Abstract
Abstract. Quantum calculations are used to determine the stability of reactive gaseous mercury (Hg(II)) compounds likely to be formed in the Br-initiated oxidation of gaseous elemental mercury (Hg(0)). Due to the absence of any evidence, current models neglect the possible reaction of BrHg with abundant radicals such as NO, NO2, HO2, ClO, or BrO. The present work demonstrates that BrHg forms stable compounds, BrHgY, with all of these radicals except NO. Additional calculations on the analogous ClHgY compounds reveal that the strength of the XHg-Y bond (for X = Cl, Br) varies little with the identity of the halogen. Calculations further suggest that HO2 and NO3 do not form strong bonds with Hg(0), and cannot initiate Hg(0) oxidation in the gas phase. The theoretical approach is validated by comparison to published data on HgX2 compounds, both from experiment and highly refined quantum chemical calculations. Quantum calculations on the stability of the anions of XHgY are carried out in order to aid future laboratory studies aimed at molecular-level characterization of gaseous Hg(II) compounds. Spectroscopic data on BrHg is analyzed to determine the equilibrium constant for its formation, and BrHg is determined to be much less stable than previously estimated. An expression is presented for the rate constant for BrHg dissociation.
Highlights
The details of gaseous elemental mercury (Hg(0)) oxidation matter because wet and dry deposition of mercury is much more efficient for reactive gaseous mercury (Hg(II), RGM) than Hg(0) (Schroeder and Munthes, 1998)
Hydroxyl radical (OH), ozone (O3), hydroperoxide (H2O2), chlorine (Cl2), and hydrochloride (HCl) were the major gas phase oxidants invoked in atmospheric models (Bollock et al 2002; Petersen et al, 2001; Seignuer et al, 1994)
As a guide to future laboratory studies of these compounds by negative ion mass spectrometry, we report electron affinities for most of these BrHgY and ClHgY compounds
Summary
The details of gaseous elemental mercury (Hg(0)) oxidation matter because wet and dry deposition of mercury is much more efficient for reactive gaseous mercury (Hg(II), RGM) than Hg(0) (Schroeder and Munthes, 1998). Sensitivity analyses show that the geographic distribution of mercury deposition is very sensitive to the assumed oxidation mechanism (Lin, et al, 2007; Seigneur et al, 2006). The mechanisms of the oxidation of Hg(0) to Hg(II) are the subject of controversy (Subir 2011; Pirrone et al, 2008; Seigneur et al, 2006; Ariya et al, 2008). Hydroxyl radical (OH), ozone (O3), hydroperoxide (H2O2), chlorine (Cl2), and hydrochloride (HCl) were the major gas phase oxidants invoked in atmospheric models (Bollock et al 2002; Petersen et al, 2001; Seignuer et al, 1994). Field data in the marine boundary layer has been analyzed to suggest a major role for Br in initiating Hg(0) oxidation (Sprovieri et al, 2010a), and recent models suggest that Br plays a dominant role in mercury cycling in the global troposphere (Holmes et al, 2009)
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