Collaborative Research: Magnitude and Pathway of Gaseous Atmospheric Mercury Deposition in Forests

Mercury (Hg) is released from both natural sources and human activities. The goal of this project is to make measurements of mercury for an entire year in two contrasting forest locations. This rich dataset will be used to better understand how Hg moves through trees, soil, and the atmosphere. Understanding the distribution of mercury is important because it is a toxic substance, which can cause negative neurological effects in humans and react with other atmospheric gases. The knowledge from this project will be useful for assessing the global distribution of a mercury so that international recognized goals for reducing Hg pollution can be achieved. Additionally, the project will provide partial support for a postdoctoral researcher, two graduate students, and one undergraduate student per year.<br/><br/>Atmospheric mercury deposits via various dry and wet (e.g., rainfall) deposition processes, including gaseous mercury uptake by plants. Mercury in vegetation is transferred to soils when plants die off or shed leaves, contributing 54-94% of mercury in soil. In the absence of direct measurements, gaseous elemental mercury (Hg[0]) deposition is inferred from litterfall and passive membranes, yet these are not ideal proxies for net Hg(0) deposition as they don?t account for re-emission. Consequently, first objective of this project is to quantify the magnitude and temporal dynamics of net gaseous dry Hg(0) deposition (sum of gross deposition minus emission) in two forests with different seasonalities, a deciduous temperate forest and an evergreen subtropical rain forest. Net Hg(0) deposition will be measured using micrometeorological measurements on large towers, the only available method for direct, non-intrusive and time-extended measurements of net Hg(0) exchange at the ecosystem level encompassing all underlying sinks and sources. The second objective is to partition Hg(0) fluxes into canopy and soil contributions via deployment of two corresponding flux systems - one above the forest canopy to measure ecosystem-level Hg(0) exchange and a second system below the canopy to quantify soil contributions. Canopy Hg(0) fluxes will be calculated by difference. Flux partitioning will provide annual, seasonal and diurnal Hg(0) sink (e.g., to canopies) and source strengths (e.g., from soils) needed to constraint Hg(0) deposition in global and regional chemical transport models. Finally, the third goal of this work is to elucidate pathways of deposition by comparing Hg(0) fluxes to those of carbon dioxide, ozone, water vapor, and carbonyl sulfide. All these trace gases have different sinks and sources in ecosystems and vary in their degree of canopy, stomatal, mesophyll and soil contributions to fluxes. Comparison among fluxes, including seasonality, diurnality and component fluxes will allow us to quantify the degree to which Hg(0) exchange is coupled to photosynthetic activity, stomatal conductance, enzymatic activity within leaves, external cuticular uptake and soil exchange.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.