Improved calibration procedures for the EM27/SUN spectrometers of the COllaborative Carbon Column Observing Network (COCCON)
Authors: Alberti Carlos , Hase Frank , Frey Matthias , Dubravica Darko , Blumenstock Thomas , Dehn Angelika , Castracane Paolo , Surawicz Gregor , Harig Roland , Baier Bianca C. , Bès Caroline , Bi Jianrong , Boesch Hartmut , Butz André , Cai Zhaonan , Chen Jia , Crowell Sean M. , Deutscher Nicholas M. , Ene Dragos , Franklin Jonathan E. , García Omaira , Griffith David , Grouiez Bruno , Grutter Michel , Hamdouni Abdelhamid , Houweling Sander , Humpage Neil , Jacobs Nicole , Jeong Sujong , Joly Lilian , Jones Nicholas B. , Jouglet Denis , Kivi Rigel , Kleinschek Ralph , Lopez Morgan , Medeiros Diogo J. , Morino Isamu , Mostafavipak Nasrin , Müller Astrid , Ohyama Hirofumi , Palmer Paul I. , Pathakoti Mahesh , Pollard David F. , Raffalski Uwe , Ramonet Michel , Ramsay Robbie , Sha Mahesh Kumar , Shiomi Kei , Simpson William , Stremme Wolfgang , Sun Youwen , Tanimoto Hiroshi , Té Yao , Tsidu Gizaw Mengistu , Velazco Voltaire A. , Vogel Felix , Watanabe Masataka , Wei Chong , Wunch Debra , Yamasoe Marcia , Zhang Lu , Orphal Johannes 
Published: April 2022

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 characterization, open-path (OP) and cell measurements. We
demonstrate that both methods can detect the small variations in 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 from 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.