Environmental Chemistry 9789350431719, 9789350243084

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ENVIRONMENTAL CHEMISTRY

II J •

"This page is Intentionally Left Blank"

ENVIRONMENTAL CHEMISTRY AJAY KUMAR BHAGI

G.R. CHATWAL

MSc. Ph.D. Reader in Chemistry Dyal Singh College Lodhi Road, New Delhi-ll0003

MSc. Ph.D

4Il

Reader in Chemistry Dyal Singh College Lodhi Road, New Delhi-JI0003

lOt GJIimalaya %blishingGJIouse MUMBAI • DELHI. NAGPUR • BANGALORE • HYDERABAD

©AUTHOR No part of this book shall be rep':oduced, reprinted or translated for any purpose whatsover without prior permission of the publisher in writing.

ISBN

: 978-93-5024-308-4

REVISED EDITION: 201 0

Published by

Mrs. Meena Pandey for HIMALAYA PUBLISHING HOUSE, "Ramdoot", Dr. Bhalerao Marg, Girgaon, Mumbai - 400004. Phones: 23860170/23863863 Fax: 022-23877178 Email: [email protected] Website: www.himpub.com

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Contents 1.

Introduction and Important Concepts in Environmental Chemistry

1

I. I 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 I. \0

General Remarks What is environmental science ? Environmental Chemistry Ecology and Environment Food Chain of Life Biome Man and his Environment Environmental Pollution Units of Concentration Environmental Segments

2 2 3 4 4 6 8 9

2.

Evolution of Earth and Bio-Geochemical Cycles

12

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

The Earth as a Planet Structural features of Earth The Evolution of Life Energy of Life Distribution of Elements Geochemical Cycles Summary and Comment on Bio-Geochemical Cycles Geochemical Accumulation of Solar Energy

12 13 13 IS IS 16 39 40

3.

Atmosphere, Meterology and Green House Effect

44

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3. \0 3.11 3.12 3.13

Introduction Evolution of the Atmosphere of the Earth M~ior Regions of the Atmosphere Composition of the Atmosphere Meterology Atmospheric Conditions and Pollution City Climate Pollution and Temperature Inversions Meteorology and Human Activities The Surface Temperature of Earth The Earth's Heat Balance The Greenhouse Effect Consequences of Greenhouse Effect

44 45 46 50 51 56 56 57 58 58 59 60 63

4.

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemistry

66

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

Introduction Nature of Light Photochemistry Types of Primary Photochemical Processes Radicals in Atmosphere Hydroxyl (OH) and Hydroperoxyl (HOi> radicals in Atmosphere Ions in Mesosphere and Lower Thermosphere Reactions of Atmospheric Nitrogen

66 67 68 69 73 73 76 79

.t.')

4.10 -1.11

5. 5.1

5.2 5.3 5.-1

55

6. 6.1

6.2 6.3 6.·1 65 6.6 6.7 6.X (l.')

6.10 (l.1 I

h.12 6 13 6.14

7. 7I 7.2

., , I ••'

7.-1

7.5 7.h

7.7 7.X

7.9 7.10 7.11 7.12

7.13 7.14 7.15 7.16

8. X.I X.2 S.3 X.-I

8.5 X./l R7 X.X

Reactions of Atmospheric Oxygen Reaetiol1s of Water in Atmosphere Reactions of Atmospheric CO 2

79

Air Pollution Intl"l'duction Air Pol!lIti,:n System I)pes oj" :,\ir Pollutants Cla~siticali()n of Air Pollution Econo\1l:c~ of Air Pollution

84

Gaseous Inorganic 'Air Pollutants Introullcti.l!1 O\id.:~ of Carbon Oxides oj" ~Ilrogen Atmo~pheric Chemistry of Nitrate Radicals Atmospheric Chemistry of Nitrous Acid HN0 2 Atmospheric Chemistry of Nitric Acid. HNO J Atmospheric Chemisty of Peroxynitric Acid. H0 2N0 2 Atmosph.:ric Chemistry of Ammonia. NH J IlydraLine in Atmosphere Oxides of Sulphur (SO) Ilydrogen Sulphide Carbon) I Sulphide and Carbon Disulphide Ozone Fluorine and its Compounds

95

81 81 84 86

92 92 93

95 95 101

110 III 112 113 113 113 113 119

120 120 122

Organic Air Pollutants Introduction Sources of 1·1) drocarbons Source~ of Other Ilydrocarhons Atmo~phenc Reactions of Hydrocarbons Atmospheric Reactions or Methane AtmospherIc Reactions of Alkenes Atmospheric Reactions of Alkynes Atmospheric reactions of Aromatic Ilydrocarbons Alkyl and Aryl Iialides as Atmosphere Pollutants Alcohols and Phenols as Atmospheric Pollutant Ethers as Atmospheric Pollutants Atmospheric Reactions of Aldehydes and Ketones Epoxides as Atmospheric Pollutants Carboxylic Acid as Atmospheric Pollutants Organosulphur Compounds as Atmospheric Pollutants Organonitrogen Compounds as Atmospheric Pollutants

125 125

Particulate Matter and Aerosols in Atmosphere Introduction 'I~ pc of Aerosols and their Sources I'h:- sical Propertie~ of Particulate Matter I\iechallisms of Particulate Formation in Atmosphere Chcmical Composition of" Ambient Particulate Matter Inorganic PartIculate' Matter Orgalllc l'artlcul;lt" :--1ath:r ChCIIll~II") 111 '\qll"ou~ ~yslel1ls--Aeros()b. Cloud~. Fogs and Rain

153 153 154 156

125 127

128 129 130

140 140 1-11

1-15 1-16

146 148

148 148

150

158 160

162 165 172

8.9 8.10 8:11 8.12 8.13

Effects of Particulate Matter on Plants Effects of Particulate Matter on Humans Effects of Particulate Matter on Materials Effects of Particulates on Solar Radiation and C:imate Particulate Matter Control

176 176 178 178 178

9. Acid Rain 9.1 Introduction 9.2 Chemistry of Acid Rain 9.3 Oxidation of S02 9.4 Oxidation of N0 2 to Nitric Acid 9.5 Role of Meteorology in Acid Rain 9.6 Acid Fogs 9.7 Dry Deposition of Acidifying Substances 9.8 Geological Effects of Acid Rain 9.Y Effects of Acid Rain on Plants 9.10 Effects of Acid Rain on Freshwater Biota

181 181 181 182 189 190 190 191 191 193 193

10. \0.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13

Smog-Classical and Photochemical Introduction Mexico City Smog-A New Phenomenon Classical Smog Photochemical Smog Chemistry of Photochemical Smog Products of the Photochemical Smog The Main Organic Cycles of Photochemical Smog Transport of Photochemical Smog Effects of Photochemical Smog on Plants Etlccts of Photochemical Smog on Humans Effects of Photochemical Smog on Materials208 Control of Photochemical Smog Chemical Control of Photochemical Smog

194 194 195 195 196 197 201 205 207 208 208

11,

Depletion of Ozone Layer Introduction' III Ozone in Atmosphere The SST Problem The Chlorofluorocarbon Problem Inter-relation between ClO. and CH 4 • NO. and HO. How Does Ozone Hole Affect Us? Ozone Depletion in Mesosphere How to overcome the ozone depletion problem ?

211 211 211 212 213 214 217 218 218

Introduction to Water Pollution Water Pollution Unpolluted Vs. Polluted Water Hydrologic Cycle Complexation in Natural Water and Waste Water Microbially Mediated Redox Reactions Eutrophication Hardness and Alkalinity Iron and Manganese Bacteria Biochemical Oxygen Demand and Chemical Oxygen Demand

219 219 220 220 222 224 224 229 231 232

11.1 11.2 11.3 11.4 11.5 11.6 11.7 1\.8

12. 12.1 12.2 12.3 12.4 12.5 1~.6

12.7 12.8 12.9

209 210

12.10 Determination of Biochemical Demand (BOD) 12.11 Classification of Water Pollutants

233 236

13.

Control of Water Pollution

248

13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10

General Aspects of Prevention and Control of Water Pollution Water Softening Water Treatment Systems Advanced Techniques of Water Treatment Removal of Metals and Metallic Ions Advanced Wastewater Treatment Methods Other Advanced Waste Treatment Methods Sewage Treatment Sterilization of Water Obtained from Effluents Water Reuse and Recycle

248 249 250 254 268 271 275 277 282 284

14. Industrial Pollution and Its Control

286

14.1 14.2 14.3 14.4

286 287 290 290

Industrial Pollution Industrial Water Wastes Environmental Pollution Due to Paper Mills Control or Reduction of Paper Mill Pollution

15. Soil Pollution and Agricultural Pollution

292

15.1 15.2 15.3 15.4 15.5 15.6 15.7

Soil Soil Types Trace Metals in Soils Organic Matter in Soil Macro-Nutrients in Soil Pollution of Soil Agricultural Pollution

292 293 294 296 297 298 300

16.

Toxicological Chemistry of Chemical Substances

304

16.1 16.2 16.3 16.4 16.5 16.6 16.7

Introduction What is a Poison? What is a Toxic Substance? How are Toxic Substances Tested? Toxic Elements and Elemental Forms Enzyme Inhibition by Toxic Metals in Man Biochemical Effects of Toxic Metals on Man Toxic Inorganic Compounds

304 304 305 308 309 310 322

17.

Environmental Chemistry of Soaps, Detergents and Pesticides

328

17.1 17.2 17.3 17.4

328

17.5 17.6

Introduction Properties of Soaps and Detergents Metabolism of Hydrocarbons. Soaps and Detergents Environmental Chemistry of Pesticides Polychlorinated Biphenyls, and Other Chloro-Organic Compounds Organophosphorus Insecticides Other Pesticides in Common Use

18.

Thermal Pollution

345

18.1 18.2 18.3 18.4 18.5

Introduction Sources of Thermal Pollution Harmful \Effects of Thermal Pollution Thermal Pollution over Urban Areas Prevention of Thermal Pollution

345 345 346 348 348

32~

329 330 340 342

19. 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10

Noise Pollution Introduction Units of Measurement of Noise (or sound) Measurements of Sound Noise Pollution Hazards or Effects of Noise Pollution Traffic Noise and its Abatement Noise from Construction and Civil Engineering Works and its Abatement Noise from industry and its Abatement Domestic Insulation against noise Aircral\ Noise and Its Abatement Control of Noise Pollution

20. 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8 20.9 20.10 20.11 20.12 20.13 20.14 20.15 20.16 20.17 20.18 20.19

Radiation and Radioactive Pollution Introduction Radiation Sources in the Environment Most Dangerous Radioactive Pollutants Radiation Units Harmful Effects of Radiation Radiation Exposure and Radiation Damage Plutonium Tritium Radiation Safety Standards Radioactive Contamination : The Effect of Radioactive Rays on Living Creatures Enrichment of Radioactive Substances Pathways of Radioactive Materials into Human Systems Radioactivity in Drinking Water Radionuclides Removal from Water by Lime-Soda Ash Softening Radioactive Wastes Atomic Waste Disposal Problems Storage of Radioactive Wastes Nuclear Reactor Accidents Nuclear Winter

21 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.10 21.11 21.12 21.13 21.14 21.15 21.16 21.17

Energy and Environment Introduction Non-Conventional and Inexhaustible Energy Resources Magnetohydrodynamic Generators Geothermal Energy Wind-Driven Power Stations Tidal Power Plants Energy from the Sea Glacier Power Plants Use of Solar Energy for Electricity Production Solar Energy for Heating and Cooling Special Problems Concerning Nuclear Energy : Natural Radioactivity Nuclear Fission and Radioactivity Nuclear Power Plant Construction Nuclear Energy and the Environment Safety of Nuclear Power Plants Nuclear Fusion Biogas or Gobar Gas

351 351 352 353 355 357 358 358 359 363

363 364 367 368 368 371 372 373 374 375 377 378 37& 379 379 . 381 382 386 386 388

388 388 389 391 393 395 396 397 398 399 40() 402 403 406 407 409 410

22.

Instrumental Techniques in Environmental Analysis

413

22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8

Introduction Neutron Activation Analysis Titrations Electrochemical Methods X-ray Analytical MClhods Mass Spectrometry Absorption Spectroscopy Chromatographic Techniques

413 413 414 416 417 419 422 428

23.

Sampling and Measurement of Air Pollutants

435

23.1 23.2 23.3 23.4 23.5 23.6

Introduction Air Pollutants Measured Sampling Analysis Techniques Analysis of Particulate Matter Analysis for Lead

435 436 436 440 449 451

24.

Sampling and Measurement of Water Pollutants or Monitoring of Water Quality

453

24.1 24.2 24.3 24.4 24.5 24.6 24.7 24.8 24.9 24.10 24.11 24.12 24.13 24.14 24.15 24.16 24.17 24.18 24.19 24.20

Introduction Chemical and Physical Properties Commonly Measured in Water Samples Aquatic Sediments Soils Pre-treatment Techniques Sample Preservation Determination of Organic Loadings Determination of Phosphates Determination of Nitrogen Compounds Determination of Toxic Metal Ions Water Analysis by Automated Procedures Mass Spectrometry High-Speed Liquid Chromatography X-Ray Fluorescence Gas Chromatography Water Test Kits Colorimetric Methods Tltrimetric Methods Neutron Activation Analysis Monitoring Techniques and Methodology of Common Parameters of Water Quality and Pollutants

453 453 454 455 455 458 458 461 462 463 468 468 469 469 470 470 471 473 475 477

25.

Marine and Coastal Pollution

495

25.1 25.2 25.3 25.4 25.5

Introduction The Coastal Pollution Natural Pollutants causing Marine and Coastal Pollution Man Made Pollutants causing Marine and Coastal Pollution Marine and Coastal Pollution Control

495 496 496 500 503

26.

Case History: Three Milg island and the China Syndrome

505

Introduction and Important Concepts in Environmental Chemistry 1.1 Gene.-al Remarks Environmental Chemistry is now a fully developed viable and exciting branch of science. It is the application of chemical principles to the maintenance and enhancement of environmental quality, thc biggest challenge facing mankind today. Scientist, professional and individuals, need to have some knowledge of environmental chemistry if they want to make a meaningful contribution to the improvement of environment. For this chemistry has a special role to playas it helps in understanding the environmental changes logically and scientifically. It was environmental calamities in past five decades that generated awareness of environmental pollution. Mercury poisoning from contaminated se;l food (Japan), recurrance of the killer smog (New York, London), massive oil spill from the super tanker (English Channel), Bhopal gas tragedy (India) are some of the examples. All thesc calamities are the result of man's activity and increasing industrialisation. Balance between the progress of science and technology and environmental damage has tilted towards later. The effect of this imbalance has been a global diaster. Scientific interest in the formation of photochemical oxidants, acid rain, and fogs and the persistence and fates of toxic chemica ls is manifest in industry as well as the common man throughout the world. These environmcntal processes are closely interrelated and involves complex chemical reactions and physical changes. These drive the chemistry not only of these anthropogenic systems but also of biogenic and geogenic emissions in nature . Thus, 's tudy of these diverse but interrelated reactions need some knowledge of ecology, biology, anthropology, geology, physics etc. along with chemistry.

1.2 What is envit'onmental science ? To understand environmental chemistry it is important to know what is environmental science? What is the meaning of environment? Environment Science in its broader sense is the science of complex interactions t!tat occur among the atmospheric, aquatic, biotic, terrestrial and anthropological environments. Scientifically, environmental science may be defined as the study of earth, water, air and liv :ng environments and the effects of technology upon them. Environmental science has in nlct evo lved flT"ll ecology, which is the study of environmental factors that affect organisms and how organi sms intereact with these factors and with each other ? 1.2.1 Meaning of Environment The term environment refer to a definable place where 3n organism lives, including both the physical and biologic features of the place. The word environment comes from the French verb environ ncr which means to surround, surroundings or something that surrounds. It include all conditions and circumstances

Environmental Chemistry

2

which influences surroundings and affecting the organism. Environment means to all those which are physical and chemical (organic and inorganic) orthe atmosphere, lithosphere and hydrosphere. It is the aggregate of external conditions that influence the life of an individual or pollution, specifically the life of human being. It ultimately detennines the quality and survival of life. Organisms and environment are in constant change. Some changes are very rapid other are slow and may take thousands of years. The interrelation between the physical environment (soil, water, air) and organismal environment (plant and animal life) constitute the study of ecology. The term micro-environment designate a functionat environment, i. e., specific ~nvironment of specific organisms. In simple words, this tenn refers to a smaIt environment and therefore it is used for the immediate environment of an organism.

1.3 Environmental Chemistry Environmental Chemistry m(lY be define(/ (IS the stlldy of chemic(ll phe/~omenon ill the ellvironment. Or

Environmental Chemistry is the Stll(1y of tile sources, reactions, transport, effects ami fates of chemical species ill hydrosphere, lithosphere, atmosphere ami biosphere. It is a multi-disciplinary science which includes many vastly different fields such as chemistry, physics, life sciences, public health, engineering, agriculture etc. Only in recent years many chemists have become deeply involved with investigations of environmental pollution. It is important for all chemists to know environmental chemistry so that no or least damage is done to the environment through chemical and related industries. Environmental chemistry helps in identifying and determination of specific pollutant . present in environment. The branch of environmental chemistry which concern with life itself can bereferrecias.£nvironmenlal biochemistry. In other words, it deals with the effects of environmental chemicals species on life. Another closely related branch is toxicological chemistry. It is the chemistry of toxic substances interaction with living organisms.

1.4 Ecology and Environment Ecology and ecosystems considers the life habitat of over two million different kinds of animals and plants and it takes into account all manner of influences and interactions among them. An ecosystem is any spatial or. organizational unit which includes living organisms and non-living substances interacting to produce an exchange of materials between the living and non-living parts. It includes populations, communities, habitats and environments, and inte~-relate the dynamic interaction of all parts of the environment focussing specially on the exchange of materials between living and non-living parts. The environment in which a particular organism lives is called its habitat. The role of an organism is called its niche. For the study of ecology one can divide environment into four categories, (i) terrestrial environment, consisting of land and biomes, such as grassland. (ii) fresh waler environment consisting of standing water habitats (lakes, reservoirs) and running water habitats (streams, rivers). (iii) Marine environment consisting of shalf.ow waters of the continental shelf and deeper water of the ocean. (iv) symbiotic environment consisting of two or more kinds of organisms exist together for their mutual benefit. Ecosystem comprises offour basic cpmponents Figure 1.1 namely (i) abiotic substances, (ii) producer organisms, (iii) consumer organisms and (iv) decomposer organisms. (i) Abiotic substances: Elements which are normally very active in biological processes, like oxygen, may be in an abiotic form readily available for living organisms like free 02 or CO 2, or they may be in an inaccessible forn1 like silicon dioxide (SiO,) in quartz, a major component of granite. The word abiotic means "without life" or ")l?n-living" e.g., HP, 2, N 2, CO 2 etc. are abioti£ when they are physically outside living organisms, but once withiri living organisms they become part of the Btoti£ world. One of the most important quality of an ecosystem is the rate of release of nutrients from solids, which regulates the rate of function of the entire system. .

°

3

Introduction and Important Concepts in Environmental Chemistry

(ii) Producer Organisms: Producer organisms are bacteria and plants which synthesize organic compounds. They are called autotrophic or self productive, in which they take inorganic compounds and prepare organic compounds and living protoplasm from them. All green plants are producer organisms including microscopic organism since they exhibit photosynthesis, and some bacteria are producers, as they may show chemosynthesis or photosynthesis. (iii) Consumer Organisms: Consumer organisms are animals that utilize the organic material directly or indirectly manufactured by plants. Consumers are unable to produce their own organic compounds for basic nutritive purposes. They are called as heterotrophic, that means different or varied in nutritional source. Primary consumers or herbivores directly consume the organic compounds of plants. Secondary consumers may be omnivores or carnivores which depend partially or entirely on other animals for food. Tertiary or quartenary consumers may be the second, or third stage predator, e.g., a hawk feeding on a weese that in turn consumes a mouse. (iv) Decomposer Organisms: Decomposer organisms are bacteria and fungi that degrade organic compounds. Their nutrition is known as saprophytic, which is associated with rotten and decaying organic material. In a way, they are the digestive organisms of an ecosystems, they reduce the complex organic molecules of dead plants ana animals to simpler organic compounds that can be absorbed by green plants as vital nutrients. They provide the final but essential link in the cycle of life. These are essential for the renewal of life, for it decomposers were not active, organic compounds would become locked into complex inorganic molecules which could not be utilized as nutrients by plants. PRIMARY PRODUCERS ... CONSUMERS r - - - - - - - - - l..~ Green Plants and I------------I~~I Herbivores

Bacteria

Plant Parasites

ABIOTIC SUBSTANCES Basic Plant Constituents

, DECOMPOSER Saprophytic Bacteria 1... L...._ _ _ _ _ _ _ I and Fungi

-------1

SECONDARY CONSUMERS Omnivores Carnivores Animal parasites Scavengers

Fig. 1.1: BasIc Components of an Ecosystem

1.5 Food Chain of Life Organisms that engage in the capture of electromagnetic energy by the process of photosynthesis are called producers. It also included chlorophyll bearing organisms of the seas such as microscopic algae. Freshwater algae and shallow, fixed plants are also important in the food chain (Fig. 1.2). But it is on land that photosynthetic plants are overwhelmingly productive. Here the grasses, herbs, shrubs and trees, which are the food-producing vascular plants, create the substances for hundreds of thousands of microorganisms, parasitic plants, and herbivorous animals as well as, ultimately, even the carnivores. Animals eating plants directly, the herbivores, called primary consumers. Thus plant feeding creatures such as insects, squirrels, rabbit, birds and cattle may be fed upon by predators such as cats, foxes, hawks, spiders, frogs, insects, worms, or man. The food chain of life becomes so complex that it becomes difficult to say who is eating whom. The scramble for survival through nourishment has been appropriately described as food web. There is another step in the web of nfe occupied by the decomposers, the organisms of decay and decomposition which convert the producers and consumers into products, some of which are ultimately re-usable by the producers.

Environmental Chemistry

4

IpHYTOPLANKTONI~--------~____

T -_ _ _ _

~ CARNIVORE 2

I· ZOOPLANKTON Fig. 1.2

1.6 Biome The climate of an area which includes temperature, rainfall, humidity, sunlight and atmospheric pressure are impOliant in determining the nature of plants and animals. Different climate conditions leads to various communities of plants and animals. The same type of plants and animals appear together in a similiar climatic conditions. The natural ecological grouping of plants and animals on the basis of climate are called biomes. This means all the ecosystems taken together in a given geographical area having the same type of climate is called biomes. A biomes has relatively homogeneously distributed flora and fauna (same type of plants and animals). Two distinct type of biomes are largely encountered on earth. They are (i) land bionles (terrestrial biomes), and (ii) water biomes (aquatic biomes). The first type include desert, grassland, tundra, tropical forest, temperature forest etc. and second type include fresh water or marine biomes. Sometimes biomes are also termed as major ecosystem of the world.

1.7 Man and his Environment The environment of man includes everything, external or internal. Physical, chemical, biological and cultural condition, and their ramifications, collectively make "the environment". Since the onset of industrial revolution, there have been important changes in the biosphere. These changes have become accelerated in past few years so that many forms of life have become threatened with extinction, and the health of man himself in danger. Every living species plant or animal is being influenced by the environment and in turn environment gets influenced by them. There is a natural check and thus the balance is being maintained between them. However, man is an exception. Man is the main culprit for polluting and creating imbalance in physical environment viz., soil, water and air. A large number of activities of man due to advancement in science and technology creating an imbalance in ecological situation. The role of chemical is one of the facets of environment that has a special influence, for the living process itself in the sense of chemistry. Although extraneous chemicals in nature have always been important to life for good or bad. They have brought about profound changes in the biosphere. The exposure of man and other living orgallisms to chemical substances such as pesticides, detergents, plastics, solvents. fuels, paints, dyes, medicines (chemical products made and disseminated for the benefit of man) may cause danger to environment by disturbing the natural balance. Some of the chemicals used are injurious when improperly used or discarded. Specially, artificially produced chemical such as plastics which are non-bio-degradable, often exceeds the limits of safety to man and other forms of life. Another natural component of the biosphere is radioactivity. It has increased as a result of man's manipulation of the atom. The effect on living organisms, including human health and genetic welfare are not fully predictable i~l the light of present knowledge. The toxicity of radiation is much more than previously thought, therefore, limits of allowable exposure are more stringent. Some of the changes in 20th century environment are due to the accelerating pace in the usage of energy and other natural resources. The use of irreplaceable resources threatens to bring about drastic changes in both the world's economy and the quality of life. Another problem is due to waste product disposal, as no satisfactory system has been found. The solution of these and related problems are the most difficult and challenging of human undertakings for like other organisms, man must live within the ecological limits of the biosphere. Some times industrial accidents take place and then there is a sudden build-up of pollution in the environment. This type of environmental pollution is more dangerous as it does not give sufficient time to the living beings to escape fi'om its effects. Examples include accidental leakage of crude petroleum fi'om

Introduction and Important Concepts in Environmental Chemistry

5

Torrey Canyon into water of English Channel polluting sea. The oil slick (thin film of oil on surface of water) resulted in disturbing marine ecology. The Bhopal Gas Tragedy in which thousands of people were affected badly and many killed due to sudden leakage of a toxic gas from a pesticide plant. 1.7.1 The Bhopal GIiS Disaster One of the most talked about industrial disaster is the accidental leakage of methyl iso-cyanate (MIC) and known as Bhopal Gas Diaster. The diaster is unprecedented in many senses. The magnitude of the ... leakage was over 40 tons from Union Carbide Corporation's pesticide factory and the devastation caused by it has made it one of the greatest industrial diasters of the century. Never before this diaster in history so many people died in one go due to exposure to industrial chemicals except in Hitler's gas chambers. Never before in history ha\le so many people been mained by a man-made diaster except in Nagasaki and Hirsohima. Apart from its li'lItgnltudg! the Bhopl1l diaster is without precedent in several other ways. It was because of the unknown nature of offending chemicals, the problem of medical treatment of the gas victims were of the kind never tackled before. Considering the enonnitf of the health damage caused due to the Bhopal diaster the scientific information available on the health status of the victims of the diaster is far from adequate. Lack of scientific information has led to an 1Iliderestitnation of damage caused due to the accidental leakage of the gas. It has also impeded the emergence of a proper line of medical treatment for the gas victims. Alongwith the estimation of health damage caused by Bhopal diaster one also consider the health care cost that would need to be borne to deal with the iarge scale and serious damage. It is understood that such health care will have to be provided to the gas-affected JjOpullltiol1 atleast for next 20 years. According to various studies on health effects of toxic gas exposute! the total estimated exposed population in 1984 was nearly 5,20,000. Of this population nearly 32,000 were In severely exposed area, 71,000 in moderately exposed area and 4,17,000 in mildly exposed area. The division of the entire gas affected area into mild, moderate and severe areas has been done on the basis of exposure-related mortality rates in the immediate aftermath of the diaster. According to the studies; the aborption tate was 7.6% in 1990 in the affected areas while in control areas the rate was 3%. The abortion rate is decreasing in the gas-affected areas gradually. Various studies have established the severity of effect on lungs due to exposure to toxic gas in Bhopal. Nearly 98% of gas-exposed population was found to be hlivlng exertional dyspnoea. Respiratory infections (73%), chest pain (42%), joint pains and easy fatigueability were the other common symptoms. Nearly 24% of the gas-affected population suffered from Reactive Airway Dysfunction Syndrome (RADS) in which the patient has paroxysmal attack of breathlessness following toxic gas inhalation. It has been found that anxiety neurosis and neurotic depression were the most common psychiatric problems among the gas-affected people. The growth of the children born to gas exposed mothers is not nornlal. It was found that the failure to grow rate is significant after 18 months. Children born to gas-exposed woman also exhibit a significantly higher delay in gross motor (control of voluntary body movements) and language sector development.

1.7.2 Minamata Diaster A serious case of water pollution occurred in Japan's Minamata Bay in 1953. Minamata is a small village on the south coast of Kyushu. The people of this village showed symptoms of mercury poisoning due to consumption of mercury contaminated fishes and sea food. This resulted in death of 45 people and 111 people faced chronic brain cell damage. Water pollution due to Hg was resulted from the disposal of industrial waste without treatment into the bay. The waste was contaminated with Hg which was used as catalyst (HgCl,) for manufacturing poly vinyl chloride (PVC). Although the amount of Hg was too low to cause any danger but fishes and bacteria accumulated mercury as methyl mercury ion. The consumption of these fishes resulted in diaster. The methyl mercury also resulted in genetic defects in children whose mother had eaten these fishes from bay in Japan. The symptoms of min am at a included sensory loss in limbs, impaired vision, hearing loss, numbress, dysphasia, chest pain, muscular tremors and convulsions leading to death.

Environmental ChemistlY

6

1.7.3 Chernobyl Diaster World's worst nuclear (radiation) pollution diaster took place in Chernobyl (USSR) on April 25, 1986. This happened due to the poor reactor designing and operator negligence. The operators disconnected the emergency core cooling system, thus, neutrotis went out of control and steam pressure built up in pipes. The ensuing explosion sent the graphite slabs of the reactor core through the root: setting it afire and spewing radioactive material. It resulted in clouds of radioactive smoke over a large area even upto 2000 km away from Chernobyl. Radiation level reached nearly 100 times than the normal. These radiations _ were harmful to flora and fauna both. The chronic health effects include skin cancer, thyroid changes, blood abnormalities, fibrosis and ulceration. The water around Chernoby was declared unsuitable-for drinking. The radiations can damage cuts in agricultural output for years to come.

1.8 Environmental Pollution Present day environmental pollution problem is the most horrible ecological crisis since the civilization on earth. Just before last century, environment was pure undisturbed and hospitable for man to live. Environmental pollution may be defined as "the unfavourable alteration of surrounding environment, wholly or largely due to by-products of man's activity, through direct or indirect effects of changes in energy patterns, radiation levels, chemical and physical constitution or abundances of organisms. These changes may affect man directly or through water supplies, agricultural or biological products, physical objects or possessions or opportunities for recreation and appreciation of nature. 1.8.1 Types of Pollution Pollution normally classified according to environment segments in which it is occurring. Sometimes it is also classified according to type of pollutant by which pollution is caused. A general classification may be natural (originates from natural processes) and artiticial (originates due to man's activities) pollution. Based upon these classifications different types of pollution those need immediate attention are : I. Air Pollution 2. Water Pollution 3. Soil Pollution 4. Marine Pollution (part of water pollution). 5. Noise Pollution 6. Radiation Pollution (Radioactivity) 7. Thermal Pollution 8. Solid-Waste Pollution (part of soil pollution). Most of the pollution that concern man occur in nature except pollution caused due to chlorinated hydrocarbons such as DDT and few short-lived radioactive isotopes. In some cases, the environmental pollution have been largely due to natural sources called natural pOI/lit ion, while in others the pollution have been largely produced by man's activities called man-generated pOI/lit ion. Natural pollution have been more impOitant on a global scale, however, man-generated pollution is usually localized and may be more important in urban and industrial areas. 1.8.2 Pollutants A pollutant may be defined as anything living and non-living or any physical agent (such as noise, heat) that in excess makes any segment or part of environment undesirable. Undesirable means; when water is no longer can be used for drinking, recreation or as a habitat for aquatic life; air is no longer can be used for breathing, for the condition of buildings exposed to it or animal and plant life: soil is no longer can be used for raising food and fibre. In common usage, "pollutant" is a term used for non-living, manmade substances or other nuisances which are present in excess (beyond a certain limit) in a particular location. Oxides of nitrogen and sulphur, carbon monoxide, smog and particulates all are examples of air pollutants, however all are produced naturally. Generally, those made by nature get widely dispersed are at low concentration and therefore are not the major threats to life. The excesses produced due to human activities when crosses a certain limit becomes a threat and thus called pollutants. According to "The Indian Environment (Protection) Act", a pollutant has been defined as any solid, liquid or gaseous substances present in such concentration as may be or tend to be injurious to environment. In addition to this, the unserviceable or residues of things we manufacture, use and throw are also regarded as pollutants. 1.8.3 Types of Pollutants The pollutants can be classified according to their physical and chemical properties. They are

Introduction and Important Concepts in Environmental Chemistry

7

as follows: (I) Gaseous Pollutants: Oxides of nitrogen (NO, N02, etc.), S02' H2S, CO, C1 2, Br2 etc. (il) Fluoride Compounds: CCll2, CClF J , CF 4 etc. (iii) Metals: Hg, Pb, Fe, Zn, Ni, Sn, Cd, etc. (iv) Complex Organic Pollutants: Benzene, benzpyrene, ether, etc. (l') Photochemical oxidants: Ozone, PAN (peroxyacetylnitrate), PBN (peroxybenzoylnitrate), aldehyde, ethylene, NOx (oxides of nitrogen), etc. (vi) Deposited Matter: Soot, smoke, dust; tar, grit, etc. (vii) Solid waste: Plastics, metal alloys; etc. (viii) Economic Poisons: Herbicides, fungicides, pesticides, namatocides, insecticides, ~odenticide, and other biocides. (ix) Fertilizers. (x) Radioactive waste. (xi) Noise (xii) Heat From the ecosystem view point, the pollutants can be classified as (i) nondegradable pollutants (ii) biodegradable pollutants. Many materials such as aluminium cans, mercuric salts, long chain phenolic compounds, plastiCS, high molecular weight artificial polymers; DDT etc., either they do not degrade (converting to smaller molecules or returnihg to nature) or degrade only very slowly in the natural environment, are termed as non-degradable pol/utants. Such nondegradable pollutants are serious threats to environment as they not only accumulate but often "biologically magnified" as they move to biogeochemical cycles and along food chains. Many times they react with other compounds in environment to produce toxins. Many materials and waste products such as domestic sewage can be rapidly decomposed by natural processes or in engineered systems (like a municipal sewage treatment plant) they are termed as biodegradable pol/utants. These pollutants or materials enhance nature's great capacity to decompose and recycle. Biodegradable pollutants possess a problem when their input into one environment part get exceeded the decomposition or dispersal capacity. 1.8.4 Contaminant Vs Pollutant A contaminant may be defined as something which causes a deviation from the normal composition of an environment. A contaminant not necessarily be a pollutant as it may not be having any adverse effect on environment. Contaminant can also be classified as pollutant if it show some detrimental effect, e.g., accidental leakage of methylisocyanate caused detrimental effect, ahd this gas is not otherwise found in atmosphere, therefore it can be regarded as contaminant as well as pollutant. In simple words contaminant does not occur naturally in the segment of environment rather it gets introduced by some human activity, affecting the composition. 1.8.5 Source, Receptor and Sink of Pollutant Source is the place from where pollutant originates. The identification of source is important since it can help in elimination of pollution. After a pollutant gets released from a source, it may act upon a receptor. This means receptor is anything which is affected by the pollutant. Man is the receptor for gaseous pollutants such as smog. Plant or vegetation is the receptor for oxides of sulphur as they die on excess exposure. Man is also receptor of photochemical smog causing irritation of the eyes and respiratory tract. Sink is the medium which is able to retain or interact with a long lived pollutant, though not necessarily indefinitely. Thus a limestone wall may be the sink for atmosphere sulphuric acid. The reaction can be written as H2S04 + CaCO J ~ CaS04 + H20 + CO 2 The above reaction fixes sulphate as part of the wall composition. Similarly oceans can be regarded as sinks for atmosphere carbon dioxide which is converted into carbonates or bicarbonates.

Environmental Chemistry

8

1.8.6. Pathway of a Pollutants The pathway which is being followed by pollutants from its source to environmental segments. It include all stages and mechanism opted by pollutants while dispersing, e.g., the tetraethyl lead present in gasoline for automobile gets dispersed in nature by a specific route called the pathway for pollutant. It is shown in Figure 1.3. Autoexhaust) PbCI 2 + PbBr2 (Released in air) Pb(C 2H S)4

I

(in gasoline)

.

Precipitation

To Food Crop and enters food chain

(

Absorbed by plants

PbCl z + PbBr2 (Soil)

Fig. 1.3: Pathway of Pb

In most of the cases the element which is responsible for pollution or toxic effect present in different chemical forms or species of inorganic or organometallic compound in environment. These different chemical species are referred as speciation. It becomes essential to identify the chemical species of a pollutant because some species of a pollutant have been found to be toxic than others, e.g., CH 3 Hg+ and (CH 3)2 Hg have been found to be more poisonous as compared to other species of mercury. 1.8.7 Synergism and Antagonism Environment in most of the cases has more than one pollutant, it is therefore expected that effect of the pollutants will also be combined. In many cases the combined effects of two or more pollutant are more severe or even qualitatively different from the individual effect of the separate pollutants-this is known as synergism. Various studies have indicated that toxicity of S02 increases many fold in presence of aerosols of soluble salts of ferrous, manganese, vanadium and chromium. Such a increase in toxicity is usually referred as potentiation. In other cases, the combined effect of pollutants may be less severe than the individual effects, this decrease in toxicity or bad effect is referred as antagoism. Cyanide in industrial wastes are quite poisonous to aquatic life, and in the presence of zinc or cadmium they are extremely poisonous (a synergistic effect), apparently due to the formation of complexes, [Zn(CN)i-, [Cd(CN)i-. However, in presence of nickel, a less toxic nickel cyanide complex [Ni(CN)J 2• is formed. The occurrence of synergistic effect makes it difficult to study the effect of individual pollutant and this makes it hard to predict the effect that might take place when certain air or water quality standard are met. The pollutant also has a threshold limit value (TLV). It refers to the permissible level of a toxic pollutant in atmosphere to which a healthy industrial worker gets exposed during eight-hour day without getting any adverse effect. For example TLV values for Bi and Zn have been 0.002 and 1.0 mg/m 3 respectively.

1.9 Units of Concentration Concentration of pollutants have been usually expressed by fractions. A concentration of one part per million (1 ppm) means one part pollutant per one million parts of the gas, liquid, or solid mixture in which the pollutant is found. In case of a gas mixture, the reference is generally to ppm by volume, while in the case of liquid and solid the reference has been generally to ppm by weight. Alternatively, because number of molecules (or moles) are proportional to their volumes, there units may be thought of as the number of 6 9 volumes of pollutant found in 10 (ppm), I0 8 (pphm), I0 (ppb) and IOi\ppt) volumes of air. Here ppm, pphm, ppb and ppt are short forms of parts per million, parts per hundred million, parts per billion and parts per trillion respectively. e.g.. 0.04 ppm means 4 pphm or 40ppb or 40,000 ppt. However, if pollutant concentration is higher, percent (i. e. part per hundred) must be used e.g.. CO concentration is automobile exhaust is expressed in percentage (%) reflecting number of CO molecules (or volumes) per 100 molecules (or volumes) of exhaust. It is important to note that the number of molecules, or volumes, of a given gaseous species forms the basis of units in atmospheric chemistry whereas in water chemistry, mass rather than volume is used as the basis for expressing concentration in ppm.

9

Introduction and Important Concepts in Environmental Chemistry

Another type of concentration unit can be used for species such as free radicals (e.g. OH.) present at a sub-ppt level. It is the number of molecules, free radical or atoms present in a given volume of air usually cubic centimeter (c.c. or cm\ For converting units of ppm, pphm, ppb or ppt to units of number cm- 3 using the ideal gas law PV = n RT. Thus, the number of molecules per cm 3 in air at 1 atmosphere pressure and 298°K is given as

n

P

V

RT

1 atm ------,-----,--= 0.0409 I

moles L- I

0.82 L atm K-Imole- x298K

Converting to units of molecules cm- 3 , we get

~=

0.0409 moles L- I x 10-3 L cm- 3 x 6.023 x 10-" molecules mole- I 0

= 2.46

X

10 19 molecules cm- 3

As we know from the definition of ppm, it is the number of pollutant molecules per 106 molecules of air. Thi~ means 1 ppm corresponds to 2.46 x 10 19 x 10-6 = 2.46 x 10 13 molecules cm-3 at 298K and one atmosphere of total pressure. From this it follow that if the concentration of the OH radical in polluted air is 0.1 ppt, it can also be expressed as 2.46 x 10 19 x 10- 12 x 0.1 = 2.46 x 10 6 molecules cm- 3 • From this relationship it is clear that 10 6 parts per = 1 ppm = 2.46 x 10 J3 particles cm- 3 10 8 parts per

= 1 pphm = 2.46

x lOll particles cm- 3

109 parts per = 1 ppb = 2.46 x 10 10 particles cm-3 10 12 parts per = 1 ppt = 2.46 x 10 7 particles cm-3 Yet another unit of measurement for gaseous species is mass per unit volume, usually 10-6g per cubic meter (~lg m- 3). As one atmosphere ;t 298K contains 0.0409 or 4.09 x 10-2 moles L- I 1 ppm 4.09 x 10-2 x 10-6 moles L- I 4.09 x 10-8 moles L-I 4.09 x 10-5 moles m-3. Let us assume the molecular weight of the pollutant is M g per mole, then I ppm = 4.09 x 10-5 x M gm- 3 = 40.9 x M ~lgm-3

Example 1.1 For 03 concentration of 0.04 ppm, it is equivalent to (40.9 x 48) x 0.04 = 79 J..lgm- 3 and for SO, concentration of 0.04 ppm, it becomes equivalent to (40.9 x 64) x 0.04 = 105 ~lg m- 3 From this it is clear that ~lg m-3 = ppm x 40.9 x M = pphm x 0.409 x M = ppb x 0.0409 x M = ppt x 4.09 x 10-5 x M For atmospheric particulate matter, concentration are expressed in mass per unit volume, ~lg m-3 or in the number of particles per unit volume, e.g., cm- 3 •

1.10 Environmental Segments It is always useful to subdivide the environment to make the studies easier. In broader sense the environment can be divided into four major parts (Figure 1.4). They are: (1) atmosphere. (2) hydrosphere (3) lithosphere and (4) biosphere. 1. Atmosphere Atmosphere refers to the protective blanket of gases which are surrounding the earth. It is able to sllstain life on ealth and saves it from the hostile environment of outer space. It is being able to absorb most of the high energy cosmic rays from outer space and major portion of the electromagnetic radiation

Environmental Chemistry

10

from the sun. It allows entry of only harmless radiations near U.v., visible, near IR (300-2500 nm) and radiowaves (0.1 0-40 ~l) and filters out U.v. radiation below 300 nm which may damage tissues. The atmosphere can be further subdivided into different region with change in altitude. The atmosphere extending upto nearly 50 km above earth's surface is called lower atmosphere and the rest is called upper atmosphere which extends out into space. The atmosphere is important in maintaining the heat balance of earth by absorbing IR radiation. Atmosphere is mainly composed of nitrogen and oxygen gases, while the minor components include argon, carbon diox;de, and other noble gases. It is the source of most wanted 0, gas essential for life on earth and CO, essential for plant photosynthesis. [t is also a major source ofllitrogen which is present in all form of life. Nitrogen fixing bacteria fixes atmospheric nitrogen to ammonia which is then transferred to different parts of biosphere, lithosphere and hydrosphere. Atmosphere is also serves as a carrier of water from ocean to land in most wonderful geological phenomena called hydrologic cycle. 2. Hydrosphere Hydrosphere refers to forms of water present in the earth system. It includes all type of water resources such as oceans, rivers, lakes, streams, glaciers, ground water, polar ice caps and other reservoirs. Most of water is 97% of earth's water lies in oceans, however, the high salt content does not allow this water to be used for human consumption. Nearly 2% of water resources gets locked in the polar ice caps and glaciers, while I % is found as fresh water. The major uses of water include irrigation. thermal power plants, domestic and industrial consumption. There are thousand of chemical species and millions of living organisms make hydrosphere as their shelter. Any change in hydrosphere, therefore, affect all including man. 3. Lithosphere Lithosphere refers to outer part of solid earth. In general, it refers to mineral encountered in the Earth's crust and to the complex and variable mixtures of minerals, organic matter, water and air making up soil. The most important part of lithosphere is outermost layer called soil, as most significant changes occurring in soil effect the environment most. 4. Biosphere Biosphere refers to a kingdom of living species and their interaction with atmosphere, hydrosphere and lithosphere. In simple words, it consists of living organisms and its surroundings in which it exist. There is a direct influence of biosphere and environment on each other. This means oxygen and carbon dioxide levels in the atmosphere are entirely based upon plant animal kingdom. This is because both of these consume and release these two gases. The word biosphere is derived as biological word intereact with environment and it has intimately related to energy flow in the environment. ATMOSPHERE

NITROGEN FIXATION DENITRIFICATION

LITHOSPHERE

Fig. 1.3: Environmental Segments

II

Introdll ction and Import alii Concepts in Environmental Chemisliy

QUESTIONS I. What is environmental chemistry ? What is its importance. 2. Deline following terms (a) Environment

(b) Ecology

(c) Habitat

(d) Terrestrial environment

(e) Symbiotic environment

(I) Ecosystem

(g) Niche

(i) Saprophytic

(j) Speciation

(k) TLV

3. Give a brief account of basic components of ecosystem. 4. Ditlerentiate between (a) Autotrophic and Heterotrophic

(b) Contaminant and Pollutant

5. Write short note on (b) Biome

(a) Food chain of life (c) Man and his environment

6. Name the gas evolved in Bhopal Gas Disaster. How it atlceted the life on people in Bhopal. 7. Which compound of mercury caused Minamata Diaster ? What were the ill effects of mercury poisoning? 8. What is environmental pollution? Name various types of environmental pollution . 9. What is a pollutant ? Name and classify various types of pollutants. ID. What do you mean by nondegradable and biodegradable pollutants? II . Deline source, receptor and sink and inter-relate these taking a suitable example. 12. Taking example of lead. explain the pathway of pollutant. 13 . DiHerentiate between synergism and antagonism. 14. Express (NO), concentration in

~gm-J

when it is D.S ppm in a sample of air.

15. How many particles of a gas pollutant per cc of (a) I ppm

(b) 5 ppt: contains at I atm and 298 K.

16. What are various environmental segments? Explain the importance of each segment.

Evolution of Earth and Bio-Geochemical Cycles 2.1 The Earth as a Planet Before going into the details of the environmental chemistry one must be familiar with evolution of earth and changes that have occurred on the earth. The origin of earth is controversial because of the various hypotheses are based upon incomplete knowledge of the solar system as it exist now. It is not possible to know the conditions that existed at the time of evolution of earth so our hypotheses mainly depends upon assumptions. It is indicated that about 4 .5 billion years ago there was a planet with a solid crust approximately 150 million kilometers from the sun. Since then major changes have occurred aLa slower rate to give present structure of earth having atmosphere, hydrosphere, lithosphere and biosphere. It has been argued that each planet in the solar system has unique features . The oxygen rich atmosphere and presence of life on Earth are important features of Earth. It was the presence of water which was more crucial to the development of life on Earth, however, the present composition of the atmosphere ensures that organisms those can make use of the oxygen in energy-releasing reactions at the Earth's surface are dominant. Earth is unique as a planet, since life exists here alone. What is life? The answer to this fundamental question is that, it is one set of instances of departure from a thermodynamic and chemical equilibrium. Were thermodynamic equilibrium to hold good without perturbation? Then life should not exist nor should oxygen rich atmosphere which sustain life. Then question is, what is perturbation and what is its influence? This can be understood in terms of inflowing energy in form of radiations within a narrow part of electromagnetic radiation between 300-800 nm. A set of delicate coincidences combines to make the perturbation effective. Why should electromagnetic radiations from sun between 300-800 nm are so signifi. cant? The answer lies in ~he quantum nature of light. According to Planck's law E = hv or Upon interling, and when,

E =

A = 300 nm A = 800 nm

~

( since v

=~ )

= 6.63 x 10- 19 J then, E = 2.48 x 10- 19 J

then,

E

It is here the first coincidence operates as the energy required to excite an electron within a molecule from one molecular orbital to another is of the order of 10- 19 to 10- 18 J. The second coincidence is the maximum intensity of sun's radiation in terms of photon output within the continuous spectrum lie around 600 nm and maximum output of energy basis occurs around 450 nm. Both of these wavelengths lie with

Evolution of t.arth and Bio-Chemical Cycles

13

in the perturbing region of electromagnetic spectrum. The consequence of thes!! two coincidences is that radiation energy can be stor!!d and converted into potential chemical energy for the use in other reactions. Moreover, the energy require to break a normal bond also of the same order so that from simpl!! molecules more complex molecules can be formed by chemical reactions and vice-versa. The quantum theory, however, does not allow the accumulation of energy to produce photo-excitation, it is a chance that evolved life as bulk of sun's energy output been of the quanta in the energy ranges 1021 20 to 10J, or 10- 16 to 1O- 17J, or alternatively had the energy required to excite electrons in molecules or break !'nerpical bonds been in the same range. Another coincidence is that Earth's orbit the sun at an average distance of approximately 1.49 x lO" m. The f, Cd

I L..

Tl

,

lithosphere ZnS, CdS (Sedimentary Rock)

i

, (

Biosphere Zn 2> in zinc containing enzymes Plant and Animal Biota

Hydrosphere Zn 2>, Cd 2•

Zn2>, Cd2•

t

Sedimentation Weathering Fig. 2.22: Bio-geochemical cycle of Zinc and Cadmium

J

I'

Evoilition of Earth and Bio-Chemical Cycles

39

then to biosphere. Sometimes unwanted in.:rease of a particular element in one particular segment can cause a problem of ecological balance and geochemical distribution; i.e., pollution. The increased quantities of cadmium which is mobilised, attributed to demand of cadmium itself and also to increased use of zinc and phosphate fertilizers. Cadmium is used in electroplating, in plastic stabilisers, in pigments, in solder, in nickel cadmium batteries etc. Zinc is used in galvisization process, in alloys, in paints, in dyes, in tyres etc. The movement of zinc also leads to dispersal of cadmium.

2.7 Summary and Comment on Bio-Geochemical Cycles Our atmosphere and its component gases take pal1 in a dynamic balance in the main, with exchange of species between air and sea and land. For example calcium carbonate is the result of the tixation of carbon dioxide from the atmosphere via solution in the sea, incorporation into the skeletons of marine organisms and finally consolidation of the deposits on the sea bed into rock. Another example of the continuous cycling between earth and atmosphere is the action of denitrifying bacteria and nitrifying symbionts which achieve a substantial balance. Interests lie in those parts of the atmospheric cycles where man's production or use of energy gives rise to possible anthropogenic perturbations of the natural cycles. One of the aim of environmental chemistry is to assess the significance of the anthropogenic terms, whether they are comparatively trivial and can therefore be ignored on a global scale, or whether the scale of anthropogenic injection into the natural cycle is large enough to give cause for concern. One area where anthropogenic perturbation could be becoming significant is the industrial fixation of nitrogen (mainly by the Haber synthesis of ammonia) in the production of m1ificial fertilizers. The concern is whether the rates of the natural denitrification processes can keep pace with the increasing total rate of fixation, which on the industrial side has been doubling every 6 years. One consequence which could follow is a decrease in the pH of the sea due to increased nitrate content, with release of carbon dioxide by perturbation of the carbonate equilibria and the inhibition of carbonate skeleton formation of marine organisms. Almost every mineral element, nutrients and gases have distinct cycles and cycle periods. The most important of these are those of oxygen, carbon and hydrogen as these three elements form COl' 02 and H20, the main compounds formed or used in photosynthesis, respiration and transpiration. These processes are interlinked and complementary so these three cycles are interlocked (Fig. 2.23). It is interesting to note that all the cycles are connected to each other directly or indirectly. In photosynthesis carbon dioxide and H20 are used releasing oxygen. When plant respire they use 0, and release CO, and H,O some of which is reused. Animals also uses 02 to produce energy from food, -releasing CO 2 and Hp. Thus 0, C and Hare interrelated in various physiological processes and cycles in the environment. The Ilatural transformation of dinitrogen into proteins provides organisms with molecules capable of undertaking a wide variety of functions, in tissue structure, enzymes, hormones, etc. The transformation also involves energy that can be utilised by the organisms. The biogeochemical cycling of nitrogen involves the breakdown of the proteins and the subsequent reversal of the reaction sequence to replace the dinitrogen originally used. Then, "Why should micro-organisms apparently use up energy converting the more stable compounds into less stable ones ?" If more stable species are produced in one half of the cycle, the continuation and completion of the cycle demands the subsequent production of less stable species, and, consequently, the energy released in one half of the cycle must be balanced by the absorption of energy in other half. The movement of the nitrogen involves (a) changes in oxidation state, (h) changes in energy, (c) reactions with other elements (particulat'ly oxygen, hydrogen and carbon). These factors should help us to understand, why the cycle keeps turning '! The major biological electron-releasing process is 20,000 mm) V2b , A. = 4,300 which "2 absorption of CO 2 is the most intense and absorbs (e) Bend Fig. 3.14: Vibration Modes of over the region 13,000-18,000 nm. This leaves an infra-red CO 2 molecule exit window between 8000-18000 nm. The vibrational modes of water have been depicted in Figure 3.15. The earth's mean temperature rose OAoC between 1880t 1940, and from 1945 it has dropped around 0.1 cc. The initial rise A. = 2,730 /~ has been correlated with an increase in the amount of CO, in H H atmosphere (Figure 3.16). · 0 f fiOSSI'1 fi.Ie I (a) Symmetric Stetching The sources of CO2 are main Iy combustlon and respirating animals, and both sources are emitting larger amount t CO 2 as population of world increasing. The plants fixes CO2 from /,,0. . . . . . "2 . A. = 6,300 atmosphere by photosynthesis, however, with increasing popula/' ........... tion the destruction of forest is also very fast, that becomes an~ other factor for increasing amount of CO2 in atmosphere as the (b) Bending utilization of CO 2 in decreasing over the years. This disturbs the 0-+ natural cycle of CO,. From fossil fuel alone, more than 2.5 x 10 13 tons of CO 2 is being emitted into the atmosphere each year. Not / ~H 1..= 2,660 all of the CO 2 emitted into atmosphere remains there; about half'; ""of it gets utilized by plant life or absorbed by water of oceans. .. Part of CO, dissolved in ocean may get precipitated or incorpo- (c) Asymmetnc stretchmg rated in marine organisms. Aquatic plants are playing an imporFig. 3.15: Vibrational modes of tant role in maintaining CO 2 equilibrium between the atmosphere Hp molecule and the surface layers of oceans. Part of CO 2 taken up by terrestrial plants gets deposited in dead vegetation and humus on the forest floor. Some of it, in the form_ of organic plant parts, has been eaten by herbivores and gets deposited on or in soil. However, much of CO 2 is left in the atmosphere. An increase in atmospheric CO, will influence the photosynthesis, and consequently plant growth by its direct fertilising effect, especialliin hot tropical environment and a longer growing season in temperate regions. The fertilising effect must be exploited by lIsing modified crop varieties and agriculture practices to compensate the disadvantages effect of temperature increase. The CO, emitted by volcanoes several billion years ago was atleast 40,000 times that still present in the air. On it global time scale, the known amount of CO 2 in limestone and fossil fuel suggest that residence time of CO 2 in atmosphere has been probably around 106 years.

°

°

y-

63

Atmosphere. Meterology and Green HOllse Effect

00 (1) II)

co

~ u c:

'ci.

0.5 0.4 0.3

E

..

320 §:

g 8 300 0 o

0.2

E ~ 0.1

1880

1900

1940

1920

1960

Year

Fig. 3.16: Changes in mean world temperature and CO 2 levels in the atlllosphere

Estimates of the rise in temperature with doubling CO? concentration to 600 pm range from 0.1 to 5.0°C with a mean around 2°e. As there is a logarithmic relationship between surface temperature and CO 2 concentration, it is likely that a temperature will be reached beyond which further increase of CO, levels will have little effect. Accordingly, a maximum increase of2.5°C for mean temperature has been suggested. Since 1945, the mean global temperature has dropped suggesting that either the greenhouse effect is not a satisfactory explanation or some other effect is beginning to dominate. One of such effect is the increase in amount of aerosols in atmosphere. Aerosols have size range from 0.1-5)lm, could reduce incoming radiations by another 5% through back scattering and any increase could lead to cooling of the atmosphere though in presence of enhanced greenhouse effect due to larger amounts of CO 2 ' Actually, if we compare the increase in atmospheric turbidity to that of CO? concentration, it is found that turbidity is increasing with the faster rate. This increase in turbidity which reduces the amount of radiations reaching earth is more important factor today. Other pollutants also have influence on change of mean temperature of earth.

3.13 Consequences of Greenhouse Effect The consequences of greenhouse effect causing heating of earth would recede of glaciers, disappearance of ice caps such as those found over Antarctic and Greenland and rise of ocean level. It has been estimated that if all the glaciers, ice caps etc. melt, there will be a rise of nearly 70 meters of water level in oceans. This clearly means that low dying coastal areas would get flooded which include cities such as Bangkok. Only a rise of I meter due to ocean warming would be able to flood low lying lands, in Bangladesh, Boston city, Bombay and West Bengal. Further, due to the much warmer tropical oceans because of increased CO, concentration, these are likely to be more hurricanes and cyclones. Warming of climate means early and faster melting of snow in mountains causing more floods during monsoon season. Another study show that if CO, content of atmosphere get doubled it may also disallow the cooling effect of particulate contaminates and as a consequence the earth's temperature may rise again. There are evidences to prove that glaciers in both hemispheres are receding due to increase in mean temperature as compared to last century. Although after 1940, global warming had subsided as far as mean temperature is concerned, however, the three warmest years in whole record being from 1980-1982. These have been some unexplained fluctuations within the long term trend. The predictions for global warming says that the temperature of earth could increase by 1.5 to 3.5°C by the year 2050. The trend in temperature which has taken place over the last century cannot be associated with the increase in concentration of CO? over same period. Average increase of 0.7°C between 1890 to 1940 but since that does it has decreased by 0.1 °c, although does not rub out the relationship between CO, concentration, however, hints at uncertainty associated with it. Increasing particulate concentrations due to man's activities increases percentage of sunlight reflected (albids) by scattering the incoming radiations, and cooling would be predicted as a consequence. Particles can also absorbs and re-emit radiations. The wavelengths absorbed is dependent upon nature of particle and its composition. In parallel to greenhouse effect argument, if this absorption predominated in i.r. region corresponding to em1h's emission, the result would be warming and cooling would ensure an absorption of sun's radiation. Particles, unlike CO, are having a relatively short life span in atmosphere (35 days). In these circumstances it is unlikely that there may occur any change in pmticulate concentration

64

Environmental Chemistry

because of man's activities and that there could not be any associated climate effect. This is not to say that tluctuations in particulate loading as, for example, take place in stratosphere following volcanic eruptions, do not have an influence on climate. The melting of ice caps has not been observed rill now, however, this may happen in next 50 years it is predicted. It is also true that some melting may take place, but total conversion of ice into liquid water seems unlikely. When the actual energy requirements are considered it is found that the energy given up when 300 lit of air is cooled 1°C would melt only one gram of ice if the ice is already at the melting temperature. The basic physics principle works here that whenever there is a evaporation due to warming cooling of surroundings takes place. As discussed above there are large number of variables involved in all of these predictions and net effect of atmospheric pollutant on the climate is not so easy to predict. They could prove to be very profound or they might cancel each other, but it is yet. to be seen.

QUESTIONS I. What is atmosphere? !-low it is useful in maintenance of life and other related activities

Oil

earth '?

2. Define (a)

Troposphere

(b) Stratosphere

(c)

.Ietstream

(d)

Thermosphere

3. !-low do you explain increase of temperature in stratosphere with altitude? 4. Why thermosphere is so hot? 5. Why temperature decreases with altitude in troposphere? 6. What was the nature of earth's atmosphere. when first primitive life molecuic was flln1led ? 7. How first living organism was formed? 8. How 02 got accumulated in atmosphere? 9. Give a brief account of major regions of atmosphere? 10. What is normal environmental lapse rate? II . What is exosphere ? 12. What is tropopause? What is its role? 13. What chemical changes takes place in troposphere? 14. Why temperature increases slightly in stratosphere? 15. What is stratosphere? What is its role? 16. Name important chemical species and present in stratosphere. What chemical changes take place involving each species present ? 17. How NO accounts for removal of 0 3 ? 18. After stratosphere. the temperature drops again. why? 19. Why thermosphere is also called ionosphere? 20. Why day temperature in thermosphere is very hot. whereas nights are very cold? 21. Why temperature falls in mesosphere with increasing altitude? 22. What is the etlect of altitude on atmospheric pressure? How much it changes for every 20 km increase in altitudc ?

-.'.

'P

If pressure at sea level is 1.0 atm. than calculate the pressure at 2 km altitude where temperature is 1Q0e.

24. What is chemosphere ? 25. Why many chcmical reactions at higher altitude occur at slow rate? 26. What is meterology ? How important is this? 27. What is temperature inversion? What is its efTect

Oil

localised pollution of air?

65

Atmosphere, Meterolog), and Green House E;ffect 28. What is adiabatic lapse rate? 29. What happens when adiabatic lapse rate of pollution gas is less than that of surrounding air? 30. How does temperature inversion takes place? 3 I. What is subsidence inversion? 32 . What is radiative inversion? 33 . What do you mean by Hadley cells? 34. How inversion temperature show their variation at different times of the day? 35. Give a brief account of frontal and advective inversion? 36. What type of inversion helps in dispersion of air coastal pollution? 37. What is the relationship between air temperature (altitude) and smoke behaviour? 38. Why fumigation is the most serious cause of localised low level air pollution in cities. 39. How winds can help in dispersal of air pollutants in urban areas? 40. Give various reasons for winds blowing? 41. How coriolis effect operates in northern hemisphere? 42. What type of atmospheric conditions increases pollutions in cities? 43 . What factor atlect the city climate

~

44 . What is the concentration of SO, pollutant in a city of 20 km length? Pollution emission rate is 150 mg/hr and wind velocity is 10 -km/hr and mixing height is 300 m.

x

lOb

45 . How photochemical smog formation is helped by inversion layer ') 46. What man's activities can cause change in climate? 47. How mean surface temperature of earth is maintained over a large period of time ? What would have happend if tht:re was no CO 2 in atmosphere? 48. " Ent:rgy transfer is crucial for earth's heat balance". Comment on the statement and describe various mechanisms of energy transfer in atmosphere. 49. Define (a) Solar constant

(b) Convection

50. Is green house effect a real pollution threat? What would happen. if CO, concentration in atmosphere goes beyond acceptable limit and decrease below optimum concentration? 51 . Why CO 2 and Hp are green house gases and not 02 and N2 ~ 52. What vibration modes of CO 2 and Hp are i.r. active? 53 . What are the main sources of CO 2 in atmospht:re ? How it is used again in atmosphere to maintain optimum level '? 54. What major consequences are expected if grcen house elfect incrt:ases the mean temperature of earth by 2°(, ?

55. How can one explain the decrease in mean temperature of earth. though CO 2 concentration is constantly incn:asing ? 56. How melting of ice can atlcct the global environment due to rise of mean tempt:rature ') Why total conversion of ice into liquid H20 seems unlikely in near future?

Chemical and Photochemical Reactions in the AtmosphereAtmospheric Chemistry 4.1 Introduction Earth's atmosphere contains many gases which are capable of undergoing chemical changes under the influence of various energy processes such as sunlight, lightning etc. The study of these chemical changes is known as atmospheric chemistry. The study of these chemical reactions is difficult because of (i) low concentration of reactants and products in atmosphere makes detection difficult, (ii) the high altitude conditions are very hard to get in the laboratory because of interfaces. Many chemical reactions need a third body which absorbs excess energy. They occur very slowly in upper atmosphere where there is a low concentration of third body. These reactions may occur fast in laboratory conditions when container walls absorbs energy or act as a catalyst. Container walls may also react chemically with reactive species under study.

i

Electromagnetic Radiations from Sun

Interchange of chemical species X, with particles

, Interchange of molecular species and particles between the atmosphere and the surface

Absorption of electromagnetic radiation by air molecule, X

I

"

Reactive excited species, X· produced

various: Products

I

Fig. 4.1: Atmospheric Chemistry Pathways

The effect of pollution is very severe on atmospheric chemistry. One of such example is fluorochlorocarbons which disturbs the ozone cycle. The extend of pollution determines how much will it effect on natural process occurring in unpolluted atmosphere. Atmospheric chemistry generally deals with (i) inorganic oxides such as CO, COl' NO l , SOl' (ii) Oxidants. 03 (iii) reductants such as CO. SOl'

67

Chemical and Photochemical Reactions in the Atmo!>phere-Atmospheric Chemistry

H2S (iv) organic compounds such as CH 4 is unpolluted atmosphere and higher alkanes, alkenes, aryl compound in polluted atmosphere. (v) photochemically active species such as NO, and formaldehyde. (vi) acids such as HN0 3, H2S04 (vii) bases such as NH3 and their salts such as NH 4 HS0 4 . (vii) atmospheric elemental gases such as 0, and N, (viii) electronically excited radicals such as OH· and NOi. (ix) solid particulate matter as site for surface reactions and (x) liquid droplets as bodies for aqueous phase reactions. The three main sources of energy providers for the atmospheric reactions are (i) radiant energy from sun, (ii) OH·, hydroxyl radical for day time reactions and (iii) NOj radical for night time atmospheric chemistry. The major atmospheric chemistry pathways can be outlined as shown in Fig. 4.1. 4.2 Nature of Light The study of photochemical reactions in atmosphere requires the knowledge of nature of light. Light is the energy emitted or absorbed in distinct quanta (or photon), the propagation of which is described by wave theory. The propagation of light waves involves both electric and magnetic forces, which gives rise to their common class name electromagnetic radiations. It is known than when sun's rays or white light passes through a prism, it is separated into its colours constituents called electromagnetic spectrum. In fact each constituent of white light is the characteristic of its energy which is related to the frequency or wavelength. The energy of single photon is given by he hE = hv =-= cv A

and for one mole

E = Nhv = N hc = Nhcv A. where E is energy, h is Planck's constant, v is frequency, c is the velocity of light, A is wavelength, v is wave number and N is Avogadro's number. Depending of the energy associated with the electromagnetic radiations, this can be classified into different regions. These are shown in Table 4.1. It is clear from the above equation of energy that energy of electromagnetic radiation can be described by wavelength (A.), wave number (IIA. or v) or frequency (v). Different units are employed to describe these values e.g. A. can be expressed in meters (m), centimeter (cm), nanometer (nm) or angstrong (A); frequency can be described in Hz or megahertz, wave number can be described in m- I or cm- I • It may be noted here than 1m = 100 cm; IA = 10-8 cm or 10- 10 m; 1nm = 10-9 m; 1Hz = 1 cycle/sec; 1MHz = 10 6 Hz. Table 4.1: The electromagnetic spectrum, with wavelengths').. and frequency v. Region

Cosmic rays Gamma rays X-rays Far-ultraviolet Ultraviolet Visible Infrared Far In ti'ared Microwave Radar Television Nuclear Magnetic Resonance Radio

A in

III 14

l'

in II:

1010- 11

10 22 19 10

10- 9

10 17 10" 10 15 10 14 10iJ

7

1010- 7 10-6 10-5 10-4 10-3 10-2

10 12

lO" 1010 8

10°

10

10

10 7

10

2

10"

Environmental Chemistry

68

From thes~ conversion factors it can be found that visible region lies between 400 nm (4000 A) to 800 nm (8000 A). UV region lies between 190nm (1900 A) to 400nm (4000A). Infrared region lies above 800nm. Knowing the wavelength of electromagnetic radiation, the energy can be found in terms of kJ/mole and vice versa e.g., when A = 200 nm the corresponding energy E will be hc E=hv=A To calculate energy in J/mole multiply it with Avogadro's number, so , hc 6.023(mole-')xl013x6.6xlO-34(JS)x3xIOM(m/s) E =N -

A

=----'-----'----"'"77---c-----'-----'--'200xlO-'l(m)

5

.. 6 x 10 J/mole .. 600 kJ/mole It is important to note have that energies of typical chemical bond are comparable with el~ergy of visible or U.V. radiations, therefore, absorption of photon of UV-visible radiation can cause bond dissociation and hence a chemical change can occur. The useful example here is of O-H bond in water. It has bond energy 464 kJ/mole, hence the electromagnetic radiation having energy corresponding to 464 kJ/mole energy can break O-H bond in H 20 molecule. The corresponding wavelength to this energy will be Nhv A=E

6.02 XJ023( mole -I) x 6.63 x 10-34 (JS) x 3.0 x I08( ms- I) 464::< 10 3 J l1!ole- 1

= 2.58

7

xl 0- m

= 258 nm

That means the photon which can cause 0-/:-1 of water molecule to break belongs to ultraviolet region. The high energy radiation from the sun is filtered but before reaching the earth's atmosphere, and in upper atmosphere UV radiations with A < 290 nm is involved in photochemical reac,tions. In troposphere radiations with A < 400 nm may, depending upon pollutants, be involved in photochemical change.

4.3 Photochemistry Photochemistry deals with chemical changes a1Jd related physical properties that are produced due to the interaction of electromagnetic radiation and m'atter. In the process, chemical species absorbs light to bring about reactions, which are known as photochemical reactions. These reactions do not take place in absence of light. Photochemical reactions are of vitaL, importance to living material and playa significant part in atmospheric pollution. The first and most import~nt step in photochemical process is the absorption of a photon or light quantum by atom, molecule, ion br radical X + hv ~ X* where X* stands for species X in its electronically excited state and hv is the quantum of light energy. Then excited species X* can take part in one of the type of primary photochemical processes, such as: 1. Photodissociation X* -~ A + B X* + A -~ M+N 2. Direct Reaction X* -~ X' + e3. Ionization X* -~ X + hv 4. Fluorescence 5. Phosphorescence 6. Collision Deactivation 7. Intermolecular Rearrangement 8. Photoisomerisation

X*

~

X* + M X* X* (cis)

X**

--:---c--;-;-:---c~)

~

J

X + hv' A + M + heat energy y metastable state

X (isomer of X) (trans)

Chemiluminescence

69

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemistry 9. Photodimerisation

2X* -~ X-X 10. Hydrogen Atom Abstraction X* + AH ~ XH + A II. Photosensitized Reactions X* + A ~ X + A* A* -~ 8 (when A itself is not directly photo excited) 12. Intramolecular Transfer X* -~ X** Except 4, 5 and 6 all other processes are accompanied with a definite chemical change. The reaction that occur following absorption of a photon are largely dependent upon the way excited species loses its excess energy. Photochemistry in atmosphere usually involves three different chemical species, they are (i) electronically excited molecules (ii) free radicals (atom molecule or ions with unpaired electron); they may some time be molecular fragments (iii) ionized atom or molecular fragments. Electronically excited molecules are produced when a stable molecule absorbs electromagnetic radiations from some part of UV-Vis or IR region of spectrum. Although molecules possess several excited states, but generally energy of electromagnetic radiation in the Triplet Excited State region of UV-Vis or IR is only sufficient to Ground State Singlet Excited State excite molecules only to several of the lowest Fig. 4.2: Dil1"en:nt Excited States energy levels. Electromagnetic radiations of lower energy than UV-Vis or IR region are not sufficient to excite molecules to bring about a chemical change and radiations with higher energy either ionise or dissociate the molecule formed. This means UVVis and IR radiations energy suits to give an chemically important electronically excited state. The nature of electronically excited state can be understood from the arrangement or distribution of electrons in a molecule. Nonnally molecule have even number of electrons which are occupying molecular orbitals with a maximum of two electrons with opposite spin. The absorption of electromagnetic radiation excite one of these electrons to a vacant orbital of higher energy see Fig. 4.2. In one case, excited electron retains a spin opposite to its former paired electron, however in another case, spin of excited electron may be reversed (called reversal of spin). The former excited state is called singlet excitep (IS) state a.J1d later as triplet excited state CP). When molecule is in any of these excited state it -becomes chemically reactive species.

i

t

i

4.4 Types of Primary Photochemical Pl"Ocesscs A variety of different types of primary photochemical processes can occur upon absorption of a·photon. These are being discussed one by one. 4.4.1 Photodissociation

The most important gas phase, tropospheric chemistry is photodissociation into reactive smaller ti·agments. The reactive intermediate produced are either atoms or free radicals such as OH, OH, and RO,. These types of species play important role in both troposphere and stratosphere chemistry. One of tile example of photodissociation is NO, + hv (290 < A < 430 nm) ~ NO + CP). The NO is in ground state, CP) is an excited triplet state of oxygen. The later is a reactive species and it immediately however, combines with 02 to form ozone. OCP) + 02 ~ 03 The above reaction takes place in the presence of third body, M, which takes away the excess energy. In another example of photodissociation in troposphere is production of spin allowed electronically excited fragments of 03 at wavelength 320 nm.

°

°

°

03 + hv ~ 0+°2

80th the 02 and are electronically excited. The chemically reactive species the hydroxyl radicals. 0+ Hp

-~

° reacts with Hp vapour producing another

20H

Environmental Chemistl)1

70

Dissociation of excited 02 molecule, the process responsible for the dominance of atomic oxygen in the upper atmosphere. Examples of other molecules dissociation in troposphere produces free radicals include HONO, HCHO and other aldehydes. In some higher molecules the photodissociation is also accompanied with intermolecular rearrangement, e.g.,

°

CH 3-CH 1-CH 2-~-H + hv --+ C)H7 + CHO C)H g + CO C 2H4 + CH)CHO

4.4.2. Direct Reaction When an electronically excited species directly reacts with another molecule to yield new species such reactions are termed as direct reaction, e.g.,

°

02 * + 0 3 --+ 202 + In the above reaction chemically reactive oxygen which is being produced can react in may different ways to produce various product. Although simplest of its reaction is it combines with another oxygen to produce 02' 0+0

4.4.3 Ionization In the ionization process the molecule loses electron and becomes a charged species. When the ionization takes by absorbing electromagnetic radiations energy (photon), this is termed as photoionization. The energy required for ionization is normally very high and it can only be achieved in ionosphere by high energy radiations, e.g., + eN 2* --+ N 2+

4.4.4 Chemiluminescence

,.'",

'

Luminescence consists of loss of energy from excited molecule by the emission of electromagnetic radiations. N0 2* --+ N0 2 + hv When the re-emission of light is instantaneous, luminenscence is called .fluorescence and when it is delayed, it is known as phosphorescence. In case the excited species which shows luminescence IS being fOll11ed in a chemical process, it is called chemiluminescence, e.g., 0) + NO --+ N0 2* + 02 N0 2* --+ N0 2 + hv (chemiluminescence)

4.4.5 Collision Deactivation

Many times electronically excited molecule loses its excess energy by simple collision in form of heat energy. This type of collision deactivation of excited molecule do not bring about any chemical change. Many of chain reactions are terminated by this process. 02 + M --+ 02 + M + heat energy

4.4.6 Intermolecular Rearrangement In many cases, the absorption of photon can cause intermolecular rearrangement, e.g., photolysis of O-nitrobenzaldehyde in vapour solution, or solid phase rearranges to o-nitrosobenzoic acid. This process may also termed as spontaneous isomerization. The reaction is exploited in chemical actinometerms to measure exposure to electromagnetic radiations. 0, H 0, OH

hOC/'NO,

V

6-

NO

+hv

(o-nitrobenzaldehyde)

(o-nitrosobenzoic acid)

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemistry

71

4.4.7 Photoisomerization Cis-trans isomerization reactions are common in photochemistry. The light photon is being absorbed by organic molecule, resulted in an excited state where the x-bond is momentarily broken. This allows a free rotation about the bond axis and cis-trans isomerization takes place. The intermediate can be said to be a biradical.

One of the photoinduced isomerisation reaction is when gaseous trans-methyl propenyl ketone convert to cis-form.

° =C.....H CH3~"""'C H.....

°

II CH3-C ..... C = C.....H H..... .....CH3

+ bv

.....CH3

(trans) (cis) Gaseous azo compounds in the trans form photo isomerize to cis-isomer.

R

'N=N

'R

+hv

~

(trans)

,

N=N

\

(cis) Here R can be alkyl or aryl. Gaseous trans-crotonaldehyde isomerises as well as rearranges to five ethyl ketene and enolcrotonaldehyde.

CH3,

/H "'C=C"""

H/

+hv

~

"-CHO

4.4.8 Photodimerisation and Cycloaddition reactions Many ene (double bond containing/compounds dimerises to give cyclic structures by absorbing a photon of light in absence of 02' The process is called cycloaddition.

These reactions are important when 02 is present as photooxidation becomes an competent reaction to photodimerization. Example include the photodimerisatioll of anthracene

+hv

)

4.4.9 Hydrogen Atom Abstraction Electronically excited (n - x*) carbonyl compounds may undergo an intramolecular H-atom abstraction, especially when the hydrogen atom in y-position. A six membered rihg is formed in the transition state. After H-atom (abstraction the molecule may break into two smaller components called Norish TypeII reactions.

Environmental Chemistry

72

OH I

3

(Cyclic transition state)

+ CH2

/C~CH ~H

- - 7 CH

t

20 2 II CH3-C-CH3

Under suitable conditions of environmental relevance, abstraction of a H-atom by an electronkaHy excited species may occur fJom another molecule rather than intramoleculary, e.g., Parathion changes to p-nitro phenol when former is used as insecticide over agriculture fields. S

t

0-P(OC 2H s)2

¢

+h'

N02 Another example of intermolecular H-atom abstraction is the solution phase photoreduction of benzophenone in the presence of H-atom donation solvent such as alcohol.

o OH OH OH II I I I Ph-C-Ph + CH 3-CH-CH3 + hv ----) Ph-G-Ph + CH 3-G-CH 3 4.4.10. Photosensitized Reactions Sometimes an electronically excited molecule can transfer its energy to another species which then undergo a photochemical change. This is known as photosensitized process. Sens (So) + hv -~ Sens (S,)

Sens (S,) Sens. (T,) + Acceptor (So)

. ) Sens CI)

I.S.c.

Energy) Sens (So) + Acceptor (T,) Transfer

Products The ideal sensitizer will be one having high rates of intersystem crossing (I.S.C.) and of energy transfer to the acceptor. The sensitizer does not undergo photochemical processes which compete with fonnation of its triplet state and energy transfer. The inter system crossing here refers to change of singlet state to triplet state. This step is important as the triplet state exists for a longer time than singlet state and reaction proceeds easily with transfer of energy. Benzophenone is a welt known triplet photosensitizer. Benzophenone by absorbing a photon of suitable energy gets excited to (n ~ 1(*) singlet state and then it undergoes I.S.c. to its triplet state. This triplet state is of sufficiently high energy (286.6 kJ/mole) and half life, so that it can transfer its electronic energy to some other suitable molecule having lower energy triplet state. The acceptor molecule in its excited triplet state can then react or be deactivated e g.. cis-I, 3-pentadiene isomerizes, giving a mixture of the cis and trans isomers. I.S.c. ) Ph 2CO (T) Energy) Ph CO (So) 2 Transfer

+

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemislly

73

>

(cis)

By similar route pesticide aldrin in benzene solution converts to photoaldrin in presence of sunlight and benzophenone. CI H

hv

sens. > (Ph,CO)

Aldrin

Photo-aldrin

One of the most important example of photosensitized reaction in atmospheric chemistry is the formation of singlet molecular oxygen through Kautsky mechanism. The mechanism is similar to that of general mechanism discussed earlier, however, in case of 10, (singlet oxygen) production, the acceptor is ground state oxygen which is triplet eO,). Therefore for energy transfer step to be spin allowed. the final state of 0, must be a singlet. It is assUliled here that sensitizers ground state is a singlet state. There are many sens-itizer in air which can form 102 such as S02' The sensitizers are need not to be always in gas phase, they may be polycyclic hydrocarbon absorbed on surfaces or present in solution. Soil surfaces also forms 10, where soil organic compounds may act as sensitizer. The surface photochemistry and that of aqueous system contailling dissolved species such as SO" 0., 0" H,O, NO" PAN etc., both in the presence or absence of reactive organic compounds are inlportant for atmospheric chemistry.

4.5 Radicals in Atmosphere Electromagnetic radiations of suitable energy may cause the fragmentation of molecule giving rise to radicals. The atom or groups produced with unpaired electrons are called free radicals.

°II

CH3-C-H Most radicals several minutes due the products of each upper atmosphere. terminated.

~

.

°II

CH3 + ~-H

are highly reactive. However, in upper atmosphere the radicals have half-lives of to its rarefied nature. Radicals can take part in many chain reactions in which one of reaction is a radical. These are very successful in chain initiation reactions occurring in In cases where one free radical reacts to another free radical chain reaction is CH; + CH;

~ C 2H 6

Most important free radical reactions are involved in smog formation. It is important to know the difference between high reactivity and instability of free radicals. An isolated free radical would be quite stable, therefore tend to have longer half lives under rarefied conditions where they can exist and travel for longer times before colliding to another chemically reactive species. However, electronically excited species have a finite life time (normally very short) as it lose its excess energy as radiation even in raretied conditions.

4.6 Hydroxyl (OH") and Hydropel'Oxyl (HOi) radicals in Atmosphere The hydroxyl and hydroperoxyl are most important reactive intermediate in atmospheric chemistry. OH· and HOi are interconverted in the atmosphere through a series of reactions involving hydrocarbons and oxides of nitrogen. Thus, sources of H0 2 are, in fact, sources of OH·, under normal tropospheric conditions where NO is present in significant amount. Possible sources of OH· and HOi in aqueous atmospheric droplets are not clearly understood.

74

Environmental Chemistry

4.6.1 Sources of OH· There are several sources of OH' (i) At higher altitude it is produced by photolysis of water H 20+hv ~ HO'+H' (ii) A major source of OH· is the photolysis of 0 3 to form excited oxygen atol11 react with Hp to produce OH·. 0 3 + hv (A. < 320 nm) ~ 02 ('L'1g) + O('D)

° ('D) which then

°

('D) + Hp ~ 20H' (iii) Photolysis of nitrous acid vapour or H,O, produces OH· directly. HONO + hv (A. < 400 nm) ~ OH· + NO H20 2 + hv (A. < 360 nm) ~ 20H· The possible sources of HN0 2 in atmosphere include the reaction of N0 2 with HP, OH· with NO, NO + N0 2 with HP, H0 2 + N0 2 reaction and direct emission from automoblies exhaust. The HP2 is formed in the atmosphere from the reaction of two hydroperoxyl radicals. HOi + HOi ~ HP2+ 02 The above reaction may form a radical water complex if reaction proceed in presence of H20. HOi + Hp E

)

(H0 2 . HP) Radical-water complex

HOi + (H02 . HP) ~ HP2 + 02 + Hp

2(H02 . HP) ~ H2 + 02 + 2Hp (iv) In presence of organic matter, hydroxyl radical is produced in abundant quantities as an intermediate in formation of photochemical smog. RH + + 02 -~ ROO· + OH· RCOO· ~ OH· + aldehyde or ketone

°

(peroxyl radical) (v) Conversion of HOi to OH· by reacting with NO.

HOi + NO

~

OH· + N0 2 This is the key reaction to the HOi and OH· interconversion. Hydroxyl radical is most frequently removed from the troposphere by reaction with either methane or CO. CH 4 + HO' -~ H 3C' + Hp CO + HO· -~ CO, + H The global concentration of hydroxyl radical changes- with season and range from 2 x 10 5 to I x 106 radicals per cm3 in troposphere. The seasonal change is due to the fact that in high humidity and larger (ID) levels is increased. This means concentration of OH· is higher in tropical incident sunlight, the regions. The southern hemisphere has 20% higher level of OH· than northern hemisphere because of greater production of anthropogenic CO in northern hemisphere consumes OH· radical.

°

4.6.2 Sources of HOi There are many direct and indirect sources for HOi productions. (i) Photolysis of formaldehyde is the major source of HOi during day light.

HCHO + hv

-~

H + HCO

H + 02 ~ H0 2 HCO + 02 -~ H0 2 + CO Here M is third body which carries away excess energy from reaction, as well as it also serves as reaction site. The importance of third bodies in sllch reactions cannot be undermined as in their absence

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemistl},

75

such reactions do not take place, From above reaction it is understood that any reaction producing HCO or H can serve as a source of H0 2' in troposphere, It also include higher aldehydes such as CH 3CHO, although their absorption cut-off (345. nm) and lower atmospheric concentration makes them less important sources of H0 2'. R-CHO + hv -~ R· + CHO HCO + 02

-~

HO l ' + CO

(ii) Reaction of CO and OH· produces H atom which can react with 02 to produce H0 2', CO + HO· ~ CO 2 + H H + 02 ~ HOi

(iii) Alkoxy radicals also serves as a source of H02' as they react with 02' RCH 20' + O2 ~ RCHO -}- H0 2' The alkoxy radicals are formed in the chain oxidation of hydrocarbons in the atmosphere initiated by OH·. Therefore, above reaction serves as one of the link between interconversion of OH· and H02', Reactions producing free radicals involve the thermal decomposition of peroxyacetylnitrate (PAN), CH 3C(0)00N0 2 , PAN can undergo series of reactions in the presence of NO to give HO l ',

°"

CH)-COONO z (

)

o

"

°" ° ° "

CH)COO + NO z

CH,COO --7 CH)CO +

o

"

CHlCO

(

)

CH; + CO 2

CH; + Oz --7 CHlOO' CH)COO' + NO --7 CH)O'+ NO z CH,O' + O2 --7 HCHO + HO; (Alkoxy radical)

(iv) Thermal decomposition of peroxynitric acid H0 2N0 2 also produces H0 2, H02 NO z ( ) 110; + NO z (v) The night time NO, reactions (nitrogen oxides in absence of light can react) occur, and many of these may produce free radicals which leads to HOi as well. Thus, the free radical concentration does not drop to zero at night. Reactions producing fi'ee radicals also involve thermal decomposition of nitrate radical (NOj) formed in the reaction of 0 3 and N0 2, The hydrogen abstraction of nitrate radical at night also leads to H02' via formation of formyl radicals and alkoxy radicals. NOj + HCHO -~ HN0 3 + HCO (formyl radical)

NOj + RH

-~

HN0 3 + R·

R + 02 RO; + NO

-~

RO;

-~

RO' + N0 2 (alkoxy radical)

HCO + 02 -~ RO' + 02

~

HOi + CO R'CHO + HOi (R' is alkyl group with one carbon less)

4.6.3 Importance of OH' and HOi The relative importance of OH· and HOi and their sources will depend upon actual conditions i.e. day or night, concentration of hydrocarbon. NO. NO} and 0)' In a polluted urban air mass during daylight

Environmental Chemistry

76

hours HNO, contributes maximum radicals, however HCHO and 0 3 contributes more in late hours of the day. In relatively unpolluted area where NO, and hydrocarbon concentration is low. there main source of radicals is photolysis of 03'

Fig. 4.3: Imporlanl reactions involving OH· and HOi radicals.

Relationship between different radicals and their important reactions have been outlined in Figure 4.3.

4.7 Ions in Mesosphel'e and Lower Thermosphere The characteristic of upper atmosphere is the presence of significant level of negative and positive ions. Due to the rarefied conditions in upper atmosphere, these ions may have longer half lives before they combine again to form neutral molecules. At altitude of 50 km more ions are prevalent, therefore the region is termed as ionosphere. The ultraviolet light has been primary producer of ions in ionosphere. In night when U.V. light no longer impinges on ionosphere, the positive and negative ions slowly combine to form neutral molecule. The process of formation of ions has been rapid in lower regions of the ionosphere, where the concentration of ions is high. Thus, lower limits of ionosphere lifts at night and makes possible the transmission of radiowaves over much greater distance. The effect of magnetic field of earth is Axis of significant on ions. The best known example of this effect is Magnetic Field discovery of Van Allen belts (Fig. 4.4). These regiol1s are having Fig. 4.4: Van Allen BellS of ions. The two belts of ionized particles which covers the earth.lf they are separation of positive and negative charges visualized as dough nuts, then the axis of earth's magnetic field in ionosphere in the efrect of magnetic extend through the holes in the doughnuts. The inner belt is field on these charges. highly energetic ions consist of mainly protons. The outer belt is mainly consists of eJectrons. 4.7.1 Primary Photoionization in the Lower Ionosphere The high energy very short wavelength solar radiations are responsible for formation of positive ions and electrons. The ionization energies required to produce ion for different molecular species and corresponding radiation wavelength to achieve this are listed in Table 4.2. From the table it can be seen that only NO can be ionized by Lyman a radiation at wavelength 121.5nm. In fact only Lyman a can only penetrate to lower ionosphere. Also NO has significant concentration contributing to the formation of ions in lower ionosphere. Since Lyman a-radiation, by virtue of localized window in absorption speclrum of 2, is the only shorl wavelength radiation to penetrate on any

°

77

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemisliy

scale into the mesosphere, it follows that photoionisation of NO must be the main source of electrons in the mesosphere. However, above 90 km altitude the rates of formation of N, + and 0,+ are appreciable due to the availability of radiations with shorter wavelength. The N, + formation rate is further enhanced by strong absorption of penetrating X-rays of wavelengths below 10 ilm. In other words NO+ formation above the mesopause is dominated by secondary conversion of N2+. Table 4.2: Ionization Energies (I{J/mole) and equivalent wavelengths (nm) SNo.

Species

1011l::(/IiOn Energy

1:·Cflllvalent Wavelellgth

I.

N (4S)

2.

o (3 p)

1314.0

9l.l

3. 4. 5. 6. 7.

H eS) N2 (X' L g+) 02 (X 3 L g-) 02 (alL'\g) NO (X 21t)

1307.2 1503.9 1166.7 1070.7 S93.3

91.6 79.6 102.6 III.S 134.0

1403.4

°

85.3

N2 + + -~ 1'10+ + N N 2+ + NO -~ NO+ + N2 The above reactions are highly exothermic as ionization energy of NO is minimum among other species. Near mesopause the X-ray producing N/ is rapidly attenulated with decreasing altitude so that Lyman a-radiation becomes the predominant soui'ce of ionization. NO does not contribute to attenuation of Lyman a-radiation as a function of altitude. The Lyman 13 line of atomic hydrogen lies at wavelength of 102.6 nm and just like Lyman a-line is enhanced well above solar irradiance at neighbouring wavelengths. This can result in photo ionization of molecular oxygen itself in mesosphere. The 0,+ which are present in mesosphere originates from photoionization of 0, ('!lg) by radiation of wavelerlgths upto III.S nm, even though the excited state is much less abundant than ground state. 4.7.2 Positive Ionic Species in Mesosphere The ions identified below SO km altitude have mass to charge (m/e) ratio of 19' and 31', \-vhile there is a considerable concentration of ions with m/e ratios in excess of 45'. The 30' ~orresponding to NO~ and less abundant 32+ corresponding to 0,+ are also detected. The last two come into existence above SOkm altitude. The identities of major ions below so km altitude are H3 0+ (19+), monohydrate proton, HsO; (37+), dihydrate proton. In the same series H+(HP)3 i.e. Hp/ (5s + 3) has also been found in atmosphere. In fact, the primary NO+ ion transfer its energy to the hydrogen atom, and that is how cluster ion [H+ (H 20),,] are formed. Before CO 2 attenuation of the Lyman 13 solar radiation was considered, it appeared to based upon the existence of high concentration of 2+ below SO km altitude. 02 + + 02 + 02 -~ 4 + 02 0/ + H20 -~ 0/. H20 + 02 ep) + 0/ -~ 0/ + 03 0/ . Hp + I-fp -~ HP+. OH -jl 02 0/ . H20 + H20 -~ H30+ + OH 'I- 02 Hp+ . OH + Hp -~ H+ (I-IP)2 + OH The concentration of cluster ion decreases above SOkm due to the fact that concentration of CP) decreases with increasing altitude thereby limiting the conversion of.04' to 02+' Hp. However, in view of the low Lyman 13 photon flux densities which would be available at SOkm altitude on account of CO, attenuation there are doubts whether this scheme really operate below SO km altitude. These are reactions of NO+ which can also produce the hydrate cluster ions. NO+ + CO 2 + M -~ NO'. CO 2 + M

°

°

T

°

°

Environmental ChemislIy

78 NO+ . CO 2 + Hp ~ NO' . Hp + Hp + M -~ NO+ . (HP)2 + Hp + M -~ NO+ (HP)3 + Hp -~ Monohydrate NO+ ion is catalysed by CO? and process under typical mesopheric conditions. NO+ + Hp + M ~

NO+. Hp + CO 2 NO' (HP)2 + M NO~ (HP)3 + M I-t (HP)3 + HN0 2 this indirect mechanism is much faster than direct NO+. Hp + M

Electron densities in upper mesosphere increase sharply above 20 km altitude which is consistent with the fact that the rate coefficient for dissociative recombination of H+ (HP)" species with electron is larger than that for the dissociative recombination of NO'. Hence the conversion from dominance of H+ (H 20)" ions below 82 km to dominance of the NO+ ion above, leads to an increase in electron loss rate with decreasing altitude in this region.

4.7.3 Negative Ionic Species in the Mesosphere Negative ions are found in upper atmosphere but largely below 90km altitude. These negative ions are mainly composed of cluster ions e.g., N0 3-(H 20)" where n range from 0-5. The two important steps in negative ions chemistry are (i) generation of negative ion (ii) hydration of negative ions. In most of the cases the initial formation of negative ions takes place the rough a three-body electron attachment to molecular oxygen. e- + 02 + 02 ~ 02- + 02 A minor concentration for negative ion formation also takes place by two body dissociative attachment of electron to ozone. e- + 03 -~ 0- + 02 After the initial generation of 2-, these are three reactions compete to each other they are:

°

02- +

(i) (ii)

° CP)

02- + 02 + 02

(iii)

~ 03 + e-~

04- + 02

The formation of 03- and 04- subsequently produce N0 3-. The first process here reverses the dissociative attachment of electrons to produce 0,- ion. Therefore, the concentration ratio of ground state oxygen atoms and ozone is critical. Abovc 80km- altitude these concentration changes oppositely as conCP) increases while concentration of 0. decreases with increasing altitude. It is therefore (i) centration reaction dominate at altitude above 80km so the I{egative ion content of atmosphere decreases. This explains that above 90km mostly electron remains unattached to any species. The transmission of radiowaves over NO longer distances in ionosphere is known. These ~ 02,M waves move round the earth by reflecting ti·om NO - 41 0 - 41 °2 + M O 03 ~ 0 3 2 the inner side of ionosphere. Negative ions are 3 N~L4 usually heavy to oscillate in resonance with / 0 ...CO z ... CO 2 the radiowave frequency and so produces lit0 3CO -____0::::...._ _ _... tie attenuation. However, electron can oscil~ C0 3 ---=-+ °2 late in resonance with the radiofrequency and NO ~NO so alternate the transmission severely. The e1ec_ 03 tron concentration of the lower ionosphere at NI 0 3 41 N0 2 night is almost zcro, due to direct ionisation . . '-----:-.. ~ N 20 4 by solar radiation and disappearance of CP). NO + This means most of the negative charge being Fig. 4.5: Important negative ion inten.:onversion reactions in form of negative ions rather than electrons, since overnight reaction (ii) dominates over (/) in the removal of 03-. This explain the improvement in long range radio transmission at night.

°

I

l4

°

°

-

Chelll ical and Photochemical Reactions in the A tmosphere-A tll10spheric Chem is/lJ!

79

Both 03- and 04- can undergo different reactions to form many more negative charge species. These have been show in Figure 4.5. Nitrate ion and its hydrate ion persists throughout the night. In mesosphere, the concentration of CO, exceeds those of NO and those reactions with CO 2 predominates. In last, N0 2- and N0 3- is formed which involves C0 3- and C04-. The subsequent hydration of the terminal N0 3- ion is likely to proceed by an indirect mechanism that is known for hydration of N0 3-. N0 3- + CO 2 + M -~ NO,-. CO 2 + M N0 3- • CO 2 + Hp ~ N0 3- . Hp + CO 2 N0 3- • Hp + Hp + M -~ N0 3- . (H 20)2 + M N0 3- • (HP)2 + Hp + M -~ N0 3- · (HP)3 + M

4.8 Reactions of Atmospheric Nitrogen The atmosphere contain 78% nitrogen by volume and is an important source of this essential element. The molecular nitrogen, N" has high dissociation energy due to the presence of triple bond between two nitrogen atom therefore remains undissociated in thermosphere even in presence of high energy U.V. radiations. However, above 100 km altitude photodissociation of N2 produces atomic nitrogen. N2 + hv -~ 2N These are other routes also possible those can produce monoatomic nitrogen such as: N2 + hv -~ N2+ + e- (in ionosphere) N2 + + -~ NO' + N NO+ + e- -~ N + 0+ + N2 -~ NO' + N The first two reactions occur in ionosphere where concentration of NO' is high. In the lowest region of ionosphere (50-85 km), NO' is formed by direct ionisation of NO. NO + hv -~ NO' + eThe NO+ by reacting with electron can further dissociate into Nand 0. The ionic species N/ IS formed through the action of galactic cosmic rays. N2 + hv -~ N/ + eThe N, + can react with monoatomic oxygen to produce NO+ and N. Other important chemical reactions viz~ nitrogen fixation etc. have already been discussed in Chapter 2 (nitrogen cycle).

°

°

4.9 Reactions of Atmospheric Oxygen The role of 0, in atmosphere is vital as it take part in energy producing reactions such as combustion, respiration and photosynthesis. Some oxidative processes like rlisting (oxidative weathering) also involve oxygen.These reactions have already been discussed in Chapter 2 (oxygen cycle). The oxygen in upper atmosphere can exist in some of those forms which are quite different from those stable in lower atmosphere. This is due to the availability of ionizing radiation and raretied conditions in upper atmosphere. The upper atmosphere has excited molecular oxygen, atomic oxygen, ozone along with molecular oxygen. The atomic oxygen is mostly found in thermosphere where it has sufficient stability. This is because of raretied conditions prevailing there and third body collision seldom takes place which is required for chemical reaction of atomic oxygen. Atomic oxygen is produced by direct dissociation of molecular oxygen by U.V. radiations. 02 + hv -~ 20 The UV radiations must have wavelength in the region 135nm to 175 nm and 240nm to 260 nm. The high enery radiations are required due to its chemical nature where two oxygen atoms are bound through double bond. Due to the photochemical dissociation in upper atmosphere the existance of 0, is almost zero. The average molecular weight of air is lower at altitude exceeding 80km due to the presei1ce

Environmental Chemistry

80

of high concentration of atomic oxygen. The division of atmosphere as homosphere and heterosphere is based upon the uniform and non-uniform molecular weight of air in lower and higher regions. The important phenomenon occuring due to the presence of atomic oxygen is air glow. Excited oxygen atom when relaxed to ground state emits radiafions of the wavelength 636, 630 and 558 part of visible light. This gives glow to air called airglow. The excited oxygen is formed when three atomic oxygen combines to form molecular oxygen

° + ° +0*° -~

01 + 0* 0+ hv Air glow is thus a faint radiation which is continuously emitted by atmosphere. This phenomenon of air glow is quite intense in infrared region though weak in visible region. The oxygen ion, 0', is formed by direct ionization of atomic oxygen by U.V. radiation. + hv -~ 0+ + e" This reaction primarily occur in ionosphere at very high altitude. The 0' ion can undergo different reactions to produce other positive species in atmosphere. 0' + 02 -~ 02 + 0' + N2 ~ NO+ + N Another species of impo11ance in ionosphere in 2" formed by high energy U.V. radiation (A = 17 103 nm) 02 + hv ~ 02t- + eThis species may also be produced when N2 + react to 02 N/ + 02 ~ N2 + 02' 4.9.1 Reactions of Ozone Ozone, 03' is another oxygen containing species having a special importance in atmosphere. The electromagnetic radiation from sun which falls on upper layers of the atmosphere has much more U.V. radiation (i.e. high energy radiation) compared to radiation reaching earth. The U.V. radiation bring about a number of photochemical changes involving various forms of oxygen like OJ and atomic oxygen 0. These reactions occur continuously. Therefore, these oxygen containing species absorbs most part of harmful U.V. radiation and serves as a shild against them. The U.V. radiation can cause skin cancer even in low levels. Ozone is formed by a photochemical reaction of molecular oxygen producing atomic oxygen which then react with another molecule of oxygen in presence of third body to produce ozone. -~

°

°

T

°

° hv -~ °03 +° +° + ° is third body like N, or 0, which absorbs excessive energy and enables the oxygen atom to remain 2

+

2

M

+

M -~

M

together in form of 0)' The ozone concentration is maximum between the altitude range of 25-35 km in stratosphere. It is nearly 10 ppm in this region. The ozone concentration in lower atmosphere is not significant because of the presence of oxidizeable aerosols. In large concentration ozone has an offensive, astringent and unpleasent odour and is sometimes noticed around electrical discharge machinery. Ozone in troposphere is a pollutant and can produce photochemical smog. One of the primary environmental pollution problem resulting due to high altitude flights of supersonic planes. The exhaust products from these aircrafts cause reactions removing ozone from atmosphere and disturbing the formation-destruction cycle of ozone. The absorption of U. V. radiation brings about its dissociation therefore the cycle is complete

° ° ° -~ ° ° Normally oxidizable material do not enter the stratosphere from troposphere due to the presence of 03 + hv -~ 2 + Ozone may also decompose by reaction with atomic o;..ygen a'; +

2

+

2

Chemical and Photochemical Reactions in the Atmosphere-Atmospheric Chemistl:v

81

tropopause which do not allow the dispersal of pollutants from one region to another. However, chlorofluorocarbon used as refrigerants, foaming agent, solvents and in aerosol spray cans can do so. These compounds undergo photochemical decomposition giving rise to chlorine atom which catalyses the decomposition of ozone.

CCIl2

UV Light>

CI

~

02 + CIO

~

02 + CI (can react again)

The above reaction which is using ozone can remove ozone blanket of ealth and exposing it to dangerous killer UV radiation.

4.10 Reactions of Water in Atmosphere The water vapour content found to very over a wide range in lower atmosphere. The normal range has been 1-3% by volume. Water vapour has ability to absorb Lr. radiation (see greenhouse effect, chapter 3). Clouds formed from water vapour are able to reflect light from sun and have a temperature lowering effect. Gaseous water in atmosphere is not known to take part in many photochemical reactions. One of the reaction with U.V. light is the dissociation of Hp which result in the formation of atomic hydrogen. Hp + hv ~ H + OH The H atom so produce can react with ozone to produce activated hydroxyl radical. These excited hydroxyl radicals emit Lr. radiation of longer wavelength. H + 0 3 ~ OH* + 02 OH* ~ OH + hv This emission is partially responsible for airglow condensed form of water vapour in form of very small droplet play vital role in atmospheric chemistry. Pollutant S02 when oxidised to S03 in atmosphere gets incorporated into droplet of water. S02 + 02 -~ 2S03 S03 + Hp -~ H 2S0 4 The formation of sulphuric acid low down the pH of water in rainfall in some areas.

4.11 Reactions of Atmospheric CO 2 Carbon dioxide is 0.0314% by volume of the atmosphere. It is a non-pollutant species of most concern to us due to its ability to absorb i.r. radiation (see greenhouse effect and earth heat balance in Chapter 3). Most of the reactions occurring in troposphere have been discussed in carbon cycle (Chapter 2).

4.11.1 The formation of carbon compounds and photosynthesis It is the process of fixing CO 2 to produce carb'on compound and 02 along with the storage of solar energy in. chemical bonds. The light is absorbed by a number of pigment molecule such as chlorophyll a and b. These pigments are concentrated in green tissue cells of higher plants. These pigments are responsible for absorbing a particular region of visible light spectrum system containing chlorophyll a absorbs red light (A - 650 nm) to give rise excited molecules containing electrons in higher energy states. These electrons are then transferred to compounds like ATP or NADPH. The A TP transfers a phosphate group to one of the organic reactant that is then react in a desired manner. The addition of phosphate to organic molecule is called phosphorylation. ROH Organic molecule

+

ATP ~ ROP0 3 Adinosine Phosphorylated triphosphate organic molecule

+

4

ADP Adinosine diphosphate

ROX + PO/ Organic Phosphate Product A TP behaves as an energy transfer agent. The energy can come directly from photosynthesis or fro'11 chemical bond energy in compounds such as fats and carbohydrates which are broken down to regenerate ATP.

Environmental Chemistry

82

Nicotinamide adenine dinucleotide phosphate (NADPH) is reduced and acts as a carrier of proton and electron. NADPH + Oxidised compound -~ NADP+ + Reduced product NADP+ can regenerate NADPH NADP+ + W + 2e- -~ NADPH During the photosynthesis the electrons and protons are furnished by water with release of oxygen. 2Hp -~ 02 + 4W + 4eThe incorporation and reduction of CO, to form various compounds involve complex series of cyclic reactions. One molecule of CO, is added to-an activated organic molecule. The product is then reduced by NADPH and rearranged, enzymes to final compound which may be a carbohydrate or lipid. The above synthesised organic molecules have stored energy in form of chemical bond therefore breakdown of these molecules into CO, and H,O during aerobic respiration. The complete decomposition of glucose to CO, and H,O is accompanied with 2900 kJ/mole of energy which was stored during photosynthesis. The liirger portion of this released energy is transferred inside the organism by the 38 molecule of ATP which are synthesised during respiration. Glucose is first converted into pyruvic acid with production of two moles of ATP. The dioxygen is not required at this stage. Further respiration can be achieved into ways aerobic (in presence of 0,) to produce CO, and H,O, and anerobic (in absence of 0, called fermentation) to produce C2 H5 0H and H20. Plants utilize starch as their main energy storage conipound, however, animal use fats. Volcanic eruptions injected enormous quantities of CO, into the atmosphere. The burning of fossil fuel is another big source of CO 2, The respiration by animaC also produces CO!, Photochemically CO, is not very active as it is mostly found in troposphere where there is only a limited U.V. light is penetrated to cause any reaction. The CO, absorbs i.r. radiation which do not have sufficient energy to induce any photochemical change. One o( the important though rare photochemical reaction of CO 2 is photodissociation reaction to produce CO at higher altitude. CO 2 + hv -~ CO +

bi

°

_QUESTIONS I. Detine, (a) atmospheric chemistry, (b) atmospheric chemistry pathways. 2. Why it is difficult to predict chemical changes of environment by studying parallel reactions in laboratory ? 3. What are the three main sources of encrgy providers for atmospheric reactions? 4. What are the energies associated with 400 and 800 nm electromagnetic radiations (in k.l/mole) '! 5. What wavelength of light bc used to break C-H bond in methane. Bond energy of methane is 390 k.l/mole. In which region of electromagnetic spectrum this light lillls ? 6. Deline giving examples (£I) Photodissociation

(b) Photodimerisation

(c)

Photoscnsitlzed reactions

(d) Photo ionisation

(e)

Photoisomerisation

(/) Chemiluminescence

7. What is the difference between tlurescence and phosphorescence? 8. Collision deactivation is rare in atmospheric chemistry, why? 9. "Hydrogen atom abstraction may give Norish type II products." Explain. 10. How collision deactivation is different tj'om photosensitized reactions? II. Differentiate between singlet and triplet excited states of 02 molecule? 12. Photodissociation of NO z may lead to formation of ozone. explain giving equations .) 13. I-low hydroxyl radicals are produced in lower atmosphere?

Chemical and Photochemical Reactions in the Atmosphere-Alfl1o~pheric ChemistlT

83

14. Explain the formation of o-nitrosobenzoic acid ti'OI11 o-nitrobenzaldehyde. 15 . Some trans-azo compounds changes into cis-azo compounds in atmosphere. why') 16. What do you mean by I.S.c. of triplet photosensitizer.

'l

How it is used in explaining photosensitized reactions .) Give one example

17. Explain. Kautsky mechanism lor the formation of singlet molecular oxygcn .) 18. How OH radical is produced in atmosphere ')

19. HoII' radical water complex and hydrogen peroxides are produ\.:ed in atmosphen:. 20. Complete the reactions

°

+ 02

~

(a)

RH +

(e)

Photolysis or formaldehyde

(b) RCOo' ~ (eI) PAN converted into HO; in various steps.

21. Why OH' concentration is higher in tropical regions

'l

22. Why southern hemisphere has greater concentration or hydroxyl radical than northern hemisphen:

23. Free radical concentration of H0 2 do not reach zero in night time (absence of light). why

'l

'l

24. With the help of chart. outline the important reactions or HO' and HO; radicals '! 25. Why lower limits of ionosphere lills at night and makes possible transmission of radiowavcs to longer

distances

'l

26. What is the efTect of earth's magnetic liekl on the ions of the mesosphere .) 27 . What is the main source of electrons in lo\\'er mesospherc

'l

28. Why concentration of cluster ions decreases above 80 km altitude .)

29. Show. how reaction of NO' can produce hydrate cluster ion H' (H 20)" '? 30. What type of reactions compcte with each other. once 02- ion is generated in upper atmosphere '!

3 I. Why electrons remains unattached to any species above 90 km altitude .) 32. With the help of chart. show important reactions of negative ions inh:rconversion. 33. Why air glow?

Air Pollution 5.1 Introduction The problem of air pollution was noted almost three centuries ago. The noted scientist John Evelyn described many of the effects of coal burning such as reduction in sunshine, morbidity and mortality from respiratory ailments, dust fall, corrosion of materials etc. In fact, the pollution of atmosphere is not a new phenomenon and existed even before man became aware of nature. In recent years, concern over air pollution has been voiced which is arising as an effect of extensive industrialization and population explosion in last century. With the growth of industries, more and more toxic substances are either used as raw material or given off as side products during manufacturing processes in the form of dust, fumes , vapours and gases to cause air pollution. These pollutants ultimately dissipate in the working environment and pose occupational health hazards. There are some half a million man made substances already present as pollutants in our total environment. These are to be controlled for our future generations, as they may end the life on the Earth. In general, the actions of people are the primary cause of pollution. The first significant change in atmosphere came with the discovery of fire . Prehistoric man built fire in his cave for cooking, heating, and to provide light. The problem of air pollution came into existence at this time. The natural pollution of air due to volcanic eruption etc . .also contribute significantly but it is beyond the control of human beings. Air pollution is basically the presence of foreign substances in air. Some specific definition of pollution are: According to WHO, "Substances put into air by the activities of man into concentration sufficient to calise harjillul effect to his health, vegetables, property or to intet/ere with the enjoyment of his property. " According to Medical Association, "Air pollution is Ihe excessive cbncentratiol1 offoreignlllalter in the air which adversely aflects the well being of individual or cause dam age to property." According to lSI , "Air pollution is the presence in ambienl atmosphere of substances, genera/~)' resulling FOIII the activity oj'mCin. sufficient con cent rat ion. present for a suflicient time and IInder circumstances which intel/ere signijicant~l' with Ihe cOlllfort, health or welfare or persuns or wilh the fidluse or enjoyment oj' property. " In simple words, "Air pollution means the presence in the outdoor atmosphere of one or more contaminants. such as dust, iilllles, gas, mist, odolll; smoke or vapour. in quantities. with characteristics, and of durations such as to be injurious to human, plant or animallije or to which unreasonab~}' intel/ere with the com/or/able enjoyment of Ilje and property. " Air pollution means different things to different people. To farmer, it may be damaged vegetation ; to common man, it may be eye irritation ; to the pilot, dangerously reduced visibility and to industries, problems of process control. The problem of air pollution also varies from place to place. Air pollution can

85

Air POI/lit ion

cause death, impair health, reduce visibility, bring about vast economic losses and contribute to the general deterioration of both urban and rural areas. It can cause intangible losses to historical monuments such as Taj Mahal which is beliewd to be badly affected by air pollution. On account of large scale industrial activities fall of acid rain is known, which may reduce forest growth, destroy environment of lakes, may cause excessive weathering and wash away essential nutrients of soil. Apart from these, the basis of ecology is affected. Large scale deforestation due to population explosion apart from creating an imbalance in oxygen proportion of the atmosphere, also affects weather and rain pattern. Industrial activities, particularly in thermal power stations, cement plants, oil refineries, metallurgical operations, steel plants. fertilizer industry, burning of fossil fuel as source of energy causes major problems of air pollution. The cases of severe air pollution in 20th century occurred in Meuse valley of Belgium in 1930; a killing smog in Donara, Pennsylvania along the Monongahela River in 1948, in which hundred people died; deadlier smog in London in 1952 in which 4000-5000 people died due to respiratory failure; "Episode 104" which blanketed all or part of 22 states east of the Mississipi, with air pollution haze in August, 1969; photochemical smog (Los Angles); Accidental leakage of MIC (melting isocyanate) in Bhopal, India (1984) killing 5000 and affecting nearly 2,00,000 people in one or other way. In early days, the effect of air pollutants was concerned to the people residing within a few kilometer of the source of emission. In recent times, it has become clear that pollutants are being transported over large distances and affecting the larger areas e.g., acid rain. Similar, global concerns include CO, and chlorofluorocarbon emissions, which may affect the balance of the atmosphere so as to cause climate changes. Concern over the affects of atmospheric pollution has widened to include damage to buildings, materials, crops, forest, human health, animals, fresh water ecosystem, plants and even the stratosphere. Sources: Automobile exhaust. industry, power plants, domestic use of fossil fuel. natural pollution due to volcanic eruption etc.

I

Control

j

Imeasures

I+-

~

Emission of primary inorganic and organic gaseous pollutants (CO, NOx' SOx' hydrocarbon. NH3 , particulate matter)

Meteorology: dispersion of clouds,fogs, rain, etc.

I

Transoformation to scondary pollutants:

,

t

,

1

I Monitoring I

I Ambient Air

•

Wet and dry deposition Acid Rain

I

I

~

.J ..

I Photochemical pollutants

I

I

Transport model

I

t .1

•

Impact on humans, animals, plants, lakes, visibility. materials. etc.

Fig. 5.1: The Air Pollution System

°

2-

H2S0 4, S04 ' 3 , PAN, H2 0 2 , free radicals. HCHO, HN02 and organiC particulates

1-

86

Environmental Chemisli)'

5.2 Air Pollution System Air pollution system involve various steps of emisison, meteorology, chemical transformations, ambient concentration (criteria and non criteria pollutants) and air quality standards, visibility and models. The air pollution system is outlined in Figure 5.1. The air pollution system starts with various sources of anthropogenic and natural emission. These are defined as primary pollutants since they are emitted directly into air from their sources. Primary pollutants include SO" NO, CO, Pb, organic pollutants and particulate matter. After they enter the atmosphere, they got dispersed and transported called meteorology. When primary pollutants are in atmosphere, physical and chemical transformations may take place to produce secondary pollutants. The primary and secondary pollutant composes ambient air. The pollutants ti'om ambient air may be removed at the earth's surface via wet or dry deposition. These can then show their affects upon humans, animals, aquatic ecosystems, vegetation and materials. From detailed knowledge of all the above steps of pollution system, a model system which predicts the concentration and effect of primary and secondary pollutants as a function of time and location can be made. These mathematical computer based models can describe the concentrations in a plume fi'om a specific point source called plume models. In a nir basin from combination of different mobile and stationary sources called air shed models, can also be formulated. Long-range transport model describing downwind from a group of sources over a large geographical area can also be made. In order to make these models work successively, their predictions must be compared to the observed concentrations of the pollutants in ambient air monitoring programs. These models can be then used to develop various c~ntrol strategy options which directly affect the air pollution system by controlling the start point i. e. the sources and primary emission.

5.2.1 Sources A major source of air pollution has been particulate and gaseous matter which gets released by burning of fossil fuels such as coal, petroleum etc. These pollutants have been artificial pollutants and they are emitted in air by four major fuel burning sources. (i) Automobiles and transportation: They have been regarded as the greatest sources of air pollution. They produce nearly 2/3 of Iota I CO and 1/2 of the total hydrocarbons and NO found in polluted air. The automobile exhaust also has leaded gas and particulate lead. (/i) Electrical power plants: Use of coal to generate electrical power produces 2/3 of total sulphur dioxide found in air.

(iii) Industrial Processors: Metallurgical plants and smelters, chemical plants, petroleum refineries, pulp and paper mills, sugar mills, cotton mills. rubber manufacturing and processing plants etc. are responsible for 1/5 of total air pollution. (iv) Heating Plants: Heating plants for homes, domestic burning of fuels etc. also contribute signi'ficantly to air pollution.

Natural contaminants are also found in air e.g., natural fog, pollen grains, bacteria and products of volcanic eruptions.

5.2.2 Emissions There are two types of sources of emissions natural and anthropogenic. The impact on air quality due to natural emissions along with anthropogenic emissions are important in ascertaining the cost-effective control strategies for polluted air basins. The estimates show that 90% of global nitrogen oxide emission and 60% of world's S02 emission are naturally produced.

A. Natural Emissions There are many natural biogenic and geogenic sources of primary pollutants known. A wide variety of parameters such as temperature, sunlight etc. can alter the emission rates. This gives large uncertainty in estimates of their emission rates. These emissions are significant in comparison to anthropogenic emissions on global scale. They generally do not play role in the atmospheric chemistry of polluted urban areas. Important primary pollutants and the sources of the natural emission are summarized below:

87

AIr POl/lII1017

(i) Sulphur Compounds. Sulphur compounds are emitted to air from volcanic eruptions, sea spray. and biogenic processes. Volcanic Eruptions: Mostly S02; H2S and CHJ-S-CH) (dimethyl sulphide) in variable amounts. Sea Spray: SO/- (some of which carried over to land masses) Biogenic: Mainly reduced form of sulphur including 1-1 2S, CH J-S-CH 3, lesser amount of CS} (carbon disulphide), CI-I.-S-S-CH, (dimethyl disulphide), COS (carbonyl sulphide) and CH.SH (methylmercaptan). These'reduced sulpiiur compounds are then oxidised in atmosphere. (ii) Particles: Natural sources of particles are mainly terrestial dust caused by winds, sea spray, biogenic emissions, volcanic eruptions and wind fires. In anthropogenic particulate emissions, particle size is important. The terrestial dust is generally of large size range (~ 2 ~lIn diameter) and is primarily composed of crustal elements, including Si, AI, Fe, Na, K,Ca, Mg. The bursting of the bubblt:s at the surface of the occan may generate particles anJ somc of these carried to land. The particle size of these are normally ~ 3 ~1I11. They a..e mainly cOl1lposed of elements found in sea water such as CI, Na, SO/-, Mg, K. Ca etc. and some organic compoun"ds, viruses. bacteria etc. The particle size coming out of volcanic eruption varies over a long range 0.0 1-5 ~un or even up to 66 ~lm. The composition is also vary and not fixed. Wildfires also produces particulate matter and it is of respirable size (0.1 to I ~lIn). Elemental carbon and other organics are formed along with some minerals also being present in these particles. (iii) Nitrogen Oxides (NO,): The major natural sources of emission for NO, are lightning, stratoscopic injection, ammonia oxidation, burning of biomass including wild fires and soil emissions. The uncertainty in emission rates are there and dependent of many factors such as light, temperature, meteorology etc. (iv) Non-methane hydrocarbons (NHMC). The major source for non-methane hydrocarbons such as isoprene and a-pinene is the biogenic processes. The magnitude and chemical composition of these emission varies largely. (v) Carbon monoxide (CO). The range of global CO emission from natural sources are ; CH 4 oxidation (60-5000 metric tons), oxidation of natural hydrocarbons (50-1300 metric tons), microbial activity in ocean (20-200 metric tons) and emission from plants (20-200 metric tons). The major sources of CO emission naturally are mainly oxidation of methane (CH 4) and other hydrocarbons in troposphere. Direct emission fi'om plants and microbial activity in ocean also contribute for CO emission in air. j

B. Anthropogenic Emissions (i) Sulphur Compounds. The most important pollutant in this category is SO,. The major ~ource of SO, is the combustion (burning) of sulphur-containing fuels. Knowing the amollllt -of sulphur and rate of fuei consumption, the emission rate can be found out. 1 The sulphur containing fuels and other sources emitting SO, (in kg per 10 kg of fuel burnt) are Hard Coal (48.2 kg). Lignite (35.6 kg), Coal coke (5.4 kg), Petro leu 111- refining (2.0 kg), Residual fuel oil (36.0 kg), Petroleum Coke (13.5 kg), Copper Smelting (2000 kg), Copper refining (350 kg), Lead produced (470 kg), Zinc produced (200 kg), Sulphuric acid (24 kg) and pulp/paper (2 kg). On global scale about 60% of the Sal produced is from coal combustion and 30% fi'om petroleum refining. (ii) Particles. Air quality parameters for particulate matter is expressed in terms of lolal slISpended parliclliale, TSP i.e. non-size fractionated particles. However, size of particle is important to assess the effect of particulate matter on health. lflarger particles inhaled, they are removed either in the head or upper respiratory tract. The respiratory system starts from the nose and connected through the tracheobronchial region which is covered with a layer of mucus. This is continuously moved upward by the motion of small hair-like projections called cilia; larger patticles deposit on the mucus, are moved up, and are ultimately swallowed. However, smaller particles of respirable size range (~ 2.5 ~lIn diameter) fi'om combustion of fossil fuels and gas-to-particle conversion reach the alveolar region where gas exchange occurs. This place

Envirunmental ChemistlY

88

is not coated with the protective mucus layer, hence the deposited particles are not cleared easily. This may cause problems like bronchitis, asthama etc. Small particles usually contain more toxic substances, therefore, they contribute more to health effects that one might employ ti'om their mass. e.g., volume of spherical pal'~icle is proportional to 1" (radius of particle), and assuming equal densities, the mass also increases with r'. A particle of diameter :2 ~lln thus weigh 0.1 % of 20 ~lln diameter particle, i.e., 1000 of the respirable small particles equal the mass of one of the large particles. The fine particles are also important from the point of view of light scattering i.e. visibility in area. Thus, an improvement in visibility impacted by air pollution is likely to be accompanied by a reduction in total particle deposition in alvelor region of the respiratory system. Most of the anthropogenic emissions of particles occur from (i) stationary fuel combustion, (ii) industrial processes, (Iii) solid waste disposal, (iv) transportation (v) other emissions (forest fire, agricultural burning, coal refuse burning) and (vi) industrial fugitive emissions including wind erosion, vehicular traffic, material handling, loading and unloading operations etc.; (vii) non-industrial fugitive emissions including unpaved and paved roads, constructi.on activities, agricultural operations, surface mining and fires. (iii) Nitrogen Oxides (NO.). High temperature combustion processes produces NO from the reaction of N2 and 02' N2 + 02 -~ 2NO This may also be produced from oxidation ofN, in the fuel. Small amounts of NO, are also produced by further oxidation of NO. HNO, may also be formed. The amount of NO, depends upon conditions under which combustion process is carried out. The two major source of NO, emissions are fossil fuel combustion (69 metric tons expressed as NO,) and biomass burning (39 metric tons) including land clearance burning and forest wild fires burning. (iv) Non-methane hydrocarbon (NHMC). Transportation and associated petroleum retining, oil, and gas production accounts for more than SO% of N HMC. Use of organic solvents in industries accounts for another 2S% of total emissions. (1') Carbon-monoxide (CO). Carbon monoxide is produced due to incomplete combustion of fossil fuels in the urban area. The automobile exahust is another major source of CO emission. In urban area 7S% of CO emission are estimated ti'om motor vehicles, 8% ti'om rail roads, aircrafts, etc., 9% from industrial processes including petroleum refining, 7.S% from solid waste combustion, O.S% ti'om electric power generation, residential and industrial fuel consumption. (vi) Lead (Pb): Gasoline combustion in molar vehicles was the greatest source' of lead emission. Lead which was used as antiknocking agent as TEL (Tetra ethyl lead) got emitted in the form of particles composed of Pb Br CI (the bromine and chlorine comes from ethylene bromide and ethylene dichloride in the fuel) and smaller amounts being emitted in the form of organolead compounds. About 33% of the mass is in very small sizes of particles (diameter < O.S ~1l11) which remains suspended for longer periods of time. About SO% is in much larger size (> S pm) which falls close to source. Nowadays lead ti'ee fuel is available, which uses BTX (Benzene-Toluene-Xylene) mixture as anti-knocking agent.

5.2.3 Mcterology Meterological parameters are extremely important in determining the dispersion and transpOit of pollutants along with studing their chemistry. The impacts of pollutan~ is therefore to a very large extent depend upon meterology. e.g during London-smog, meterological condi\ions were such that pollutants were effectively contained in small volume leading to high pollutant concentration. For details see chapter 3.

5.2.4 Chemical Trallsformations The atmospheric chemical transformations is the most important step of environmental pollution system. The chemistry of air pollution needs special emphasis to understand its functioning and elimination. The chemistry in troposphere is more concerned to pollution chemistry. Primary pollutant, NO, reacts in presence of NMHC and other organics (NMOC, non-methane organic compoullds) and sunlight to produce secondary pollutants. These include photooxidants such as 0" PAN, HNO" HCHO etc. • .'

89

Air Pol/lIlion Solid line NOz Dotted Line NO Dashed Line oxidant

,

0.44

II I I I I I I I I I I

0.40 E

2: 0.36

I

.5

I

I

I

c:

~ 0.32

I

I II

I " I

g 0.28

I I

I

I

g o u 0.24

I

I

I I

I I

I I

I

I

I I

I

0.20

I

I II

I " I

I

I

0.16

I

I I I

I I

0.12

I

I

I

I •

0.08 0.04

•

•

\

• •

.

.................. : ---------------------------, 3 AM

,r..

#

6 AM

,,

I

V

.. ..

...

I

I

.

••••

9 AM

......................

12 NOON

3 PM

• •••••,

'

6 PM

••••• .....

....... _----_ ..... 9 PM

Time

Fig. 5.2: Variation of NO, NO, and total oxidant during the day time

NMOC + NO, + hv -~ 0,.' + PAN + HCHO +..... Some important features of time-concentration curves (Figure 5.2) for NO, NO, and total oxidants (such as 0,) in ambient air are: (i) In early morning, the concentration of NO rises and i'eaches a maxima at the time "of peak automobile traffic time; (ii) thereafter rise in concentration of NO, starts and reaches maximum; and (iii) oxidant such as 0, level, whose concentration is low in the earli morning, increases about noon when NO concentration drops to a low level. The maxima of 03 curve appears after maxima of NO}. The maxima for total oxidant may shift to afternoons or evening depending upon meteorological factors such as winds. The chemistry responsible for these changes can be understood in following way. It was suggested by Blacet in 1952, the photolysis of N0 2 produces ozone as shown by equation. N0 2 + hv (A :: 430 nm) -~ NO +

°

0+0, ~ 0, -

.'

NO emitted from automobile exhaust reacts with 03' reproducing NO} consumed in the formation of 0 ...' NO + 03 -~ NO} + O} Because of the above reaction NO and 0, cannot exist together in high concentration as they combine to produce NO,. This explains the nature of cu'~ves shown in Figure 5.2. The 0 1 cannot show its peak until NO's concentnition has fallen significantly. . There are three basic questions regarding the photochemical air pollution. They are: I. How NO is oxidised to N0 2 ? 2. What role organic compounds play in photochemical air pollution? 3. What are those reactions which consumes organic compounds from air? The answer to first question was initially suggested in terms of thermal oxidation reaction. 2NO + 02 -~ 2N0 2

Environmental Chemistry

90

The above reaction is well demonstrated at laboratory scale when colourless NO is instaneously oxiJised to NO, (brown) in air. From kinetic studies it is found that the above reaction is of second order which means rate of reaction increases as the square of the NO concentration. If one lower the concentration of NO to ppm level, then rate of reaction drops to the point where the rate becomes almost zero. Therefore, possibility of thermal oxidation of NO is not important in the atmosphere. However, if during a pollution episode the concentration of NO is high then thermal oxidation can take place. From above discussion, it is evident that in ambient photochemical smog formation, thermal oxidation of NO could not explain the relative rapid conversion of NO to N0 2• It can only be explained on the basis of photolysis. In order to have an understanding for second and third questions regarding fate of organic compounds, it is found that observed rate of organic compound (e.g. propylene) loss is based upon two reactions. 0 3 + C/1 6 -~ Products o + C3H6 -~ Products However, a large fraction of hydrocarbon loss cannot be accounted by considering only above two reactions, rather they must be involving free radical reactions of alkyl (R), alkyl peroxy (ROi), alkoxy (RO'), hydroxyl, (OR), hydroperoxyl (HOi), and hydrogen (H). These reaction may also involve NO oxidation. The chain reaction involving hydroxyl radicals and hydroperoxyl radical Catl also convert NO to NOr OH'+CO

~

H+C0 2

H+02 ~ HOi H0 2 + NO ~ 6f:i"':+ ....- NO,'~-

Organic compounds can play similar role that is played by CO for the conversion of NO to NO, (See 4.6). Thus, OR" initiate the chain reactions by attack on organic compounds, these reaction are propagated as shown in Figure 5.3.

CHO CH'F

------A--------

CH 3CH 2CHzO ·• ..

NO z NO Fig. 5.3: Oxidation of Propane initiated by OH'

5.2.5 Ambient Air : Air Quality Standards Meterological effects and chemical as well as physical transformation results in the formation of ambient ail: The criteria and non-criteria pollutants are to be indicated based upon their concentrations. The major pollutants are criteria pollutants, however, trace pollutants are non-criteria pollutants. Table 5.1, list some of the common criteria and non-criteria pollutants and their ranges of concentration normally acceptable for unperturbed life. Table 5.1: Criteria and Non-criterill pollutants with ranges of concentration S.No.

Name of Pol/lltallt

Range

0/ eoncentrat iun

Criteria Pol/lltants CO

~

0.2 - 50 pplll

S02 03

~

I ppb - 2pplll

N0 2

~

Non-methane hydrocarbons (NMHC)

65 ppb - 1.5 pplll

Total suspended Particles (TSP)

5 - 1500 I-Igm- 3 I x 10-4 - 10 ~Ig m- 3

1. 2. 3. 4. 5. 6.

7.

Lead

0.0 I - 0.5 pplll I ppb - 0.5 pplll

91

Air Po/hilton I

SNo.

Name of Pol/utant

Range of concentratUJ/1

Non-Criteria Pol/wants

I. 2. 3. 4. 5. 6. 7. 8. 9. 10.

NO

0.02 - 2

x

10] ppb

Peroxyacetylnitrate (PAN)

0.05 - 70 ppb

HN0 3 HN0 2

< 0.1 - 50 ppb

NO; (nitrate radical)

< 5 - 8 ppb

NP5 (dinitrogen pentaoxide) NH] HCHO

:::: 15ppb < 0.02 - 100 ppb < 0.5 - 75 ppb

HCOOH

< 20 ppb

CHjOH

:::: 40 ppb

< 0.Q3 - 8 ppb

5.2.6 Visibility High pollution level is usually accompanied with poor visibility. It is caused due to scattering and absorption of light by pollutants. The sum of scattering and absorption by gases and particles is the total light extinction, which is dependent upon both the wavelength of the light and scattering angle. This means, it depend upon position of the sun, so that "haze" due to air pollution may appear to have different colours depending upon the conditions. As NO? absorbs visible light, it can therefore, causes considerable reduction in visibility. As A < 430 nm, hence it acts as a tilter for blue portion of visible light. The brownish colour sky of polluted urban areas at the time of sunset is partly due to presence of NO, in air. Scattering due to air pollutants is negligible compared to Rayleigh scattering by N? and 0, as they are the major components of air. Particulate matter suspended in air scatter and absorbs light to a greater extent than gaseous pollutants. Small particles (0.0 I - I ~lIn) diameter contributes to greatest amount of light scattering due to pollutants. Elemental carbon is the only significant light absorbing species in particles. The scattering particles are normally nitrates and sulphates. With increase in relative humidity (H,O particles) also scatters light therefore reduces visibility. 5.2.7 Models There has been special emphasis on the development of models which can predict ambient pollution concentration at specitic location and time in recent years. If one knows abOUt sources, emission rates, chemical transformation reactions involved in conversion of primary pollutants to secondary pollutants meteorology and tropography. One can construct a model. Three types of models which have been developed and most popular. They are (i) plume models (ii) airshed models (iii) long range transport models (LRT). (i) Plume Models: This model attempts to stimulate the dispersion and chemical transformations of a plume down wind from a single point source or cluster group of point sources such as thermal power plant and industrial complex. (ii) Airshed Models: Airshed models treats the etrect of a diverse set of stationary and mobile sources scattered throughout the given geographical area with dimensions not less than 100 km. From this Illodel prediction are made for various pollutant concentration within the air basin and down-wind. Airshed models are best used at the present ,time to predict relative changes in pollutant concentrations expected -trom given changes in emissiol1S'. (iii) Long-Range Transport Models: These models are quite similar to airshcd models with one major ditlcrence of great..::r geographical area being covered here which is not le~s than I(JOO km. These models are used in a~sessing the acid deposition problem. These may also be used for predicting efrects of various controls strategies on reducing the impact. For all these models to provide information, three basic informations are needed regarding emissions, meterology, and chemical transformations. The computational task of simulation becomes difficult due to

92

Enl'ironl11f!nla/ ('hf!misllJ'

enormous variations in these three basic components. The simplification of these computation are normally done by ignoring the intermediate steps involved in diffellent processes. However, approximations and ignoring of steps is being done on the basis of some rationale, and not otherwise.

5.2.8 Monitoring There are several methods of identifying air pollution with their merits and demerits. The common ones are: (i) Sensory recognition (ii) Physical measurement of pollutant (iii) Effects on plants, animals and materials. (i) Sensory Recognition: Normally first awareness of an air pollution problem is through some effects on the individual. These are: (a) Strong and Unusual odour; (b) Reduction in visibility; (c) Eye irritation; (d) Acid taste (sour) in mouth; (e) Feel of grit under foot. (ii) Physical Measurement. Although sensory recognition provide the first indication of air pollution, it is often not possible to detect trace quantities of many air-borne toxic substances or the presence of radioactive matter through the senses. It is therefore, requil1e to analysis the sample air qualitatively and quantitatively. Physical measurements by standard methods of sampling and analysis will be taken up separately. (iii) Effects on plnnts, animnls and materials. Effect of air pollutants can be observed on the growth of plants, health of animals and deleterious effect on materials. This means plants, animals and materials can act as indicator of some specific air pollutants.

5.3 Types of Air Pollutants There are two types of air pollutants, primary pollllltants such as CO, NO" hydrocarbons, SO" particultes and secondary pollutants such as H2S0~, S04 2 -, HN0 3 , N0 3-, photochemical oxidants (03' PAN, H,O" free radicals), others (HCHO, HNO" organic particles). However, for the sake of simplicity these pollutants have been grouped as gaseous inorganic air pollutants, particulate matter, organic air pollutants and smog. These will be dealt separately. Some special topics such as acid rain, ozone layer destruction and pollution due to automobile exhaust and smog formation will be given special attention.

5.4 Classification of Ail' Pollution Man's activities has caused substances to be released into the atmosphere, however, nature of these releases kept changing with time. Measures to control the impact of these emissions have been built up in individual areas mainly in terms of preventing all ill-effects in locality of each source of emission. However, there are certain pollutants transported fast and show ill-effects on worldwide basis. Keeping this in view, air pollution can be classified as being global, regional (upto - 1000 km fi'om source) or local (up to about ) 00 km from the source). A. Global Air Pollution Carbon dioxide normal balance set up by nature is being gradually affected by increasing quantities of CO, emitted during burning of fossil fuels and deforestation pr..ide consumes OH : CO + OH -~ H + CO 2 H + 02 + M -~ H0 2 + M \

The net reaction is

HO l + 0]

-~

OH + 202

CO + 0,.,

~

COl +

°

1

Therefore, complete oxidation of one methane molecule in the NO-poor atmosphere produces one carbon dioxide molecule and leads to the losses of about 3.5 HO molecules and 0.7 ozone molecules. This scenario in NO-poor atmosphere can be modified depending, in' part, on atmospheric hydroxyl concentrations, pathways of CH,OlH (a catalytic intermediate) production and removal by precipitation scavenging, and atmospheric carbon monoxide concentrations. It is important to reiterate that hydroxyl is the major sink for both methane and carbon monoxide in the atmosphere, just as methane and carbon monoxide are the major sink for hydroxyl. Therefore, perturbations to anyone of the three species leads to changes in the background levels of the other two.

7.6 Atmosphel"ic Reactions of Alkenes 7.6.1. Reactions with OR' radical An electron deticient free radical, OH adds to the double bond to form a radical adduct:

OH+ -::C=C(

-~

For long chain alkenes containing weak secondary and tertiary allylic C-H bonds, hydrogen atom abstraction competes with addition. This is because, the bond dissociation energy tor allylic C-H (480 kJ mor l ) is less compared to most of the other C-H bonds (90-100 kJ mol-I).

131

O'gGnic Air Pullutants H

I I

CH)-C-CI-I = CH, + 01-1"

-

-~

H

/

Allylic hydrogen However, the abstraction is negligible in comparison to addition therefore can be ignored at ambient tropospheric pressure and temperatures. The addition of OJ-I" to the alkene is exothermic (32 kcal mole-I) so that the aduct contains excess energy when initially formed. This energized adduct may decompose back to reactants, be stabilized by collision with another molecule, or, for the larger alkenes, fragment into smaller species. The decomposition back to reactants can predominate for the smaller alkenes, so that the addition reaction is not observed. Competing pathways such as abstraction are then relatively more important. For asymmetrical alkenes such as propene, addition of OR" may occur at either end of the double bond, giving rise to different radical adducts: OH· + CHFH

= CH 2

~

CH 3CH-CHPH

H

I

.

CH3