Heterogeneous oxidation of I₂O₄ at the air-water interface as an effective source of I₂O₅

Higher-order iodine oxides are pivotal to iodine chemistry and atmospheric aerosol formation. Among them, I₂O₅ stands out as a thermodynamically stable yet highly reactive species central to iodine cycling. However, its formation mechanisms remain elusive, as the identified gas- and particle-phase pathways are kinetically limited under ambient conditions. Given the iodine enrichment and the enhanced reactivity intrinsic to the air–water interface, we investigated the heterogeneous formation of I₂O₅ through oxidation of I₂O₄ by O₃ using Born–Oppenheimer molecular dynamics simulations and quantum chemical calculations. Our results show that the free-energy barrier of interfacial reaction, relative to the gas phase, decreases from 18.9 to 8.3 kcal mol⁻¹ for the O₃-mediated pathway, implying that the interfacial mechanism is more favorable. This acceleration of reactions may stem from activating the reactants and stabilizing the transition states through interfacial water. These findings identify a previously overlooked heterogeneous pathway for I₂O₅ formation, highlighting the air–water interface as an active chemical microenvironment for iodine oxidation and providing molecular insights into atmospheric iodine cycling and iodine-mediated aerosol formation.