Improved mechanistic model of the atmospheric redox chemistry of mercury


V. Shah, D.J. Jacob, C.P. Thackray, X. WANG, E.M. Sunderland, T.S. Dibble, A. Saiz-Lopez, I. Cernusak, V. Kello, P.J. Castro, R. Wu, and C. Wang. Submitted. “Improved mechanistic model of the atmospheric redox chemistry of mercury.” Environmental Science & Technology.


We present a new chemical mechanism for Hg0 /HgI /HgII 22 atmospheric cycling, including recent 23 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 Hg0 24 by Br atoms and OH radicals, with subsequent oxidation of HgI 25 by ozone and radicals, respeciation of gaseous HgII in aerosols and cloud droplets, and speciated HgII 26 photolysis in the gas 2 27 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 Hg0 28 concentrations and HgII wet deposition fluxes. Hg0 29 is oxidized almost equally through Br and OH. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII 30 by OH. BrHgIIOH and HgII 31 (OH)2 are the initial products, which re-speciate in aerosols/cloud and volatilize to photostable forms. Reduction of HgII to Hg0 32 takes place largely through photolysis of aqueous HgII-organic complexes. 71% of model HgII 33 deposition is to the oceans. Major 34 uncertainties for atmospheric Hg chemistry modeling include the concentrations of Br atoms, the stability and reactions of HgI , and the speciation of HgII 35 in aerosols/cloud with implications for 36 photoreduction.