Quantification of SO_2 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 Na_2SO_4(aq) as a function of pH. The observed formation of SO_3–•, SO_4–•, and HSO_4– at pH < 3.5 without the addition of other oxidants indicates that an efficient oxidation pathway takes place involving direct interfacial electron transfer from SO_2 to O_2 on the surface of aqueous microdroplets. Compared to the well-studied sulfate formation kinetics via oxidation by H2O2(aq), the interfacial SO_4^(2–) 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 H_2O_2(aq) for a 1.0 ppbv concentration of H_2O_2(g) interacting with 50 μg/m^3 of aerosols. This finding highlights the relative importance of the interfacial SO_2 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 SO_2 to sulfate in mega-cities and nearby regions with high PM2.5 haze aerosol loadings