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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
28 Oct 2016
Review status
A revision of this discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
Eddy covariance carbonyl sulphide flux measurements with a quantum cascade laser absorption spectrometer
Katharina Gerdel, Felix M. Spielmann, Albin Hammerle, and Georg Wohlfahrt Institut of Ecology, University of Innsbruck, Innsbruck , Austria
Abstract. The trace gas carbonyl sulphide (COS) has lately received growing interest in the eddy covariance (EC) community due to its potential to serve as an independent approach for constraining gross primary production and canopy stomatal conductance. Thanks to recent developments of fast-response high-precision trace gas analysers (e.g. quantum cascade laser absorption spectrometers (QCLAS)), a handful of EC COS flux measurements have been published since 2013. To date, however, a thorough methodological characterisation of QCLAS with regard to the requirements of the EC technique and the necessary processing steps has not been conducted. The objective of this study is to provide for the first time a rigorous analysis of the most widely used QCLAS model for making defensible EC COS flux measurements. Data were collected from May to October 2015 at a temperate mountain grassland in Tyrol, Austria. Analysis of the Allan variance of high-frequency concentration measurements revealed laser drift to occur under field conditions after an averaging time of around 50 s. We thus explored the use of two high-pass filtering approaches (linear detrending and recursive filtering) as opposed to block averaging for covariance computation. Spectral analysis revealed considerable noise at higher frequencies, appearing to influence the high-frequency region of co-spectra (and therefore covariances). By applying a finite-impulse response filter we removed the noise-affected spectral region. The effects of this digital high- and low-pass filtering, and additional low-pass filtering due to the eddy covariance system design, were corrected for using a site-specific reference model co-spectrum and a series of transfer functions. An independent validation of these post-processing steps was achieved by comparison of the CO2 and H2O flux measurements obtained with the QCLAS against those obtained with a closed-path infrared gas analyser. While the validation showed good correspondence and minor statistical differences between the three different high-pass filtering approaches – the benefits of high pass filtering clearly emerged as a reduction of the random flux uncertainty and a higher fraction of data passing the applied QA/QC criterions. We conclude that the most widely used QCLAS can be used to make defensible COS flux measurements provided the appropriate corrections are applied.

Citation: Gerdel, K., Spielmann, F. M., Hammerle, A., and Wohlfahrt, G.: Eddy covariance carbonyl sulphide flux measurements with a quantum cascade laser absorption spectrometer, Atmos. Meas. Tech. Discuss.,, in review, 2016.
Katharina Gerdel et al.
Katharina Gerdel et al.
Katharina Gerdel et al.


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