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 ΔG_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 SO₂ at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.