A fault-tolerant eddy covariance system for measuring CH4 fluxes

Eddy covariance flux measurements of methane, other trace gases, and fog droplets often involve modern analyzers that use digital signal processing. This allows to eliminate several potential sources of noise by using the digital signal directly in combination with a digital sonic anemometer. Some specific knowledge on timing issues and fault tolerance of such a digital data acquisition is however needed. Here we show how such a system works successfully with a modern cavity ringdown spectrometer measuring CH4 concentrations. This system has evolved over the past 10 years and has successfully been used for eddy covariance flux measurements with other sensors as well. In a field deployment during 5 days in July 2008 we measured methane fluxes from a sealed landfill (Lindestock near Liestal, Switzerland) that contains organic substances that lead to event-driven effluxes of methane at relatively low rates. During 4 days mostly net CH4 emissions were measured with daily median values between 1.6 and 11.0 mu g CH4 m(-2) s(-1), whereas during 1 day primarily uptake (methane oxidation) was measured with a median flux of -0.62 mu g CH4 m(-2) s(-1). Detailed analyses indicate that this new CH4 flux system is able to measure this magnitude of fluxes. The performance of the off-axis integrated-cavity output spectrometer was quantified at 0.35 ppb at optimum integration time, with a white-noise level of 1.5 ppb Hz(-1/2), which corresponds to a precision of +/- 0.08% of the background concentration at 1-Hz resolution. Depending on the sampling rate of the chemical analyzer in comparison to the sampling rate of the sonic anemometer it was found that the variance measured by oversampling the digital signal is as good or even better than the downsampling with respect to an ideal sensor. It is therefore concluded that switching from partial digital or analog data acquisition to fully digital data acquisition is highly recommended for eddy covariance flux measurements with modern instrumentation. (C) 2010 Elsevier B.V. All rights reserved.