Evaluating atmospheric mercury (Hg) uptake by vegetation in a chemistry-transport model

Mercury (Hg), a neurotoxic heavy metal, is transferred to marine and terrestrial ecosystems through atmospheric transport. Recent studies have highlighted the role of vegetation uptake as a sink for atmospheric elemental mercury (Hg(0)) and a source of Hg to soils. However, the global magnitude of the Hg(0) vegetation uptake flux is highly uncertain, with estimates ranging 1000–4000 Mg per year. To constrain this sink, we compare simulations in the chemical transport model GEOS-Chem with a compiled database of litterfall, throughfall, and flux tower measurements from 93 forested sites. The prior version of GEOS-Chem predicts median Hg(0) dry deposition velocities similar to litterfall measurements from Northern hemisphere temperate and boreal forests (∼0.03 cm s(−1)), yet it underestimates measurements from a flux tower study (0.04 cm s(−1)vs. 0.07 cm s(−1)) and Amazon litterfall (0.05 cm s(−1)vs. 0.17 cm s(−1)). After revising the Hg(0) reactivity within the dry deposition parametrization to match flux tower and Amazon measurements, GEOS-Chem displays improved agreement with the seasonality of atmospheric Hg(0) observations in the Northern midlatitudes. Additionally, the modelled bias in Hg(0) concentrations in South America decreases from +0.21 ng m(−3) to +0.05 ng m(−3). We calculate a global flux of Hg(0) dry deposition to land of 2276 Mg per year, approximately double previous model estimates. The Amazon rainforest contributes 29% of the total Hg(0) land sink, yet continued deforestation and climate change threatens the rainforest's stability and thus its role as an important Hg sink. In an illustrative worst-case scenario where the Amazon is completely converted to savannah, GEOS-Chem predicts that an additional 283 Mg Hg per year would deposit to the ocean, where it can bioaccumulate in the marine food chain. Biosphere–atmosphere interactions thus play a crucial role in global Hg cycling and should be considered in assessments of future Hg pollution.