Paper No: 8 Atmospheric Processes Module: 12 Atmospheric Chemistry

Development Team

Principal Investigator Prof. R.K. Kohli & Prof. V.K. Garg &Prof. Ashok Dhawan Co- Principal Investigator Central University of Punjab, Bathinda

Dr. Sunayan Saha Paper Coordinator Scientist, ICAR-National Institute of Abiotic Stress Management, Baramati, Pune Dr. Puneeta Pandey Content Writer Central University of Punjab, Bhatinda Dr. Amrita Daripa Content Reviewer ICAR-National Bureau of Soil Survey and Land Use Planning,

Nagpur, Maharashtra

1 Anchor Institute Central University of Punjab

Atmospheric Processes Environmental Sciences Atmospheric Chemistry

Description of Module

Subject Name Environmental Sciences

Paper Name Atmospheric Processes

Module Name/Title Atmospheric Chemistry

Module Id EVS/AP-VIII/12

Pre-requisites Basic knowledge of elementary physics and chemistry

 Understand the structure of the atmosphere and its thermal stratification  Know the chemical composition of the atmosphere Objectives  Know the thermo-chemical and photochemical reactions occurring in the atmosphere  Understand the mechanism of formation of smog, its reactions and effects Keywords Atmosphere, Electromagnetic spectrum, Thermochemical, Photochemical, Smog

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TABLE OF CONTENTS 1. Aim of the Module 2. Introduction 3. Chemical composition of the earth atmosphere

3.1 Nitrogen (N2)

3.2 Oxygen (O2) 3.3 Argon (Ar)

3.4 Carbon Dioxide (CO2) 3.5 Trace Elements 4. Thermochemical reactions in atmosphere 4.1 Electromagnetic spectrum 4.1.1 Gamma rays 4.1.2 X-rays 4.1.3 Ultra-Violet light 4.1.4 Visible light 4.1.5 Infrared light 4.1.6 Microwaves 4.1.7 Radiowaves 4.2 Reactions taking place in earth’s atmosphere 4.2.1 Troposphere 4.2.2 Stratosphere 4.2.3 Mesosphere 4.2.4 Thermosphere 5. Photochemical reactions in atmosphere 5.1 Smog formation 5.1.1 Hydrocarbons and photochemical smog 5.1.2 Photochemical reactions of methane 5.1.3 Mechanism of smog formation 5.1.4 Nitrate radical 5.1.5 Photolyzable compounds in the atmosphere 5.1.6 Inorganic products from smog 5.2 Effects of smog 5.2.1 Human health and comfort 5.2.2 Damage to materials 5.2.3 Effects on the atmosphere 5.2.4 Toxicity to plants

6. Summary

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1. Aim of the Module

After going through this module, you shall be able to:  Understand the structure of the atmosphere and its thermal stratification  Know the chemical composition of the atmosphere  Know the thermo-chemical and photochemical reactions occurring in the atmosphere  Understand the mechanism of formation of smog, its reactions and effects 2. Introduction

The earth’s atmosphere is a mixture of gases and aerosols; some of which may be in rather fixed proportions throughout the atmosphere; while, some may vary in proportion depending on altitude and region. The layer of the atmosphere in which its gaseous composition is generally uniform (does not change with altitude) is called as ‘homosphere’ and extends up to an altitude of 80 km. At altitudes beyond 80 km, the chemical constituents of air change significantly with height and hence that layer is known as ‘heterosphere’.

Nitrogen (N2) by far is the most abundant of all gases present in earth's atmosphere. About 3.5

times less than that of N2 is the quantum of oxygen (O2) gas in atmosphere and these two gases comprise about 99 % of dry air volume. Within the rest about 1 %, many different gases and non

gaseous constituents are accommodated and some of which like CH4 and CO2 are known to produce huge influence on our planet through chain of events starting with the rise of temperature. The nature of chemical interaction among these atmospheric constituents under the ambience of solar light and or earth radiation in different layers of the atmosphere is what comprises the subject matter of this “atmospheric chemistry” module.

3. Chemical composition of the earth atmosphere

Principle gases composing the earth’s atmosphere is given in Table 1 followed by their brief description.

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Table 1: Gaseous composition of earth’s atmosphere (Battan, 1984)

S. No. Constituents Percent by volume of Concentration dry air (ppm) of air

1 Nitrogen 78.08 780800

2 Oxygen 20.94 209400

3 Argon 0.934 9340

4 Neon 0.00182 18.2

5 Helium 0.000524 5.24

6 Methane 0.00015 1.5

7 Krypton 0.000114 1.14

8 Hydrogen 0.00005 0.5

9 Important Variable gases

A Water vapour 0-5

B Carbon Dioxide (CO2) 0.034 340

C Carbon Monoxide --- <100 (CO)

D Sulfur Dioxide (SO2) --- 0-1

E Nitrogen Dioxide --- 0-0.2

(NO2)

F Ozone (O3) --- 0-10

3.1. Nitrogen (N2)

Nitrogen gas constitutes 78% by volume of atmosphere. Nitrogen in its native form is inert; however, it can be converted into nitrites and nitrates during nitrogen cycling. Ammonification and denitrification during this cycle converts these nitrogenous compounds back to ammonia and then

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nitrogen; which eventually reaches back to atmosphere. The residence time of nitrogen in atmosphere is 100 million years.

3.2. Oxygen (O2)

It occupies 21% by volume of atmosphere. It plays a major role in respiration and combustion.

The turnover time of CO2 in the atmosphere is 3000 years.

3.3. Argon (Ar)

It is an inert gas that occupies about 1% by volume of atmosphere.

3.4. Carbon Dioxide (CO2)

This gas occupies 0.038% (376 ppm) of the atmosphere. The concentration of this gas has been rising since the last century due to enhanced anthropogenic activities. It is also a potent greenhouse gas

since it traps long-wave solar radiations. The turnover time of O2 in the atmosphere is 4 years.

3.5. Trace Elements

This includes various elements and compounds in varying proportions such as Water Vapour

(H2O), Sulfur Dioxide (SO2), Nitrogen Dioxide (NO2), Carbon Monoxide (CO), Ozone (O3),

Chlorofluorocarbons (CFCs), Methane (CH4), dust and other particulate matter. Water vapour is also a potent greenhouse gas, the constitution of which varies in the atmosphere with a residence time of 11

days. SO2 and NO2 are by-products of fossil fuel combustion and automobile exhausts. They can lead to formation of nitric acid and sulfuric acid in the atmosphere, thus contributing to ‘acid rain’. Ozone gas lies in highest concentration in stratosphere and plays the vital role of absorbing ultraviolet radiations; in troposphere, it acts as a secondary pollutant causing respiratory ailments and eye irritations. CFCs cause depletion of the ozone layer in the stratosphere; hence, it is of prime

environmental concern. CH4 is another significant greenhouse gas which is even more potent than

CO2. Dust and other particulate matter are airborne solids suspended in air; also known as ‘Aerosols’. Aerosols play an important role in scattering and reflecting solar radiations, thus affecting the albedo. Major sources include sea salt from evaporated sea spray, wind-blown dust, debris from volcanoes and 6

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fires, and from anthropogenic sources. Particulates can also act as cloud condensation nuclei (CCN) onto which water vapor condenses, thus affecting the rainfall pattern.

4. Thermochemical reactions in atmosphere

4.1. Electromagnetic spectrum

The earth’s radiation budget is regulated by regular input of solar energy. At a temperature of about 6000K, the sun radiates an enormous amount of energy, but the earth intercepts only about 5x10- 10 of it since the earth subtends a small angle when viewed from the sun. The average flux of the solar radiation at outer limit of earth’s atmosphere, falling on a surface perpendicular to the incoming rays, is called solar constant.

All electromagnetic waves travel at the speed of light i.e. 300,000,000 metres per second. Various spectra of electromagnetic radiations are described below:

4.1.1. Gamma rays

Gamma rays are high frequency waves, carrying a large amount of energy given off by stars, and some radioactive substances. Gamma rays are used in ‘Radiotherapy’ to kill cancer cells.

Figure 1: Electromagnetic Spectrum (www.earthobservatory.nasa.gov)

4.1.2. X-rays

X-rays are very high frequency waves, and carry a lot of energy. They are useful in medicine and industry to see inside things since they can pass through most substances.

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4.1.3. Ultra-Violet light

It is emitted by sun and also made by special lamps. UV light is used in sterilizing microbial contamination in surgical equipments, operation theatres and laboratories; detecting forged bank notes in shops, and hardening some types of dental filling.

4.1.4. Visible light

Visible light lies in the range of 0.4-0.7µm and includes radiations that can be perceived by human eye. White light is made up of various wavelengths ranging from violet to red (the colours of rainbow).

4.1.5. Infrared light

Infra-red waves are released as heat, because they're given off by hot objects, and can be felt as warmth. Infra-Red waves are also given off by stars, lamps, flames and anything else that's warm, i.e., which has temperature above absolute zero (273K). Infra-red waves are used for remote controls for TVs and video recorders, to help heal sports injuries by physiotherapists, in burglar alarm systems, and for weather forecasting by infrared satellite data.

4.1.6. Microwaves

They are basically extremely high frequency radio waves with wavelength ranging from 1mm to 1m. They find application in cooking, mobile phones, traffic speed cameras, and for radar, which is used by aircraft, ships and in weather forecasting.

4.1.7. Radiowaves

These are the lowest frequencies in the electromagnetic spectrum, and are used mainly for communications. They are divided into- Long Wave (around 1~2 km in wavelength); Medium Wave (around 100m in wavelength) and VHF, which stands for "Very High Frequency" and has wavelengths of around 2m; used in radio stations, civilian aircraft and taxis. UHF stands for "Ultra High Frequency", and has wavelengths of less than a meter. It's used for Police radio communications,

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military aircraft radios and television transmissions. Large doses of radio waves are believed to cause cancer, leukaemia and other disorders.

4.2. Reactions taking place in earth’s atmosphere

4.2.1. Troposphere

Weather related phenomenon occurs in troposphere.

4.2.2. Stratosphere

Reactions occurring in stratosphere are summarized below:

N2O + hν N2+O

N2O +O NO+NO

N2O +O3 NO2+O2

H2+O OH+H

CH4+O OH+CH3

4.2.3. Mesosphere

In mesosphere, the following reactions take place:

H2O+hν n OH + H

H2+O OH+H

4.2.4. Thermosphere

This layer is characterized by the following reactions:

N+O2 NO+O

N+NO N2 +O

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5. Photochemical reactions in Atmosphere

The most significant feature of atmospheric chemistry is the occurrence of photochemical reactions resulting from the absorption by molecules of light photons, designated hν. (The energy, E, of a photon of visible or ultraviolet light is given by the equation, E = hν, where h is Planck’s constant and ν is the frequency of light, which is inversely proportional to its wavelength. Ultraviolet radiation has a higher frequency than visible light and is, therefore, more energetic and more likely to break chemical bonds in molecules that absorb it. One of the most significant photochemical reactions is the

one responsible for the presence of ozone in the stratosphere, which is initiated when O2 absorbs highly energetic ultraviolet radiation in the wavelength ranges of 135-176 nanometers (nm) and 240- 260 nm in the stratosphere:

O2 + hν O + O

The oxygen atoms produced by the photochemical dissociation of O2 react with oxygen

molecules to produce ozone, O3,

O + O2 + M O3 + M

Where, M is a third body, such as a molecule of N2, which absorbs excess energy from the reaction. The ozone that is formed is very effective in absorbing ultraviolet radiation in the 220-330 nm wavelength range, which causes the temperature increase observed in the stratosphere. The ozone serves as a very valuable filter to remove ultraviolet radiation from the sun’s rays. If this radiation reached the earth’s surface, it would cause skin cancer and other damage to living organisms.

5.1. Smog formation

Smog is recognized as a major air pollution problem in many areas of the world in the presence of ultraviolet light, hydrocarbons, and nitrogen oxides. However, originally, this term was used to describe the unpleasant combination of smoke and fog together with sulfur dioxide in the city of London. The term ‘smog’ is generally used to denote a photochemically oxidizing atmosphere. Since sulfur dioxide is a reducing compound; therefore, such smog is a reducing smog or sulfurous smog.

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Further, automobiles are a major source of hydrocarbons and nitrogen oxides, the principle components that help in smog formation.

5.1.1. Hydrocarbons and photochemical smog

The most abundant hydrocarbon in the atmosphere is methane, CH4, released from underground sources as natural gas and produced by the fermentation of organic matter. Methane is one of the least reactive atmospheric hydrocarbons and is produced by diffuse sources, so that its participation in the formation of pollutant photochemical reaction products is minimal. The most significant atmospheric pollutant hydrocarbons are the reactive ones produced as automobile exhaust emissions. In the presence of NO, under conditions of temperature inversion, low humidity, and sunlight, these hydrocarbons produce undesirable photochemical smog manifested by the presence of particulate matter, ozone, and aldehydes.

5.1.2. Photochemical Reactions of Methane

Some of the major reactions involved in the oxidation of atmospheric hydrocarbons may be understood by considering the oxidation of methane.

Like other hydrocarbons, methane reacts with oxygen atoms (generally produced by the

photochemical dissociation of NO2 to O and NO) to generate the all-important hydroxyl radical and an alkyl (methyl) radical. Despite its low reactivity, methane is so abundant in the atmosphere that it accounts for a significant fraction of total hydroxyl radical reactions.

CH4 + O H3C• + HO• (1)

The methyl radical produced reacts rapidly with molecular oxygen to form very reactive peroxyl radicals,

H3C• + O2 + M (energy-absorbing third body, usually N2 or O2) H3COO• + M (2)

in this case, methyl peroxyl radical, H3COO•. Such radicals participate in a variety of subsequent chain reactions, including those leading to smog formation.

The hydroxyl radical reacts rapidly with hydrocarbons to form reactive hydrocarbon radicals, 11

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CH4 + HO• H3C• + H2O (3)

in this case, the methyl radical, H3C•.

The following are more reactions involved in the overall oxidation of methane:

H3COO• + NO H3CO• + NO2 (4)

This is an important reaction in smog formation because the oxidation of NO by peroxyl radicals is the

predominant means of regenerating NO2 in the atmosphere after it has been photochemically dissociated to NO.

H3CO• + O3 various products (5)

H3CO• + O2 CH2O + HOO• (6)

H3COO• + NO2 + M CH3OONO2 + M (7)

H2CO + hν photodissociation products (8)

Hydroxyl radical, HO•, and hydroperoxyl radical, HOO•, are ubiquitous intermediates in photochemical chain-reaction processes. These two species are known collectively as odd hydrogen radicals. Reactions such as (1) and (3) are abstraction reactions involving the removal of an atom, usually hydrogen, by reaction with an active species. Addition reactions of organic compounds are also common. Typically, hydroxyl radical reacts with an alkene such as propylene to form another reactive free radical. Organic free radicals undergo a number of chemical reactions.

The hydroxyl radical may react with other organic compounds, maintaining the chain reaction. Gas-phase reaction chains commonly have many steps. Furthermore, chain-branching reactions take place in which a free radical reacts with an excited molecule causing it to produce two new radicals. Chain termination may occur in several ways, including reaction of two free radicals,

2HO• H2O2 (9)

adduct formation with nitric oxide or nitrogen dioxide (which, because of their odd numbers of electrons, are themselves stable free radicals),

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HO• + NO2 + M HNO3 + M or reaction of radical with a solid particle surface. (10)

Hydrocarbons may undergo heterogeneous reactions on particles in the atmosphere. Dusts composed of metal oxides or charcoal have a catalytic effect upon the oxidation of organic compounds. Metal oxides may enter into photochemical reactions. For example, zinc oxide photosensitized by exposure to light promotes oxidation of organic compounds.

5.1.3. Mechanism of Smog Formation

In atmospheres that receive hydrocarbon and NO pollution accompanied by intense sunlight and stagnant air masses, oxidants tend to form. Photochemical oxidant is a substance in the atmosphere capable of oxidizing iodide ion to elemental iodine. The primary oxidant in the

atmosphere is ozone. Other atmospheric oxidants include H2O2, organic peroxides (ROOR'), organic hydroperoxides (ROOH), and peroxyacyl nitrates such as peroxyacetyl nitrate (PAN).

Nitrogen dioxide, NO2, is 15% as efficient as O3 in oxidizing iodide to iodine (0). Peroxyacetyl nitrate and related compounds containing the -C(O)OONO2 moiety, such as peroxybenzoyl nitrate (PBN), are produced photochemically in atmospheres containing alkenes and NOx. As shown in

Figure 2, smoggy atmospheres show characteristic variations with time of day in levels of NO, NO2, hydrocarbons, aldehydes and oxidants.

Figure 2: Generalized plot of atmospheric concentrations of species involved in smog formation as a function of time of day (Manahan, 2000)

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Examination of the figure shows that shortly after sunrise, the level of NO in the atmosphere

decreases while that of NO2 rises. During midday, the levels of aldehydes and oxidants become relatively high. The concentration of total hydrocarbons in the atmosphere peaks sharply in the morning, then decreases during the remaining daylight hours. An overview of the processes responsible for this behavior is summarized in Figure 3.

However, certain facts such as the rapid increase in NO2 concentration and decrease in NO

concentration under photodissociation conditions of NO2 to O and NO could not be explained. Further, disappearance of alkenes and other hydrocarbons was much more rapid than could be explained by

their relatively slow reactions with O3 and O. These anomalies are explained by chain reactions

involving the inter-conversion of NO and NO2, the oxidation of hydrocarbons, and the generation of reactive intermediates, particularly hydroxyl radical (HO•). Figure 6 shows the overall reaction scheme for smog formation, which is based upon the photochemically initiated reactions that occur in an atmosphere containing nitrogen oxides, reactive hydrocarbons, and oxygen.

Figure 3: Generalized scheme for the formation of photochemical smog (Manahan, 2000)

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The time variations in the levels of hydrocarbons, ozone, NO and NO2 are explained by the following overall reactions: 1. Primary photochemical reaction producing oxygen atoms: NO2 + hν (λ < 420 nm) NO + O (11) 2. Reactions involving oxygen species (M is an energy-absorbing third body):

O2 + O + M O3 + M (12)

O3 + NO NO2 + O2 (13)

Because the latter reaction is rapid, the concentration of O3 remains low until that of NO falls

to a low value. Automotive emissions of NO tend to keep O3 concentrations low along freeways. 3. Production of organic free radicals from hydrocarbons, RH: O + RH R• + other products (14)

O3 + RH R• + and/or other products (15) (R• is a free radical which may or may not contain oxygen.) 4. Chain propagation, branching, and termination by a variety of reactions such as the following:

NO + ROO• NO2 + and/or other products (16)

NO2 + R• products (for example, PAN) (17) The latter kind of reaction is the most common chain-terminating process in smog because

NO2 is a stable free radical present at high concentrations. Chains may terminate also by reaction of free radicals with NO or by reaction of two R• radicals, although relatively low concentrations of radicals compared to molecular species make the latter uncommon. A large number of specific reactions are involved in the overall scheme for the formation of photochemical smog. The formation of atomic oxygen by a primary photochemical reaction (Reaction 11) leads to several reactions involving oxygen and nitrogen oxide species:

O + O2 + M ---> O3 + M (18)

O + NO + M ---> NO2 + M (19)

O + NO2 ---> NO + O2 (20)

O3 + NO ---> NO2 + O2 (21)

O + NO2 + M ---> NO3 + M (22)

O3 + NO2 ---> NO3 + O2 (23) 15

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There are a number of significant atmospheric reactions involving nitrogen oxides, water, nitrous acid, and nitric acid:

NO3 + NO2 ---> N2O5 (24)

N2O5 ---> NO3 + NO2 (25)

NO3 + NO ---> 2NO2 (26)

N2O5 + H2O ---> 2HNO3 (27) Very reactive HO• radicals can be formed by the reaction of excited atomic oxygen with water,

O* + H2O ---> 2HO• (28) by photodissociation of hydrogen peroxide,

H2O2 + hν (λ < 350 nm) ---> 2HO• (29) or by the photolysis of nitrous acid,

HNO2 + hν ---> HO• + NO (30) Among the inorganic species with which the hydroxyl radical reacts are oxides of nitrogen,

HO• + NO2 ---> HNO3 (31)

HO• + NO + M ---> HNO2 + M (32) and carbon monoxide,

CO + HO• + O2 ---> CO2 + HOO• (33) The last reaction is significant in that it is responsible for the disappearance of much atmospheric CO and because it produces the hydroperoxyl radical HOO•. One of the major inorganic reactions of the hydroperoxyl radical is the oxidation of NO:

HOO• + NO ---> HO• + NO2 (34) For purely inorganic systems, kinetic calculations and experimental measurements cannot

explain the rapid transformation of NO to NO2 that occurs in an atmosphere undergoing

photochemical smog formation and predict that the concentration of NO2 should remain very low.

However, in the presence of reactive hydrocarbons, NO2 accumulates very rapidly beginning with photodissociation. It may be concluded, therefore, that the organic compounds form species which

react with NO directly rather than with NO2.

When alkane hydrocarbons, RH, react with O, O3, or HO• radical,

RH + O + O2 ---> ROO• + HO• (35) 16

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RH + HO• + O2 ---> ROO• + H2O (36) reactive oxygenated organic radicals, ROO•, are produced. Alkenes are much more reactive, undergoing reactions with hydroxyl radical, very rapid Oxidation products. Radical adduct (where R may be one of a number of hydrocarbon moieties or an H atom) with oxygen atoms. These oxidation

products react with O3 to form Primary ozonide. Aromatic hydrocarbons, Ar-H, may also react with O and HO•. Addition reactions of aromatics with HO• are favored. The product of the reaction of benzene with HO• is phenol. Hydroxyl radical (HO•), which reacts with some hydrocarbons at rates that are almost diffusion-controlled, is the predominant reactant in early stages of smog formation.

Significant contributions are made by hydroperoxyl radical (HOO•) and O3 after smog formation is well underway. One of the most important reaction sequences in the smog-formation process begins with the

abstraction by HO• of a hydrogen atom from a hydrocarbon and leads to the oxidation of NO to NO2 as follows:

RH + HO• ---> R• + H2O (37)

The alkyl radical, R•, reacts with O2 to produce a peroxyl radical, ROO•:

R• + O2 ---> ROO• (38)

This strongly oxidizing species very effectively oxidizes NO to NO2,

ROO• + NO ---> RO• + NO2 (39)

This explains the conversion of NO to NO2 in an atmosphere in which the latter is undergoing photodissociation. The alkoxyl radical product, RO•, is not stable as compared to ROO•. In cases where the oxygen atom is attached to a carbon atom that is also bonded to H, a carbonyl compound is likely to be formed. The rapid production of photosensitive carbonyl compounds from alkoxyl radicals is an important stimulant for further atmospheric photochemical reactions. In the absence of extractable hydrogen, cleavage of a radical containing the carbonyl group occurs. Another reaction that can lead to the oxidation of NO is of the following type:

RCOOO• + NO + O2---> ROO• + NO2 + CO2 (40) Peroxyacyl nitrates (PAN) are highly significant air pollutants formed by an addition reaction with

NO2:

RCOOOO• + NO2 --- > RCOONO2 (41) 17

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When R is the methyl group, the product is peroxyacetyl nitrate. Although it is thermally unstable, peroxyacetyl nitrate does not undergo photolysis rapidly, reacts only slowly with HO• radical, and has a low water solubility. Therefore, the major pathway by which it is lost from the atmosphere is thermal decomposition. Alkyl nitrates and alkyl nitrites may be formed by the reaction of alkoxyl radicals (RO•) with nitrogen dioxide and nitric oxide, respectively:

RO• + NO2 --- > RONO2 (42) RO• + NO --- > RONO (43)

Addition reactions with NO2 such as these are important in terminating the reaction chains involved in smog formation. Since NO2 is involved both in the chain initiation step (Reaction 11) and the chain termination step, moderate reductions in NOx emissions alone may not curtail smog formation and in some circumstances may even increase it. As shown in Reaction 39, the reaction of oxygen with alkoxyl radicals produces hydroperoxyl radical. Peroxyl radicals can react with one another to produce reactive hydrogen peroxide, alkoxyl radicals, and hydroxyl radicals: HOO• + HOO• ---> H2O2 + O2 (44) HOO• + ROO• ---> RO• + HO• + O2 (45) ROO• + ROO• ---> 2RO• + O2 (46) 5.1.4. Nitrate Radical

First observed in the troposphere in 1980, nitrate radical, NO3, is now recognized as being an important atmospheric chemical species, especially at night. This species is formed by the reaction

NO2 + O3 ---> NO3 + O2 (47)

and exists in equilibrium with NO2:

NO2 + NO3 + M ---> N2O5 + M (energy-absorbing third body) (48)

Levels of NO3 remain low during daylight, typically with a lifetime at noon of only about 5 seconds, because of the following two dissociation reactions:

NO3 + hν (λ< 700 nm) ---> NO + O2 (49)

NO3 + hν (λ < 580 nm) ---> NO2 + O (50)

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NO3 is responsible for much of the atmospheric chemistry that occurs at night. The nitrate radical adds across the double bonds in alkenes leading to the formation of reactive radical species that participate in smog formation. 5.1.5. Photolyzable Compounds in the Atmosphere It may be useful at this time to review the types of compounds capable of undergoing photolysis in the troposphere and thus initiating chain reaction. Under most tropospheric conditions,

the most important of these is NO2:

NO2 + hν (λ < 420 nm) ---> NO + O (51) In relatively polluted atmospheres, the next most important photodissociation reaction is that of carbonyl compounds, particularly formaldehyde:

•CH2O + hν (λ < 335 nm) ---> H• + HCO (52) Hydrogen peroxide photodissociates to produce two hydroxyl radicals: HOOH + hν (λ < 350 nm) ---> 2HO• (53) Finally, organic peroxides may be formed and subsequently dissociate by the following reactions, starting with a peroxyl radical:

H3COO• + HOO• ---> H3COOH + O2 (54)

H3COOH + hν (λ < 350 nm) ---> H3CO• + HO• (55) It should be noted that each of the last three photochemical reactions gives rise to two free radical species per photon absorbed. Ozone undergoes photochemical dissociation to produce excited

oxygen atoms at wavelengths less than 315 nm. These atoms may react with H2O to produce hydroxyl radicals.

5.1.6. Inorganic Products from Smog Two major classes of inorganic products from smog are sulfates and nitrates. Inorganic sulfates and nitrates, along with sulfur and nitrogen oxides, can contribute to acidic precipitation, corrosion,

and adverse health effects. Although the oxidation of SO2 to sulfate species is relatively slow in a clean atmosphere, it is much faster under smoggy conditions. During severe photochemical smog conditions, oxidation rates of 5-10% per hour may occur, as compared to only a fraction of a percent per hour under normal atmospheric conditions. Thus, sulfur dioxide exposed to smog can produce very 19

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high local concentrations of sulfate, which can aggravate already bad atmospheric conditions. Several

oxidant species in smog can oxidize SO2. Among the oxidants are O3, NO3, and N2O5, as well as reactive radical species, particularly HO•, HOO•, O, RO•, and ROO•. The two major primary reactions are oxygen transfer (Reaction 56), or addition (Reaction 57).

SO2 + O (from O, RO•, ROO•) ---> SO3 ---> H2SO4, sulfates (56)

An example is HO• adds to SO2 to form a reactive species which can further react with oxygen, nitrogen oxides, or other species to yield sulfates, other sulfur compounds, or compounds of nitrogen:

HO• + SO2 ---> HOSOO• (57)

Addition of SO2 to RO• or ROO• can yield organic sulfur compounds.

5.2. Effects of Smog The harmful effects of smog can be summarized as follows: 5.2.1. Human health and comfort Ozone produced during smog causes coughing, wheezing, bronchial constriction, and irritation to the respiratory mucous system in individuals. In addition, oxidants such as peroxyacyl nitrates and aldehydes found in smog are eye irritants. 5.2.2. Damage to materials Materials such as polymers and rubber are adversely affected by smog components. Rubber has a high affinity for ozone and causes cracking and ageing of rubber by oxidizing and breaking double bonds in the polymer. 5.2.3. Effects on the atmosphere Various organic compounds and particulate matter (aerosols) are produced from smog. The organic compounds chiefly include organic acids, alcohols, aldehydes, ketones, esters and organic nitrates. Aerosol particles are known to affect the visibility of the atmosphere during smog formation. 5.2.4. Toxicity to plants The three major oxidants involved in smog are ozone, PAN, and nitrogen oxides. Of these, PAN has the highest toxicity to plants, attacking younger leaves and causing “bronzing” and “glazing” of their surfaces. Nitrogen oxides occur at relatively high concentrations during smoggy conditions,

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but their toxicity to plants is relatively low. Alkyl hydroperoxides occur at low levels under smoggy conditions causing adverse genetic effects such as DNA damage. Alkyl hydroperoxides are formed under smoggy conditions by the reaction of alkyl peroxy radicals

with hydroperoxy radical, HO2•, as shown for the formation of methyl hydroperoxide below:

H3CO2• + HO2• ---> H3COOH + O2 (58) The low toxicity of nitrogen oxides, PAN, hydroperoxides and other oxidants present in smog render ozone as the greatest threat to plants under smog conditions.

6. Summary

Thus, at the end of the module, you shall have gained an understanding about the following and would be able to answer questions related to the structure and composition of the atmosphere, thermochemical and photochemical reactions occurring in the atmosphere, mechanism of smog formation- sources, causes and effects of smog.

References

Books  Battan Louis J. (1984). Meteorology, 2nd Edition, Prentice Hall International, Inc, New Jersey, U.S.A. ISBN: 0133411230.  De A.K. (2017). Environmental Chemistry. 8th Edition, New Age International Publishers, New Delhi. ISBN: 9789385923890.  Manahan, Stanley E. (2000). Environmental Chemistry, 7th Edition, Lewis Publishers, Boca Raton: CRC Press, LLC, 2000. ISBN: 1566704928.  Subramanian V. (2011). A textbook of Environmental Chemistry. I.K. International Publishing House Pvt. Ltd., New Delhi. ISBN: 97893811, pp. 61-82.

Weblinks  http://teachertech.rice.edu/Participants/louviere/struct.html  http://climate.ncsu.edu/edu/k12/.AtmStructure  http://www.albany.edu/faculty/rgk/atm101/structur.htm  weather.cod.edu/sirvatka/1110/Unit1_1110.pdf  www.earthobservatory.nasa.gov

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 www.forbes.com  www.mirror.co.uk ------

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Atmospheric Processes Environmental Sciences Atmospheric Chemistry