HMS/Sulfate Ratio as a Constraint on Aqueous Sulfur Chemistry Under Cold and Dark Conditions
JAMES CAMPBELL, Michael Battaglia, Kayane Dingilian, Jason St. Clair, Meeta Cesler-Maloney, William Simpson, Athanasios Nenes, Rodney J. Weber, Jingqiu Mao,
University of Alaska Fairbanks Abstract Number: 448
Working Group: Aerosol Chemistry
AbstractFairbanks, Alaska is a subarctic city with fine-particle (PM2.5) concentrations that often exceed air quality regulations in winter due to weak dispersion caused by strong atmospheric inversions, local emissions, and the unique chemistry occurring under cold and dark conditions. Sulfate accounts for roughly 20% of PM2.5 mass in wintertime Fairbanks. Hydroxymethanesulfonate (HMS) is comparable to PM2.5 sulfate (HMS/sulfate molar ratio of 26-41%), and comprises a significant fraction of overall PM2.5 mass concentration (2.8-6.8% by mass) during polluted episodes. This high relative fraction of HMS is substantially higher than observed elsewhere, likely due to low temperatures and high levels of precursors. Here, we developed a box model that couples aerosol thermodynamics (using the ISORROPIA-II model) that explicitly calculates the liquid water uptake, inorganic speciation and acidity with aerosol aqueous chemistry (F0AM-aq) to study the secondary production of sulfate and HMS in Fairbanks winter. We find that both HMS and sulfate production are self-limiting owing to the rapid change of aerosol acidity as a result of sulfate production. While uncertainties exist in the aerosol pH, ionic strength, aqueous diffusion and chemistry, we find that the HMS/sulfate ratio depends primarily on the branching ratio of several sulfur (IV) reaction pathways, providing a unique constraint on aqueous sulfur chemistry under cold and dark conditions. Subsequent work will focus on the role of HMS and organic species on ALWC and its impact on acidity and aqueous chemistry, by replacing ISORROPIA-II with its most recent version (ISORROPIA-Lite) that can consider such interactions.