Improved Sciamachy Wfdoas Total Ozone Retrieval: Steps towards Homogenising Long-term Total Ozone Datasets from Gome, Sciamachy, and Gome2

Currently three European satellite spectrometers, GOME1/ERS2 (1995-present), SCIAMACHY/ENVISAT (2002-present), and GOME2 (2006-present) provide a near global decadal data set of total ozone covering more than a decade. This data are very valuable for the detection of recovery in stratospheric ozone as expected from the levelling off of stratospheric chlorine. In order to detect such a signal beyond the level of the large inter-annual variability is challenging and requires high accuracy in retrieved ozone with a stability to within one percent (relative bias) over a decade and absolute accuracy to within a few percent. A particular challenge will be to homogenise data sets from different instruments. In this paper we discuss important retrieval issues that are deemed important in order to avoid excessive homogenisation of multiple data sets (if homogenisation is understood as bias removal). The relevant issues discussed here are the choice of ozone cross-sections, the use of a-priori O3 profile climatologies, and level-1 spectral calibration.