First intercalibration of column-averaged methane from the total carbon column observing network and the network climate for the detection of atmospheric composition change

We present the first intercalibration of dry-air column-averaged mole fractions of methane (XCH₄) retrieved from solar Fourier transform infrared (FTIR) measurements of the Network for the Detection of Atmospheric Composition Change (NDACC) in the mid-infrared (MIR) versus near-infrared (NIR) soundings from the Total Carbon Column Observing Network (TCCON). The study uses multi-annual quasi-coincidentMIR and NIR measurements from the stations Garmisch, Germany (47.48°N, 11.06°E, 743ma.s.l.), andWollongong, Australia (34.41°S, 150.88°E, 30ma.s.l.). Direct comparison of the retrieved MIR and NIR XCH4 time series for Garmisch shows a quasi-periodic seasonal bias leading to a standard deviation (stdv) of the difference time series (NIR-MIR) of 7.2 ppb. After reducing time-dependent a priori impact by using realistic site- and timedependent ACTM-simulated profiles as a common prior, the seasonal bias is reduced (stdv = 5.2 ppb). A linear fit to the MIR/NIR scatter plot of monthly means based on sameday coincidences does not show a y-intercept that is statistically different from zero, and the MIR/NIR intercalibration factor is found to be close to ideal within 2-σ uncertainty, i.e. 0.9996(8). The difference time series (NIR-MIR) do not show a significant trend. The same basic findings hold for Wollongong. In particular an overall MIR/NIR intercalibration factor close to the ideal 1 is found within 2-σ uncertainty. At Wollongong the seasonal cycle of methane is less pronounced and corresponding smoothing errors are not as significant, enabling standard MIR and NIR retrievals to be used directly, without correction to a common a priori. Our results suggest that it is possible to set up a harmonized NDACC and TCCON XCH₄ data set which can be exploited for joint trend studies, satellite validation, or the inverse modeling of sources and sinks.