Preprints
https://doi.org/10.5194/amt-2021-395
https://doi.org/10.5194/amt-2021-395

  09 Dec 2021

09 Dec 2021

Review status: this preprint is currently under review for the journal AMT.

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

Carlos Alberti1, Frank Hase1, Matthias Frey2, Darko Dubravica1, Thomas Blumenstock1, Angelika Dehn3, Gregor Surawicz4, Roland Harig4, Johannes Orphal1,a, and the EM27/SUN-partners team Carlos Alberti et al.
  • 1Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
  • 2National Institute for Environmental Studies (NIES), Tsukuba, Japan
  • 3ESA / ESRIN, Frascati, Italy
  • 4Bruker Optics GmbH, Ettlingen, Germany
  • anow at: Division 4 “Natural and Built Environment”, Karlsruhe Institute of Technology (KIT)
  • See section: Team list

Abstract. In this study, an extension on the previously reported status of the COllaborative Carbon Column Observing Network’s (COCCON) calibration procedures incorporating refined methods is presented. COCCON is a global network of portable Bruker EM27/SUN FTIR spectrometers for deriving column-averaged atmospheric abundances of greenhouse gases. The original laboratory open-path lamp measurements for deriving the instrumental line shape (ILS) of the spectrometer from water vapour lines have been refined and extended to the secondary detector channel incorporated in the EM27/SUN spectrometer for detection of carbon monoxide (CO). The refinements encompass improved spectroscopic line lists for the relevant water lines and a revision of the laboratory pressure measurements used for the analysis of the spectra. The new results are found to be in good agreement with those reported by Frey et al. (2019), and discussed in detail. In addition, a new calibration cell for ILS measurements was designed, constructed and put into service. Spectrometers calibrated since January 2020 were tested using both methods for ILS characterisation, open path (OP) and cell measurements. We demonstrate that both methods can detect the small variations of ILS characteristics between different spectrometers, but the results of the cell method indicate a systematic bias of the OP method. Finally, a revision and extension of the COCCON network instrument-to-instrument calibration factors for XCO2, XCO, and XCH4 is presented, incorporating 47 new spectrometers (of 83 in total by now). This calibration is based on the reference EM27/SUN spectrometer operated by the Karlsruhe Institute of Technology (KIT) and spectra collected by the collocated TCCON station Karlsruhe. Variations in the instrumental characteristics of the reference EM27/SUN during 2014 to 2017 were detected probably arising from realignment and the dual-channel upgrade performed in early 2018. These variations are considered in the evaluation of the instrument-specific calibration factors in order to keep all tabulated calibration results consistent.

Carlos Alberti et al.

Status: open (until 19 Feb 2022)

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Carlos Alberti et al.

Carlos Alberti et al.

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Short summary
Space-borne greenhouse gas missions require ground-based validation networks capable of providing fiducial reference measurements. Here, considerable refinements of the calibration procedures for the Collaborative Carbon Column Observing Network (COCCON) are presented. Laboratory and solar side-by-side procedures for the characterization of the spectrometers have been refined and extended. Revised calibration factors for XCO2, XCO, and XCH4 are provided, incorporating 47 new spectrometers.