Performance Evaluation of an Integrated Open-Path Eddy Covariance System in a Cold Desert Environment

Abstract

AbstractPerformance evaluation of an integrated eddy covariance (EC) instrument called the IRGASON, with a separated EC for reference, was conducted in a desert riparian Populus euphratica stand in the lower Tarim River basin in northwestern China. The separated EC consisted of an open-path gas analyzer and a sonic anemometer separated by 20 cm. The IRGASON integrates an open-path gas analyzer and a sonic anemometer into the same sensing volume, thus eliminating sensor separation in comparison to the traditional open-path EC setup. Integrating the infrared gas analyzer’s sensing head into the sensing volume of the sonic anemometer had negligible effects on wind speed and friction velocity observations of the IRGASON. Physiologically unreasonable daytime CO2 uptake was observed by both systems during the cold winter season (mean air temperature of −6.7°C), when the trees were dormant without any photosynthetic activities. The mean midday CO2 flux was −1.65 and −1.61 μmol m−2 s−1 for the IRGASON and the separated EC setup, respectively. No evidence was found for sensor self-heating as the cause of the apparent uptake CO2 flux. Instead, the uptake CO2 flux appeared to be an artifact of the spectroscopic effect of the IRGASON’s gas analyzer. After adjusting for this spectroscopic effect using a relationship with the sensible heat flux, the wintertime IRGASON CO2 flux became physiologically reasonable (mean value of −0.04 μmol m−2 s−1).