1. Air pollution and weather are linked in two ways. One concerns the influence that weather conditions have on the dilution and dispersal of air pollutants. The second connection is the reverse and deals with the effect that air pollution has on weather and climate.
2. Air is never perfectly clean. Volcanic ash, salt particles, pollen and spores, smoke, and windblown dust are all examples of “natural air pollution.” Although some types of air pollution are recent creations, others, such as London’s infamous smoke pollution, have been around for centuries. One of the most tragic air-pollution episodes occurred in London in December 1952, when more than 4000 people died.
3. Air pollutants are airborne particles and gases that occur in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the environment. Pollutants can be grouped into two categories: (1) primary pollutants, which are emitted directly from identifiable sources, and (2) secondary pollutants, which are produced in the atmosphere when certain chemical reactions take place among primary pollutants. The major primary pollutants include, particulate matter (PM), sulfur dioxide, nitrogen oxides, volatile organic compounds (VOCs), carbon monoxide, and lead. Atmospheric sulfuric acid is one example of a secondary pollutant. Air pollution in urban and industrial areas is often called smog. Photochemical smog, a noxious mixture of gases and particles, is produced when strong sunlight triggers photochemical reactions in the atmosphere. A major component of photochemical smog is ozone.
4. Although considerable progress has been made in controlling air pollution, the quality of the air we breathe remains a serious public health problem. Economic activity, population growth, meteorological conditions, and regulatory efforts to control emissions all influence the trends in air pollution. The Clean Air Act of 1970 mandated the setting of standards for four of the primary pollutants—particulates, sulfur dioxide, carbon monoxide, and nitrogen oxides—as well as the secondary pollutant ozone. In 2004, emissions of the five major primary pollutants in the United States were about 54 percent lower than 1970. In 1990 Congress passed the Clean Air Act Amendments, which further tightened controls on air quality. Regulations and standards regarding the provisions of the Clean Air Act Amendments of 1990 are periodically established and revised.
5. The most obvious factor influencing air pollution is the quantity of contaminants emitted into the atmosphere. However, when air-pollution episodes take place, they are not generally the result of a drastic increase in the output of pollutants; instead, they occur because of changes in certain atmospheric conditions. Two of the most important atmospheric conditions affecting the dispersion of pollutants are (1) the strength of the wind and (2) the stability of the air. The direct effect of wind speed is to influence the concentration of pollutants. Atmospheric stability determines the extent to which vertical motions will mix the pollution with cleaner air above the surface layer. The vertical distance between Earth’s surface and the height to which convectional movements extend is called the mixing depth. Generally the greater the mixing depth, the better the air quality. Temperature inversions represent a situation in which the atmosphere is very stable and the mixing depth is significantly restricted. When an inversion exists and winds are light, diffusion is inhibited and high pollution concentrations are to be expected in areas where pollution sources exist. Surface-temperature inversions form because the ground is a more effective radiator than the air above. Inversions aloft are associated with sinking air that characterizes centers of high air pressure.
6. In most areas within several hundred kilometers of large centers of human activity, the pH value of precipitation is much lower than the usual value found in unpopulated areas. This acidic rain or snow, formed when sulfur and nitrogen oxides produced as by-products of combustion and industrial activity are converted into acids during complex atmospheric reactions, is called acid precipitation. The atmosphere is both the avenue by which offending compounds travel from sources to the sites where they are deposited and the medium in which the combustion products are transformed into acidic substances. Beyond possible impacts on health, the damaging effects of acid precipitation on the environment include the lowering of pH in thousands of lakes in Scandinavia and eastern North America. Besides producing water that is toxic to fish, acid precipitation has also detrimentally altered complex ecosystems.