Atmospheric Deposition of Mercury
Dr. Jane Guentzel
Coastal Carolina University

The Importance of Chemical Speciation, Climate, and Meteorology

Mercury exists in many different physical and chemical forms in the environment and it is the interconversions between these species that mediate its distribution patterns and biogeochemical cycling. The most widely known conversion is the biological transformation of inorganic Hg (II) to organic (methyl) Hg and its subsequent biomagnification in piscivorous fish, which poses a risk to higher trophic level organisms and humans who consume these fish. As the atmosphere is considered the dominant pathway for the delivery of inorganic Hg to aquatic ecosystems,1 this presentation will discuss the chemical species of Hg in the atmosphere; the sources of Hg to the atmosphere; and the transport and deposition of inorganic Hg to aquatic ecosystems.

In the atmosphere, mercury exists predominantly in the zero oxidation state as gaseous elemental Hg (Hgo) and in the +2 oxidation state as particulate Hg (Hgp) or as reactive gaseous Hg (Hg(II)). Gaseous elemental Hg comprises 97-99% of the total mercury found in the atmosphere and has a residence time on the order of 1 year.1,2 The remaining 1-3% is comprised of Hg(II) and Hgp, with residence times on the order of days to weeks.2 Reactive gaseous Hg can be formed in the atmosphere through the oxidation of gaseous elemental Hg by ozone3,4 or halogen radicals in the marine boundary layer and troposphere.5,6 Hg(II) is incorporated into cloud droplets or becomes attached to particulate material and is scavenged from the atmosphere by wet and dry deposition processes. Hgp can dry deposit to surfaces, be incorporated into cloud droplets, or be scavenged by precipitation.

These various species of mercury in the atmosphere originate from natural processes (25-30%) and anthropogenic activities (60-75%).7 Natural or background sources of atmospheric mercury, mainly in the form of Hgo, include emissions from volcanoes, soils, vegetation, and the ocean.8 It has been estimated that 20-30% of the current oceanic emissions originates from mercury mobilized by natural sources, with the remaining 70-80% derived from recycled anthropogenic Hg.7 Forest fires may emit Hgo and some partially oxidized species.8. Estimates of contributions from natural sources are limited by our uncertainties regarding the amount of Hg in the pre-industrial environment as well as uncertainties in estimating the amount of anthropogenic Hg that is recycled by the ocean and terrestrial environment.9

Modeling calculations estimate that anthropogenic emissions have tripled the concentration of mercury in the atmosphere and surface ocean over the last century.7 Anthropogenic sources of mercury include fossil fuel combustion (coal, oil, gas), waste incineration, chloro-alkali production, metal extraction processes, and cement production. These sources emit Hgo, Hgp, and Hg(II), which can cycle within the atmosphere and be deposited to ecosystems mainly as Hg(II)and Hgp. The distance that anthropogenically mobilized Hg is transported prior to deposition is determined largely by the speciation of Hg that is emitted.8 Hg(II) and Hgp deposit locally (50 km), while a significant fraction of Hgo can be transported over long distances ( 10,000 km) and enter the global mercury cycle.8 This mercury is subsequently available for oxidation to Hg(II) in the troposphere and marine boundary layer, resulting in a global or "background" contribution of Hg(II) to mercury deposition.5,10

Chemical speciation, climate, and meteorology influence the extent to which local and or global sources contribute to Hg deposition. The sub-tropical climate and complex meteorology of Southern Florida provide us with a rather unique environment to investigate Hg deposition. The annual rainfall volumes across Southern Florida range from 128-150 cm, with greater than 70% of the rainfall occurring during the rainy season (May-Oct.).10 The summertime wet season in Southern Florida is characterized by the almost daily occurrence of tall convective thunderstorms (12-16 km) and daily ventilation of background air by the strong synoptic southeasterly winds associated with the North Atlantic trade winds. Findings from the Florida Atmospheric Mercury Study (FAMS) suggest that the annual deposition of Hg in Southern Florida is mediated by long range transport of Hg (mainly Hg(II) resulting from the oxidation of Hgo in the global atmosphere) coupled with strong convective thunderstorm activity during the wet season. Model calculations indicate that long range transport accounts for 54-70% of the summertime rainfall Hg deposition, with the remaining 30-46% attributable to local anthropogenic Hgp and Hg(II) emissions.10 It is important to recall that 60-70% of the Hgo in the modern atmosphere results from industrial activity7 and reductions in Hg deposition will likely require reductions in local and global Hg emissions.

References
1 Fitzgerald, W.F., Mason, R.P., and Vandal, G.M. (1991) Water Air Soil Poll. 56, 745-768.
2 Lindqvist, O., Johansson, M., Aastrup, A., Andersson, L., Iverfeldt, A., Meili, M., and Timm, B. (1991) Water Air Soil Poll. 55, 1-262.
3 Hall, B. 1995 Water Air Soil Pollut. 80, 301-315.
4 Munthe, J. (1992) Atmos. Environ. 26A, 1461-1468.
5 Mason, R.P., Guey-Rong, S., and Lawson, Nichole. (2001) Abstract submitted to the Symposium on Methylmercury: Impacts on Wildlife and Human Health. Charleston, South Carolina, 2001.
6 Lindberg, S.E., Brooks, S., Lin, C.J., Scott, K.J., Landis, M.S., Stevens, R.K., Goodsite, M., and Richter, A. (2002) Environ. Sci. Technol. 36, 1245-1256.
7 Mason, R.P., Fitzgerald, W.F., Morel, F.M.M. (1994) Geochim. Cosmochim. Acta. 58, 3191.
8 Porcella, D.B., Chu, P., and Allan, M.A. (1996) Inventory of North American Hg Emissions to the Atmosphere; Relationship to the Global Hg Cycle. In; Global and Regional Mercury Cycles: Sources, Fluxes, and Mass Balances, 179-190.
9 EPMAP (Expert Panel on Mercury Atmospheric Processes) (1994) Mercury Atmospheric Processes: A Synthesis Report. EPRI/TR-104214. EPRI, Palo Alto, CA 94304, 23p.
10 Guentzel, J.L., Landing, W.M., Gill, G.A., and Pollman, C.D. (2001) Environ. Sci. Technol.35, 863-873.

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