Heterogeneous Reaction of SO2 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 SO2. In this study, the kinetics of SO2 reactions on MnO2 with different morphologies (alpha, beta, gamma and delta) was investigated using flow tube reactor and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Under dry conditions, the reactivity towards SO2 uptake was highest on delta-MnO2 but lowest on beta-MnO2, with a geometric uptake coefficient (gamma(obs)) of (2.42 +/- 0.13) x 10(-2) and a corrected uptake coefficient (gamma(c)) of (1.48 +/- 0.21) x10(-6) for the former while gamma(obs) of (3.35 +/- 0.43) x10(-3) and gamma(c) of (7.46 +/- 2.97) x10(-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 SO2. The maximum sulfate formation rate was reached at 25% RH and 45% for delta-MnO2 and gamma-MnO2, respectively, possibly due to their different crystal structures. The results suggest that morphologies and RH are important factors influencing the heterogeneous reaction of SO2 on mineral aerosols, and that aqueous oxidation process involving transition metals of Mn might be a potential important pathway for SO2 oxidation in the atmosphere.