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

Abstract

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 Hg^II 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%.