Determination of SCIAMACHY Level-1 Data Versions Using Nadir Ozone Profile Retrievals during 2003–2011

In this paper we compare and evaluate the three most recent versions of the SCIAMACHY Level-1 (L1) product. The comparison is based on the retrieval of the nadir ozone profiles from the SCIAMACHY UV reflectance spectra, showing the impact of L1 calibration improvements. Ozone profile retrieval from nadir-viewing satellite instruments operating in the ultraviolet-visible range requires accurate calibration of Level-1 (L1) radiance data. We retrieve nadir ozone profiles from the SCIAMACHY instrument that flew on board Envisat using the Ozone ProfilE Retrieval Algorithm (OPERA) developed at KNMI with a focus on stratospheric ozone. We study and assess the quality of these profiles and compare retrieved L2 products from L1 SCIAMACHY data versions from the years 2003 to 2011 without further radiometric correction. From validation of the profiles against ozone sonde measurements, we find that the v8 performs better than v7 and v7mfac due to correction for the scan-angle dependency of the instrument's optical degradation.

Validation for the years 2003 and 2009 with ozone sondes shows deviations of SCIAMACHY ozone profiles of 0.8-15% in the stratosphere (corresponding to pressure range ∼ 100-10 hPa) and 2.5-100 % in the troposphere (corresponding to pressure range ∼ 1000-100 hPa), depending on the latitude and the L1 version used. Using L1 v8 for the years 2003-2011 leads to deviations of ∼ 1-11 % in stratospheric ozone and ∼ 1-45 % in tropospheric ozone. Retrieving the instrument slit function in the UV range for the v8 data set gives an improvement of a few percent in the solar data through the mission length. The measured reflectance spectra show that the degradation in v8 is still significant for some wavelengths, because the ratio of the measured to simulated reflectance below 300 nm can range from ∼ 1.1 to 1.4.

The SCIAMACHY L1 v8 data can still be improved upon in the 265-330 nm range used for ozone profile retrieval. The slit function can be improved with a spectral shift and squeeze, which leads to a few percent residue reduction compared to reference solar irradiance spectra. Furthermore, studies of the ratio of measured to simulated reflectance spectra show that a bias correction in the reflectance for wavelengths below 300 nm appears to be necessary.