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. In this study, an extension on the previously reported status of theCOllaborative Carbon Column Observing Network's (COCCON) calibrationprocedures incorporating refined methods is presented. COCCON is a globalnetwork of portable Bruker EM27/SUN FTIR spectrometers for derivingcolumn-averaged atmospheric abundances of greenhouse gases. The originallaboratory open-path lamp measurements for deriving the instrumental lineshape (ILS) of the spectrometer from water vapour lines have been refinedand extended to the secondary detector channel incorporated in the EM27/SUNspectrometer for detection of carbon monoxide (CO). The refinementsencompass improved spectroscopic line lists for the relevant water lines anda revision of the laboratory pressure measurements used for the analysis ofthe spectra. The new results are found to be in good agreement with thosereported by Frey et al. (2019) and discussed in detail. In addition, a newcalibration cell for ILS measurements was designed, constructed and put intoservice. Spectrometers calibrated since January 2020 were tested using bothmethods for ILS characterization, open-path (OP) and cell measurements. Wedemonstrate that both methods can detect the small variations in ILScharacteristics between different spectrometers, but the results of the cellmethod indicate a systematic bias of the OP method. Finally, a revision andextension of the COCCON network instrument-to-instrument calibration factorsfor XCO2, XCO and XCH4 is presented, incorporating 47 newspectrometers (of 83 in total by now). This calibration is based on thereference EM27/SUN spectrometer operated by the Karlsruhe Institute ofTechnology (KIT) and spectra collected by the collocated TCCON stationKarlsruhe. Variations in the instrumental characteristics of the referenceEM27/SUN from 2014 to 2017 were detected, probably arising from realignmentand the dual-channel upgrade performed in early 2018. These variations areconsidered in the evaluation of the instrument-specific calibration factorsin order to keep all tabulated calibration results consistent.

Improved calibration procedures for the EM27/SUN spectrometers of the COllaborative Carbon Column Observing Network (COCCON)