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 (< 100 ng g−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.
Response of Tallgrass Prairie to Management in the U.S. Southern Great Plains: Site Descriptions, Management Practices, and Eddy Covariance Instrumentation for a Long-Term Experiment
