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

Abstract

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 (I2O X, 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€¯ p g€¯ < 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 Y 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 O X loss rate1 (748g€¯Tgg€¯O X g€¯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 I2O X ( 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 O X 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.