As a result of removal of natural vegetation, industrialization and increase in the number of automobiles, the air over big cities is heavily contaminated with dust, smoke and poisonous gases like carbon monoxide, sulphur dioxide and oxides of nitrogen.
The contamination of air with dust, smoke and harmful gases, which adversely affect human beings and other living organisms, is called air pollution. The substances whose presence in air produces air pollution are called air pollutants.
Some of the major air pollutants are carbon monoxide, excess carbon dioxide, sulphur dioxide, oxides of nitrogen, lower organic molecules, dust and smoke. Polluted air produces a large number of harmful effects on the living and non-living things.
Sources of Pollutants in the Air
Automobiles, industries and thermal power plants are the major sources of air pollutants from human activities. It may be mentioned here that pollution is caused not only by the activities of man but also by natural processes. For example:
- Volcanic eruptions release large amounts of gases and particulate matter in the air
- Forest fires release CO2 and smoke
- Decomposition of plant and animal residue
- Pollen grains, storms
- Methane gas from marshes
However, the contribution from these natural processes is within tolerable limits. On the other hand, the contribution from man-made sources is much larger.
Types of Air Pollutants
On the basis of particle size, there are three major categories of air pollutants: gaseous pollutants, particulate pollutants and aerosols.
1. Gaseous pollutants consist of atoms, molecules and include harmful gases, which can freely mix with air without settling down. Some examples of gaseous pollutants of air are carbon monoxide, carbon dioxide, sulphur dioxide, hydrogen sulphide, nitrogen oxides and hydrocarbons.
2. Particulate pollutants include finely divided solids as well as liquids having particle size from 10-4 to 10-3 cm. Particulates are harmful to the living as well as non-living things. The examples of particulate pollutants in the air are: dust, smoke, clouds, fumes, mist, spray and smog.
3. Aerosols are suspensions of fine particulate matter in the air. Aerosols have particle size smaller than particulates. Their particle size ranges from 10-7 cm to 10-4 cm. Aerosols can be either liquid or solid particles. They are small enough to remain suspended in the atmosphere for long periods of time. Smoke, fine dust, fog, clouds are examples of aerosols.
Particulates and aerosols serve as collectors of chemically active sulphur oxides, nitrogen oxides, ozone, hydrocarbons and other pollutants and are serious health hazards.