LAUSR

Abstract. Direct measurements of the net ecosystem exchange (NEE) of gaseous elemental mercury ( Hg0 ) are important to improve our understanding of global Hg cycling and, ultimately, human and wildlife Hg exposure. The lack of long-term, ecosystem-scale measurements causes large uncertainties in Hg0 flux estimates. It currently remains unclear whether terrestrial ecosystems are net sinks or sources of atmospheric Hg0 . Here, we show a detailed validation of direct Hg0 flux measurements based on the eddy covariance technique (Eddy Mercury) using a Lumex RA-915 AM mercury monitor. The flux detection limit derived from a zero-flux experiment in the laboratory was 0.22 ng m −2  h −1 (maximum) with a 50 % cutoff at 0.074 ng m −2  h −1 . We present eddy covariance NEE measurements of Hg0 over a low-Hg soil (41–75 ng Hg g −1 in the topsoil, referring to a depth of 0–10 cm), conducted in summer 2018 at a managed grassland at the Swiss FluxNet site in Chamau, Switzerland (CH-Cha). The statistical estimate of the Hg0 flux detection limit under outdoor conditions at the site was 5.9 ng m −2  h −1 (50 % cutoff). We measured a net summertime emission over a period of 34 d with a median Hg0 flux of 2.5 ng m −2  h −1 (with a −0.6 to 7.4 ng m −2  h −1 range between the 25th and 75th percentiles). We observed a distinct diel cycle with higher median daytime fluxes (8.4 ng m −2  h −1 ) than nighttime fluxes (1.0 ng m −2  h −1 ). Drought stress during the measurement campaign in summer 2018 induced partial stomata closure of vegetation. Partial stomata closure led to a midday depression in CO2 uptake, which did not recover during the afternoon. The median CO2 flux was only 24 % of the median CO2 flux measured during the same period in the previous year (2017). We suggest that partial stomata closure also dampened Hg0 uptake by vegetation, resulting in a NEE of Hg0 that was dominated by soil emission. Finally, we provide suggestions to further improve the precision and handling of the “Eddy Mercury” system in order to assure its suitability for long-term NEE measurements of Hg0 over natural background surfaces with low soil Hg concentrations ( −1 ). With these improvements, Eddy Mercury has the potential to be integrated into global networks of micrometeorological tower sites (FluxNet) and to provide the long-term observations on terrestrial atmosphere Hg0 exchange necessary to validate regional and global mercury models.