Photoactivation of Chlorine and Its Catalytic Role in the Formation of Sulfate Aerosols

We present a novel mechanism for the formation of photocatalytic oxidants in deliquescent NaCl particles, which can greatly promote the multiphase photo-oxidation of SO₂ to produce sulfate. The photoexcitation of the [Cl^--H₃O⁺-O₂] complex leads to the generation of Cl and OH radicals, which is the key reason for enhancing aqueous-phase oxidation and accelerating SO₂ oxidation. The mass normalization rate of sulfate production from the multiphase photoreaction of SO₂ on NaCl droplets could be estimated to be 0.80 × 10^-4 μg·h^-1 at 72% RH and 1.33 × 10^-4 μg·h^-1 at 81% RH, which is equivalent to the known O₃ liquid-phase oxidation mechanism. Our findings highlight the significance of multiphase photo-oxidation of SO₂ on NaCl particles as a non-negligible source of sulfate in coastal areas. Furthermore, this study underscores the importance of Cl^- photochemistry in the atmosphere.