Suitability of quantum cascade laser spectroscopy for CH<sub>4</sub> and N<sub>2</sub>O eddy covariance flux measurements

Abstract

Abstract.A quantum cascade laser spectrometer was evaluated for eddy covariance flux measurements of CH 4 and N 2 O using three months of continuous measurements at a field site. The required criteria for eddy covariance flux measurements including continuity, sampling frequency, precision and stationarity were examined. The system operated continuously at a dairy farm on peat grassland in the Netherlands from 17 August to 6 November 2006. An automatic liquid nitrogen filling system for the infrared detector was employed to provide unattended operation of the system. The electronic sampling frequency was 10 Hz, however, the flow response time was 0.08 s, which corresponds to a bandwidth of 2 Hz. A precision of 2.9 and 0.5 ppb Hz −1/2 was obtained for CH 4 and N 2 O, respectively. Accuracy was assured by frequent calibrations using low and high standard additions. Drifts in the system were compensated by using a 120 s running mean filter. The average CH 4 and N 2 O exchange was 512 ngC m −2 s −1 (2.46 mg m −2 hr −1 ) and 52 ngN m −2 s −1 (0.29 mg m −2 hr −1 ). Given that 40% of the total N 2 O emission was due to a fertilizing event.

A new method for continuous measurements of oceanic and atmospheric N<sub>2</sub>O, CO and CO<sub>2</sub>: performance of off-axis integrated cavity output spectroscopy (OA-ICOS) coupled to non-dispersive infrared detection (NDIR)

“…Thus, for σ 2 A = 1.4 × 10 −3 ppb 2 N 2 O and σ 2 A = 5.9 × 10 −4 ppb 2 CO the corresponding detection limits of the DLT-100 are 37 ppt for N 2 O and 24 ppt for CO. Detection limits in the order of 50 and 60 ppt have been reported for N 2 O measurements using quantum cascade laser (QCL) spectrometers (Nelson et al, 2004;Kroon et al, 2007). On the other hand, our values are higher than the 10 ppt detection limit reported by Zellweger et al (2012) for CO measurements with the same type of OA-ICOS analyzer.…”

“…When higher integration times are considered, however, drift tends to increase and σ 2 A starts to rise, indicating a reduced performance of the system. This means that during the time phase dominated by random noise, σ 2 A is equivalent to the statistical variance of the measurements and therefore its square root provides an estimation of the detection limit (Werle et al, 1993;Kroon et al, 2007). Since the LI-6252 and similar models are well-established instruments that have been extensively tested as part of autonomous systems for CO 2 measurements (Pierrot et al, 2009) the analysis here is centered on the DLT-100 analyzer.…”

A case study of eddy covariance flux of N<sub>2</sub>O measured within forest ecosystems: quality control and flux error analysis

“…In order to suppress the instrumental drift and its potential effect on the estimated flux values, the N 2 O flux was also calculated after applying an autoregressive running mean filter (RMF, McMillen, 1988) to the sampled signals. Although this approach was adopted previously for post-processing drifting concentration signals (Billesbach et al, 1998;Kormann et al, 2001;Kroon et al, 2007), the choice of the high pass time constant is not straightforward and objective selection methods are rare in literature (Werle, 2009). Methods based on signal auto-correlation coefficient and spectral analysis likely fail to give a reliable estimation of the timescale at which the drift effect becomes important, because of non-stationarity nature of the low frequency signal noise.…”

“…Moreover the performance and stability of fast response N 2 O gas analyzer strongly depends on the instrumental drift, which typically characterizes TDL and QCL spectrometers (Werle et al, 1993;Nelson et al, 2002). Previous studies performed under field conditions (Eugster et al, 2007;Kroon et al, 2007;Neftel et al, 2009) already noted that the laser drift can cause an over or under-estimation of EC flux. However they did not perform a thorough investigation on the effect of the drift on the calculated flux values.…”

“…N 2 O emissions are episodic in nature, showing high spatial and temporal variability, due to large variation in soil properties such as soil moisture, availability of nitrogen and easily decomposable organic matter (Ambus and Christensen, 1995;Pihlatie et al, 2005;Silver et al, 2005). Emission bursts of short duration, typically occurring after fertilizer application, or associated to thawing and rain events (Flechard et al, 2005, Kroon et al, 2007, Pihlatie et al, 2009, are followed by long periods of small fluxes, when also uptakes of N 2 O have been reported (Flechard et al, 2005). Moreover the performance and stability of fast response N 2 O gas analyzer strongly depends on the instrumental drift, which typically characterizes TDL and QCL spectrometers (Werle et al, 1993;Nelson et al, 2002).…”