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

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 Hg⁰ 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 Gardsjon watershed in Sweden. Our data show that Hg⁰ 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 ~10-300 ng m^-2 h^-1, and over the boreal forest floor in Sweden of ~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 Hg⁰ 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 Hg⁰ between the atmosphere and terrestrial systems. Transpiration of Hg⁰ 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%.