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Air pollutants are substances that adversely affect the environment by interfering with climate, the physiology of plants, animal species, entire ecosystems, as well as with human property in the form of agricultural crops or man-made structures. We list climate at the top of the list to reflect the fact that global climate change has been recognized as one of the most important environmental challenges to be faced by humanity in the 21st century. In this context certain climate forcing agents—the most important one being carbon dioxide—which otherwise cause no harm to living organisms, should be added to the list of “classic” pollutants, along with such compounds as oxides of nitrogen or sulfur. On the other hand, climate research has linked certain compounds long recognized as air pollutants (for instance black carbon) to the warming of climate, thus providing one more reason for their control.
Air pollutants can originate from natural or anthropogenic (man-made) sources, or both. Examples of natural sources of pollution include volcanic eruptions or wind erosion. Emissions from internal combustion engines are an exemplary source of anthropogenic pollution. Some sources of pollution, such as forest fires, can be related to both natural phenomena and human activities.
Atmospheric reactions can transform primary pollutants into different chemical species. These reactions can produce both harmless compounds and secondary air pollutants that may be more harmful than their precursors.
The world’s most important air pollutants, their sources, and known or suspected environmental effects are listed in Table 1 (after ).
|Pollutant||Natural Source||Anthropogenic Source||Environmental Effect|
|Nitrogen oxides (NO + NO2)||Lightnings, soil bacteria||High temperature fuel combustion—motor vehicles, industrial, and utility||Primary pollutants that produce photochemical smog, acid rain, and nitrate particulates. Destruction of stratospheric ozone. Human health impact.|
|Particulates||Forest fires, wind erosion, volcanic eruption||Combustion of biofuels such as wood, and fossil fuels such as coal or diesel||Reduced atmospheric visibility. Human health impact. Black carbon particulates contribute to global warming.|
|Sulfur dioxide||Volcanic eruptions and decay||Coal combustion, ore smelters, petroleum refineries, diesel engines burning high-sulfur fuels||Acid rain. Human health impact.|
|Ozone||Lightning, photochemical reactions in the troposphere||Secondary pollutant produced in photochemical smog||Damage to plants, crops, and man-made products. Human health impact.|
|Carbon monoxide||Unnoticeable||Rich & stoichiometric combustion, mainly from motor vehicles||Human health impact|
|Carbon dioxide||Animal respiration, decay, release from oceans||Fossil fuel and wood combustion||Most common greenhouse gas|
|Non-methane hydrocarbons (VOC)||Biological processes||Incomplete combustion, solvent utilization||Primary pollutants that produce photochemical smog|
|Methane||Anaerobic decay, cud-chewing animals, oil wells||Natural gas leak and combustion||Greenhouse gas|
|Chlorofluorocarbons (CFC)||None||Solvents, aerosol propellants, refrigerants||Destruction of stratospheric ozone|
Governments and international organizations have been taking actions to protect the quality of air, as well as—in more recent years—to control emissions of climate forcing agents. Ambient air quality standards and guidelines, issued by environmental protection authorities, are instrumental in achieving the air quality objective. An example of such legislation is set by the US National Ambient Air Quality Standards (NAAQS) adopted by the Environmental Protection Agency (EPA). The NAAQS apply to both human health (primary standard) and public welfare (secondary standard). Primary standards protect sensitive members of the human population from adverse health effects of criteria air pollutants. Secondary standards protect the public welfare from any known or anticipated adverse effects associated with the presence of a pollutant in the ambient air. Welfare effects include effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility, climate, damage to and deterioration of property, hazards to transportation, as well as effects on economic values and personal comfort and well-being.
Under the US Clean Air Act of 1990, the NAAQS standards set maximum ambient concentration limits for six criteria pollutants including: