Verification of Sciamachy Level 1 and 2 Near-ir Nadir Data Products by Wfm–doas Analysis

Initial results are presented concerning the verification of operational SCIAMACHY near–infrared (NIR) Level 1 nadir radiance spectra and corresponding Level 2 near–realtime (NRT) BIAS data products such as CH4, CO, and H2O total column amounts. The focus of this study is channel 8 (2265–2380 nm). The verification approach used here is based on applying an independent retrieval method (WFM–DOAS) to the Level 1 data followed by a comparison of the retrieved trace gas columns with the corresponding operational BIAS products. The analyses is based on orbits being part of the “verification orbits data sets” made available by ESA/ESTEC for initial operational data products verification purposes. They reflect state–of–the–art operational processing of SCIAMACHY data. Verification data VERSION 1 were made available end of August 2002. VERSION 2 data have been distributed on 25 November, 2002. The spectral absorption features of the strong NIR absorbers CH4, H2O, and CO2 are clearly visible in these spectra. The accuracy of these spectra, however, does not seem to be good enough to enable a high quality quantitative retrieval and to identify the weak absorbers CO and N2O. The corresponding NRT BIAS products (CH4, CO, H2O, N2O) cannot be considered scientifically useful at present. They suffer significantly from the quality of the spectra (e.g., from residual dark signal calibration errors and dead/bad pixels not rejected by pixel mask) and probably also from Level 1–2 processing problems. In comparison, the accuracy of the WFM–DOAS derived columns is significantly higher. In order to improve the dark signal correction special nadir states (“nadir/dark–states”) have been executed on November 24, 2002. They provide quasi real–time (within approx. 1 minute) dark signal measurements. An initial investigation using these measurements is also presented.