An update on the natural sources and sinks of atmospheric mercury
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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