Global observations and modeling of atmosphere–surface exchange of elemental mercury: a critical review

Abstract

Abstract. Reliable quantification of air-surface fluxes of elemental Hg vapor (Hg 0 ) is crucial for understanding mercury (Hg) global biogeochemical cycles. There have been extensive measurements and modeling efforts devoted to estimating the exchange fluxes between the atmosphere and various surfaces (e.g., soil, canopies, water, snow, etc.) in the past three decades. However, large uncertainties remain due to the complexity of Hg 0 bidirectional exchange, limitations of flux quantification techniques and challenges in model parameterization. In this study, we provide a critical review on the state of science in the atmosphere-surface exchange of Hg 0 . Specifically, the advancement of flux quantification techniques, mechanisms in driving the air-surface Hg exchange and modeling efforts are presented. Due to the semi-volatile nature of Hg 0 and redox transformation of Hg in environmental media, Hg deposition and evasion are influenced by multiple environmental variables including seasonality, vegetative coverage and its life cycle, temperature, light, moisture, atmospheric turbulence and the presence of reactants (e.g., O 3 , radicals, etc.). However, the effects of these processes on flux have not been fundamentally and quantitatively determined, which limits the accuracy of flux modeling.We compile an up-to-date global observational flux database and discuss the implication of flux data on the global Hg budget. Mean Hg 0 fluxes obtained by micrometeorological measurements do not appear to be significantly greater than the fluxes measured by dynamic flux chamber methods over unpolluted surfaces (p = 0.16, one-tailed, Mann-Whitney U test). The spatiotemporal coverage of existing Hg 0 flux measurements is highly heterogeneous with large data gaps existing in multiple continents (Africa, South Asia, Middle East, South America and Australia). The magnitude of the evasion flux is strongly enhanced by human activities, particularly at contaminated sites. Hg 0 flux observations in East Asia are comparatively larger in magnitude than the rest of the world, suggesting substantial re-emission of previously deposited mercury from anthropogenic sources. The Hg 0 exchange over pristine surfaces (e.g., background soil and water) and vegetation needs better constraints for global analyses of the atmospheric Hg budget. The existing knowledge gap and the associated research needs for future measurements and modeling efforts for the air-surface exchange of Hg 0 are discussed.

Isotopic Composition of Gaseous Elemental Mercury in the Marine Boundary Layer of East China Sea

“…In addition, soil GEM emission is another factor for the highly negative GEM δ 202 Hg observed in China. Soil GEM emissions are much higher in China than that in the rest of the world (Wang et al, ; Zhu et al, ). GEM emissions from naturally Hg‐enriched soils in China have significantly negative δ 202 Hg (−4.64‰ to −1.10‰, mean [±1σ] = −2.84 ± 0.67‰, n = 50) and positive Δ 199 Hg (−0.06‰ to 0.88‰, mean [±1σ] = 0.27 ± 0.18‰, n = 50; Fu et al, ).…”

Chemical cycling and deposition of atmospheric mercury in polar regions: review of recent measurements and comparison with models

“…3.2.1), suggesting that the dynamic daily cycle of deposition and re-emission at the air/snow interface is not captured by the models. The bidirectional exchange of Hg (0) is complex and influenced by multiple environmental variables (e.g., UV intensity, temperature, atmospheric turbulence, presence of reactants) limiting the accuracy of flux modeling (Zhu et al, 2016). The work carried out by Durnford et al (2012) in the Arctic and by Zatko et al (2016) in Antarctica could be good starting points for future research.…”

Gaseous elemental mercury (GEM) fluxes over canopy of two typical subtropical forests in south China

“…However, the present estimates of natural Hg emission from waters, soils, and vegetation are poorly constrained and have large uncertainties, with the values larger than anthropogenic emission (e.g., 2000 t yr −1 : Lindqvist et al, 1991; 5207 t yr −1 : Pirrone et al, 2010; 4080-6950 t yr −1 : UNEP, 2013; 4380-6630 t yr −1 : Zhu et al, 2016). The reliable quantification of natural Hg source, specifically GEM exchange between terrestrial ecosystem and the atmosphere would contribute to the understanding of global and regional Hg cycling budgets (Pirrone et al, 2010;Wang et al, 2014b;Song et al, 2015).…”