A compact and stable eddy covariance set-up for methane measurements using off-axis integrated cavity output spectroscopy
Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences, Department of Hydrology and Geo-environmental Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
Abstract. A DLT-100 Fast Methane Analyser (FMA) from Los Gatos Research (LGR) Ltd. is assessed for its applicability in a closed path eddy covariance field set-up. The FMA uses off-axis integrated cavity output spectroscopy (ICOS) combined with a highly specific narrow band laser for the detection of CH4 and strongly reflective mirrors to obtain a laser path length of 2×10³ to 20×10³ m. Statistical testing, a calibration experiment and comparison with high tower data showed high precision and very good stability of the instrument. The measurement cell response time was tested to be 0.10 s. In the field set-up, the FMA is attached to a scroll pump and combined with a Gill Windmaster Pro 3 axis Ultrasonic Anemometer and a Licor 7500 open path infrared gas analyzer. The power-spectra and co-spectra of the instrument are satisfactory for 10 Hz sampling rates. The correspondence with CH4 flux chamber measurements is good and the observed CH4 emissions are comparable with (eddy covariance) CH4 measurements in other peat areas.
CH4 emissions are rather variable over time and show a diurnal pattern. The average CH4 emission is 50±12.5 nmol m−2 s−1, while the typical maximum CH4 emission is 120±30 nmol m−2 s−1 (during daytime) and the typical minimum flux is –20±2.5 nmol m−2 s−1 (uptake, during night time).
Additionally, the set-up was tested for three measurement techniques with slower measurement rates, which could be used in the future to make the scroll pump superfluous and save energy. Both disjunct eddy covariance as well as slow 1 Hz eddy covariance showed results very similar to normal 10 Hz eddy covariance. Relaxed eddy accumulation (REA) only matched with normal 10 Hz eddy covariance over an averaging period of at least several weeks.