Abstract
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 analyzer (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. The fluctuations of the CH4 concentration measured at 10 Hz appear to be small and their standard deviation is comparable to the magnitude of the signal noise (±5 ppbv). Consequently, the power spectra typically have a white noise signature at the high frequency end (a slope of +1). 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. We furthermore evaluate the complications of combined open and closed-path measurements when applying the Webb-Pearman-Leuning (WPL) corrections (Webb et al., 1980) and the consequences of a phase lag between the water vapor and methane signal inside the closed path system. The results of diurnal variations of CH4 concentrations and fluxes are summarized and compared to the monthly results of process-based model calculations.
Highlights
Methane (CH4) is the predominant hydrocarbon in the Earth’s atmosphere
We focus on data processing using frequency-dependent response functions with special attention for the complications that arise under low flux conditions, the combination of open and closed-path measurements when applying the WebbPearman-Leuning (WPL) corrections (Webb et al, 1980) and the consequences of a phase lag between the water vapor and methane signal inside the closed path system
We have shown the feasibility of eddy covariance (EC) measurements at low CH4 flux levels with a Fast Methane analyzer (FMA) during an experiment in a Ponderosa pine plantation at the Blodgett Forest site in California
Summary
Measurements performed with the EC method need several corrections that mostly increase the turbulent flux. 3.2 we discuss the general corrections related to the geometry of the EC set-up and the physical separation and limitations of the individual sensors For some of these corrections we study the frequency-dependent cospectral transfer functions as described by Moore (1986). With Twq the net system cospectral transfer function associated with sensors of vertical velocity w and quantity q, Cwq is the atmospheric cospectrum of w and q at natural frequency f (Hz). It is the product of the response functions associated with sensor frequency response, size and separation. The latter corrections are not taken into account traditionally and merit extra attention
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