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

Nitrate radical (NO 3 ) 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 NO 3 –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, K c ( T ), for NO 3 + Hg(0) = NO 3 Hg(I). Our kinetic analysis finds that under the conditions of Peleg et al., NO 3 could not have contributed significantly to the formation of Hg(II). In addition, we added NO 3 -initiated oxidation of Hg(0) into the GEOS-Chem global model of atmospheric Hg. Despite that the model indicates that NO 3 oxidizes Hg(0) to Hg(I) about three times faster than either Br or OH radicals, NO 3 Hg(I) falls apart so fast that NO 3 -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 NO 3 + 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 NO 3, even if this reaction proceeded with the collision rate constant.