Intercomparability of XCO2 and XCH4 from the United States TCCON sites

The Total Carbon Column Observing Network (TCCON) has become the standard for long-term columnaveraged 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 2 sigma 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.