Comparative measurements of CO2 flux over a forest using closed-path and open-path CO2 analysers

An eddy covariance system using a closed-path CO(2)analyser was constructed for long-term CO(2)flux measurements above a forest, and its total frequency response was valuated experimentally. The amplitude and phase responses of the system were examined through a preliminary test, in which a prescribed pattern of CO2 fluctuation was input to the system. The result showed that the amplitude of the output from the system was attenuated as frequency increased, with a half-power frequency of 0.3 Hz. The phase was delayed by the air sampling through a long tube, but the delay in phase decreased as frequency increased. We then presented a new technique for the correction of flux loss due to the inadequate system response for the eddy covariance measurements of CO2 flux. Using the present system and the correction technique, diurnal variations in CO2 flux were measured over a temperate deciduous forest on three days in 1997. The results were compared with the CO2 fluxes measured with a fast response open-path gas analyser. The CO2 fluxes from the closed-path system agreed with those from the open-path system after the Webb, Pearman and Leuning correction was made for the latter. In the present test over a forest, the contribution of the frequency-response correction to the CO2 fluxes was small and its averaged percentage was only 3% in the daytime. However, the percentage would likely increase, if the system were applied to a shorter vegetation site where high frequency components are more important. The comparison confirmed that we can obtain correct measurements of CO(2)flux using the present closed-path system and the correction technique.