Recent advances in understanding and measurement of Hg in the environment: Surface-atmosphere exchange of gaseous elemental mercury (Hg~0)

摘要

The atmosphere is the major transport pathway for distribution of mercury (Hg) globally. Gaseous elemental mercury (GEM, hereafter Hg~0) is the predominant form in both anthropogenic and natural emissions. Evaluation of the efficacy of reductions in emissions set by the UN's Minamata Convention (UN-MC) is critically dependent on the knowledge of the dynamics of the global Hg cycle. Of these dynamics including e.g. red-ox reactions, methylation-demethylation and dry-wet deposition, poorly constrained atmosphere-surface Hg~0 fluxes especially limit predictability of the timescales of its global biogeochemical cycle. This review focuses on Hg~0 flux field observational studies, namely the theory, applications, strengths, and limitations of the various experimental methodologies applied to gauge the exchange flux and decipher active sub-processes. We present an in-depth review, a comprehensive literature synthesis, and methodological and instrumentation advances for terrestrial and marine Hg~0 flux studies in recent years. In particular, we outline the theory of a wide range of measurement techniques and detail the operational protocols. Today, the most frequently used measurement techniques to determine the net Hg~0 flux (＞95% of the published flux data) are dynamic flux chambers for small-scale and micro-meteorological approaches for large-scale measurements. Furthermore, top-down approaches based on Hg~0 concentration measurements have been applied as tools to better constrain Hg emissions as an independent way to e.g. challenge emission inventories. This review is an up-dated, thoroughly revised edition of Sommar et al. 2013 (DOI: 10.1080/10643389.2012.671733). To the tabulation of ＞100 cited flux studies 1988-2009 given in the former publication, we have here listed corresponding studies published during the last decade with a few exceptions (2008-2019). During that decade, Hg stable isotope ratios of samples involved in atmosphere-terrestrial interaction is at hand and provide in combination with concentration and/or flux measurements novel constraints to quantitatively and qualitatively assess the bi-directional Hg~0 flux. Recent efforts in the development of relaxed eddy accumulation and eddy covariance Hg~0 flux methods bear the potential to facilitate long-term, ecosystem-scale flux measurements to reduce the prevailing large uncertainties in Hg~0 flux estimates. Standardization of methods for Hg~0 flux measurements is crucial to investigate how land-use change and how climate warming impact ecosystem-specific Hg° sink-source characteristics and to validate frequently applied model parameterizations describing the regional and global scale Hg cycle.