Correcting apparent off-season CO2 uptake due to surface heating of an open path gas analyzer: progress report of an ongoing study

A number of Eddy Covariance flux sites with open-path analyzers have observed small apparent CO₂ uptake outside the growing season when such uptake is generally considered physiologically unlikely. In this study, experiments were conducted to resolve this issue by examining the influence of the surface heating of the LI-7500 open-path CO₂/H₂O analyzer on the CO₂ flux measured using the Eddy Covariance technique. As a result, an additional correction due to surface heating was introduced as a part of the sensible heat term of the Webb-Pearman-Leuning correction. Effects of this additional term on CO₂ fluxes during both off-season and growing season have been examined separately on hourly and seasonal integration time scales. Significant reductions in apparent CO₂ uptake during off-season periods were observed, while fluxes during the growing season were not noticeably affected. Processes contributing to the instrument surface energy budget were studied in one laboratory and two field studies, which are still in progress. We evaluated heating of the instrument surface due to heat dissipated by the electronics over a wide range of ambient temperatures, heating of the surface due to incoming solar radiation during daytime, long-wave radiation cooling of the instrument at night, and influence of wind-induced convective exchange near the instrument surface. A method for data correction was developed by estimating parameters for these processes. The additional term in the Webb-Pearman-Leuning correction may be useful for future year-round CO₂ flux research, as well as, for past studies. The proposed correction method can be applied to pre-existing data, which may help to provide more accurate interpretations of previously collected information.