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

The multiphase oxidation of SO 2 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 2 oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO 2 to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O 2 that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, 3 SO 2 + H 2 O + 1/2O 2 → HSO 4 - + H + (298 K, 1 M), was Δ G r = -107.8 kcal·mol -1 . Moreover, calculations revealed that this was a multistep reaction involving submerged, small energy barriers (∼10 kcal·mol -1 ). These results indicate that photochemical oxidation of SO 2 at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.