Net ecosystem exchange of atmospheric gaseous elemental mercury (GEM) over an evergreen needle-leaf forest

Mercury (Hg) is a global pollutant and large amounts of Hg are emitted to the atmosphere (>6,0008,000 Mg Hg yr-1) by anthropogenic activities, biomass burning, geogenic degassing and legacy emissions from land and oceans. Up to two-thirds of terrestrial Hg emissions are deposited back onto land, predominantly through vegetation uptake of Hg. Atmospheric gaseous elemental mercury (GEM) deposition through foliage uptake accounts for 90% of the total vegetation uptake. Here, we present ongoing measurements of ecosystematmosphere GEM exchange using a tower-based flux-gradient approach in a high-latitude coniferous forest starting in November 2020. We aim to quantify the magnitude, seasonality and diel patterns of GEM exchange fluxes and characterize controlling variables and relationships to carbon dioxide exchange. A major challenge was to detect small GEM gradients between the two inlets above the canopy (at 23 and 28 m) and a second gradient above the forest floor (2.6 m and 1.3 m). Measured GEM concentration differences were extremely small with a median of -0.00028 ng m-3 and -0.0018 ng m-3 (November 2020-June 2021) for forest canopy and floor, equal to 0.02% and 0.16% of background atmospheric concentrations. Calculated canopy GEM fluxes showed a median flux of -3.84×10-14 ng m-2 hr-1 and cumulative GEM flux exchange from November 2020 to June 2021 showed that the forest to be a GEM sink of 8.4 µg m-2. The cumulative flux was driven by a pronounced summertime GEM deposition of 11.9 µg m-2 between May to June and periods of emissions prior to the onset of the vegetation period in April. Ongoing work is focused on completing a full year of flux measurements and comparison of GEM forest deposition to measurements of wet (rain and snow) deposition and to patterns measured at a nearby deciduous forest.