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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 Aug 2019

Submitted as: research article | 06 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

First eddy covariance flux measurements of gaseous elemental mercury (Hg0) over a grassland

Stefan Osterwalder1,2, Werner Eugster3, Iris Feigenwinter3, and Martin Jiskra1 Stefan Osterwalder et al.
  • 1Environmental Geosciences, University of Basel, 4056 Basel, Switzerland
  • 2CNRS, IRD, IGE, University Grenoble Alpes, 38058 Grenoble, France
  • 3Institute of Agricultural Sciences, ETH Zurich, 8092 Zurich, Switzerland

Abstract. Direct measurements of the net ecosystem exchange (NEE) of gaseous elemental mercury (Hg0) are crucial to improve the understanding of global Hg cycling und ultimately human and wildlife Hg exposure. The lack of long-term, ecosystem-scale measurements causes large uncertainties in Hg0 flux estimates. Today it remains unclear whether terrestrial ecosystems are net sinks or sources of atmospheric Hg0. Here we show a detailed validation of the eddy covariance technique for direct Hg0 flux measurements (Eddy Mercury) based on a Lumex mercury monitor RA-915AM. 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 % cut-off at 0.074 ng m−2 h−1. The statistical estimate of the Hg0 flux detection limit under real-world outdoor conditions at the site was 5.9 ng m−2 h−1 (50 % cut-off). We present the first successful eddy covariance NEE measurements of Hg0 over a low-Hg level soil (41–75 ng Hg g−1 topsoil [0–10 cm]) in summer 2018 at a managed grassland at the Swiss FluxNet site in Chamau, Switzerland (CH-Cha). We measured a net summertime re-emission over a period of 34 days with a median Hg0 flux of 2.5 ng m−2 h−1 (−0.6 to 7.4 ng m−2 h−1, range between 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 which led to a midday depression in CO2 uptake which did not recover during the afternoon. Thus, the cumulative net CO2 uptake was only 8 % of the net CO2 uptake during the same period in the previous year 2017. We suggest that partial stomata closure dampened Hg0 uptake by vegetation, resulting in a NEE of Hg0 dominated by soil re-emission. Finally, we give 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). The system, improved in the suggested way, has the potential to be integrated in global networks of micrometeorological tower sites (FluxNet) and provide the long-term observations on terrestrial atmosphere Hg0 exchange necessary to validate regional and global mercury models.

Stefan Osterwalder et al.
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Stefan Osterwalder et al.
Stefan Osterwalder et al.
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Short summary
Direct mercury (Hg) flux studies are crucial to improve our understanding of terrestrial Hg cycling and human Hg exposure. We tested a new system to measure Hg fluxes using the eddy covariance technique. Our Eddy Mercury system revealed a net Hg re-emission flux from grassland. We concluded that the prevailing dry conditions resulted in low uptake of CO2 and Hg. Eddy Mercury shows potential to address some of the largest uncertainties in global Hg cycling through long-term flux measurements.
Direct mercury (Hg) flux studies are crucial to improve our understanding of terrestrial Hg...