Aerosols: Chemistry, Environmental Impacts and Health Effects: Chemistry, Environmental Impact and Health Effects [1 ed.] 9781608766567, 9781606929254

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Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved. Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved. Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

AEROSOLS: CHEMISTRY, ENVIRONMENTAL IMPACT AND HEALTH EFFECTS

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

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Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved. Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

AEROSOLS: CHEMISTRY, ENVIRONMENTAL IMPACT AND HEALTH EFFECTS

DANIEL H. PERETZ

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

EDITOR

Nova Science Publishers, Inc. New York

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Copyright © 2009 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA

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Published by Nova Science Publishers, Inc.    New York

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CONTENTS Preface

vii

Research and Review Studies Chapter 1

Photochemical Synthesis of Aerosol Particles Hiroshi Morita

Chapter 2

Influence of Atmospheric Conditions on Aerosol Inhalation during a Forest Fire in Spain A.I. Calvo, A. Castro, C. Palencia and R. Fraile

Chapter 3

Nanofibre Filters in Aerosol Filtration Pirjo Heikkilä and Ali Harlin

Chapter 4

Kinetics and Mechanism of So2 Oxidation by O3 on the Surface of Aluminum Oxide Particles Maofa Ge, Shengrui Tong, Weigang Wang and Shi Yin

Chapter 5

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Chapter 6

Chapter 7

Chapter 8

Characterization of Indoor and Outdoor Bioaerosols in Urban, Industrial and Rural Sites Using Conventional and DNA (Fingerprint) Based Methods M. Negrin, M.T. Del Panno, C.G. Terada and A. Ronco

1

33 69

91

109

Urban Aerosol Observations and Comparisons of Daytime and Nighttime Characteristics William Porch, Terry Galloway, Alan Roche and Steven Massie

127

Neural Network Model for Forecasting Atmospheric Particulate Levels Maria Ragosta and Gianluigi Gioscio

149

Interaction of Solar Radiation with Non-Spherical Inhomogeneous Aerosol Particles Miroslav Kocifaj

161

Short Communications Real Time Monitoring of Indoor Aerosols Jyh-Shyan Lin and Chuen-Jinn Tsai

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vi

Contents Evaluation of Saharan Dust Events for a Correct Estimation of Air Quality Alessandro Buccolieri, Giovanni Buccolieri, Nicola Cardellicchio and Angelo Dell’Atti Some Aerosol Distributions Induced by the Artificial Gravity in Plasma Polymerization System A.V. Andreeva, I. Kutsarev and V.I. Zyn

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Index

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201

217 227

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PREFACE Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are smoke, oceanic haze, air pollution, smog and CS gas. In general conversation, "aerosol" usually refers to an aerosol spray can or the output of such a can. The word aerosol derives from the fact that matter "floating" in air is a suspension (a mixture in which solid or liquid or combined solid-liquid particles are suspended in a fluid). To differentiate suspensions from true solutions, the term sol evolved—originally meant to cover dispersions of tiny (sub-microscopic) particles in a liquid. With studies of dispersions in air, the term aerosol evolved and now embraces both liquid droplets, solid particles, and combinations of these. An aerosol may come from sources as various as a volcano or an aerosol can. this new book focuses on the chemistry, environmental impact and health effects of aerosols. Chapter 1 - Using photochemical reactions of gaseous molecules, ultrafine particles are synthesized in the gas phase from some binary and/or ternary gaseous mixtures involving 2propenal (acrolein), carbon disulfide (CS2), glyoxal, organosilicon compounds, and organometal compounds. UV and visible light from either a medium pressure mercury lamp, a nitrogen gas laser, or a Nd:YAG laser is used to initiate photochemical reactions of the gaseous molecules, leading to the production of spherical aerosol particles. Chemical structures of the sedimentary aerosol particles thus produced are dependent on wavelength and intensity of the exciting light, and the particle size is effectively controlled by regulating the reaction rate and the reaction time as is exemplified in a gaseous mixture of iron pentacarbonyl (Fe(CO)5) and CS2. External magnetic field also influences chemical reactions during aerosol particle formation, resulting in the acceleration or deceleration of nucleation reactions during aerosol particle formation. Although the magnetic field effect on particle formation processes has not yet been thoroughly understood, its use gives us a new way to control the properties of the aerosol particles. As a technical advantage of the photochemical method, particle-wires are produced by controlling the convection of entire gaseous sample, and magnetic particles are synthesized upon exposure to Nd:YAG laser light. Production and manipulation of the particle-wires will develop a new field in nanotechnology. The photochemical method is further applied to synthesize spherical particles from solid materials. Laser-ablated solid materials such as poly(dimethylsilane) successfully produce spherical fine particles with the aid of photochemical reaction of gaseous molecules such as trimethylsilyl azide. Technical advantages of the method are discussed briefly.

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Daniel H. Peretz

Chapter 2 - Fine aerosol size distribution was measured in September 2000 during one of the eight days of a forest fire in Villablino, Spain. A laser spectrometer was used to determine the range of particle sizes: following their corresponding refractive index corrections, it was possible to establish an interval between 0.09 to 27.64 μm. The samples were collected at three different points distributed throughout the valley, following the direction in which the smoke plume advanced. The atmospheric conditions changed dramatically from the morning to the afternoon, with a subsidence inversion setting in during the day. Two modes (fine and coarse) were identified and defined for particle number, surface and volume distribution, and at all sampling points an increase in diameter caused by thermal inversion was observed. Estimated PM10 concentration varied between 16 μg m-3 and 27 μg m-3 in the morning to between 61 μg m-3 and 166 μg m-3 in the afternoon, after the thermal subsidence inversion had set in. This fact was most noticeable in fraction PM2.5, the most dangerous one for human health. Three calculation methods were applied to respirable dust fractions. The inhabitants of Villablino were exposed to high levels of particle concentration, especially during the thermal inversion, and the limits established by Spanish regulations (50 μg m-3) were clearly exceeded. Chapter 3 - Performance of a fibrous filter is mainly dependent on the size of the aerosol particles, the velocity of the airflow, the size of the fibres and the packing density of the filter media. High-performance filters useful for micro- and nanoparticle separation require ultrafine fibres with diameters less than one micrometer. The use of nanofibres improves filtration performance, especially in HEPA (High Efficiency Particulate Air) and ULPA (Ultra Low Penetration Air) filtration, since the ability of the nanofibres to capture sub-micron particles is better than that of the larger fibres used in conventional filters. Existing HEPA and ULPA filters are mostly made of glass fibre paper. Other viable nanoporous materials are ceramic membranes, expanded polytetrafluoro ethylene, and, of course, polymeric nanofibres. The use of the polymeric nanofibres instead of glass fibres has many advantages. Electrospinning is a method that can be used in the production of polymeric nanofibres. The small fibre diameter, small pore size, and high surface area of the electrospun nanofibre web are properties that are advantageous for filtration applications. Electrospun nanofibre webs are proven to be effective in stopping aerosol particles, and simulation of unsteady-state filtration has endorsed the efficiency of nanofibre filtration media. The dramatic increase in filtration efficiency due to a thin coating layer of electrospun fibres on conventional filter media can be seen with a relatively small or almost immeasurable decrease in permeability. Another interesting feature of the electrospun webs that are advantageous to filtration applications is the possibility to add functionality to electrospun fibres. This chapter is an abridged and revised version of the PhD thesis of Pirjo Heikkilä, “Nanostructured Fibre Composites and Materials for Air Filtration” [1]. The thesis contains an extended summary that consists of a literature review and an experimental section, and it is based on six scientific papers published in refereed journals [2-7]. Chapter 4 - Sulfate particles play a key role in the air quality and the global climate, but the heterogeneous formation mechanism of sulfates on surfaces of atmospheric particles is not well established. Gas-phase sulfur oxides can react with mineral aerosol to form particulate sulfate. Aluminum oxide, which is one of the most important components of mineral aerosol and is often used as a model oxide for the study of heterogeneous reactions, may contribute significantly to the sulfate formation by heterogeneous processes. This paper

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Preface

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presents a study on the oxidation of SO2 by O3 on basic, neutral, and acidic Al2O3. Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), the formation of sulfite and sulfate on the surface was identified. The results showed that in the presence of O3, SO2 can be oxidized to sulfate on the surface of three types of Al2O3 particles. The reactive uptake coefficient for SO2 oxidation by O3 on Al2O3 with different acidity varied in the order of basic Al2O3 > neutral Al2O3 > acidic Al2O3. A two-stage mechanism that involves adsorption of SO2 followed by O3 oxidation was proposed and the adsorption of SO2 on the Al2O3 surface was the rate determined step. The proposed mechanism can well explain the experimental results. It was found that the heterogeneous reaction might be an important pathway for sulfate formation in the atmosphere. Chapter 5 - At present, bioaerosols are one major issue in air quality studies due to their human- related adverse health effects as well as regarding their important role in ecosystem functioning. The present study approaches the characterization of indoor (homes) and outdoor bioaerosols in a sampling program carried during 2006–2007 in rural, urban and industrial sites within La Plata area, Buenos Aires, Argentina. Culturable airborne fungal and bacterial communities were collected on DG18 agar and R2 agar plates, respectively, using a singlestage SKC sampling device. Fungal genera were identified based on their morphological characteristics, while bacterial study or community structure was performed by denaturing gradient gel electrophoresis (DGGE) of PCR 16srDNA bacterial amplifications. Indoor and outdoor total bioaerosol concentrations of all sites were comparable to other reports (GM values between 102 and 103 CFU m-3); however, the highest individual fungi indoor value reached GM 26.495 CFU m-3. Analysis of fungal genera by principal component analysis (PCA) evidenced distributions of some genera associated with indoor (Wallemia, Penicillium, Eurotioum, Aspergillus sp) and outdoor (Cladosporium, Alternaria, Fusarium and yeasts) ambience. Site characteristics did not significantly affect distribution, seasonal sampling time being the main influence. With regards to bacterial populations, indoor and outdoor concentrations did not significantly vary, but instead, a more consistently differentiation did arise between sites. When bacterial community structure was analyzed by means of DGGE, banding profiles from all sites revealed the existence of a great diversified culture-based airborne community, while construction of similarity dendrograms exposed patterns with little differentiation among studied sites or between indoor and outdoor ambiences. Chapter 6 - Understanding of atmospheric aerosol optical properties and their effect on urban and global climate is biased by lack of observations at night. Sunlight scattering and extinction provide satellite and automated sun-photometer observations on a global and local scale. Nighttime observations are very rare and usually include only point measurements of dry aerosol physical and optical properties. We have analyzed aerosol stellar extinction data over many years at an observatory in Oakland, California. This observatory was located for many years at a relatively low altitude below the nocturnal inversion layer depth (104 m). It was recently moved to a higher location (470 m) approximately at or slightly below the San Francisco Bay Area boundary layer depth. The results of our analysis show that both the aerosol light scattering magnitude and light scattering properties as a function of wavelength are different at night than during the day. This is caused by the nighttime increase in relative humidity and associated growth of hydroscopic aerosols. This growth in aerosol and nighttime haze reinforces the nocturnal inversion by heating the air both through the latent heat of condensation as well as by the trapping of infrared surface radiation. This in turn, causes more pollution trapping on the morning of the following day. This effect has not been

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included in urban pollution models. It is also expected that similar effects of smaller magnitude can be expected over the marine boundary layer where day / night differences in relative humidity are significant and thereby have an effect on global climate. We also show that for coastal regions (like the San Francisco Bay Area) the optical properties of the aerosols change depending on the season. As the inner regions of California warm in summer clouds and marine aerosols are drawn in from the ocean throughout the night. In the late fall, winter and early spring, aerosols tend to be dominated by urban pollution and fewer clouds are advected from the ocean. Chapter 7 - Procedures based on artificial neural network (ANN) have been applied with success to forecast levels of atmospheric pollutants. These techniques show a capability to make regressive approximation of non-linear functions in high-dimensional space and they are more flexible in comparison to traditional statistical techniques. In this paper we present a short review of recent applications of ANN models for forecasting atmospheric particulate levels and the results of a study carried out to forecast hourly levels of PM10 in urban area starting from data measured in N-previous days. In particular we analyze PM10 hourly concentrations measured from March 2001 to February 2002 in three stations of the air quality monitoring network of Potenza town (southern Italy). The applied ANN model is a feed-forward multi-layer perceptron (MLP) with an only hidden layer. The conjugate gradient learning algorithm is used. The learning capability of the model and the average goodness of the prediction are evaluated by Mean Absolute Percentage Error (MAPE) and by the number of concentration values that the model is not able to predict (NP). The results indicate that, in the study area, a simple model of ANN is able to forecast PM10 hourly levels with a good approximation but the quality of data, in terms of presence of data missing, represents the main limit of these forecasting techniques at local scale. In order to improve the model performance increasing the number of input variables, the results suggest not only to take into account meteorological parameters but also to better characterize the dynamic features of emission source pattern. Chapter 8 - Some aspects of light scattering by non-spherical inhomogeneous aerosol particles are discussed. A special attention is paid to the composite particles that may scatter the light depending on their internal topologies. It is shown that the character of internal mixing of individual materials preferably affects the polarization features as well as the scattering patterns. This has an evident consequence for solving the inverse problems of atmospheric optics. Typically, the kernels of integral equations (like scattering cross section, or phase function for a single particle) change with altering chemistry and internal topology of the particles. It implies the ambiguity of determined microphysical properties of aerosol particles, such as the size distribution, the mean refractive index, or the prevailing aspect ratio. To overcome complexities with computing the optical behaviour of inhomogeneous aggregates the hypothetically homogeneous particles are frequently employed in numerical models. The refractive indices of “virtually” homogeneous particles are traditionally determined in agreement with an effective medium theory. Nevertheless, this approach is very questionable and often fails in simulating the observed optical effects. Therefore the exact computational techniques need to be applied to interpret the measured optical data correctly (i.e. to get the information on realistic microphysical properties of aerosol particles). Short Communication 1 - This study investigated the effects of different indoor air pollutants such as smoking, cooking and incense burning on indoor air quality. Before the experiments, the indoor particulate concentration was evaluated using two types of aerosol

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Preface

xi

monitoring devices, the TSI, Inc. Model 8520 DustTrak Aerosol Monitor (DustTrak), and the MSP Corp., Mode 200, Personal Environmental Monitor (PEM). These two instruments were collocated indoors simultaneously to assess the comparability of the sampling methods. The results show that PM10 and PM2.5 concentrations measured by PEM sampler were less than that of DustTrak sampler. The difference between PM10 and PM2.5 concentrations measured by PEM and DustTrak increase with increasing particulate concentrations. Whereas the DustTrak sampler is well correlated with PEM sampler in measuring PM10 and PM2.5 concentration for a R2 value of 0.92 and 0.89, respectively. As a consequence, real-time motoring of PM10 and PM2.5 concentrations by DustTrak sampler is applicable and was used to evaluate indoor air quality. When there is no indoor pollutant, the PM2.5/PM10 ratio is 0.78 ± 0.04 indoors. It means that most indoor particulates are less than 2.5 μm in aerodynamic diameter. If there exists indoor pollutant sources, the monitored particulates are less than 2.5 μm mostly, especially in smoking and incense burning experiments where the PM2.5/PM10 ratio is above 0.90, while the ratio is 0.77 in cooking experiment. Furthermore, it was found that the PM10 concentration indoors is less than that outdoors. The average I/O ratio (indoor to outdoor PM10 concentrations) of 24-hour PM10 concentration is 0.58. Short Communication 2 - The Sahara desert and dry lands around the Arabian peninsula are the main source of airborne dust, with some contributions from Iran, Pakistan and India and generally the transport of dust from these areas is well-known as transport of Saharan dust. Sandstorms, or dust storms, are usually the result of convection atmospheric currents, which form when warm, lighter air rises and cold, heavier air sinks. These dust can be transported over thousands of kilometers by atmospheric currents and these are frequently registered in America, in Asia and, above all, in Southern Europe, such as Spanish and Italy, primarily during transition periods between spring and autumn. The inputs of Saharan dust events cause other meteorological conditions, such as thunderstorms, and also affect the composition and the concentration of both atmospheric particulate matter and heavy metals of lithospherical source. Therefore, their evaluation is very important for a correct estimation of air quality. During these natural phenomena heavy metals concentrations of crustal source, such as aluminum, iron, manganese and titanium, may reach concentrations in excess of the background concentrations by up to one or two orders of magnitude with consequent health risks. In this chapter we present and discuss the Saharan dust events registered during the sampling of the particular matter PM10 from two urban sites of Northern Mediterranean coasts, in Salento (Apulia, Southern Italy), which are about eight hundred kilometers away from the Northern Africa coast. For the record of the events of Saharan dust were used the satellite images MODIS (Moderate Resolution Imaging Spectroradiometer). In particular, we show the temporal variation of the concentration of particulate matter PM10 and of cadmium, chromium, copper, iron, manganese, nickel, lead, titanium, vanadium and zinc, present in this fraction, in order to demonstrate the important role of the study of Saharan dust events in the correct estimation of quality air and to establish correlation between these events and the variation of both PM10 and metals. Short Communication 3 - Polymer aerosol generated in a glow discharge has now proved to be exclusively useful in some areas of nano/micro technology for the synthesis of previously unknown powder materials with unique nano-scale structures and morphologies, as well as optical, thermal, catalytic, mechanical, structural and other properties [1].

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The polymer clusters forming inside a discharge zone fall down, becoming more heavy and sensitive to gravity during their growth. The gravitational drift of the polymeric clusters creates characteristic vertical distribution of integral mass of the polymer contained in all the particles. This distribution will be manifested as the characteristic cuneiform profile of the polymer film grown at a vertical substrate. This suggests that the effect can be used as a method of studying the disperse phase by measuring parameters of the film profile. But corresponding laws and connections between action of gravity, sedimentation of the polymer and properties of the disperse phase and film are indirect or even unknown. Modeling is seemingly a suitable tool to this purpose. An adequate model can allow developing new efficient experimental and analytical approaches fit for studying aerosol systems. One of such models and corresponding experimental and analytical techniques have been developed and presented briefly in this work.

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RESEARCH AND REVIEW STUDIES

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In: Aerosols: Chemistry, Environmental Impact … Editor: Daniel H. Peretz

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Chapter 1

PHOTOCHEMICAL SYNTHESIS OF AEROSOL PARTICLES Hiroshi Morita∗ Graduate School of Advanced Integration Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

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ABSTRACT Using photochemical reactions of gaseous molecules, ultrafine particles are synthesized in the gas phase from some binary and/or ternary gaseous mixtures involving 2-propenal (acrolein), carbon disulfide (CS2), glyoxal, organosilicon compounds, and organometal compounds. UV and visible light from either a medium pressure mercury lamp, a nitrogen gas laser, or a Nd:YAG laser is used to initiate photochemical reactions of the gaseous molecules, leading to the production of spherical aerosol particles. Chemical structures of the sedimentary aerosol particles thus produced are dependent on wavelength and intensity of the exciting light, and the particle size is effectively controlled by regulating the reaction rate and the reaction time as is exemplified in a gaseous mixture of iron pentacarbonyl (Fe(CO)5) and CS2. External magnetic field also influences chemical reactions during aerosol particle formation, resulting in the acceleration or deceleration of nucleation reactions during aerosol particle formation. Although the magnetic field effect on particle formation processes has not yet been thoroughly understood, its use gives us a new way to control the properties of the aerosol particles. As a technical advantage of the photochemical method, particle-wires are produced by controlling the convection of entire gaseous sample, and magnetic particles are synthesized upon exposure to Nd:YAG laser light. Production and manipulation of the particle-wires will develop a new field in nanotechnology. The photochemical method is further applied to synthesize spherical particles from solid materials. Laser-ablated solid materials such as poly(dimethylsilane) successfully produce spherical fine particles with the aid of photochemical reaction of gaseous ∗ E-mail address: [email protected]. Mailing address: Misora 4-12-16, Yotsukaido, Chiba 284-0023, Japan

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2

Hiroshi Morita molecules such as trimethylsilyl azide. Technical advantages of the method are discussed briefly.

INTRODUCTION Existence of aerosols in stratosphere and troposphere influences human health and global warming [1]. In particular, aerosol particles less than 2.5 μm may cause severe lung injury by inhalation [2]. These harmful particles are commonly produced from gas-to-particle conversion or combustion of organic materials which produces mutagens such as polycyclic aromatic hydrocarbons. Heterogeneous chemical processes between polar stratosphere clouds and gaseous halogen-containing species destroy the ozone layer catalytically [3, 4]. Thus, elucidation of chemical processes of aerosol particle formation is a key to solve human health and global warming problems. Chemistry of aerosol particle formation can also apply to synthesize spherical particles utilized in industry. Ultrafine and nano-size particles involving metal clusters can be used as a building block of nano-wire and nano-devices in addition to digital dots in high density recording materials [5]. These particles can be synthesized in the gas phase using photochemical reactions of organometal compounds [6]. For the practical use, we need to give some chemical and physical properties to the particles such as electric conductivity and magnetism. Control of the particle size is also required. In this chapter, I report on the synthetic method of aerosol particles using photochemical reactions of gaseous molecules and explain the characteristics of the aerosol particles produced by the present method. I emphasize how the chemical reactions change depending on wavelength and intensity of the exciting light and how an external magnetic field influences the nucleation reaction during aerosol particle formation. The present method is further developed to particle formation from solid materials, and some technical advantages of the method are discussed briefly.

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1. Preparation of Gaseous Mixtures In order to initiate nucleation reaction during aerosol particle formation, a gaseous mixture involves at least one kind of photo-reactive molecules such as 2-propenal (acrolein) (AC), carbon disulfide (CS2), and glyoxal. To lend any physical and/or chemical properties to the particles, some organosilicon compounds (allyltrimethylsilane (ATMeSi), trimethylsilyl azide (TMSAz), and trimethyl(2-propynyloxy)silane (TMPSi)), organogermanium compounds (tetraethenylgermane, tetraethylgermane), and organometal compounds (iron pentacarbonyl (Fe(CO)5), cobalt tricarbonyl nitrosyl (Co(CO)3NO)) are added to a gaseous sample. The chemicals used in this work are usually liquid. The liquid samples were degassed by freeze-pump-thaw cycles in the dark and purified by vacuum distillation immediately before use. To prepare a gaseous mixture, each vapor was introduced successively into a

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Photochemical Synthesis of Aerosol Particles

3

cylindrical or a cross-shaped Pyrex cell (inner diameter 35 or 20 mm) equipped with quartz windows through a vacuum line equipped with a capacitance manometer (Edwards Barocel Type 600). The background pressure of the irradiation cell was less than 8×10-5 Torr (1 Torr = 133.3 Pa), and the leakage into the evacuated cell from the atmosphere was carefully controlled to be less than 1 Torr in a day. The partial pressure of each gaseous molecule in the irradiation cell was determined from the diagnostic band intensity of FT-IR spectrum of pure vapor.

2. Light Irradiation on Gaseous Mixtures The gaseous samples were irradiated with UV light. Light source is a medium pressure mercury lamp (Ushio UM-452, 450W) combined with some appropriate filters (UV29, UV31, UVD33S), or pulsed lasers such as a nitrogen gas laser (Lumonics HE-440, pulse width 10 ns, 337.1 nm) and a Nd:YAG laser (Continuum Surelite I-10, pulse width 6 ns, 355 nm at the third harmonic, 266 nm at the fourth harmonic). By the choice of exciting wavelength and intensity, we can select which molecules are initially excited in the gaseous mixture. Magnetic field was applied during light irradiation using a helium-free superconducting magnet (Toshiba TM-5SP) or an electromagnet (Tokin SEE-10D) to study the magnetic field effect on chemical processes. Experimental setup of exciting light source and the magnet is schematically shown in Fig. 1.

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3. Measurements The nucleation process during aerosol particle formation can be investigated by measuring monitor (He-Ne laser) light intensity scattered by the aerosol particles formed under UV light irradiation. Light intensity scattered perpendicularly to the incident direction of the monitor light was measured with a combination of a photomultiplier tube (EMI 6256S or Hamamatsu R212) and a lock-in amplifier (SRS SR-530) through a Y-52 (or R-62) filter as shown in Fig. 1. The aerosol particles were deposited on a glass plate and/or Cu substrate at the bottom of the irradiation cell. To investigate morphology of the deposits, scanning electron microscope (SEM) images were recorded with a JEOL JSM 6060 scanning electron microscope, and SEM-EDS, with a Philips XL30 CP/EDAX scanning electron microscope. X-ray photoelectron spectra (XPS) were measured with a Gammadata Scienta ESCA 310 electron spectrometer using monochromatized Al Kα (hν=1486.6 eV) radiation for electron excitation. The sedimentary aerosol particles were mixed with KBr powder to prepare KBr pellets and FT-IR spectra of the sedimentary particles embedded in the pellets were measured with a Nicolet NEXUS 470 FT-IR spectrometer.

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Hiroshi Morita

4

i

k

j

g f

h

l

S

e N

c

a

d

b

Figure 1. Experimental setup used for photochemical production of aerosol particles. a: exciting light source (Hg lamp, N2 gas laser, or Nd:YAG laser), b: electromagnet (or helium-free superconducting magnet), c: Pyrex cell, d: He-Ne laser, e: lens, f: glass filter (Y-52), g: sector, h: photo-interrupter, i: high-voltage power supply, j: photomultiplier tube, k: lock-in amplifier, l: personal computer.

RESULTS AND DISCUSSION 1. Aerosol Particle Formation from Acrolein (AC) Vapor and its Sensitization AC molecule has an n-π* absorption band in the 330 nm region [7, 8]. Under light irradiation at 313 nm, formyl radical (HCO) is produced via photodecomposition reaction (1) and initiates polymerization reaction of AC vapor (200 Torr) to produce a continuous translucent deposit on the walls of the reaction cell [9].

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CH2=CH-CHO + hν → C2H3 + HCO

(1)

Although quantum efficiency of polymerization of AC vapor is estimated to be very low (6.2 × 10-3) at 313 nm [10], spherical aerosol particles with a mean diameter of 0.75 μm are produced from pure AC vapor at a pressure of 41 Torr (yield 0.2 mg) under light irradiation at 313 nm for 4 h using a medium pressure mercury lamp through a UV-31 and a UVD33S filters [11]. At an excitation wavelength longer than 313 nm, quantum efficiency to produce HCO radical becomes negligibly small, and hence polymerization reaction of AC vapor may not proceed. In spite of the unfavorable experimental conditions, N2 laser light (337.1 nm, 12 Hz, 2.3 mJ/pulse) was exposed on pure AC vapor at a pressure of 80 Torr. Upon exposure to N2 laser light for 2.5 h, AC vapor produced spherical sedimentary aerosol particles with a mean diameter of 1 μm (yield 0.5 mg) (Fig. 2) [12, 13]. Aerosol particles fell down on a glass plate (placed at the bottom of the irradiation cell) with a reproducible sedimentary pattern due to

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convection of the gaseous sample, showing that the sedimentary particles were actually formed in the gas phase. From the energetics, two-photon absorption is necessary to induce the nucleation reaction during aerosol particle formation. To investigate the nucleation process in detail, intensity of the monitor (He-Ne laser) light scattered by aerosol particles as formed under light irradiation was measured; the results are shown in Fig. 3. Under irradiation with N2 laser light at an energy of 1.2 - 4.3 mJ/pulse, increasing rate (Rs) of the scattered light intensity was proportional to the square of the laser intensity. Considering that the scattered light intensity is proportional to the number of aerosol particles as in the present case where the particle size distribution of aerosol particles is almost constant regardless of the irradiation time, the increasing rate (Rs) is proportional to increasing rate of the number of initiating radicals, showing that nucleation of gaseous AC molecules needs two-photon energy. ArF excimer laser light (193 nm) (6.24 eV) can excite AC molecule into the S2 (π-π*) state from which AC molecule dissociates into an ethylidene diradical (CH3CH) and CO in SF6 or in helium gas [14, 15] and into C2H3 and HCO radicals in a supersonic molecular beam [16, 17]. Using two-photon energy at 337.1 nm (7.36 eV), AC molecule is excited to a higher excited state through which the decomposition reaction (1) takes place efficiently.

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5 μm Figure 2. SEM image of sedimentary aerosol particles deposited from pure AC vapor (80 Torr) upon exposure to N2 laser light at an energy of 2.3 mJ/pulse. Original magnification of SEM, 2000×.

In order to improve the efficiency of particle formation from AC, utilization of a sensitization mechanism is practically important. As a candidate to generate HCO radical by one-photon decomposition, formaldehyde (H2CO) was chosen [8, 13]. H2CO was prepared by pyrolysis of paraformaldehyde (Merck, extra pure) at 120℃ under vacuum and mixed with AC vapor immediately before use. Under irradiation with N2 laser light, a gaseous mixture of H2CO and AC produced sedimentary aerosol particles with a mean diameter of 0.6 μm more efficiently (yield 0.4 mg) than pure AC vapor (yield ≤ 0.1 mg). Under the present experimental conditions, pure H2CO vapor did not polymerize into poly(oxymethylene) thermally over 5 h, nor did it produce any aerosol particles under N2 laser light irradiation.

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The product yield increased with increasing partial pressure of H2CO (from 3 to 6 Torr) and with increasing laser intensity between 1.2 - 4.8 mJ/pulse. These results indicate that the nucleation reaction of the particle formation is initiated via one-photon decomposition of H2CO molecule. H2CO in the lowest singlet excited state (A 1A2 (n-π*)) decomposes into H and HCO through the ground state singlet (S0) or triplet (T1) surface of H2CO [18-22]. The threshold energy for the decomposition is estimated to be 86 kcal/mol (3.73 eV) [18, 19, 22], and below the threshold energy, the yield of HCO radical drops down quickly to 0.8% at 339.2 nm (3.65 eV) [19]. Although one-photon energy at 337.1 nm (3.68 eV) is still a little lower than the threshold energy, substantial production of HCO radical can be expected by the one-photon process, and HCO radical thus formed can initiate the nucleation reaction of AC molecules.

(A)

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(B)

Figure 3. (A) He-Ne laser light intensity scattered by the aerosol particles produced from pure AC vapor (40 Torr) upon exposure to N2 laser light at an energy of (a) 4.3, (b) 3.8, (c) 3.0, (d) 2.2, and (e) 1.2 mJ/pulse. (B) Rs value (i.e., increasing rate of the scattered light intensity) (□ ) and (Rs)1/2 value (● ) as a function of N2 laser light intensity.

2. Magnetic Field Effect on Aerosol Particle Formation from a Gaseous Glyoxal/AC Mixture

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Glyoxal (GLY) molecule has an n-π* absorption band in the 390 - 460 nm region, and under visible light irradiation at 435.8 nm, GLY in the n-π* state produces CO and H2CO [23]. Hence, GLY may be used as a photochemical sensitizer in particle formation process. GLY monomer was prepared by pyrolysis of glyoxal trimeric dihydrate (Merck) in the presence of P2O5 at 120-150℃ in a vacuum. A gaseous mixture of GLY (3.3 Torr) and AC (34 Torr) was prepared and irradiated with a medium pressure mercury lamp at 435.8 nm for 1 h [24]. The gaseous mixture produced white and spherical aerosol particles with a mean diameter of 0.94 μm in a cylindrical cell (inner diameter 35 mm, length 200 mm) (Fig. 4), whereas pure AC vapor did not produce any deposits under the same experimental conditions, and pure GLY vapor deposited a very faint film on a glass plate at the bottom. Photochemical sensitization during aerosol particle formation was successfully achieved by adding GLY molecule. Due to the increased efficiency of aerosol particle formation, aerosol particles produced from the gaseous mixture deposited uniformly over the whole glass plate at the bottom. (a)

2 μm

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(b)

Figure 4. (a) SEM image of sedimentary aerosol particles deposited from a gaseous mixture of GLY (3.3 Torr) and AC (34 Torr) under light irradiation at 435.8 nm for 1 h, and (b) particle size distribution therefrom. Original magnification of SEM, 5000×.

In the nucleation reaction during aerosol particle formation from the gaseous mixture of GLY and AC, incorporation of AC molecules is essential. Upon light excitation at 435.8 nm, only GLY molecule is excited to the singlet n-π* state (1GLY) and efficiently intersystem-

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crosses to the triplet manifold (3GLY), followed by several chemical pathways such as decomposition into CO and H2CO. Among the chemical reactions from 3GLY, only a small fraction of GLY molecules participates to polymerization [23, 25-27]. In order to explain the unexpected excess production of CO, an intermediate complex, M, between 3GLY and GLY in the ground state is postulated [25, 28]. It is further supported from opto-acoustic measurement that the long-lived complex, M, relaxes only by a bimolecular encounter of the complex to produce photoproducts and heat [29]. GLY + hν → 1GLY → 3GLY

(2)

3

(3)

GLY + GLY → M

M + M → Products + Heat release

(4)

Following this scheme, polymerization reaction between GLY molecules may proceed via a bimolecular encounter of the long-lived intermediate complex, M, accompanying heat release [25, 28, 29]. Considering the fact that AC in the ground state may form a complex, M’, with excited GLY in the triplet state, the nucleation reaction during aerosol particle formation from a gaseous mixture of GLY and AC may be initiated via a bimolecular encounter of the complex, M’, accompanying heat release: 3

GLY + AC → M’

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M’ + M’ → Nucleation + Heat release

(5) (6)

Nucleation process of gaseous mixtures of GLY (0.3 - 1.6 Torr) and AC (34 Torr) was investigated by measuring scattered light intensity of the monitor (He-Ne laser) light. The results are shown in Fig. 5. With increasing partial pressure of GLY, the scattered light intensity became stronger and the induction period to detect scattered light became shorter (from 23 to 4 min). These results indicate that electronically excited GLY initiates chemical reaction with AC to produce aerosol particles under light irradiation at 435.8 nm. In the presence of a magnetic field of 0.53 T, aerosol particles were produced from a gaseous mixture of GLY (1.3 Torr) and AC (40 Torr) under light irradiation at 435.8 nm [24]. The He-Ne laser light intensity scattered by the aerosol particles formed under a magnetic field of 0.53 T became stronger and the induction period to detect scattered light became shorter (to 50 s from 140 s) (Fig. 6), indicating that the magnetic field accelerated the nucleation reaction and increased the number of aerosol particles. Furthermore, the convection of aerosol particles was influenced considerably with the application of a magnetic field. These are the first observation for the magnetic field effect on the nucleation reaction during aerosol particle formation. Based on the reactions (5) and (6), acceleration of the nucleation reaction under a magnetic field may be accompanied by larger heat release to disturb inhomogeneous spatial distribution of heat which is originally induced by spatially inhomogeneous exciting light intensity inside the irradiation cell. This may change the

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convection of gaseous molecules and hence, may induce the change in the sedimentation pattern of the aerosol particles as is shown in Fig. 7. As is well known [30-35], magnetic field enhances the intersystem crossing (ISC) rate between 1GLY and 3GLY. Hence, the chemical reaction through 3GLY (reaction (5)) may be enhanced with the application of a magnetic field. However, quantum efficiency of ISC of GLY is reported to be near unity at a pressure of 0.2 Torr [27], and 0.7 at a pressure of 20 Torr [23]. Considering the high efficiency of ISC of 1GLY without a magnetic field, it is not expected that the formation of 3GLY and hence, the formation of M’ in reaction (5) is increased significantly with the application of a magnetic field. Instead, we propose that the bimolecular reaction of the complex, M’ (reaction (6)) is accelerated with the application of a magnetic field.

Figure 5. He-Ne laser light intensity scattered by the aerosol particles produced from a gaseous mixture of GLY and AC (34 Torr) under light irradiation at 435.8 nm. Partial pressure of GLY is (a) 0.3, (b) 1.3, and (c) 1.6 Torr.

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Figure 6. He-Ne laser light intensity scattered by the aerosol particles in a small cross-shaped cell (volume 50 cm3) produced from a gaseous mixture of GLY (1.3 Torr) and AC (40 Torr) under light irradiation at 435.8 nm (a) in the presence and (b) in the absence of a magnetic field of 0.53 T.

(a)

(b)

(c)

(d)

hν

hν

hν

hν

Figure 7. Sedimentation pattern (viewed from the bottom of the cell) of the aerosol particles produced from a gaseous mixture of GLY (1.8 Torr) and AC (40 Torr) under light irradiation at 435.8 nm in an electromagnet. The magnetic field is (a) 0, (b) 0.18, (c) 0.35, and (d) 0.53 T. One arm of a cross-shaped cell (volume 300 cm3) is directed vertically between the poles of an electromagnet.

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3. Excitation Wavelength Dependence of Aerosol Particle Formation from a Gaseous CS2 /Glyoxal Mixture 3.1. Particle Formation from Pure CS2 Vapor It is well recognized that CS2 is polymerized under UV light irradiation and produces aerosol particles and a thin film. Chemical structure of the aerosol particles produced from pure CS2 vapor is described as (CS2)x polymer [36, 37] , a blend of C polymer and S polymer [38], (CS)x polymer [39-41], or a mixture of (CS2)x and (C3S2)x [42]. These different views are based on IR spectral analysis and theoretical calculation on the structure and still compete in the structure assignment. CS2 molecule has an absorption band in the 270 - 350 nm region. The electronic structure of excited states of CS2 is rather complicated in view of analysis of its absorption spectrum [43-46]. Lower excited states come from the electron configuration (πg)3(πu). The molecular geometry is linear (of D∞h symmetry) in the ground state, and is bent (of C2v symmetry) in lower excited states [44]. The 1Δu and 3Δu states in the linear molecule split into the 1A2 and 1 B2 states, and the 3A2 and 3B2 states, respectively, by the Renner-Teller interaction. The longest wavelength absorption (called Kleman’s R system) corresponds to the transition to the B2 spin component of the 3A2 state [45,46]. The transition to the 1B2 state is observed as strong bands (V system) in the 290-330 nm region, whereas the transition to the 1A2 state is electronically forbidden: only the hot bands are observed in the 334-350 nm region (T system) [44]. N2 laser light (337.1 nm) can excite CS2 to the v2’ = 3 and 5 levels of the 1A2 state from the v2” =1 and 2 levels, respectively [44]. Under irradiation with N2 laser light at an energy of 1.8 mJ/pulse for 3 h, aerosol particles were produced from pure CS2 vapor (50 Torr) [47]. FT-IR spectrum of the deposited aerosol particles showed IR bands of ν(C=C) [48] at 1600 and 1440 cm-1, ν(C-C) at 1224 and 1257 cm-1, ν(C=S) at 1066 cm-1, and ν(C-S) at 818 cm-1, which is similar to that of the (CS2)x polymer deposited from gaseous CS2 under light irradiation at 313 nm [37]. The XPS analysis was performed on the sedimentary aerosol particles deposited on a Cu sheet from pure CS2 (50 Torr) [47]. The stoichiometry of atoms is S1.00 C3.00 O1.11 (Cu0.31) for the deposits, and the result is summarized in Table 1. The particles deposited from CS2 contain sulfur atoms in a (-C-S-)n bonding (and partly in elemental sulfur) as a major contribution (72%). About 22% of sulfur can be assigned to sulfur in >C=S or Cu-S bonding, and remaining ~6% to sulfidic and/or S-O- moiety [49-51]. These latter two contributions likely result from a reaction of S with Cu substrate and with atmospheric oxygen. In the C 1s spectrum of the aerosol particles, carbon belonging to the (-C-S-)n bonding was detected at 285.5 eV. The binding energies (284.4, 287.1 and 288.5 eV) of components of the C 1s spectrum are compatible with carbon contained in C-C and C-H bonds (major components) and C-O and C=O bonds (minor components). The atomic ratio of S and C atoms pertinent to the C-S bonding was evaluated using the experimentally determined stoichiometry of atoms to be 0.72:0.81 for the particles deposited from pure CS2. The value which is very close to unity clearly supports that the major chemical species of polymeric particles is (-C-S-)n polymer as reported previously [52]. The chemical structure of the sedimentary aerosol particles produced from CS2 is strongly suggested to be highly crosslinked (-C-S-)n polymer.

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Table 1. XPS characterization of aerosol particles deposited from pure CS2 (50 Torr). Line designation

S 2p3/2

C 1s

Binding energy / eV 162.0 163.7 167.9 284.4 285.5 287.1 288.5

Population / % 22 72 6 59 27 7 7

Assignment [49-51] >C=S(a) , Cu-S (CS)n polymer S-O C-C, C-H (CS)n polymer C-O C=O

(

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a) this assignment is based on our measurements of S 2p3/2 binding energies for 4,4’bis(dimethylamino)thiobenzophenone and 10-methylacridine-9-thione used as standards.

3.2. Particle Formation from a Gaseous CS2 / Glyoxal Mixture upon Exposure to 435.8 nm Light As discussed above, each of GLY and CS2 molecules can initiate the nucleation reaction separately during light irradiation. Under light irradiation at 435.8 nm, a gaseous mixture of GLY (4 Torr) and CS2 (40 Torr) produced aerosol particles at the early stage of light irradiation, followed by the deposition of a thin film as the final product [53]. Because pure CS2 vapor does not absorb light at 435.8 nm and only GLY is excited to the n-π* state, it is clear that GLY molecules initiate the nucleation reaction during aerosol particle formation. In support for this, the product yield increased almost linearly to the partial pressure of GLY above 1.5 Torr. Monitor (He-Ne laser) light intensity scattered by aerosol particles formed under light irradiation was measured for a gaseous mixture of GLY (3.5 Torr) and CS2 (60 Torr). The result is shown in Fig. 8 (curve (a)). Scattered light was detected only for the first 20 min, indicating that the aerosol particle formation was a dominant process only in the early stage of photochemical reaction, and the aerosol particles deposited on a glass plate contributed to form a thin film in the later stage. As in the case of a gaseous GLY/AC mixture, with increasing partial pressure of GLY (from 2.0 to 4.3 Torr), scattered light intensity became stronger (about twice) and the induction period to detect scattered light became shorter (from 180 to 150 s). These results further support that electronically excited GLY molecule induces chemical reactions of CS2 to initiate the nucleation reaction during aerosol particle formation. Under light irradiation at 435.8 nm, external magnetic field accelerated the nucleation reaction as in the case of a gaseous GLY/AC mixture. As is shown in Fig. 8 (curve (b)), monitor (He-Ne laser) light intensity scattered by the aerosol particles formed under a magnetic field of 0.51 T became stronger, and the induction period to detect scattered light became shorter (from 80 to 40 s). Furthermore, by the application of a magnetic field, the convection of aerosol particles was influenced considerably, indicating that heat release was increased by the application of a magnetic field. For the gaseous mixture of GLY and CS2 where only GLY molecules are excited, we may propose a complex formation, M”, between 3GLY and CS2 in the ground state through a chemical interaction between C=O bond of 3GLY and C=S bond of CS2. As in the case of a

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gaseous mixture of GLY and AC, a bimolecular encounter of the complex, M” initiates the nucleation reaction in the gas phase with heat release. 3

GLY + CS2 → M”

(7)

M” + M” → Nucleation + Heat release

(8)

As in the case of a gaseous mixture of GLY and AC, the magnetic field effect is well understood by the acceleration of the bimolecular reaction of the complex, M” (reaction (8)).

Intensity / arb. unit

0.06 b 0.04 a 0.02

b

a b a

0.00 0

20 40 Irradiation time / min

60

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Figure 8. He-Ne laser light intensity scattered by the aerosol particles produced from a gaseous mixture of GLY (3.5 Torr) and CS2 (60 Torr) under light irradiation at 435.8 nm (a) in the absence and (b) in the presence of a magnetic field of 0.51 T.

3.3. Particle Formation from a Gaseous CS2 / Glyoxal Mixture upon Exposure to 313 nm Light In contrast to the excitation at 435.8 nm, UV light irradiation at 313 nm for 3 h on a gaseous mixture of CS2 (60 Torr) and GLY (6 Torr) resulted in the production of only spherical aerosol particles of brownish yellow color (product yield: 0.6 mg) [54]. SEM image of the sedimentary particles is shown in Fig. 9. Particle size of the sedimentary aerosol particles was dependent on the inner diameter of a cylindrical irradiation cell being used. The particle size distribution is shown in Fig. 10. With decreasing diameter of the irradiation cell (from 35 mm to 20 mm), particles became smaller and the mean diameter of the sedimentary particles reduced to 0.43 from 0.76 μm. This is due to the fact that aerosol particles generated under light irradiation travel along the cylindrical cell wall due to convection of the entire gaseous sample. Throughout the entire travel period, the aerosol particles continue to grow and finally collide with the substrate at the bottom of the irradiation cell within one convection cycle. Hence, in a cylindrical cell with a smaller diameter, the convection period is shorter, resulting in a shorter propagation time. The particle size dependence on the cell diameter is characteristic of the aerosol particles which are synthesized in the gas phase by photochemical reactions.

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3.3 μm Figure 9. SEM image of sedimentary aerosol particles produced from a gaseous mixture of CS2 (60 Torr) and GLY (6.0 Torr) under UV light irradiation at 313 nm for 3 h. Original magnification of SEM, 2950 ×.

Number of particles

50

b

40 30

a

20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Diameter / μm

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Figure 10. Particle size distributions of sedimentary particles deposited from a gaseous mixture of CS2 and GLY under UV light irradiation at 313 nm for 3 h in a cylindrical irradiation cell with an inner diameter of (a) 35 mm and (b) 20 mm. Respective partial pressures of CS2 and GLY are (a) 60 and 6, and (b) 40 and 4 Torr.

The nucleation process during aerosol particle formation was monitored by measuring the He-Ne laser light intensity scattered by the aerosol particles formed under light irradiation at 313 nm. The result for a gaseous mixture of CS2 (60 Torr) and GLY (6 Torr) is shown in Fig. 11(a). Scattered light was observed during the whole period (3 h) under light irradiation, in contrast to the case of visible light irradiation at 435.8 nm [53] where scattered light was observed only in the early stage of the photochemical reaction within the first ~20 min. As shown in Fig. 11, with increasing partial pressure of CS2, the scattered light intensity increased and the induction period to detect the scattered light became shorter (75, 40, and 21 s, respectively, for a CS2 partial pressure of 20, 40, and 60 Torr). The product yield also increased (0.2, 0.3, and 0.6 mg) with increasing partial pressure of CS2 (20, 40, and 60 Torr, respectively). The increase of the product yield, together with the shortening of the induction

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period to detect scattered light clearly shows that the nucleation reaction is initiated by CS2 in the n-π* excited state, although a small amount of GLY molecules are excited.

0.08

a

Intensity / arb. unit

0.04 0.00 0.04

b 0.02 0.00 0.02

c

0.01 0.00

0

30

60 90 Irradiation time / min

120

150

Figure 11. He-Ne laser light intensity scattered by the aerosol particles produced from a gaseous mixture of CS2 and GLY (6.0 Torr) under UV light irradiation at 313 nm. Partial pressure of CS2 is (a) 60, (b) 40, and (c) 20 Torr.

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Based on the above discussion, the nucleation reaction during aerosol particle formation from a gaseous mixture of CS2 and GLY under light irradiation at 313 nm may proceed as follows. UV light at 313 nm excites CS2 molecules predominantly (reaction (9)), and excited CS2 molecules react with both CS2 [37, 39, 40, 42, 48, 52] and GLY in the ground electronic state (reactions (10) and (11)). Following the previous discussion, we may propose that excited CS2 and GLY form a long-lived species (M#) (reaction (11)), and a bimolecular encounter of M# induces the nucleation reaction accompanying heat release (reaction (12)). The long-lived species (M#) can also react with excited CS2 molecules (reaction (13)) to initiate the nucleation reaction. (9) CS2 + hν → CS2* CS2* + CS2 → (CS2)2* → (CS)2 + S2 → Nucleation

(10)

CS2* + GLY → M# M# + M# → Nucleation + Heat release M# + CS2* → Nucleation + Heat release

(11) (12) (13)

M# + GLY → Nucleation + Heat release

(14)

On the addition of 6 Torr of GLY to pure CS2 vapor, the product yield of the sedimentary particles increased from 0.1 to 0.6 mg. This result clearly shows that reaction (11) is more

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efficient than reaction (10). Chemical structure of the long-lived species has not yet been established, but a biradical intermediate is a possible candidate [28]. To demonstrate the change in chemical structure of the sedimentary aerosol particles due to different nucleation reactions, FT-IR spectrum of the sedimentary aerosol particles deposited under light irradiation at 313 nm is compared with the spectrum of the deposits produced under light irradiation at 435.8 nm (Fig. 12). The spectrum under light irradiation at 313 nm showed strong bands at 1068, 1670, and 1720 cm-1. Of these, the band intensities at 1068 and 1670 cm-1 increased with increasing partial pressure of CS2. The 1068 cm-1 band can be assigned to ν(C=S) [37, 48] and the 1670 cm-1 band to ν(C=O) of GLY bonded to CS2. The FT-IR spectra confirmed that under light irradiation at 313 nm, CS2 molecules were incorporated significantly into the sedimentary particles. 0.3

a

0.2

Absorbance

0.1 0.0 1.0

b 0.5 0.0

2200

2000

1800

1600

1400

1200 -1 Wave number / cm

1000

800

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Figure 12. FT-IR spectra of deposits produced from (a) a gaseous mixture of CS2 (60 Torr) and GLY (6.0 Torr) under light irradiation at 313 nm for 3 h and (b) a gaseous mixture of CS2 (60 Torr) and GLY (3.2 Torr) under light irradiation at 435.8 nm for 8 h.

3.4. Magnetic Field Effect on Aerosol Particle Formation upon Exposure to 313 nm Light Depending on the excitation wavelengths, the nucleation reactions of the gaseous mixture of CS2 and GLY changed as discussed above. Hence a different magnetic field effect on the nucleation reaction may be observed for the case of the excitation wavelength at 313 nm. To observe any change in magnetic field effect, the scattered light intensity of He-Ne laser light was measured for a gaseous mixture of CS2 (60 Torr) and GLY (6 Torr) under light irradiation at 313 nm. With the application of a magnetic field, the induction period to detect scattered light became longer from 21 s (at 0 T) to 26 s (at 0.3 T), and further to 45 s (at 0.5 T), accompanied by the decrease in the scattered light intensity. This result indicated that the

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magnetic field decelerated the nucleation reaction between CS2 and GLY in contrast to the case of visible light irradiation at 435.8 nm [53]. Deceleration of the nucleation reaction under a magnetic field resulted in a decrease in the product yield of the sedimentary aerosol particles obtained by weighing (0.6, 0.4 and 0.2 mg, respectively, under a magnetic field of 0, 0.3, and 0.5 T). The magnetic field dependence of the chemical structure of the sedimentary particles was investigated by measuring the FT-IR spectra. The spectra of the sedimentary particles deposited from a gaseous mixture of CS2 (40 Torr) and GLY (4 Torr) in a superconducting magnet are shown in Fig. 13. With increasing magnetic field, band intensities at 1068 cm-1 and 1670 cm-1 decreased, showing that the incorporation of CS2 molecules decreased during aerosol particle formation. Furthermore, XPS analysis of the C 1s and O 1s spectra clearly showed that the C and O atoms in the >C-OH and -CH2-O- chemical bonds were abundant in the particles and their amount increased under a magnetic field of 5 T, showing that the contribution of chemical species originating from GLY increased in the sedimentary aerosol particles under a magnetic field.

0.10

a

0.05

Absorbance

0.2

b

0.1 0.08

c

0.04 0.10

d

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0.05 0.00

2000

1500

1000

500

-1

Wave number / cm

Figure 13. FT-IR spectra of the sedimentary particles produced from a gaseous mixture of CS2 (40 Torr) and GLY (4.0 Torr) under light irradiation at 313 nm for 3 h in the presence of a magnetic field of (a) 0, (b) 1, (c) 3, and (d) 5 T.

As to the magnetic field effect on gaseous CS2 molecules, it is reported that the fluorescence intensity from the 1A2 and 1B2 states decreases by 40 and 30 %, respectively, by the application of a magnetic field of 0.61 T [55]. This is attributed to depopulation in the

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singlet excited states of CS2 by fast intramolecular relaxation [56, 57]. Based on these results, the magnetic field effect on aerosol particle formation can be understood qualitatively when the excited CS2 in reactions (9), (11), and (13) is assigned to be the CS2 in the fluorescing state of the singlet excited state. Depopulation in the fluorescing state under a magnetic field results in a decrease in production of M# in reaction (11) which subsequently decelerates the nucleation reaction via reaction (12). Depopulation in the fluorescing state also reduces the incorporation of chemical species originating from CS2 via reaction (13) compared with those originating from GLY via reaction (14). It is worth mentioning that the aerosol particle formation process from pure CS2 vapor is not influenced by the application of a magnetic field [58]. This strongly suggests that the nucleation reaction of pure CS2 takes place via an excited state of CS2 different from the fluorescing state, and a magnetic field influences the nucleation reaction only between CS2 and GLY.

4. Synthesis of Composite Aerosol Particles Involving Organometal Compound 4.1. Composite Particle Formation from a Gaseous Fe(CO)5 / CS2 Mixture Organometal compounds, especially, metal carbonyls are photoreactive and can form a variety of chemical bonds between organic molecules [59]. Organometal compounds incorporated into the aerosol particles will give to the particles some physical and chemical properties characteristic of metal-metal bond formation. From pure Fe(CO)5 vapor (1.0 Torr), two kinds of morphologically different deposits,

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i.e., hexagonal rod with a mean size of 2.8×2.4 μm and crystalline particles with a mean size of 0.46 μm are produced under UV light irradiation at 313 nm for 10 min [60]. The major chemical species of the deposits is Fe2(CO)9 [61, 62] in addition to a slight amount of Fe3(CO)12 [63]. Pure CS2 vapor at a pressure of 2 Torr does not produce any deposits because of its low pressure [64]. Taking these into account, a gaseous mixture of Fe(CO)5 (1.1 Torr) and CS2 (1.9 Torr) was prepared and irradiated with light at 313 nm. The gaseous mixture produced spherical aerosol particles with a mean diameter of 0.31 μm. The morphological change of the deposits clearly shows that CS2 molecules are incorporated into the photochemical reactions of Fe(CO)5. Addition of some reactive organic molecules (such as CS2 and ATMeSi) is useful to produce spherical particles containing organometal compound. To investigate the chemical structure of the particles, FT-IR spectrum of the sedimentary particles deposited from the gaseous mixture was measured (Fig. 14(a)) [60]. The spectrum exhibited the C≡O stretching bands at 2005, 2037, and 2080 cm-1, but not the C=O stretching band at 1825 cm-1 which was observed with Fe2(CO)9 (Fig. 14(b)). These results indicate that Fe-C(=O)-Fe chemical bond is not formed by the incorporation of CS2 molecules. In addition, FT-IR bands observed with the sedimentary aerosol particles produced from pure CS2 vapor (Fig. 14(c)) almost disappeared, supporting that the bond formation between Fe(CO)4 (originating from Fe(CO)5 [65]) and CS2 molecules is the primary chemical reaction of the gaseous mixture.

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1.6

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0.8 0.6 0.4 0.2

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Figure 14. FT-IR spectra of deposits produced from (a) a gaseous mixture of Fe(CO)5 (1.1 Torr) and CS2 (2.0 Torr), (b) pure Fe(CO)5 vapor (1.0 Torr), and (c) pure CS2 vapor (50 Torr) under light irradiation at 313 nm for (a) 30 min, (b) 10 min, and (c) 2 h.

SEM-EDS analysis of the sedimentary particles on the atomic ratio (1 : 0.15) of Fe to S atom also supports that CS2 molecules are actually incorporated into the aerosol particles. CS2 molecules are known to be activated by transition metal complexes in the liquid phase [59, 66]. For iron carbonyls, π-coordination of CS2 molecule and σ-coordination through a sulfur atom are reported [67]. Although the present experiment was done in the gaseous phase, Fe(CO)4 species produced from excited Fe(CO)5 may react with CS2 through either π- or σ-coordination. Fe (CO)5 + hν → Fe(CO)4 + CO

(15)

Fe(CO)4 + CS2 → (Fe(CO)4・CS2)

(16)

σ-Coordinated CS2 can ligate another Fe atom to connect two Fe atoms as is observed for Co2(CN)10CS2 [59, 66]. (Fe(CO)4・CS2) + Fe(CO)5 → Fe(CO)4-S-C(=S)- Fe(CO)4 + CO

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(17)

Hiroshi Morita

20

For the above complex, ν(C=S) and ν(C-S) bands are expected to appear at 840 - 980 cm-1 region [66]. A thiocarbonyl group can add to Fe(CO)4 to form Fe(CO)4(CS) [68-70], and combine two transition metal atoms by end-to-end bridging of a thiocarbonyl, M-C≡S-M [71]. Considering that CS2 molecules are polymerized in the gas phase mainly as (C-S)n under UV light irradiation [47], photo-excited Fe(CO)5 may react with CS2 to bridge two iron atoms through a thiocarbonyl group. CS2 + hν → CS2*

(9)

CS2* + CS2 → 2CS + S2

(18)

Fe(CO)4 + CS + Fe(CO)5 → Fe(CO)4-C≡S- Fe(CO)4 + CO

(19)

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FT-IR band of an end-to-end bridging thiocarbonyl has not yet been identified, although it is expected to appear near 1100 cm-1 [71]. Judging from FT-IR spectrum of the sedimentary particles in Fig. 14 where several bands are observed in 900 - 1100 cm-1 region, two Fe atoms may possibly be connected either by σ-coordinated CS2 (reaction (17)) and/or by end-to-end bridging of a thiocarbonyl (reaction (19)). From SEM-EDS analysis of the sedimentary particles, the atomic ratio of Fe to C atom was determined to be 1: 2.5, and that of Fe to O atom, to be 1: 2.7 [64]. Since the substrate used in the present experiment is not free from the surface contamination and oxidation, the atomic ratios of C and O atoms to Fe atom are maximum values. These values strongly suggest that iron carbonyls are present as Fe(CO)3 and/or Fe(CO)2 groups in the sedimentary particles. Considering a variety of possible structures of metal carbonyl complexes containing S atoms, the sedimentary aerosol particles are believed to be composed of several chemical species, and the chemical species with sulfur bridged Fe atoms is one of the plausible chemical structures of the particles.

4.2. Control of Chemical Composition by Post Exposure with UV Light Chemical structure of sedimentary particles can be changed by exposing UV light on the deposited particles (i.e., by post-exposure). The 313 nm light from a medium pressure mercury lamp was irradiated for 2 h on the sedimentary particles which were produced from a gaseous mixture of Fe(CO)5 (1.1 Torr) and CS2 (1.9 Torr) [64]. FT-IR spectra before and after the post-exposure are shown in Fig. 15. In Fig. 15(b), the ≈2000 cm-1 (2080, 2037, and 2005 cm-1) bands assigned to ν(C≡O) and the 607 and 576 cm-1 bands assigned to δ(Fe-C-O) decreased their intensities compared to the intensities of ≈1650 and ≈1000 cm-1 bands originating from CS2. This shows that terminal C≡O groups are evolved by the postexposure.

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0.4

21

a

0.2

Absorbance

0.0

b

0.4 0.2 0.0

c

0.4 0.2 0.0

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Figure 15. FT-IR spectra of sedimentary particles deposited from a gaseous mixture of Fe(CO)5 (1.1 Torr) and CS2 (1.9 Torr) under light irradiation at 313 nm for 10 min (a) before and (b) after postexposure with a mercury lamp (313 nm) for 2 h, and (c) after post-exposure with pulsed Nd:YAG laser light (266 nm) for 1 h.

The fourth harmonic (266 nm) of pulsed Nd:YAG laser light was also used for the postexposure [64]. Under direct irradiation of YAG laser light (100 mJ/pulse･cm2), the particles with a mean diameter of 310 nm were decomposed into smaller particles with a mean

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diameter of 30 nm. With decreasing laser power to 32 mJ/pulse･cm2, the particles tended to fuse together to form bigger deposits. In order to avoid the above-mentioned effect of intense laser light, the laser light was defocused using a concave lens to reduce the energy to 5 mJ/pulse･cm2 and was irradiated on the particles. FT-IR spectrum of the particles after the post-exposure for 1 h is shown in Fig. 15(c). The ≈2000 cm-1 (2080, 2037, and 2005 cm-1) bands assigned to ν(C≡O) and the 607 and 576 cm-1 bands assigned to δ(Fe-C-O) decreased their intensities (to less extent than the result in Fig. 15(b)), showing that the terminal C≡O groups were evolved by the post-exposure. Population of atoms in the particles analyzed by SEM-EDS was 21.0, 5.0, 38.2, and 35.8 At.% for Fe, S, C, and O atoms, respectively, after the post-exposure, in comparison to the corresponding values of 15.9, 2.4, 39.0, and 42.7 At.%, respectively, before the post-exposure. Before the post-exposure, atomic ratio of C (and O) to Fe atom is roughly 2.5 : 1, but it reduces to 1.7 : 1 after the post-exposure, indicating

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22

Hiroshi Morita

that CO groups are evolved by the post-exposure. Furthermore, atomic ratio of Fe to S reduces to 4.2 : 1 (from 6.5 : 1) after the post-exposure. This strongly suggests that upon exposure to 266 nm light, volatile fragments such as Fe(CO)n (n = 1-3) are produced and evolved in addition to CO species.

5. Control of Particle Size In the synthesis of ultrafine particles in the gas phase, light irradiation initiates the nucleation reaction and induces the convection of entire gaseous sample. The aerosol particles produced under light irradiation travel along the cylindrical cell wall due to convection of the entire gaseous sample. Throughout the entire travel period, aerosol particles continue to grow by colliding with excited molecules (gas-to-particle conversion) during nucleation mode and then by colliding with other particles (coagulation) during accumulation mode, and finally collide with the substrate at the bottom of the irradiation cell within one convection cycle. Hence, the particle size is expected to change by varying the exciting light intensity, light irradiation time, and ambient temperature which regulate the reaction rate and the reaction time. In order to evaluate the effect of these experimental parameters on the particle size, aerosol particles were produced from a gaseous mixture of Fe(CO)5 (1.4 Torr) and CS2 (1.6 Torr) under different experimental conditions [64]. In the present experimental set-up, light intensity decreases along the propagation direction of light, and the mean diameter of particles deposited at the rear side of the cylindrical cell is smaller by 20 % than the one deposited at the front side of the cell. Hence, the sedimentary particles deposited at the front side of the irradiation cell were used to determine and compare the mean diameter of the particles produced under various experimental conditions. At first, in order to reduce the reaction rate during aerosol particle formation, UV light

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intensity from a mercury lamp was reduced from 5.4 to 1.4 mJ/cm2･s at the front side of the irradiation cell. The mean diameter of the sedimentary particles reduced to 215 nm from 270 nm using a cylindrical cell with inner diameter of 20 mm. Furthermore, ambient temperature was lowered to 5℃ from 22℃. This caused a decrease in the mean diameter to 172 nm from 215 nm. These results clearly show that the particle size of the aerosol particles is actually controlled by changing the chemical reaction rate. In a cylindrical cell with a larger diameter, the period of convection of entire gaseous sample becomes longer under stationary light irradiation. This causes the longer propagation time during aerosol particle formation. An example has already been discussed for a gaseous mixture of CS2 (60 Torr) and GLY (6 Torr) in Fig. 10 where the mean diameter was reduced to 0.43 μm from 0.76 μm by decreasing the diameter of the cylindrical irradiation cell from 35 to 20 mm. For a gaseous mixture of Fe(CO)5 and CS2, the mean diameter of the sedimentary particles decreased from 310 nm to 270 nm (to less extent) by decreasing the inner diameter of the cell from 35 to 20 mm under UV light intensity of 5.4 mJ/cm2·s. For the gaseous mixture of Fe(CO)5 and CS2, aerosol particles grow very fast and the gravitational sedimentation of the grown-up particles takes place concurrently with the convection. Due to this effect, any change in the convection period was not fully realized by the aerosol particles. In the nucleation mode during aerosol particle formation, the gas-to-particle conversion takes place only when the gaseous molecules are irradiated with UV light. Hence, the

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Photochemical Synthesis of Aerosol Particles

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propagation time of the nucleation mode can be controlled directly by changing the irradiation time of UV light. On a gaseous mixture of Fe(CO)5 (1.1 Torr) and CS2 (1.9 Torr), UV light irradiation in a short period was repeated several times with an interval of 60 s to obtain enough amount of the product for further analysis. From the measurement of monitor (He-Ne laser) light intensity scattered by the aerosol particles, the convection period of the gaseous sample in a cylindrical cell with an inner diameter of 35 mm was estimated to be ~ 50 s. Hence, the interval (60 s) allowed complete sedimentation of the formed aerosol particles onto the substrate. The fixed irradiation time was shortened from 120 s to 1 s, and the particle size distribution of the deposited particles was measured. The results are shown in Fig. 16. With shorter irradiation time, the mean diameter of the sedimentary particles became smaller until to 58 nm. In Fig. 17, the mean diameter is plotted as a function of the cube root of the irradiation time. A good linear relationship holds for the particles with mean diameters of less than 100 nm, to which the nucleation mode of particle growth dominates. The linear relationship shows that the number of molecules incorporated into the particles is roughly proportional to the reaction time. These results demonstrate that in the photochemical method, the size of aerosol particles is effectively controlled by changing the irradiation time. Easy controllability of the particle size by this way is an advantage of the photochemical method.

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Number of particles

30 20 10 0 30 20 10 0

a

b

40

c

20 0 60

d

30 0 60 40 20 0 90 60 30 0

e

f 0

50 100 150 200 250 300 350 400 450 Diameter / nm

Figure 16. Particle size distributions of sedimentary particles deposited from a gaseous mixture of Fe(CO)5 (1.1 Torr) and CS2 (1.9 Torr) under light irradiation at 313 nm for (a) 120, (b) 60, (c) 10, (d) 5, (e) 2.5, and (f) 1 s. The mean diameter is (a) 220, (b) 190, (c) 130, (d) 97, (e) 77, and (f) 58 nm.

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Particle size / nm

250 200 150 100 50 0

0

1

2

3

4

( Irradiation time / s)

5

6

1/3

Figure 17. Mean diameters of the sedimentary particles as a function of the cube root of the irradiation time.

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6. Formation of Linearly Aggregated Fine Particles In contrast to Fe(CO)5, pure Co(CO)3NO vapor (2 - 5 Torr) produces highly aggregated sedimentary particles with a mean diameter of 80 nm (yield 0.1 mg) under UV light irradiation at 313 nm [72, 73]. Allyltrimethylsilane (2-propenyltrimethylsilane) (ATMeSi) is a reactive molecule, but it does not absorb light longer than 210 nm [11]. Hence, pure ATMeSi vapor at a pressure of 10 Torr does not produce any deposits under UV light irradiation at 313 nm [73]. On the other hand, a gaseous mixture of Co(CO)3NO (1.8 Torr) and ATMeSi (2.4 Torr) produced spherical aerosol particles of dark grey color with a mean diameter of 0.23 μm (in a cylindrical Pyrex cell with an inner diameter of 35 mm) under UV light irradiation at 313 nm for 10 min (yield 0.1 mg) [73]. The morphological change of the sedimentary particles clearly shows that ATMeSi molecules are incorporated into the photochemical reactions of Co(CO)3NO molecules. During aerosol particle formation, a gaseous mixture of Co(CO)3NO (1.5 Torr) and ATMeSi (1.4 and 9.8 Torr) was irradiated with a medium pressure mercury lamp only for several minutes repeatedly (≈five times). This intermittent light irradiation induced a modulated convectional flow of the entire gaseous sample to result in a deposition of linearly aggregated particles. The SEM images of the linearly aggregated particles are shown in Fig. 18. The particle-wires reach to 250 μm in length with a mean diameter of 0.2 μm. The formation of particle-wire is characteristic of the sedimentary particles produced by the photochemical method. Namely, the sedimentary particles maintain the photochemical reactivity even after deposition on a substrate [74, 75], and hence, two neighboring particles on the substrate can be connected in each other by a chemical bond through chemical reaction [5]. The particle-wire is flexible, and can be connected to another particle-wire so as to form a longer wire in any shape. Production of particle-wires is a key technology to construct nanodevices.

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7. Formation of Magnetic Particles Upon exposure to the third harmonic of intense Nd:YAG laser light (355 nm, 35 mJ/pulse), a gaseous mixture of Fe(CO)5 and trimethylsilyl azide (TMSAz) and of Fe(CO)5 and CS2 produced ultrafine particles with diameter of 50 ~ 100 nm. The particles were aggregated in each other, and were attracted towards a permanent magnet. Thus, we have succeeded in producing magnetic particles using the photochemical method. In a preliminary HRTEM analysis, crystalline nano-domain of metals is detected inside the particles. The details will be reported elsewhere.

(a)

50 μm

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(b)

1 μm Figure 18. SEM images of linearly aggregated sedimentary particles produced from a gaseous mixture of (a) Co(CO)3NO (1.5 Torr) and ATMeSi (1.4 Torr) and (b) Co(CO)3NO (1.5 Torr) and ATMeSi (9.8 Torr) under light irradiation at 313 nm with a medium pressure mercury lamp. Original magnification of SEM, (a) 430×, (b) 10,000×.

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Hiroshi Morita

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8. Particle Formation from Solid Materials Until this section, the photochemical method to produce spherical particles is applied only to gaseous mixtures. In order to synthesize various kinds of spherical particles, it is preferable to utilize solid materials. Synthesis of ultrafine and fine particles from solid materials generally needs the melting and/or vaporization of solid materials. For this purpose, laser ablation of solid materials has frequently been used as a convenient and powerful method [76-81]. It is well known that depending on the laser light intensity and laser pulse duration, chemical and physical phenomena of laser ablation change dramatically [82]. Hence, in actual application of laser ablation to a specific purpose, determination of the experimental conditions is essential. We have developed a synthetic method to produce spherical particles from solid materials by the aid of laser ablation and photochemical reaction of ambient gaseous molecules. The method was applied to poly(dimethylsilane) (PDMS) in the atmosphere of trimethylsilyl azide (TMSAz) [83]. Chemical reactivity of TMSAz towards PDMS was utilized to produce spherical fine particles [75]. A small amount (40 - 60 mg) of PDMS powder (Wako, E.P. grade) was put into a small aluminum chamber placed on a glass plate at the bottom of a cross-shaped irradiation cell (for both arms, inner diameter 35 mm, length 130 mm). The small chamber (8 × 8 mm, H = 15 mm) had an aperture (5.9 φ) on the top surface, and the third harmonic (355 nm) of pulsed Nd:YAG laser light (pulse width 6 ns, repetition rate 10 Hz, energy 32 mJ/pulse) was irradiated on the PDMS powder vertically across the aperture (Fig. 19). The laser-ablated materials escaped from the chamber across the aperture and scattered on the glass plate. Without filling TMSAz vapor, the PDMS powder produced crystalline deposits as the major product (Fig. 20(a)) together with a small amount of spherical particles with a diameter of ~1 μm as a minor product. TMSAz (20 Torr) does not absorb light at 355 nm, and do not produce any solid material under laser light irradiation at 355 nm (energy 36 mJ/pulse) for 5 min. Taking this into account, PDMS powder in the presence of TMSAz vapor (20 Torr) was irradiated with 355 nm laser light (energy 36 mJ/pulse) for 5 min. Laser-ablated PDMS produced only spherical particles over the whole glass plate with a mean diameter of 1.3 μm (Fig. 20(b)). FT-IR spectrum of the particles was measured and the spectrum is shown in Fig. 21, together with the spectra of PDMS powder without laser ablation and of TMSAz vapor. In addition to the bands at 1250, 832, 743, 691, and 630 cm-1 ascribed to PDMS and the bands at 1259 and 834 cm-1 ascribed to TMSAz [84], a new band was observed at 1036 cm-1, assignable to ν(C-N). Considering that antisymmetric stretching band of the azido group, νa(N3) at 2153 cm-1 became very weak, it was strongly suggested that TMSAz reacted with PDMS accompanying the decomposition of the azido group to produce the trimethylsilyl nitrene intermediate. Through chemical reactions between the nitrene and laser-ablated PDMS, spherical particles were successfully formed. The PDMS powder emits fluorescence at 362 nm. The PDMS/TMSAz particles thus obtained show a fluorescence peak at 344 nm, being shifted to shorter wavelength by 18 nm than that of PDMS powder. As mentioned above, by the aid of laser ablation, spherical fine particles are successfully produced from solid PDMS by the photochemical method. Chemical reactions between laser-

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Photochemical Synthesis of Aerosol Particles

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ablated materials and gaseous molecules play an important role in producing spherical fine particles. YAG laser (355 nm)

a b

c

d e

f

Figure 19. Schematic diagram of laser ablation. a: quartz prism, b: quartz window, c: aluminum chamber, d: aperture (5.9 φ), e: PDMS powder, f: glass plate.

(a)

2 μm

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(b)

2 μm Figure 20. SEM images of laser-ablated PDMS (a) without filling TMSAz vapor and (b) in the atmosphere of TMSAz vapor (20 Torr). Original magnification of SEM, (a) and (b) 6,000×.

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Absorbance

0.08 (a)

0.06 0.04 0.02

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0.00 2.0 (b)

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Figure 21. FT-IR spectra of (a) particles produced by laser ablation of PDMS in the atmosphere of TMSAz vapor (20 Torr), (b) PDMS powder, and (c) TMSAz vapor (5 Torr).

CONCLUSION

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The photochemical method to produce spherical aerosol particles in the gas phase has some technical advantages as follows. (1) Chemical structure of the particles can be controlled by choosing the wavelength and intensity of the exciting light. A typical example so far studied is a gaseous CS2/GLY mixture. Sometimes, two-photon excitation of gaseous molecules (such as AC) is required to produce aerosol particles. In this case, photochemical sensitization is effective to increase the product yield. Some organometal compounds produce magnetic particles via multiphoton process. (2) Magnetic field influences the chemical reactions of some gaseous mixtures, resulting in a change of chemical compositions of the spherical particles. (3) Chemical compositions of the particles are also changed by the post-exposure with UV light. (4) The particle size can be controlled by regulating the photochemical reaction rate and reaction time. (5) The particles can be connected in each other by a chemical bond, leading to the formation of linearly aggregated particles. This technique can be developed to produce particle-wires applicable to build a nano-device. (6) By the aid of laser ablation, the photochemical method can produce spherical particles from solid materials.

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By analyzing the chemical compositions of aerosol particles synthesized by the photochemical method, the photochemical reaction pathways during aerosol particle formation can be elucidated. Moreover, the production and manipulation of the aerosol particles will develop a new field of nanotechnology.

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30 [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51]

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Hiroshi Morita K. F. Freed, Chem. Phys. Lett. 37 (1976) 47-50. H. G. Kuettner, H. L. Selzle, and E. W. Schlag, Chem. Phys. Lett. 48 (1977) 207-211. H. G. Kuettner, H. L. Selzle, and E. W. Schlag, Chem. Phys. 28 (1978) 1-9. H. G. Kuettner, H. L. Selzle, and E. W. Schlag, Israel J. Chem. 16 (1977) 264-276. F. C. Wen, T. McLaughlin, and J. L. Katz, Phys. Rev. A26 (1982) 2235-2242. J. J. Colman and W. C. Trogler, J. Am. Chem. Soc. 117 (1995) 11270-11277. A. Matsuzaki, J. Mol. Struct. (Theochem) 310 (1994) 95-103. K. Ernst and J. J. Hoffman, Chem. Phys. Lett. 68 (1979) 40-43. K. Ernst and J. J. Hoffman, Phys. Lett. 87A (1981) 133-136. R. Tomovska, Z. Bastl, V. Vorlíček, K. Vacek, J. Šubrt, Z. Plzák, and J. Pola, J. Phys. Chem. B107 (2003) 9793-9801. F. Cataldo, Inorg. Chim. Acta 232 (1995) 27-33. B. Kleman, Can. J. Phys. 41 (1963) 2034-2063. Ch. Jungen, D. N. Malm, and A. J. Merer, Can. J. Phys. 51 (1973) 1471-1490. A. E. Douglas and E. R. V. Milton, J. Chem. Phys. 41 (1964) 357-362. J. T. Hougen, J. Chem. Phys. 41 (1964) 363-366. H. Morita, K. Semba, Z. Bastl, J. Šubrt, and J. Pola, J. Photochem. Photobiol. A171 (2005) 21-26. P. B. Zmolek, H. Sohn, P. K. Gantzel, and W. C. Trogler, J. Am. Chem. Soc. 123 (2001) 1199-1207. G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers: the Scienta ESCA 300 Database, J. Wiley and Sons, 1992. C. D. Wagner and J. A. Taylor, J. Electron Spectrosc. Relat. Phenom. 28 (1982) 211217. D. Briggs and M.P. Seah, (eds.), Practical Surface Analysis Vol. 1, Auger and X-ray Photoelectron Spectroscopy, J. Wiley and Sons, Chichester, 1990, p. 595. H. Sontag, A. C. TAM, and P. Hess, J. Chem. Phys. 86 (1987) 3950-3958. H. Morita, F. Matsubayashi, and A. Nozue, Mol. Phys. 101 (2003) 2569-2574. H. Morita, S. Kanaya, and Z. Bastl, Mol. Phys. 104 (2006) 3003-3009. H. Orita, H. Morita, and S. Nagakura, Chem. Phys. Lett. 81 (1981) 29-32. T. Imamura, S. Nagakura, H. Abe, Y. Fukuda, and H. Hayashi, J. Phys. Chem. 93 (1989) 69-74. S. Ikeda, H. Abe, and H. Hayashi, Chem. Phys. Lett. 257 (1996) 507-512. H. Morita, R. Nozawa, and Z. Bastl, Mol. Phys. 104 (2006) 1711-1717. I. S. Butler and A. E. Fenster, J. Organomet. Chem. 66 (1974) 161-194. H. Morita, Y. Takeyasu, H. Okamura, and H. Ishikawa, Sci. Technol. Adv. Mater. 7 (2006) 389-394. I. S. Butler, S. Kishner, and K. R. Plowman, J. Mol. Struct. 43 (1978) 9-15. F. A. Cotton and J. M. Troup, J. Chem. Soc., Dalton Trans. (1974) 800. M. Poliakoff and J. J. Turner, J. Chem. Soc. A (1971) 654-658. H. Morita, Y. Takeyasu, and J. Šubrt, J. Photochem. Photobiol. A197 (2008) 88-93. M. Poliakoff and J. J. Turner, J. Chem. Soc. Dalton Trans. (1973) 1351-1357. M. C. Baird, G. Hartwell, and G. Wilkinson, J. Chem. Soc. A (1967) 2037-2040. M. C. Baird and G. Wilkinson, J. Chem. Soc. A (1967) 865. W. Pets, J. Organomet. Chem. 146 (1978) C23-25. W. Pets, J. Organomet. Chem. 270 (1984) 81-91.

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W. Pets, Inorg. Chim. Acta 201 (1992) 203-206. B. D. Dombek and R. J. Angelici, J. Am. Chem. Soc. 96 (1974) 7568-7569. K. Abe and H. Morita, J. Photopolym. Sci. Technol. 19 (2006) 135-140. H. Morita, H. Sakano, and C. Yamano, J. Photopolym. Sci. Technol. 20 (2007) 117123. H. Morita and H. Tonooka, J. Photopolym. Sci. Technol. 14 (2001) 203-206. H. Morita, K. Imura, and T. Sano, J. Photopolym. Sci. Technol. 15 (2002) 77-82. Y. Kawamura, K. Toyoda, and S. Namba, Appl. Phys. Lett. 40 (1982) 374-375. R. Srinivasan and W. J. Leigh, J. Am. Chem. Soc. 104 (1982) 6784. R. Srinivasan and V. Mayne-Banton, Appl. Phys. Lett. 41 (1982) 576-578. D. Bauerle, “laser Processing and Chemistry”, Springer, Berlin (1996). D. Dijkkamp, T. Venkatesan, X. D. Wu, S. A. Shaheen, N. Jisrawi, Y. H. Min-Lee, W. L. McLean, and M. Croft, Appl. Phys. Lett. 51 (1987) 619-621. G. B. Blanchet, C. R. Fincher, Jr., C. L. Jackson, S. I. Shah, and K. H. Gardner, Science 262 (1993) 719-721. H. Niino, J. Ihlemann, S. Ono, and A. Yabe, J. Photopolym. Sci. Technol. 13 (2000) 167-173. H. Morita and T. Tanabe, J. Photopolym. Sci. Technol. 21 (2008) 305-310. J. R. Durig, J. F. Sullivan, A. W. Cox, Jr., and B. J. Streusand, Spectrochim. Acta 34A (1978) 719-730.

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Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved. Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

In: Aerosols: Chemistry, Environmental Impact … Editor: Daniel H. Peretz

ISBN 978-1-60692-925-4 © 2009 Nova Science Publishers, Inc.

Chapter 2

INFLUENCE OF ATMOSPHERIC CONDITIONS ON AEROSOL INHALATION DURING A FOREST FIRE IN SPAIN A.I. Calvo, A. Castro, C. Palencia and R. Fraile∗ Departamento de Física, Universidad de León, Spain

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ABSTRACT Fine aerosol size distribution was measured in September 2000 during one of the eight days of a forest fire in Villablino, Spain. A laser spectrometer was used to determine the range of particle sizes: following their corresponding refractive index corrections, it was possible to establish an interval between 0.09 to 27.64 μm. The samples were collected at three different points distributed throughout the valley, following the direction in which the smoke plume advanced. The atmospheric conditions changed dramatically from the morning to the afternoon, with a subsidence inversion setting in during the day. Two modes (fine and coarse) were identified and defined for particle number, surface and volume distribution, and at all sampling points an increase in diameter caused by thermal inversion was observed. Estimated PM10 concentration varied between 16 μg m-3 and 27 μg m-3 in the morning to between 61 μg m-3 and 166 μg m-3 in the afternoon, after the thermal subsidence inversion had set in. This fact was most noticeable in fraction PM2.5, the most dangerous one for human health. Three calculation methods were applied to respirable dust fractions. The inhabitants of Villablino were exposed to high levels of particle concentration, especially during the thermal inversion, and the limits established by Spanish regulations (50 μg m-3) were clearly exceeded.

Keywords: forest fire, fine mode, coarse mode, respirable dust fraction, thermal inversion

*Roberto Fraile: Phone: +34 987 29 15 43; fax: +34 987 29 19 45; e-mail: [email protected].

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1. INTRODUCTION Aerosols are the focus of an increasing interest due to the serious effect they have on various aspects, climate and human health being among the most important ones. Aerosols can either absorb or scatter light. In this way they directly influence the earth’s radiation balance (Schwartz, 1996; Andreae and Crutzen, 1997; Seinfeld and Pandis, 1998; Schwartz and Busek, 2000; Andreae, 2001; Jacobson, 2001; Mallet et al., 2005; Gu et al., 2006; Generoso et al., 2007) and contribute to climate change. Aerosols are also the main source of cloud condensation nuclei (CCN) (Kaufman and Nakajima, 1993; Lohmann and Feichter, 2005; Kawamoto, 2005; Sun and Ariya, 2006; Rissman et al., 2007). On the other hand, biomass burning in the form of large forest fires is one of the global sources of aerosol particles (von Hoyningen-Huene et al., 1998) liberating large quantities of particulate matter into the troposphere (Ikegami et al., 2001) and producing between approximately 3–150 Tg of particles per year in the world (Hinds, 1999). Biomass burning is considered to be the second most important source of anthropogenic aerosols (Crutzen et al., 1979; IPCC, 1995) with potential effects on precipitation, cloud properties, and radiative balance (Penner et al., 1992; IPCC, 1996; Hobbs et al., 1997; Lin et al., 2006). Aerosol size distribution influences the dynamics of aerosol population (Vakeva et al., 2001), their production and removal processes, size transformation, lifetime, optical properties and radiative effects (Huebert et al., 1996). Size distribution of atmospheric aerosols strongly depends on the sources and sinks as well as on the meteorological processes prevailing during their lifetime (Suzuki and Tsunogai, 1988; Ito, 1993; Fraile et al., 2006). Aerosol size is the major determining factor on atmospheric behavior of aerosol particles, controlling residence time, removal mechanisms of aerosol bound contaminants from the troposphere (Zufall and Davidson, 1997; Kaupp and McLachlan, 1999), and environmental impact (Wardoyo et al., 2007; Calvo et al., 2008). Factors like moisture content, fuel variability, burning processes and burning conditions determine the size of particles released during biomass burning, and the majority of particles are less than 2.5 μm in diameter (Ferge et al., 2005; Wieser and Gaegauf, 2005). However, quantitative knowledge of the relationship with particle size distribution is very limited. As has been stated above, many of the particles produced by forest fires are very small, and it has been demonstrated that exposure to fine particles, PM2.5, plays an important role in human health, especially in respiratory diseases (Maynard and Maynard, 2002; US EPA 2004, Dai et al., 2006). Several studies report the relationship between forest fires and health impact (Duclos et al., 1990; Copper et al., 1994; Smith et al., 1996; Aditama, 2000), especially in the case of wildland firefighters (Liu et al., 1992; Reinhardt and Ottmar, 2004; Booze et al., 2004). Numerous studies deal with the ageing processes of particles produced by biomass burning (Liousse et al., 1995; Anderson et al., 1996; Reid et al., 1998; Fiebig et al., 2003; Hobbs et al., 2003). All of them conclude that the ageing of smoke particles can have a significant effect on their physical and chemical (and consequently radiative) properties. In addition, smoke particles increase in size as they are transported in the atmosphere (Reid et al., 1999). This increase may occur during the first few hours following emission, as reported by Abel et al. (2003), or over a period of days, as stated by Reid et al. (1999). Various mechanisms can explain smoke particle ageing, including hygroscopic water uptake (Hobbs

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et al., 1997), condensation of volatile organic species (Reid et al., 1998), and coagulation (Westphal and Toon, 1991; Radke et al., 1995; Fiebig et al., 2003). There are several studies on pollution in valleys (Kuhns et al., 2003; Strawbridge and Snyder, 2004; Ta et al., 2004) which draw attention to the topography, the meteorological conditions, the entrance of desert dust, anthropogenic pollution and other factors that contribute greatly to the low quality of the air in valleys. In Spain, 1978, 1985, 1989 and 1994 were the years in which the largest areas were burnt, with 4395 km2, 4845 km2, 4267 km2 and 4376 km2 of surface area being burnt, respectively. However, if we consider the incidence of fires, the years with the highest number of fires were 1995, 1997, 2000 and 2005 with 25,827, 22,446, 24,118 and 25,492 cases, respectively (Ministry of the Environment, 2006). One of the main reasons for variation in the land area affected by forest fires in Spain is climate change. The years when most land was affected were years of persistent drought, especially during the summer months. In the province of León (Spain), the trend is similar to that observed in the rest of Spain. In 1998 there were 694 forest fires in the province and 37.4 km2 were burnt; in 1999 there were 757 forest fires that devastated 25.7 km2 of wooded land; in 2000 there were 1050 forest fires that affected 19.0 km2; and in 2005 there were 920 forest fires and 28.2 km2 were burnt (Ministry of the Environment, 2006). Characteristics of burning biomass particles have been studied in several regions of the world, showing that particle size varied among regions due to a variety of factors including vegetation, moisture content and weather conditions. The majority of these studies were carried out taking measurements while flying over the smoke plume in a plane (e.g., Hobbs et al., 2003) and/or in laboratory conditions (e.g., Wardoyo et al., 2007). Studies carried out at ground level in the vicinity of the fire are less common (e.g., Cachier et al., 1995). This type of study is almost non-existent in Spain: hence the importance of the study presented here. The main objective of the present chapter is to describe how the amount of respirable dust inhaled by the people living in the valley during a forest fire in Villablino (province of León) increased significantly due to the sudden appearance of a thermal subsidence inversion. Furthermore, this study analyzes and develops a better general understanding of the influence the physical relief and environmental conditions of an area has on aerosol size distribution. The number concentration and size distribution of the particles were measured at different distances from the fire with a laser spectrometer during one of the eight days of the fire.

2. STUDY ZONE The district of Villablino (42º56′20′′ N, 6º19′00′′ W) lies in the region of Laciana, to the northwest of the province of León (Spain) (Fig. 1), in the Cantabrian Mountain Range with altitudes between 900 and 1300 meters. It is a mountainous land with high valleys, exploited for agricultural purposes, forestry and industry, especially coal mining. On September 6, 2000, an intentional forest fire was started which lasted for eight days and affected a vast area including the districts of Villablino and Palacios del Sil (Fig. 2a). The land burnt (10.1 km²) included shrub (7.61 km²) and wooded land (2.69 km²) (Fig. 2b). The sampling of aerosols at different points close to the fire was carried out on September 7.

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A.I. Calvo, A. Castro, C. Palencia et al.

Figure 1. Study area in the province of León (Spain).

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(a)

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(b)

Figure 2. a) Orography of the study area (Valle del Sil); b) Photograph of the area affected by the fire (right side of the valley), taken four years after the event.

3. METHODOLOGY

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3.1. Measurement Equipment Continuous particle number/size distributions were measured using a passive cavity aerosol spectrometer probe, PMS Model PCASP-X. The PCASP measures particle size distribution for diameters ranging from 0.1 to 10 μm in 31 channels, derived from the light scattering properties of the particles at a wavelength of 633 nm between angles of 35º and 135º. The PCASP-X shows an adjustable sample volume of 1.0-3.0 cm3 s-1 that must be established for each measurement carried out. The sheath airflow setting is 15 cm3 s-1 and the maximum count rate is 10 000 s-1. The instrument was calibrated by the manufacturer, using polystyrene latex particles of known size, with a refractive index of 1.58– 0i. These particles are highly efficient at scattering radiation, as is indicated by the high values for the real component of refractive index and completely non-absorbing (no imaginary component). Ambient particles are generally less efficient scatterers of radiation, but display some absorptive properties. Consequently, as Guyon et al. (2003) report in their study, optical counters such as the PCASP-X, generally underestimate the true diameter of ambient aerosol, and the measured size distributions should be corrected for the refractive index of sampled particles. Guyon et al. (2003) have shown that the refractive index of ambient aerosol particles may be subject to large intra- and inter-day variations, as a function of aerosol sources, age, and relative humidity (RH). Here, we are presenting PCASP size distributions corrected with a fire aerosols refractive index (1.49-0.013i) using Mie theory and implemented with a computer

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A.I. Calvo, A. Castro, C. Palencia et al.

code developed by Bohrenm and Huffman (1983). This value corresponds to AERONET sunphotometer retrieval at 675 nm in Palencia—in north-western Spain, near to the study area- (41°N, 4°W) on 06/08/2003. This value can be taken to be representative of the burning of the same biomass type. These refractive index values are consistent with those for the composite aged regional haze dominated by biomass burning aerosol (Haywood et al., 2003). The real part of the refractive index also agrees with that found by Yamasoe et al. (1998) for biomass burning aerosol in Brazil. After correction, the mean particle size range went from 0.1-10 μm to 0.09 – 24 µm (Table 1). In addition to refractive index diameter correction, a further series of corrections were performed following the manufacturer’s instructions for counts carried out by the spectrometer in order to establish the exact number of particles by unit of volume sampled in each channel:

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a) The value of the sample volume has to be adjusted according to the altitude of the sampling point. We used a correction for the adjustment of flowmeter versus altitude, as recommended by the manufacturer. In this case, the measurements were carried out at different points, so a different correction factor was applied for each sampling point as shown in Table 2. b) Activity is defined as the span of time during which the instrument is analyzing particles. It includes both the transit event time as well as the processing time of all particles. It is read out as a number from 0-100 Hz, which corresponds to the percentage of the total time used for analyzing particles. Depending on the activity registered during data collection, each measurement has to be adjusted by means of an algorithm. 100% activity represents the maximum number of particles the probe is capable of measuring per second (10,000 particles per second). In addition to these corrections, the probe was calibrated by a specialised firm, Particle Metrics INC (Boulder, USA), some weeks before sampling was carried out. The data were gathered with the isokinetic probe of the spectrometer three meters above the ground. Each sampling lastedten minutes, with subsamplings every 60 seconds. Consequently, ten subsamplings were obtained for each sampling. After carrying out all the necessary corrections on the number of counts registered, the number of particles per cubic centimeter can be established for each one of the 31 channels in each subsampling. The mean of the 10 subsamplings is considered the final value for a particular sampling point. With the sole aim of enabling the comparison of measurements collected during our study both with those collected during other studies, and with air quality standards, we calculated the mass concentration (μg m-3) of PM10, using 1.35 g cm-3 (Reid and Hobbs, 1998) as the single particle density value for all channels and assuming the associated error factor. In the case of the background measurement, taken in an area far from sources of pollution, the density of clean air (1.29 g cm-3) was used. However, this calculation later proved to be useful in other respects as well, highlighting the changes produced in the smoke plume due to different vertical dispersion with and without thermal inversion, and facilitating an interpretation of the findings. In addition, a SODAR system was used to remotely measure the vertical turbulence structure and the wind profile of the lower layer of the atmosphere. The SODAR SR 1000 has a pulse frequency of five tones, around 2 150 Hz, a pulse power of 300 W, a pulse repetition

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period of 8 s, and a maximum range of 1 250 m. The system was located at about 60 km southeast of the forest fire. Every thirty minutes we gathered data relating to wind speed and wind direction at an altitude of 275 m above ground level. Although the SODAR system could not provide us with precise data about the exact study area, it did give us an idea of overall atmospheric circulation in the general area. Table 1. Diameter ranges included in each channel, and intervals between each channel (μm) of the PCASP-X instrument when calibrated with latex spheres (n=1.58-0i) and assuming the refractive index of biomass burning aerosols particles (n=1.49-0.013i). Midpoint latex diameter and midpoint biomass burning diameter ratio (τlatex/ τaerosol) is also shown, showing that from channel 10 onwards, the PCASP-X will tend to underestimate the size of biomass burning aerosols particles due to the effects of particulate absorption.

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n= 1.58-0i

n= 1.49-0.013i

Channel

Range

Interval

Range

Interval

PCASP

(μm)

(μm)

(μm)

(μm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

0.10 - 0.12 0.12 - 0.14 0.14 - 0.16 0.16 - 0.18 0.18 - 0.20 0.20 - 0.23 0.23 - 0.26 0.26 - 0.30 0.30 - 0.35 0.35 - 0.40 0.40 - 0.45 0.45 - 0.50 0.50 - 0.60 0.60 - 0.70 0.70 - 0.80 0.80 - 0.90 0.90 - 1.00 1.00 - 1.20 1.20 - 1.40 1.40 - 1.60 1.60 - 1.80 1.80 - 2.00 2.00 - 2.30 2.30 - 2.60 2.60 - 3.00 3.00 - 3.50 3.50 - 4.00 4.00 - 5.00 5.00 - 6.50 6.50 - 8.00 8.00 - 10.00 >10.00

0.09-0.11 0.11-0.13 0.13-0.15 0.15-0.17 0.17-0.19 0.19-0.22 0.22-0.25 0.25-0.28 0.28-0.36 0.36-0.44 0.44-0.50 0.50-0.58 0.58-0.71 0.71-0.85 0.85-1.03 1.03-1.15 1.15-1.27 1.27-1.38 1.38-1.50 1.50-2.14 2.14-2.43 2.43-2.80 2.80-3.49 3.49-4.39 4.39-5.33 5.33-6.59 6.59-9.02 9.02-13.44 13.44-18.75 18.75-23.86 23.86-31.41 > 31.41

0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.08 0.08 0.06 0.08 0.13 0.14 0.18 0.12 0.12 0.11 0.12 0.64 0.29 0.37 0.69 0.9 0.94 1.26 2.43 4.42 5.31 5.11 7.55

0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.04 0.05 0.05 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.20 0.20 0.20 0.20 0.20 0.30 0.30 0.40 0.50 0.50 1.00 1.50 1.50 2.00

τlatex/ τaerosol

1.10 1.08 1.07 1.06 1.06 1.05 1.04 1.06 1.02 0.94 0.90 0.88 0.85 0.83 0.80 0.78 0.79 0.83 0.90 0.82 0.74 0.73 0.68 0.62 0.58 0.55 0.48 0.40 0.36 0.34 0.33

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A.I. Calvo, A. Castro, C. Palencia et al.

Table 2. Geographic location of the sampling points given in order of distance from the fire (C, B, and D), times of the samplings (morning-with 0- and afternoon-with 1-) and altitude correction factor for the PCASP-X.

SAMPLING TIME LOCATION LONGITUDE LATITUDE POINTS (UTC)

Background measurement

0852

C0

1226

C1

1554

D0

1307

D1

1625

B0

1150

B1

1650

DISTANCE ALTITUDE FROM ALTITUDE CORRECTION THE FIRE (m) FACTOR (m)

(º)

(º)

Barrios de Luna Reservoir

5º54’00”W

42º53’55”N

39000

1200

0.866

Villarino del Sil

6º21’45”W

42º54’34”N

2700

1066

0.887

km 55 between Villarino and Cuevas del Sil

6º22’36”W

42º54’01”N

3200

900

0.879

Cuevas del Sil

6º23’36”W

42º53’43”N

3800

890

0.878

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3.2. Sampling Points and Data Gathering On September 7, the Department for the Environment of the Junta de Castilla y León (Regional Government) informed us early in the morning of the forest fire close to Villablino, in the province of León. We drove to the study area in order to evaluate aerosol emission. The fire was located on the right-hand side of the valley, at an approximate altitude of 1,500 m. A four-wheel drive vehicle was used to reach each of the selected sampling points, all located at the lowest part of the valley, with altitudes of 890 m, 900 m, and 1,066 m. A diesel generator was carried in a trailer, whilst computer and laser spectrometer equipment was carried in the interior of the vehicle. In order to evaluate aerosol emission at each sampling point, the spectrometer was positioned at a height of 3 m, using an elevated platform on the vehicle roof. Before arriving we carried out a background measurement close to the Barrios de Luna reservoir, about 39 km from the fire; an unpopulated area with no industries in the vicinity. Before deciding on the points for the aerosol samplings, we checked that the air was calm. A light wind was blowing down the valley and the smoke plume was drifting in a southwesterly direction. It was therefore decided that the sampling points should be located down the valley in the same direction. There were three different sampling points, providing a

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total of 6 observations (Fig. 3) as for each sampling point one observation was taken in the morning and another one in the afternoon. The sampling points were denominated B, C and D (Table 2), adding 0 if the measurement was carried out in the morning before thermal inversion set in (B0, C0 and D0), and 1 if it was carried out in the afternoon after thermal inversion appeared (B1, C1 and D1). The sampling point sites were as follows: Cuevas del Sil (point B), Villarino del Sil (point C) and at km 55 on the road between Villarino del Sil and Cuevas del Sil (point D). Two measurements were taken at two other points; at one, a measurement was taken in the morning, and at the other, a measurement was taken in the afternoon. These measurements have not been included in the overall study, except for the occasional comment, as the prime objective was to compare samples taken at the same point before and after thermal inversion, and these measurements do not contribute to this aim.

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Figure 3. Map of the study zone with the sampling points (morning & afternoon), plume direction and direction of surface wind.

Apart from the background measurement, the laser spectrometer was used to carry out three samplings in the morning and early afternoon, between 1150 and 1307 UTC, and another three samplings later in the afternoon, between 1554 and 1650. The reason for this was that in only two hours the meteorological conditions changed dramatically. The development of the fire did not change much, but the valley was suddenly filled with smoke and visibility was very much reduced. Because of this sudden change, it was decided to carry out additional samplings in the afternoon at the same three points which had been sampled in the morning, with the aim of investigating whether size distribution underwent any changes. The forest fire appeared to show the same intensity and speed of advance throughout the whole sampling period, but it should be noted that during its advance it moved closer to point C, and when the afternoon measurement was taken, this sampling point was only 200 – 300 m from the fire. The down-valley direction of the plume remained constant.

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Additional measurements carried out in situ included temperature at surface level, and direction and speed of surface wind for all sampling points. The SODAR provided available altitude data for areas closest to the study area (Table 3). The sampling points were not very far from each other, but carrying all the material and re-installing it was time-consuming, and at least 30-60 minutes elapsed between one measurement and the next. Table 3. Meteorological data at the sampling points. SAMPLING POINTS

TIME

LOCATION

SURFACE TEMPERATURE

SURFACE WIND

SURFACE WIND

WIND DIRECTION

WIND SPEED

Background measurement

0852

Reservoir Barrios de Luna

30

NW

Calm-1.0

323

1.2

C0

1226

Villarino del Sil

35

-

Calm

187

2.5

C1

1554

36

-

Calm

172

3.1

D0

1307

35

SW - NE

Calm-2.6

180

2.7

D1

1625

35

-

Calm

172

3.1

B0

1150

35

SE - NW

Calm-2.2

191

1.8

km 55 between Villarino and Cuevas del Sil

4. METEOROLOGICAL ANALYSIS

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4.1. Synoptic Analysis Synoptic analysis showed the presence of high pressure both on the ground and at high levels, which began on September 1st and remained stable until September 17 throughout the Iberian Peninsula. There was great stability and no rain. In particular, the synoptic maps in Fig. 4 show that an Atlantic high covered the study zone when the fire began (September 6). The next day a large part of the Iberian Peninsula lay under the influence of a flat low located in the western Mediterranean area. With diurnal warming the synoptic map at 1200 GMT (not shown here) shows a thermal low on the Peninsula, whereas at high levels there was an intense high of warm air. Twelve hours later, the situation was clearly one of high pressures, with several high pressure areas in the north of the Peninsula generating great atmospheric stability. This impeded the dispersion of pollutants, in particular with respect to the smoke particles produced by the forest fire. The average maximum and minimum temperature on the study day and the previous day and the following day registered at the weather station in León was 30ºC and 14ºC, respectively. These temperatures are unusually high for the study zone at that time of year.

Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

Influence of Atmospheric Conditions on Aerosol Inhalation . . .

(a)

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(b)

Figure 4 (Continued)

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43

44

A.I. Calvo, A. Castro, C. Palencia et al.

(c)

Figure 4. Synoptic maps: (a) 6th September, (b) 7th September, (c) 8th September at 0000 UTC at ground level.

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4.2. Radiosoundings The University of Wyoming provided the data of the radiosoundings carried out over Madrid (40º 30′00′′ N, 03º 34′ 48′′ W), to the southeast of the study zone, at a distance of approximately 360 km. Fig. 5 gives a graphic representation of vertical temperature profiles and dew point temperature in the atmospheric layers that lie closer to the ground. Analysis of the radiosounding carried out on September 7 at 0000 UTC (Fig. 5a) shows a radiation inversion that disappeared in the hours following sun-rise and therefore did not affect smoke plume dispersal measurements taken in the morning. However, the sounding carried out at 1200 UTC (Fig. 5b) shows a particularly interesting subsidence inversion located at 648 m above ground level, caused by the high over the Iberian Peninsula. Based on the Holzworth methodology (Holzworth, 1972), we also analyzed the altitude of the maximum mixing layer on the study day, the previous day and the following day (Table 4) corresponding to maximum temperature at surface level. Stability varies with altitude, so both soundings show a certain conditional instability from surface level to an altitude of 9 km, whereas at higher levels there were intervals of extremely high stability. At an altitude of 275 m above ground level (data obtained by the SODAR in Carrizo de la Ribera, less than 60 km to the SE of the fire), the wind was light, and the Madrid sounding confirmed that winds were very light in general at all levels in the atmosphere This is one of the identifying features of thermal inversion.

Aerosols: Chemistry, Environmental Impacts and Health Effects : Chemistry, Environmental Impact and Health Effects, Nova Science Publishers, Incorporated, 2008. ProQuest Ebook

Influence of Atmospheric Conditions on Aerosol Inhalation . . . (a)

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(b)

Figure 5. Madrid (Spain) radiosounding on September 7th 2000 (a) 0000 UTC (b) 1200 UTC. (Data source: The University of Wyoming).

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A.I. Calvo, A. Castro, C. Palencia et al.

5. ANALYSIS OF RESULTS The data gathered by the PCASP-X were used to obtain the number of particles per cubic centimeter in each of the 31 channels, and to estimate the concentration in each channel for each of the six samplings (plus the background measurement) carried out close to the fire.

5.1 Analysis of Atmospheric Conditions A visual analysis, carried out by the personnel responsible for data collection, indicated that visibility was excellent in the morning, with completely clear skies, and that the smoke plume was moving down the valley, indicative of the valley's influence. Under the existing atmospheric conditions, smoke plume dispersal was equally horizontal and vertical (rising and descending). The abrupt change observed in situ by researchers between morning and afternoon conditions led to a cloud of immobile smoke throughout the valley and surrounding areas. As a result of this observation it was decided to take further measurements in order to compare them with those taken in the morning, as visibility was drastically reduced in the afternoon. The first in situ interpretation of the change was that a strong thermal inversion had set in over a period of less than two hours, at around 15 UTC, with the immediate result that rising vertical dispersion of aerosols released by the fire was hugely inhibited. Thermal inversion was later confirmed in a laboratory analysis of radiosounding data from areas close to the study area: Madrid, La Coruña and Santander. The latter two areas, although not included in this report, also showed signs of the presence of thermal inversion. Whilst the radiosoundings are not specifically focused on the study area, they do provide an idea of the great atmospheric stability present that day throughout the Iberian Peninsula.

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5.2 Morning Study (Without Thermal Inversion) A) Particle Size Spectrum In the measurements carried out in the morning (C0, D0 and B0), with no thermal inversion, it can be observed (Fig. 6a) that the smallest particles found in channel 1 (0.09-0.11 μm) showed particle number values between 887 cm-3 at sampling point B and 478 cm-3 at the sampling point D. Outside the fire zone (background measure) the value registered in this channel was only 333 cm-3. If we turn to the total number of particles registered, we can see that this varies between 1926±58 particles cm-3 at sampling point D0 and 3645±50 particles cm-3 at sampling point B0. In the measurements, over 99% of the particles have a diameter under 0.4 μm, and the average number of particles with a diameter under 0.2 μm was 88%. All particle size distributions were of the lognormal type, and their average geometric mean diameter with their geometric standard deviation was 0.15 μm and 1.40 respectively. The particle size distributions have the same shape in all the three measurements carried out in the morning (C0, D0 and B0).

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Influence of Atmospheric Conditions on Aerosol Inhalation . . .

47

(a) 100000 B0

10000 -3

dN/d(logD) (cm )

C0

1000 D0

100

BACKGROUND

10 1 0,1 0,01 0,1

1

10

100

Diameter (μm)

(b) 100000 B1

10000 -3

dN/d(logD) (cm )

C1

1000

D1

100

BACKGROUND

10 1 0,1 0,01 0,1

1

10

100

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Diameter (μm)

Figure 6. Aerosol size spectrum in Villablino on September 7 and background sampling in (a) the morning, without thermal inversion and (b) the afternoon, with thermal subsidence inversion.

B) Total Estimated Particle PM10 Concentration Estimated PM10 concentration varied between 16 μg m-3 at point D0 and 27 μg m-3 at point B0 (Table 5), in spite of the fact that D0 was located 600 m further away from the fire. A possible explanation could be that point D was situated laterally to the right of the plume axis. The background measurements produced a value of 12 μg m-3, which is to say that in spite of the presence of a fire, concentrations registered at ground level for the different points barely doubled background concentrations. We believe that this finding could be explained by

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A.I. Calvo, A. Castro, C. Palencia et al.

the fact that the lack of thermal inversion in the morning meant that there was efficient horizontal and vertical dispersion of the plume, so that only a small amount of smoke from the plume reached the sampling points, which were situated at ground level. In short, no significant differences were found in estimated concentrations for the different sampling points, and these concentrations did not exceed the maximum daily value of 50 μg m-3 established by the European and Spanish regulations on air quality -European Directive 1999/30/CE, reflected in Spanish legislation by the Real Decreto (Royal Decree) 1073/2002. Table 5. Total experimental number concentration (mean± standard deviation), geometric mean diameter, experimental geometric standard deviation (σg) of number distribution and total estimated concentration (mean ± standard deviation). Total number Concentration (particles cm-3)

Geometric mean diameter (μm)

Background

991

0.14

1.38

Total estimated concentration (D