Improved GOMOS/Envisat retrievals: focus on the upper troposphere and the lower stratosphere

International audienceGlobal Ozone Monitoring by Occultation of Stars (GOMOS) on board Envisat has performed about 440 000 night-time occultations during 2002–2012. Self-calibrating measurement principle, good vertical resolution and the wide vertical range from the troposphere up to the lower thermosphere make GOMOS profiles interesting for different analyses. Vertical profiles of ozone, NO2, NO3 and aerosols are retrieved from night-time UV-VIS spectrometer measurements.The GOMOS ozone data are of high quality in the stratosphere and the mesosphere, but the current operational retrieval algorithm (IPF v.6) is not optimized for retrievals in the upper troposphere–lower stratosphere (UTLS). In particular, validation of GOMOS profiles against ozonesonde data has revealed a substantial positive bias (up to 100%) in the UTLS region. The retrievals in the UTLS are challenging because of low signal-to-noise ratio and the presence of clouds.In this work, we present two advanced GOMOS algorithms, which are optimized for retrievals in the UTLS. One of the algorithms relies on the operational two-step inversion (the spectral inversion followed by the vertical inversion), while another uses one-step approach, i.e., direct fitting the profiles of all constituents. In the new retrievals, a special attention is paid to outlier filtering, the choice of aerosol extinction model, and the vertical resolution.The validation of new retrieved ozone profiles with ozonesondes has shown dramatic reduction of GOMOS ozone biases in the UTLS, for both algorithms. The new GOMOS ozone profiles are also in a very good agreement with measurements by MIPAS, ACE-FTS and OSIRIS satellite instruments in the UTLS. The known geophysical phenomena in the UTLS ozone are well reproduced with the advanced retrievals. In addition, we present GOMOS ozone profiles during the unprecedented stratospheric ozone loss in the Arctic in 2011.These studies have been performed in the framework of ESA-funded ALGOM (GOMOS Level 2 algorithm evolution studies) project. New GOMOS data will be made available via the ESA website