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Abstract. We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transportmodel and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneouschemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model toinclude mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixingratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison tosurface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytimemeasurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very largemissing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a globalmean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global meantropospheric 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 ozonesimulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere. 

Abstract. We present a comprehensive simulation of tropospheric chlorinewithin the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmosphericchemistry. The simulation includes explicit accounting of chloridemobilization from sea salt aerosol by acid displacement of HCl and by otherheterogeneous processes. Additional small sources of tropospheric chlorine(combustion, organochlorines, transport from stratosphere) are also included.Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl,HOCl, ClNO3, ClNO2, and minor species, is produced by theHCl+OH reaction and by heterogeneous conversion of sea salt aerosolchloride to BrCl, ClNO2, Cl2, and ICl. The modelsuccessfully simulates the observed mixing ratios of HCl in marine air(highest at northern midlatitudes) and the associated HNO3decrease from acid displacement. It captures the high ClNO2 mixingratios observed in continental surface air at night and attributes thechlorine to HCl volatilized from sea salt aerosol and transported inlandfollowing uptake by fine aerosol. The model successfully simulates thevertical profiles of HCl measured from aircraft, where enhancements in thecontinental boundary layer can again be largely explained by transport inlandof the marine source. It does not reproduce the boundary layer Cl2mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in thedaytime, low at night); the model is too high at night, which could be due touncertainty in the rate of the ClNO2+Cl- reaction, but we haveno explanation for the high observed Cl2 in daytime. The globalmean tropospheric concentration of Cl atoms in the model is 620 cm−3and contributes 1.0 % of the global oxidation of methane, 20 % ofethane, 14 % of propane, and 4 % of methanol. Chlorine chemistryincreases global mean tropospheric BrO by 85 %, mainly through theHOBr+Cl- reaction, and decreases global burdens of troposphericozone by 7 % and OH by 3 % through the associated bromine radicalchemistry. ClNO2 chemistry drives increases in ozone of up to8 ppb over polluted continents in winter.