Photolysis of frozen iodate salts as a source of active iodine in the polar environment

Abstract. Reactive halogens play a key role in the oxidation capacity of the polar troposphere. However, sources and mechanisms, particularly those involving active iodine, are still poorly understood. In this paper, the photolysis of an atmospherically relevant frozen iodate salt has been experimentally studied using infrared (IR) spectroscopy. The samples were generated at low temperatures in the presence of different amounts of water. The IR spectra have confirmed that, under near-ultraviolet–visible (UV–Vis) radiation, iodate is efficiently photolysed. The integrated IR absorption coefficient of the iodate anion on the band at 750 cm⁻¹ has been measured to be A  =  9.8 ± 0.5  ×  10⁻¹⁷ cm molecule⁻¹. The photolysis rate of the ammonium iodate salt was measured by monitoring the decay of ammonium or iodate IR bands (1430 and 750 cm⁻¹ respectively) in the presence of a solar simulator. The absorption cross section of the liquid solutions of ammonium iodate at wavelengths relevant for the troposphere (250 to 400 nm) has been obtained and used to estimate the photolytic quantum yield for the frozen salt. Finally, using an atmospheric model, constrained with the experimental data, we suggest that the photolysis of iodate in frozen salt can potentially provide a pathway for the release of active iodine to the polar atmosphere.