especially in internal combustion engines of automobiles and trucks, partially oxidizes atmospheric nitrogen to the compounds nitric oxide (NO), and nitrogen dioxide (NO2) (collectively referred to as NOx). Once released to the atmosphere, sulfur and nitrogen oxides are oxidized further to sulfuric acid and nitric acid. The other major human activity leading to acid rain is the smelting of sulfide ores of various metals—including iron, copper, and lead—which releases SO2 to the atmosphere.

Sulfur dioxide and NOx have residence times in the atmosphere of up to a few days, by which time they can spread hundreds of kilometers from their sources. In this way, they are able to create broad patches of air pollution over, for instance, much of Europe and most of eastern North America (National Research Council, 1981). The arctic haze phenomenon also is caused by sulfate aerosols, which are carried north in winter from industrial sites in Eurasia (Barrie, 1986; Welch et al., 1991). The polar ice caps show evidence of increasing deposition of sulfate and nitrate beginning with the industrial revolution (Boutron and Delmas, 1980; Wolf and Peel, 1985).

Acid rain is neutralized readily if it falls on fertile soils rich in bases, the more so if the soils contain particles of calcium carbonate weathered from soft limestone rocks. If, however, it falls on infertile soils, poor in bases and derived from hard, slowly weathering rocks such as granite and quartzite, the acid is neutralized only partly by soil bases and by microbial and plant uptake of nitrate. The remainder, chiefly sulfuric acid, is washed into streams and lakes, where further neutralization may take place owing to sulfate reduction in lake sediments and loss of nitrate by algal assimilation or denitrification in the sediments (Cook et al., 1986; Brezonik et al., 1987; Rudd et al., 1988) (see Box 3-8).

If neither soils nor aquatic sediments can neutralize acid rain completely, the pH of the lake or stream water declines, causing a variety of deleterious effects. marked changes occur in the species abundance and composition of all types of aquatic communities, including open-water and shoreline algae, larger aquatic plants, open-water microscopic zooplankton, bottom-living invertebrates, and fish. For instance, increasing acidity can interfere with spawning, so that the population fails to reproduce (Dillon et al., 1984). Alternatively, embryonic development or the development of species in juvenile life stages may not take place normally (Rosseland and Staurnes, 1994). Acidification of lake and stream water also releases toxic forms of aluminum (Al 3+ and AlOH2+) that clot the mucus on fish gills, interfering with their function. Finally, acidification can alter food webs. For example, acidification may lead to a reduction in prey species of minnows and invertebrates so that predators such as lake trout starve to death (Schindler et al., 1985). Minns et al. (1990, 1992) estimate that acid precipitation has eliminated many species of organisms in thousands of sensitive lakes in eastern Canada alone. They further



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