Atmospheric Pollution

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Introduction to Atmospheric Pollution

Air is never found clean due to natural and manmade pollution. Gases such as carbon monoxide, nitrogen oxides, Sulphur oxides, hydrogen sulphide and many volatile organic compounds are continuously released into the atmosphere due to natural and human activities. Addition of new chemical substance which disturbs the normal composition of air by building undesirable components which causes harm to humans, vegetation and other organisms. This phenomenon is called Air Pollution. The chemical substance causing air pollution is called Air Pollutant. About 90% of air pollution problems are caused by the pollutants such as carbon monoxide, Sulphur oxides, volatile organic compounds, nitrogen oxides and suspended particulate matter (such as carbon, dust, pollen, metal, etc.)

Types of Air Pollutant

There are two types of air pollutants

  • Primary Air Pollutants
  • Secondary Air Pollutants

Primary Air Pollutants

Those harmful chemical substance that enters directly into the air as a result of human activities or natural events are called primary pollutants. The primary pollutants are:

(i) Carbon Oxides (CO and CO2)

(ii) Nitrogen Oxides (NO)

(iii) Sulphur Oxides (SO2)

(iv) Hydrocarbons

(v) Suspended Particulate Matter

Secondary Air Pollutants

A secondary Air Pollutant is a harmful chemical that forms in the air due to a chemical reaction between two or more air components or a primary pollutant and one or more air components.

The common secondary pollutants are SO3, H2SO4, NO2, N2O, HNO3, H2O2, O3, nitrate and Sulphate salts, etc.

Tropospheric Pollution

The tropospheric pollution occurs because of the presence of undesirable solid or gaseous particles in air. The pollutants may be broadly classified into two major types:

  • Gaseous Air Pollutants: These include oxides of Sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other ox1dants.
  • Particulate Pollutants: These are dust, fumes, mist, spray, smoke, etc.

Some Common Air Pollutants
Although there are many sources of air pollutants, the common pollutants come from combustion processes. Let us discuss the major air pollutants.

Gaseous Air Pollutants

Carbon Monoxide (CO)

It is a colorless, odorless and tasteless gas above 81 K. It is heavier than air and is not soluble in water. It is one of the most serious air pollutants. It is highly poisonous to living beings because of its ability to block the delivery of oxygen to the organs and the tissues.
The main sources of air pollution of CO are the automobile engines and defective furnaces.

The main contribution of CO in air pollution is from human activity. Of the total carbon monoxide content in, atmosphere, about 74% is contributed by automobile exhaust (motor vehicles, air craft, rail, road). About 16% is contributed by forest fires and agriculture burning (burning of forest debris, crop residues, weeds and other vegetation). Industrial processes mainly iron and steel industries, paper, petroleum industries contribute about 9.6% of CO in atmosphere.

Effects of CO on Plants

Carbon monoxide has detrimental effects on plants when exposed for longer times. It inhibits the nitrogen fixing ability of bacteria. It also affects leaf drop, leaf curling, decrease in leaf size and premature ageing of the plants.

Health Problems caused by Air PollutionFig- Health Problems caused by Air Pollution

Carbon Dioxide (CO2)

Carbon dioxide is a normal and essential component of the atmosphere. Animals exhale it. Therefore, it is vital to all forms of plant and animal life. It is normally not a pollutant. It is not an important contributor to acid rain. It can cause acidity when it dissolves in water, but it is relatively less soluble in water than the oxides of nitrogen and Sulphur. However, human activities are changing the balance established by nature's carbon cycle. With the increased use of fossil fuels, a large amount of carbon dioxide gets released into the atmosphere. As a result, the concentration of CO2 in the atmosphere is increasing to an alarming extent. Therefore, there is need to keep balance of CO2 in the atmosphere.

Effects of CO2 

Normally carbon dioxide is harmless and is not a pollutant. However, the increasing concentration of CO2 may affect the atmosphere, causing undesirable change in climate. The increased concentration of CO2 in atmosphere is due to human activities. The massive use of fuels has greatly increased the amount of CO2 entering the atmosphere. However, the amount of CO2 removed from the atmosphere by photosynthesis process is being decreased by cutting forests particularly tropical jungles. As a result of these activities, there has been increase in CO2 in the atmosphere. Though CO2 is not a toxic, but the excess of it can lead to increase in earth's temperature. This effect is known as Greenhouse Effect.

Oxides of Nitrogen (NOx)

Three oxides of nitrogen namely nitrous oxide (N 2O), nitric oxide (N O) and nitrogen dioxide (NO2) occur in the atmosphere. Nitrous oxide is produced by microbiological process in soils and is a component of unpolluted air. The gas is chemically unreactive and does not influence chemical reactions in the lower atmosphere. However, it undergoes photochemical reactions at higher altitudes (in stratosphere)

Nitric oxide (NO) is colorless, odorless gas and nitrogen dioxide (NO2) is reddish brown gas having pungent suffocating odor. These two are important pollutants in air.
The irritant red haze in traffic and congested places is due to oxides of nitrogen.

  • Harmful Effects of NOx: The irritant red haze in the traffic and congested places is due to the oxides of nitrogen. NO is biologically less active and less toxic than NO2 and therefore, does not have a significant adverse effect on human health even at high concentrations. However, it is oxidized by oxygen or ozone to nitrogen dioxide which is extremely toxic to living tissue. Like CO, NO binds to hemoglobin and decreases oxygen transport efficiency of blood. The oxides of nitrogen cause damage to the leaves of plants. Exposure of plants to NOx causes leaf. Spotting and breakdown of plant tissues. Excessive concentration (10 ppm) of NO causes decrease in the rate of photosynthesis.

Sulphur Oxides (SOx)

Oxides of Sulphur are probably the most harmful of the common gaseous pollutants. Sulphur dioxide is the main pollutant among Sulphur oxides. It is a colorless gas with a pungent odor. It is generally accompanied by little of SO3 obtained by partial oxidation of SO2

Effects of SO2
Sulphur dioxide and its compounds (formed from SO2 such as SO3, H2 SO4 or sulphates) are dangerous air pollutants. These have dangerous effects on human life. SO2 effects respiratory tract producing nose, eye and lung irritation. SO2 has been considered the most serious single air pollutant causing many health hazards. Atmospheric SO2 is also harmful for plants. It damages vegetable crops and affects plant growth and nutrient quality of plant products

Sulphur dioxide is also considered to cause cough, shortness of breath and spasm of larynx.

Hydrogen Sulphide (H2S)

Hydrogen Sulphide is rapidly oxidized to SO2, therefore, .H2S and SO2 co-exist in the atmosphere.

Sources of H2S

The important sources of H2S are :
(i) The main source of H2S is the natural decay of animals and vegetable matter.
(ii) Volcanic activity is also the primary source of H2S
(iii) It can be produced in the atmosphere by the reduction of sulphates and organo sulphur compounds.
(iv) Many industrial processes such as paper mills, oil refineries natural gas plants and chemical manufacturing plants containing sulphur add H2S to the atmosphere.

Harmful effects of H2S
Hydrogen sulphide has rotten egg smell. It is very toxic and it is objectionable even at low concentrations. Mild exposure to H2S can cause giddiness. It is dangerous for plants also and affects the leaves and reduces their growth.

Chlorine, Cl2

Chlorine gas does not occur as an air pollutant on a large scale. However, it can be quite harmful and dangerous on a local scale. It is very poisonous gas.

  • Sources of Chlorine: As such it does not occur as air pollutant. It can cause local pollution due to release of Cl2 in atmosphere from industrial processes such as plastic industries and water treatment plants. The compound of chlorine, HCl is emitted from a number of sources. Some chlorinated plastics also release HCl on combustion. Some of the chlorine present in coal is released as HCI during combustion.
  • Effects of Chlorine: Chlorine is poisonous and toxic gas. It causes irritation in mucus membrane. It dissolves in atmospheric water droplets giving hydrochloric acid and hypochlorous acid.

H2O + Cl→ HCL + HCIO
The exposure of chlorine may cause fatalities.

  • Control of Chlorine Pollution: Attempts are made to avoid pollution of atmosphere by chlorine. The effluents from the industries are checked for the presence of Cl2. Excess of free chlorine during water treatment is avoided.

Particulates in Air Pollution

Particulates: The small sized solid particles and liquid droplets which range in size from 2 x 10-10 m to 5 x 10-4 m are collectively called as Particulates. These particles are quite small and cannot be seen by the naked eye. The particulate matter in the range 0.001 to 10 mm range is commonly visible as suspended in the air near the sources of pollution. These particulates include carbon (soot), metals, metal salts, cement dust, fly ash, hydrocarbons, soil dust, pulverized coal, asbestos dust, mineral particles, etc.

Types of Particulates and their Sources

There are four types of non-viable particulates in the atmosphere injected into the atmosphere through human activity.

Smoke: Small soot particles consisting of mixture of solid and liquid or just solid which are mainly formed at the time of combustion of organic matter. For Examples, oil smoke, tobacco smoke and Carbon smoke are typical examples of this type of particulate materials. Their common sources are carbon particles, forest fire, coal refuse burning, inefficient burning of fuel in vehicles.

Industrial SmokeFig – Industrial Smoke

These are suspensions of liquid particles in air which arise because of chemical reactions and condensation of vapours in air.

Fumes: Fumes are form of condensed vapours usually formed by the condensation of vapours during boiling, distillation, sublimation, and several other chemical reactions. For Examples, nonmetallic oxides, metals, organic solvents etc.

Dust: Suspended (in air) solid particles having diameter of about 1pm in diameter are called dust particles. These particles enter into atmosphere from natural, domestic and industrial sources. These enter the atmosphere by volcanic eruptions, blowing of dust by wind, mining operations, crushing and grinding, etc.

Construction work largely contributes to release of dust particlesFig- Construction work largely contributes to release of dust particles

Control of Particulates

Various methods are employed to reduce the presence of carbon and other particulates in the atmosphere.

  • Effluent gases from the industries are led into a chamber where the velocities of these gases decrease so that dust or droplets get settled.
  • The carbon and other dust particles are removed by electrostatic precipitator. This method is based on the principle that the particles acquire electrical charges when subjected to an electric field.
  • The solid, liquid or gaseous particulates can be removed by spraying water from spray chambers.
  • Large amount of particles can be collected in cyclone collectors.


Smog is a mixture of smoke, dust particles and small drops of fog. It is a major air pollutant in big cities. This is the best known example of air pollution that occurs in many cities throughout the world. The small drops of smog contains poisonous gases produced by burning of fuels in homes, factories and automobiles.

Smoke  +  Fog = Smog

Smog has become a common scenario in New DelhiFig- Smog has become a common scenario in New Delhi

Effects of Smog

  • Pungent smelling smog containing ozone is very toxic causing respiratory problems.
  • Aldehydes and peroxyacylnitrate (PAN) components of photochemical smog cause irritation of eyes and affects respiratory tract of human beings.
  • It reduces visibility causing nose, throat and eye irritation. It may lead to several chronic diseases of eyes, heart and lungs.
  • It affects human health and comfort.
  • Photochemical smog affects plant growth and damages plants.
  • Materials are also adversely affected by Acid Rain

Acid Rain

The combustion of fuels contributes significantly to atmospheric pollution. The burning of fossil fuels gives CO2. The gaseous CO2 dissolves in water droplets to form weak acid, carbonic acid.
Rain water normally has pH of 5.6 due to the formation of H+ ions from the reaction of rain water with carbon dioxide present in the" atmosphere. If pH of water reduces below 5.6 then the rain water is considered as acid rain.

The oxides of nitrogen and Sulphur present in acid rain undergo many photochemical reactions in atmosphere and form HNO3 and H2SO4 acids. During rains, these acids fall to the earth with rain. This polluted rain is called acid rain.

Formation of Acid RainFig- Formation of Acid Rain

Harmful effects of Acid Rain: Acid rain is very damaging. It is toxic to vegetation, human life and aquatic life. Some of its harmful effects are:

  • It causes extensive damage to buildings and sculptural materials of marble, limestone, slate, etc. These materials react with rain water and get damaged. As a result of acid rain, the invaluable statues and buildings deteriorate. The Taj Mahal of India is facing the same problem.
  • The acid rain has also caused elimination of life from some fresh water lakes by destroying the living bodies.
  • The rain water also corrodes metals.
  • It damages leaves of trees and plants and retard the growth of forests.

Green House Effect

Sun emits various kind of light having different wavelength consisting of ultra-violet (λ < 400 nm), visible region (λ = 400 – 700 nm) and infra-red (λ >700 nm). Out of these the harmful ultra-violet radiations are absorbed by the ozone layer in the stratosphere and warm the air rather than coming to the surface of the earth. But the increase in the Industrialization has resulted in production of greenhouse gases. Production of Chlorofluorocarbon has resulted in thinning of protective ozone layer especially in the polar region which results in melting of polar ice and increase of temperature worldwide. This heating of the earth due to trapped radiation is called greenhouse effect. The gases which can trap infra-red radiation given by the sun to produce greenhouse effect leading to heating up the environment are called greenhouse gases. For Example, CO2 CH4, N2O, CFCI3 etc. These gases absorb thermal IR radiation and therefore, would redirect back more and more outgoing thermal IR energy causing increase in the average surface temperature. This phenomenon will be referred to as the enhanced greenhouse effect to distinguish its effect from the one that has been operating in nature for millions of years.

Pictorial Representation of Greenhouse EffectFig – Pictorial Representation of Greenhouse Effect

Major causes of Greenhouse effect or Global Warming
Modern human activities are releasing large quantities of CO2 in the atmosphere. The major activities which contribute towards this are:

  • Burning of fossil fuels
  • Cultivation of soil (large quantities of methane gas is released in paddy fields)
  • Chlorofluorocarbons (CFC’s) have also the greatest potential to cause global warming due to their greater efficiency of absorbing thermal IR radiations. Each molecule of CFC has the potential to cause the same extent of global warming as do tens of thousands molecules of CO2.

Melting of Polar IceFig- Melting of Polar Ice

The net effect of increased level of all these greenhouse gases is increased greenhouse effect or global warming which may lead to increased global temperature. If nothing is done to control the concentration of these gases into the atmosphere, than average temperature will increase. It has been observed that the average temperature of the earth has increased by about 1°F due to greenhouse effect.

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