Negligible Role of Nitrate Radical in Directly Initiating Oxidation of Hg(0) to Hg(II) in the Atmosphere

Nitrate radical (NO3) has been suggested to participate in oxidizing gaseous Hg(0) in the atmosphere, most convincingly in the field study by Peleg et al. (Environ. Sci. Technol. 2015, 49, 14008–14018). This conclusion has been hard to reconcile with the two-step mechanism of Hg(0) conversion to Hg(II) via Hg(I), due to the low value reported for the NO3–Hg(I) bond energy (∼5 kcal mol–1). Using a high level of computational quantum chemistry, we find this bond energy to be 6.5 kcal mol–1, and we use standard statistical mechanics to compute the equilibrium constant, Kc(T), for NO3 + Hg(0) = NO3Hg(I). Our kinetic analysis finds that under the conditions of Peleg et al., NO3 could not have contributed significantly to the formation of Hg(II). In addition, we added NO3-initiated oxidation of Hg(0) into the GEOS-Chem global model of atmospheric Hg. Despite that the model indicates that NO3 oxidizes Hg(0) to Hg(I) about three times faster than either Br or OH radicals, NO3Hg(I) falls apart so fast that NO3-initiated oxidation has a negligible impact on atmospheric Hg cycling. Finally, we reanalyze an experiment that reported an upper limit to the rate constant for the NO3 + Hg(0) reaction. We argue that this experiment would not have been able to detect the loss of Hg(0) due to gas-phase reaction with NO3, even if this reaction proceeded with the collision rate constant.