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.