Retrieval of trace gas vertical columns from SCIAMACHY/ENVISAT near-infrared nadir spectra: first preliminary results

The European environmental satellite ENVISAT was successfully launched on 1st of March 2002. The UV/visible/near-infrared grating spectrometer SCIAMACHY is part of ENVISAT's atmospheric science payload. SCIAMACHY observes the atmosphere in nadir, limb, and solar and lunar occultation viewing geometries with moderate spectral resolution (0.2–1.5 nm). At the University of Bremen a modified DOAS algorithm (WFM-DOAS) is being developed primarily for the retrieval of CH4, CO, CO2, H2O, N2O, and O2 total column amounts from ratios of SCIAMACHY nadir radiance and solar irradiance spectra in the near-infrared and visible spectral regions. First preliminary results concerning this activity are presented. SCIAMACHY is currently (September 2002) in its commissioning phase and only preliminary, i.e., not yet fully calibrated, Level 1 data products are available, generated mainly for initial Level 0 to 1 processing verification purposes. A method aimed at improving the retrieval in case of systematic artifacts resulting from, e.g., residual calibration errors, is presented. This study focuses on methane vertical column retrieval using channel 8 (2260–2385 nm). One of the major scientific objectives of the SCIAMACHY methane measurements is to derive information on methane (surface) sources and sinks. Such an application requires a relative radiometric accuracy close to the signal-to-noise performance of the instrument (S/N ∼50–100 in channel 8 for albedo 0.1 and solar zenith angle 60°) and an accurate and fast retrieval algorithm. This study presents first steps undertaken to reach this ambitious goal, focusing on the retrieval algorithm.