Water Promotes the Oxidation of SO₂ by O₂ over Carbonaceous Aerosols

Severe haze episodes typically occur with concurrent high relative humidity. Here, the vital role of water in promoting the oxidation of SO₂ by O₂ on carbonaceous soot surfaces was identified at the atomic level by first-principles calculations. Water molecules can dissociate into surface hydroxyl groups through a self-catalyzed process under ambient conditions. The surface hydroxyl groups, acting as facilitators, can significantly accelerate the conversion of SO₂ to SO₃ (precursor of particulate sulfate) over soot aerosols by reducing the reaction barriers. Specifically, the hydroxyl groups activate the reactants and stabilize the transition states and products through hydrogen-bonding interactions, making the reactions both thermodynamically and kinetically more favorable at room temperature. The findings indicate that atmospheric humidity plays an important role in enhancing the atmospheric oxidation capacity, thus exacerbating SO₂ oxidation and severe haze development. Also, this study unravels a mechanism of surface hydroxyl-assisted O₂ and H₂O dissociation over metal-free carbocatalysts under normal conditions.