Stratospheric Extinction Profiles from SCIAMACHY Solar Occultation
Abstract. The SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY) instrument on ENVISAT provided between August 2002 and April 2012 measurements of solar and Earthshine spectra from the UV to the SWIR spectral region in multiple viewing geometries. We present a new approach to derive stratospheric aerosol extinction profiles from SCIAMACHY solar occultation measurements based on an onion peeling method similar to the Onion Peeling DOAS (Differential Optical Absorption Spectroscopy) retrieval, which has already been successfully used for the derivation of greenhouse gas profiles. Since the retrieval of aerosol extinction requires as input measured transmissions in absolute units, an improved radiometric calibration of the SCIAMACHY solar occultation measurements has been developed, which considers various instrumental and atmospheric effects specific to solar occultation. The extinction retrieval can in principle be applied to all wavelengths measured by SCIAMACHY. As a first application, we show results for 452 nm, 525 nm and 750 nm. The whole SCIAMACHY solar occultation time series has been processed, covering a latitudinal range of about 50–70° N. Reasonable extinctions are derived between about 15 and 30 km with typically larger uncertainties at higher altitudes due to decreasing extinction. Comparisons with collocated SAGE~II and SCIAMACHY limb aerosol data products revealed a good agreement with essentially no mean bias. However, depending on altitude differences of up to ± 20–30 % to SAGE II at 452 nm and 525 nm are observed. These differences are mainly caused by systematic vertical oscillations in the SCIAMACHY occultation data. The agreement with SCIAMACHY limb data is even better (typically within 5–10 % between 17 and 27 km). Major volcanic eruptions as well as occurrences of PSCs can be identified in the time series of extinction data and related anomalies. Influence of the Quasi-Biennial-Oscillation (QBO) are visible above 25 km. Estimated linear changes of extinction between 2003 and 2011 reach 20–30 % per year at 15 km, mainly because all relevant volcanic eruptions (above 50° N) occurred after 2006.