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 CH4 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.
Highlights
Understanding the temporal dynamics of methane emission at the global scale requires continuous and long-term field measurements of CH4 fluxes at representative sites, where the relationships between landscape-scale flux measurements and their environmental drivers can be investigated (Bartlett and Harriss, 1993; Bubier and Moore, 1994)
This paper demonstrates the benefits of using infrared laser absorption spectroscopy for in situ, fast and high precision ambient methane mixing ratio measurements
The compact and cryogen-free instruments can be operated unattended in the field to provide continuous measurements of CH4 mixing ratios. Their fast response provides the opportunity to investigate the temporal dynamics of the mechanisms controlling ecosystem-atmosphere CH4 exchange
Summary
Understanding the temporal dynamics of methane emission at the global scale requires continuous and long-term field measurements of CH4 fluxes at representative sites, where the relationships between landscape-scale flux measurements and their environmental drivers can be investigated (Bartlett and Harriss, 1993; Bubier and Moore, 1994). The general problems associated with chamber flux measurements are leaks, inhibition of fluxes through concentration build-up and pressure effects, which are well known limitations of this method (Matthias et al, 1978; Bain et al, 2005; Camarda et al, 2009). Micrometeorological methods like eddy covariance (EC) integrate trace gas exchange over extended areas (typically hundreds of m2) – appropriate for landscape scale studies. These techniques rely on fast response, field-deployable and high-sensitivity instruments, which can rapidly resolve small (preferably better than 0.1%) concentration changes in CH4 at ambient level. EC flux measurements of CH4 required frequent re-calibration and/or liquid nitrogen for the analyzers (Verma et al, 1992; Fowler et al, 1995; Friborg et al, 1997; Werle and Kormann, 2001) leading to logistic limitations for unattended field deployment
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