Impact of sulfur dioxide oxidation by Stabilized Criegee Intermediate on sulfate

Abstract

We revise the Carbon Bond chemical mechanism to explicitly represent three Stabilized Criegee Intermediates (SCIs) and their subsequent reactions with sulfur dioxide, water monomer, and water dimer, and incorporate the reactions into the Community Multiscale Air Quality model. The reaction of sulfur dioxide with SCI produces sulfuric acid which partitions into sulfate. We examine the impact of sulfur dioxide oxidation by SCI on sulfate using two different measured rate constants for the reaction of sulfur dioxide and SCI. When we use the higher rate constant and emissions estimates from the Biogenic Emissions Inventory System, it enhances monthly mean sulfate in summer by ∼20% in biogenically active areas. Enhancements are driven primarily by SCI produced from the reactions of biogenically derived alkenes and ozone. The use of the lower rate constant only marginally enhances sulfate since it is 65 times lower than the higher rate constant. We performed several sensitivity analyses to investigate the impacts of uncertain biogenic emissions and SCI loss rates. When we use the higher rate constant and emissions estimates from the Model of Emissions of Gases and Aerosols from Nature, it enhances monthly mean sulfate by ∼75%. A simulation using the lowest reported rate constant for the reaction of SCI and water indicated the maximum enhancement of sulfate from this chemistry was up to 4 μg/m3 over a 24-h period in some locations in the Southeastern U.S. Predictions without the SCI reaction are lower than observed sulfate while predictions with the SCI reaction improve the agreements with observations.