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methylation. Methylmercury bioaccumulates in organisms and biomagnifies up food chains, <br /> particularly in aquatic food chains. Organisms exposed to McHg in their food can build up <br /> concentrations that are many times higher than the ambient concentrations in the environment. <br /> Inorganic atmospheric mercury occurs in three forms: <br /> • Elemental mercury vapor(Hg(0)), also referred to as gaseous elemental mercury (GEM); <br /> • Gaseous divalent mercury, Hg(II), also referred to as reactive gaseous mercury (RGM) or <br /> gaseous oxidized mercury; <br /> • Particulate mercury, Hg(p), also referred to as particle bound mercury (PBM); PBM can be <br /> directly emitted or can form when RGM adsorbs on atmospheric particulate matter. <br /> In the global atmosphere, Hg(0) accounts for more than 90 percent of total mercury, on average, <br /> while both RGM and PBM typically account for less than 5 percent(EPRI 2014). The reactive <br /> form of mercury (RGM) is often deposited to land or water surfaces much closer to their sources <br /> due to its chemical reactivity and high water solubility. PBM is transported and deposited at <br /> intermediate distances depending on aerosol diameter or mass. Within the atmosphere, <br /> numerous physical and chemical transformations of mercury can occur depending on many <br /> factors. <br /> The various forms of mercury have very different physical and chemical characteristics, resulting <br /> in large differences in their removal rates from the atmosphere, and consequently, in their <br /> atmospheric lifetimes (EPRI 2014). GEM has a lifetime on the order of several months to more <br /> than a year because of its low reactivity, low water solubility, and slow deposition rate. Thus, it <br /> is considered a global pollutant since it is transported over long distances. On the other hand, the <br /> lifetimes of both RGM and PBM are much smaller, ranging from a few hours to days,because <br /> they are removed efficiently by dry and wet deposition, particularly RGM. Thus,mercury is a <br /> pollutant at all scales ranging from global to local. <br /> Mercury is emitted by both natural and anthropogenic sources. Natural sources include <br /> volcanoes, geothermal sources, and exposed naturally mercury-enriched geological formations. <br /> These sources may also include re-emission of historically deposited mercury as a result of <br /> evasion from the surface back into the atmosphere, fires, meteorological conditions, as well as <br /> changes in land use and biomass burning. Anthropogenic sources of mercury include burning of <br /> fossil fuels,incinerators, mining activities,metal refining, and chemical production facilities. <br /> Once mercury is emitted from the smoke stacks at the Craig Station it is transported some <br /> distance through the atmosphere before deposition on the land scape takes place. Apportioning <br /> the deposition of mercury based on emissions from multiple emissions sources is a complicated <br /> endeavor. Currently no requirement or program exists for modeling the source apportionment of <br /> mercury emissions. Regional scale photochemical modeling that accounts for simulated <br /> chemical transport, dispersion within the atmosphere, and chemical interactions of pollutants <br /> within the atmosphere are required for such an effort. Independent from the project under <br /> consideration in this consultation, an effort to conduct such a mercury deposition modeling effort <br /> 44 <br />