FOLIAR EXCHANGE OF MERCURY VAPOR: EVIDENCE FOR A COMPENSATION POINT P. J. HANSON 1, S.E. LINDBERG 1, T.A. TABBERER1, J.G. OWENS 1, AND K.-H. KIM1 1Envirortmental Sciences Division, Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, Tennessee 37831-6034, U.S.A. Abstract. Historical studies for crop and weed species documented elemental Hg vapor (Hg~ deposition to foliage, but they used ltg ~ concentrations that were orders of magnitude higher than levels now known to occur under background conditions, possibly creating artificially high gradients between the atmosphere and landscape surfaces. Measurements of Hg~ exchange with white oak (Quercus alba L.), red maple (Acer rubrum L.), Norway spruce (Picea abies L.), and yellow-poplar (Liriodendron tulipifera L.) foliage were conducted in an open gas exchange system that allows for simultaneous measurements of CO2, H20 and Hg~ exchange under controlled environmental conditions. When Hg ~ concentrations were held at 0.5 to 1.5 ng m-X,red maple (Acer rubrum L.), Norway spruce (Picea abies L.), yellow-poplar (Liriodendron tulipifera L.), and white oak (Quercus alba L.) foliage exhibited mean Hg ~ emissions of 5.5, 1.7, 2.7, and 5.3 ng m-2 h- t, respectively. At Hg~ concentrations between 9 and 20 ng m-3 little net exchange of Hg~ was observed. However at concentrations between 50 and 70 ng m-3 the Hg ~ was deposited to foliage at rates between 22 and 38 ng m-2 h-~. These data suggest that dry foliar surfaces in terrestrial forest landscapes may be a dynamic exchange surface that can function as a source or sink dependent on the magnitude of current Hg~ concentrations. These data provide evidence of species-specific compensation concentrations (or compensation points) for Hg ~ deposition to seedling foliage in the 10-25 ng m-3 range. 1. Introduction Mercury (Hg) emissions from the combustion of fossil fuels have been identified as a potential source of Hg accumulating in natural food chains (Lindqvist, 1985; Schroeder et al., 1989) and the problem is of special concern for aquatic systems (Swain et al., 1992). Deposition of elemental Hg vapor (Hg ~ to terrestrial forest landscapes may also represent a significant sink within the biogeochemical cycle of Hg (Lindberg et al., 1992), but quantitative data describing rates of exchange with woody plant foliage and other forest landscape surfaces are not available. Forested landscapes are made up of a variety of surfaces with different and often highly variable surface characteristics. Foliar surfaces typically make up 4-8 times the surface area of bare ground, and bark/stem surfaces account for an additional 1.5 ground area equivalents. Because of their stomata and physiologically active mesophyll cells, foliar surfaces represent a diurnally changing sink for many trace gases (Taylor et al., 1988). Previous studies for crop and weed species documented Hg ~ deposition to foliage (Du and Fang, 1982; Browne and Fang, 1978), but they used Hg ~ concentrations that were orders of magnitude higher than levels now known to occur under background conditions (0.5-3 ng m-3), possibly creating artificially high gradients between the atmosphere and landscape surfaces. It is not clear that the mechanisms responsible for Hg ~ uptake at high concentrations remain active near ambient levels. Other trace gases such as NO and NH3 are known to exhibit different deposition characteristics as a function of ambient concentrations (Farquhar et al., 1980; Johansson, 1987). Water, Air, and Soil Pollution 80: 373-382, 1995. 9 1995 Kluwer Academic Publishers. Printed in the Netherlands.