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).

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

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