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

T. Sherwen, M. J. Evans, L. Carpenter, S. J. Andrews, R. T. Lidster, B. Dix, T. K. Koenig, R. Volkamer, A. Saiz-Lopez, C. Prados-Roman, A. S. Mahajan, C. Ordóñez

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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 = 2, 3, 4) chemistry, photolysis, deposition and parametrised heterogeneous reactions. In comparisons with recent Iodine Oxide (IO) observations the iodine simulation shows an average bias of +66 % available surface observations in the marine boundary layer (outside of polar regions), and of +73 % within the free troposphere (350 <hPa <900) over the eastern Pacific. Iodine emissions (3.8 Tg yrminus;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 IY burden (contributing up to 70 . The iodine chemistry leads to a significant global tropospheric O3 burden decrease (9.0 compared to standard GEOS-Chem (v9-2). The iodine-driven OX loss rate (748 Tg OX yrminus;1) is by photolysis of HOI (78 , photolysis of OIO (21 , and reaction of 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 parameters and conclude that the simulation is sensitive to choices in parameterisation 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. This is a significant impact and so halogen chemistry needs to be considered in climate and air quality models.

Original languageEnglish
Pages (from-to)20957-21023
Number of pages67
JournalAtmospheric Chemistry and Physics Discussions
Issue number15
Publication statusPublished - 5 Aug 2015

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