Estimates of Sulfate Deposition in the Middle Eastern United States: 1975, 1990, and 2010

ABSTRACT

Deposition of airborne sulfates and nitrates is considered to be an important factor in the decline of certain fish populations and of forest health in the eastern United States. A solution has been sought through legislation that mandates significant reductions in anthropogenic emissions of oxides of sulfur and nitrogen, precursors of sulfates and nitrates. This paper looks at the changes in sulfate deposition in the middle eastern United States from 1975 to 1990 and 2010 due to SO₂ emission reductions brought about by 1970, 1977, and 1990 clean-air legislation. Recently developed emission inventories, together with a statistical, long-range transport model, were used to construct the estimates. The results show that sulfate deposition decreased over the middle eastern United States by about 25% between 1975 and 1990 and will decrease another 45% between 1990 and 2010—reductions commensurate with the overall decreases in SO₂ emissions. The results further indicate that about half of the sulfur deposited in the environmentally sensitive southern Appalachian Mountains comes from states not a part of the southern Appalachian region. These estimates contain a measure of uncertainty due to uncertainty in emission estimates, the interannual variability of meteorology, and uncertainty arising from model assumptions and choices of oxidation and deposition rates. The model was able to reproduce about 10% of the variability in observations of wet sulfate deposition at over 100 sites over a 12-year period.