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Volume 16, issue 2 | Copyright

Special issue: Marine trace gases and aerosols over tropical oceans (AMT/ACP...

Atmos. Chem. Phys., 16, 1161-1186, 2016
https://doi.org/10.5194/acp-16-1161-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 02 Feb 2016

Research article | 02 Feb 2016

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

T. Sherwen1, M. J. Evans1,2, L. J. Carpenter1, S. J. Andrews1, R. T. Lidster1, B. Dix3, T. K. Koenig3,4, R. Sinreich3, I. Ortega3,4, R. Volkamer3,4, A. Saiz-Lopez5, C. Prados-Roman5, A. S. Mahajan6, and C. Ordóñez7 T. Sherwen et al.
  • 1Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, York, YO10 5DD, UK
  • 2National Centre for Atmospheric Science (NCAS), University of York, York, YO10 5DD, UK
  • 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-021, USA
  • 5Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, 28006, Spain
  • 6Indian Institute of Tropical Meteorology, Maharashtra, 411008, India
  • 7Met Office, FitzRoy Road, Exeter, EX1 3PB, UK

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 (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 (350hPa < p < 900hPa) over the eastern Pacific. Iodine emissions (3.8Tg 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 (748TgOXyr−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.

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 PAN = peroxyacetyl nitrate, PPN = peroxypropionyl nitrate, MPN = methyl peroxy nitrate, and MPN = peroxymethacryloyl nitrate.

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Short summary
Using a global chemical transport model (GEOS-Chem) with additional iodine emissions, chemistry, and deposition we show that iodine is responsible for ~ 9 % of global ozone loss but has negligible impacts on global OH. Uncertainties are large in the chemistry and emissions and future research is needed in both. Measurements of iodine species (especially HOI) would be useful. We believe iodine chemistry should be considered in future chemistry-climate and in air quality modelling.
Using a global chemical transport model (GEOS-Chem) with additional iodine emissions, chemistry,...
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