Global Tropospheric Halogen Chemistry and its Impacts on Ozone, OH, and Aerosols (Invited Presentation)

Global tropospheric observations of halogen (Cl, Br, I) radicals and their precursors indicate concentrations sufficiently high to significantly affect the concentrations of oxidants (ozone, OH) and aerosols, and to drive the atmospheric cycling of mercury. The chemical mechanisms controlling halogen radical concentrations are still not well understood and include a critical role of complex heterogeneous chemistry. Successive generations of halogen radical mechanisms have been developed in the GEOS-Chem global 3-D model of atmospheric chemistry to reflect rapidly increasing knowledge. Here we will present our most recent mechanism and its implementation in GEOS-Chem to describe coupled Cl-Br-I chemistry in the troposphere and stratosphere. Major improvements over previous mechanisms include (1) explicit tracking of sea-salt chlorine and its sink from acid displacement and heterogeneous chemistry, (2) mechanistic representation of sea-salt debromination, (3) mechanistic updates to heterogeneous chemistry including explicit simulation of alkalinity and cloud/aerosol pH. Model results can match most major features in the global tropospheric observations of halogen radicals and their precursors. However, inconsistency between different observational data sets for BrO concentrations is a major limitation in our ability to test the model bromine chemistry. Our best model estimate finds that halogen chemistry drives a moderate sink of tropospheric ozone at extratropical latitudes that is consistent with ozonesonde observations. Global OH concentrations also decrease, improving agreement with OH proxies. Acid displacement of HCl by HNO₃ in aerosol is an important global process affecting HNO₃ and nitrate aerosol levels. Application of our halogen mechanism to a regional GEOS-Chem study for China, where most of boundary layer chlorine is from anthropogenic sources, shows significant effects on NO_x, PM_2.5, and ozone levels.