In addition to air pollution effects on human health, impacts on ecosystem form and function are also a serious concern. Moreover, because ecosystems often supply society with valuable services (such as cleaning and purifying water), damage to ecosystems from air pollution can exact a significant economic as well as an environmental cost (Daily 1997). Terrestrial, aquatic, and coastal ecosystems are exposed to air pollution via atmospheric substances (such as O3), or by deposition of substances (such as acids, nutrients, and metals). In terrestrial ecosystems, air pollution deposition affects plant physiology; microbial processes; biogeochemical cycles of substances, such as nitrogen; and plant community dynamics. In aquatic ecosystems, acidic deposition results in acidification of waterways, the mobilization of trace metals in surface waters, and ultimately, the loss of aquatic biodiversity (Driscoll et al. 2001a). Atmospheric deposition is also a major source of mercury to some aquatic ecosystems in North America. When mercury is present as methylmercury in sufficient quantities in the food chain, this contaminant is toxic to humans and animals (NRC 2000a). In addition, atmospheric deposition of nitrate and ammonium might be an important source of nitrogen in coastal regions, contributing to eutrophication, increased or harmful algal blooms, hypoxic and anoxic bottom waters, loss of sea grasses, and reduced fish stocks (Fisher and Oppenheimer 1991; D’Elia et al. 1992; Boynton et al. 1995; Paerl 1997; Castro and Driscoll 2002).
Protection of visibility in national parks and wilderness areas has traditionally played a smaller but nonetheless important role in driving air quality regulation. Scenic vistas in most U.S. parklands are often diminished by haze that reduces contrast, washes out colors, and renders distant landscape features indistinct or invisible. Haze degrades visibility primarily through scattering or absorption of light by fine atmospheric particles (NRC 1993a; Watson 2002).
Air pollution can discolor or damage commonly used building materials and works of art. In addition, such pollutants as sulfate can accelerate the natural weathering process of materials, including metals, painted surfaces, stone, and concrete.
As the health, ecological, and economic impacts of air pollution in the United States have become increasingly evident through more sophisticated scientific approaches, the nation has endeavored to protect air quality through increasingly complex and ambitious legislation (Table 1-1). The federal government’s first major efforts in this regard began in 1955 with the Air Pollution Control Act. These efforts were enhanced over the next 15 years through a series of enactments, including the Clean Air Act (CAA)