Advanced total column ozone retrieval from hyperspectral UV satellite instruments

Abstract

This study exploits nadir spectral measurements in 325-335nm range to infer total column ozone (TCO) from the Global Ozone Monitoring Experiment (GOME) and SCaning Imaging Absorption spectroMeter for Atmospheric CHartographY(SCIAMACHY) instruments. TCO from these two instruments retrieved using the Weighting Function Differential Optical Absorption Spectroscopy (WFDOAS) are presented. Unlike in standard DOAS where the fitting procedure results inozone slant column which needs to be converted into vertical column (TCO) by using air mass factor (AMF), in our novel approach direct retrieval of vertical column amounts of ozone is possible by fitting vertically integratedozone weighting function to the sun-normalized radiances. Other implementations include proper modelling of the Ring effect including Raman correction for ozone absorption, the implicit use of the effective albedo and effective sceneheight accounting for cloud effects, and an ozone temperature correction. The new algorithm has been extensively validated with ground-based Dobson andBrewer TCO measurements. In general, the agreement between GOME WFDOAS and ground stations data is very good and the validation shows that the retrieval accuracy of WFDOAS is within the uncertainty of current ground-based instruments. Better agreements are observed in the comparison withBrewer measurements than with Dobson measurements. This may be explained to a large extent by the neglect of ozone temperature correction in the standard retrieval for Dobson and Brewer. Temperature correction has a larger effect onthe Dobson results. The accuracy of WFDOAS retrievals makes GOME data very attractive for evaluating the ground-based network data. Eight years of GOME data are used to assess the quality of the WOUDC-archived Brewer data. It is shown that monitoring of Brewer data quality and identification ofproblems in Brewer instruments are possible. This method can also be applied to other ground-based instruments. Despite improvement with WFDOAS, somewhat larger differences between satellite TCO and ground based measurements remain at high latitudes under low sun conditions. The persistent TCO differences are due in part to profile shape sensitivity of satellite TCO retrieval algorithms. Improved ozone and temperature profile climatologies, which is prepared and presented in this thesis, will lead to improved satellite ozone measurements. The effect of ozone and temperature profiles on TCO retrievals is explored. The studydemonstrates that an improved and updated ozone and temperature climatological profiles can reduce the systematic errors in the retrieved GOME TCO, inparticular at high solar zenith angles. For SCIAMACHY application, the only change of algorithm is for calculating the reference spectra online in each iteration during retrievals rather than using look-up tables. Scaled SCIAMACHY FM ozone cross-section and wavelength pre-shifted by 0.016nm, as determined from our comprehensive investigation performed in this thesis, is used for the calculation of reference spectra and the Ring spectra. Improved cloud products and the new ozone and temperature profile climatologies are used. Quality of theretrieved TCO is assessed by correlative measurements from GOME and selected Brewer spectrophotometers. In general a good agreement is achieved apart from a scan angle dependent offset of about -1% to -2%. The reason for the offset is not understood but part of the offset might be related to thecalibration errors in the level 1 data. Nevertheless, combined data set from GOME and SCIAMACHY as retrieved from WFDOAS algorithm presented in this thesis will be valuable for long-term ozone studies.