Global simulation of tropospheric halogen chemistry

Abstract:

Halogen (Cl, Br, I) radicals and their precursors affect tropospheric chemistry in a number of ways, such as depleting oxidants (ozone, OH), oxidizing volatile organic compounds, and driving the atmospheric cycling of mercury. The mechanisms controlling halogen radical concentrations are still not well understood. Halogen mechanisms with complex gaseous and heterogeneous chemistry have been developed in the GEOS-Chem global 3-D model of atmospheric chemistry. However, all previous developments appeared to generate excessive free-tropospheric BrO in the extratropics when sea salt aerosol (SSA) debromination, the dominant global source of tropospheric bromine, was included. Here we will present our most recent mechanism in GEOS-Chem to describe coupled Cl-Br-I chemistry in the troposphere and stratosphere. With a series of chemistry updates, the model now solves the previous issue of overestimating free-tropospheric BrO with SSA debromination. We compare the model simulation with a series of observations of halogen radicals and their precursors, including those during the Atmospheric Tomography Mission (ATom), which provides the first aircraft measurements of halogen species at global scale. Model results can reproduce most major features in the observations, however, inconsistency between different observational data sets gives a limitation in our ability to test the model. The impact of halogen chemistry on tropospheric oxidants is analyzed using our best model estimate.