First intercalibration of column-averaged methane from the Total Carbon Column Observing Network and the Network for the Detection of Atmospheric Composition Change

[摘要] We present the first intercalibration of dry-air column-averaged molefractions of methane (XCH₄) retrieved from solar Fourier transforminfrared (FTIR) measurements of the Network for the Detection of AtmosphericComposition Change (NDACC) in the mid-infrared (MIR) versus near-infrared(NIR) soundings from the Total Carbon Column Observing Network (TCCON). Thestudy uses multi-annual quasi-coincident MIR and NIR measurements from thestations Garmisch, Germany (47.48° N, 11.06° E,743 m a.s.l.), and Wollongong, Australia (34.41° S, 150.88° E,30 m a.s.l.).

Direct comparison of the retrieved MIR and NIR XCH₄ time series forGarmisch shows a quasi-periodic seasonal bias leading to a standarddeviation (stdv) of the difference time series (NIR–MIR) of 7.2 ppb. Afterreducing time-dependent a priori impact by using realistic site- andtime-dependent ACTM-simulated profiles as a common prior, the seasonal biasis reduced (stdv = 5.2 ppb). A linear fit to the MIR/NIR scatter plot ofmonthly means based on same-day coincidences does not show a y-intercept thatis statistically different from zero, and the MIR/NIR intercalibrationfactor 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 significanttrend. The same basic findings hold for Wollongong. In particular an overallMIR/NIR intercalibration factor close to the ideal 1 is found within 2-σ uncertainty. At Wollongong the seasonal cycle of methane is less pronouncedand corresponding smoothing errors are not as significant, enabling standardMIR and NIR retrievals to be used directly, without correction to a common apriori.

Our results suggest that it is possible to set up a harmonized NDACC andTCCON XCH₄ data set which can be exploited for joint trend studies,satellite validation, or the inverse modeling of sources and sinks.

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