Fast Sulfate Formation Initiated by the Spin-Forbidden Excitation of SO₂ at the Air–Water Interface

The multiphase oxidation of SO2 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 SO2 oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO2 to its triplet states using UVA photons at an air–water interface, followed by reactions with water and O2 that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, 3SO2 + H2O + 1/2O2 → HSO4– + H+ (298 K, 1 M), was ΔGr = −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 SO2 at the air–water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions