A compact and stable eddy covariance set-up for methane measurements using off-axis integrated cavity output spectroscopy

Abstract

Abstract.A Fast Methane Analyzer (FMA) is assessed for its applicability in a closed path eddy covariance field set-up in a peat meadow. The FMA uses off-axis integrated cavity output spectroscopy combined with a highly specific narrow band laser for the detection of CH 4 and strongly reflective mirrors to obtain a laser path length of 2-20×10 3 m. Statistical testing and a calibration experiment showed high precision (7.8×10 −3 ppb) and accuracy (<0.30%) of the instrument, while no drift was observed. The instrument response time was determined to be 0.10 s. In the field set-up, the FMA is attached to a scroll pump and combined with a 3-axis ultrasonic anemometer and an open path infrared gas analyzer for measurements of carbon dioxide and water vapour. The power-spectra and co-spectra of the instruments were satisfactory for 10 Hz sampling rates.Due to erroneous measurements, spikes and periods of low turbulence the data series consisted for 26% of gaps. Observed CH 4 fluxes consisted mainly of emission, showed a diurnal cycle, but were rather variable over. The average CH 4 emission was 29.7 nmol m −2 s −1 , while the typical maximum CH 4 emission was approximately 80.0 nmol m −2 s −1 and the typical minimum flux was approximately 0.0 nmol m −2 s −1 . The correspondence of the measurements with flux chamber measurements in the footprint was good and the observed CH 4 emission rates were comparable with eddy covariance CH 4 measurements in other peat areas.Additionally, three measurement techniques with lower sampling frequencies were simulated, which might give the possibility to measure CH 4 fluxes without an external pump and save energy. Disjunct eddy covariance appeared to be the most reliable substitute for 10 Hz eddy covariance, while relaxed eddy accumulation gave reliable estimates of the fluxes over periods in the order of days or weeks.

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

“…Precisions of 3.8 × 10 −2 ppb Hz −1/2 for N 2 O and 2.4 × 10 −2 ppb Hz −1/2 for CO were achieved. These values are comparable to those reported for other OA-ICOS analyzers using the same measurement rate (Hendriks et al, 2008;Zellweger et al, 2012), and are lower compared to those obtained by Kroon et al (2007) for a QCL analyzer operated at 10 Hz (0.5 ppb Hz −1/2 ). The results also evidence an improvement in the precision achieved for N 2 O and CO measurements by employing OA-ICOS in comparison with the use of gas chromatographic methods (Weiss, 1981;Bates et al, 1995;Bange et al, 1996a).…”

Field intercomparison of four methane gas analyzers suitable for eddy covariance flux measurements

“…One of the participating analyzers, TGA-100A (Campbell Scientific Inc., USA) (e.g., Billesbach et al, 1998), is an older closed-path model, and is based on the same principle as the gas analyzers used in the early studies in the 1990s. The two other closed-path models, RMT-200 (Los Gatos Research, USA) (e.g., Baer et al, 2002;Hendriks et al, 2008;Eugster and Pluess, 2010;Tuzson et al, 2010), G1301-f (Picarro Inc., USA), and an early open-path prototype, Prototype-7700 (LI-COR Biosciences, USA) (e.g., McDermitt et al, 2010;Dengel et al, 2011;Detto et al, 2011), are new state-of-the-art instruments. The three closed-path units operated with occasional breaks throughout the measurement campaign, whereas Prototype-7700 malfunctioned permanently at 11 June due to water leakage which damaged internal electronics of the instrument.…”

Robust, spatially scanning, open-path TDLAS hygrometer using retro-reflective foils for fast tomographic 2-D water vapor concentration field measurements

“…There are instruments for gas concentration measurements (Babilotte et al, 2010;Crosson, 2008;McManus et al, 2008) or even flux measurements (McDermitt et al, 2011;Hendriks et al, 2008;Tuzson et al, 2010) that are well-suited for field applications but that do not provide a 2-D-concentration distribution. Certain variants of direct TDLAS (tunable diode laser absorption spectroscopy) are powerful techniques for field applications, as no reference gas measurement is needed for calibration (Buchholz et al, 2014) they are based on robust, industrially available components.…”