Quantification of SO2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation.

Sulfate formation on the surface of aqueous microdroplets was investigated using a spray-chamber reactor coupled to an electrospray ionization mass spectrometer that was calibrated using Na2SO4(aq) as a function of pH. The observed formation of SO3-•, SO4-•, and HSO4- at pH < 3.5 without the addition of other oxidants indicates that an efficient oxidation pathway takes place involving direct interfacial electron transfer from SO2 to O2 on the surface of aqueous microdroplets. Compared to the well-studied sulfate formation kinetics via oxidation by H2O2(aq), the interfacial SO42- formation rate on the surface of microdroplets was estimated to be proportional to the collision frequency of SO2 with a pH-dependent efficiency factor of 5.6 × 10-5[H+]3.7/([H+]3.7+10-13.5). The rate via the acidic surface reactions is approximately 1-2 orders of magnitude higher than that by H2O2(aq) for a 1.0 ppbv concentration of H2O2( g) interacting with 50 µg/m3 of aerosols. This finding highlights the relative importance of the interfacial SO2 oxidation in the atmosphere. Chemical reactions on the aquated aerosol surfaces are overlooked in most atmospheric chemistry models. This interfacial reaction pathway may help to explain the observed rapid conversion of SO2 to sulfate in mega-cities and nearby regions with high PM2.5 haze aerosol loadings.