Air Pollution: Sources and Control (with diagram)

We depend on air for our respiratory needs. So, air pollution causes injury to all living organisms.

In case of plants, the growth and yield of crops are reduced and cause premature death. In animals including man, serious metabolic and respiratory diseases are manifested due to air pollution.

Air pollution is also called as atmospheric pollution. The atmosphere is an invisible layer of gases that surround the earth.

The atmosphere extends from the surface of earth upto 650 killometers. The lower most layer of atmosphere is known as troposphere which extends upto 8-10 km near the poles and 18-20 km near equator. Air pollution is largely confined to the lower atmosphere i.e. troposphere.

The air comprises of four gases (99.99%) and a small amount of water vapour. These four gases are: Nitrogen (78.08%), Oxygen (20.95%), Argon (0.93%) and Carbon dioxide (0.03%). The oxygen and carbon dioxide are the two gases of air which directly interact with various biotic components through respiration and photosynthesis.

Sources of Air Pollution:

Air pollutants are gases, liquids and solids.

The major sources of air pollutants are:

(i) Transportation (42%)

(ii) Fuel combustion in stationary sources (21%)

(iii) Industrial processes (14%),

(iv) Solid waste disposal (5%),

(v) Forest fires (8%) and

(vi) Miscellaneous sources including radioactive fallout (10%).

So far, six pollutants, that account for most of the air pollution worldwide are carbon monoxide (CO), sulphur dioxide (SO₂), nitrogen oxides (NO_x), Ozone (O₃), particulate matters (PM₁₀) and lead. They may come from natural sources or from human activities. Natural sources of air pollution are volcanic eruption, discharge of spores, conidia, endospores etc. of airborne micro-organisms, pollens of certain flowers, dust particles suspended in air.

Man Made Sources are, burning of fossil fuels (coal, natural gases, kerocine, petroleum products, etc.), burning of firewood for domestic purpose, automobile exhausts, smokes of domestic and industrial sources, particulate matters and aerosol etc.

A brief description of atmospheric pollutants released through manmade sources is given below:

1. Gases:

A large amount of air pollution results from burning of coal and oils in furnaces and steel plants. They are burnt to produce heat energy along with gaseous and solid waste products. Gases produced during fuel combustion are CO, CO₂, SO₂, various oxides of nitrogen (NO, NO₂, N₂O₄) and assorted hydrocarbons. Carbon monoxide (CO) is produced due to incomplete combustion of the carbon content of fossil fuels. Carbon dioxide (CO₂) is produced due to complete combustion of carbon content.

Hydrogen sulphide (H₂S) is produced from refineries, sewage treatment and pulp mills. CO alone contributes to 47% air pollution. S0₂ is produced due to burning of coal and oil and from industrial processes. Nitrogen oxides are formed when fuel is burnt at very high temperature such as in industrial plants and transportation vehicles. Nitrogen monoxide (NO) is produced during combustion in the engine cylinder.

When this gas passes out of the engine, it cools down and combines with more oxygen to form NO₂ and N₂O₄. This mixture of gases is generally called as oxides of nitrogen (NO_x). Ozone (O₃) is another air pollutant which is a major component of photochemical smog which is formed from NOx, VOCs and oxygen in the presence of sunlight and heat.

2. Particulate Matters:

These are solid particles and liquid droplets suspended in air. They may be settled down where particle size is more than 10 μm or remain suspended in air when particle size is below 10 μm. Particulate matter in the size range of 0.01 μm to 50 μm or less in size), aerosols (less than 1 μm) flash and dust 0.25 to 500 μm), grit (more than 500 μm).

Dust and flash cover the leaf surface reducing photosynthetic ability of leaves. Particulate matter below 5 μm size is usually deposited in respiratory tract. Smoke and fog reduce air visibility, photosynthetic efficiency of plants and cause respiratory distress and allergy in human beings.

Fine dust particles released from cotton mills, floor mill or asbestos factory can cause serious respiratory problems and even may lead to cancer. Air borne microorganisms released during sneezing of diseased persons can spread air borne diseases. Jet aeroplanes release aerosol which contain CFC and can cause ozone layer depletion in stratosphere.

3. Toxic Chemicals:

Some highly toxic chemicals are emitted directly from different source^. For example, Arsenic emitted from coal and oil furnace and also from glass manufacturing units are directly delivered to air which is highly toxic. Similarly, C₆H₆ from refineries and motor vehicles, cadmium from smelters, burning waste, and coal and oil furnaces are some of the highly toxic chemicals acting as air pollutants.

4. Secondary Pollutants:

As stated earlier, these are formed from primary pollutants through wide range of photochemical reactions and cause greater damage than primary pollutants.

When hydrocarbons from exhaust are exposed to light, alkanes, ethylenes, unsaturated hydrocarbons, aldehydes and aromatics are formed. One of these compounds is benzopyrene which induces cancer in man. Two other photochemically originated pollutants are peroxybenzoil nitrate and peroxyacetyl nitrate (PAN).

Some other secondary pollutants for air pollution are sulphur dioxide (2H₂S + 3O₂ →2SO₂ + 2H₂O), Sulphuric acid mist (2SO₂ + O₂→ 2SO₃, SO₃ + H₂O→ H₂SO₄) and smog (Hydrocarbons + NO₂ + sunlight→ Peroxyocetyl nitrate + HCHO + OH).

The word ‘smug’ was first used in 1905 to describe the combination of smoke and fog in London which totally obscured visibility for few hours. This smog resulted from S0₂, soot and tarry materials released into the atmosphere by burning of high-sulphur coal.

Some other secondary air pollutants are:

(a) NO: Formed by dissociation of N0₂ to atomic oxygen and NO.

(b) NO₂: formed in sunlight from NO (NO + O → NO₂)

(c) OH (Hydroxy radical): Formed in sunlight from hydrocabons and nitrogen oxides. It reacts with other gases to form acid droplets.

(d) HONO (Nitrous acid): Formed from NO₂ and water vapours.

(e) HN0₃ (Nitric acid): Formed from NO₂ and is a major component of acid rain.

Effects of Air Pollution:

The effect of air pollution has been extensively studied in man and animals, in plants and in climatic changes.

A few of them are described below:

1. Acute health hazard:

Smog in Donora, Pennsylvania along Monogahila River in 1948 resulted 6,000 illness and 20 deaths in a population of 14,000. Photochemical smog in London in 1952 and 1956 has caused eye and throat irritation. In India, a most terrible effect of air pollution has been witnessed in Bhopal on December 3, 1984. Leakage of Methyl Isocyanate from Union Carbide factory caused number of deaths in a few minutes.

2. Chronic diseases:

Incidence of respiratory diseases in Delhi is about 12 times higher than the national average. Carbon monoxide if present in air can combine with blood haemoglobin 7 to 10 times faster than O₂. CO concentration 30 ppm for 4 hours can convert 5% of body haemoglobin into carboxy haemoglobin. Prolonged exposure to CO could cause death due to lack of O₂ supply to living cells of the body.

Diseases like bronchitis, lungs cancer and emphysema are caused by air pollution. N0₂ in air causes bronchitis and lowers resistance to influenza. SO₂ obstructs breathing and irritates eyes. Silicon tetrafloride irritates lungs. Nitric acid, nitrous acid and sulphuric acid initiate respiratory disease. Photochemical smog causes eye irritation and headache. Constant exposure to peroxyacetyl nitrate (PAN) aggravates asthma and can damage lungs.

Air pollution produces offensive odours and gives general discomfort, anxiety or suffering to people. Long term exposure to benzene causes low WBC count and leukaemia. Similarly, long term exposure to arsenic may cause lung and skin cancer. Exposure to cadmium damages kidney and lungs and weakens bones. Prolonged exposure to nickel may cause lung cancer. Exposure to lead causes highpertension and impair growth. Long term exposure to manganese may contribute to Purkinson’s disease.

3. Reduction in Visibility:

Smokes, fumes, fog and particulates in air absorb solar radiation and reduce the quantity of solar radiation reaching the earth surface. Smokes and fumes increase atmospheric turbidity. Particulates absorb and reflect incoming solar radiation, thus reducing 15 to 20% of total radiation reaching the earth surface. Cause of many accidents is reduction of visibility due to smoke and fog in the atmosphere.

4. Effects on plants:

Air pollution has devastating effect on plants, ultimately resulting in lower yield. It damages the crops and trees. According to an estimate there has been 5 to 10% crop loss due to ozone pollution. SO₂ causes bleaching of leaves, chlorosis, growth suppression and yield reduction. PAN produces glazing, silvering or bronzing on the lower surface of leaves. Hydrogen fluoride (HF) could also cause chlorosis, dwarfing, leaf abscission and lower yield. Chloride (Cl₂) develops bleaching spots and leaf abscission. Ethylene (C₂H₄) causes leaf abnormalities and withering, flower dropping and failure of the flower to open.

5. Effect on Climate:

Particulates of air play a vital role in producing temperature changes and air movements. Solid particulates take part in cloud formation. Since urban air pollution is more, there is increased particulate matter in air, increased cloud formation (upto 10% in comparison to rural areas) and 10% more wet days. There is increased amount of mist, fog and smog in industrial areas.

Control of Air Pollution:

Atmospheric pollution can be controlled effectively by using some of the following techniques.

1. Use of tall chimneys:

Industries should be asked to build up high chimneys for escape of smoke, fumes so that harmful gases may not spread in the lower layer of atmosphere.

2. Use of CNG:

Automobiles in Delhi account for 50% of air pollution and 90% of CO are released to air from automobiles. Recently Delhi Administration has emphasized use of Compressed Natural Gas’ (CNG) in place of petrol and diesel to reduce air pollution.

3. Removal of pollutant from fuel:

A lead compound, tetraethyl lead (TEL) is mixed in petrol for smooth and easy running of the vehicles. But the exhaust is leaded gas and particulate lead. Lead mixed air when inhaled, is injurious for kidney, liver and blood. When mixed with food and water, it may lead to poisoning. Therefore unleaded petrol must be available in the petrol pumps.

4. Use of catalytic converters:

Removal of pollutants from fossil fuel with be possible by use of catalytic converters in two, three and four wheelers. The catalytic converter has expensive metals like platinum, paladium and rhodium as catalysts. When the poisonous exhaust gases pass through catalytic converter, unburnt hydrocarbons are converted into carbon dioxide and water. Carbon monoxide and nitric oxide are changed to carbon dioxide and nitrogen gas respectively.

Smoking tobaco is injurious to health because its smoke contains nearly seven polycyclic hydrocarbons and radioactive polonium-210 which are said to be carcinogenic. An average smoker has the risk of developing and dying from lung cancer ten times more than a nonsmoker.

5. Use of scrubber: (Fig. 9.1) A scrubber can remove gases like sulphur dioxide and ammonia. In a scrubber, the exhaust is passed through a spray of water or lime.

6. Use of electrostatic precipitator (Fig. 9.1):

For removing particulate matter from air, electrostatic precipitator is used, which can remove 99% particulate matter present in the exhaust of thermal power plants. It has electrodes with supply of several thousand volts of electric current, which produce a corona that releases electrons. These electrons attach to particles giving them net negative charge. The collecting plates are positively charged and attract the negatively charged particles.

7. Proper treatment of Organic Wastes:

Public awareness regarding air pollution potential of sewage and many other solid wastes will help in reducing air pollution. It should be mandatory for municipalties to carry-out proper treatment of sewage and other wastes before disposal.

8. Development of green covers:

More effort should be made for extensive green coverage development because the green plants serve as sinks for air pollutants. Many plant species have been evaluated for their scavenging potential against air pollutants.

9. Bioremediatieon:

Many countries have started the use of microorganisms to treat air, water and instrial wastes. Japan is exploring various uses of biovemediation. For example, use of microorgnisms to manufacture advanced biodegradable polymers or to produce clean burning fuel like hydrogen.

10. Pollution Control at source is a better preventive measure. This would facilitate not releasing pollutant into air.

In India, the Air (Prevention and control of Pollution) Act came into force in 1981 but was amended in 1987 to include noise as an air pollutant. Government has established pollution Control Boards in State Headquarters. In spite of all reactions, public awareness is the basic need to fight against pollution.