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

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 = 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 similar to + 90% with available surface observations in the marine boundary layer (outside of polar regions), and of similar to + 73% within the free troposphere (350 hPa < p < 900 hPa) over the eastern Pacific. Iodine emissions (3.8 Tgyr(-1)) are overwhelmingly dominated by the inorganic ocean source, with 76% of this emission from hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric I-Upsilon burden (contributing up to 70 %). The iodine chemistry leads to a significant global tropospheric O-3 burden decrease (9.0 %) compared to standard GEOS-Chem (v9-2). The iodine-driven O-X loss rate(1) (748 TgO(X) yr(-1)) is due to photolysis of HOI (78 %), photolysis of OIO (21 %), and re-action between IO and BrO (1 %). Increases in global mean OH concentrations (1.8 %) 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 = 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 14.4% compared to a simulation without iodine and bromine chemistry in the model, and a small increase in OH (1.8 %). This is a significant impact and so halogen chemistry needs to be considered in both climate and air quality models.