Atmospheric Chemistry and Physics Discussions
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2009
10.5194/acpd-9-5201-2009
http://www.atmos-chem-phys-discuss.net/9/5201/2009/
http://www.atmos-chem-phys-discuss.net/9/5201/2009/acpd-9-5201-2009.html
http://www.atmos-chem-phys-discuss.net/9/5201/2009/acpd-9-5201-2009.pdf
5201
5229
2009-02-26
Eddy covariance methane measurements at a Ponderosa pine plantation in California
C. J. P. P. Smeets
c.j.p.p.smeets@uu.nl
R. Holzinger
I. Vigano
A. H. Goldstein
T. Röckmann
Institute for Marine and Atmospheric research Utrecht, Princetonplein 5, 3584 CC, Utrecht, The Netherlands
University of California, Berkeley, Department of Environmental Science, Policy and Management, USA
Long term methane flux measurements have been mostly performed with plant or
soil enclosure techniques on specific components of an ecosystem. New fast
response methane analyzers make it possible to use the eddy covariance (EC)
technique instead. The EC technique is advantageous because it allows
continuous flux measurements integrating over a larger and more
representative area including the complete ecosystem, and allows fluxes to be
observed as environmental conditions change naturally without disturbance. We
deployed the closed-path Fast Methane Analyser (FMA) from Los Gatos Research
Ltd and demonstrate its performance for EC measurements at a Ponderosa pine
plantation at the Blodgett Forest site in central California. CH<sub>4</sub>
concentrations measured at 10 Hz showed a relatively high noise level that
was caused by a software related problem. Nevertheless, in the frequency
range important for turbulent exchange, the cospectra of CH<sub>4</sub> compare very
well with all other scalar cospectra confirming the quality of the FMA
measurements are good for the EC technique. The low-pass filtering
characteristics of our closed-path system and the use of the
Webb-Pearman-Leuning (WPL) corrections for a combination of open and
closed-path sensors are discussed using a large ensemble of cospectra. The
diurnal variation of the methane concentration was up to 60 ppbv with an
average of 1843 ppbv. Concentrations increased from morning to late
afternoon as upslope flow from the valley below carried polluted air to the
site, and then decreased through the night as downslope flow carried cleaner
air from aloft. The fluxes were consistently directed downward with a well
defined diurnal pattern, averaging −35±40 ng m<sup>−2</sup> s<sup>−1</sup> during
the daytime. The detection limit of the system was estimated at
22 ng m<sup>−2</sup> s<sup>−1</sup>. The average CH<sub>4</sub> deposition during the daytime
was higher than the average value for warm temperate forests in a recent
global inventory and the results from a process-based model study.
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