mercury derived from the atmosphere or terrestrial landscape (Munthe et al., 2007). Although this form has been measured in rain, the concentrations are extremely low, and the atmosphere is not thought to be a direct source to ecosystems (Sakata and Marumoto, 2005; Hammerschmidt et al., 2007).

Hg is released to the atmosphere by both natural and anthropogenic sources. Natural sources emit almost exclusively Hg (0) (Bagnato et al., 2007; Gustin et al., 2008) ; anthropogenic sources emit varying combinations of Hg (0), RGM, and Hgp (Pacyna et al., 2003b, 2006a). Atmospheric RGM may also be generated by reactions of Hg (0) with oxidants such as O3, OH, and reactive halogens such as Cl, Cl2, Br, and BrO (Lin et al., 2006; Hynes et al., 2008; Ariya et al., 2009). These reactions, which may occur in the surface boundary layer, the free troposphere, and stratosphere, and are poorly understood.

To evaluate the potential for long-range transport of Hg once emitted from a natural or anthropogenic source, the atmospheric lifetime (mean time in the air before being removed) of the different forms needs to be considered. Gaseous Hg (0) has an estimated atmospheric lifetime of months to more than a year (Lindberg et al., 2007), thus a molecule of Hg in a fish may have its origin from a source far removed. Episodic events of elevated air Hg concentrations recorded at the Mt. Bachelor Observatory in central Oregon (Jaffe et al., 2005a, Figure 1.3; Weiss-Penzias et al., 2006) and in aircraft measurements (Friedli et al., 2003; Swartzendruber et al., 2008) have been linked to air masses passing over Asia. The form of Hg transported in these events is predominantly Hg (0). Elevated air Hg concentrations have also been measured in plumes associated with fires and industrial sources (Edgerton et al., 2006; Ebinghaus, 2008; Finley et al., 2009). In contrast RGM and Hgp are water soluble and have high deposition velocities, resulting in efficient removal from the atmosphere by dry and wet deposition (Schroeder and Munthe, 1998).

Field and laboratory data have shown that Hg (0) is recycled between the air and terrestrial and aquatic surfaces, and this can occur over the course of a day (Gustin et al., 2008). Limited field studies using stable isotopes and laboratory experiments have indicated that 5 to 40 percent of Hg added to an ecosystem as HgCl2 is released in the short term (Hintelmann and Evans, 1997; Hintelmann et al., 2001, 2002; Lindberg et al., 2002; Amyot et al., 2004; Ericksen et al., 2005; Xin et al., 2007). This recycling of Hg between the air and surfaces results in long-term availability of Hg to ecosystems and complicates source attribution.

Human and Ecosystem Health The major human and ecosystem health threat associated with long-range transport of Hg arises from the fact that Hg in the air is deposited to watersheds where it may enter aquatic systems

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