Field intercomparison of two optical analyzers for CH<sub>4</sub> eddy covariance flux measurements

Abstract

Abstract. Fast response optical analyzers based on laser absorption spectroscopy are the preferred tools to measure field-scale mixing ratios and fluxes of a range of trace gases. Several state-of-the-art instruments have become commercially available and are gaining in popularity. This paper aims for a critical field evaluation and intercomparison of two compact, cryogen-free and fast response instruments: a quantum cascade laser based absorption spectrometer from Aerodyne Research, Inc., and an off-axis integrated cavity output spectrometer from Los Gatos Research, Inc. In this paper, both analyzers are characterized with respect to precision, accuracy, response time and also their sensitivity to water vapour. The instruments were tested in a field campaign to assess their behaviour under various meteorological conditions. The instrument's suitability for eddy covariance flux measurements was evaluated by applying an artificial flux of CH 4 generated above a managed grassland with otherwise very low methane exchange. This allowed an independent verification of the flux measurements accuracy, including the overall eddy covariance setup and data treatment. The retrieved fluxes were in good agreement with the known artificial emission flux, which is more than satisfactory, given that the analyzers were attached to separate sonic anemometers placed on individual eddy towers with different data acquisition systems but similar data treatment that are specific to the best practice used by the involved research teams.

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

“…Humidity and temperature fluctuations affect gas flux measurements by causing changes in air density (Webb et al, 1980;Massman and Tuovinen 2006;Ibrom et al, 2007b) and, for gas analysers based on laser absorption spectrometry, also by altering the shape of the gas absorption line (McDermitt et al, 2010;Neftel et al, 2010;Tuzson et al, 2010). Corrections for these effects are referred to as density and spectroscopic correction, respectively.…”

“…For a more thorough site description, see e.g. Beljaars and Bosveld (1997) and van Ulden and Wieringa (1996).…”

“…In this context, it is important to characterise the currently available CH 4 analysers and to assess their performance. A few inter-comparison studies exist (Detto et al, 2011;Peltola et al, 2013;Tuzson et al, 2010), but none of them included such a wide range of models currently available on the market as this study.…”

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.…”

Development of a cavity-enhanced absorption spectrometer for airborne measurements of CH<sub>4</sub> and CO<sub>2</sub>

“…This instrument and its predecessor model, the Fast Methane Analyser (FMA), previously have been successfully used for ground-based mole fraction and eddy covariance flux measurements (Hendriks et al, 2008;Tuzson et al, 2010). However, great care must be taken if the instrument is to be used on an aircraft due to the range and rapidly varying conditions that it will be subjected to, which can significantly alter its precision and stability.…”