02 Jun 2021

02 Jun 2021

Review status: this preprint is currently under review for the journal ACP.

Global tropospheric halogen (Cl, Br, I) chemistry and its impact on oxidants

Xuan Wang1, Daniel J. Jacob2, William Downs2, Shuting Zhai3, Lei Zhu4, Viral Shah2, Christopher D. Holmes5, Tomás Sherwen6,7, Becky Alexander3, Mathew J. Evans6,7, Sebastian D. Eastham8, J. Andrew Neuman9,10, Patrick Veres9,10, Theodore K. Koenig10,11, Rainer Volkamer10,11, L. Gregory Huey12, Thomas J. Bannan13, Carl J. Percival13,a, Ben H. Lee3, and Joel A. Thornton3 Xuan Wang et al.
  • 1School of Energy and Environment, City University of Hong Kong, Hong Kong SAR, China
  • 2School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
  • 3Department of Atmospheric Sciences, University of Washington, Seattle, USA
  • 4School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, China
  • 5Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, Florida, USA
  • 6Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
  • 7National Centre for Atmospheric Science, University of York, York, UK
  • 8Laboratory for Aviation and the Environment, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
  • 9NOAA Chemical Sciences Laboratory (CSL), Boulder, Colorado, USA
  • 10Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
  • 11Department of Chemistry, University of Colorado, Boulder, CO, USA
  • 12School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, Georgia, USA
  • 13School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
  • anow at: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

Abstract. We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transport model and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneous chemistry and its pH dependence in our simulation leads to less effective recycling and mobilization of bromine radicals, and enables the model to include mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixing ratio is 0.19 ppt, lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison to surface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytime measurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very large missing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a global mean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global mean tropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane, 8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %, NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozone simulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere.

Xuan Wang et al.

Status: open (until 14 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Xuan Wang et al.


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Short summary
Halogen radicals have a broad range of implications for tropospheric chemistry, air quality, and climate. We present an updated a new mechanistic description and comprehensive simulation of tropospheric halogens in a global 3-D model, and compare the model results with surface and aircraft measurements. We find that halogen chemistry decreases the global tropospheric burden of ozone by 11 %, NOx by 6 %, and OH by 4 %.