Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere without Violating Physical Laws.

In 2005, Calvert and Lindberg (Calvert, J. G.; Lindberg, S. E. Atmos. Environ. 2005, 39, 3355-3367) wrote that the use of laboratory-derived rate constants for OH + Hg(0) "...to determine the extent of Hg removal by OH in the troposphere will greatly overestimate the importance of Hg removal by this reaction." The HOHg• intermediate formed from OH + Hg will mostly fall apart in the atmosphere before it can react. By contrast, in laboratory experiments, Calvert and Lindberg expected HOHg• to react with radicals (whose concentrations are much higher than in the atmosphere). Yet, almost all models of oxidation of Hg(0) ignore the argument of Calvert and Lindberg. We present a way for modelers to include the OH + Hg reaction while accounting quantitatively for the dissociation of HOHg•. We use high levels of quantum chemistry to establish the HO-Hg bond energy as 11.0 kcal/mol and calculate the equilibrium constant for OH + Hg = HOHg•. Using the measured rate constant for the association of OH with Hg, we determine the rate constant for HOHg• dissociation. Theory is also used to demonstrate that HOHg• forms stable compounds, HOHgY, with atmospheric radicals (Y = NO2, HOO•, CH3OO•, and BrO). We then present rate constants for use in modeling OH-initiated oxidation of Hg(0). We use this mechanism to model the global oxidation of Hg(0) in the period 2013-2015 using the GEOS-Chem 3D model of atmospheric chemistry. Because of the rapid dissociation of HOHg•, OH accounts for <1% of the global oxidation of Hg(0) to Hg(II), while Br atoms account for 97%.