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Data collected in the eastern US between 1990 and 2009 at 34paired dry and wet monitoring sites are examined. A goal is to evaluate theair quality impacts occurring between 1990 and 2009 that are associated withconcurrent legislatively mandated changes in emissions. Four 5-year periods,1990–1994 (P1), 1995–1999 (P2), 2000–2004 (P3), and 2005–2009 (P4) areconsidered, with a primary focus on P1-to-P4 changes. Results suggest thatlegislatively mandated air pollution mitigation strategies have beensuccessful in improving air quality and reducing atmospheric deposition inthe eastern US. Respective P1-to-P4 reductions of estimated sulfur dioxide (SO2) andnitrogen oxides (NOx) emissions in the eastern US are 50 and 42%.Corresponding behavior of the following metrics associated with theseemissions reductions is examined: monitored atmospheric concentrations ofSO2, aerosol sulfate (SO4), and oxidized sulfur (S); dry, wet, andtotal deposition of S; monitored atmospheric concentrations of nitric acid(HNO3), aerosol nitrate (NO3), and their sum, oxidized nitrogen(OxN); dry, wet, and total deposition of OxN; monitored atmosphericconcentration of aerosol ammonium (NH4); dry, wet, and total depositionof NH4; summed monitored atmospheric concentration of oxidized andreduced nitrogen (N); dry, wet, and total deposition of N; wet deposition ofhydrogen ion (H+); monitored atmospheric concentration of ozone(O3); dry deposition of O3; and the summed monitored atmosphericconcentration of aerosol NO3, SO4, and NH4 (Clean AirStatus and Trends Network particulate matter – CASTNET PM).Other metrics (e.g., ratios of dry to total deposition) are also considered. Selected period-to-period changes of air quality and deposition metrics atsite, regional, and seasonal scales are discussed. As an example, despiteP1-to-P3 reductions in estimated emissions of both SO2 and NOx, aerosolNO3 concentration increased in the east, with widespread wintertimenumerical increases in both aerosol NO3 concentration and CASTNET PM.However, a reversal of this behavior is associated with continuing P3-to-P4reductions of SO2 and NOx emissions. Thus, additional P3-to-P4reductions of these emissions, especially NOx, appear to have made progressin altering the chemical regime of the wintertime eastern US atmosphere sothat future emissions reductions and their resulting reductions in aerosolconcentrations may no longer be accompanied by sub-linear changes (or actualincreases) in CASTNET PM

Air quality and atmospheric deposition in the eastern US: 20 years of change