Atmospheric Gaseous Elemental Mercury Fluxes at Harvard Forest EMS Tower 2019-2020

In terrestrial ecosystems, dry deposition of atmospheric gaseous elemental mercury (GEM) is considered the dominant source of mercury accounting for 54% to 94% of mercury loads observed in soils, yet direct quantification of GEM deposition across forests is largely missing. The goal of this project is to quantify atmosphere-surface exchange of GEM at Harvard Forest for one full year, providing the first such record in a non-polluted forest. GEM exchange is measured using micrometeorological techniques using a large measurement tower, the only available method for direct, non-intrusive and time-extended measurements of net GEM exchange at the ecosystem level encompassing all underlying sinks and sources. A second objective was to partition GEM fluxes into canopy and soil contributions via deployment of two corresponding flux systems: one system was deployed above the forest canopy to measure ecosystem-level GEM exchange; a second system was deployed below the canopy to quantify soil contributions. This dataset contains an 18-month record of gaseous elemental mercury concentrations and fluxes measured at the EMS tower at Harvard Forest from May 2019 to August 2020.

This dataset is released to the public under Creative Commons CC0 1.0 (No Rights Reserved). Please keep the dataset creators informed of any plans to use the dataset. Consultation with the original investigators is strongly encouraged. Publications and data products that make use of the dataset should include proper acknowledgement.

Above-forest GEM gradients were measured at heights of 24.1 m and 30.8 m (and 29 m before May 15, 2019). Forest floor GEM gradients were measured at 0.4 m and 1.2 m above the ground. Two pairs of air inlets each (total of four inlet lines) were used to draw air to instruments inside the field laboratory through ¼” perfluoroalkoxy (PFA) tubes and 47-mm PFA inlet filter holders and 0.2 μm PFA inlet filters. Inlet filters were changed every one to three months and lines were wrapped for light protection to avoid photochemical reactions. Two valve control systems with three-way solenoid valves with Teflon®-wetted flow paths (NResearch, West Caldwell, NJ, US) were used to switch between the two gradient inlets every 10 minutes and allowing a concentration measurement with each of the two sampling traps of the analyzers (traps A and B) at each inlet height, therefore avoiding trap biases. Lines not being sampled were flushed by a pump to avoid stagnant air in lines. Two ambient air mercury analyzers (Model 2537B and 2537X, Tekran Inc., Toronto, Canada) were used for gradient measurements, one for above-canopy (whole-ecosystem) fluxes and one for forest floor GEM fluxes. We used internal calibration systems to perform autocalibrations every 49 hours. Internal calibrations were verified prior to and after field deployments with an independent calibration source (Model 2500, Tekran Inc.).