FOLIAR EXCHANGE OF MERCURY-VAPOR - EVIDENCE FOR A COMPENSATION POINT

Historical studies for crop and weed species documented elemental Hg vapor (Hg-o) deposition to foliage, but they used Hg-o 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-o exchange with white oak (Quercus alba L.), red maple (Acer rubrum L,.), Norway spruce (Picea abies I..), and yellow-poplar (Liriodendron tulipifera L.) foliage were conducted in an open gas exchange system that allows for simultaneous measurements of CO2, H2O and Hg-o exchange under controlled environmental conditions. When Hg-o concentrations were held at 0.5 to 1.5 ng ms, red maple (Acer rubrum L,.), Norway spruce (Picea abies L.), yellow-poplar (Liriodendron tulipifera L..), and while oak (Quercus alba L..) foliage exhibited mean Hg-o emissions of 5.5, 1.7, 2.7, and 5.3 ng m(-2) h(-1), respectively. At Hg-o concentrations between 9 and 20 ng m(-3) little net exchange of Hg-o was observed. However al concentrations between 50 and 70 ng m(-3) the Hg-o was deposited to foliage at rates between 22 and 38 ng m(-2) h(-1). 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-o concentrations. These data provide evidence of species-specific compensation concentrations (or compensation points) for Hg-o deposition to seedling foliage In the 10-25 ng m(-3) range.