The role of chlorine in tropospheric chemistry

We present a comprehensive simulation of tropospheric chlorine within the GEOS-Chem global 3-D model of oxidant-aerosol-halogen atmospheric chemistry. The simulation includes explicit accounting of chloride mobilization from sea-salt aerosol by acid displacement of HCl and by other heterogeneous processes. Additional sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are small in comparison. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl, HOCl, ClNO3, ClNO2, and minor species, is produced by the HCl + OH reaction and by heterogeneous conversion of sea-salt aerosol chloride to BrCl, ClNO2, Cl2, and ICl. The model simulates successfully the observed mixing ratios of HCl in marine air (highest at northern mid-latitudes) and the associated HNO3 decrease from acid displacement. It captures the high ClNO2 mixing ratios observed in continental surface air at night with chlorine of sea salt origin transported inland as HCl and fine aerosol. It simulates successfully the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be explained by transport inland of the marine source. It does not reproduce the boundary layer Cl2 mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the daytime, low at night); the model is too high at night compared to WINTER observations, which could be due to uncertainty in the rate of the ClNO2 + Cl− reaction, but we have no explanation for the daytime observations. The global mean tropospheric concentration of Cl atoms in the model is 620 cm−3 and contributes 1.0 % of the global oxidation of methane, 20 % of ethane, 14 % of propane, and 4 % of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85 %, mainly through the HOBr + Cl− reaction, and decreases global burdens of tropospheric ozone by 7 % and OH by 3 % through the associated bromine radical chemistry. ClNO2 chemistry drives increases in ozone of up to 8 ppb over polluted continents in winter.