Adsorption of SO2 on mineral dust particles influenced by atmospheric moisture

Moisture plays a crucial role in the heterogeneous formation of sulfur compounds on mineral dust particles. The heterogeneous uptake of SO2 under various humidity conditions, however, is not well explained. In this study, heterogeneous reaction of SO2 on hematite particles are investigated using an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The interaction of SO2 with H2O is divided into the initial moisture absorption stage and the subsequent heterogeneous oxidation stage, indicating physical adsorption and chemical conversion of SO2, respectively. Uptake coefficients were estimated by Brunauer-Emmett-Teller (BET) surface area and geometric surface area. For unreacted (fresh) particles, the coefficients peak at 33% RH and then decline with increasing RH in both stages, implying the competition between SO2 and large amount of adsorbed H2O besides the promoting effect of water. For hematite after in-situ exposure to SO2 (sulfated particles), the coefficients increase as a function of RH in moisture absorption stage, indicating SO2 adsorption with particle hygroscopic growth. Nevertheless, the coefficients exhibit similar variation with that for fresh particles in heterogeneous oxidation stage, highlighting the competition effect after H2O accumulation. Generally, H2O plays both positive and negative roles in the adsorption of SO2. Reaction on sulfated hematite is significantly influenced by previously formed sulfate compounds with low hygroscopicity. Moreover, sulfate products may influence particle acidity after H2O introduction, and further result in diverse existence forms of S(IV) species. The extremely great uptake capacity of sulfated particles in moisture absorption stage may offer opportunities to explain the severe haze in high RH.