Elsevier

Applied Geochemistry

Volume 23, Issue 3, March 2008, Pages 482-493
Applied Geochemistry

An update on the natural sources and sinks of atmospheric mercury

https://doi.org/10.1016/j.apgeochem.2007.12.010Get rights and content

Abstract

This paper summarizes recent advances in the understanding of the exchange of Hg between the atmosphere and natural terrestrial surfaces including substrates (soil, rocks, litter-covered surfaces and weathered lithological material) and foliage. Terrestrial landscapes may act as new sources of atmospheric Hg, and as repositories or temporary residences for anthropogenically and naturally derived atmospheric Hg. The role of terrestrial surfaces as sources and sinks of atmospheric Hg must be quantified in order to develop regional and global Hg mass balances, and to assess the efficacy of regulatory controls on anthropogenic point sources in reduction of human Hg exposure.

Continued field research has allowed for refinement of emission estimates for geothermal and volcanic, and Hg mineralized areas in the western USA to ∼1.2–3.0, and 10–20 Mg/a, respectively. The emission estimate for areas of Hg mineralization in the western USA includes only identified Hg deposits and occurrences, and since other areas of geologic Hg enrichment such as Au and Ag deposits are not considered, the range in values is most likely an underestimate. Laboratory and field measurements have improved understanding of air–surface Hg exchange associated with soils with low or natural background concentrations of Hg (<100 ppb), litter-covered forest floors, and foliar surfaces, all of which have large spatial coverage. Deposition of atmospheric Hg and re-emission are important processes occurring at these surfaces on diel and seasonal time scales. Foliage is a significant sink for atmospheric elemental Hg, however, the net flux associated with low Hg containing soils is uncertain. Mass balances developed for soil–air exchange using measured fluxes and estimated deposition indicate that over a year background soils may exhibit no net flux. This suggests that the residence time for elemental Hg in the air is on the order of hours to weeks. Short term exchange would result in a homogenous air Hg concentration due to constant mixing and in an apparent calculated residence time that is most likely too long (one year). Recycling of atmospheric Hg between natural background soils and foliar surfaces also provides a mechanism for long-term atmospheric contamination and continued deposition in pristine ecosystems well after anthropogenic sources are controlled.

Introduction

Natural emissions of Hg to the atmosphere occur from areas with ongoing geologic activity (volcanic and geothermal) and from substrates with elevated Hg concentrations (>100 ppb) due to mineralization as a result of geologic processes that occurred in the past. Geologically-derived naturally Hg-enriched areas are in general concentrated in broad global belts that coincide with major plate tectonic boundaries (cf. Pennington, 1959, Johanson and Boyle, 1972). However, there are Hg-enriched areas outside these boundaries, i.e. areas of Fe mineralization in Michigan and Zn mineralization in Missouri and Canada (Rytuba, 2003, Rytuba, 2005) and Siberia (Obolenskiy, 1996). Some component of emission from these naturally enriched areas includes re-emission of Hg deposited from the atmosphere.

Other “natural” terrestrial sources of atmospheric Hg include emission from background or low-Hg containing soils/substrates (<100 ppb) and foliar surfaces, and biomass burning. Emission from these sources is hypothesized to be predominantly re-emission of Hg deposited from the air by wet and dry processes derived from both anthropogenic and natural sources. Important identified natural sinks for atmospheric Hg are soils, plant foliage, and regions where the atmospheric chemistry facilitates formation of reactive gaseous Hg (RGM) (i.e. polar regions, marine boundary layer).

Estimates of global natural source emissions range from 800 to 3000 Mg/a (Nriagu, 1989, Lindqvist et al., 1991, Mason and Sheu, 2002, Seigneur et al., 2001). Although estimates for individual anthropogenic point source emissions have an uncertainty of up to 50% (Pai et al., 1998, Pacyna et al., 2001), the range in published values for global anthropogenic releases (∼2000–2400 Mg/a) is small compared to that reported for natural sources. Gustin and Lindberg (2005) indicated that the estimated Hg emissions to the atmosphere were significantly greater than known sinks, highlighting the uncertainties in the global Hg mass balance. This paper will summarize some recent advances in the authors’ assessment of natural terrestrial sources and sinks for atmospheric Hg. Refinements in their understanding of air–substrate and air–foliar Hg exchange and the impacts of these advancements on assessment of the fate and transport of Hg in the environment will be discussed.

Section snippets

Natural geologic sources of atmospheric Hg

Estimation of Hg releases from volcanoes is difficult due to spatial and temporal variability in activity, and the overall lack of data (Gustin and Lindberg, 2005). Emission estimates are typically developed by applying the few available Hg/SO2 mass ratios to measured SO2 concentrations derived for volcanic systems. Mercury/SO2 ratios span four orders of magnitude (cf. Pyle and Mather, 2003) and reflect the fact that Hg and SO2 content of volcanic gases can vary as a function of the eruptive

Background soils

Soils with low or background Hg concentrations cover large spatial areas, therefore understanding their role as a net source or sink of atmospheric Hg as well as the parameters controlling the exchange is important. In most pristine ecosystems the soil Hg pool is the largest reservoir of Hg (Krabbenhoft et al., 2005). In areas without natural Hg-enrichment by geologic processes, Hg in the surface soils consists of a component derived from parent rock material plus that contributed by wet and

The role of vegetation

The importance of foliar uptake of atmospheric Hg° has been demonstrated by numerous laboratory and field studies (c.f. Frescholtz et al., 2003, Ericksen et al., 2003, Millhollen et al., 2006a, Millhollen et al., 2006b, Rasmussen et al., 1991). Research has shown that plant uptake is dependent upon plant species and age, as well as air Hg concentrations. It is now thought that foliar uptake and sequestration is an important net sink for atmospheric Hg° and emissions from foliage do not reflect

Other natural sources and sinks

Because the volatility of Hg is well known and its accumulation in foliage has been demonstrated, it should come as no surprise that biomass burning is a source of atmospheric Hg. Measurement of Hg, CO and CO2 concentrations in plumes of fires have been applied to estimate Hg released during biomass burning. Global emission estimates of Hg released are 200–1000 Mg/a (Brunke et al., 2001, Friedli et al., 2001). Friedli et al. (2003) estimated approximately 3.7 ± 1.9 Mg/a were released by fires in

Estimating the contribution of Hg to the atmosphere from natural sources

Scaling up emissions measured from areas naturally enriched in Hg to larger areas has been done using empirical data to develop algorithms between Hg flux and environmental or substrate parameters. Area average fluxes of 2–440 ng/m2 h (2–110 kg/a) for regions of ∼1–900 km2 have been reported (Gustin, 2003, Rasmussen et al., 1998, Coolbaugh et al., 2002, Zehner and Gustin, 2002, Ferrara et al., 1998). The size of the Hg-mineralized area included in the scaling exercise can influence the area average

Conclusions

The Hg research community is making steady progress towards understanding natural source Hg emissions. It appears that the amount emitted from volcanoes and geothermal areas from the conterminous United States is small (∼3 Mg/a) while releases from naturally Hg-enriched substrates is at the least 10–20 Mg/a. This range in values is based solely on areas of Hg mineralization in the western USA and should not be considered representative of the entire country. Additionally, the empirical databases

Acknowledgements

The following agencies provided support for research presented in this manuscript – US EPA STAR and EPSCoR programs, EPRI, National Science Foundation-Atmospheric Sciences Program, and the US Geological Survey. We also thank our many collaborators and graduate and undergraduate students whose diligent work has helped move the research forward. Rekha Pillai assembled GIS databases for Hg scaling estimates.

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