Figure 1. Weighted average pH in precipitation in Europe, July 1972 to March Data from Schaug, 1975, 1976. 1975. (From Tollan 1977). Figure 2. Weighted average pH in precipitation in North America, 1972-75. (From Tollan 1977). marily by distance from identifiable sources and by having sulfuric and nitric acids as the principal contaminants. Acid deposition is a better term for these phenomena. Rain and snow over most of the United States east of the Mississippi River have average annual acid concentrations of from 25 to 80 eq/l (pH 4.6 to 4.1), with much of the Northeast receiving precipitation with an annual average acid concentration of pH near 4.1. Unpolluted rain and snow should be slightly acidic with 2.5 eq/l (pH 5.6), so that current acidity of precipitation, averaged through the years, is ten to thirty times greater than unpolluted precipitation (Likens et al. 1979). EMISSIONS AND AIR QUALITY The acidity of precipitation in the northeastern U.S., below pH 5.6, is due almost entirely to sulfuric and nitric acids (H2SO4 and HNO3) (Likens and Bormann 1974; Galloway et al. 1976). But the ratio of these two acids has been changing. Between 1964-65 and 1972-73, H2SO4 decreased from 83% to 66% of the total acids while HNO3 increased from 15% to 30% (Likens 1976). These acids are derived primarily from oxides of sulfur and nitrogen emitted into the atmosphere from fossil fuel combustion, ore processing and metal refining, and the amount of NOx emitted has been increasing more rapidly than SOX emissions. Figures 3 and 4 present estimated national emissions trends for SOx and NOX. From 1940 to 1976, the data are based on a 1978 report from the Environmental Protection Agency. From 1985 to 2000, estimates of emissions were obtained from several future energy scenarios which are currently being analyzed and reviewed at Brookhaven National Laboratory, under the sponsorship of DOE's Office of Technology Impacts. These scenarios are based on the Second National Energy Plan (NEP II), which predicts a total energy use in the US in the year 2000 of about 120 Quads (1 Quad = 1015 Btu). The future-year data were adjusted to correspond to the 1975 EPA figures in order to present continuous trends. The shaded bands on Figure 4 represent the variations among scenarios; for sulfur oxides, these variations were much larger, due primarily to the uncertainties of retrofit controls on existing power plants, and thus were not shown. The SOx emissions shown for 1985-2000 are thus probably an upper limit. Between 1940 and 1960 the estimated total U.S. emission of sulfur oxides (SOX) to the atmosphere increased very little, from 21.9 million tons per year |