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 afte r (1995-1997) implementation of Phase I of the Clean Air Act Amendments (CA AA) of 1990 for the eastern US, shows a significant reduction in SO2 emissi ons for most states, except for Texas, North Carolina, Illinois, Florida, a nd Alabama. However, of the major NOx emitting states, only two eastern sta tes (New York and Pennsylvania) show significant declines in NOx. A pattern of large declines in SO2 emissions (> 20%) after CAAA implementation, and large declines in precipitation SO42- and H+, as well as air concentrations of SO2 and SO42- (components of dry deposition), exists for most regions o f the eastern US. In most cases, the emission/concentration relations are c lose 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 trajectori es are used for the six other eastern US regions that were studied. The sou thern Appalachian Mountain region, an acid-sensitive area receiving high le vels of acidic deposition, has not seen an appreciable improvement in preci pitation acidity. This area has also shown the least improvement in wet and dry sulfur concentrations, of the areas examined. Precipitation base catio ns (Ca2+ and Mg2+) show a pattern of either increasing or level concentrati ons when comparing 1990-1994 to 1995-1998 data, for six of the seven region s examined. Ammonium concentrations have generally changed < 10%, except fo r the Illinois and southern Appalachian Mtn. regions, which increased >15%. (C) 2001 Elsevier Science Ltd. All rights reserved.