Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury.

We present a new chemical mechanism for Hg/Hg/Hg atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg by Br and OH, subsequent oxidation of Hg by ozone and radicals, respeciation of Hg in aerosols and cloud droplets, and speciated Hg photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg concentrations and Hg wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg to Hg. Ozone is the principal Hg oxidant, enabling the efficient oxidation of Hg to Hg by OH. BrHgOH and Hg(OH), the initial Hg products of Hg oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of Hg to Hg takes place largely through photolysis of aqueous Hg-organic complexes. 71% of model Hg deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of Hg, and speciation and photoreduction of Hg in aerosols and clouds.