Using Observations to Constrain Bidirectional Exchange of Elemental Mercury Between Atmosphere and Vegetation

The exchange of elemental mercury (Hg⁰) between the atmosphere and terrestrial ecosystems is a critical component of the global mercury cycle. Vegetation acts as both sources and sinks for Hg⁰, influencing atmospheric Hg concentrations much like a pump. Previously, studies have used models to assess the bidirectional exchange process of Hg⁰ by introducing compensation points for the cuticle, stomata, and soil. However, evaluations of flux trends using these models against field observations of net Hg⁰ exchange are limited, preventing an assessment of their performance over heterogeneous global environments. In this study, we adjusted and validated a bidirectional model based on field observations and applied it to evaluate the impacts of vegetation on Hg⁰ fluxes globally. By adjusting the compensation points and resistances for soil, cuticular, and stomatal exchange, the normalized mean error (NME) of the adjusted model was reduced to one-fifth of that of the base model. We used the model to diagnose varying contributions from soil, cuticle, and stomatal exchanges at different sites. Based on the model, sites that received higher dry deposition of Hg^II and Hg^P exhibited higher Hg⁰ release fluxes through the cuticle, with fluxes reaching up to 20 ng/m²/h. Sites with higher soil Hg concentrations, such as those with over 150 ng/g soil Hg, predominantly showed soil emissions, with fluxes exceeding 24 ng/m²/h. In contrast, sites with low atmospheric mercury concentrations (less than 1 ng/m³) and relatively low soil Hg (below 80 ng/g) exhibited deposition fluxes, generally dominated by stomata. Globally, higher soil emissions were simulated in northern South America, and central and southern Africa, regions characterized by higher soil Hg concentrations and lower soil organic matter (SOM, below 2%). High cuticular emissions were simulated in southern South America, influenced by high dry deposition of Hg^II and Hg^P, with deposition rates over 0.5 ng/m²/h. This research highlights a potential emission source of Hg⁰ from vegetation through the cuticle, partially counteracting stomatal uptake. This finding may further affect our understanding of the role of vegetation in the global mercury cycle, providing critical insights for global mercury management and mitigation strategies.