Iodine Activation from Iodate Reduction in Aqueous Films via Photocatalyzed and Dark Reactions

Iodine in the atmosphere destroys ozone and can nucleate particles by formation of iodic acid, HIO₃. Recent field observations suggest iodate recycles from particles sustaining significant gas-phase IO radical concentrations (0.06 pptv) in aged stratospheric air, and in elevated dust plumes. However, laboratory evidence for iodine activation from aerosols is currently missing. Here, a series of coated-wall flow tube (CWFT) experiments test for iodine release from thin aqueous films containing iodate. Photocatalyzed reactions were studied using iron(III) citrate (Fe-Cit), Arizona Test Dust (ATD), and Fe₂O₃, along with the dark reaction of iodate with H₂O₂ at 90% RH and 293 K. Fresh films were separately irradiated with visible and UV-A light, and the efficient release of molecular iodine, I₂, was observed from all irradiated films containing photocatalysts. For films with Fe-Cit, visible light reduced larger amounts of iodate than UV-A light, activating ∼40% of iodate as I₂. The formation of oxygenated volatile organic compounds (OVOC) and iodinated OVOC was also observed. Dark exposure of films to H₂O₂ led to I₂ release in smaller amounts than suggested by Bray-Liebhafsky kinetics, consistent with H₂O₂ salting-out in the films, or possibly other reasons. Photochemical activation is enhanced by dust proxies in the film, and by aging the film with H₂O₂ in the dark prior to irradiation. These findings help explain recent field observations of elevated IO radical concentrations in lofted dust layers, and warrant the inclusion of photocatalyzed iodate reduction in atmospheric models.