US EPA

769549 

Journal Article 

Air/surface exchange of mercury vapor over forests—the need for a reassessment of continental biogenic emissions 

Lindberg, SE; Hanson, PJ; Meyers, TP; Kim, KH 

1998 

1 

Atmospheric Environment
ISSN: 1352-2310
EISSN: 1873-2844 

32 

5 

895-908 

English 

10.1016/S1352-2310(97)00173-8 

WOS:000073337900010 

http://linkinghub.elsevier.com/retrieve/pii/S1352231097001738
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Atmospheric sources are significant in the cycling of Hg in the biosphere, but there are few reliable data on air:surface exchange of Hg in terrestrial systems. We developed a tower-based micrometeorological gradient method for measuring gas-phase Hg(o) fluxes over soils and vegetation. We describe here results of the modified Bowen ratio approach from three separate flux sampling campaigns: over a mature deciduous forest at the Walker Branch Watershed in Tennessee, over a young pine plantation in Tennessee, and over the boreal forest floor at the Lake Gordsjon watershed in Sweden. Our data show that Hg(o) exchange over these surfaces is bidirectional, but is primarily characterized by emissions from plants and soil. Dry deposition (foliar uptake) is less frequent, of generally lower magnitude, and may be enhanced by surface wetness. We measured emissions over tree canopies in Tennessee in the range of similar to 10-300 ng m(-2) h(-1), and over the boreal forest floor in Sweden of similar to 1-4 ng m(-2) h(-1). Fluxes were influenced by temperature, solar radiation, and atmospheric turbulence. The ability of trees to emit Hg from soil pools has now been established. Others have proposed a significant biotic re-emission of Hg(o) from the oceans, and our data provide the first direct evidence of a similar process in terrestrial systems. These data have been combined with results from chamber studies to estimate the overall flux of gas-phase Hg(o) between the atmosphere and terrestrial systems. Transpiration of Hg(o) represents a previously unmeasured mobilization of Hg from the continents to the troposphere. Including this new source term could increase current estimates of so-called natural emissions by over 100%. (C) 1998 Elsevier Science Ltd. All rights reserved. 

micrometeorological studies; atmosphere/surface exchange processes; emission; dry deposition; biogeochemical cycling; modeling; trace metals; air toxics; boreal forest; Sweden