Intercomparability of X_CO2 and X_CH4 from the United States TCCON sites

Abstract. The Total Carbon Column Observing Network (TCCON) has become the standard for long-term column-averaged measurements of CO₂ and CH₄. Here, we use a pair of portable spectrometers to test for intra-network bias among the four currently operating TCCON sites in the United States (US). A previous analytical error analysis has suggested that the maximum 2σ site-to-site relative (absolute) bias of TCCON should be less than 0.2 % (0.8 ppm) in X_CO2 and 0.4 % (7 ppb) in X_CH4. We find here experimentally that the 95 % confidence intervals for maximum pairwise site-to-site bias among the four US TCCON sites are 0.05–0.14 % for X_CO2 and 0.08–0.24 % for X_CH4. This is close to the limit of the bias we can detect using this methodology.