First Results on the DOAS-Retrieval of OClO from SCIAMACHY Nadir Measurements

The Scanning Imaging Absorption Spectrometer for \nAtmospheric Chartography was launched successfully \nonboard ENVISAT on March 1, 2002. It observes solar \nradiation transmitted, backscattered from the atmosphere \nand reflected from the ground in nadir, limb and occultation \nviewing modes. Chlorinedioxide (OClO), an important \nindicator for stratospheric chlorine activation, can be \nmeasured in the UV spectral range by Differential Optical \nAbsorption Spectroscopy (DOAS). \nFirst results of the DOAS retrieval of OClO slant column \ndensities (SCDs) from the SCIAMACHY measurements \nare presented. The influence of several parameters like the \nwavelength range chosen as fitting window or the reference \nspectra included in the fit on the quality of the retrieval is examined. It is found that a proper correction of \npolarisation features in the spectra is essential for a good \nDOAS analysis of OClO. The OClO SCDs derived from SCIAMACHY are compared to measurements of the Global Ozone Monitoring Experiment (GOME) which has successfully measured OClO since 1995. SCIAMACHY flies in the same orbit, but measures approx. 30 minutes earlier than GOME. As OClO shows a strong diurnal variation, this leads to differences in the observed column densities, which may be useful to investigate the photochemistry of OClO and related compounds. Also, the spatial resolution of SCIAMACHY is higher (30*60 km^2 compared to 40*320 km^2 for GOME), which will allow a more detailed study of small scale effects like e.g. chlorine activation in mountain waves.