GOMOS one-step retrieval algorithm

Global Ozone Monitoring by Occultation of Stars (GOMOS) is a satellite instrument onboard the ENVISAT platform that was in operation during 2002{2012. During these years, GOMOS observed about 880 000 vertical profiles of ozone, NO 2 , NO 3 and aerosols. The GOMOS measurement principle is relatively simple based on the stellar-occultation technique. In this paper, we present an alternative retrieval algorithm for processing the GOMOS measurements. The presented algorithm is based on the so-called one-step approach, where both the spectral and the vertical inversions are executed simultaneously. This approach has several attractive features. In particular, the one-step approach allows a better use of the smoothness prior information and, unlike in the operative algorithm, the prior given to one specie affects the other species too. This feature is critical when going near the detection limit, especially in the upper troposphere lower stratosphere (UTLS) region. The main challenge in the GOMOS one-step algorithm is to find the correct smoothness priors for the different species at different altitudes. In this paper, we give a technical description of the one-step retrieval algorithm and discuss the differences between this and the operative algorithm. In the case study part of this paper, we compare the one-step and the operative ozone retrievals in Arctic region during the exceptional ozone-depletion conditions in spring 2011. We show that the quality of the ozone profiles can be improved by introducing the one-step algorithm. The improvement is drastic in the lower stratosphere at 15{20km altitude.