Abstract
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 %, 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.
Highlights
Halogen radicals have a broad range of implications for tropospheric oxidant chemistry
Extensive referencing will be made to Sherwen et al (2016b), who implemented the previous representation of tropospheric halogen chemistry in GEOS-Chem, and to Wang et al (2019), who described an earlier version of the mechanism implemented here
This is due to the updated reactive uptake of HOBr in Sect. 2.2, which results in less efficient mobilization of bromine radicals from sea salt aerosol (SSA)
Summary
Halogen radicals (chlorine, bromine, iodine) have a broad range of implications for tropospheric oxidant chemistry. They originate from sea salt aerosol (SSA), emitted halogen gases, and transport from the stratosphere, and they cycle rapidly with inorganic non-radical reservoirs (Platt and Hönninger, 2003; Finlayson-Pitts, 2003; Saiz-Lopez and von Glasow, 2012; Simpson et al, 2015; Wang et al, 2019). A number of global modeling studies have explored the importance of halogen chemistry in the troposphere (von Glasow et al, 2004; Saiz-Lopez et al, 2006; Ordóñez et al, 2012; Long et al, 2014), but there remain large uncertainties in sources and chemical mechanisms. Shah et al (2021) examine the impact of our simulated Br and Cl atom concentrations in a new redox mechanism for atmospheric mercury
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