This paper describes a new ozone column retrieval algorithm and its application to SCIAMACHY measurements. The TOSOMI algorithm is based on the Differential Optical Absorption Spectroscopy (DOAS) technique and implements several improvements over older algorithms. These improvements include aspects like (i) the explicit treat-5 ment of rotational Raman scattering, (ii) an improved air-mass factor formulation which is based on a simulation of the reflectivity spectrum and a subsequent DOAS fit

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... this simulated spectrum, (iii) the use of an improved ozone climatology and a column dependent air-mass factor, (iv) the use of daily varying ECMWF temperature profile analyses. The results of three validation exercises are reported. The TOSOMI columns 10 are compared with an extensive set of ground-based observations (Brewer, Dobson) for the years 2003 and 2004. Secondly, a direct comparison for January-June 2003 with two new GOME retrievals, GDP Version 4 and TOGOMI, is presented. Third, data assimilation is used to study the dependence of the TOSOMI columns with retrieval parameters such as the viewing angle, cloud fraction and geographical location. 15 These comparisons show a good consistency on the percent level between the GOME and SCIAMACHY algorithms. The present TOSOMI implementation (v0.32) shows an offset of about -1.5% with respect to ground-based observations and the GOME retrievals. and Arctic ozone hole in particular (WMO, 2003). A European contribution to this 4430 ACPD 5, 4429-4475, 2005 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Print Version Interactive Discussion EGU long-term ozone column monitoring consists of Global Ozone Monitoring Instrument (GOME) on ERS-2 (Burrows et al., 1999a), SCIAMACHY on Envisat (Bovensmann et al., 1999) and OMI on EOS-AURA (Veefkind et al., 2002) . Apart from the long-term record and coverage it is in particular the perceived high accuracy (on the few percent level) of these UV-VIS instruments which contributes to their popularity. An important 5 trend-monitoring requirement for the ozone data sets is the long-term stability which should be on the order of 1% to be able to document changes in the ozone layer and the possible recovery resulting from the reduction of ozone depleting substances. However, the development of high-quality retrieval algorithms is non-trivial and requires detailed instrument knowledge and a careful description of atmospheric pro-10 cesses. During 2004 a new version of the TOMS algorithm, Version 8, has become available (Bhartia and Wellemeyer, 2004) . Similarly, a new reprocessing has become available for GOME, GOME Data Processor (GDP) Version 4 (Spurr et al., 2004) . Both new algorithms implement substantial improvements over older TOMS and GOME processor versions. The GDP-4 is based on the GDOAS algorithm developed at BIRA-15 IASB. GDOAS was one of three new DOAS-type algorithms developed for GOME in response to an ESA Invitation to Tender. The other two algorithms are the weighting function DOAS developed by the University of Bremen (Coldewey-Egbers et al., 2004; Weber et al., 2004) and the TOGOMI algorithm developed by KNMI (Valks and van Oss, 2003). All three algorithms were shown to improve considerably upon GOME 20 GDP Version 3. The present operational ESA algorithm for SCIAMACHY is based on the older GDP Version 2.7 of GOME. Known problems with this algorithm were an important motivation for us to develop our own SCIAMACHY total ozone retrieval algorithm. A second important motivation for an independent implementation is the verification and 25 validation of the official ESA ozone column products. Our algorithm is called "Total Ozone retrieval scheme for SCIAMACHY based on the OMI DOAS algorithm" (TO-SOMI). It is based on the TOGOMI retrieval algorithm, and has been coupled to the SCIAMACHY level-1 measurements with only minor modifications to the code. The 4431 ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract 25 4433 ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract 25 4446 ACPD 5, 4429-4475, 2005 Abstract 5, 4429-4475, 2005 Abstract 5, 4429-4475, 2005 Abstract ACPD 5, 4429-4475, 2005 Abstract 5, 4429-4475, 2005 Abstract 5, 4429-4475, 2005