10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

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Exploration of Potentially Increasing SO2 Oxidation in the Eastern United States

BRET SCHICHTEL, Anthony Prenni, Jenny Hand, Scott Copeland, Kristi Gebhart, John Vimont, William Malm, Jeffrey Collett, National Park Service

     Abstract Number: 1522
     Working Group: Aerosol Chemistry

Abstract
In the early 1990s, ammoniated sulfate accounted for about two-thirds of the fine particulate mass in the rural eastern U.S. as measured by the Interagency Monitoring of Protected Visual Environments (IMPROVE) program. Particulate sulfate also had a distinct seasonal cycle, with summer concentrations a factor of 2 larger than in winter. In the ensuing 25 years, sulfur dioxide (SO2) emissions have decreased by about a factor of 10, resulting in well-documented decreases in SO2 measured by the Clean Air Status and Trends Network (CASTNET) program and particulate sulfate measured by the IMPROVE and CASTNET programs. The reductions in particulate sulfate were commensurate with the SO2 emission reductions until about 2008, when the trends diverged, after which time smaller decreases in particulate sulfate occurred for every unit decrease in SO2 emissions. The ratios of ambient sulfate to SO2 also increased in CASTNET measurements. In addition, particulate sulfate seasonality was substantially reduced throughout the eastern U.S. One explanation for this recent nonlinear response of particulate sulfate to changes in SO2 emissions is that the SO2 to sulfate oxidation may have become more efficient, resulting in a greater fraction of SO2 being converted to sulfate. Alternatively, these observations may be driven by increased background sulfate concentrations from international sources or primary emissions of particulate sulfate. In this presentation we will explore these possible causes and present the available evidence suggesting that the fraction of SO2 oxidized to sulfate in the eastern U.S. has increased over the past 10 years.