Air in the atmosphere is never pure and clean. It contains many chemical substances which arise because of natural or human activity. These substances include:
- gases like CO, CO2, SO2, NO2, H2S, NO, hydrocarbons (CxHy) along with many other volatile organic compounds and
- suspended particulate matter such as dust, smoke, fumes, and also some radioactive material.
The presence of these substances in low concentration does not cause any major problem but when their concentration becomes high they produce harmful effects and becomes pollutants. For example, SO2 is pollutant when its concentration in the atmosphere exceeds the normal limit of 2 × 10-4 ppm. Similarly, CO becomes pollutant when its concentration exceeds the normal limit of 1 x 10-4 ppm.
Air pollution may thus, be referred to as any condition in which the. chemical substance already present in air or those which are added or formed in air; gradually build up their concentration in the atmosphere to such an extent that is undesirable and produce harmful effects on man and his environment.
Air pollution can be caused both by natural as well human activities.
Three main natural activities responsible for air pollution are:
- Wild fires. These release oxides of carbon CO, CO2 in air through combustion.
- Volcanic Eruptions. The volcanic eruptions release CO2, SO2, SO3, HCI, H2S, water vapours, particulate matter, etc into air.
- Duststorms. Windblown dust or windstorms also cause the release of particulate matter in air.
The human activities which cause air pollution are:
- Burning of octane, fossil fuels like coal, oil in limited supply of oxygen to release CO and H2O(g)·
- Vehicular exhausts release CO, oxides of nitrogen.
- Combustion of sulphur rich ores which release oxides of sulphur in air.
Let us discuss major air pollutants, their source, their harmful effects and the control.