Air pollution may be defined as: “Contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere” Air pollution can occur naturally or by anthropogenic activities in the form of solid particles, liquid droplets, or gases. The pollutants can be divided into primary and secondary with respect to their origin. Primary pollutants are those which are directly emitted from a process, such as ash from volcanic eruptions, dangerous gases from car exhausts or factories, etc. Secondary pollutants are usually derived from primary pollutants after their reaction or interaction in the air like ground-level ozone.
Some of the major primary pollutants and their major sources are as follows:
1. Sulphur Dioxide (SO2): Produced by volcanoes, in various industrial processes from combustion of coal and petroleum.
2. Nitrogen Oxide (NOx): Emitted from high temperature combustion, can be seen as the brown haze dome above or downwind of cities.
3. Carbon Monoxide (CO): Formed by incomplete combustion of fuel such as gas, coal or wood and from vehicular exhaust.
4. Carbon Dioxide (CO2): A naturally existing greenhouse gas (GHG) in the atmosphere, it is also emitted from combustion.
5. Volatile Organic Compounds (VOCs: An outdoor air pollutant, mainly divided into methane and non-methane (NMVOCs). Methane is an efficient GHG but other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone and in prolonging the life of methane in the atmosphere, although the effect varies depending on local air quality.
6. Particulate Matter (PM) or fine particles: Formed by both natural and man-made activities. Naturally, they originate from volcanoes, dust storms, forest and grassland fires, living vegetation, sea spray and alternatively through burning of fossil fuels in vehicles, power plants and various industrial processes.
7. Chlorofluorocarbons (CFCs): Comes from coolants and refrigerants but most of the products emitting it are banned now.
8. Ammonia (NH3): Emitted in agricultural processes, from organic waste in biogas plants and landfills.
9. Radioactive pollutants: They are produced by nuclear explosions, war explosives, and natural processes such as radioactive decay.
Secondary pollutants and their sources:
1. Photochemical Smog: A mixture of smoke and SO2 in coal burning areas result in classic smog but now it’s also from vehicular and industrial emissions, which is acted by sunlight to form secondary pollutants. They also combine with the primary emissions to form photochemical smog.
2. Ground level Ozone (O3): Formed from NOx and VOCs by photochemical reactions in the presence of sunlight. At abnormally high concentrations brought about by human activities (largely the combustion of fossil fuel), it is a pollutant, and a constituent of smog.