HOSO₂ Released from Mineral Dust: A Novel Channel of Heterogeneous Oxidation of Sulfur Dioxide during Dust Storms

Mineral dust has been recognized as an emerging source of radicals that may significantly influence the fate of environmental pollutants. While the production of hydroxyl (OH) radicals from an aqueous dust surface has been recently clarified, other radical-produced channels have been little understood. Here, we propose a novel channel of hydroxysulfonyl (HOSO2) radical production from the aqueous surface of α-Fe2O3, a ubiquitous iron mineral, during the heterogeneous oxidation of sulfur dioxide (SO2). The reactive force field molecular dynamic (ReaxFF-MD) simulations and the density functional theory (DFT) calculations disclosed that the OH groups bound to the α-Fe2O3 surface can oxidize the adsorbed SO2 into HOSO2 radicals in the presence of the surface water layer. The HOSO2 radical can be released from the dust surface and subsequently contribute to the gaseous sulfuric acid (H2SO4) formation. Our kinetic modeling revealed that, despite most SO2 converting to interfacial sulfate through competing reactions on the aqueous dust surface, the abundant surface-bound OH groups and high dust particle concentration during dust storms may enable substantial gaseous H2SO4 production. The level of gaseous H2SO4 production from the HOSO2-released channel is likely comparable to that from the traditional gas-phase OH oxidation pathway. This study demonstrates that mineral-produced radicals may play a critical role in transforming atmospheric pollutants and hence modulate the impacts of mineral dust on local and regional air quality.