Heterogenous Chemistry of I₂O₃ as a Critical Step in Iodine Cycling

Global iodine emissions have been increasing rapidly in recent decades, further influencing the Earth’s climate and human health. However, our incomplete understanding of the iodine chemical cycle, especially the fate of higher iodine oxides, introduces substantial uncertainties into atmospheric modeling. I₂O₃ was previously deemed a “dead end” in iodine chemistry; however, we provide atomic-level evidence that I₂O₃ can undergo rapid air-water or air-ice interfacial reactions within several picoseconds; these reactions are facilitated by prevalent chemicals on seawater such as amines and halide ions, to produce photolabile reactive iodine species such as HOI and IX (X = I, Br, and Cl). The heterogeneous chemistry of I₂O₃ leads to the rapid formation of iodate ions (IO₃^-), which is the predominant soluble iodine and its concentration cannot be well explained by current chemistry. These new loss pathways for atmospheric I₂O₃ can further explain its absence in field observations and its presence in laboratory experiments; furthermore, these pathways represent a heterogeneous recycling mechanism that can activate the release of reactive iodine from oceans, polar ice/snowpack, or aerosols. Rapid reactive adsorption of I₂O₃ can also promote the growth of marine aerosols. These findings provide novel insights into iodine geochemical cycling.