Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury

We present a new chemical mechanism for Hg-0/Hg-I/Hg-II 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-0 by Br and OH, subsequent oxidation of Hg-I by ozone and radicals, respeciation of Hg-II in aerosols and cloud droplets, and speciated Hg-II 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-0 concentrations and Hg-II wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg-0 to Hg-II. Ozone is the principal Hg-I oxidant, enabling the efficient oxidation of Hg-0 to Hg-II by OH. (BrHgOH)-O-II and Hg-II(OH)(2), the initial Hg-II products of Hg-0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of Hg-II to Hg-0 takes place largely through photolysis of aqueous Hg-II-organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of Hg-I, and speciation and photoreduction of Hg-II in aerosols and clouds.

Automated summary

A new chemical mechanism for the atmospheric cycling of Hg-0/Hg-I/Hg-II is proposed and implemented in a global atmospheric chemistry model for comparison with observations. The model successfully explains the oxidation and deposition processes of Hg in the atmosphere, with major uncertainties including the concentration of Br, stability and reactions of Hg-I, and speciation and photoreduction of Hg-II in aerosols and clouds.