Suitability of quantum cascade laser spectroscopy for CH₄ and N₂O eddy covariance flux measurements

Abstract. A quantum cascade laser spectrometer was evaluated for eddy covariance flux measurements of CH₄ and N₂O 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 CH₄ and N₂O, 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 CH₄ and N₂O 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 N₂O emission was due to a fertilizing event.