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Air pollutants are generally defined as substances of gaseous, solid or liquid nature emitted into the atmosphere by both natural and anthropogenic (human-caused) sources, that are capable of causing harm to the environment in general, and living organisms in particular.
In other words, these pollutants are the agents that change the normal functioning of the atmosphere and have an negative impact on all life forms including animals and plants.
Possibly one of most important characteristics of these pollutants is their transboundary nature. They know no borders and they can travel long distances from their source of origin to the places that are not related to them in any shape or form.
An excellent example of that is the Arctic Haze, a visible reddish brown haze found above the Arctic mainly in winter months and caused by pollutants in the air travelling from global sources thousands of miles away, specifically from Asia.
This contributes to warming the Arctic faster than any other region in the world, causing the sea ice to melt and destabilising the environment. (1)
Pollutants in the air can also be of primary or secondary nature.
Primary air pollutants are the ones that are emitted directly into the atmosphere by the sources (such as power-generating plants).
Secondary air pollutants are the ones that are formed as a result of reactions between primary pollutants and other elements in the atmosphere, such as ozone.
Once in the air, primary pollutants may undergo chemical reactions with other elements and become part of different compounds, and thus transforming into secondary pollutants.
There are two ways for primary and secondary pollutants to be removed from the atmosphere (this process is called deposition).
The first one is wet deposition. It works as follows. Pollutants make way into clouds or other precipitation and then get deposited on to the surface of the Earth by way of rain (specifically, acid rain).
The second one is dry deposition, during which pollutants are deposited directly on to the surface of the Earth and vegetation, such as plants and trees of tropical rainforests.
Among the most important pollutants are sulphur dioxide (SO2), nitric oxide (NO), ozone, volatile organic compounds (VOCs) and other pollutants which are discussed in more detail in the Air Pollutants article.
Last update: April 2008
