The goal of this study was to measure net gaseous elemental mercury (GEM) exchange fluxes using a micrometeorological flux-gradient approach over a remote deciduous temperate forest in Massachusetts. We aimed 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 concentration differences between the two inlets above the canopy (at 24 and 31 m). Measured GEM concentration differences were in fact very small with a median of 0.0015 ng m^-3, equal to 0.1% of atmospheric concentrations and about four times lower than previous measurements conducted over grasslands/shrubs. Detailed analysis of null gradients, i.e. potentially systematic biases in concentrations between the inlet lines, along with analysis of spectral patterns and autocorrelations showed that measured gradients were real, statistically highly significant and characterized by strong diel patterns.

Calculated daily GEM fluxes generally were positive (i.e., emissions) during the winter months (December through February), with median rates of emissions between 1.1 to 4.5 ng m^-2 hr^-1. All other months, specifically summer and fall months, experienced negative GEM fluxes with monthly median net deposition between -2.0 to -8.3 ng m^-2 hr^-1. Cumulative  GEM flux exchange over the duration of a full year showed that the forest was a strong GEM sink, driven by a pronounced summertime and fall GEM deposition (23.9 µg m^-2 between May to November). We attribute the pronounced summertime and fall GEM uptake to GEM assimilation by vegetation during peak vegetation period. Detailed analysis of diel patterns in July, August, September, and October supported pronounced midday deposition GEM enhancement, in accordance to similar daytime deposition maxima for CO₂. Calculating an annual mass balance, we estimate gaseous GEM deposition (e.g., dry deposition) to account to 72% of the total annual Hg deposition of 24.6 µg m^-2 at this forest and exceeding wet deposition three to four-fold.