Web Release Date: July 4,
Investigation of Mercury Exchange
between Forest Canopy Vegetation
and the Atmosphere Using a New
Dynamic Chamber
and
Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6083, Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario, Canada, K9J 7B8, and U.S. Geological Survey-WRD, 8505 Research Way, Middleton, Wisconsin 53562
Received for review February 27, 2006
Revised manuscript received May 2, 2006
Accepted May 15, 2006
Abstract:
This paper presents the design of a dynamic chamber system that allows full transmission of PAR and UV radiation and permits enclosed intact foliage to maintain normal physiological function while Hg(0) flux rates are quantified in the field. Black spruce and jack pine foliage both emitted and absorbed Hg(0), exhibiting compensation points near atmospheric Hg(0) concentrations of ~2-3 ng m-3. Using enriched stable Hg isotope spikes, patterns of spike Hg(II) retention on foliage were investigated. Hg(0) evasion rates from foliage were simultaneously measured using the chamber to determine if the decline of foliar spike Hg(II) concentrations over time could be explained by the photoreduction and re-emission of spike Hg to the atmosphere. This mass balance approach suggested that spike Hg(0) fluxes alone could not account for the measured decrease in spike Hg(II) on foliage following application, implying that either the chamber underestimates the true photoreduction of Hg(II) to Hg(0) on foliage, or other mechanisms of Hg(II) loss from foliage, such as cuticle weathering, are in effect. The radiation spectrum responsible for the photoreduction of newly deposited Hg(II) on foliage was also investigated. Our spike experiments suggest that some of the Hg(II) in wet deposition retained by the forest canopy may be rapidly photoreduced to Hg(0) and re-emitted back to the atmosphere, while another portion may be retained by foliage at the end of the growing season, with some being deposited in litterfall. This finding has implications for the estimation of Hg dry deposition based on throughfall and litterfall fluxes.
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