Atmospheric Chemistry and Physics Discussions
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2007
10.5194/acpd-7-11587-2007
http://www.atmos-chem-phys-discuss.net/7/11587/2007/
http://www.atmos-chem-phys-discuss.net/7/11587/2007/acpd-7-11587-2007.html
http://www.atmos-chem-phys-discuss.net/7/11587/2007/acpd-7-11587-2007.pdf
11587
11619
2007-08-06
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 de 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 DLT-100 Fast Methane Analyser (FMA) from Los Gatos Research (LGR) Ltd. is
assessed for its applicability in a closed path eddy covariance field
set-up. The FMA uses off-axis integrated cavity output spectroscopy (ICOS)
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
2×10³ to 20×10³ m. Statistical testing, a
calibration experiment and comparison with high tower data showed high
precision and very good stability of the instrument. The measurement cell
response time was tested to be 0.10 s. In the field set-up, the FMA is
attached to a scroll pump and combined with a Gill Windmaster Pro 3 axis
Ultrasonic Anemometer and a Licor 7500 open path infrared gas analyzer. The
power-spectra and co-spectra of the instrument are satisfactory for 10 Hz
sampling rates. The correspondence with CH<sub>4</sub> flux chamber measurements
is good and the observed CH<sub>4</sub> emissions are comparable with (eddy
covariance) CH<sub>4</sub> measurements in other peat areas.
<br><br>
CH<sub>4</sub> emissions are rather variable over time and show a diurnal pattern.
The average CH<sub>4</sub> emission is 50±12.5 nmol m<sup>−2</sup> s<sup>−1</sup>,
while the typical maximum CH<sub>4</sub> emission is 120±30 nmol m<sup>−2</sup> s<sup>−1</sup> (during daytime)
and the typical minimum flux is –20±2.5 nmol m<sup>−2</sup> s<sup>−1</sup> (uptake, during night time).
<br><br>
Additionally, the set-up was tested for three measurement techniques with
slower measurement rates, which could be used in the future to make the
scroll pump superfluous and save energy. Both disjunct eddy covariance as
well as slow 1 Hz eddy covariance showed results very similar to normal 10 Hz
eddy covariance. Relaxed eddy accumulation (REA) only matched with normal
10 Hz eddy covariance over an averaging period of at least several weeks.
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