Articles | Volume 16, issue 17
Atmos. Chem. Phys., 16, 11283–11299, 2016
Atmos. Chem. Phys., 16, 11283–11299, 2016

Research article 13 Sep 2016

Research article | 13 Sep 2016

Surface–atmosphere exchange of ammonia over peatland using QCL-based eddy-covariance measurements and inferential modeling

Undine Zöll1,*, Christian Brümmer1, Frederik Schrader1, Christof Ammann2, Andreas Ibrom3, Christophe R. Flechard4, David D. Nelson5, Mark Zahniser5, and Werner L. Kutsch6 Undine Zöll et al.
  • 1Thünen Institute of Climate-Smart Agriculture, 38116 Braunschweig, Germany
  • 2Swiss Federal Research Station Agroscope ART, 8046, Zürich, Switzerland
  • 3Technical University of Denmark, Department of Environmental Engineering, Bygningstorvet, 2800 Kgs. Lyngby, Denmark
  • 4INRA, Agrocampus Ouest, UMR1069 SAS, 35042 Rennes, France
  • 5Aerodyne Research, Inc., Billerica, Massachusetts, USA
  • 6Integrated Carbon Observation System (ICOS), Head Office, University of Helsinki, Finland
  • *previously published under my name: Undine Richter

Abstract. Recent advances in laser spectrometry offer new opportunities to investigate ecosystem–atmosphere exchange of environmentally relevant trace gases. In this study, we demonstrate the applicability of a quantum cascade laser (QCL) absorption spectrometer to continuously measure ammonia concentrations at high time resolution and thus to quantify the net exchange between a seminatural peatland ecosystem and the atmosphere based on the eddy-covariance approach. Changing diurnal patterns of both ammonia concentration and fluxes were found during different periods of the campaign. We observed a clear tipping point in early spring with decreasing ammonia deposition velocities and increasingly bidirectional fluxes that occurred after the switch from dormant vegetation to CO2 uptake but was triggered by a significant weather change. While several biophysical parameters such as temperature, radiation, and surface wetness were identified to partially regulate ammonia exchange at the site, the seasonal concentration pattern was clearly dominated by agricultural practices in the surrounding area. Comparing the results of a compensation point model with our measurement-based flux estimates showed considerable differences in some periods of the campaign due to overestimation of non-stomatal resistances caused by low acid ratios. The total cumulative campaign exchange of ammonia after 9 weeks, however, differed only in a 6 % deviation with 911 and 857 g NH3-N ha−1 deposition being found by measurements and modeling, respectively. Extrapolating our findings to an entire year, ammonia deposition was lower than reported by Hurkuck et al. (2014) for the same site in previous years using denuder systems. This was likely due to a better representation of the emission component in the net signal of eddy-covariance fluxes as well as better adapted site-specific parameters in the model. Our study not only stresses the importance of high-quality measurements for studying and assessing land surface–atmosphere interactions but also demonstrates the potential of QCL spectrometers for continuous observation of reactive nitrogen species as important additional instruments within long-term monitoring research infrastructures such as ICOS or NEON at sites with strong nearby ammonia sources leading to relatively high mean background concentrations and fluxes.

Short summary
Accurate quantification of atmospheric ammonia concentration and exchange fluxes with the land surface has been a major metrological challenge. We demonstrate the applicability of a novel laser device to identify concentration and flux patterns over a peatland ecosystem influenced by nearby agricultural practices. Results help to strengthen air quality monitoring networks, lead to better understanding of ecosystem functionality and improve parameterizations in air chemistry and transport models.
Final-revised paper