Addressing the influence of instrument surface heat exchange on the measurements of CO₂ flux from open-path gas analyzers

• Published in: Global change biology 2008-08, Vol.14 (8), p.1854-1876
• Main Authors: BURBA, GEORGE G, McDERMITT, DAYLE K, GRELLE, ACHIM, ANDERSON, DANIEL J, XU, LIUKANG
• Format: Article
• Language: English
• Subjects: Animal, plant and microbial ecology; Atmospheric chemistry; Biological and medical sciences; carbon budget; Carbon dioxide; carbon dioxide exchange; Climate change; closed path; CO2 flux; CO₂ flux; density correction; Fundamental and applied biological sciences. Psychology; gas analyzer; General aspects. Techniques; heat exchange; heat transfer; Measurement techniques; Methods and techniques (sampling, tagging, trapping, modelling...); open path; sensible heat flux; surface heating; Thermodynamics; WPL
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/j.1365-2486.2008.01606.x

There is a growing concern in the flux community that using the eddy covariance method with open-path CO₂ analyzers often leads to measurements of an apparent ecosystem CO₂ uptake during off-season periods, especially in cold climates. Such uptake has not been observed when measurements were made with closed-path analyzers, chambers, or profile methods, suggesting it is an artifact due in some way to the use of open-path analyzers. In this study, a series of laboratory tests and field experiments were conducted to determine the magnitude of the instrument surface heat exchange in the open path and its relationship with the measured CO₂ flux. Results showed that (1) the surface of an open-path instrument became substantially warmer than ambient due to electronics and radiation load during daytime, while at night, radiative cooling moderated temperature increases in the path; (2) high-frequency temperature measurements inside the path were correlated with vertical wind speed producing sensible heat flux inside the instrument path exceeding the ambient heat flux by up to 14%; (3) enclosing the open-path instrument eliminated the sensible heat flux in the path, and caused measured CO₂ flux to match a closed-path reference; (4) using sensible heat flux measured directly inside the open path in the WPL term instead of the ambient sensible heat flux also led to a match in CO₂ flux between open-path instrument and closed-path reference; and (5) correcting previously collected open-path CO₂ flux data was possible by estimating the instrument heating effect with a semi-empirical model using standard weather variables. Results showed that all proposed techniques led to a significant reduction in apparent CO₂ uptake during off-season periods and to a reduction of the underestimation of CO₂ release in other periods. Close agreement between the open-path measurements and closed-path references was achieved in all cases.