, R.K., 42, 64, 65, 124, 127, 128, 185, 436, 450, 451, 453, 456 Tan, D., 174 Tanaka, K., 7 Tang, X., 31, 39, 44, 60, 465, 468, 472, 473, 490 Tang, Y., 436 Taniguchi, N., 64 Tanimoto, H., 6, 21, 22, 24, 175 Tanner, D., 48, 124, 334

Tanner, D.J., 63, 64, 174, 175, 410 Tans, P.P., 376, 412 Tapia, A., 249, 155, 157, 170, 391 Tapia-Valle, A., 398 Taraborrelli, D., 208, 331 Tarantola, S., 479 Tarasick, D., 20, 21, 23 Tarasick, D.W., 22, 33, 48, 411 Taylor, J., 49 Telenta, B., 482 Templeton, E.M.J., 48, 410 Teruel, M.A., 274, 287, 393, 395, 397, 398, 399, 405 Tevault, P., 60 Tezaki, A., 316 Theloke, J., 350, 358, 359, 365 Thévenet, R., 400, 403, 436 Theys, N., 48 Thiel, S., 66, 482, 483 Thiemens, M.H., 412, 413 Thomas, W., 99 Thompson, A.M., 7, 48 Thompson, J.E., 63, 64 Thompson, K.C., 136, 155, 159, 170, 384 Thompson, R., 44 Thomson, A., 31, 49 Thomson, D.S., 492 Thornton, J.A., 124, 127, 377 Thorseth, T.M., 66, 482, 483 Thouret, V., 21 Thudium, J., 6, 17, 19 Thüner, L.P., 123, 157, 352, 353, 358, 359, 364, 365, 366, 367, 473 Tian, J., 491 Tie, X., 31 Tiedje, J.M., 44 Tignor, M., 114 Tillmann, R., 101, 110, 141, 331 Tilmes, S., 48 Timonen, R.E., 104 Tobias, D.J., 48 Toby, S., 131, 133, 145 Toft, A., 83 Tokiwa, Y., 454 Tokuhashi, K., 142, 151, 153 Tomlinson, J.M., 491 Tonachini, G., 343 Tong, Z., 406, 408 Tonokura, K., 316, 333, 334, 335 Tonse, S., 58 Toole, M., 492 Toon, O.B., 423 Topaloglou, C., 483 Topping, D.O., 491 Torrent-Sucarrat, M., 107 Tost, H., 412 Toumi, R., 16, 18 Trainer, M., 21, 52, 55, 173, 174, 492 Treacy, J., 101, 102 Trenberth, K.E., 8, 57

Author Index Trevitt, A.J., 104 Trimborn, A.M., 491 Troe, J., 46, 47, 61, 99, 116, 117, 118, 119, 120, 121, 122, 123, 126, 136, 145, 208, 316, 317, 318, 319, 322, 324, 325, 326, 327, 329, 332, 333, 334, 335, 337, 339, 340, 341, 342, 346, 376, 378, 379, 380, 383, 384, 405, 406, 407, 410, 413, 414, 416, 417, 418, 419, 420, 423, 471, 491 Trolier, M., 63, 64 Tromp, T.K., 44 Tsimpidi, A.P., 493 Tsou, J.J., 36 Tuazon, E.C., 69, 99, 102, 111, 141, 146, 147, 150, 151, 322, 346, 357, 358, 454 Tucceri, M.E., 295, 333, 346 Tuck, A.F., 41 Tulet, P., 493 Turner, W.V., 108, 435 Turnipseed, A.A., 63, 64, 124 Turpin, B.J., 471, 492 Tyagi, B., 30 Tyndall, G.S., 9, 13, 14, 15, 18, 37, 39, 40, 44, 57, 65, 67, 118, 119, 121, 122, 123, 124, 128, 129, 130, 132, 161, 163, 172, 178, 180, 184, 185, 189, 297, 300, 305, 316, 317, 318, 320, 321, 325, 326, 327, 328, 329, 331, 332, 336, 338, 339, 341, 346, 347, 349, 350, 352, 353, 355, 356, 358, 359, 360, 361, 362, 363, 364, 365, 369, 371, 372, 373, 374, 380, 381, 383, 384, 388, 399, 400, 401, 405, 406, 408, 410, 412, 413, 449 Tyndall, J., 56 Uchimaru, T., 142, 151, 153 Ufer, T., 443, 445 Ukeguchi, H., 131, 133 Ulbrich, I.M., 491, 492, 493 Ullerstam, M., 157, 322, 384, 405 Ullmann, K., 410 UN, 25 UNECE, 7 University of Leeds, 467, 468, 469 Upadhyaya, H.P., 448 U.S. Department of Commerce, 28, 31, 54, 55, 56 U.S. EPA, 28, 29, 30 Uselman, W.M., 117, 118 Utembe, S.R., 469 Vaattovaara, P., 491 Vaghjiani, G.L., 63, 64 Vallina, S.M., 412 Van Hoosier, M.E., 427 Van Roozendael, M., 48 Van Weele, M., 65, 119 Vandenberk, S., 203, 206, 301, 303, 308 Varutbangkul, V., 323 Vas, G., 492 Vasu, S.S., 99, 104, 106, 107, 108 Vaughan, S., 332 Vázquez, G.J., 118, 119 Večeřa, A., 454 Veefkind, P., 33

571

Velasco, E., 175 Vereecken, L., 106, 108, 138, 141, 142, 146, 148, 203, 206, 207, 208, 295, 301, 303, 308, 323, 345, 346, 350, 351, 352, 353, 354, 355, 356, 357, 358, 362, 364, 366, 367, 368, 369, 372, 384, 496 Vermeylen, R., 492 Vet, R.J., 19, 20 Veyret, B., 339, 341, 342 Viarengo, S., 16 Viidanoja, J., 157 Villa, A., 360, 361 Villanueva, F., 155, 157, 170, 249, 391 Villena, G., 454, 455 Villenave, E., 338, 339, 345 Vingarzan, R., 23, 24 Viskolcz, B., 347, 349, 350, 352, 353, 355, 361, 362, 363, 364, 365, 366, 368 Vogel, B., 108, 125, 454 Vogel, H., 125, 454 Vogt, R., 122, 377 Voigt, S., 10, 62 Volkamer, R., 175, 469, 492, 493 Volke, A., 18, 412 Volpe, C.J., 174 Volz, A., 17, 18, 174 Volz-Thomas, A., 6, 17, 19, 21, 22, 24, 101, 108, 175 Von Sydow, L.M., 44 Vuuren, D.P., 31, 49 Waddington, D.J., 341, 342 Wahner, A., 83, 101, 110, 174, 352, 353, 358, 359, 364, 365, 366, 367, 454, 473 Wagner, T., 48, 409, 411 Wagner, V., 127, 467, 468, 469 Wahner, A., 125, 126 Walcek, C.J., 472, 473 Walega, J.G., 28, 99, 334, 410, 412 Walker, J.C.G., 6, 44 Walker, K.A., 33 Walker, R.W., 372 Wallington, T.J., 8, 37, 42, 43, 44, 47, 67, 71, 83, 87, 89, 93, 94, 121, 122, 124, 125, 130, 132, 133, 136, 138, 141, 146, 147, 148, 150, 151, 153, 154, 155, 157, 158, 159, 160, 161, 162, 163, 172, 173, 178, 180, 184, 185, 189, 192, 203, 205, 297, 211, 218, 226, 228, 230, 249, 251, 253, 262, 265, 266, 274, 276, 277, 278, 280, 287, 295, 296, 297, 300, 305, 316, 317, 318, 319, 326, 327, 328, 329, 330, 331, 333, 334, 335, 338, 339, 340, 341, 347, 349, 350, 352, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 368, 369, 371, 372, 373, 374, 380, 381, 383, 384, 385, 386, 387, 388, 389, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 403, 405, 408, 425, 427, 428, 429, 431, 432, 433, 434, 435, 437, 438, 439, 440, 442, 446, 447, 448, 449, 451, 452, 453, 454, 456, 457, 459, 464, 473 Walsh, T., 36 Wang, B., 328, 330, 357 Wang, B.S., 328 Wang, C., 482 Wang, C.C., 174

572

Author Index

Wang, F., 343 Wang, H., 31 Wang, K., 93, 162 Wang, L., 388 Wang, Q., 343 Wang, R.H.J., 173, 178 Wang, S., 31, 454 Wang, T., 31 Wang, T.J., 469 Wang, W., 48, 83, 93, 162, 343, 387, 492 Wang, W.C., 41 Wang, W.G., 388 Wang, W.H., 384 Wang, X.M., 469 Wang, Y., 18, 31, 377, 410 Wängberg, I., 141, 143, 162, 439 Wanger, N.L., 377 Wantuck, P.J., 360, 361 Waring, R., 109, 136 Warmbt, W., 17, 18, 19 Warneck, P., 60, 366, 367, 431, 442, 443, 449 Warneke, C., 52, 55, 492 Warwick, N.J., 377 Washida, N., 203, 208, 209, 344, 371 Washington, W., 18 Watanabe, K., 174 Watanabe, T., 174 Waterland, R.L., 43, 44, 83, 437 Watson, C.E., 23 Watson, J., 492 Watson, L.A., 469 Watson, R.T., 63, 64 Watterson, J.D., 492 Watts, S.F., 412 Waygood, S.J., 130, 145 Wayne, R.P., 1, 9, 109, 130, 131, 133, 136, 142, 143, 145, 150, 155, 157, 159, 163, 170, 332, 333, 334, 384 WBCSD, 54 Weaver, J., 342 Webb, A., 32 Webb, A.R., 66, 482, 483 Webb, P.J., 492 Weber, R.J., 492 Wegener, R., 101, 110, 331 Weibring, P., 334, 410 Weichman, M.L., 352, 353, 364 Weilnstein-Lloyd, J.B., 108 Weimer, S., 491, 492 Weinheimer, A.J., 21, 48, 410 Weiss, R.F., 173, 178 Weitkamp, E.A., 493 Weller, G., 428 Wells, J.R., 150, 153 Welz, O., 99, 104, 106, 107, 108 Wendisch, M., 66, 482, 483 Wenger, J.C., 101, 102, 109, 110, 157, 345, 469, 492 Wenig, M., 411 Wennberg, P.O., 124, 173, 208, 322, 331, 346, 471, 496 van der Werf, G.R., 31, 49

Werle, P., 483 Wert, B., 175 Wesely, M.L., 124 Wetzel, M.A., 66, 482 Whalley, L.K., 46, 119, 173, 174, 177, 494, 495 Wheeler, M., 407, 410 White, A.B., 175, 454 Whitten, G.Z., 467, 468, 470, 471 Whitten, R.C., 423 WHO, 6, 7 Whyte, L.J., 327 Wiebe, H.A., 360, 361 Wiedensohler, A., 490, 492, 493 Wiedimyer, C., 331 Wiedmer, A., 44 Wieprecht, W., 108, 435 Wiesen, E., 437, 437 Wiesen, P., 93, 125, 274, 352, 353, 358, 359, 364, 365, 366, 367, 394, 395, 397, 398, 399, 436, 437, 454, 473 Wiesenfeld, J.R., 63, 64 Wild, O., 6, 44 Wildt, J., 492 van Wile, M., 427 Wilkinson, J.G., 479 Wille, U., 145 Williams, E.J., 127 Williams, E.L., II, 123, 174, 175, 454, 492 Williams, J., 208, 331 Williams, L.R., 491 Williams, P., 491, 492 Williams, P.I., 491 Willmott, C.J., 44 Wilson, K.R., 491 Wilson, M.R., 131, 133 Wilson, R.C., 29 Wilson, R.R., 124, 456 Wilson, S.R., 39, 44 Wilson, W.E., 60 Wine, P.H., 116, 117, 120, 121, 126, 127, 128, 129, 130, 277, 317, 323, 378, 383, 406, 408, 411, 414, 417, 420, 439, 457, 471, 491 Winer, A.M., 58, 127, 147, 154, 208, 320, 347, 352, 364, 369, 454 Wingen, L.M., 48, 125, 384 Winn, K.R., 360, 361 Winterhalter, R., 103 Wirtz, K., 101, 102, 103, 345, 436, 437, 469, 492, 493 Wistaler, A., 101, 110, 139, 141, 157 Witte, J.C., 22, 23 WMO, 25, 32, 33, 37, 38, 39, 40, 41, 42, 44, 58, 427 Wofsy, S.C., 5 Wohltmann, I., 33 Wojcik-Pastuszka, D., 104 Wojtal, P., 125 Woldridge, S.T., 437 Wolfe, G.M., 124, 127, 377 Wolff, E., 48, 409, 411 Wolford, G., 57 Wood, E.C., 491

Author Index Wood, S.W., 48 Wooldrige, P.J., 122, 124, 139, 141, 331 Woolley, A., 341, 342 Worsnop, D.R., 43, 106, 108, 414, 491, 492 Worth, J.J.B., 6, 44 Wu, F., 330, 333, 357 Wu, W.S., 31 Wuebbles, D.J., 41 Wyche, K.P., 83, 101 Xiang, B., 436 Xiaoyana, T., 31 Xie, M., 343 Xie, S., 490 Xing, J.-H., 386, 387, 388 Xiuji, Z., 31 Xu, J., 31, 108 Xu, W.Y., 31 Xu, X., 18 Xu, X.B., 31 Xu, Y., 83 Yamaji, K., 31 Yamanaka, T., 333, 334, 401 Yamano, D., 333, 334, 335 Yan, X., 31 Yang, H., 472 Yang, X., 48, 377 Yang, Y., 479, 487 Yang, Y.-J., 479 Yantosca, R.M., 412, 413, 466, 490 Yarnal, B., 15 Yarwood, G., 67, 71, 72, 83, 85, 96, 97, 98, 99, 109, 111, 133, 136, 138, 141, 146, 147, 150, 151, 161, 192, 203, 205, 211, 218, 372, 380, 384, 410, 425, 454, 464, 467, 468, 471, 473 Yatavelli, R.L.N., 124 Yee, L.D., 331 Yeung, L.Y., 318, 319, 344 Yocke, M., 467, 468, 471 Yokelson, R.J., 127, 128 Yokouchi, Y., 175 Yoshida, M., 174 Yoshida, Y., 377 Yoshino, A., 174 Yoshino, K., 9 Young, R.H., 63 Young, V.I., 175 Yttri, K.E., 493 Yu, Q., 31

573

Zabarnick, S., 360, 361, 362, 365, 366 Zabel, F., 99, 123, 316, 341, 350, 358, 359, 365, 473 Zádor, J., 343, 344, 469 Zamora, R.J., 175 Zanis, P., 39, 44 Zavala, M., 493 Zaveri, R.A., 491, 492, 493 Zawodny, J.M., 36 Zellner, R., 347, 348, 350, 351, 352, 353, 355, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 443, 445, 473 Zeng, G., 22, 44, 58 Zeng, T., 31, 377 Zetzsch, C., 178, 184, 304, 352, 353, 358, 359, 364, 365, 366, 367, 473 Zhang, D., 319 Zhang, J., 323 Zhang, J.Y., 323 Zhang, L., 349, 364, 367, 371, 372 Zhang, P., 343 Zhang, Q., 468, 491, 492, 493 Zhang, Q.J., 490, 492 Zhang, R., 136, 319 Zhang, R.Y., 319, 384 Zhang, S.W., 352 Zhang, T., 343 Zhang, X.C., 491 Zhang, X.Y., 491 Zhang, Y., 31, 48 Zhang, Y.H., 490 Zhang, Y.M., 491, 492 Zhao, C., 31 Zhao, H., 18 Zhao, Z., 136, 148, 155, 157, 169 Zheng, M., 63 Zheng, W., 48, 334, 410 Zhong, X., 398, 400, 405 Zhou, G., 31 Zhou, S., 93 Zhou, X., 108, 175 Zhou, Y.Z., 352 Zhou, Z., 343 Zhu, L., 420, 436, 490 Zhu, T., 93, 341, 372 Ziemann, P.J., 141, 321, 493 Ziemke, J.R., 22, 23 Zimmer, A., 449 Zimmerman, P., 49 Zinoviev, N.S., 33 Zogg, C.K., 107

SUBJECT INDEX

The abbreviations following a given compound name indicate the type of information given at the page cited:  k(Br), k(BrO), k(Cl), k(ClO), k(HO), k(IO), k(NO3), k(O3), indicate rate coefficients for the compound’s reaction with Br, BrO, Cl, ClO, HO, IO, NO3, and O3, respectively. CS, absorption cross section. j, photolysis frequency. M(Br), M(Cl), M(HO), M(NO3), M(O3), mechanisms of reaction with Br, Cl, HO, NO3, O3, respectively. Q, quantum yield of photodecomposition. τ, photochemical lifetime. Y(HO), yield of HO radicals formed in reactions with O3. Y(PCC), yield of primary carbonyl compounds formed in O3 reactions. Y(SCI), yield of stabilized Criegee intermediates formed in O3 reactions.

acenaphthalene, k(HO), 217; k(NO3), 197 acenaphthene, k(HO), 217; k(NO3), 147 acetaldehyde, CS, 430; j, 435; k(Br), 410; k(Cl), 393; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 acetic acid, k(Cl), 396; k(HO), 278 acetic acid anhydride, k(HO), 279 acetone, CS, 426; j, 444; k(Cl), 395; k(HO), 267; k(NO3), 158; τ, 460; Q, 443 2-acetylbenzaldehyde, j, 461; k(HO), 260; τ, 461 acetylcedrene, k(HO), 274; k(NO3), 158; k(O3), 92 acetylchloride, j, 452; τ, 460 acetylene. See ethyne acid rain, 412 acrolein, CS, 436; j, 437; k(Br), 410; k(Cl), 393; k(NO3), 156; M(HO), 264; Q, 436; τ, 459 acrylic acid, k(Cl), 396; k(HO), 278; k(O3), 92 actinic flux, 120–370 nm as a function of altitude, 8; solar zenith angle, and overhead ozone column effect on, 10 acyclic alcohols, effect of CH 3-substitution on k(O3), 88 aerosol, modeling of secondary inorganic and organic formation, 490 Air Resources Board (ARB), 6 air quality modeling, 465; CMAQ, 466; carbon bond mechanism, 469; DDM, 479; development of, 465; EKMA, 469; future developments in, 496 ; Green’s function, 479; methods of sensitivity and process analysis, 477; MCCM, 473; MCM, 468; potential deficiencies in, 493; RADM, 469; SAPRC, 469; WRF, 466 alkenes, k(O3), 72 alloisolongifolene, k(NO3), 136 allyl acetate, k(Cl), 398; k(HO), 285; k(O3), 92 allylic H-atoms, different types of, 205

2-amino-2-methyl-1-propanol, k(HO), 289 anthracene, k(HO), 218 atmospheric trace gases, vertical profile of, 13, 14; major sources of: CO, 54; NOx, 54; VOC’s, 48; NMHC, 49 azulene, k(NO3), 147 benzaldehyde, j(HO only) 459; j(total), 459; k(Cl), 393; k(HO), 257; k(NO3), 157; τ(total), 459; τ(HO only), 459 benzene, CS, 426; k(Br), 409; k(Cl), 385; k(HO), 213; k(NO3), 147 benzene oxide, k(HO), 240; k(NO3), 153; τ, 461 1,2-benzenediol, k(NO3), 150 1,4-benzodioxan, k(HO), 241; k(NO3), 153 2,3-benzofuran, k(O3), 91 benzoic acid, k(HO), 278 benzyl alcohol, k(HO), 23; k(NO3), 150; k(O3), 89 benzyl chloride, k(NO3), 147 cis-bicyclo[4.4.0]decane, k(Cl), 382; k(HO), 183 trans-bicyclo[4.4.0]decane, k(HO), 183 bicyclo[2.2.1]- 2,5-heptadiene, k(HO), 198; k(NO3), 135; k(O3), 78 bicyclo[2.2.1]heptane, k(HO), 182 bicyclo[ 2,2,1]-2-heptene, k(HO), 198; k(NO3), 135; k(O3), 78 cis-bicyclo[4.3.0]nonane, k(HO), 186 trans-bicyclo[4.3.0]nonane, k(HO), 186 bicyclo[2.2.2]octane, k(HO), 182 bicyclo[3.3.0]octane, k(HO), 182 bicyclo[2.2.2]-2-octene, k(HO), 195; k(NO3), 135; k(O3), 78 biphenyl, k(HO), 217 bis(difluoromethyoxy)difluoromethane, k(HO), 245

576

Subject Index

bis(difluoromethoxy)tetrafluoroethane, k(HO), 245 bornyl acetate, k(HO), 283 Br-atom reactions with organic compounds, 405; reactions with alkanes, 406; alkenes and alkynes, 406; aromatic hydrocarbons, 407; alcohols and ethers, 407; aldehydes, 407; ketones, acids, and esters, 409; role in polar surface ozone depletion, 409 bromine, j, 459; τ, 459 bromoacetone, CS, 449; j, 450; τ, 460 1-bromobutane, k(HO), 188 2-bromo-2-butene, k(NO3), 143 3-bromo-1-butene, k(NO3), 143 4-bromo-1-butene, k(NO3), 143 1-bromo-1-chloro -2,2,2-trifluoroethane, k(HO), 189 1-bromo-2-chloro -1,1,2-trifluoroethane, k(HO), 189 bromochlorodifluoromethane, k(HO), 189 bromochloromethane, k(HO), 189 bromodichloromethane, k(HO), 189 bromodifluoromethane, k(HO), 188 2-bromodiphenyl ether, k(HO), 248 2,2'-bromodiphenyl ether, k(HO), 248 3-bromodiphenyl ether, k(HO), 248 4-bromodiphenyl ether, k(HO), 248 bromoethanal, k(Br), 410 bromoethane, k(HO), 183 2-bromoethyl methyl ether, k(Cl), 390; k(HO), 245 1-bromohexane, k(HO), 188 1-bromo -1,2,2,2-tetrafluoroethane, k(HO), 188 1-bromo -2,2,2-trifluoroethane, k(HO), 189 bromomethyl methyl ether, k(Cl), 390 1-bromopentane, k(HO), 188 1-bromopropane, k(HO), 188 2-bromopropane, k(HO), 188 3-bromo-1-propene, k(NO3), 42; k(O3), 83 bromotrifluoromethane, k(HO), 189 BrO reactions with HO 2 and RO2, 333 1,3-butadiene, k(Br), 408; k(HO), 195; k(NO3), 135; k(O3), 74; M(O3), 87; Y(HO), 102 butanal, CS, 430; j, 435; k(Br), 410; k(Cl), 393; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 2,3-butandione, CS, 450; j, 451; k(Cl), 395; k(HO), 269; τ, 460 butane, CS, 426; k(Cl), 381; k(HO), 179; k(NO3), 132; M(NO3), 131 1,2-butanediol, k(HO), 221 1,3-butanediol, k(HO), 221 2,3-butanediol, k(HO), 221 butanoic acid, k(HO), 278 1-butanol, k(Cl), 386; k(HO), 219; k(NO3), 149 2-butanol, k(Cl), 386; k(HO), 219; k(NO3), 149 2-butanone, CS, 441; j, 448; k(Cl), 395; k(HO), 267; M(HO), 275; τ, 460 2-butenal, CS, 436; j, 4327; k(HO), 256; k(NO3), 156; k(O3), 91; M(O3), 91 3-butenal, k(O3), 91 1-butene, CS, 426; k(Br), 408; k(Cl), 384; k(HO), 193; k(NO3), 134; k(O3), 72; Y(HO), 101; Y(SCI), 105; Y(PCC), 111 cis-2-butene, k(Cl), 384; k(HO), 193; k(NO3), 134; k(O3), 72; Y(HO), 101; Y(PCC), 111

trans-2-butene, k(Br), 408; k(Cl), 384; k(HO), 193; k(NO3), 134; k(O3), 72; M(Cl), 384; Y(HO), 101; Y(SCI), 105; Y(PCC), 111 E-butenedial, j, 459; k(HO), 257; τ, 459 Z-butenedial, j, 459; k(HO), 257; τ, 459 2-buten-1-ol, k(Cl), 386; k(HO), 221; k(NO3), 149; k(O3), 89; M(O3), 88 3-buten-1-ol, k(Cl), 386; k(HO), 221; k(NO3), 149; k(O3), 89 3-buten-2-ol, k(Cl), 386; k(HO), 221; k(NO3), 149; k(O3), 89 3-butene-2-one, CS, 450; j, 460; k(HO), 269; k(O3), 91; M 451; τ, 460 2-butoxyethanol, k(HO), 232; k(NO3), 152 1-butoxy-2-propanol, k(HO), 233 iso-butyl acetate, k(Cl), 397; k(HO), 282 n-butyl acetate, k(Cl), 397; k(HO), 282 sec-butyl acetate, k(Cl), 397; k(HO), 282 tert-butyl acetate, k(Cl), 397; k(HO), 282 n-butyl acrylate, k(Cl), 398; k(HO), 285; k(NO3), 162; k(O3), 92 tert-butylbenzene, k(HO), 215 n-butyl n-butanoate, k(Cl), 397; k(HO), 283 tert-butyl chloride, k(HO), 187 n-butyl formate, k(Cl), 397; k(HO), 281 tert-butyl formate, k(Cl), 397; k(HO), 281 n-butyl methacrylate, k(Cl), 398; k(HO), 286 iso-butyl nitrate, k(Cl), 399; k(HO), 289 n-butyl nitrate, j, 455; k(Cl), 399; k(HO), 289; τ, 461 sec-butyl nitrate, k(HO), 289 tert-butyl nitrate, CS, 453; j, 455; τ, 461 iso-butyl nitrite, k(HO), 293; τ, 460 n-butyl nitrite, j, 460; τ, 460 sec-butyl nitrite, k(HO), 293 tert-butyl nitrite, j, 460; k(Cl), 400; k(HO), 293; τ, 460 n-butyl propionate, k(Cl), 397; k(HO), 283 tert-butyl toluene, k(NO3), 147 iso-butyl vinyl ether, k(HO), 234 tert-butyl vinyl ether, k(HO), 234 1-butyne, k(HO), 203; k(NO3), 143 2-butyne, k(HO), 203; k(NO3), 143 California Air Resources Board (CARB), 6 camphene, k(HO), 198; k(NO3), 135; k(O3), 79; Y(HO), 103; Y(SCI), 105; Y(PCC), 112 camphenilone, k(HO), 272 camphor, k(HO), 273; k(NO3), 158 carbamates, k(NO3), 163 carbon tetrachloride, k(HO), 119; mean global mixing ratios of, 41 carbonyl dibromide, j, 452; τ, 460 carbonyl disulfide (SCS), atmospheric reactions of, 420 carbonyl sulfide (OCS), atmospheric chemistry of, 421; CS, 422; j, 423; k(HO), 416; τ, 459; photodissociation of, in the stratosphere, 422; source of stratospheric sulfuric acid, 422 2-carene, k(HO), 199; k(NO3), 135; k(O3), 80 3-carene, k(HO), 199; k(NO3), 135; k(O3), 80; Y(HO), 103 caronaldehyde, k(HO), 260; k(NO3), 157 carvomethene, k(O3), 81

Subject Index β-caryophyllene, k(HO), 202; k(NO3), 136; k(O3), 82; Y(HO), 103 catalytic cycles of ozone decay induced by: BrOx, 36; ClOx, 36; HOx, 35; NOx, 36 α-cedrene, k(HO), 201; k(NO3), 135; k(O3), 82; Y(HO), 103 CF 3CFHCF2O(CH2)3OCF2CFHCF3, k(Cl), 391; k(HO), 246 C xF2x+1CH=CH2 (x = 2, 4, 8), k(O3), 83 C xF2x+1CH2CHO (x = 1, 4, 8), k(Cl), 393; k(HO), 261 C xF2x+1CH2CH2OH, (x = 1,4,6,8), k(Cl) 387 C xF2x+1CHO (x = 1, 2, 3, 4), k(HO), 261 C xF2x+1CHO (x = 1, 4, 6), k(Cl), 393 C xF2x+1CH(OH)2, (x = 1,3,4), 387 C xF2x+1C(O)OH (x = 2, 3, 4), k(HO), 278 CF 3OCF(CF3)CF2OCF2OCF3, k(HO), 245 n-C4F9O(CH2)3O-n-C4F9, k(HO), 246 Chapman mechanisms of stratospheric ozone formation and decay, 33, 36 CH 2=CHC(O)OCH2CH2CF2CF2CF2CF2CF2CF3, k(Cl), 398 C xH2x+1CH2OH (x = 2,3,4,6) chemical activation of RO radicals, 358 chemical mechanisms for air quality modeling, 465 CHF 2CF2CH2OH, k(Cl), 387 CHF 2CF2OCH2CF3, k(NO3), 152 CH 2=CHC(O)OCH2CH2(CH2)3CF3, k(Cl) (4:2 fluorotelomer acrylate), 398; CHF 2O(CF2CF2O)m(CF2O)nCHF2 (m = 0–7; n = 0–5), k(Cl), 391 CH 3OCH2CF3, k(NO3), 152 CH 3[OCF(CF3)CF2]m (OCF2)nOCF3, k(Cl), 391 CH 3O(CF2CF2O)mCH3, m = 1,2,3), k(Cl), 391; k(HO), 245 CF 3[OCF(CF3)CF2]m(OCF2)nOCF3, k(Cl), 391 CF 3CFHCF2O(CH2)3OCF2CFHCF3, k(Cl), 391 n-C4F9O(CH2)3O-n-C4F9, k(Cl), 391 cyclo-[(CF3)2CF]CFCF2C(OCH2CH3)(C(CF3)2O, k(Cl), 391 2-chloroacetaldehyde, k(HO), 261 1-chloroacetone, CS, 449; j, 450; k(Cl), 395; k(HO), 274; τ, 460 chloroacetylchloride, j, 452; τ, 460 4-chloro-1-buten-3-one, k(HO), 270 1-chlorodibenzodioxin, k(HO), 246 2-chlorodibenzodioxin, k(HO), 246 1-chlorobutane, k(HO), 87 1-chloro-1-butene, k(NO3), 143 1-chloro-2-butene, k(NO3), 143; k(O3), 83 2-chloro-1-butene, k(NO3), 143 2-chloro-2-butene, k(NO3), 143 3-chloro-1-butene, k(NO3), 143; k(O3), 83 4-chloro-1-buten-3-one, k(HO), 270 2-chloroethanal, k(Cl), 393 2-chloroethanol, k(Cl), 387 1-chloroethene, k(Br), 408; k(NO3), 142; k(O3), 83; M(NO3), 144 2-chloroethyl methyl ether, k(HO), 245 2-chloro -2,2-difluoroacetaldehyde, k(Cl), 393; k(HO), 262

577

1-chloro -1,1-difluoroethane, k(HO), 188 chlorodifloromethane, k(Cl), 383; k(HO), 188; mean global mixing ratios of, 41 chlorodifluoromethyl 2,2,2-trifluoroethyl ether, k(Cl), 391 2-chloroethyl methyl ether, k(Cl), 390 2-chloro-2-fluoroacetaldehyde, k(HO), 262 chlorodifluoromethyl -2,2,2-trifluoroethyl ether, k(Cl), 391 2-chloro -1,1,2-trifluoroethyl difluoromethyl ether, k(Cl), 391 2-chloro -1,1,2-trifluoroethyl methyl ether, k(Cl), 391; k(HO), 245 1-chloro -2,2,2-trifluoroethyl difluoromethyl ether, k(Cl), 391; k(HO), 245 1-chloro -2,2,2-trifluoroethyl ethyl ether, k(Cl), 391; k(HO), 245 chlorofluoromethane, k(Cl), 383; k(HO), 188 1-chlorohexane, k(HO), 187 chloromethyl methyl ether, k(Cl), 390 1-chloro-2-methyl-2-propene, k(NO3), 143; k(O3), 83 3-chloro-2-methylpropene, k(NO3), 143 1-chloro-3-methyl-2-butene, k(O3), 83 1-chloropentane, k(HO), 187 1-chloropropane, k(HO), 187 2-chloropropane, k(HO), 187 2-chloro-1-propene, k(O3), 83 3-chloro-1-propene, k(NO3), 142; k(O3), 83 1-chloro -1,3,3,3-tetrafluoroethane, k(HO), 188 1-chloro -2,2,2-trifluoroethane, k(HO), 188 chlorotrifluoromethane, k(HO), 181; mean global mixing ratios of, 38 cyclo-C6H11-O•, reactions of, 371 1,8-cineole, k(HO), 240; k(NO3), 153 citronellol, k(HO), 222, k(NO3), 150 Cl-atom, rate coefficients for reactions of with, alkanes, 380; haloalkanes, 380; alkenes, 383; aromatic hydrocarbons, 384; alcohols, 386; ethers, 388; aldehydes, 392; ketones, 392; organic acids, 394; esters, 396; N-atom containing compounds, 399 climate change, 57 ClO reactions with HO 2 and RO2, 333 CO, major sources, 50 52, 376; major sinks of, 376; M(HO), 376; reaction with HO, 376 CO 2, major sources and sinks of, 368 condensed chemical mechanisms (carbon bond, SAPRC, RADM/RACM), 469 α-copaene, k(HO), 201; k(NO3), 136; k(O3), 82; Y(HO), 103 m-cresol, k(HO), 223; k(NO3), 150; k(O3), 89 o-cresol, k(HO), 222; k(NO3), 150; k(O3), 89 p-cresol, k(HO), 223; k(NO3), 150; k(O3), 89 Criegee intermediates, fragmentation reactions of, CH 2OO‡, 94; CH3CH2OO‡, 97, 99; (CH3)2COO‡, 97, 100; CH2OO and CH3CHOO + H2O, 107; HO radical formation, 98; other trace gases, 108; stabilized (SCI), reactions of, 94; reaction with SO2, 106; structure of, 97; yield of SCI from some unsaturated compounds, 105 crotonaldehyde, CS, 436; k(Cl), 393; k(NO3), 156 cycloalkenes, k(O3), 84 cyclobutanecarbaldehyde, k(HO), 255; k(NO3), 156

578

Subject Index

cyclobutanone, j, 460; k(HO), 270; τ, 460 cyclobutyl methyl ketone, k(NO3), 158 cis-cyclodecene, k(O3), 79; Y(HO), 102 1,3-cycloheptadiene, k(HO), 197; k(NO3), 139; k(O3), 77 cycloheptane, k(HO), 182 1,3,5-cycloheptatriene, k(HO), 197; k(NO3), 135; k(O3), 77 cycloheptene, k(HO), 197; k(NO3), 135; k(O3), 77; Y(HO), 102 1,3-cyclohexadiene, k(HO), 197; k(NO3), 135; k(O3), 77 1,4-cyclohexadiene, k(HO), 197; k(NO3), 135; k(O3), 77 cyclohexane, k(Cl), 382; k(HO), 181; k(NO3), 132 cyclohexanecarbaldehyde, k(HO), 255; k(NO3), 156 cyclohexanol, k(HO), 220 cyclohexanone, k(Cl), 395; k(HO), 271 cyclohexene, k(Cl), 384; k(HO), 196; k(NO3), 135; k(O3), 76; M(O3), 86; Y(HO), 102; Y(SCI), 105 2-cyclohexen-1-one, k(O3), 92 cyclohexyl nitrate, k(HO), 290 cis,cis-1,3-cyclooctadiene, k(O3), 79 1,5-cyclooctadiene, k(O3), 79 cyclooctane, k(HO), 182 1,3,5,7-cyclooctatetraene, k(O3), 79 cis-cyclooctene, k(O3), 78; Y(HO), 102 cyclopentane, k(Cl), 382; k(HO), 181 cyclopentanecarbaldehyde, k(HO), 255; k(NO3), 156 cyclopentanol, k(HO), 220 cyclopentanone, k(Cl), 395; k(HO), 270 cyclopentene, k(HO), 196; k(NO3), 135; k(O3), 76; Y(HO), 102; Y(SCI), 105 cyclopentyl nitrate, j, 455; τ, 461 cyclopropane, k(Cl), 382; k(HO), 181 cyclopropanecarbaldehyde, k(HO), 255; k(NO3), 156 cyclopropanone, j, 460; τ, 460 p-cymene, k(HO), 215; k(NO3), 147 1,1,1,2,2,5,5,6,6,6-decafluorohexane, k(HO), 187 1,1,1,2,3,4,4,5,5,5-decafluoropentane, k(HO), 187 decanal, k(NO3), 156 decane, k(Cl), 381; k(HO), 180; k(NO3), 132 2-decanone, k(HO), 268 1-decene, k(HO), 195; k(NO3), 134; k(O3), 74; Y(PCC), 112 cis-5-decene, k(O3), 74; Y(PCC), 112 trans-5-decene, k(O3), 74; Y(SCI), 105; Y(PCC), 112 dibenzo- p-dioxin, k(HO), 242; k(NO3), 153 dibenzofuran, k(HO), 242; k(NO3), 153 1,2-dibromo-3-chloropropane, k(HO), 189 dibromodifluoromethane, k(HO), 189 2,4-dibromodiphenyl ether, k(HO), 249 3,3'-dibromodiphenyl ether, k(HO), 249 4,4'-dibromodiphenyl ether, k(HO), 249 1,2-dibromoethane, k(HO), 188 dibromomethane, k(HO), 187 1,2-dibromo-1,1,2,2-tetrafluorothane, k(HO), 189 di- n-butoxymethane, k(HO), 233 di- sec-butoxymethane, k(HO), 233 di- iso-butoxymethane, k(HO), 233 di-isobutyl ether, k(HO), 231 di- n-butyl ether, k(Cl), 389; k(HO), 231

di- tert-butyl ether, k(Cl), 389; k(HO), 231; k(NO3), 152 2,2-dichloroacetaldehyde, CS, 438; k(Cl), 393; k(HO), 261 1,1-dichloroacetone, k(Cl), 395; k(HO), 274 1,3-dichloroacetone, k(Cl), 395 dichloroacetylchloride, j, 452; τ, 460 2,3-dichlorodibenzodioxin, k(HO), 247 2,7-dichlorodibenzodioxin, k(HO), 247 2,8-dichlorodibenzodioxin, k(HO), 247 2,2-dichloroethanal, k(Cl), 393 1,1-dichloroethane, k(HO), 187 1,2-dichloroethane, k(HO), 187 1,1-dichloroethene, k(Br), 408; k(NO3), 142; k(O3), 83 Z-1,2-dichloroethene, k(NO3), 142; k(O3), 83 E-1,2-dichloroethene, k(NO3), 142 dichloroacetylchloride, j, 460; τ, 460 1,2-dichloro-1,1-difluoroethane, k(HO), 188 dichlorodifluoromethane, k(HO), 189; mean global mixing ratios of, 38 2,2-dichloroethyl methyl ether, k(Cl), 390; k(HO), 245 2,2-dichloro-2-fluoroacetaldehyde, k(Cl), 393; k(HO), 262 2,2-dichloro-2-fluoroethanal k(Cl), 393 1,1-dichloro-1-fluoroethane, k(HO), 188 dichlorofluoromethane, k(HO), 188 dichloromethane, k(Cl), 383; k(HO), 187; k(NO3), 133 2,2-dichloromethyl methyl ether, k(Cl), 390 1,1-dichloro-2,2,3,3,3-pentafluoropropane, k(HO), 188 1,3-dichloro-1,1,2,2,3-pentafluoropropane, k(HO), 188 1,3-dichloropropane, k(HO), 187 1,1-dichloropropene, k(NO3), 142 1,3-dichloropropene, k(NO3), 142 2,3-dichloropropene, k(NO3), 142 1,1-dichloro-1,3,3,3-tetrafluoropropane, k(HO), 188 1,2-dichloro-1,1,2,-trifluoroethane, k(HO), 188 1,1-dichloro-2,2,2-trifluoroethane, k(HO), 188 1,1-dichloro-1,2,2-trifluoropropane, k(HO), 188 diethoxymethane, k(HO), 232 1,2-diethoxyethane, k(HO), 233 2,2-diethoxypropane, k(HO), 233 3,4-diethyl-2-hexene, k(O3), 74 1-(difluoromethoxy)-2-(difluoromethoxy) difluoromethox y)- 1,1,2,2-tetrafluoroethane, k(HO), 245 di-isopropoxymethane, k(HO), 233 2,3-dimethylbenzaldehyde, k(HO), 258 2,4-dimethylbenzaldehyde, k(Cl), 393; k(HO), 258 2,5-dimethylbenzaldehyde, k(Cl), 393; k(HO), 259 2,6-dimethylbenzaldehyde, k(HO), 259 3,4-dimethylbenzaldehyde, k(HO), 259 3,5-dimethylbenzaldehyde, k(Cl), 393; k(HO), 259 1,4-dimethoxybutane, k(HO), 233 1,3-dimethoxypropane, k(HO), 232 2,2-dimethoxypropane, k(HO), 232 diethylene glycol n-butyl ether, k(HO), 233 diethylene glycol divinyl ether, k(NO3), 152; k(O3), 90 diethylene glycol ethyl ether, k(HO), 233 diethyl ether, k(Br), 410; k(Cl), 389; k(HO), 231; k(NO3), 152; M(HO), 251 3,4-diethyl-2-hexene, k(O3), 74; Y(PCC), 42

Subject Index 2,2-difluoroacetaldehyde, CS, 438; j, 439; k(HO), 261; k(NO3), 156; τ, 459 1,1-difluorodimethyl ether, k(HO), 242 1,1-difluoroethane, k(Cl), 383; k(HO), 186 1,2-difluoroethane, k(Cl), 383; k(HO), 186 2,2-difluoro-1-ethanol, k(Cl), 386 1,1-difluoroethene, k(NO3), 142 2,2-difluoroethyl 2,2-difluoroethyl ether, k(HO), 244 2,2-difluoroethyl trifluoromethyl ether, k(Cl), 390 difluoromethane, k(Cl), 383; k(HO), 186 difluoromethyl ether, k(Cl), 389; k(HO), 242 difluoromethyl 2,2,3,3-tetrafluoropropyl ether, k(HO), 244 difluoromethyl trifluoroacetate, k(Cl), 398 9,10-dihydroanthracene, k(NO3), 147 2,3-dihydrobenzofuran, k(HO), 241; k(NO3), 153 2,5-dihydrofuran, k(HO), 238; k(O3), 90 2,3-dihydro-5-methylfuran, k(NO3), 153 4,5-dihydro-2-methylfuran, k(O3), 90 dihydromyrcenol, k(NO3), 150 1,2-dihydroxybenzene, k(O3), 80 3,4-dihydroxy-3-hexene-2,5-dione, k(HO), 270; k(O3), 92 1,3-dihydroxy-3-methylbenzene, k(O3), 90 1,2-dihydroxy-3-methylbenzene, k(O3), 90 1,1-dihydroxy-4-methoxybenzene, k(O3), 90 diiodomethane, k(NO3), 133 1,1-dimethoxyethene, k(O3), 90 1,2-dimethoxybenzene, k(HO), 241; k(NO3), 153 1,3-dimethoxybenzene, k(NO3), 153 1,4-dimethoxybenzene, k(NO3), 153 dimethoxymethane, k(Cl), 389; k(HO), 232 1,2-dimethoxypropane, k(HO), 232 N,N-dimethylacetamide, k(Cl), 399; k(HO), 288; k(NO3), 163 dimethyl adipate, k(HO), 284 2-(dimethylamino)ethanol, k(HO), 289; k(O3), 93 2,3-dimethylbenzaldehyde, k(HO), 258 2,4-dimethylbenzaldehyde, k(Cl), 393; k(HO), 258 2,5-dimethylbenzaldehyde, k(Cl), 393; k(HO), 259 2,6-dimethylbenzaldehyde, k(HO), 259 3,4-dimethylbenzaldehyde, k(HO), 259 3,5-dimethylbenzaldehyde, k(Cl), 393; k(HO), 259 2,3-dimethyl-1,3-butadiene, k(Br), 408; k(HO), 196; k(NO3), 135; k(O3), 75; Y(HO), 103 2,3-dimethylbutane, k(Cl), 381; k(HO), 179; k(NO3), 132 3,3-dimethylbutanal, k(HO), 254; k(NO3), 156 2,3-dimethyl-2-butanol, k(Cl), 386; k(HO), 220 3,3-dimethyl-2-butanone, k(HO), 267 2,2-dimethylbutene, k(HO), 179 2,3-dimethyl-1-butene, k(O3), 73; Y(HO), 101; Y(PCC), 112 3,3-dimethyl-1-butene, k(HO), 194; k(NO3), 134; k(O3), 73; Y(PCC), 111 2,3-dimethyl-2-butene, k(Br), 408; k(HO), 194; k(NO3), 134; k(O3), 73; Y(HO), 102; Y(SCI), 105; Y(PCC), 111 dimethyl carbonate, k(Cl), 398; k(HO), 286 1,2-dimethyl-1-cyclohexene, k(NO3), 135; Y(HO), 102 2,3-dimethylcyclohexene, k(NO3), 135

579

2,2-dimethyl-3-(2,2-dimethylvinyl) oxirane, k(NO3), 153 dimethyl ether, CS, 426; k(Cl), 389; k(HO), 231; k(NO3), 152 dimethyldisulfide (CH 3SSCH3), atmospheric reactions of, 420; k(Cl), 416; k(HO), 416; k(NO3), 416 N,N-dimethylformamide, k(Cl), 399; k(HO), 288; k(NO3), 163 2,5-dimethylfuran, k(Cl), 389; k(HO), 238; k(NO3), 153 2,3-dimethylfuran, k(NO3), 153 2,4-dimethylfuran, k(NO3), 153 2,5-dimethylfuran, k(Cl), 389 dimethyl glutarate, k(HO), 284 E,Z-5,5-dimethyl-1,3-hexadiene, k(O3), 75 2,5-dimethyl-1,5-hexadiene, k(HO), 196; k(O3), 75 2,5-dimethyl-2,4-hexadiene, k(HO), 196; k(O3), 75 3,5-dimethyl-3-hexanol, k(HO), 220 3,5-dimethyl-1-hexyn-3-ol, k(HO), 222 trans-2,2-dimethyl-3-hexene, k(O3), 73; Y(PCC), 112 cis/trans-3,4-dimethyl-3-hexene, k(O3), 79; Y(PCC), 112 trans-2,5-dimethyl-3-hexene, k(O3), 73; Y(PCC), 112 dimethylketene, k(HO), 274; k(O3), 92 2,3-dimethylnaphthalene, k(HO), 217 N,N-dimethylnitramine, k(HO), 295 N,N-dimethylnitrosamine, j, 460; k(HO), 295; τ, 460 cis-3,7-dimethyl-1,6-octadiene, k(O3), 75 trans-3,7-dimethyl-1,6-octadiene, k(O3), 75 2,6-dimethyl-2,6-octadien-8-ol, k(HO), 222; k(O3), 89 3,7-dimethyl-1,6-octadien-3-ol, k(HO), 221; k(NO3), 149; k(O3), 89 3,7-dimethyl-1,3,6-octatriene, k(NO3), 75 3,7-dimethyl-6-octenal, k(HO), 261; k(O3), 91 3,7-dimethyl-6-octen-1-ol, k(HO), 222; k(O3), 89 2,6-dimethyl-7-octen-2-ol, k(HO), 221; k(O3), 89 cis-2,3-dimethyloxirane, k(NO3), 152 2,3-dimethylpentanal, k(HO), 255 2,3-dimethylpentane, k(HO), 179 2,4-dimethylpentane, k(Cl), 381; k(HO), 179; k(NO3), 132 2,4-dimethylpentanol, k(HO), 220 2,4-dimethyl-3-pentanone, k(HO), 267 3,3-dimethyl-1-pentene, k(NO3), 140 2,3-dimethyl-2-pentene, k(HO), 194 3,4-dimethyl-1-pentene, k(NO3), 134 4,4-dimethyl-1-pentene, k(NO3), 134 trans-4,4-dimethyl-2-pentene, k(HO), 194 2,4-dimethyl-2-pentene, Y(PCC), 112 2,3-dimethylphenol, k(HO), 223; k(NO3), 150 2,4-dimethylphenol, k(HO), 223; k(NO3), 150 2,5-dimethylphenol, k(HO), 224; k(NO3), 150 2,6-dimethylphenol, k(HO), 224; k(NO3), 150 3,4-dimethylphenol, k(HO), 224; k(NO3), 150 3,5-dimethylphenol, k(HO), 225; k(NO3), 150 2,2-dimethylpropanal, CS, 435, j, 459; k(Br), 410; k(Cl), 393; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 2,2-dimethylpropane, k(Cl), 381; k(HO), 129 2,2-dimethyl-1-propanol, k(HO), 219 N,N-dimethylpropionamide, k(HO), 288; k(NO3), 163 2,2-dimethyl-1-propyl nitrate, k(HO), 290 β,β-dimethylstyrene, k(HO), 216

580

Subject Index

dimethyl succinate, k(HO), 284 dimethyl sulfide (CH 3SCH3), atmospheric reactions of, 418; k(Br), 416; k(BrO), 416; k(Cl), 416; k(ClO), 416; k(HO), 416; k(IO), 416; k(NO3), 416; M(HO), 419 dimethylsulfone [CH 3S(O)(O)CH3], k(Cl), 416; k(HO), 416 dimethylsulfoxide [CH 3S(O)CH3], k(Br), 416; k(BrO), 416; k(Cl), 416; k(ClO), 416; k(HO), 416; k(NO3), 416 1,4-dioxane, k(HO), 236 1,4-dioxane, k(Cl), 389 1,3-dioxepane, k(HO), 237 1,3-dioxolane, k(Cl), 389; k(HO), 236 di- n-pentyl ether, k(Cl), 389; k(HO), 231 diphenyl ether, k(HO), 241 di- n-propoxymethane, k(HO), 233 di- iso-propyl ether, k(Cl), 389; k(HO), 231; k(NO3), 152 di -n-propyl ether, k(Cl), 389; k(HO), 231; k(NO3), 152 di -2,2,2-trifluoroethyl ether, k(Cl), 390 DNA, region of maximum light absorption by, 2 dodecane, k(HO), 180 1-dodecene, k(NO3), 134; k(O3), 74 Earth’s atmosphere air pressure as a function of altitude in, 12; air temperature as a function of altitude in, 12; chemical composition of, 11; concentration of trace gases in, 13, 14; distribution of actinic flux in, 8; ionosphere, 12; height of atmospheric regions of troposphere, 12; light absorption by oxygen in, 9; mesopause, 12; mesosphere, 12; stratopause, 12; stratosphere, 12; thermosphere, 12; tropopause, 12; vertical distribution of ozone in, 15 effects of [H 2O], temperature, and cloud-cover on ozone development, 481 emissions (anthropogenic), of: CO, 51; VOC, 51; NOx, 49; CH2O, 484; CO, 485 3,4-epoxy-1-butene, k(O3), 40 3,4-epoxy-cyclohexene, k(NO3), 153 ethanal. See acetaldehyde ethane, CS, 426; k(Cl), 381; k(HO), 179; k(NO3), 132 ethanol, k(Cl), 386; k(HO), 219; (NO3), 149 ethene, CS, 426; k(Br), 408; k(Cl), 384; k(HO), 193; k(NO3), 134; k(O3), 72; M(Br), 406; M(HO), 192; Y(HO), 99; Y(SCI), 105; Y(PCC), 111 2-ethoxy -2,5-bis(perfluoropropane-2-yl)tetrahydrofuran, k(HO), 246 2-ethoxyethanol, k(HO), 232 2-ethoxyethyl acetate, k(HO), 234 2-ethoxyethyl isobutyrate, k(HO), 234 2-ethoxyethyl formate, k(HO), 233 ethoxymethyl acetate, k(HO), 233 ethoxymethyl formate, k(HO), 234 3-ethoxy -1,1,1,2,3-pentafluoro-2-(trifluoromethyl)hexane, k(Cl), 390 2-ethoxy -3,3,4,4,5-pentafluoro-tetrahydro2,5-bis[1,2,2,2-tetrafluoro-1-(trifluoromethyl) ethyl]-furan, k(HO), 246

3-ethoxy -1,1,1,2,3,4,4,5,5,6,6,6-dodecafluoro-2(trifluoromethyl)hexane, k(Cl), 391; k(HO), 245 3-ethoxy-1-propanol, k(HO), 232 4-ethoxytoluene, k(NO3), 153 ethyl acetate, k(Cl), 397; k(HO), 281; k(NO3), 162 ethyl acrolate, k(Cl), 398; k(HO), 285; k(NO3), 162 ethylbenzene, k(Cl), 385; k(HO), 213; k(NO3), 147 2-ethylbutanal, k(HO), 254; k(NO3), 196 ethyl n-butanoate, k(Cl), 397; k(HO), 283 ethyl n-butyl ether, k(Cl), 389; k(HO), 231 ethyl tert-butyl ether, k(Cl), 389; k(HO), 134; k(NO3), 152 ethyl chloride, k(HO), 187 ethyl crotonate, k(Cl), 398; k(HO), 285 ethyl 3-ethoxypropionate, k(HO), 234 ethyl formate, k(Cl), 397; k(HO), 281; k(NO3), 162 ethylene glycol diacetate, k(Cl), 398; k(HO), 284 ethylene glycol diformate, k(Cl), 398; k(HO), 284 ethylene glycol divinyl ether, k(HO), 234; k(NO3), 152; k(O3), 90 ethylene glycol monovinyl ether, k(HO), 234; k(NO3), 152; k(O3), 90 ethylene oxide, k(Cl), 389; k(HO) 235 2-ethyl-1-butene, k(NO3), 134; k(O3), 73; Y(PCC), 111 ethyl ethylcarbomate, k(NO3), 163 ethyl formate, k(Cl), 397 2-ethyl furan, k(Cl), 389; k(HO), 238 2-ethylhexanal, k(Cl), 393 ethylhydroperoxide, k(Cl), 387 ethyl iodide, k(HO), 188 ethylketene, k(HO), 274; k(O3), 92 ethyl lactate, k(HO), 286 ethyl methacrylate, k(Cl), 398; k(HO), 286; k(NO3), 162; k(O3), 93 ethyl methylcarbamate, k(NO3), 161 ethyl nitrate, CS, 453; j, 455; k(Cl), 399; k(HO), 289; τ, 460 ethyl nitrite, j, 460; k(Cl), 400; k(HO), 293; τ, 460 ethyl oxirane, k(HO), 235 ethyl 1-propenyl ether, k(O3), 90 ethyl propionate, k(Cl), 397; k(HO), 282; k(NO3), 162 2-ethyltoluene, k(HO), 214 3-ethyltoluene, k(HO), 214 4-ethyltoluene, k(HO), 214; k(NO3), 147 ethyl 2,2,2-trifluoroacetate, k(Cl), 398; k(HO), 287 ethyl vinyl ether, k(HO), 234; k(NO3), 152; k(O3), 90; M(O3), 93 ethyl vinyl ketone, k(Cl), 395; k(NO3), 158 ethyne, k(Br), 408; k(Cl), 384; k(HO), 203; k(NO3), 145; M(HO), 208 explicit computer generated mechanisms, 468 fluoranthene, k(HO), 217 fluorene, k(NO3), 147 fluoroacetone, CS, 449; k(HO), 274 fluoroethane, k(HO), 186 2-fluoro-1-ethanol, k(Cl), 386 fluoroethene, k(NO3), 142 fluoromethane, k(Cl), 383; k(NO3), 186 1-fluoro-2-propanone, k(Cl), 395

Subject Index 3-fluoropropene, k(NO3), 142 formaldehyde, CS, 426, 430; j, 432; k(Br), 410; k(Cl), 393; k(HO), 254; k(NO3), 156; M(HO), 432; Q, 431; τ, 459, 462; importance as a source of HOx in polluted troposphere, 433 formamide, k(NO3), 163 formic acetic anhydride, k(HO), 279; k(Cl), 396 formic acid, CS, 326; k(Cl), 396; k(HO), 278 formic acid anhydride, k(HO), 279 formic propionic anhydride, k(Cl), 396 2-formylbenzaldehyde, k(HO), 260 formyl bromide, j, 452; τ, 460 formyl chloride, j, 452; k(Cl), 393; k(HO), 261; τ, 460 formyl fluoride, k(Cl), 393; k(HO), 261 N-formyl pyrrolidinone, k(HO), 280 furan, k(Cl), 389; k(HO), 237; k(NO3), 153; k(O3), 90; M(NO3), 151 3H-furan-2-one, k(HO), 284; k(NO3), 162 2-furfural, k(Cl), 289; k(HO), 239 3-furfural, k(Cl), 289; k(HO), 239 geraniol, k(HO), 222; k(NO3), 149 global warming potential (GWP), 41 glycidaldehyde, k(HO), 237; j, 459; τ, 459 glycolaldehyde, k(Cl), 393 glyoxal, CS, 439; j, 439; k(Cl), 393; k(HO), 256; Q, 440; τ, 459 Haagen-Smit, early studies of tropospheric ozone formation, 5 halocarbons, effect on stratospheric ozone concentrations, 38 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1decanol, k(HO), 225 2,2,3,3,4,4,4-heptafluorobutanal, k(HO), 226 2,2,3,3,4,4,4-heptafluorobutan-1-ol, k(Cl), 387; k(HO), 226 2,2,3,3,4,4,4-heptafluorobutyric acid, k(HO), 226 2,2,3,3,4,4,4-heptafluoro-1,1-dihydroxybutane, k(Cl), 387; k(HO), 226 1,1,1,2,3,3,3-heptafluoropropane, k(HO), 226 heptafluoro- n-propyl formate, k(HO), 287 1,1,2,2,3,3,3,-heptafluoropropyl methyl ether, k(Cl), 390 1,1,2,2,3,3,3-heptafluoropropyl 1,2,2,2-tetrafluoroethyl ether, k(Cl), 390 heptanal, j, 435; k(Cl), 393; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 heptane, k(Cl), 381; k(HO), 179; k(NO3), 132 1-heptanol, k(Cl), 386; k(HO), 220 4-heptanol, k(HO), 220; k(NO3), 149 2-heptanone, k(HO), 267 E-2-heptenal, k(Cl), 393; k(HO), 256; k(NO3), 157 Z-2-heptenal, k(HO), 256; k(NO3), 157 1-heptene, k(HO), 194; k(NO3), 134; k(O3), 73; Y(HO), 102; Y(PCC), 112 trans-2-heptene, k(HO), 194 2-heptyl nitrate, k(Cl), 399 3-heptyl nitrate, k(HO), 290 hexadecane, k(HO), 180 2,4-hexadiendial, k(O3), 91

581

trans-2-trans-4-hexadiene, k(HO), 257; k(NO3), 135; k(O3), 75 cis-2,trans-4-hexadiene, k(HO), 257; k(O3), 75 hexanal, j,; k(Cl), 393; k(NO3), 156; Q, 434; τ, 459 hexane, k(HO), 174; k(NO3), 132 cis-3-hexenyl acetate, k(NO3), 162; k(O3), 92 cis-hex-3-ene-2,5-dione, k(O3), 91 trans-hex-3-ene-2,5-dione, k(O3), 91 cis-3-hexen-1-ol, k(HO), 221 hex-5-ene-2-one, k(O3), 91 hex-4-ene-3-one, k(O3), 91 cis-/trans-1,3,5-hexatriene, k(HO), 196; k(O3), 75 cis-3-hexenyl acetate, k(NO3), 162; k(O3), 92 cis-hex-3-ene-2,5-dione, k(O3), 91 trans-hex-3-ene-2,5-dione, k(O3), 91 cis,2-trans-4-hexadiene, k(HO), 195; k(O3), 91 cis,cis-2,4-hexadienedial, k(HO), 257 cis,trans-2,4-hexadiendial, k(HO), 257 trans,2-trans-4-hexadiene, k(O3), 75; k(NO3), 135; k(HO), 195 trans-1,3-hexadiene, k(HO), 195 trans-1,4-hexadiene, k(HO), 195 1,5-hexadiene, k(HO), 195 hexafluoroacetone, j, 450; τ, 460 3,3,4,5,5,5-Hexafluorobutan-1-ol, k(Cl), 387 1,1,1,2,2,4-hexafluorobutane, k(HO), 187 1,1,1,4,4,4-hexafluorobutane, k(HO), 187 1,1,1,3,3,3-hexafluoro-2-(fluoromethyl)propane, k(Cl), 390 1,1,1,3,3,3-hexafluoro-2-methoxypropane, k(Cl), 390 1,1,1,3,3,3-hexafluoro-2-trifluoromethoxypropane, k(Cl), 390 1,1,1,3,3,3-hexafluoroisopropyl fluoromethyl ether, k(Cl), 390; k(HO), 245 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, k(Cl), 390; k(HO), 243 1,1,1,3,3,3-hexafluoroisopropyl trifluoromethyl ether, k(Cl), 390; k(HO), 243 1,1,1,2,2,3-hexafluoropropane, k(HO), 186 1,1,1,3,3,3-hexafluorolpropane, k(HO), 186 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, k(Cl), 390; k(HO), 244 1,1,2,3,3,3-hexafluoropropyl 2,2,2-trifluoroethyl ether, k(HO), 244 1,1,2,3,3,3-hexafluoropropyl trifluoromethyl ether, k(HO), 244 1,1,2,3,3,3-hexafluoropropyl ethyl ether, k(Cl), 390; k(HO), 244 1,1,2,3,3,3-hexafluoropropyl methyl ether, k(Cl), 390; k(HO), 244 1,1,2,3,3,3-hexafluoropropyl-2,2,3,3-tetrafluoropropyl ether, k(HO), 245 1,1,2,3,3,3-hexafluoropropyl-2,2,3,3,3-pentafluoropropyl ether, k(HO), 245 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethyl cyclopenta[γ]-2-benzopyran, k(HO), 242 hexanal, j, 435; k(Cl), 393; k(HO), 254, k(NO3), 156; Q, 434; τ, 459 2,5-hexandione, k(HO), 269

582

Subject Index

hexane, k(Cl), 381; k(HO), 179; k(NO3), 132 1-hexanol, k(Cl), 386; k(HO), 219 2-hexanol, k(HO), 219 2-hexanone, j, 448, 460; k(Cl), 395; (HO), 267; τ, 448, 460 3-hexanone, k(Cl), 395; k(HO), 267 cis-1,3,5-hexatriene, k(HO), 196 cis-/trans-1,3,5-hexatriene, k(HO), 196; k(O3), 75 trans-1,3,5-hexatriene, k(HO), 196 E-2-hexenal, k(Cl), 393; k(HO), 256; k(NO3), 157; k(O3), 91 E,E-2,4-hexadiendial, k(HO), 257 cis-hex-3-ene-2,5-dione, j, 460; k(O3), 91; τ, 460 trans-hex-3-ene-2,5-dione, j, 460; k(O3), 91; τ, 460 1-hexene, k(HO), 194; k(NO3), 134; k(O3), 73; Y(HO), 102; Y(PCC), 111 cis-2-hexene, k(NO3), 134; k(O3), 73 trans-2-hexene, k(NO3), 134; k(O3), 73 cis-3-hexene, k(NO3), 134; k(O3), 73; Y(HO), 102; Y(PCC), 111 trans-3-hexene, k(NO3), 134; k(O3), 73; Y(HO), 102; Y(PCC), 111 cis-3-hexene-2,5-dione, j, 460; k(HO), 270; τ, 460 trans-3-hexene-2,5-dione, j, 460; k(HO), 270; τ, 460 trans-3-hexen-1-ol, k(NO3), 149 cis-2-hexen-1-ol, k(NO3), 149; k(O3), 89 trans-2-hexen-1-ol, k(NO3), 149 cis-3-hexen-1-ol, k(NO3), 149; k(O3), 89 cis-4-hexen-1-ol, k(NO3), 149 4-hexen-2-one, k(HO), 269 4-hexen-3-one, k(Cl), 395; k(HO), 269, k(O3), 91 5-hexen-2-one, k(Cl), 395; k(HO), 269; k(O3), 91 5-hexene-2-one, k(Cl), 395; k(O3), 91 cis-3-hexenyl acetate, k(HO), 285; k(NO3), 162; k(O3), 92 2-hexyl nitrate, k(HO), 290 3-hexyl nitrate, k(HO), 290 1-hexyne, k(NO3), 145 2-hexyne, k(NO3), 145 HO radical, CS, 173 fraction reaction of with various trace gases in polluted air scenario; measured and model simulated concentrations of, 175–177; and measurements of, 173; mechanisms of reactions of with: alkanes, 178; halogen substituted alkanes, 168, 178; alkenes (mono-alkenes, 191, 205; dienes, 206; alkynes, 204; aromatic hydrocarbons, 209); alcohols, 228; (acylic alcohols, 211; diols, 227; aromatic alcohols, 228; unsaturated alcohols, 228; halogen substituted alcohols, 229); ethers (acyclic, 230; difunctional, 251; vinyl ethers, 251; cyclic polyethers, 257; halogen substituted ethers, 251); aldehydes (acyclic, 263; unsaturated, 263; aromatic, 265; halogen substituted, 265); ketones (acyclic, 266; hydroxyketones, 276; unsaturated, 276; halogen substituted, 276); organic acids, 276; acid anhydrides, 276; esters (acyclic, 277; unsaturated, 279; halogen substituted esters, 279); N-containing oxygenates, 280 rate coefficients for reactions of with alkanes, 179; cyclic alkanes, 181; halogen substituted alkanes, 185; alkenes (mono-alkenes, 191; dienes,

206); alkynes, 208; aromatic hydrocarbons, 209; alcohols, 211 (acylic alcohols, 211; diols, 227; aromatic alcohols, 228; unsaturated alcohols, 228; halogen substituted alcohols, 229); ethers (acyclic, 230; difunctional, 251; vinyl ethers, 251; cyclic, 251; polyethers, 251; halogen substituted ethers, 251); aldehydes, 253 (acyclic, 263; unsaturated, 263); aromatic, 265; halogen substituted, 265; ketones, 265 (acyclic, 266; hydroxyketones, 276; unsaturated, 276; halogen substituted ketones, 276); organic acids, 276; acid anhydrides, 276; esters (acyclic, 277; unsaturated, 279; halogen substituted, 279); N-containing oxygenates, 280 rates of generation of from HONO, ozone, and CH 2O, 457; yields of, in ozone-alkene reactions, 113 HO, HO 2, comparison of modeled and measured values of HO2/HO ratio, 494; maximum values of as function of H2O and temperature, 483; mechanism of formation in forested regions, 207 HO 2, RO2, kinetics of the reactions of HO 2 + RO2, 317, 318, 326 RO 2 + NO, 320 HO 2 + NO2, 324 RO 2 + NO2, 325; organic nitrate formation, 320 HO 2 + HO2, 326 RO 2 + HO2, 327; mechanisms and products of, 329 RO 2 + NO3, 332 HO 2 + NO3, 331, 332 HO 2 + ClO, 332 HO 2 + BrO, 333 HO 2 + O3, 335 RO 2 + O3, 335 RO 2 + RO2, 339; products of, 341 RO 2 + R'O2, 339; products of, 341 RO 2 unimolecular reaction, RO2 ® QOOH isomeriztion, 343 α-hydroxy-peroxy radicals, decomposition reactions, 346 α-humulene, k(O3), 82; Y(HO), 103 hydrogen peroxide, j, 459; k(Cl), 387 hydrogen sulfide, atmospheric oxidation of, 414 hydroxacetaldehyde, CS, 438; k(HO), 256 hydroxyacetone, CS, 449; j, 450; k(Cl), 395; k(HO), 268; τ, 460 2-hydroxybutanal, k(HO), 256 3-hydroxybutanal, k(HO), 256 1-hydroxy-2-butanone, k(HO), 268; k(NO3), 158 3-hydroxy-2-butanone, k(HO), 268; k(NO3), 158 4-hydroxy-2-butanone, k(HO), 268; k(NO3), 158 5-hydroxy-2-heptanone, k(HO), 268 6-hydroxy-3-heptanone, k(HO), 268 1-hydroxy-4-heptanone, k(HO), 268 3-hydroxy-2-hexanone, k(HO), 268 4-hydroxy-3-hexanone, k(HO), 268; k(NO3), 158 6-hydroxy-3-hexanone, k(HO), 268 3-hydroxy-3-methyl-2-butanone, k(HO), 268; k(NO3), 158hydroxymethylhydroperoxide, 461 4-hydroxy-4-methyl-2-pentanone, k(HO), 268 2-hydroxy-1-methylpropanal, k(HO), 256 5-hydroxy-2-octanone, k(HO), 269

Subject Index 6-hydroxy-3-octanone, k(HO), 269 7-hydroxy-4-octanone, k(HO), 269 5-hydroxy-2-pentanone, k(HO), 268 2-hydroxypropanal, k(HO), 256 3-hydroxypropanal, k(HO), 256 indane, k(HO), 215; k(NO3), 147 indene, k(HO), 215: k(NO3), 147 indole, k(NO3), 147 inorganic halogen species, sources in troposphere, 377; kinetics of the major reactions of interconversion, 378, 379 interelationships between O 3 formation, VOC, NOx etc., 466 iodine, j, 449; τ, 449 iodine-atom reactions with organic compounds, 411 iodoethane, k(NO3), 188 iodomethane, k(HO), 188 iodomethyl methyl ether, k(Cl), 391 1-iodopropane, k(HO), 188 2-iodopropane, k(HO), 188 3-iodo-1-propene, k(NO3), 142; k(O3), 83 b-ionone, k(HO), 273; k(NO3), 158; k(O3), 92 isochroman, k(HO), 241 isolongifolene, k(NO3), 136 isopleths, Simulated, 477; HNO3 478; HO, 478; HO2, 478; H2O2, 478; O3, 478; PAN, 478 isoprene. See 2-methyl-1,3-butadiene isopropenyl acetate, k(O3), 92; M(O3), 96 isopropenyl-6-oxo-heptanal, k(HO), 261; k(NO3), 157; k(O3), 91 2-isopropoxyethanol, k(HO), 232 isopropyl isobutyl ether, k(HO), 231 j, tau(greek symbol), 459 j-values, 425, 427 Junge sulfuric acid layer, stratosphere, 421 ketene, CS, 441; j, 448; k(Cl), 395; k(HO), 274; k(NO3), 158; k(O3), 92; τ, 460 Kuznets curve, 53, 56 Leighton relationship, 47; 474 limonaketone, k(NO3), 158 limonene, k(HO), 199; k(NO3), 135; k(O3), 80; Y(HO), 103; Y(PCC), 112 linalool, k(HO), 221; k(NO3), 149 longifolene, k(HO), 202; k(NO3), 136; k(O3), 83 Los Angeles Air Pollution Control District, 6 major products of the reactions of HO with: alkanes, 179; alkenes, 193; aromatic hydrocarbons, 209; alcohols, 211; aldehydes, 253; ketones, 265; acids, 276; esters, 277; N-containing oxygenates, 280; NO3 with alkanes, 132; alkenes, 140; alcohols, 148; aldehydes, 156; ketones, 158; esters, 160; N-containing oxygenates, 163; O3 with alkenes, 72; usaturated oxygenates, 89 major sources of the atmospheric trace gases: CO, 52; NOx, 51, 52; VOCs, 25

583

Master Chemical Mechanism, 468 maximum HO, HO2, and HO/O isopleths, 483 maximum ozone and maximum HNO3 isopleths, 482 measures of ozone formation reactivity: MOIR, 487; MIR, 487; POCP, 489 mechanism of HO radical formation, O(1D) + H2O, 61 mechanisms for air chemistry modeling: condensed mechanisms, 469; detailed explicit (MCM), 468; EKMA, 467, 468; Carbon Bond (CB041, CB05, CB06), 467, 468; RADM1/2, 468; RACM1/2, 468; SAPRC-99; SAPRC07C, 468; CMAQ, 472; MCCM, 473; WRF, 473; MCMv3.2; MCMv3.1, 467 mechanisms of ozone photodecomposition, 61 mechanisms of ozone reactions in the troposphere, 67 mechanisms of reactions of HO 2 and RO2 radicals, 315: HO2-NO, 317, 318; RO2-NO, 317, 318; HO2-RO2, 317 mechanisms of photodecomposition, aldehydes, 428; ketones, 440; carbonyl halides, 451; formyl halides, 451; acetyl halides, 451; alkyl nitrates, 452; peroxyacyl nitrates, 453 mesopause, region as defined by U.S. standard atmosphere, 12 mesosphere, region as defined by U.S. standard atmosphere, 12 meteorology, effects on air quality, 14 meteorology, effects on ozone ambient concentrations, 14 methacrolein, CS, 436; j, 437; k(Br), 410; k(Cl), 393; k(NO3), 156; M(HO), 264; M(O3), 94; Q, 436; τ, 459; structures of hydroperoxy radicals formed from, 322 methacrylic acid, k(O3), 92; M(O3), 95; methanal. See formaldehyde methane, CS, 426; k(Cl), 381; k(HO), 179; k(NO3), 132; M(HO), 172 methanethiol, atmospheric oxidation of, 414 methanol, CS, 426; k(Cl), 386; k(HO), 219; k(NO3), 149 methoxybenzene, k(HO), 240; k(NO3), 153 3-methoxy-1-butanol, k(HO), 232 trans-4-methoxy-3-buten-2-one, k(HO), 270; k(O3), 91 2-methoxyethyl acetate, k(HO), 234 2-methoxyethyl formate, k(HO), 234 methoxymethyl acetate, k(HO), 233 methoxymethyl formate, k(HO), 233 1-methoxy-2-propanol, k(NO3), 152; k(HO), 232 4-methoxytoluene, k(NO3), 153 N-methylacetamide, k(HO), 288 methyl acetate, k(Cl), 397; k(HO), 281; k(NO3), 162 methyl acrylate, k(Cl), 398; k(HO), 285; k(NO3), 162; k(O3), 92 methyl tert-amyl ether, k(HO), 231; k(NO3), 152 2-methylbenzaldehyde, k(NO3), 157 3-methylbenzaldehyde, k(NO3), 157 4-methylbenzaldehyde, k(NO3), 157 3-methyl -1,2-benzenediol, k(NO3), 150 4-methyl -1,2-benzenediol, k(NO3), 150 methyl bromide, k(Cl), 383; k(HO), 187 3-methyl -1,2-butadiene, k(HO), 145

584

Subject Index

2-methyl -1,3-butadiene, k(Br), 408; k(HO), 195; k(O3), 135; k(O3), 74; Y(HO), 102; peroxy hydroxy isoprene nitrate isomers structures, 139 2-methylbutanal, k(HO), 254; k(NO3), 156 3-methylbutanal, j, 435; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 2-methylbutane, k(Cl), 381; k(HO), 179; k(NO3), 132 methyl n-butanoate, k(Cl), 397 2-methyl-2-butanol, k(Cl), 386; k(HO), 219 3-methyl-1-butanol, k(Cl), 386; k(HO), 219 3-methyl-2-butanol, k(Cl), 386 3-methyl-2-butanone, k(Cl), 395; k(HO), 267; k(NO3), 158 trans-2-methyl-2-butenal, k(O3), 91 2-methyl-2-butenal, k(O3), 91 3-methyl-2-butenal, k(Cl), 393; k(O3), 91 2-methyl-1-butene, k(Br), 408; k(HO), 193; k(NO3), 134; k(O3), 72; Y(HO), 101; Y(PCC), 111 3-methyl-1-butene, k(HO), 193; k(NO3), 134; k(O3), 72; Y(PCC), 111 2-methyl-2-butene, k(Br), 408; k(Cl), 384; k(HO), 194; k(NO3), 134; k(O3), 72; Y(HO), 101; Y(PCC), 111 methyl E-2-butenoate, k(NO3), 162 3-methyl-2-buten-1-ol, k(Cl), 386; k(HO), 221; k(NO3), 149 3-methyl-3-buten-1-ol, k(Cl), 386; k(HO), 221; k(NO3), 149 3-methyl-3-buten-2-ol, k(Cl), 386; k(NO3), 149 2-methyl-3-buten-2-ol, k(Cl), 386; k(HO), 221; k(NO3) k(O3), 89 3-methyl-3-buten-2-one, k(HO), 269; k(NO3), 158 2-methyl-3-butene-1-ol, k(Cl), 386; k(HO), 221 methyl tert-butyl ether, k(Cl), 389; k(NO3), 152 2-methyl-1-butyl nitrate, k(HO), 290 3-methyl-1-butyl nitrate, k(HO), 290 3-methyl-2-butyl nitrate, k(HO), 290 methyl butyrate, k(NO3), 162; k(HO), 283 methyl crotonate, k(Cl), 398; k(HO), 285 methylchloride, k(Cl), 383; k(HO), 187; k(NO3), 133; mean global mixing ratios of, 41 1-methylcycloheptene, k(O3), 77 methylcyclohexane, k(Cl), 382; k(HO), 181 methylcyclohexane carboxylate, k(Cl), 398 1-methyl-1-cyclohexene, k(HO), 198; k(NO3), 135; k(O3), 76; Y(HO), 102; Y(SCI), 105 3-methyl-1-cyclohexene, k(O3), 77 4-methyl-1-cyclohexene, k(O3), 77 3-methyl-2-cyclohexen-1-one, k(HO), 271 4-methyl-3-cyclohexen-1-one, k(HO), 272; k(NO3), 158; k(O3), 92 1-methyl-1-cyclooctene, k(O3), 78 3-methyl-1-cyclooctene, k(O3), 79 methylcyclopentane k(HO), 181 1-methyl-1-cyclopentene, k(O3), 76 3-methyl-1-cyclopentene, k(O3), 76 2-methyl-1-decene, k(O3), 74 methyl 2,2-difluoroacetate, k(Cl), 398; k(HO), 287 methyl 3,3-dimethylacyloate, k(Cl), 398 4-methyl -1,3-dioxane, k(HO), 237 1-methyl -1,2-dinitrooxy cyclohexane, j, 461; k(HO), 291; τ, 461

methylenecyclobutane, k(NO3), 135; k(O3), 75 methylenecycloheptane, k(NO3), 135 methylenecyclohexane, k(NO3), 135; k(O3), 76; Y(SCI), 105 3-methylenecyclohexene, k(NO3), 135 methylenecyclopentane, k(NO3), 135; k(O3), 78 methylenecyclopropane, k(NO3), 135; k(O3), 75 3-methylene-7-methyl -1,6-octadiene, k(O3), 75 4-methylenehex-5-enal, k(HO), 257; k(NO3), 157; k(O3), 91 methyl ethyl carbamate, k(NO3), 163 methyl ethyl ether, k(Cl), 389; k(HO), 231; k(NO3), 152 5-(1-methylethyl)-bicyclo[3.1.0]hexane-2-one, k(NO3), 158 methyl fluoride, k(Cl), 583 methyl fluoroformate, k(HO), 287 N-methylformamide, k(Cl), 399; k(HO), 288; k(NO3), 163 methyl formate, CS, 426; k(Cl), 397; k(HO), 281; k(NO3), 162 2-methylfuran, k(Cl), 389; k(HO), 238; k(NO3), 153 3-methylfuran, k(Cl), 389; k(HO), 238; k(NO3), 153; k(O3), 90 5-methyl-2-furfural, k(Cl), 389; k(HO), 239 methylglyoxal, CS, 439; j, 441; k(Cl), 393; k(HO), 256; Q, 440; τ, 459 6-methyl-5-hepten-2-ol, k(HO), 221 methyl n-heptanoate, k(Cl), 398 2-methyl-1-heptene, k(O3), 75 6-methyl-5-hepten-2-one, k(HO), 270; k(NO3), 158; k(O3), 91 3( E)-4-methylhex-3,5-dienal, k(HO), 257; k(NO3), 157; k(O3), 91 3( Z)-4-methylhex-3,5-dienal, k(HO), 257; k(NO3), 157; k(O3), 91 E,E-2-methyl-2,4-hexadiendial, k(HO), 257 E,Z-5-methyl-1,3-hexadiene, k(O3), 75; Y(HO), 103 2-methyl -1,5-hexadiene, k(HO), 196 2-methylhexane, k(Cl), 381 methyl n-hexanoate, k(Cl), 397; k(HO), 283 5-methyl-2-hexanone, k(Cl), 395; k(HO), 267 2-methyl-1-hexene, k(NO3), 134 3-methyl-1-hexene, k(NO3), 134 4-methyl-1-hexene, k(NO3), 134 5-methyl-1-hexene, k(NO3), 134 methylhydroperoxide, j, 461; k(Cl), 387; τ, 461 methyl iodide, k(HO), 188; k(NO3), 133 methyl isobutyrate, k(HO), 283 3-methyl-2-isopropyl-1-butene, k(O3), 74; Y(PCC), 112 methylketene, k(HO), 274; k(O3), 91 methyl lactate, k(HO), 286 methyl mercaptan (CH 3SH), atmospheric reactions of, k(HO), 416; k(Br), 416; k(BrO), 416; k(Cl), 416; k(NO3), 416 methylmethacrylate, k(Cl), 398; k(HO), 285; k(NO3), 162 methyl 2-methylbutanoate, k(Cl), 397 methyl 2-methylpropionate, k(Cl), 397 methyl E-2-methyl-2-butenoate, k(NO3), 162 methyl 3-methyl-2-butenoate, k(NO3), 162 methyl methyl carbamate, k(NO3), 163

Subject Index 1-methylnaphthalene, k(HO), 216 2-methylnaphthalene, k(HO), 217 methyl nitrate, CS, 453; j, 455; k(Cl), 399; k(HO), 289; τ, 460 methyl nitrite, j, 460; k(Cl), 400; k(HO), 293; τ, 460 2-methyl-1-nitronaphthalene, j, 461; k(HO), 295; k(NO3), 165; τ, 461 2-methyl-1-nonene, k(NO3), 134 2-methyl-1-octene, k(NO3), 134; k(O3), 74 methyl oxirane, k(HO), 235 2-methyl -1,3-pentadiene, k(HO), 196; k(O3), 75 2-methyl -1,4-pentadiene, k(HO), 195; k(O3), 75 4-methyl -1,3-pentadiene, CS, 436; k(HO), 196 2-methylpentanal, k(HO), 254; k(NO3), 156 3-methylpentanal, k(HO), 254; k(NO3), 156 4-methylpentanal, k(HO), 254; k(NO3), 156 2-methyl -2,4-pentandial, k(HO), 221 2-methylpentane, k(Cl), 381; k(HO), 179; k(NO3), 132 3-methylpentane, k(Cl), 381; k(HO), 179; k(NO3), 132 3-methyl -2,4-pentanedione (keto), k(HO), 269 3-methyl -2,4-pentanedione (enol), k(HO), 269 methyl n-pentanoate, k(Cl), 397; k(HO), 283 2-methyl-2-pentanol, k(HO), 219 4-methyl-2-pentanol, 219 3-methyl-2-pentanone, k(HO), 267 4-methyl-2-pentanone, k(Cl), 395; k(HO), 267 2-methyl-1-pentene, k(HO), 194; k(NO3), 134; k(O3), 72; Y(PCC), 111 2-methyl-2-pentene, k(HO), 194; k(NO3), 134; k(O3), 73 3-methyl-1-pentene, k(NO3), 134; k(O3), 72; Y(PCC), 111 cis-3-methyl-2-pentene, k(NO3), 134; k(O3), 73 trans-3-methyl-2-pentene, k(NO3), 134; k(O3), 73 trans-4-methyl-2-pentene, k(HO), 194 4-methyl-1-pentene, k(NO3), 134; k(O3), 73; Y(PCC), 111 3-methyl-1-penten-1-ol, k(O3), 89 3-methyl-1-penten-3-ol, k(HO), 222 3-methyl-3-penten-2-one, k(HO), 270; (NO3), 158 4-methyl-3-penten-2-one, k(HO), 269; k(NO3), 158; k(O3), 91 methyl n-pentoate, k(HO), 283 2-methyl-2-pentyl nitrate, k(HO), 290 3-methyl-2-pentyl nitrate, k(HO), 290 methyl pivalate, k(Cl), 397; k(HO), 283 2-methylpropanal, j, 435; k(Br), 410; k(Cl), 393; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 2-methylpropane, k(Cl), 381; k(HO), 179; k(NO3), 132 2-methyl-1-propanol, k(Cl), 386; k(HO), 219 2-methyl-2-propanol, k(Cl), 386; k(HO), 219 2-methyl-2-propenal, j, 437; k(HO), 256; k(O3), 91 2-methylpropene, k(Cl), 384; k(HO), 193; k(NO3), 134; k(O3), 72; Y(HO), 111; Y(SCI), 105; Y(PCC), 111 2-methyl-2-propen-1-ol, k(Cl), 386; k(HO), 221 2-methyl-2-(2-propenyl) oxirane, k(NO3), 153 N-methylpropionamide, k(HO), 288 1-methyl-2-pyrolidinone, k(NO3), 163; k(HO), 288; M(NO3), 161 methyl salicylate, k(Cl), 398; k(HO), 286 α-methyl styrene, k(HO), 216; Y(HO), 103 trans-β-methyl styrene, k(HO), 216; Y(HO), 103

585

N-methyl succinimide, k(HO), 288; k(NO3), 163 2-methyl-tetrahydrofuran, k(HO), 236 2-methyl-1-tridecene, k(NO3), 134; k(O3), 74 methyl 2,2,2-trifluoroacetate, k(Cl), 398; k(HO), 287 methyl 2,2,2-trifluoroethyl ether, k(NO3), 152 2-methyl-1-undecene, k(NO3), 134; k(O3), 74 methyl vinyl ether, k(HO), 234; k(NO3), 152 methyl vinyl ketone, CS, 450; j, 451; k(Br), 410; k(Cl), 395; Y(HO), 103; k(NO3), 158; τ, 460 2-methyl-2-vinyl oxirane, k(NO3), 153 5-methyl-5-vinyl tetrahydrofuran-2-ol, k(HO), 239; k(NO3), 153; k(O3), 91 MIR, 487; comparison of values with MOIR, 488; comparison with POCP values, 490; values of for selected organic species, 489 modeling atmospheric chemistry, potential deficiencies in, 493; future developments, 496 MOIR, 487; comparison of values with MIR, 488 Montreal Protocol and stratospheric ozone depletion, 40 myrcene, k(HO), 196; k(NO3), 135; k(O3), 75; Y(HO), 103 naphthalene, k(HO), 216 1,4-naphthoquinone k(HO), 271; k(NO3), 158; k(O3), 92 α-neoclovene, k(NO3), 136 nitric acid, CS, 453; j, 455; τ, 459 o-nitrobenzaldehyde, j, 461; τ, 461 nitrobenzene, j, 461; k(Cl), 400; k(HO), 294; τ, 461 1-nitrobutane, k(Cl), 400; k(HO), 293 nitroethane, k(Cl), 400; k(HO), 293 nitroethene, k(HO), 294 nitromethane, k(Cl), 400; k(HO), 293 1-nitronaphthalene, k(NO3), 163; k(HO), 294 2-nitronaphthalene, k(NO3), 163; k(HO), 294 α-nitrooxyacetone, j, 461; k(HO), 293; τ, 461 1-nitrooxy-2-butanol, k(Cl), 400; k(HO), 292 2-nitrooxy-1-butanol, k(Cl), 400; k(HO), 291 3-nitrooxy-1-butanol, k(Cl), 400; k(HO), 292 4-nitrooxy-1-butanol, k(Cl), 400; k(HO), 292 4-nitrooxy-2-butanol, k(Cl), 400; k(HO), 292 1-nitrooxy-2-butanol-3-ene, k(HO), 292 4-nitrooxy-1-butanol-2-ene, k(HO), 292 1-nitrooxy-2-butanone, j, 461; k(HO), 293; τ, 461 3-nitrooxy-2-butanone, k(HO), 293 2-nitrooxy-1-cyclopentanol, k(HO), 292 2-nitrooxyethanol, j, 461; τ, 461 6-nitrooxy-1-hexanol, k(Cl), 400; k(HO), 292 2-nitrooxypentane, j, 461; τ, 461 2-nitrooxy-1-pentanol, k(Cl), 400; k(HO), 292 1-nitrooxy-2-pentanol, k(Cl), 400; k(HO), 292 4-nitrooxy-1-pentanol, k(Cl), 400; k(HO), 292 5-nitrooxy-2-pentanol, k(Cl), 400; k(HO), 292 1-nitrooxy-2-propanol, k(Cl), 400; k(HO), 292 2-nitrooxy-1-propanol, k(Cl), 400; k(HO), 291 1-nitropentane, k(Cl), 400; k(HO), 294 o-nitrophenol, k(NO3), 163 1-nitropropane, k(HO), 293; k(Cl), 400 2-nitropropane, k(HO), 293 3-nitro-1-propene, k(HO), 294

586

Subject Index

m-nitrotoluene, k(HO), 294 nitrous acid, CS, 457; role of in ozone generation, 454; τ, 459 NO reactions with O3 and HO2, 116 NO2, CS, 118; P, 117; Q, 118; reaction of with HO, 120; HO2, 121; RO2, 121; RC(O)O2, 121; rate coefficients for reactions with various peroxy radicals, 122; τ, 459 NO3, CS, 128; j, 459; formation reaction of, 126; major products of reactions with alkenes, 140; acyclic dienes, 140; cyclic dienes, 140; unsaturated biogenic species, 141; mechanisms of reaction with: NO, 130; alkanes, 130; haloalkanes, 131; alkenes, 194; alkynes, 194; haloalkenes, 142; aromatic hydrocarbons, 146–147; oxygenates, 146; alcohols, 148; ethers, 151; aldehydes, 154; ketones, 157; organic acids, 159; esters, 169; N-atom containing oxygenates, 163; P, 127; Q, 128; τ, 459 N2O5, formation reaction, 126; dissociation reaction, 126; reaction with aqueous aerosols, 116; reaction with halide ions, 126 NOx and VOC, indicator ratios, Stockwell, 481; Sillman, 481 NOx emissions, 49, 50 n-/iso-nonafluorobutyl ethyl ether, k(Cl), 390 n-/iso-nonafluorobutyl methyl ether, k(Cl), 390 2,2,3,3,4,4,5,5,5-nonafluoro-1,1-dihydroxybutane, k(Cl), 387 2,2,3,3,4,4,5,5,5-nonafluorohexanal, k(HO), 225 3,3,4,4,5,5,6,6,6-nonafluorohexanol, k(Cl), 387 3,3,4,4,5,5,6,6,6-nonafluorohexyl acrylate, k(HO), 287 2,2,3,3,4,4,5,5,5-nonafluoropentanal, k(HO), 261 2,2,3,3,4,4,5,5,5-nonafluoropentan-1-ol, k(Cl), 225 2,2,3,3,4,4,5,5,5-nonafluoropentanoic acid, k(HO), 278 nonanal, k(HO), 255; k(NO3), 156 nonane, k(Cl), 382; k(HO), 180; k(NO3), 132 2-nonanone, k(HO), 268 1-nonene, k(HO), 195; k(NO3), 134; k(O3), 74 nopinone, k(NO3), 158; k(HO), 272 O 3 rate coefficients for reactions with mono-alkenes, 67; dienes, 86; trienes, 86; cyclic alkenes, 84; unsaturated oxygenates, 87; alcohols, 87; esters, 96; ethers, 88; aldehydes, 93; ketones, 94; aromatic and other ketones, 94; organic acids, 96 ocimene, k(HO), 196; k(NO3), 135; k(O3), 75; Y(HO), 103 octachlorodibenzodioxin, k(HO), 248 1,1,2,2,3,3,4,4-octafluoropentane, k(HO), 187 1-( 1,2,3,4,5,6,7,8-octahydro-2,3,8,8-tetramethyl2-naphthalene)ethanone, k(HO), 273 octanal, k(NO3), 156 octane, k(Cl), 382; k(HO), 180; k(NO3), 132 1-octanol, k(Cl), 386; k(HO), 220 2-octanone, k(HO), 267 E-2-octenal, k(NO3), 157 1-octene, k(HO), 194; k(NO3), 134; k(O3), 73; Y(HO), 102; Y(SCI), 105; Y(SCI), 105; Y(PCC), 112 cis-4-octene, k(O3), 73; Y(PCC), 112

trans-4-octene, k(HO), 194; k(O3), 73; Y(PCC), 112 3-octyl nitrate, k(HO), 290 odd nitrogen cycling in the atmosphere, 115 OH. See HO OTNE, 1-( 1,2,3,4,5,6,7,8-octahydro-2,3,8,8-tetramethyl2-naphthalenyl)ethanone, k(HO), 273; k(NO3), 158 oxepane, k(HO), 236 oxetane, k(HO), 235 oxides of nitrogen, overview of NO, NO2 atmospheric chemistry, 114; relation for NO/NO2 ratio, 114 E,Z-4-oxo-2-pentenal, k(HO), 256; j, 437 2-oxobutyl nitrate, j, 460; τ, 460 2-oxocyclohexyl-1-nitrate, j, 461; k(HO), 293; τ, 461 cis-4-oxo-2-pentenal, j, 459; τ, 459 trans-4-oxo-2-pentenal, j, 459; k(HO), 270; k(O3), 91; τ, 459 2-oxopropyl nitrate, j, 461; τ, 461 oxygen, CS, 9; Schuman-Runge bands of, 9; photolysis rates of versus altitude, latitude, 11 ozone ambient air quality standards for, NAAQS, 6; WHO, 6 average surface concentrations of at: Mauna Loa, Hawaii, 26; Barrow, Alaska, 26; American Samoa, 26; South Pole, 27; 247 sites in the U.S., 30 assessing influence of NOx and VOC’s on generation of, 473 Chapman-like, oxygen only calculation of, 36 computer assessment of effects on generation of: [H 2O], temperature, and clouds, 481 concentration of, versus altitude, 15; measurements from instrument intercomparison, 36 CS, Chappuis band of, 10, 62; Hartley band, 10, 62 discovery of, 3 distribution of in the atmosphere, 36 effects of on generation of: [CH 2O], 484; [CO], 484; [H2O], 482; [NO3] and [N2O5], 484; [NOx], 484; [HONO], 485; [SO2], 485; temperature, 481; [VOC], 478; clouds, 482 highly significant parameters effecting it in air quality modeling, 480 impact of inorganic trace gases on, 375 importance of in the atmosphere, 1 instrumentation for measurement of, Dobson’s spectrometer, 3; Schönbein’s starch-iodine paper, 16; spectrophotometric method, 16; satellite instrumentation, 22 j(O1D), 65; effects of temperature on, 66; ozone column, 66; solar zenith angle, 66; altitude, 67; measured and calculated at Boulder Colorado, 69 light absorption by, in Hartley Band (210–290 nm), 10; Huggins Bands (310–350 nm), 7, 11; Chappuis Bands (400–850nm), 7, 10, 11 measures of ozone formation reactivity, MOIR, 487; MIR,487; POCP, 489 mechanism of its destruction in the troposphere, 47 mechanism of its formation in the unpolluted stratosphere, 33; in the lower polluted atmosphere, 44

Subject Index mechanisms of its reactions with organic compounds: acyclic mono-alkenes, 68; cyclic alkenes, 84, 86; dienes, 86, 87; unsaturated oxygenates, 87; unsaturated alcohols, 87, 88; unsaturated ethers, 88; unsaturated aldehydes, 83; unsaturated ketones, 94; unsaturated organic acids, 95; unsaturated esters, 96 minimum concentrations of in Antarctic Ozone Hole, 32 mixing ratio of, versus altitude, 15 mixing ratios of, and ClO in ozone hole, 40 National Ambient Air Quality Standards (NAAQS) for, 6 photolysis frequencies of , comparison of measured and calculated values, 64 photolysis rate, contribution of from different absorption bands and altitude, 11 Q of O( 1D) formation in, 63, 64; temperature dependence of, 65 radiative forcing by, 6, 42 rate coefficients of with: alkenes, 72; haloalkenes, 83; cyclic alkenes, 84; dienes, 86; unsaturated alcohols, 81; unsaturated aldehydes, 93; unsaturated ketones, 94; unsaturated esters, 96 stratospheric measurements of, at Halley Bay (Antarctica), 5 surface concentration measurements of at: American Samoa, 21; Arkona, Germany, 17; Barrow, Alaska, 20; Cape Grim, Australia, 25; Cape Point, South Africa, 25; Composite, 1878 to 2008, 17; Houston, Sugarland, Baytown, Texas, 28; Jungfraujoch, Switzerland, 17; Long Island, New York area, 29; Los Angeles, California, 27; Mace Head, Ireland, 17, 24; Mauna Loa, Hawaii, 21; Mexico City, 30; Montsouris, France, 17; New York, Northern New Jersey, 29; Niwot Ridge, Colorado, 23; Pacific MBL, 17; Ragged Point, Barbados, 23; South Coast Air Basin (California), 28; South pole, 20; Summit, Greenland, 24; Tudor Hill, Burmuda, 22; Tutuila, Storhold, Vetmannaeyjar Iceland, 22 zonally averaged concentration of versus latitude and altitude, 34 ozone and climate, 7 ozone depletion potential (ODP), 41 ozone hole, 39 ozone and radiative forcing, 42 ozonide, alternative fragmentation modes, 85 PAN. See peroxyacetyl nitrate 1,1,1,2,2-pentachloroethane, k(HO), 187 pentadecane, k(HO), 180 1,2-pentadiene, k(HO), 195 1,3-pentadiene, k(HO), 195; k(O3), 75 E-1,3-pentadiene, k(HO), 195; k(NO3), 135; k(O3), 75; Y(HO), 103 Z-1,3-pentadiene, k(NO3), 135; k(O3), 75; Y(HO), 103 1,4-pentadiene, k(HO), 195; k(NO3), 135; k(O3), 75 1,1,1,3,3-pentafluorobutane, k(HO), 186 1,1,1,2,2-pentafluorodimethyl ether, k(Cl), 389; k(HO), 242 1,1,1,2,2-pentafluoroethane, k(HO), 186; k(Cl), 383 pentafluoroethyl formate, k(HO), 286

587

1,1,2,2,2-pentafluoroethyl methyl ether, k(Cl), 390; k(HO), 243 2,2,3,4,4-pentafluorooxetane, k(HO), 243 1,1,1,2,3-pentafluoropropane, k(HO), 186 1,1,2,3,3-pentafluoropropane, k(HO), 186 1,1,1,3,3-pentafluoropropane, k(HO), 186 1,1,2,2,3-pentafluoropropane, k(HO), 186 2,2,3,3,3-pentafluoropropanol, k(HO), 226 Z-1,2,3,3,3-pentafluoro-1-propene, k(O3), 83 E-1,2,3,3,3-pentafluoro-1-propene, k(O3), 83 2,2,3,3,3-pentafluoropropionic acid, k(HO), 278 2,2,3,3,3-pentafluoropropyl difluoromethyl ether, k(HO), 244 2,2,3,3,3-pentafluoropropyl methyl ether, k(Cl), 390; k(HO), 243 pentanal, CS, 430; j, 435; k(Cl), 393; k(HO), 254; k(NO3), 156; Q, 434; τ, 459 pentane, k(Cl), 381; k(HO), 179; k(NO3), 132 2,4-pentanedione, k(HO), 269 E-2-pentanoic acid, k(O3), 92 1-pentanol, k(Cl), 386; k(HO), 219 2-pentanol, k(Cl), 386; k(HO), 219 3-pentanol, k(Cl), 386; k(HO), 219 2-pentanone, CS, 441; j, 448; k(Cl), 395; k(HO), 267; τ, 460 1-penten-3-one k(HO), 269; k(O3), 91 2-penten-4-one, M(O3), 95 1-penten-3-ol, k(Cl), 386; k(HO), 221; k(NO3), 149; (O3), 89 3-penten-2-one, k(Cl), 395; k(HO), 269; k(NO3), 158; k(O3), 91 cis-2-penten-1-ol, k(Cl), 386; k(HO), 221; k(NO3), 149; k(O3), 89 E-2-pentenal, k(Cl), 393; k(HO), 256; k(NO3), 156; k(O3), 91 1-pentene, k(Cl), 384; k(HO), 193; k(NO3), 184; k(O3), 72; Y(HO), 102; Y(SCI), 105; Y(PCC), 111 cis-2-pentene, k(HO), 193; k(NO3), 134; k(O3), 72; Y(HO), 101 trans-2-pentene, k(HO), 193; k(NO3), 134; k(O3), 72; Y(HO), 101 4-penten-2-ol, k(Cl), 386 2-pentenoic acid, k(O3), 92 2-pentoxy radical reaction pathways, 348 4-pentyl acetate, k(HO), 285 n-pentyl nitrate, k(Cl), 399; k(HO), 289 2-pentyl nitrate, CS, 453; j, 455; k(Cl), 399; k(HO), 290; τ, 461 3-pentyl nitrate, CS, 453; j, 455; k(HO), 290; τ, 461 n-pentyl nitrite, k(Cl), 400; k(HO), 293 1-pentyne, k(NO3), 123 perfluoroalkyl formates ( n-CxF2x+1 OC(O)H, x = 4, 5, 7,10), k(Cl), 398 perfluorobuta -1,3-diene, k(O3), 83; k(NO3), 143 perfluorobutyl ethyl ether, k(Cl), 390; k(HO), 245 perfluoro- n-butyl formate, k(Cl), 398 perfluorobutyl methyl ether, k(Cl), 390; k(HO), 244 perfluoroethyl perfluoroisopropyl ketone, j, 450; τ, 460 perfluoroisobutyl ethyl ether, k(HO), 245; perfluoroisobutyl methyl ether, k(HO), 244

588

Subject Index

perfluoroisopropyl methyl ether, k(HO), 244 perfluorooxetane, k(HO), 243 perfluoropentyl methyl ether, k(HO), 244 perfluoropropene, k(NO3), 143; k(O3), 83 perfluoropropyl formate, k(Cl), 398 perfluoropropyl methyl ether, k(Cl), 390; k(HO), 244 perfluoropropyl 1,2,2,2-tetrafluoroethyl ether, k(Cl), 390 peroxyacetyl nitrate (PAN), CS, 456; formation processes of, 121; j, 456; loss processes of, 121; k(Cl), 400; k(HO), 295; rate coefficients for dissociation of, 121; τ, 461 peroxymethacryloyl nitrate, k(HO), 295; k(NO3), 163; k(O3), 93 peroxynitrates, thermal decomposition, 123 peroxynitric acid, CS, 453; j, 455; τ, 459 peroxypropionyl nitrate (PPN), CS, 456; j, 456; k(Cl), 400; τ, 461 α-phellandrene, k(HO), 199; k(NO3), 135; k(O3), 80 β-phellandrene, k(HO), 199; k(NO3), 135; k(O3), 80; Y(HO), 103; Y(PC), 112 phenanthrene, k(HO), 218; k(NO3), 147 phenol, k(Cl) 386; k(HO), 222; k(NO3), 150 3-phenyl-2-propenal, k(HO), 259 photodecomposition of aldehydes, 425; mechanism of, formaldehyde, 428; higher acyclic aldehydes, 433; unsaturated aldehydes, 435; halogen-atom substituted aldehydes, 437; acylic dials, 438 photodecomposition of ketones, 440; acetone, 442; larger acylic ketones, 446; HO- and halogen atom-substituted ketones, 448; difunctional ketones, 449 photodecomposition of some nitrogen-containing compounds, 451; photodecomposition of the oxygenates, 425; summary of processes in troposphere, 457 photolysis frequency of molecules, 425 phthalaldehyde, j, 461; τ, 461 trans-pinane, k(HO), 183 α-pinene, k(Br), 408; k(HO), 200; k(NO3), 135; k(O3), 81; Y(HO), 103; Y(SCI), 105 β-pinene, k(Br), 408; k(HO), 200; k(NO3), 135; k(O3), 81; Y(HO), 103; Y(SCI), 105; Y(PCC), 113 pinonaldehyde, CS, 439; j, 441; k(Cl), 393; k(HO), 260; k(NO3), 157; τ, 459 pivaldehyde, j, 435, 459; Q, 434; τ, 459 POCP, 489 polar surface ozone depletion, 409 probability of measuring or modeling a given range of ozone mixing ratios, 496 products and mechanisms of HO 2-NO and RO2-NO reactions, 318, 320 1,2-propadiene, k(HO), 195; k(O3), 74 propanal, CS, 430; j, 435; k(Br), 410; k(Cl), 393; k(HO), 254; k(NO3), 154; Q, 434; τ, 459 propane, CS, 426; k(Cl), 381; k(HO), 174; k(NO3), 132 1,2-propanediol, k(HO), 221 1-propanol, k(Cl), 386; k(HO), 219; k(NO3), 149; M(HO), 226 2-propanol, k(Br), 410; k(Cl), 386; k(HO), 219; k(NO3), 149 2-propenal, k(HO), 256; k(NO3), 156; k(O3), 91; M(HO), 264

propene, CS, 426; k(Br), 408; k(Cl), 384; k(HO), 193; k(NO3), 134; k(O3), 72; M(HO), 205; Y(HO), 101; Y(SCI), 105; Y(PCC), 111 2-propen-1-ol, k(Cl), 386; k(HO), 221; k(NO3), 149; k(O3), 89; M(Cl), 388; M(NO3), 148 iso-propenyl acetate, k(HO), 285; M(O3), 96 propionic acid, k(Cl), 396; k(HO), 278 2-propoxyethanol, k(HO), 232 iso-propyl acetate, k(Cl), 397; k(HO), 182; k(NO3), 162 n-propyl acetate, k(Cl), 397; k(HO), 282; k(NO3), 162 n-propylbenzene, k(HO), 213 iso-propylbenzene, k(HO), 213 n-propyl n-butanoate, k(Cl), 397; k(HO), 283 iso-propyl iso-butyrate, k(HO), 283 propylene oxide, k(Cl), 389 iso-propyl formate, k(Cl), 397; k(HO), 281 n-propyl formate, k(Cl), 397; k(HO), 281; k(NO3), 162 iso-propyl nitrate, CS, 453; j, 455; k(Cl), 399; k(HO), 289; τ, 461 iso-propyl nitrite, j, 460; τ, 460 n-propyl nitrate, CS, 453; j, 455; k(Cl), 399; k(HO), 289; τ, 460 n-propyl nitrite, j, 460; k(Cl), 400; k(HO), 293; τ, 460 n-propyl propionate, k(Cl), 397; k(HO), 282 n-propyl vinyl ether, k(HO), 234; k(NO3), 152; k(O3), 90 propyne, k(Br), 408; k(HO), 203; k(NO3), 145; M(HO), 209 2-propyn-1-ol, k(HO), 222 pyrrole, k(HO), 289; k(NO3), 147 pyruvic acid, CS, 450; j, 451; k(HO), 278; τ, 460 quadricyclane, k(HO), 184 RC(O)O 2NO2, rate coefficients for thermal decomposition of, 325 RO rate coefficients for reaction with O2, 348; unimolecular decomposition, 349; unimolecular isomerization, 352, 353; M, 358; chemical activation of, 358 decomposition modes of alkoxy radicals formed from HO attack on 5- and 6-carbon alkanes, 373; alkoxy radical decomposition reactions, 349 RO reaction mechanisms for CH3O•, 360; C2H5O•, 361; n-C3H7O•, 361; iso-C3H7O•, 362; CH3CH2CH2CH2O•, 363; CH3CH2CH(O•)CH3, 364; (CH3) 3CO•, 365; (CH3)2CHCH2O•, 365; n-C5H11O•, 366; CH3CH2CH2CH(O•)CH3, 366; M, 348; CH3CH2CH(O•)CH2CH3, 367; (CH3) 3CCH2O•, 367; CH3CH2C(O•)(CH3)2, 368; CH3CH2CH(CH3)CH2O•, 368; CH3CH2CH2C(O•)(CH 3)2, 369; (CH3)2CHCH CH2CH2O•, 370; CH3CH2CH2CH2CH2CH 2O•, 2 369; CH3CH2CH2CH2CH(O•)CH3, 368; CH3CH2CH(O•)CH2CH2CH3, 369; (CH3)2CHCH2CH2CH(O•)CH3, 368; CH3CH2CH2CH2C(O•)(CH 3)2, 370; (CH3)2CHCH2CH2C(O•)(CH3) 2, 370; cyclopentoxy, 371; cyclohexoxy, 371

Subject Index RO 2 rate coefficients for reactions with HO2, 318; products of, 329; BrO, 333; ClO, 333; IO, 333; NO, 324; alkyl nitrate formation in, 320; NO2, 324; RO2, 336; O3, 335; RO2, 336; Russell mechanism for; R’O2, 336; RO2 → QOOH isomerization, 343 RO2NO2, rate coefficients for thermal decay of, 325 Russell mechanism in RO2-RO2 reactions, 337 sabinaketone, k(HO), 272; k(NO3), 158 sabinene, k(HO), 299; k(NO3), 135; k(O3), 81; Y(HO), 103; Y(PC), 113 SARs, yield of HO radical formed in the alkene-O 3 reactions, 110; alkene-O3 reactions, 108; alkanes-NO3, 164; hydroxycarbonyls-NO3, 166; alkenes, haloalkenes, alkynes-NO3, 167; dienes-NO3, 167; unsaturated oxygenated-NO3, 169; alkanes-Cl, 400; saturated alcohols, ethers-Cl, 400; aldehydes, ketones, esters-Cl, 403; unsaturated compounds-Cl, 405 secondary aerosol formation modeling of, 493 sensitivity coefficient, 478 sensitivity of ozone formation to VOC and NOx, 473 SO2, atmospheric reactions of, 412; solution phase processes of, 412; HO reaction with, 413 solar flux. See actinic flux solar zenith angle, vs. local time, latitude for: January 1, 463; March 20, 463; March 22, 429; September 21, 463; September 23, 429; June 20, 463; June 22, 429; December 22, 430 stratopause, region, U.S. standard atmosphere, 12 stratosphere, region, U.S. standard atmosphere, 12 structures of isomeric peroxy radical derived from isoprene oxidation, 319; hydroxy isoprene nitrate isomers, 322; hydroperoxy radicals formed from methyl vinyl ketone, methacrolein, 322 styrene, k(Cl), 385; k(HO), 219; k(NO3), 147; M(NO3), 146; Y(HO), 103; Y(PCC), 112 sulfur compounds, atmospheric reactions of, 412; sources of, 412; rate coefficients for the major reactions in the atmospheric oxidation of, 416 sulfuric acid, mechanisms of formation in troposphere, 412; in the stratosphere, 421 α-terpinene, k(HO), 200; k(NO3), 135; k(O3), 81 γ-terpinene, k(HO), 201; k(NO3), 135; k(O3), 81 α-terpineol, k(HO), 222; k(NO3), 135, 149; k(O3), 85 terpinolene, k(HO), 201; k(NO3), 135; k(O3), 81; Y(HO), 103; Y(PCC), 113 1,2,3,4-tetrachlorodibenzodioxin, k(HO), 247 1,1,2,2-tetrachloroethane, k(HO), 187 1,1,2,2-tetrachloro-1,2-difluoroethane, k(HO), 189 1,1,2,2-tetrachloroethene, k(Br), 408; k(NO3), 142; k(O3), 83 tetradecane, k(HO), 180 1-tetradecene, k(NO3), 134; k(O3), 74 1,1,1,2-tetrafluoroethane, k(HO), 186 1,1,2,2-tetrafluoroethane, k(HO), 186 tetrafluoroethene, k(NO3), 142 1,1,2,2-tetrafluoroethyl difluoromethyl ether, k(HO), 243

589

1,1,2,2-tetrafluoroethyl ethyl ether, k(Cl), 390; k(HO), 243 1,1,2,2-tetrafluoroethyl methyl ether, k(Cl), 390; k(HO), 243 1,1,2,2-tetrafluoroethyl 1,3,3,3-tetrafluoroethyl ether, k(HO), 243 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, k(Cl), 390; k(HO), 244 1,1,2,2-tetrafluoroethyl trifluoromethyl ether, k(Cl), 390; k(HO), 243 1,2,2,2-tetrafluoroethyl difluoromethyl ether, k(Cl), 389; k(HO), 245 1,2,2,2-tetrafluoroethyl formate, k(Cl), 398; k(HO), 286 1,2,2,2-tetrafluoroethyl methyl ether, k(HO), 243 1,2,2,2-tetrafluoroethyl trifluoromethyl ether, k(Cl), 390; k(HO), 243 tetrafluoromethane, k(HO), 189 2,2,3,3-tetrafluoropropan-1-ol, k(Cl), 386 1,3,3,3-tetrafluoro-1-propene, k(O3), 83 2,3,3,3-tetrafluoro-1-propene, k(O3), 83 2,2,3,3-tetrafluoropropyl methyl ether, k(Cl), 390; k(HO), 243 tetrahydrofuran, k(Br), 410; k(Cl), 389; k(HO), 235; k(NO3), 152 tetrahydropyran, k(Br), 410; k(Cl), 589; k(HO), 236 tetralin, k(HO), 215; k(NO3), 147 2,2,3,3-tetramethylbutane, k(Cl), 382; k(HO), 180; k(NO3), 132 tetramethylfuran, k(NO3), 153 tetramethyloxirane, k(NO3), 152 thermosphere, region U.S. standard atmosphere, 12 m-tolualdehyde, k(Cl), 393; k(HO), 258 o-tolualdehyde, k(Cl), 393; k(HO), 258 p-tolualdehyde, k(Cl), 393; k(HO), 258 toluene, CS, 426; k(Br), 409; k(Cl), 385; k(HO), 213; k(NO3), 147; M(HO), 210, 212 toluene oxide, j, 461; k(HO), 240; k(NO3), 153; τ, 461 trace atmospheric gases, concentrations of, 14 tribromomethane, k(HO), 187 2,2,2-trichloroacetaldehyde, CS, 438; j, 459; τ, 459; k(Br), 410; k(Cl), 393; k(HO), 261 1,1,1-trichloroacetone, k(Cl), 395; k(HO), 274 trichloroacetyl chloride, j, 452; τ, 460 1,1,1-trichloroethane, k(Cl), 383; k(HO), 187; mean global mixing ratios of, 178 1,1,2-trichloroethane, k(HO), 187 3,3,3-trichloroethanol, k(Cl), 387 1,1,2-trichloroethene, k(Br), 408; k(NO3), 142; k(O3), 83 1,1,2-trichloro-1,2-difluoroethane, k(HO), 188 1,1,2-trichloro-2,2-difluoroethane, k(HO), 188 trichlorofluoromethane, k(HO), 189 trichloromethane, k(Cl), 383; k(HO), 187; k(NO3), 133 1,1,2-trichloro-1,2,2-trifluoroethane, k(NO3), 189 tricyclene, k(HO), 183 tricyclo[3.3.1.1 3.7]decane, k(HO), 183 tricyclo[5.2.1.0 2.6]decane, k(HO), 183 1,1,2,2,3,3,4,4,5,5,6,6,6-tridecafluorohexane, k(HO), 187 2,2,3,3,4,4,5,5,6,6,7,7,7-tridecafluoro-1-hexanol, k(Cl), 387 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol, k(Cl), 387 tridecane, k(HO), 180 1-tridecene, k(O3), 79

590

Subject Index

trifluoroacetaldehyde, j, 439; k(HO), 261; τ, 459 trifluoroacetic acid (TFA), k(Cl), 396; k(HO), 278 1,1,1-trifluoroacetone, j, 450; k(Cl), 395; k(HO), 274; τ, 460 trifluoroacetylchloride, j, 460; τ, 460 4,4,4-trifluorobutanal, k(Cl), 393 2,2,2-trifluoroethyl chlorodifluoromethyl ether, k(HO), 243 2,2,2-trifluoroethyl difluoromethyl ether, k(Cl), 390; k(HO), 243 2,2,2-trifluoro-1,1-dihydroxyethane, k(Cl), 386 bis-2,2,2-trifluoroethyl ether, k(HO), 244 1,1,1-trifluoromethyl ethyl ether, k(HO), 242 1,1,1-trifluoroethane, k(HO), 186 1,1,2-trifluoroethane, k(HO), 186 1,1,2-trifluoroethene, k(NO3), 142 2,2,2-trifluoro-1-ethanol, k(Cl), 387 1,2,2-trifluoroethyl trifluoromethyl ether, k(OH), 243; k(Cl), 390 2,2,2-trifluoroethyl difluoromethyl ether, k(Cl), 390 2,2,2-trifluoroethyl methyl ether, k(Cl), 390; k(HO), 243 2,2,2-trifluoroethyl-2,2,2-trifluoroacetate, k(Cl), 398; k(HO), 287 trifluoromethyl ethyl ether, k(Cl), 390; k(HO), 243 trifluoroiodomethane, k(HO), 189 1,1,1-trifluoroethane, k(Cl), 383; k(HO), 186 trifluoromethane, k(Cl), 383; k(HO), 186 trifluoromethyl formate, k(Cl), 398; k(HO), 286 trifluoromethyl methyl ether, k(Cl), 389 trifluoromethyl perfluorovinyl ether, k(Cl), 391 3,3,3-trifluoropropanal, k(HO), 186 1,1,1-trifluoropropane, k(HO), 186 3,3,3-trifluoropropanol, k(Cl), 225 3,3,3-trifluoro-1-propene, k(O3), 83 1,1,2-trifluoro-2-(trifluoromethoxy)ethane, k(Cl), 390 trimethoxymethane, k(Cl), 389; k(HO), 233 1,1,3-trimethoxypropane, k(HO), 233 1,2,3-trimethylbenzene, k(Br), 409; k(HO), 214; k(NO3), 147 1,2,4-trimethylbenzene, k(HO), 214; k(NO3), 147 1,3,5-trimethylbenzene, k(HO), 215; k(NO3), 147 2,2,3-trimethylbutane, k(Cl), 381; k(HO), 177; k(NO3), 132

2,3,3-trimethyl-1-butene, k(O3), 72; Y(PCC), 112 2,2,3-trimethyl-cyclobutyl-1-ethanone, k(NO3), 158 3,5,5-trimethyl-2-cyclohexene-1-one, k(HO), 271 2,2,4-trimethylpentane, k(Cl), 382; k(HO), 180; k(NO3), 132 2,3,4-trimethylpentane, k(HO), 180 2,4,4-trimethyl-1-pentene, k(O3), 74 2,4,4-trimethyl-2-pentene, k(O3), 74; Y(PCC), 112 2,3,5-trimethylphenol, k(HO), 224 1,7,7-trimethyltricyclo[2.2.1.02,6]heptane, k(HO), 183 1,3,5-trioxane, k(Cl), 389; k(HO), 237 Troe relation, 120 tropopause, region, U.S. standard atmosphere, 12 troposphere, region, U.S. standard atmosphere, 12 undecane, k(HO), 180 1-undecene, k(HO), 195; k(O3), 74 U.S. Environmental Protection Agency (USEPA), 6, 28, 29, 30, 52, 56, 471, 472, 473 U.S. Standard Atmosphere, temperature, pressure vs. altitude, 12 vinyl acetate, k(Cl), 398; k(HO), 285; k(NO3), 162; k(O3), 92; M(NO3), 160 vinyl chloride, M(NO 3), 144 4-vinyl-cyclohexane, Y(PCC), 112 4-vinylcyclohexene, k(O3), 78 vinyl oxirane, k(NO3), 152 vinyl propionate, k(Cl), 398; k(HO), 285 VOC ambient concentrations near Los Angeles, California, 55 m-xylene, k(Br), 409; k(Cl), 385; k(HO), 213; k(NO3), 147 o-xylene, CS, 426; k(Br), 409; k(Cl), 385; k(HO), 213; k(NO3), 147 p-xylene, k(Br), 409; k(Cl), 385; k(HO), 143; k(NO3), 147 yield of HO radicals from O3-alkene reactions, 101 yield of primary products of: O3-alkene reactions, 72; NO3-alkene reactions; 152 yield of stabilized Criegee intermediates in O3-alkene reactions, 105