The gas-phase formation of iodic acid, HIO₃, efficiently nucleates particles, and accumulates iodate, IO₃^-, in particles. Iodate is the most thermodynamically stable form of iodine, and atmospheric models currently accumulate oxidized iodine as particulate iodate. Field measurements support iodate as the primary iodine reservoir in the stratosphere, but find that most iodine resides in gas-phase throughout the troposphere. The co-existence of particulate iodate and gas-phase IO radicals in 10 year aged stratospheric air also suggests the existence of recycling pathways of iodate back to the gas-phase. In addition, observations of IO radicals in the free-troposphere of South American coastal deserts, show dust as a source of gas-phase iodine, but laboratory evidence for iodine activation from dust particles is currently missing. This work presents an overview of known iodate reduction pathways and tests the release of iodine from multiphase chemistry of thin aqueous films containing iodate as aerosol proxies. A series of coated-wall-flow-tube (CWFT) experiments were performed to study the reduction of iodate from dark reactions with H₂O₂ and photosensitized reactions using Iron(III)-citrate (Fe-cit), Arizona Test Dust (ATD), and Fe₂O₃, in aqueous thin films containing atmospherically relevant iodate concentrations at 80% RH and 293 K. Molecular iodine, I₂, was measured by Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS), and is found to be efficiently released to the gas-phase. Chemical Ionization Mass Spectrometry (CIMS) is used to measure I₂ and other gas-phase iodine products, including organic iodine species. The atmospheric implications are discussed in context of recent field observations of elevated IO radicals in lofted dust layers, and opportunities the laboratory findings present in developing atmospheric models.

Henning Finkenzeller

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