A review of uncertainties in atmospheric modeling of mercury chemistry I. Uncertainties in existing kinetic parameters – Fundamental limitations and the importance of heterogeneous chemistry

Abstract

Mercury and its related compounds are widely recognized as global pollutants. The accurate atmospheric modeling of its transport and fate has been the subject of much research throughout the last decade. Atmospheric gas, aqueous and heterogeneous chemistry are expected to occur for Hg-containing specie sand accurate implementation of their chemical parameters is essential for realistic modeling of mercury cycling. Although significant progress has been made, the current state of knowledge of mercury chemistry exhibits numerous uncertainties. The objective of this two-part review is to explore the sources of uncertainty from the viewpoint of mercury chemistry. In this first part, we assess the discrepancy that exists in the currently available mercury kinetic parameters for the gas and aqueous phases. Theoretical and experimental approaches of rate constant determination exhibit various levels of limitation and accuracy. We present an overview of the available techniques and the assumptions and shortcomings associated with these methods in order to assist the atmospheric modellers. We review specific mercury oxidation and reduction reactions that have been investigated and are commonly implemented in mercury models with respect to the uncertainties associated with them. We reveal that for most of these mercury reactions our current state of knowledge reflects a lack of proper under-standing of their mechanisms. Atmospheric heterogeneity is a topic of great importance and we elaborate upon it in part II of this review.

Recent Advances in Atmospheric Chemistry of Mercury

“…The existence of BrO radical has been reported in the upper troposphere [62,63] and over the southeastern US [5,64]. Due to the rapid exchange between Br and BrO radicals [65] and the much slower oxidation rate of Hg 0 by BrO [18] the field observations of BrO seem to support the dominant oxidation of Hg 0 by Br in the atmosphere. Shah et al [5] found that sensitivity simulations using the GEOS-Chem chemical transport model by either increasing Br concentrations, or using a faster rate constant for the oxidation reaction of Hg 0 byBr, resulted in a better agreement between the modeled results and the aircraft observations.…”

“…Table 1 lists the gas-phase oxidation reactions of Hg 0 and the reported rate coefficients. As shown in Figure 2 in Subiret al [18], large variations exist in the current reported rate constants of gas-phase oxidation reactions of Hg. Here we discuss recent advances in the understanding of the oxidation of Hg 0 by halogen atoms and NO3 radicals, as well as determining the major oxidant(s) in the atmosphere.…”

“…Theoretical studies estimated the rate constants to be from 9.8 × 10 −13 to 2.1 × 10 −12 cm 3 molec −1 s −1 , at 1 atm and 298 K [3,21,23]. Theoretical and experimental rate constants for mercury oxidation by Br and Cl atoms at 298 K have previously been compared in detail in Table A.2 by Subir et al [18].…”

“…However, whether Br, O 3 /OH or multiple oxidants are the major oxidants of Hg 0 in the global atmosphere is unclear. To address this important question, more studies on reducing the large uncertainties in rate constants, understanding the heterogeneous oxidation reactions of Hg 0 , as well as improving the treatment of chemical mechanisms in atmospheric Hg models and the accuracy of mercury emission inventories are needed [8,18,48].…”

“…The chemistry of atmospheric mercury was previously reviewed in detail by Lin and Pehkonen [16], Ariya et al [17], Subir et al [18] , Lin et al [14] and Ariya et al [4] . Several excellent contributions have been made to our understanding in this area since 2015.…”