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

Neuman, Jonathan Andrew; Koenig, Theodore K; Volkamer, Rainer

Citeable URL: https://scholar.colorado.edu/concern/articles/4m90dw916

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 efficient 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 (parts per trillion), 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 %, NO_x 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.

Full List of Authors:

Xuan Wang^1,2, Daniel J. Jacob³, William Downs³, Shuting Zhai⁴, Lei Zhu⁵, Viral Shah³, Christopher D. Holmes⁶, Tomás Sherwen^7,8, Becky Alexander⁴, Mathew J. Evans^7,8, Sebastian D. Eastham⁹, J. Andrew Neuman^10,11, Patrick R. Veres¹⁰, Theodore K. Koenig^11,12, Rainer Volkamer^11,12, L. Gregory Huey¹³, Thomas J. Bannan¹⁴, Carl J. Percival^14,a, Ben H. Lee⁴, and Joel A. Thornton⁴
- ¹School of Energy and Environment, City University of Hong Kong, Hong Kong SAR, China
- ²City University of Hong Kong Shenzhen Research Institute, Shenzhen, China
- ³School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
- ⁴Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
- ⁵School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen, China
- ⁶Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, Florida, USA
- ⁷Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, UK
- ⁸National Centre for Atmospheric Science, University of York, York, UK
- ⁹Laboratory for Aviation and the Environment, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
- ¹⁰NOAA Chemical Sciences Laboratory (CSL), Boulder, Colorado, USA
- ¹¹Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
- ¹²Department of Chemistry, University of Colorado, Boulder, Colorado, USA
- ¹³School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, Georgia, USA
- ¹⁴School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
- ^anow at: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

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