Eddy correlation measurement of CO2 flux using a closed-path sensor: Theory and field tests against an open-path sensor

We have examined the potential of using a closed-path sensor to accurately measure eddy fluxes of CO₂. Five inlet tubeflow configurations were employed in the experimental setup. The fluxes of CO₂ were compared against those measured with an open-path sensor. Sampling air through an intake tube causes a loss of flux, due to the attenuation of CO₂ density fluctuations. Adjustments need to be made to correct for this loss and to account for density effects due to the simultaneous transfer of heat and water vapor. Theory quantifying these effects is discussed.The raw CO₂ flux measured with the closed-path sensor was smaller than that measured with the open-path sensor by about 15% (on average) for the turbulent tubeflow configurations with a short (3 m) intake tube, by 31% for turbulent tubeflow with a longer (6 m) intake tube and by 24% for laminar tubeflow. The difference was, in part, caused by tube attenuation of the CO₂ density fluctuations and inadequate sensor time response. The elimination of the flux adjustment for the simultaneous transfer of sensible heat (i.e., the attenuation of ambient temperature fluctuations in the intake tube) generally accounted for the rest of this difference.The raw flux measured with the closed-path sensor was corrected for frequency response and density effects. Except in the case of laminar tubeflow, the corrected closed-path flux agreed consistently with the corrected open-path flux within a few percent (<5%). These results suggest that closed-path sensors, with appropriate corrections, can be used to measure CO₂ flux accurately. Recommendations are included on selecting an optimum flow configuration to minimize the effect of sampling air through a tube.Published as Paper No. 9938, Journal Series, Nebraska Agricultural Research Division.