We present an updated mechanism for tropospheric halogen (Clĝ€¯+ĝ€¯Brĝ€¯+ĝ€¯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 0.19ĝ€¯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 0.08ĝ€¯ppt, and shows consistency with surface and aircraft observations. The modeled global mean tropospheric concentration of Cl atoms is 630ĝ€¯cm-3, contributing 0.8ĝ€¯% of the global oxidation of methane, 14ĝ€¯% of ethane, 8ĝ€¯% of propane, and 7ĝ€¯% of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11ĝ€¯%, NOx by 6ĝ€¯%, and OH by 4ĝ€¯%. 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.
Bibliographical noteFunding Information:
Financial support. This research has been supported by the City
Acknowledgements. This work was supported by the City University of Hong Kong New Research Initiatives (grant no. 9610470) and the National Natural Science Foundation of China (grant no. 42005083) and was partially supported by the Shenzhen Research Institute, City University of Hong Kong. Work at Harvard was supported by the EPA STAR Program (grant no. 84001401). The authors thank Michael Le Breton for CAST measurements and Kelvin H. Bates for helpful discussions.