Strong Acceleration of SO2 Autoxidation at the Subsurface of Deliquesced Aerosols with Surface-Active Organic Acids.

Although the uncatalyzed oxidation of SO2 by O2 (autoxidation) is a ubiquitous SO2 conversion pathway in the troposphere, its atmospheric significance has long been overlooked due to its low reaction rate. Here we show that SO2 autoxidation is largely accelerated by surface-active organic acids such as dicarboxylic acids (DCAs) in deliquesced organic aerosol particles. Specifically, the sulfate formation rate in DCA particles (i.e., glutaric, succinic, and malonic acids) is 2-3 orders of magnitude higher than in inorganic particles under identical buffered pH and ionic strength conditions, and is 4-5 orders of magnitude higher than in solutions. We find that the reaction rate is proportional to the surface-to-volume ratio (1/R) of individual deliquesced aerosols by TEM-EDS, suggesting that the aerosol surface is involved. Combining experimental results with theoretical simulations, we reveal that the enrichment of DCAs at the air-water interface of aerosols enhances SO2 autoxidation by promoting the surface uptake of SO2 and accelerating the oxidation of HSO3- by O2 within a distinct subsurface region. These findings suggest that the subsurface of aerosols may serve as a previously unrecognized hotspot for multiphase oxidation of SO2, and that organic-acid-accelerated SO2 autoxidation can occur at rates comparable to the H2O2 oxidation pathway.