Evaluating the performance of commonly used gas analysers for methane eddy covariance flux measurements: the InGOS inter-comparison field experiment

[摘要] The performance of eight fast-response methane (CH₄) gas analyserssuitable for eddy covariance flux measurements were tested at a grasslandsite near the Cabauw tall tower (Netherlands) during June 2012. Theinstruments were positioned close to each other in order to minimise theeffect of varying turbulent conditions. The moderate CH₄ fluxes observedat the location, of the order of 25 nmol m⁻² s⁻¹, provided asuitable signal for testing the instruments' performance.

Generally, all analysers tested were able to quantify the concentrationfluctuations at the frequency range relevant for turbulent exchange and wereable to deliver high-quality data. The tested cavity ringdown spectrometer(CRDS) instruments from Picarro, models G2311-f and G1301-f, were superior toother CH₄ analysers with respect to instrumental noise. As an open-pathinstrument susceptible to the effects of rain, the LI-COR LI-7700 achievedlower data coverage and also required larger density corrections; however,the system is especially useful for remote sites that are restricted in poweravailability. In this study the open-path LI-7700 results were compromiseddue to a data acquisition problem in our data-logging setup. Some of theolder closed-path analysers tested do not measure H₂O concentrationsalongside CH₄ (i.e. FMA1 and DLT-100 by Los Gatos Research) and thiscomplicates data processing since the required corrections for dilution andspectroscopic interactions have to be based on external information. Toovercome this issue, we used H₂O mole fractions measured by other gasanalysers, adjusted them with different methods and then applied them tocorrect the CH₄ fluxes. Following this procedure we estimated a bias ofthe order of 0.1 g (CH₄) m⁻² (8% of the measured mean flux)in the processed and corrected CH₄ fluxes on a monthly scale due tomissing H₂O concentration measurements. Finally, cumulative CH₄fluxes over 14 days from three closed-path gas analysers, G2311-f (PicarroInc.), FGGA (Los Gatos Research) and FMA2 (Los Gatos Research), which weremeasuring H₂O concentrations in addition to CH₄, agreed within3% (355–367 mg (CH₄) m⁻²) and were not clearly differentfrom each other, whereas the other instruments derived total fluxes whichshowed small but distinct differences (±10%,330–399 mg (CH₄) m⁻²).

[学科分类] 地球化学与岩石