12 Atmospheric Chemistry

Puneeta Pandey

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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

  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
  1. 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.

  1. Chemical composition of the earth atmosphere

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

 

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

 

  1. Thermochemical reactions in atmosphere
  • 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 5×10- 10of 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.

 

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.

 

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, 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

  1. 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. 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,

 

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.

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),

 

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.

 

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.

 

 

The time variations in the levels of hydrocarbons, ozone, NO and NO2  are explained by the

 

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)

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)

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:

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:

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

 

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 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)

 

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,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.

 

  1. 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.

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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.

 

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