Substantial Iodide Activation during the Dark Freezing of Aqueous Droplets

Molecular iodine (I₂) significantly influences atmospheric oxidation capacity, driving new particle formation and contributing to ozone depletion in polar regions. However, its formation mechanisms in these cold environments, particularly at night, remain poorly understood. Here, we present experimental evidence that the dark freezing of iodide-containing bulk solutions and droplets activates iodide (I^-) without any added catalysts or oxidants. This I^- activation is markedly enhanced during the freezing of iodide-containing droplets, leading to substantial I₂ production in both the gas and the aqueous phases. The production follows a Langmuir-Hinshelwood formalism, indicating a surface-mediated process at the frozen air-water interface. A detailed mechanistic investigation reveals that the production of I₂ is driven by the hydroxyl radicals (OH·) and hydrogen peroxide (H₂O₂) formed at the frozen interface, which subsequently oxidize I^-. We attribute the formation of OH· and H₂O₂ to the high electric field possibly induced by the Workman-Reynolds freezing potential. This work identifies a previously unrecognized nighttime formation mechanism of I₂ in atmospheric snow and ice, which has implications for atmospheric multiphase chemistry in cold environments.