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Discussion papers
https://doi.org/10.5194/acp-2018-1088
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1088
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Oct 2018

Research article | 19 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

The role of chlorine in tropospheric chemistry

Xuan Wang1, Daniel J. Jacob1,2, Sebastian D. Eastham3, Melissa P. Sulprizio1, Lei Zhu1, Qianjie Chen4, Becky Alexander5, Tomas Sherwen6,7, Mathew J. Evans6,7, Ben H. Lee5, Jessica D. Haskins5, Felipe D. Lopez-Hilfiker8, Joel A. Thornton5, Gregory L. Huey9, and Hong Liao10 Xuan Wang et al.
  • 1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
  • 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
  • 3Laboratory for Aviation and the Environment, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
  • 4Department of Chemistry, University of Michigan, Ann Arbor, Michigan, USA
  • 5Department of Atmospheric Sciences, University of Washington, Seattle, USA
  • 6Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
  • 7National Centre for Atmospheric Science, University of York, York, UK
  • 8Paul Scherrer Institute, Villigen, Switzerland
  • 9School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, GA, USA
  • 10School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing, China

Abstract. We present a comprehensive simulation of tropospheric chlorine within the GEOS-Chem global 3-D model of oxidant-aerosol-halogen atmospheric chemistry. The simulation includes explicit accounting of chloride mobilization from sea-salt aerosol by acid displacement of HCl and by other heterogeneous processes. Additional sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are small in comparison. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl, HOCl, ClNO3, ClNO2, and minor species, is produced by the HCl + OH reaction and by heterogeneous conversion of sea-salt aerosol chloride to BrCl, ClNO2, Cl2, and ICl. The model simulates successfully the observed mixing ratios of HCl in marine air (highest at northern mid-latitudes) and the associated HNO3 decrease from acid displacement. It captures the high ClNO2 mixing ratios observed in continental surface air at night with chlorine of sea salt origin transported inland as HCl and fine aerosol. It simulates successfully the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be explained by transport inland of the marine source. It does not reproduce the boundary layer Cl2 mixing ratios measured in the WINTER aircraft campaign (1–5ppt in the daytime, low at night); the model is too high at night compared to WINTER observations, which could be due to uncertainty in the rate of the ClNO2 + Cl reaction, but we have no explanation for the daytime observations. The global mean tropospheric concentration of Cl atoms in the model is 620cm−3 and contributes 1.0% of the global oxidation of methane, 20% of ethane, 14% of propane, and 4% of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85%, mainly through the HOBr + Cl reaction, and decreases global burdens of tropospheric ozone by 7% and OH by 3% through the associated bromine radical chemistry. ClNO2 chemistry drives increases in ozone of up to 8ppb over polluted continents in winter.

Xuan Wang et al.
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Xuan Wang et al.
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Short summary
Chlorine radicals have a broad range of implications for tropospheric chemistry, air quality, and climate. We present a comprehensive simulation of tropospheric chlorine in a global 3-D model, which includes explicit accounting of chloride mobilization from sea-salt aerosol. We find the chlorine chemistry contributes 1.0 % of the global oxidation of methane, and decreases global burdens of tropospheric ozone by 7 % and OH by 3 % through the associated bromine radical chemistry.
Chlorine radicals have a broad range of implications for tropospheric chemistry, air quality,...
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