50.

What do you understand by air pollution? Discuss the types of air pollutants. 

Air pollution is defined as the addition of undesirable materials into the atmosphere either due to the natural phenomenon or due to human activity on the earth which adversely affects the quality of air and hence causing harm to humans, other animals, vegetation and materials. The chemical substance causing pollution is called air pollutant. About 90% of air pollution problems are caused by the pollutants such as carbon monoxide, sulphur dioxide, sulphuric acid, nitrogen oxides, volatile organic compounds and suspended particulate matter.

The natural sources of air pollution are volcanic eruptions, vegetable decay, forest fires (caused by lightning), evaporation of volatile organic compounds from leaves and wind erosion of soil.
Man made pollutants are carbon dioxide (CO₂), oxides of nitrogen (NO, NO₂), sulphur dioxide (CO₂), hydrocarbons, aerosols etc.

Air is also polluted by burning of fuels (coal, oil, gasoline, etc.) in power and industrial plants and in motor vehicles.
Types of air pollutants: There are two types of air pollutants:
(i) Primary air pollutants
(ii) Secondary air pollutants

(i) Primary air pollutants: A primary air pollutant is a harmful chemical substance that directly enters the air as a result of natural events or human activities. For example carbon oxides (CO and CO₂), nitrogen oxides (NO, NO₂), sulphur oxides (SO₂), hydrocarbons and suspended particulate matter.

(ii) Secondary air pollutants: A secondary air pollutant is a harmful material which is formed in the air due to a chemical reaction between two or more air components or a primary pollutant and one or more air components in the atmosphere. For example sulphur dioxide (primary pollutant) reacts with oxygen gas (in the atmosphere) to form secondary pollutant sulphur trioxide (SO₃).

Sulphur trioxide formed may react with water vapours in air to form sulphuric acid.

Sulphuric acid is also a secondary pollutant. Other secondary pollutants are SO₃, H₂SO₄, NO₂, HNO₃, H₂O₂ etc.