Terrestial Hg fluxes: Is the next exchange up, down, or neither?

Abstract

Natural terrestrial sources of atmospheric mercury (Hg) include geologically enriched substrate, volcanoes, geothermal areas, forest fires, vegetation, and "background soils" (soils that have low concentrations of Hg (<0.lμg Hg/g) and have not been enriched by geologic processes). Emissions from the latter three are probably dominated by re-emission of previously deposited atmospheric Hg derived from anthropogenic and natural sources. Natural terrestrial sinks include soils, plant foliage, and regions where the atmospheric chemistry facilitates formation of reactive gaseous Hg (RGM) (i.e. Polar Regions). Modeled estimates of natural source emissions in the literature range from 800 to 3000 Mg/y (Nriagu, 1989; Lindqvist et al., 1991; Mason et al, 1994; Mason and Sheu, 2002; Lamborg et al., 2002; Seigneur et al., 2001; Bergan et al., 1999) (Table 1). Most of these were derived by difference, using measured air Hg concentrations, wet deposition rates, and anthropogenic emissions estimates in mass balance models, and are based on precious few (if any) actual flux measurements from substrate. Mercury is also known to be dry deposited although this is often not considered. Lindberg et al. (1998) used measured terrestrial flux data to develop an estimate of emissions from forests and background soils ranging from 1400 to 3400 Mg/y. However, their estimate did not include Hg fluxes from naturally geologically enriched substrates, volcanoes, geothermal systems and fires; which, as new data described below demonstrates, will add significantly to the emissions. Point source anthropogenic emission estimates remain highly uncertain (up to 50%) (Pai et al, 1998; Pacyna et al., 2001); yet, the range applied in global mass balance models is more limited than that for natural sources (∼2000 to 2400 Mg/y) (Bergan et al., 1999; Mason and Sheu, 2002; Lamborg et al, 2002; Seigneur et al, 2004) (Table 1). Dastoor and Larocque (2004) concluded that known anthropogenic emissions can account for only ∼1/3 of the measured concentration of gaseous elemental Hg (Hg) at ground level, and suggested that natural sources and re-emission account for 2/3 of the total estimated 6400 Mg emitted each year. Pirrone et al. (2001) suggested that natural and anthropogenic emissions in Europe were approximately equal.(Table presented) Global estimates for wet deposition to the continents are 2000 Mg/yr with dry deposition estimates at 1000 Mg/yr (Mason and Sheu, 2002) (Table 1). This is not enough to balance the estimated 6000 to 6600 Mg/y (Bergan et al., 1999; Mason and Sheu, 2002; Lamborg et al., 2002; Seigneur et al, 2004) emitted. The arctic sink (see below) appears to account for only a few hundred Mg/y (Schroeder et al., 2003). Recent models indicate that the ocean could be a sink; however, as Mason and Sheu (2002) point out, there is limited and variable data for assessing the role of the ocean in the Hg biogeochemical cycle. Mason et al. (1994) first suggested that the net ocean flux was zero; however, Mason and Sheu (2002) revised this estimate, based on new data on concentrations of RGM in the marine boundary layer, and concluded that the ocean is a net sink (∼500 Mg/y). Lamborg et al. (2002) assumed significantly lower ocean evasion rates than Mason and Sheu (2002) (820 Mg/y versus 2665 Mg/y, respectively), and suggested the ocean is a net sink (1230 Mg/y). These differences in numbers illustrate that the quantitative role of the ocean remains uncertain. Based on current data and the uncertainty associated with flux estimates, we appear to have too many sources and not enough sinks for atmospheric Hg. If air concentrations are not increasing, then we must be missing or underestimating some important sinks, or perhaps Hg is constantly recycled between terrestrial ecosystems and the atmosphere. The latter process would give Hg the overall appearance of exhibiting a long residence time when it actually is rather short for any particular atom. In this paper we will summarize our current understanding of natural terrestrial sources and sinks for atmospheric Hg. Advances in our ability to measure atmospheric Hg concentrations and speciation have allowed us to improve the estimates of emissions from geologically enriched substrates, assess the potential significance of atmospheric Hg exchange with background soils, and allowed us to generate data that will help us understand the significance of vegetation in Hg cycling.