Computational chemistry re-interprets lab and field studies of nitrate radical initiation of oxidation of Hg(0)

Mercury, a potent neurotoxin, predominately exist in the atmosphere as Hg(0). Elemental mercury can be oxidized into Hg(II) and readily undergo either dry or wet deposition to enter the ecosystem. Redox chemistry of mercury in the atmosphere are still not completely understood. A recent observational study has suggested that NO₃ radical may play a role in nighttime oxidation of Hg(0). Therefore, we explored reaction (1) oxidizing Hg(0) to Hg(I):
Hg + ●NO₃ → ●HgNO₃ (1)
followed by two types of reactions oxidizing ●HgNO₃ to Hg(II):
●HgNO₃ + O₃ → NO₃HgO● + O₂ (2)
●HgNO₃ + Y → NO₃HgY (3)
where Y = BrO, NO₂, NO₃, HO₂, and O₂. In determining the Hg-NO₃ bond energy, advanced quantum calculations were used. The bond between Hg and NO₃ is very weak: only 6.5 kcal/mol. Reactions (2) and (3) were studied using CCSD(T)//DFT: an approach that gives reliable thermodynamics with somewhat less precision than was attempted for the Hg-NO₃ bond energy. We discovered that HgNO₃ radical closely mimics the reactivity of both HgBr and HgOH, in that it undergoes reactions (2) and (3) without a barrier. However, given the low bond energy of HgNO₃ (6.5 kcal/mole) versus HgOH (11.0 kcal/mole) and HgBr (15.5 kcal/mole), it is clear that O₃ is the only reactant with a chance of oxidizing HgNO₃ to Hg(II).
In the lone experimental study of reaction (1), Sommar et al., (1997) reported an upper limit to k₁ of 4 × 10^-15 cm³ molecule^-1 s^-1. Our analysis indicates that Sommar et al. would not have found evidence for reaction regardless of the value of the rate constant, so their reported upper limit is not valid.
Nighttime loss of Hg(0) or formation of gaseous Hg(II) has been attributed to NO₃ in several studies. Most notably. Peleg et al. (2015) observed a correlation between nighttime [NO₃] and formation of gaseous Hg(II). We carried out kinetic simulations of the conditions of Peleg et al., using their highest reported values of [Hg], [O₃], and [NO₃]. Regardless of the value of k₁ and with k₂ as high as 10^-10 cm³ molecule^-1 s^-1, respectively, the total extent of Hg(II) production was less than 1 pg/m³. This is much less than the average nighttime value (27 pg/m³) reported by Peleg et al. during their study. As a result, we conclude that the high Hg(II) values observed by Peleg et al. were not the result of atmospheric oxidation of Hg(0).