Iodic acid formation and yield from iodine photolysis at the CERN CLOUD chamber

Iodine oxoacids are key species involved in the cycling of iodine between the gas- and aerosol phases. Iodic acid (HIO₃) nucleates particles more efficiently than sulfuric acid and ammonia at comparable concentrations, and grows them at comparable rates, but the formation mechanism of HIO₃ is essentially unknown. As a result, atmospheric models of iodine chemistry are currently incomplete. Proposed precursors for iodine oxoacids include iodine atoms and higher iodine oxides (e.g., I₂O₂, I₂O₃, I₂O₄), but theoretical predictions have not currently been assessed under experimental conditions that approximate the open ocean marine atmosphere. We present results from laboratory experiments at the CLOUD chamber that observe rapid oxoacid formation from photolysis of iodine (I₂) at green wavelengths, in the presence of ozone and variable relative humidity (0-80%). Under these (soft) experimental conditions iodine oxide (IO) radical concentrations closely approximate those found in the remote marine boundary layer. A chemical box model is constrained by measurements of I₂, ozone, RH, photolysis frequencies (i.e., I₂, IO, OIO, HOI, I_xO_y) and known losses of gases to particles and the chamber walls, and evaluated using time resolved measurements of IO, OIO, and I_xO_y species in the chamber. Hypothesized mechanisms for HIO₃ formation - either proposed in the literature or motivated from our observations - are then discussed in terms of their ability to explain the observed amounts (yield), and the temporal evolution of HIO₃. Finally, the atmospheric relevance of the laboratory findings is assessed in context of unique field measurements at the Maido Observatory, La Reunion, during spring 2018, where IO radicals and HIO₃ were measured simultaneously in the remote free troposphere.