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

The mechanism of elemental mercury (Hg0) re-emission from vegetation to the atmosphere is currently poorly understood. In this study, we investigated branch-level Hg0 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 Hg0 and its Hg content increases with leaf age and mass. Atmospheric Hg0 is the dominant source of foliar Hg and the involvement of HgII is not supported by isotopic evidence. Upon Hg0 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 Hg0 evasion from foliage supports that Hg0 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 Hg0 on foliar surface. An isotopic differential mass balance model indicates that the proportion of foliar Hg0 efflux to uptake gradually increase from emergence to senescence with an average flux ratio of 30%.