Concerning the measurement of atmospheric trace gas fluxes with open- and closed-path eddy covariance systems: The density terms and spectral attenuation [Chapter 7]

Abstract

Atmospheric trace gas fluxes measured with an eddy covariance sensor that detects a constituent's density fluctuations within the in situ air need to include terms resulting from concurrent heat and moisture fluxes, the so called 'density' or 'WPL corrections' (Webb et al. 1980). The theory behind these additional terms is well established. But, virtually no studies to date have examined the constraints imposed on the theory by different instrumentation technologies and by limitations inherent to eddy covariance systems. This study extends the original WPL theory by examining how eddy covariance instrumentation, particularly spectral attenuation and an instrument's basic technology, influences the application of this theory to flux measurement. Specific issues discussed here include the importance of static pressure fluctuations to the WPL theory, the possible systematic overestimation of the WPL vapor term, and the transfer functions associated with signal processing and volume averaging effects of a fast-response closed-path CO2/H2O sensor. This different perspective on the WPL theory suggests that current methods of applying the WPL theory, particularly with closed-path systems, can yield significant biases in the annual carbon balance derived from eddy covariance technology and can cause the surface energy imbalance to increase with increasing wind speed. Furthermore, it is suggested that spectral corrections should be made before applying the WPL theory to estimate fluxes and that high frequency point-by-point conversions from mass density to mixing ratio is not the preferred method for estimating fluxes by eddy covariance.

Citation

Massman, W. J. 2004. Concerning the measurement of atmospheric trace gas fluxes with open- and closed-path eddy covariance systems: The density terms and spectral attenuation [Chapter 7]. In: Lee, X.; Massman, W. J.; Law, B. E. Handbook of Micrometeorology: A Guide to Surface Flux Measurements. Dordrecht, Netherlands: Springer. p. 87-111.