Atmospheric Pollutants

Many of the air pollutants are naturally present in the atmosphere in low concentration. They become pollutants when their concentration is relatively high. For example, SO₂ is considered as pollutant when its concentration in the atmosphere exceeds the normal limit of 2 x 10^-4 ppm. Similarly, CO acts as pollutant when its concentration exceeds the normal limit of 1 x 10^-1 ppm.

Inorganic Gases

Carbon Monoxide (CO)

This is a colorless, odorless and invisible gas. It is largely the product of incomplete combustion of carbonaceous matter in internal combustion engines (vehicles). Defective industrial furnaces such as blast furnaces also create CO by the reaction of carbon dioxide with coke, at high temperatures. Other processes that release carbon monoxide include incomplete combustion of agricultural matter, slash and burn farming practices and a few natural processes such as forest fires, volcanic activity, lightning etc.

Of the total amount of CO present in the atmosphere, about 73 per cent is contributed by vehicular exhausts; industrial processes of steel, paper and petroleum industries contribute about 9.5 per cent; about 17 per cent is caused by natural and manmade forest fires and burning agricultural produce, including slash burning.

Harmful effects of carbon monoxide

The toxic effects of carbon monoxide on human beings and animals arise from its reversible combination with haemoglobin (H_b) in red blood cells, which cause displacement of oxygen. This results in oxygen starvation or anoxia. The maximum permissible concentration of CO in the atmosphere is 40 ppm for an exposure of 8 hours. A higher concentration of CO, or a longer exposure starts causing adverse effect. In small amounts CO causes headaches and fatigue, in larger doses it can be lethal.

The organisms, which are affected by a pollutant, are called 'receptor', while elements or organisms that can consume a pollutant are known as 'sink'. Recent researches have shown that soil is capable of removing large amounts of CO from the atmosphere by the action of some soil micro-organisms. The micro-organisms act as a sink for CO.

In thickly populated urban areas, the concentration of CO is considerably high because of heavy vehicular traffic and insufficient availability of soil to act as sink, while the concentration of the gas is comparatively small in the countryside.

Carbon dioxide (CO₂)

 

Carbon dioxide is a natural constituent of the atmosphere and is vital to all forms of plant life. Animals and plants give it out in the process of respiration. However it is released mainly during the combustion of fuels at homes, factories, power stations, etc. An estimated seven billion tons of carbon from human-generated carbon dioxide is going into the atmosphere each year.

Green plants in the process of photosynthesis remove carbon dioxide in large amounts. The ocean is also a very large 'sink' taking up about two billion tons. In spite of this removal, concentration of carbon dioxide has increased considerably in the atmosphere and about three billion tons stay there.

Harmful effects of CO₂

The high concentration of CO₂ in atmosphere has been the main component of the green house effect that has caused global warming i.e. the rise of atmospheric temperature in recent years.

Oxides of sulphur

These are acidic, corrosive, poisonous gases that come from combustion of sulphur-bearing fuels such as coal and oil. These gaseous pollutants are released into the atmosphere through volcanic eruptions (natural activity).

Man-made sulphur oxide pollutants are produced mainly by industrial plants that burn fuels with sulphur impurities or produce sulphide ore. The common oxide of sulphur emitted by pollution sources is sulphur dioxide (SO₂). Natural activity contributes about 66% of total SO₂ and the rest is due to human activity.

A part of SO₂ undergoes photolytic and catalytic oxidation in the atmosphere to form SO₃. Thus, SO₂ present in the atmosphere is generally contaminated with SO₃. The SO₃ so formed, changes to H₂SO₄ in the presence of moisture. This acid comes down from the atmosphere in the form of sulphuric acid rain.

Harmful effects of SO₂

 

SO₂ is poisonous and irritates the respiratory system of animals and humans. A continuous exposure to concentration of 0.2 ppm lasting for 3-4 days or even 0.5ppm for full day has been reported to damage the lungs and increase the rate of mortality. In plants and grasses, SO₂ also produces leaf injuries (called necrotic blotching). Sulphur dioxide damages barley, alfalfa, cotton and wheat.

Exposure to SO₂ causes deterioration of fabrics (cotton, rayon) paper and leather. Being corrosive it has damaging effects on metals such as iron, steel, aluminium and on building materials such as lime stone, marble, roof slate etc. Marble structures, limestone and geological sources of ammonia act as sink for atmospheric SO₂.

Oxides of nitrogen

Nitrogen and oxygen forms five oxides of nitrogen. Out of which only three, namely; nitrogen dioxide (NO₂), nitric oxide (NO) and nitrous oxide (N₂O) are present in the atmosphere in significant amounts. Of these only NO and NO₂ are atmospheric pollutants.

Significant amount of NO and NO₂ are emitted into the atmosphere by natural activity, which cause considerable pollution. NO₂ is also produced in small amounts by microbiological processes in soil.

Man-made production of oxides of nitrogen from N₂ and O₂ in the atmosphere occur when fuel such as coal, oil and natural gas, are burnt in industries at high temperature (exceeding 1200^oC).

These oxides are also produced during combustion of gasoline or diesel in automobiles.

Harmful effects of oxides of nitrogen

Inhaling of the nitrogen oxides by humans results in pulmonary odema and hemorrhage. Several sensitive plants get adversely affected by NO_x even at concentration of 1 ppm for a day or 0.35 ppm for a few months. The affected plants start shedding their leaves and fruit. Nitrogen oxides cause fading of textile dyes and deterioration of cotton and nylon fabrics.

Hydrogen sulphide

The natural sources of H₂S are volcanic activity and biological decay of protein matter in stagnant water and swamps. A large number of industries such as petroleum refining, manufacture of paper, manufacture of sulphur dyes, tanning, etc., also produce considerable amount of H₂S.

Harmful effect of H₂S

H₂S gas is extremely toxic to humans. In higher atmospheric concentrations, it can be fatal. The reaction of H₂S with essential proteins is primarily responsible for its toxic effects.