Abstract
Abstract. The exchange of gaseous elemental mercury, Hg(0), between the atmosphere and terrestrial surfaces remains poorly understood mainly due to difficulties in measuring net Hg(0) fluxes on the ecosystem scale. Emerging evidence suggests foliar uptake of atmospheric Hg(0) to be a major deposition pathway to terrestrial surfaces. Here, we present a bottom-up approach to calculate Hg(0) uptake fluxes to aboveground foliage by combining foliar Hg uptake rates normalized to leaf area with species-specific leaf area indices. This bottom-up approach incorporates systematic variations in crown height and needle age. We analyzed Hg content in 583 foliage samples from six tree species at 10 European forested research sites along a latitudinal gradient from Switzerland to northern Finland over the course of the 2018 growing season. Foliar Hg concentrations increased over time in all six tree species at all sites. We found that foliar Hg uptake rates normalized to leaf area were highest at the top of the tree crown. Foliar Hg uptake rates decreased with needle age of multiyear-old conifers (spruce and pine). Average species-specific foliar Hg uptake fluxes during the 2018 growing season were 18 ± 3 µg Hg m−2 for beech, 26 ± 5 µg Hg m−2 for oak, 4 ± 1 µg Hg m−2 for pine and 11 ± 1 µg Hg m−2 for spruce. For comparison, the average Hg(II) wet deposition flux measured at 5 of the 10 research sites during the same period was 2.3 ± 0.3 µg Hg m−2, which was 4 times lower than the site-averaged foliar uptake flux of 10 ± 3 µg Hg m−2. Scaling up site-specific foliar uptake rates to the forested area of Europe resulted in a total foliar Hg uptake flux of approximately 20 ± 3 Mg during the 2018 growing season. Considering that the same flux applies to the global land area of temperate forests, we estimate a foliar Hg uptake flux of 108 ± 18 Mg. Our data indicate that foliar Hg uptake is a major deposition pathway to terrestrial surfaces in Europe. The bottom-up approach provides a promising method to quantify foliar Hg uptake fluxes on an ecosystem scale.
Highlights
Mercury (Hg) is a toxic pollutant ubiquitous in the environment due to long-range atmospheric transport
Needles older than 1 year (y2, y3) continue to accumulate Hg albeit at a slower rate than younger needles (y0, y1). This finding is in agreement with previous studies that reported positive trends of Hg concentration in spruce needles from age classes y1 to y4 (Hutnik et al, 2014; Navrátil et al, 2019; Ollerova et al, 2010)
The temporal and vertical variation of foliar Hg uptake fluxes are consistent with the notion that stomatal uptake represents the main deposition pathway to atmospheric Hg(0)
Summary
Mercury (Hg) is a toxic pollutant ubiquitous in the environment due to long-range atmospheric transport. The residence time of Hg in the atmosphere and its transfer to land and ocean surfaces mainly depends on its speciation (Driscoll et al, 2013). Gaseous elemental mercury Hg(0) is the dominant form (> 90 %) of atmospheric Hg (Sprovieri et al, 2017), exhibiting a residence time of several months to more than a year (Ariya et al, 2015; Saiz-Lopez et al, 2018). Atmospheric Hg will be transferred to water and land surfaces by wet or dry deposition. In the wet deposition process, Hg(0) is oxidized in the atmosphere to water-soluble Hg(II) and washed down to the Earth surface by precipitation (Driscoll et al, 2013). Wet deposition fluxes of Hg(II) to terrestrial surfaces are well constrained and direct measurements are coordinated in regional and interna-
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