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.