GEOS-Chem model simulations with iodine chemistry sensitivity studies

Background Tropospheric reactive halogens influence the oxidizing capacity of the atmosphere. Although previous studies have shown that iodine exerts the strongest impact on tropospheric ozone concentrations compared to chlorine and bromine, the impact of the recycling of aerosol iodide back to the gas phase on oxidants has not been estimated prior to this work. Here, we explicitly represent aerosol iodine speciation in a chemical transport model, including soluble organic iodine (SOI), iodate, and iodide, and allow for interconversion among these species. We find that aerosol iodine speciation, interconversion, and recycling substantially affect modeled oxidant abundances. Across model sensitivity studies, tropospheric Ox (the odd-oxygen family for ozone) and Oz (the odd-oxygen family for the HOx family, including OH) burdens vary by up to 10% and 5%, respectively. These ranges arise from uncertainties in heterogeneous iodine, demonstrating a need for future laboratory experiments and field observations in this area.