Heterogeneous Reaction of SO(2) on Manganese Oxides: the Effect of Crystal Structure and Relative Humidity

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.