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Substantial Iodide Activation during the Dark Freezing of Aqueous Droplets.

Junwei Song,Linyu Gao,Matthieu Riva,D. Donaldson,C. George

Published 2026 in Environmental Science and Technology

ABSTRACT

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.

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