ABSTRACT
Human-caused emissions of S, N, and Hg have been relatively high in the past in some areas but have decreased over the past few decades throughout much of the United States in response to emissions controls that resulted from the Clean Air Act and subsequent federal and state regulations. Particularly important have been controls on large power plants and motor vehicles, changes in industrial processes, and changes in fuels (i.e., coal to oil and gas). In general, the states that have had the highest levels of S and N emissions have shown the largest reductions in response to national Acid Rain Program regulations and other U.S. Environmental Protection Agency (EPA) programs, including the Clean Air Interstate Rule (Burns et al. 2011).
