Atmospheric Mercury Speciation
The behavior of mercury in the atmosphere depends upon its form, or specie. Elemental mercury (Hgo) is typically not very reactive with global lifetime of a few months to a year and is thought to be transported significantly in the troposphere. Reactive gaseous mercury (RGM) species, are not well characterized chemically but are thought to be gaseous Hg(II)-bearing molecules such as HgCl2(g). RGM species are notable for being quickly deposited from the atmosphere to the surface and are thought to be readily available for conversion to methylmercury, a highly toxic form of mercury. Particulate mercury (Hg-P) is also quickly deposited and is often found in high concentrations near combustion sources. Although much lower in proportion than Hgo, the greater reactivity and deposition rates of RGM and Hg-P make them a larger environment concern. Chemical reactions that occur in the atmosphere can transform mercury between these various species.
Figure 1. USGS Mobile Mercury Laboratory operated by the USGS Wisconsin Water Science Center. Instruments include an atmospheric mercury speciation unit; NOx, SOx, O3, and PM2.5 analyzers; a meteorological station; and a wet deposition sampler. Photo by James Robinson, USGS.
Recently, USGS researchers from the Energy Resources Team and the Water Resources Discipline have started characterizing tropospheric Hgo, RGM, and Hg-P concentrations in southern Alabama, near the Gulf of Mexico. The project relies upon the USGS Mobile mercury laboratory (Figure 1) which measures near real-time concentrations of the three mercury species (Figure 2). Potential sources of mercury to the region include a nearby coal-fired power plant and industrial emissions from nearby cities such as Mobile, Alabama; Pensacola, Florida; and New Orleans, Louisiana. Natural processes are also being investigated that could cause conversion of Hgo to RGM or Hg-P.
Figure 2. Plot of preliminary RGM, Hg-P, and Hgo concentration data determined by the atmospheric mercury speciation unit on the USGS Mobile Mercury Laboratory deployed at Weeks Bay, Alabama from 6/6/05 to 6/16/05 (Julian days 157 to 167). The gap in the data is a result a shut down during tropical storm Arlene.
Additional total suspended particulate samplers were used to collect atmospheric particulates on filters that are analyzed for mercury as well as a variety of other constituents (Figure 3). Combining the mercury speciation data with multivariate statistical methods and air mass trajectory modeling, potential sources of mercury and other trace metals can potentially be identified as well as processes that produce RGM and Hg-P.
Figure 3. High-volume (left) and low-volume (right) total suspended particulate samplers deployed at Gulf Shores State Park, Alabama. The high volume sampler collects particles on a large filter located under the triangular lid. The high volume filters were analyzed for mercury and soluble components including Cl and SO4. The low volume sampler contains two filters housed in filters packs at each end of the tripod crossbar. One filter is analyzed for mercury and the other for other trace metals. Photo by Allan Kolker, USGS.
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U.S. Geological Survey
