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Manganese oxides from anthropogenic sources can promote the formation of sulfate through catalytic oxidation of SO 2 . In this study, the kinetics of SO 2  reactions on MnO 2  with different morphologies (α, β, γ and δ) was investigated using flow tube reactor and  in situ  Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Under dry conditions, the reactivity towards SO 2  uptake was highest on δ-MnO 2  but lowest on β-MnO 2 , with a geometric uptake coefficient (γ obs ) of (2.42 ± 0.13) ×10 –2  and a corrected uptake coefficient (γ c ) of (1.48 ± 0.21) ×10 −6  for the former while γ obs  of (3.35 ± 0.43) ×10 −3  and γ c  of (7.46 ± 2.97) ×10 −7  for the latter. Under wet conditions, the presence of water altered the chemical form of sulfate and was in favor for the heterogeneous oxidation of SO 2 . The maximum sulfate formation rate was reached at 25% RH and 45% for δ-MnO 2  and γ-MnO 2 , respectively, possibly due to their different crystal structures. The results suggest that morphologies and RH are important factors influencing the heterogeneous reaction of SO 2  on mineral aerosols, and that aqueous oxidation process involving transition metals of Mn might be a potential important pathway for SO 2  oxidation in the atmosphere.

Heterogeneous Reaction of SO2 on Manganese Oxides: the Effect of Crystal Structure and Relative Humidity