Elsevier

Atmospheric Environment

Volume 45, Issue 32, October 2011, Pages 5664-5676
Atmospheric Environment

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

https://doi.org/10.1016/j.atmosenv.2011.04.046Get rights and content

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 species and 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 understanding of their mechanisms. Atmospheric heterogeneity is a topic of great importance and we elaborate upon it in part II of this review.

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

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