Impacts of air pollution
Human activities and natural processes release chemicals into the atmosphere and can result in air pollution. Once in the atmosphere, the chemicals are subjected to various atmospheric processes that control their transport, and can alter their chemical and physical form. The eventual environmental impact of chemicals released to the atmosphere is therefore greatly influenced by these processes.
The impacts of air pollution can range from poor air quality in the close vicinity of a source, to the disruption of natural chemical cycles and physical processes that occur on a global scale. The properties that are attributable to the different environmental impacts of some of the major pollutants are shown in the table below.
|Lead, volatile organic compounds, sulphur dioxide, nitrogen oxides, carbon monoxide, benzene, 1,3-butadiene, fine particles*.
|These chemicals are emitted directly into the atmosphere and are known as primary pollutants.
(An exception is NO2 which is mainly a secondary pollutant, rapidly formed from NO released to the atmosphere)
|Reduction of local air quality - affecting human health and vegetation growth, and causing damage to materials.
|Sulphur dioxide, nitrogen oxides, hydrochloric acid.
|After release into the atmosphere these compounds can be deposited close to their source. Alternatively chemical reactions can convert them into other acidic compounds that can be transported long distances before deposition to the surface.
|Acid deposition - leads to degradation of the terrestrial environment.
|These chemicals are termed secondary pollutants, because they are formed by reactions involving primary pollutants (see above).
Increased concentrations of these compounds occur because of the disruption of natural chemical cycles due to the action of sunlight on enhanced concentrations of volatile organic compounds and nitrogen oxides.
Impacts generally occur at a distance from the source, as the chemical reactions take time to produce the oxidants.
Photochemical oxidants - reduce local air quality (as above).
|Fine particles (generally <2.5µm in diameter)
|Unlike primary particles, which are emitted directly by various processes, secondary particles are formed in the atmosphere from chemical reactions mainly involving SO2 and NOx. These secondary particles can be transported long distances.
|Formation of secondary particles - human health impacts
|Carbon dioxide, methane, nitrous oxide, halocarbons.
|These chemicals are only slowly removed from the atmosphere and therefore their concentrations are continually increasing because their emission rate is greater than their removal rate.
Each chemical can effectively absorb long wave radiation, which results in a warming of the atmosphere.
|Enhanced greenhouse effect - leading to greater climate change.
|These chemicals can build up in the atmosphere like the greenhouse gases above. They remain in the atmosphere long enough to be transported to the upper part of the atmosphere, where the protective 'ozone layer' is concentrated. They can disrupt natural chemical reactions, leading to ozone destruction.
|Destruction of stratospheric ozone - causing increased UV radiation at the earth´s surface.
*fine particles can be both primary and secondary pollutants.