Photoenhanced sulfates formation by the heterogeneous uptake of SO₂ on non-photoactive mineral dust

Abstract. Heterogeneous uptake of SO₂ on mineral dust is a predominant formation pathway of sulfates, whereas the contribution of photo-induced SO₂ oxidation to sulfates on the dust interfaces still remains unclear. Here, we investigated heterogeneous photochemical reactions of SO₂ on five mineral oxides (SiO₂, kaolinite, Al₂O₃, MgO, and CaO) without photocatalytic activity. Light significantly enhanced the uptake of SO₂, and its enhancement effects negatively depended on the basicity of mineral oxides. The initial uptake coefficient (γ_0,BET ) and the steady-state uptake coefficient (γ_s,BET ) of SO₂ positively relied on light intensity, relative humidity (RH) and O₂ content, while they exhibited a negative relationship with the initial SO₂ concentration. Rapid sulfate formation during photo-induced heterogeneous reactions of SO₂ with all mineral oxides was confirmed to be ubiquitous, and H₂O and O₂ played the key roles in the conversion of SO₂ to sulfates. Specially, ³SO₂ was suggested to be the trigger for photochemical sulfate formation. Atmospheric implications supported a potential contribution of interfacial SO₂ photochemistry on non-photoactive mineral dust to atmospheric sulfate sources.