Fast Sulfate Formation Initiated by the Spin-Forbidden Excitation of SO 2 at the Air-Water Interface.

The multiphase oxidation of SO to sulfate in aerosol particles is a key process in atmospheric chemistry. However, there is a large gap between the observed and simulated sulfate concentrations during severe haze events. To fill in the gaps in understanding SO oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, SO + HO + 1/2O → HSO + H (298 K, 1 M), was Δ = -107.8 kcal·mol. Moreover, calculations revealed that this was a multistep reaction involving submerged, small energy barriers (∼10 kcal·mol). These results indicate that photochemical oxidation of SO at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.