The industrial use of mercury in the elemental form (Hg°) and the behavior of most Hg species during combustion of fuels and wastes assures that Hg is emitted to the
atmosphere primarily in gaseous forms. These gases deposit locally and also undergo long range transport and deposition to remote areas, where they readily bioaccumulate in the toxic, methylated form [See figure].
The widespread occurrence of atmospheric Hg suggests that there are numerous processes leading to the airborne dispersion of Hg from the earth's surface. The atmospheric burden of Hg is known to consist of both natural and anthropogenic sources, and previous estimates of the global Hg budget indicate that these sources are almost equally important contributors to the atmospheric Hg burden. However, the individual sources and processes by which Hg is emitted from natural surfaces are not well understood. Oceanic sources of Hg are thought to comprise the dominant portion of the natural Hg budget, but quantitative information concerning the role of terrestrial ecosystems is less available. Several aspects of the Hg issue are discussed in the proceedings of a recent workshop in Canada. [Canadian Mercury Network]
The EPRI (Electric Power Research Institute) MASE (Mercury Air/Surface Exchange) project was initiated at Oak Ridge National Laboratory to quantify the deposition and emission of Hg in both terrestrial and aquatic ecosystems. We and our colleagues at the NOAA ATDD [NOAA ATDD] have developed and extensively tested the micrometeorological modified Bowen ratio (MBR) method which uses vertical gradients of vapor-phase Hg and reference trace gas concentrations with simultaneously measured turbulent-mixing parameters to quantify area-averaged Hg fluxes. To facilitate these observations we developed a multiple replicate sampling system consisting of a six-port-manifold connected to six separate mass flow controllers (MFCs) to obtain highly precise quantifications of gas phase elemental mercury vapor [Replicate Sampling System]. We have also developed and tested refluxing mist chambers to quantify the speciation of gaseous Hg, particularly reactive, water soluble compounds of divalent Hg. This species is important because it exhibits a water solubility far greater than that of the dominant form of atmospheric Hg (i.e., metallic vapor).
We have completed successful applications of our gradient flux methods over soils contaminated with Hg wastes and municipal sludge, background forest and agricultural soils, forest vegetation, lake waters in temperate and boreal forests, and subtropical wetlands. In addition, we have recently completed studies using alternative flux methods: portable Teflon and polycarbonate dynamic flux chambers over soils, plants, and waters, a dynamic branch chamber for field studies, and a controlled-environment laboratory chamber for tree saplings. Most of these studies have been published (see link below). The work over sludge-amended soils identified the first known terrestrial emissions of methyl mercury. Several new projects are now underway, and further information can be found at:
[http://www.esd.ornl.gov/programs/ACA/index.html].
Please direct questions regarding the MASE project to: Steven E. Lindberg
(lindbergse@ornl.gov)