Gas chromatography vs. quantum cascade laser-based N<sub>2</sub>O flux measurements using a novel chamber design

Abstract

Abstract. Recent advances in laser spectrometry offer new opportunities to investigate the soil–atmosphere exchange of nitrous oxide. During two field campaigns conducted at a grassland site and a willow field, we tested the performance of a quantum cascade laser (QCL) connected to a newly developed automated chamber system against a conventional gas chromatography (GC) approach using the same chambers plus an automated gas sampling unit with septum capped vials and subsequent laboratory GC analysis. Through its high precision and time resolution, data of the QCL system were used for quantifying the commonly observed nonlinearity in concentration changes during chamber deployment, making the calculation of exchange fluxes more accurate by the application of exponential models. As expected, the curvature values in the concentration increase was higher during long (60 min) chamber closure times and under high-flux conditions (FN2O > 150 µg N m−2 h−1) than those values that were found when chambers were closed for only 10 min and/or when fluxes were in a typical range of 2 to 50 µg N m−2 h−1. Extremely low standard errors of fluxes, i.e., from ∼ 0.2 to 1.7 % of the flux value, were observed regardless of linear or exponential flux calculation when using QCL data. Thus, we recommend reducing chamber closure times to a maximum of 10 min when a fast-response analyzer is available and this type of chamber system is used to keep soil disturbance low and conditions around the chamber plot as natural as possible. Further, applying linear regression to a 3 min data window with rejecting the first 2 min after closure and a sampling time of every 5 s proved to be sufficient for robust flux determination while ensuring that standard errors of N2O fluxes were still on a relatively low level. Despite low signal-to-noise ratios, GC was still found to be a useful method to determine the mean the soil–atmosphere exchange of N2O on longer timescales during specific campaigns. Intriguingly, the consistency between GC and QCL-based campaign averages was better under low than under high N2O efflux conditions, although single flux values were highly scattered during the low efflux campaign. Furthermore, the QCL technology provides a useful tool to accurately investigate the highly debated topic of diurnal courses of N2O fluxes and its controlling factors. Our new chamber design protects the measurement spot from unintended shading and minimizes disturbance of throughfall, thereby complying with high quality requirements of long-term observation studies and research infrastructures.

Analysis of uncertainty for N2O fluxes measured with the closed‐chamber method under field conditions: Calculation method, detection limit, and spatial variability.

“…We therefore conclude that the ''collapse'' in the flux reliability below 5 g N 2 O-N ha -1 d -1 is most likely caused by an increase in the relative error in determining N 2 O concentration increases via GC as they become smaller. By repeatedly (n = 10) analyzing gas mixtures with N 2 O concentrations in a range from 0.274 to 1 ppm, it was determined that the coefficients of variation decrease with increasing concentration from 3.66 to 0.84%, which is consistent with the maximum acceptable coefficient of variation used by other authors for N 2 O concentration determinations via GC (Brü mmer et al, 2017). This explains why the uncertainty in flux estimate decreases with flux magnitude.…”

A novel injection technique: using a field-based quantum cascade laser for the analysis of gas samples derived from static chambers

“…Once the injection line had been established, the flow rate was reduced from an initial 15 L min -1 used for EC to 1 L min -1 for manual injections, based on Lebegue et al (2016), Savage et al (2014) and Brümmer et al (2017). The reduction in flow was monitored using a RMA-SSV flow meter (Dwyer Instruments, PTY.…”

“…Recent developments in the technology of fast-response analysers have enabled e.g. tunable diode laser absorption spectrometers, Fourier transform infrared spectrometers and, in particular, continuous-wave quantum cascade laser absorption spectrometers (QCL) to be coupled to automated chambers (Brümmer et al, 2017;Cowan et al, 2014;Savage et al, 2014) or eddy covariance (EC) systems (Nemitz et al, 2018;Nicolini et al, 2013). Despite these recent advances in analyser technology, our understanding of the micro-and macro-scale processes that lead to the emission of N2O has remained limited.…”

Memory effects on greenhouse gas emissions (CO<sub>2</sub>, N<sub>2</sub>O and CH<sub>4</sub>) following grassland restoration?

“…Even though more complex in technology and assumptions made before carrying out measurements, the eddy covariance (EC) technique has become a valuable tool to derive ecosystem integrated CO2 and H2Ovapour exchange across the globe (Baldocchi, 2014;Eugster and Merbold, 2015). The technique has been further extended to continuous measurements of CH4 and N2O with the development of easy field-deployable fast-response analyzers during the last decade (Brümmer et al, 2017;Felber et al, 2015;Kroon et al, 2007;Nemitz et al, 2018a). Each of the two approaches has its strengths and weaknesses and it is beyond the scope of this study to discuss each of them in detail.…”

“…Each of the two approaches has its strengths and weaknesses and it is beyond the scope of this study to discuss each of them in detail. However, we refer to a set of reference papers highlighting the advantages and disadvantages of each technique separately (chambers: (Ambus et al, 1993;Brümmer et al, 2017;Pavelka et al, 2018a); eddy covariance: (Baldocchi, 2014;Denmead, 2008;Eugster and Merbold, 2015;Nemitz et al, 2018a).…”