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

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 SO₄^2- 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 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.