A Spatial Assessment of Current and Future Foliar Hg Uptake Fluxes Across European Forests

Atmospheric mercury (Hg) is deposited to land surfaces mainly through vegetation uptake. Foliage stomatal gas exchange plays an important role for net vegetation Hg uptake, because foliage assimilates Hg via the stomata. Here, we use empirical relationships of foliar Hg uptake by forest tree species to produce a spatially highly resolved (1 km(2)) map of foliar Hg fluxes to European forests over one growing season. The modeled forest foliar Hg uptake flux is 23 +/- 12 Mg Hg season-1, which agrees with previous estimates from literature. We spatially compared forest Hg fluxes with modeled fluxes of the chemical transport model GEOS-Chem and find a good overall agreement. For European pine forests, stomatal Hg uptake was shown to be sensitive to prevailing conditions of relatively high ambient water vapor pressure deficit (VPD). We tested a stomatal uptake model for the total pine needle Hg uptake flux during four previous growing seasons (1994, 2003, 2015/2017, 2018) and two climate change scenarios (RCP 4.5 and RCP 8.5). The resulting modeled total European pine needle Hg uptake fluxes are in a range of 8.0-9.3 Mg Hg season(-1) (min-max). The lowest pine forest needle Hg uptake flux to Europe (8 Mg Hg season-1) among all investigated growing seasons was associated with unusually hot and dry ambient conditions in the European summer 2018, highlighting the sensitivity of the investigated flux to prolonged high VPD. We conclude, that stomatal modeling is particularly useful to investigate changes in Hg deposition in the context of extreme climate events.