Stable Isotope Evidence Shows Re-emission of Elemental Mercury Vapor Occurring after Reductive Loss from Foliage

Abstract

The mechanism of elemental mercury (Hg 0 ) re-emission from vegetation to the atmosphere is currently poorly understood. In this study, we investigated branch-level Hg 0 atmosphere-foliage exchange in a pristine evergreen forest by systematically combining Hg isotopic composition, air concentration and flux measurements to unravel process information. It is found that the foliage represents a diurnally changing sink for atmospheric Hg 0 and its Hg content increases with leaf age and mass. Atmospheric Hg 0 is the dominant source of foliar Hg and the involvement of Hg II is not supported by isotopic evidence. Upon Hg 0 uptake, maturing foliage becomes progressively enriched in lighter Hg isotopes and depleted in odd mass isotopes. The measured isotopic composition of foliage Hg and isotopic shift caused by Hg 0 evasion from foliage supports that Hg 0 emitted from foliage is derived from Hg previously metabolized and bound in the leaf interior then subsequently recycled after reduction, and not merely a retroflux of recently deposited Hg 0 on foliar surface. An isotopic differential mass balance model indicates that the proportion of foliar Hg 0 efflux to uptake gradually increase from emergence to senescence with an average flux ratio of 30%.