Analyzer temperature sensitivity leads to an underestimation of CO2 by the open-path eddy covariance system in winter

The open-path eddy covariance (OPEC) system is widely employed for direct measurement of CO₂ exchange between terrestrial ecosystems and the atmosphere, offering high accuracy in CO₂ observations. However, the performance of OPEC in cold environments, particularly in alpine regions, remains a key topic of research. This study, based on in-situ observations near Lhasa in the central-southern Tibetan Plateau, China (altitude: 3560.8 m), compares CO₂ mixing ratio (Xc) measurements obtained from OPEC and closed-path eddy covariance (CPEC) systems during both cold and warm seasons, with CPEC serving as the benchmark. Our results show that OPEC significantly underestimates Xc during the winter, with average discrepancies of 18.33 ppm and 27.75 ppm across two distinct observational periods. In contrast, during the warm season, Xc measurements from both systems are highly consistent. Further analysis indicates that this underestimation is primarily due to temperature-sensitive biases in the pressure measurements of the OPEC system. To mitigate this issue, we developed a semi-empirical correction model based on air temperature and Xc to adjust the OPEC data. After applying the correction, the adjusted OPEC Xc values align closely with CPEC measurements (y = x - 0.05 and y = x, RMSE = 1.67 and 1.02, at operational temperatures of 30°C and 5°C, respectively). Our findings highlight the importance of correcting OPEC-measured Xc in cold seasons to improve the accuracy of CO₂ concentration measurements in eddy covariance systems.