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Volume 16, issue 18
Atmos. Chem. Phys., 16, 12239-12271, 2016
https://doi.org/10.5194/acp-16-12239-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 12239-12271, 2016
https://doi.org/10.5194/acp-16-12239-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Sep 2016

Research article | 29 Sep 2016

Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem

Tomás Sherwen1, Johan A. Schmidt2, Mat J. Evans1,3, Lucy J. Carpenter1, Katja Großmann4,a, Sebastian D. Eastham5, Daniel J. Jacob5, Barbara Dix6, Theodore K. Koenig6,7, Roman Sinreich6, Ivan Ortega6,7, Rainer Volkamer6,7, Alfonso Saiz-Lopez8, Cristina Prados-Roman8,b, Anoop S. Mahajan9, and Carlos Ordóñez10 Tomás Sherwen et al.
  • 1Wolfson Atmospheric Chemistry Laboratories (WACL), Department of Chemistry, University of York, York, YO10 5DD, UK
  • 2Department of Chemistry, University of Copenhagen, Universitetsparken, 2100 Copenhagen O, Denmark
  • 3National Centre for Atmospheric Science (NCAS), University of York, York, YO10 5DD, UK
  • 4Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany
  • 5School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
  • 6Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
  • 7Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-021, USA
  • 8Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, 28006, Spain
  • 9Indian Institute of Tropical Meteorology, Maharashtra, 411008, India
  • 10Dpto. Física de la Tierra II, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
  • anow at: Joint Institute For Regional Earth System Science and Engineering (JIFRESSE), University of California Los Angeles, Los Angeles, CA, 90095, USA
  • bnow at: Atmospheric Research and Instrumentation Branch, National Institute for Aerospace Technology (INTA), Madrid, Spain

Abstract. We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde.

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We present a simulation of tropospheric Cl, Br, I chemistry within the GEOS-Chem CTM. We find a decrease in tropospheric ozone burden of 18.6 % and a 8.2 % decrease in global mean OH concentrations. Cl oxidation of some VOCs range from 15 to 27 % of the total loss. Bromine plays a small role in oxidising oVOCs. Surface ozone, ozone sondes, and methane lifetime are in general improved by the inclusion of halogens. We argue that simulated bromine and chlorine represent a lower limit.
We present a simulation of tropospheric Cl, Br, I chemistry within the GEOS-Chem CTM. We find a...
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