N 2 O exchange over managed grassland: Application of a quantum cascade laser spectrometer for micrometeorological flux measurements § Albrecht Neftel a, *, Christof Ammann a , Cornelia Fischer a , Christoph Spirig a , Franz Conen b , Lukas Emmenegger c , Bela Tuzson c , Susanne Wahlen c a Agrosocope Reckenholz-Ta ¨nikon Research Station ART, Reckenholzstrasse 191, 8046 Zu ¨rich, Switzerland b Institute of Environmental Geosciences, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland c Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, 8600 Du ¨bendorf, Switzerland 1. Introduction In recent years operational flux measurement networks have been setup to monitor biosphere–atmosphere exchange of trace gases with the aim to improve our understanding of the underlying processes and to estimate annual exchange rates on regional to global scale. Observations also serve to validate models used to predict climate change related issues (Sto ¨ ckli et al., 2008). Up to now these flux networks have been limited to CO 2 , water and energy exchange. The new Integrated Carbon Dioxide Observation System (ICOS) program aims to complement operational flux measurement systems with other important greenhouse gases, such as CH 4 and N 2 O(http://www.icos-infrastructure.ipsl.jus- sieu.fr). In this paper we focus on the eddy covariance (EC) method to determine net N 2 O exchange between grassland and the atmosphere. We used a novel, cryogenic free, continuous wave quantum cascade laser based absorption spectrometer (QCL-AS) for flux measurements in the field. We are especially interested in the bi-directional nature of the N 2 O exchange (see Neftel et al., 2007) and whether the precision, accuracy and time response for the reliable measurement of uptake of N 2 O can be achieved. Exchange of N 2 O between terrestrial ecosystems and the atmosphere is an important component in the global N 2 O budget. Evaluation of mitigation options in the area of land management require good process understanding, which must be supported by long-term flux measurements. Many experimental data on N 2 O flux originate from measurements with closed chambers covering areas usually smaller than 1 m 2 . Recently a growing number of publications based on eddy covariance measurements integrating over areas between 10 3 and 10 5 m 2 appeared, because fast and sensitive analytical systems based on absorption spectroscopy are now available (Edwards et al., 2003; Pihlatie et al., 2005; Eugster et al., 2007; Neftel et al., 2007). Either type of approach quantifies the net exchange of N 2 O between the soil surface and the atmosphere. The net exchange represents the sum of all sources and sinks within the soil profile (Clough et al., 1999, 2005, 2006; Neftel et al., 2000, 2007). Sources of N 2 O are located in aerobic and anaerobic soil microsites. Production of N 2 O in the soil results from the microbiological processes of nitrification and denitrification. Nitrifiers operate under aerobic conditions. Denitrification is favored by wet conditions with 70–90% water filled pore space, high nitrate concentrations and warm temperatures. Agricultural and Forest Meteorology 150 (2010) 775–785 ARTICLE INFO Article history: Received 12 December 2008 Received in revised form 22 July 2009 Accepted 23 July 2009 Keywords: N 2 O flux Eddy covariance method Quantum cascade laser Field measurements ABSTRACT A novel quantum cascade laser absorption spectrometer (QCL-AS) was tested to monitor N 2 O exchange fluxes over an intensively managed grassland using the eddy covariance approach. The instrument employs a continuous wave quantum cascade laser to scan over the absorption features of N 2 O, CH 4 and water vapor at 7.8 mm. The precision of the N 2 O flux measurements was determined to be 0.2 nmol m 2 s 1 but the accuracy can easily be affected by water vapor interferences twice as large. These water vapor interferences are not only due to the respective gas dilution effect but also due to an additional cross-sensitivity of the N 2 O analyzer to water vapor (0.3 ppb N 2 O/% H 2 O). Both effects cause a negative bias of similar magnitude (0.3 nmol m 2 s 1 N 2 O flux/mmol m 2 s 1 H 2 O flux) in the flux measurements. While the dilution (or density) correction is a well known and routinely applied procedure, the magnitude of the analyzer cross-talk may depend on the specific instrumental setup and should be empirically determined. The comparison with static chamber measurements shows the necessity of the cross-talk correction; otherwise the QCL-AS based eddy covariance system would yield unrealistically large uptake of N 2 O. ß 2009 Elsevier B.V. All rights reserved. § Contribution to the special issue ‘‘CH 4 and N 2 O fluxes’’ to be published in Agricultural and Forest Meteorology. * Corresponding author. Tel.: +41 44 377 7504; fax: +41 44 377 7201. E-mail address: Albrecht.Neftel@art.admin.ch (A. Neftel). Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet 0168-1923/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2009.07.013