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Iodines impact on tropospheric oxidants: a global model study in GEOS-Chem

Sherwen, T and Evans, MJ and Carpenter, LJ and Andrews, SJ and Lidster, RT and Koenig, TK and Volkamer, R and Saiz-Lopez, A and Prados-Roman, C and Mahajan, AS and Ordóñez, C (2016) Iodines impact on tropospheric oxidants: a global model study in GEOS-Chem. Atmospheric Chemistry and Physics, 16. pp. 1161-1186.

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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 parametrized heterogeneous reactions. In comparisons with recent iodine oxide (IO) observations, the simulation shows an average bias of  ∼ +90 % with available surface observations in the marine boundary layer (outside of polar regions), and of  ∼ +73 % within the free troposphere (350 hPa  <  p  <  900 hPa) over the eastern Pacific. Iodine emissions (3.8 Tg yr−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 rate1 (748 Tg OX yr−1) is due to photolysis of HOI (78 %), photolysis of OIO (21 %), and reaction 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.

Item Type: Article
Additional Information: Copyright of this article belongs to Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 License.
Subjects: Meteorology and Climatology
Depositing User: IITM Library
Date Deposited: 08 Apr 2017 07:02
Last Modified: 08 Apr 2017 07:02

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