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During the rapid development of the GOME-1 satellite spectrometer, the GOME Science Advisory Group (GSAG) and Characterization and Calibration Sub-group recommended to measure the temperature dependent trace gas absorption spectra under representative in-flight conditions with the GOME Flight Model.

For these measurements the IFE/IUP-UB spectroscopy team developed a mobile absorption spectroscopy set-up called CATGAS.

Measurements were then made with CATGAS and the GOME-1 FM spectrometer, yet at one atmosphere pressure and at temperatures different from instrumental temperatures as observed on ERS-2. Absolute absorption cross-sections were derived by the IUP/IFE-UB team from these GOME-FM measurements for NO2, O3 and SO2 at their own expense. This required laboratory measurements at the University of Bremen too.

Subsequently an alternative study at the IUP/IFE-UB, supported by the DLR and the ESA, yielded high resolution absorption spectra of NO2and O3 at a selected temperatures. One of the intentions of this study was to provide the data to investigate the accuracy of combining an accurate knowledge of the instrument response function coupled with high resolution gas absorption spectra to reduce the requirement for the measurement of trace gas spectra with the FM during calibration. In the case of unstable absorbers (e.g., free radicals as BrO and OClO) it is difficult to measure reference spectra using the Flight Models at all, as either time resolved measurement modes for the FM would be needed or impractically high absorber concentrations were required.

In practice at least for the dominant absorbers in the spectral range measured by GOME, the error of the reference spectra has not been reduced by using the two step procedure. The reason is that it is not feasible to measure the instrument response function of the spectrometers with sufficient accuracy. So in spite of their limitations the CATGAS - GOME-1-FM spectra, when adjusted to account for the different dispersion between one atmosphere and vacuum, still provide the best reference spectra for the GOME-1 analyses. The CATGAS - GOME-1-FM spectra have therefore been used as the reference spectra in the generation of GOME Level 2 data products.

The above result has been confirmed by a study commissioned by ESA and entitled "A Comparison between Predicted and Measured GOME Spectra". This investigation has shown explicitly that significant errors can be introduced if the high resolution reference spectrum convoluted with the instrument response function, rather than absorption spectra measured with the Flight Models themselves are used as reference spectra for DOAS retrieval algorithms. These errors are dependent on the knowledge of the characterisation of the instrument response function.

For the above reasons, the SCIAMACHY calibration program measured both high resolution trace gas reference spectra of the relevant gases (using the IUP/IFE-UB Fourier Transform Spectrometer set-up), accurate instrument response functions and - using the CATGAS set-up - a limited set of trace gas spectra of the majority of the target gases at a variety of temperatures and pressures with the SCIAMACHY FM itself.

Institute of Environmental Physics (IUP)
University of Bremen, Otto-Hahn-Allee 1
28359 Bremen Germany