Abstract
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 (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 SO2 uptake was highest on δ-MnO2 but lowest on β-MnO2, with a geometric uptake coefficient (γobs) of (2.42 ± 0.13) ×10–2 and a corrected uptake coefficient (γc) of (1.48 ± 0.21) ×10−6 for the former while γobs of (3.35 ± 0.43) ×10−3 and γc 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 SO2. The maximum sulfate formation rate was reached at 25% RH and 45% for δ-MnO2 and γ-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.
Highlights
Sulfate species contribute substantially to tropospheric aerosols, with a significant cooling effect on the global climate by scattering solar radiation and acting as cloud condensation nuclei (CCN)[1]
The presence of trace manganese oxide derived from mineral dust, fossil fuel deposits, fuel-oil fly ash, metal processing industry, etc. may have a significant effect on the SO2 oxidation rate through a redox chemistry process[19, 37]
Γ- and δ-MnO2 had a similar spherical morphology composed of nanowires ranging 10–20 nm in diameter
Summary
Sulfate species contribute substantially to tropospheric aerosols, with a significant cooling effect on the global climate by scattering solar radiation and acting as cloud condensation nuclei (CCN)[1]. Research found that conversion of SO2 to sulfate species was closely associated with mineral dust, accounting for 50–70% of aerosol sulfate in the vicinity of the dust source regions[17] This positive correlation seemed to show an important role in the haze formation occurring in China in recent years[12]. A recent inclusion of parameterization into models simulation involving Fe3+-catalyzed SO2 heterogeneous oxidation in aerosol water successfully reproduced the rapid sulfate growth during haze days in China[11] Both iron and relative humidity played key roles in promoting the uptake of SO2 to aerosol surface, with a high reactive uptake coefficient of 0.5 × 10−4 assuming enough alkalinity in the catalytic reaction[11]. The results could help understand the role of Mn in the heterogeneous formation of sulfate
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