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 NaSO(aq) as a function of pH. The observed formation of SO, SO, and HSO 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 to O on the surface of aqueous microdroplets. Compared to the well-studied sulfate formation kinetics via oxidation by HO(aq), the interfacial SO formation rate on the surface of microdroplets was estimated to be proportional to the collision frequency of SO with a pH-dependent efficiency factor of 5.6 × 10[H]/([H]+10). The rate via the acidic surface reactions is approximately 1-2 orders of magnitude higher than that by HO(aq) for a 1.0 ppbv concentration of HO( g) interacting with 50 μg/m of aerosols. This finding highlights the relative importance of the interfacial SO 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 to sulfate in mega-cities and nearby regions with high PM2.5 haze aerosol loadings.
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