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We present a global simulation of tropospheric iodine chemistry within the GEOS-Chem chemical transport model. This includes organic and inorganic iodine sources, standard gas-phase iodine chemistry, and simplified higher iodine oxide (I2OX, X Combining double low line 2, 3, 4) chemistry, photolysis, deposition, and parametrized heterogeneous reactions. In comparisons with recent iodine oxide (IO) observations, the simulation shows an average bias of g1/4 +90g€¯% with available surface observations in the marine boundary layer (outside of polar regions), and of g1/4 +73g€¯% within the free troposphere (350g€¯hPag€¯ < g€¯pg€¯ < g€¯900g€¯hPa) over the eastern Pacific. Iodine emissions (3.8g€¯Tg yr−1) are overwhelmingly dominated by the inorganic ocean source, with 76g€¯% of this emission from hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric IY burden (contributing up to 70g€¯%). The iodine chemistry leads to a significant global tropospheric O3 burden decrease (9.0g€¯%) compared to standard GEOS-Chem (v9-2). The iodine-driven OX loss rate1 (748g€¯Tgg€¯OXg€¯yrg'1) is due to photolysis of HOI (78g€¯%), photolysis of OIO (21g€¯%), and reaction between IO and BrO (1g€¯%). Increases in global mean OH concentrations (1.8g€¯%) by increased conversion of hydroperoxy radicals exceeds the decrease in OH primary production from the reduced O3 concentration. We perform sensitivity studies on a range of parameters and conclude that the simulation is sensitive to choices in parametrization of heterogeneous uptake, ocean surface iodide, and I2OX (X Combining double low line 2, 3, 4) photolysis. The new iodine chemistry combines with previously implemented bromine chemistry to yield a total bromine- and iodine-driven tropospheric O3 burden decrease of 14.4g€¯% compared to a simulation without iodine and bromine chemistry in the model, and a small increase in OH (1.8g€¯%). This is a significant impact and so halogen chemistry needs to be considered in both climate and air quality models. 1 Here OX is defined as O3 + NO2 + 2NO3 + PAN + PMN+PPN + HNO4 + 3N2O5 + HNO3 + BrO + HOBr + BrNO2+2BrNO3 + MPN + IO + HOI + INO2 + 2INO3 + 2OIO+2I2O2 + 3I2O3 + 4I2O4, where PANg€¯ Combining double low line g€¯peroxyacetyl nitrate, PPNg€¯ Combining double low line g€¯peroxypropionyl nitrate, MPNg€¯ Combining double low line g€¯methyl peroxy nitrate, and MPNg€¯ Combining double low line g€¯peroxymethacryloyl nitrate.Peer Reviewe

Iodine's impact on tropospheric oxidants: a global model study in GEOS-Chem