Upper and Lower Limits of the Rate of SO2 Oxidation In Wintertime Atmosphere Over The Eastern United States

A set of constraints on the rate of oxidation of SO2 during wintertime conditions over the Eastern United States is presented, based on measurements taken during a series of night and day flights on a C-130 aircraft from Feb 3 to Mar 13, 2015 over the Eastern coastal region of the United States during the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign. In the Eastern United States, there are fewer reactant sinks for gaseous SO2 during the winter, and as a result the atmospheric lifetime is measurably longer. Photochemical oxidation of SO2 is extremely slow during winter due to significantly reduced OH concentrations compared to summer. The long photochemical lifetime enables analysis of rates of proposed alternative sulfur oxidation mechanisms, such as heterogeneous uptake. An examination of the SO4/SO2 ratio and the mixing ratio will be used to determine the upper and lower limits of the rate of SO2 oxidation, along with the branching ratios of the gas-phase and heterogeneous removal. A correlation of SO2 to the co-emitted compounds measured in the region will allow for the accurate assignment of intercepted plumes to individual power plants and urban areas. A combination of HYSPLIT trajectory modeling, use of several chemical clocks, and analysis of the wind speed along the path of the plume is used to constrain the travel time from the source to the intercepted plume.