A similar process produces nitric acid from nitrogen oxides. Sulfur and nitrogen oxides are emitted into the air through smokestacks at power plants and tailpipe emissions from cars, buses, and trucks. Electric utilities accounted for more than 75 percent of sulfur oxides emitted nationwide in 1990. The total amount decreased about 24 percent between 1980 and 1989, however, thanks to the increased use of low-sulfur coal and scrubbers that remove much of the sulfur compounds before they are emitted into the air. Emissions of nitrogen oxides, which stood at 22.1 million tons in 1985, also contribute to rain acidity.
Nitrogen oxides come more from automobiles than from electric power plants.
Amounts of nitrogen oxides have dropped by 5 percent during the last decade. The concentration is partially caused by wind patterns transporting pollutants from midwestern power plants, which primarily burn high-sulfur coal mined in the East. Power plants in the West burn mostly western low-sulfur coal. The proximity and plentiful availability of higher sulfur coal has made it difficult for utilities in the Midwest and Northeast to switch to lower sulfur coals to clean up emissions without significantly raising utility rates. Soil type also plays a critical role in acid rain's effects.