ReviewA review of uncertainties in atmospheric modeling of mercury chemistry I. Uncertainties in existing kinetic parameters – Fundamental limitations and the importance of heterogeneous chemistry
Highlights
► We assess discrepancies in the existing mercury gas and aqueous phase rate constants. ► Assumptions in experimental and theoretical methods of rate determination are discussed. ► Uncertainties for specific mercury redox reactions are evaluated. ► Our current state of knowledge lacks proper understanding of most mercury reaction mechanisms. ► The available rate constants are not realistic due to atmospheric heterogeneity.
Introduction
Mercury is widely recognized as a global pollutant and a serious health and environmental hazard (Pirrone and Mahaffey, 2005, Pirrone and Mason, 2009, Selin, 2009, Watras and Huckabee, 1994). Released to the environment from a multitude of natural and anthropogenic sources, mercury is redistributed globally via a complex combination of chemical, physical, and biological processes throughout the atmosphere and the rest of the environment. Fig. 1 depicts some of the pathways by which mercury is chemically transformed and transported throughout the atmosphere. Processes such as oxidation/reduction reactions, complex formation, phase transitions, biodegradation, and surface and heterogeneous interactions with aerosols, clouds, snow, and ice convert mercury from one species to another. Physical mechanisms such as wind flux, runoff, and dry and wet deposition transport elemental and oxidized mercury species from one place to another (Pirrone and Mahaffey, 2005, Pirrone and Mason, 2009).
Given that selected mercury species, such as organomercury, are serious contaminants of the ecosystem and can have detrimental effects on human health (Flora et al., 1994, Selin, 2009), understanding the detailed processes that govern the overall transport and cycling of mercury species has been a chief concern amongst environmental and atmospheric scientists. Accordingly, with the aim to simulate its transport and cycling, this recent decade has seen a surge of scientific research in the realm of atmospheric chemistry related to mercury (Pirrone and Mahaffey, 2005, Pirrone and Mason, 2009). Despite the extensive effort, however, accurate prediction of mercury transport at both regional and global scales remains challenging.
Uncertainty in mercury modeling arises from many sources including: (1) inaccuracies present in existing chemical kinetic parameters, (2) inadequate representation of chemical processes and lack of characterization of mercury species critical in describing the cycling of mercury in the atmosphere, (3) lack of detailed mercury chemical speciation in field studies (4) difficulties at the boundary layer, (5) challenges with mercury transport and deposition mechanisms, (6) emission inventory, and (7) inherent assumptions in chemical models. The objective of this two-part review is to survey the sources of uncertainty associated with the modeling of mercury chemistry. Herein, we examine the discrepancies that exist within the currently available mercury kinetic data for gas and aqueous phase reactions (1). The chemical processes that are missing and/or inadequately implemented in mercury models (2), including the heterogeneous interaction of mercury (see Fig. 1) with aerosol, snow, ice, water droplets, vegetation surfaces, and dissolved organic matters (DOM), as well as reduction pathways, are discussed in part II.
Most of the different mercury gas and aqueous phase reaction rate constants available display large discrepancies (see Fig. 2) or are too limited to substantiate. Several recent publications have explored and addressed these kinetic studies (Ariya and Peterson, 2005, Ariya et al., 2009b, Hynes et al., 2009, Lin et al., 2006). We herein extend the existing reviews beyond comparative studies of rate coefficients and focus on sources of uncertainties in field, experimental and theoretical studies for the available mercury reaction rate constants. We further discuss the implication of these uncertainties on modelling studies.
Section snippets
Chemical kinetics – fundamentals of experimental and theoretical methods
The determination of rate constants, both theoretical and experimental, is complex and requires that assumptions be made. Usage of different kinds of measurement techniques and theoretical methods of calculations leads to different levels of uncertainty. A comprehensive knowledge of the assumptions pertaining to both experimental and theoretical techniques is thus necessary. In this section, we present a detailed overview of the limitations and assumptions associated with these approaches. A
Uncertainties in the existing mercury (Hg0) oxidation reaction
Oxidation of elemental mercury, Hg0 to HgI and HgII species, is an important pathway in mercury cycling in the atmosphere because the oxidized species exhibit different physical and chemical properties. For instance, among other processes, vapor pressure, solubility (Clever et al., 1985, Hepler and Olofsson, 1975) and photochemistry (Kunkely et al., 1997, Zhang, 2006) of oxidized mercury compounds differ greatly from that of elemental mercury. Several experimental and theoretical investigations
Uncertainties in the existing mercury reduction rate constants
Reduction of mercury (HgI and HgII species) is assumed to occur in the aqueous phase, where mercury can exist as free ions or complexes (Pirrone and Mahaffey, 2005, Pirrone and Mason, 2009). Table A.3 provides aqueous reduction reactions and their corresponding rate constants that are generally incorporated in atmospheric mercury modeling (Hedgecock and Pirrone, 2005, Lin et al., 2006, Seigneur et al., 2009, Seigneur et al., 2006). The importance of the reduction of mercury with hydroperoxyl
Concluding remarks
There are several factors that lead to uncertainties in the existing mercury reaction rate constants and thereby in the modeling of atmospheric mercury cycling. Table 2 summarizes the uncertainties associated with these reactions. The theoretical calculation of rate constants involving mercury requires extensive treatment of the relativistic effects and accurate determination of bond energies of reactants, reaction intermediates, and products. Furthermore, assumptions associated with TST and
References (98)
- et al.
Gas phase elemental mercury: a comarison of LIF Detection techniques and study of the kinetics of reaction with the hydroxyl radical
Journal of Photochemistry and Photobiology A: Chemistry
(2003) - et al.
Mechanisms of mercury removal by O3 and OH in the atmosphere
Atmospheric Environment
(2005) - et al.
Oxidation of atomic mercury by hydroxyl radicals and photoinduced decomposition of methylmercury in the aqueous phase
Atmospheric Environment
(2001) - et al.
Atmospheric oxidation of elemental mercury by ozone in the aqueous phase
Atmospheric Environment
(1986) - et al.
Photophysics and photochemistry of mercury complexes
Coordination Chemistry Reviews
(1997) - et al.
Scavenging of gaseous mercury by acidic snow at Kuujjuarapik, Norther Quebec
Science of the Total Environment
(2006) - et al.
aqueous free radical chemistry of mercury in the presence of iron oxides and ambient aerosol
Atmospheric Environment
(1997) - et al.
The chemistry of atmospheric mercury: a review
Atmospheric Environment
(1999) - et al.
scientific uncertainties in atmospheric mercury models I: model science evaluation
Atmospheric Environment
(2006) Surface catalyzed reaction of Hg + Cl2
Chemical Physics Letter
(1979)