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

Abstract

In 2005, Calvert and Lindberg 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/mole, and calculate the equilibrium constant for OH + Hg = HOHg•. Using the measured rate constant for 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 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%.

BrHgO• + CO: Analogue of OH + CO and Reduction Path for Hg(II) in the Atmosphere

“…Mercury is a neurotoxin which is transported globally by the atmosphere . Mercury deposits from the atmosphere to ecosystems most efficiently when it is in the form of Hg­(II) compounds. , Unfortunately, we are still uncovering major aspects of the redox chemistry of atmospheric mercury. − This complicates efforts to predict the impact of emissions reductions on the spatial distribution of mercury entry into ecosystems. , …”

“…It is believed that atomic bromine is the major agent initiating oxidation of Hg(0). ,− Horowitz et al updated a global model of atmospheric mercury to incorporate recent advancements in the mechanism and kinetics of Hg(0) oxidation by gas-phase Br . They found oxidation to proceed so rapidly as to underpredict [Hg(0)], which inspired a search for reduction pathways to add to models.…”

Earth system modeling of mercury using CESM2: part 1. atmospheric model CAM6-Chem/Hg v1.0

“…The oxidation with OH radicals is an important pathway when considering the proportion of oxidation rates, consisting with previous studies (Selin et al, 2007;Shah et al, 2021). Indeed, recent research has found that the main products HOHg I and Hg(OH) 2 formed by the oxidation of Hg 0 by OH are relatively stable (Dibble et al, 2020). The reservoir of global atmospheric Hg is 3896 Mg, of which Hg 0 and Hg II are 3566 Mg and 330 Mg, respectively (for Hg II , 122 Mg and 208 Mg in the gaseous and particulate phases, respectively).…”

Theoretical Study of the Monohydration of Mercury Compounds of Atmospheric Interest

“…Scientists currently believe that Hg(0) oxidation is mainly initiated by atomic bromine, to yield stable Hg­(II) compounds such as BrHgOH, BrHgOOH, and BrHgONO. ,,− BrHgONO has two conformers (syn and anti) and an isomer (BrHgNO 2 ) . Photolysis of BrHgONO , and BrHgOOH can produce the radical BrHgO.…”

“…Photolysis of BrHgONO , and BrHgOOH can produce the radical BrHgO. Initiation of Hg(0) oxidation by hydroxyl radical leads to a Hg­(I) radical intermediate: HgOH . Goal of this paper is to investigate the monohydration of these seven mercury compounds.…”