Air pollution is the presence in the atmosphere of any substance at a concentration great enough to produce an undesirable effect on humans, animals, vegetation, or materials, or to significantly alter the natural balance of any ecosystem. Air pollutants can be solids, liquids, or gases, and can have local, regional, and global impacts.
At urban scales, air pollution is frequently referred to as photochemical smog. "Smog" is a contraction of the words "smoke" and "fog," and was originally used to describe air pollution caused by coal burning in London. Urban smog is photochemical because many of the chemicals found in urban air are formed by chemical reactions driven by sunlight. Among the many air pollutants in urban smog that are produced by photochemical reactions, one of the most abundant is ozone, O 3 . In contrast to the ozone found in the upper atmosphere (stratospheric ozone), which protects the planet from ultraviolet radiation , ground level or tropospheric ozone is a lung irritant and a danger to human health. It is also responsible for crop damage and is suspected of being a contributor to forest decline in Europe and in parts of the United States. Ground level ozone and other photochemical pollutants are formed in urban atmospheres by the reactions of oxides of nitrogen (mainly NO and NO 2 ) in the presence of hydrocarbons. Oxides of nitrogen are byproducts of combustion processes. At the high temperatures generated during combustion, some of the N 2 and O 2 in air is converted to oxides of nitrogen and, in general, the higher the combustion temperature, the greater are the amounts of oxides of nitrogen produced. Hydrocarbons are emitted from natural sources and as a result of activities utilizing organic solvents, coatings, or fuel. These hydrocarbons and oxides of nitrogen participate in reactions that yield, not only ozone, but also aldehydes, hydrogen peroxide, peroxyacetyl nitrate (C 2 H 3 NO 5 ), nitric acid, and molecular species of low volatility that accumulate in fine particles suspended in the atmosphere. Although many of these constituents of photochemical smog have environmental impacts, fine particulate matter (PM) presents the greatest health endangerment in most urban areas.
Solid and liquid phase material in the atmosphere is variously referred to as particulate matter, particulates, particles, and aerosols. These terms are often used interchangeably, but all refer to particles with diameters between approximately 1 nanometer (3.9 × 10 −8 inches) and 10 micrometers (39.4 × 10 −5 inches) that remain suspended in the atmosphere for long periods. The greatest threats to health are associated with the smallest particles because they have the greatest likelihood of becoming deposited deep within the respiratory system.
Somewhat counterintuitively, particles of about 1 micrometer (39.4 × 10 −6 inches) in size can remain suspended in the atmosphere much longer than gases. Particles much larger than 1 micrometer (39.4 × 10 −6 inches) will, of course, quickly settle out of the atmosphere because of gravity. The smallest particles will coagulate and coalesce quickly, forming larger particles. But particles of approximately 1 micrometer (39.4 × 10 −6 inches) in diameter do not grow as quickly as smaller particles and can remain suspended in the atmosphere for a week or more. It is not unusual, for example, for Saharan dust or particle plumes from Asia to be detected in the United States. Consequently, particulate matter is a continental to global scale air pollution problem.
Also, unlike ozone and other gas phase pollutants that are specific chemical species, particulate matter is a collection of chemical species defined mainly on the basis of particle size. The chemical constituents that make up particulate matter vary with particle size. Windblown dust is a main contributor to particles larger than 10 micrometers (39.4 × 10 −5 inches) in diameter, whereas sulfates, nitrates, and organic compounds are the main constituents of smaller particles that can penetrate deeply into the respiratory system and engender health effects. Organic particles can be emitted directly as soot from combustion processes or can be formed when large hydrocarbon molecules react with oxidants in the atmosphere and form chemicals that condense onto particles. Sulfate particles are formed via a series of reactions that convert sulfur dioxide, SO 2 , which is released into the atmosphere by the combustion of sulfur containing fuels, into sulfuric acid. Nitrate particles are formed via reactions that convert oxides of nitrogen, which are released into the atmosphere by combustion processes, into nitric acid. If particles containing sulfuric acid, nitric acid, and/or organic compounds
retain their acidity and are washed out of the atmosphere by rainfall, the rainfall becomes acid rain . Figure 1 shows acidity of rainfall averages in the United States and provides a sense of the continental scale of particulate matter air pollution.
The continental and global scale of air pollution problems is not limited to particulate matter. Emissions of greenhouse gases cause global climate change. The presence in the stratosphere of ozone-depleting compounds has created polar ozone holes. Atmospheric releases caused by volcanic eruptions and fires have global effects. Atmospheric particles also influence climate and rainfall. The challenges of reducing air pollution call for a sophisticated understanding of atmospheric chemistry, applied at local, regional, continental, and global scales.
SEE ALSO Atmospheric Chemistry .
David T. Allen
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