Regional-scale impacts of Phase I of the Clean Air Act Amendments in the USA: the relation between emissions and concentrations,both wet and dry

A comparison of data records in the 1990s, both before (1991–1994) and after (1995–1997) implementation of Phase I of the Clean Air Act Amendments (CAAA) of 1990 for the eastern US, shows a significant reduction in SO₂ emissions for most states, except for Texas, North Carolina, Illinois, Florida, and Alabama. However, of the major NO_x emitting states, only two eastern states (New York and Pennsylvania) show significant declines in NO_x. A pattern of large declines in SO₂ emissions (>20%) after CAAA implementation, and large declines in precipitation SO₄^2? and H⁺, as well as air concentrations of SO₂ and SO₄^2? (components of dry deposition), exists for most regions of the eastern US. In most cases, the emission/concentration relations are close to 1 : 1 when the source region based on 15-h back trajectories is used for the New England region, and source regions based on 9-h back trajectories are used for the six other eastern US regions that were studied. The southern Appalachian Mountain region, an acid-sensitive area receiving high levels of acidic deposition, has not seen an appreciable improvement in precipitation acidity. This area has also shown the least improvement in wet and dry sulfur concentrations, of the areas examined. Precipitation base cations (Ca²⁺ and Mg²⁺) show a pattern of either increasing or level concentrations when comparing 1990–1994 to 1995–1998 data, for six of the seven regions examined. Ammonium concentrations have generally changed <10%, except for the Illinois and southern Appalachian Mtn. regions, which increased >15%.