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Paper No and Title 4,

Module No and Title 18, Composition of

Module Tag CHE_P4_M18

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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TABLE OF CONTENTS

1. Learning Outcomes 2. Introduction 3. Composition of ’s atmosphere 3.1 : formation of particulate , physical and chemical and its effects 3.2 Ions and radicals with reactions: hydroxyl and hydroperoxy in atmosphere and reaction with trace . 4. Summary

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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1. Learning Outcomes-:

After studying this module, you will:

1. Be able to differentiate between air and atmosphere. 2. Know about chemical composition of earth’s atmosphere. 3. Know the physical and chemical properties of particles, ions and radicals. 4. Know the effect of particles, ions, and radicals. 5. Understand the reactions of ions and radicals. 6. Know the variation in earth’s atmosphere with in various levels of atmospheric layers.

2. Introduction

The atmosphere of the earth is the blanket of surrounding gases maintained in its place by earth’s gravity. of all the planets of our are different but some planets have similarities between their atmospheres. This includes the four innermost planets , , Earth and Mars (the ‘terrestrial’ planets) and the four outer planets , , Uranus and (the ‘ giant’ planets).

3. Composition of earth’s atmosphere

The earth’s atmosphere is composed of air. , , and , constitute the major gases of the air. This is the of gases, vapour and particles.

Fig1. Composition of atmosphere

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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The chemical composition of the earth’s atmosphere varies with altitude, and profile of the different regions of atmosphere and also due to the penetration of incoming solar . (Fig 2)

Fig2. Different regions of the atmosphere

3.1 Particles

Particles are very important constituent of the atmosphere. They are small sized solid and liquid matter in the atmosphere of earth. The number of particles per unit of air varies from several hundred per cubic centimetre in pure air to more than 105 per cubic centimetre in polluted air. Their size is in the range of 0.1-10µ.The particulate level ranges from10 µg/m3 in clean air to 60-2000 µg/m3 in the polluted urban areas. Colloidal sized solid and liquid present in the earth’s atmosphere are known as . Particles of natural origin with diameter less than 0.2 µ are called Aitken particles or Aitken nuclei, after a pioneer of science, John Aitken. The sources of aerosols define their composition. -blown comes from mineral and other material blown from the earth’s crust; this particulate is -absorbing. The second largest contributor of global aerosol is salt and consists mainly of originating directly from the (); other constituents of sea salt reflect the composition of sea water, and thus include magnesium, sulphate, calcium, potassium, etc. sea spray aerosols contain both inorganic as well as organic compounds, which affect their chemistry. The particles which occur naturally in the atmosphere are , fog, and .

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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The size and chemical of the particles are more important than the particulate material present in the earth’s atmosphere.

3.1.1 Formation of particulate matter

1). Inorganic particulate matter-: Metal oxides form a major group of inorganic particles in the atmosphere. They originate from fuel . Iron particles, for example, originate in the combustion of - containing coal. Calcium oxide particles also come from coal combustion. Part of the CaCO3 in the ash fraction of coal is converted to calcium oxide and discharged through the stack.

Combustion of leaded gasoline in automobiles releases lead and lead halide particles in the exhaust. Aerosol are formed from sulphuric acid, which in turn is obtained by oxidation of sulphur dioxide which collects vapour to form small liquid droplets.

In the presence of basic air such as or calcium oxide, salts are formed.

Table -1. Some constituents of inorganic particulate matter and their origins

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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2). Organic particulate matter-: Organic particulate matter is organic in nature and is obtained from number of sources such as emission from vegetation, automobiles, combustion of fuels etc. It has been estimated that the average formula for the organic particulate matter is C32H48O3.8S0.083X0.06 (X = halogen) and a size ranges up to 1µ. Such particulate matter is a potential hazard. One class of components of organic particulate matter, polycyclic aromatic hydrocarbons (PAH) have carcinogenic effect. They commonly occur in upper atmosphere at about 20 µg/m3 level. They originate from the pyrolysis of higher paraffin’s present in fuels and material. High molecular weight compounds are pyrolysed during combustion to yield C10H22, which undergoes further pyrolysis:

The PAH compounds are usually adsorbed on soot particles. Soot is produced on combustion of fuel in automobiles and power . In urban areas it accounts for half of the total particulate load. The content of soot is 1-3% and oxygen content about 5-10% due to partial surface oxidation. Soot acts as a carrier for toxic organics due to its large surface area. The size range of these particles is in between 0.1 – 20µ.

3.1.2 Physical and Chemical Properties of Particulate Matter (PM)

The size and shape of particles comes under physical properties. These particles include liquid droplets, regular or irregular shaped crystals or aggregates of odd shape. Particles are often classified by size, e.g. the particles having diameter less than and up to 10µm are named as PM10 and PM2.5 refers to those of diameter up to 2.5µm.

• The chemical composition of these particles vary a lot they can be dilute water of acids or salts or can be organic liquids and even materials present in the earth's crust (dust) or soot (unburned ) and even toxic metals. • Optical properties such as light , absorption and help in determination of dust, smoke and haze . Physical and chemical properties of ambient PM, in turn, decide their optical properties • Particles produced by any PM source have specific physical, chemical and optical properties. Hence PM should be viewed as containing several pollutants since each type of aerosol has its unique properties and sources and different control is required.

3.1.3 Effects of Particulate Matter

• They affect the atmosphere by forming and fog • They help in balancing the temperature of the atmosphere on earth through light . • They serve as nuclei for the development of crystals and water droplets. • They provide a surface for many atmospheric reactions like neutralization of water droplets, oxidation reactions (e.g. catalytic effect of tiny particles of metal oxides), photochemical oxidation reaction etc. • Particulate matter is responsible for having adverse effects on human health, especially fine particles such as PM10 and PM2.5, which enter the through the respiratory tract (such particles are called Respirable Particulate

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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Matter or RPM). PM10, when inhaled, penetrates deep into the , bypassing the body’s respiratory filters and can give rise to chronic respiratory diseases such as bronchitis, bronchial asthma and pulmonary oedema. PM2.5 is more dangerous than PM10 as they can affect the exchange of gases inside the lungs by reaching the peripheral regions of bronchioles when inhaled. due to particulate matter is recognized by WHO to be responsible for disease named chronic obstructive pulmonary disease and cancer, in addition to infections of the . The removal of particulate matter from exhaust gases is an important strategy for air pollution control.

3.2 Ions and Radicals

The upper layer of atmosphere known as contain significant numbers of and + + + positive ions (O2 , O , NO ). They persist for long durations without recombining to give neutral species due to rarefied conditions in the upper atmosphere. In the ionosphere the ions are primarily produced by light. At , when there is no UV light, the positive ions present in the atmosphere combine with free electrons and results in the formation of original neutral species. The upper layer atmosphere of earth is strongly influence by the magnetic of earth. The best known example of this phenomenon is found in the Van Allen belts discovered in 1958, consisting of two ionised belts surrounding the earth. The inner belt and outer belt is consists of protons and electrons respectively. The electromagnetic radiation in the atmosphere which are high in may also produce free radicals which are or groups of atoms with unpaired electrons. Free radicals are very reactive. The half- of radicals at high may be of several minutes or longer than that. Radicals are used in chain reaction, in which at least one of the products is a free radical. Finally the chain may be terminated by the combination of two free radicals:

Free radicals contribute in the formation of photochemical smog.

3.2.1 Hydroxyl radicals in the atmosphere

Hydroxyl radical (.OH ) is the product of reaction between water present in the atmosphere and excited oxygen which are formed by photodissociation of present in earth’s atmosphere. In the hydrogen and hydroperoxy radicals are produced by number of reaction as follow:

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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Large number of radicals and atoms can be produced rapidly by the reactions of these two radicals. These species are fundamental to the chemistry of atmosphere in gas-phase. Due to the high reactivity of these species they are naturally found in very low concentrations. Their typical atmospheric concentrations of these radicals are: hydroxyl radical 7 X 105 per cm3; hydroperoxy radical 2 X 107 per cm3. These radicals are very high in reactivity therefore they have a short residence . The OH radical resists less than 1s in the atmosphere while that of .HO2 radical is around 1 minute.

3.2.2 Reactions of hydroxyl and hydro per oxy radicals with atmospheric trace gases

Trace gases in the earth’s atmosphere when react with the hydroxyl radical affect the composition of the atmosphere.

1). Reaction with sulphides

The sulphur gases produced biologically are present in the atmosphere in the form of sulphides such as hydrogen sulphide, dimethyl sulphide, and carbon disulphide. They easily react with highly reactive hydroxyl radical present in the atmosphere.

(a) Dimethyl sulphide: Dimethyl sulphide reacts with hydroxyl radical to produce methyl sulphonic acid as depicted below.

Out of the methyl sulphonic acid (CH3SO3H) so produced, majority of the product remains in the earth’s atmosphere and only a small amount get oxidized through sulphur dioxide.

(b) Hydrogen sulphide:

The product so formed is oxidized through SO2 by number of reactions. The SO2 formed is then oxidized by OH and HO2 radicals:

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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Bisulphite radical (HSO3) and SO3 react with OH radical and water respectively to form sulphuric acid, which is the final oxidation product of atmospheric sulphur.

(c) Carbon disulphide (CS2)

It get oxidised by OH radical to form carbonyl sulphide and sulphur dioxide in equal proportions. The reaction of CS2 with oxygen atoms in atmosphere is more important than its reaction with OH radical.

2). Reaction with carbon monooxide-:

When reacts with Hydroxyl radical and hydrogen radical are produced.

3). Reaction with -:

Formaldehyde is result of many reactions taking place in atmosphere. The formaldehyde available is oxidized by OH radical as follows:

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere

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4). Reaction with -:

Methane can be obtained from surface of earth naturally. Methane is oxidized to methyl radical and water by OH radical. Methyl radical so formed is then converted into CH3O2, which then reacts with hydroperoxy radical as follows:

5). Reaction between hydroxyl hydroperoxy radicals-:

Hydroxyl radicals react with each other. Hydrogen peroxide can also be formed by reaction between two hydroperoxy radicals:

H2O2 is soluble in water and it is a strong oxidizing agent. In the atmosphere hydrogen peroxide plays an important role in oxidation processes.

4. Summary

• Atmosphere is a segment of environment, and very close to earth. • There are many layers in atmosphere like troposphere, , , etc. Our atmosphere is made by these layers. • Atmosphere is composition of many gases, particles, and ions. Environment is affected by these gases. • In atmosphere reactions take place and form different type of composition. • Atmosphere is an important segment of environment and it is our responsibility to clean it. We must do our best efforts to keep it clear and less-polluted.

CHEMISTRY PAPER No. 4: Environmental Chemistry MODULE No. 18: Composition of atmosphere