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 = NO 2, HOO •, CH 3 OO •, 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%.