Atmos. Chem. Phys. Discuss., 9, 5201-5229, 2009
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Eddy covariance methane measurements at a Ponderosa pine plantation in California
C. J. P. P. Smeets1, R. Holzinger1, I. Vigano1, A. H. Goldstein2, and T. Röckmann1
1Institute for Marine and Atmospheric research Utrecht, Princetonplein 5, 3584 CC, Utrecht, The Netherlands
2University of California, Berkeley, Department of Environmental Science, Policy and Management, USA

Abstract. 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. CH4 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 CH4 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−2 s−1 during the daytime. The detection limit of the system was estimated at 22 ng m−2 s−1. The average CH4 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.

Citation: Smeets, C. J. P. P., Holzinger, R., Vigano, I., Goldstein, A. H., and Röckmann, T.: Eddy covariance methane measurements at a Ponderosa pine plantation in California, Atmos. Chem. Phys. Discuss., 9, 5201-5229, doi:10.5194/acpd-9-5201-2009, 2009.
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