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Towards an improved CO2 retrieval algorithm for SCIAMACHY on ENVISAT

Michael Buchwitz(1), Maximilian Reuter(1), Oliver Schneising(1), Heinrich Bovensmann(1) and John P. Burrows(1)

(1) University of Bremen FB1, Otto Hahn Allee 1, 28334 Bremen, Germany


At the Institute of Environmental Physics (IUP) of the University of Bremen scientific retrieval algorithms are being developed to extract information about important atmospheric parameters from the spectra as measured by the SCIAMACHY instrument onboard ENVISAT. One of the algorithms, the Weighting Function Modified Differential Optical Absorption Spectroscopy (WFM-DOAS or WFMD) algorithm, has been optimized to retrieve information about column-averaged mixing ratios of the two most important anthropogenic greenhouse gases, carbon dioxide (CO2) and methane (CH4). WFMD is very fast and sufficiently accurate to retrieve global atmospheric methane distributions, which have already been analysed in terms of their information content on methane surface emissions. Mainly for two reasons, methane retrieval is less challenging compared to CO2 retrieval: (i) Methane is typically more variable and (ii) light path (scattering) related errors are smaller compared to CO2 as methane can be normalized by CO2 retrieved from spectrally near-by located absorption lines, which results in a significant cancellation of scattering related errors. As a consequence, scattering related CO2 retrieval errors are larger than for CH4 and may exceed one percent. WFMD considers first order scattering effects by including a low order polynomial in the spectra fit. Second order scattering effects influencing the relative depth of absorption lines arising from residual undetected cloud cover or aerosol variability are not fully considered. In order to reduce scattering related CO2 retrieval errors a new retrieval algorithm is under development, the Bremen optimal EStimation DOAS (BESD) algorithm. BESD will aim at extracting and using more information on atmospheric scatterers from the SCIAMACHY spectra and also to consider a-priori knowledge on atmospheric parameters influencing the CO2 retrieval such as aerosols and clouds. The status of this ongoing activity will be presented.