Air Pollution

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Humans can only live about five minutes without air. Dry air is generally a homogeneous mixture (solution) of nitrogen gas (78%), oxygen gas (21%), and traces of other gases. Some of the other gases are waste products of human activities and are known as air pollutants.

Air pollution may be defined as an atmospheric condition where the presence of some substances in the atmosphere beyond certain concentration, causes harmful effects on man and his environment. Pollution of air is caused due to contamination of the air by noxious materials usually given off by machines (factories, automobiles, refrigerators, aerosol sprays, etc.).

The part of atmosphere nearest the Earth's surface and extending to a height of about 80 km, is referred here. This is called the homosphere and consists of homogeneous mixture of various constituents. It constitutes about 99.9% of the total atmosphere. The substances causing air pollution can be divided into following categories:

Inorganic gases

Carbon monoxide, sulphur oxides, hydrocarbons and nitrogen oxides etc.

Particulate matter

Dust, smoke, fumes, etc.

Miscellaneous

Others pollutants including lead, arsenic, asbestos, mercury, beryllium, cadmium fluoride, plutonium and organic pesticides.

Radioactive materials

Fallouts from Nuclear power generation plants and nuclear bombs.

Primary and Secondary Pollutants

Primary Pollutants

These are pollutants, which are emitted directly from the sources. Some examples are particulate matter such as ash, smoke, dust, fumes of inorganic gases such as carbon monoxide, sulphur dioxide, hydrogen sulphide, nitric oxide, ammonia, hydrocarbons and radioactive substances.

Secondary pollutants

These are pollutants that form in the atmosphere due to chemical interactions among primary pollutants and normal atmospheric constituents. Some examples are: ozone, nitrogen dioxide, various sulphate and nitrate salts, sulphur trioxide, aldehydes and ketones.

The chemical reactions that convert primary pollutants into secondary pollutants in the atmosphere are oxidation, dissociation and dissolution. For example,

Primary Pollutants

Secondary Pollutants

SO₂ SO₃

Sulphur dioxide  in atmosphere   Sulphur trioxide

 

H₂SSO₂ + H₂O

Hydrogen sulphide  in atmosphere

NO₂ 

Nitrogen dioxide  in atmosphere  Nitric oxide

 

SO₂ + H₂O H₂SO₃

Sulphur dioxide                  Sulphurous acid

SO₃ + H₂OH₂SO₄

Sulphur trioxide                        Sulphuric acid