Changes in air quality and atmospheric deposition in the eastern United States: 1990-2004

Data collected in the eastern United States (U.S.) between 1990 and 2004 at 34 dry and paired wet monitoring sites are examined. A goal is to evaluate the air quality impacts occurring between 1990 and 2004 resulting from legislatively mandated changes in emissions. Three 5-year periods, 1990-1994 (P1), 1995-1999 (P2), and 2000-2004 (P3) are considered. Period-to-period changes in selected pollutant metrics are examined, focusing on P1-to-P3 changes. Data are composed from reported weekly measurements into estimates of means for year, site, and season. The mean squared error derived from analysis of variance applied to these means for atmospheric concentration, dry deposition velocity, precipitation rate, and dry, wet and total deposition is used to examine differences between periods for seasons and predefined regional groupings of sites. Results suggest that relationships exist at the current scale between changes in both concentration and deposition of relevant atmospheric pollutants and changes in SO2 emissions that are generally less than 1:1 and that these disparities are more pronounced for SO42- (a reaction product) than SO2 (the primary pollutant). Coincident timing and location suggest that legislatively mandated summertime reductions in estimated NOx emissions contributed strongly to observed reductions of atmospheric HNO3 concentration and dry deposition in the eastern U. S. Less than 1: 1 relationships are also indicated at the current scale between changes in both concentration and deposition of the relevant measured secondary atmospheric pollutants, HNO3 and NO3-, and changes in NOx emissions. In the face of P1-to-P3 reductions in estimated emissions of both SO2 and NOx, wintertime changes in the sum of atmospheric SO42-, NO3-, and NH4+ concentrations, relative to those for corresponding SO42- concentrations, range from reductions that are less than 1:1 to actual increases.