Foliar Exchange of Mercury Vapor: Evidence for a Compensation Point

Abstract

Historical studies for crop and weed species documented elemental Hg vapor (Hg°) deposition to foliage, but they used Hg° 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, H2O and Hg° exchange under controlled environmental conditions. When Hg° concentrations were held at 0.5 to 1.5 ng m-³, 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-¹respectively. 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–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° 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.