Atmospheric Chemistry and Physics
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2008
10.5194/acp-8-431-2008
http://www.atmos-chem-phys.net/8/431/2008/
http://www.atmos-chem-phys.net/8/431/2008/acp-8-431-2008.html
http://www.atmos-chem-phys.net/8/431/2008/acp-8-431-2008.pdf
431
443
2008-01-31
A compact and stable eddy covariance set-up for methane measurements using off-axis integrated cavity output spectroscopy
D. M. D. Hendriks
dimmie.hendriks@falw.vu.nl
A. J. Dolman
M. K. van der Molen
J. van Huissteden
Vrije Universiteit Amsterdam, Faculty of Earth and Life Sciences Department of Hydrology and Geo-environmental Sciences De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
A Fast Methane Analyzer (FMA) is assessed for its applicability in a closed
path eddy covariance field set-up in a peat meadow. The FMA uses off-axis
integrated cavity output spectroscopy combined with a highly specific narrow
band laser for the detection of CH<sub>4</sub> and strongly reflective mirrors to
obtain a laser path length of 2–20×10<sup>3</sup> m. Statistical testing
and a calibration experiment showed high precision (7.8×10<sup>−3</sup> ppb)
and accuracy (<0.30%) of the instrument, while no drift was
observed. The instrument response time was determined to be 0.10 s. In the
field set-up, the FMA is attached to a scroll pump and combined with a
3-axis ultrasonic anemometer and an open path infrared gas analyzer for
measurements of carbon dioxide and water vapour. The power-spectra and
co-spectra of the instruments were satisfactory for 10 Hz sampling rates.
<br><br>
Due to erroneous measurements, spikes and periods of low turbulence the data
series consisted for 26% of gaps. Observed CH<sub>4</sub> fluxes consisted
mainly of emission, showed a diurnal cycle, but were rather variable over.
The average CH<sub>4</sub> emission was 29.7 nmol m<sup>−2</sup> s<sup>−1</sup>, while the
typical maximum CH<sub>4</sub> emission was approximately 80.0 nmol m<sup>−2</sup> s<sup>−1</sup>
and the typical minimum flux was approximately 0.0 nmol m<sup>−2</sup> s<sup>−1</sup>. The correspondence of the measurements with flux chamber
measurements in the footprint was good and the observed CH<sub>4</sub> emission
rates were comparable with eddy covariance CH<sub>4</sub> measurements in other
peat areas.
<br><br>
Additionally, three measurement techniques with lower sampling frequencies
were simulated, which might give the possibility to measure CH<sub>4</sub> fluxes
without an external pump and save energy. Disjunct eddy covariance appeared
to be the most reliable substitute for 10 Hz eddy covariance, while relaxed
eddy accumulation gave reliable estimates of the fluxes over periods in the
order of days or weeks.
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