Field intercomparison of two optical analyzers for CH4 eddy covariance flux measurement

Fast response optical analyzers based on laserabsorption spectroscopy are the preferred tools to measurefield-scale mixing ratios and fluxes of a range of trace gases.Several state-of-the-art instruments have become commer-cially available and are gaining in popularity. This paperaims for a critical field evaluation and intercomparison oftwo compact, cryogen-free and fast response instruments: aquantum cascade laser based absorption spectrometer fromAerodyne Research, Inc., and an off-axis integrated cavityoutput spectrometer from Los Gatos Research, Inc. In thispaper, both analyzers are characterized with respect to preci-sion, accuracy, response time and also their sensitivity to wa-ter vapour. The instruments were tested in a field campaignto assess their behaviour under various meteorological con-ditions. The instrument’s suitability for eddy covariance fluxmeasurements was evaluated by applying an artificial flux ofCH4generated above a managed grassland with otherwisevery low methane exchange. This allowed an independentverification of the flux measurements accuracy, including theoverall eddy covariance setup and data treatment. The re-trieved fluxes were in good agreement with the known artifi-cial emission flux, which is more than satisfactory, given thatthe analyzers were attached to separate sonic anemometersplaced on individual eddy towers with different data acquisi-tion systems but similar data treatment that are specific to thebest practice used by the involved research teams.ISSN:1867-1381ISSN:1867-854