ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-12-1517-2012 Calibration of column-averaged CH<sub>4</sub> over European TCCON FTS sites with airborne in-situ measurements Geibel M. C. 1 6 Messerschmidt J. 2 7 Gerbig C. 1 Blumenstock T. 3 Hase F. 3 Kolle O. 1 Lavrič J. V. 1 Notholt J. 2 Palm M. 2 Rettinger M. 4 Schmidt M. 5 Sussmann R. 4 Warneke T. 2 Feist D. G. 1 Max Planck Institute for Biogeochemistry (MPI-BGC), Jena, Germany Institute of Environmental Physics (IUP), University of Bremen, Bremen, Germany IMK-ASF, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany IMK-IFU, Karlsruhe Institute of Technology (KIT), Garmisch-Partenkirchen, Germany Laboratoire des Sciences du Climat et l'Environnement (LSCE), Gif-sur-Yvette, France now at: Department for Applied Environmental Research (ITM), Stockholm University, Stockholm, Sweden now at: California Institute of Technology, Pasadena, CA, USA 17 01 2012 12 1 1517 1551 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/12/1517/2012/acpd-12-1517-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/12/1517/2012/acpd-12-1517-2012.pdf

In September/October 2009, six ground-based Fourier Transform Spectrometers (FTS) of the Total Carbon Column Observation Network (TCCON) in Europe were calibrated with aircraft in-situ measurements for the first time. The campaign was part of the Infrastructure for Measurement of the European Carbon Cycle (IMECC) project. <br><br> During this campaign aircraft in-situ profiles of CO<sub>2</sub>, CH<sub>4</sub>, CO and H<sub>2</sub>O (from continuous measurements) as well as N<sub>2</sub>O, H<sub>2</sub>, and SF<sub>6</sub> (from flasks) were taken close to the FTS sites. The aircraft data had a vertical coverage ranging from approximately 300 to 13 000 m, corresponding to ~80% of the total atmospheric column seen by the FTS. <br><br> This study summarizes the calibration results for CH<sub>4</sub>. Using similar methods, the resulting calibration factor of 0.978 ± 0.002 (±1 σ) from the IMECC campaign agreed very well with the results that Wunch et al. (2010) had derived for TCCON instruments in North America, Australia, New Zealand, and Japan. By adding the data of the previous calibration of Wunch et al. (2010), the uncertainty of the calibration factor could be reduced by a factor of three. <br><br> A careful analysis of the calibration method used by Wunch et al. (2010) revealed that the incomplete vertical coverage of the aircraft profiles can lead to a bias in the calibration factor. This bias can be compensated with a new iterative approach that we developed. Using this improved method, we derived a significantly lower calibration factor of 0.974 ± 0.002 (±1 σ). This corresponds to a correction of all TCCON CH<sub>4</sub> measurements by roughly −7 ppb.

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