ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-5975-2011 NDACC/SAOZ UV-visible total ozone measurements: improved retrieval and comparison with correlative ground-based and satellite observations Hendrick F. 1 Pommereau J.-P. 2 Goutail F. 2 Evans R. D. 3 Ionov D. 2 4 Pazmino A. 2 Kyrö E. 5 Held G. 6 Eriksen P. 7 Dorokhov V. 8 Gil M. 9 Van Roozendael M. 1 Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium LATMOS, CNRS, and University of Versailles Saint Quentin, Guyancourt, France Earth System Research Laboratory/Global Monitoring Division, National Oceanic and Atmospheric Administration (NOAA), Boulder, Colorado, USA Department of Atmospheric Physics, Research Institute of Physics, St. Petersburg State University, St. Petersburg, Russia Arctic Research Center, Finnish Meteorological Institute (FMI), Sodankyla, Finland Meteorological Research Institute, Universidade Estadual Paulista, Bauru, S.P., Brazil Danish Meteorological Institute, Copenhagen, Denmark Central Aerological Observatory, Moscow, Russia Instituto de Tecnica Aerospacial (INTA), Torrejón de Ardoz, Spain 24 06 2011 11 12 5975 5995 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/5975/2011/acp-11-5975-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/5975/2011/acp-11-5975-2011.pdf

Accurate long-term monitoring of total ozone is one of the most important requirements for identifying possible natural or anthropogenic changes in the composition of the stratosphere. For this purpose, the NDACC (Network for the Detection of Atmospheric Composition Change) UV-visible Working Group has made recommendations for improving and homogenizing the retrieval of total ozone columns from twilight zenith-sky visible spectrometers. These instruments, deployed all over the world in about 35 stations, allow measuring total ozone twice daily with limited sensitivity to stratospheric temperature and cloud cover. The NDACC recommendations address both the DOAS spectral parameters and the calculation of air mass factors (AMF) needed for the conversion of O<sub>3</sub> slant column densities into vertical column amounts. The most important improvement is the use of O<sub>3</sub> AMF look-up tables calculated using the TOMS V8 (TV8) O<sub>3</sub> profile climatology, that allows accounting for the dependence of the O<sub>3</sub> AMF on the seasonal and latitudinal variations of the O<sub>3</sub> vertical distribution. To investigate their impact on the retrieved ozone columns, the recommendations have been applied to measurements from the NDACC/SAOZ (Système d'Analyse par Observation Zénithale) network. The revised SAOZ ozone data from eight stations deployed at all latitudes have been compared to TOMS, GOME-GDP4, SCIAMACHY-TOSOMI, SCIAMACHY-OL3, OMI-TOMS, and OMI-DOAS satellite overpass observations, as well as to those of collocated Dobson and Brewer instruments at Observatoire de Haute Provence (44° N, 5.5° E) and Sodankyla (67° N, 27° E), respectively. A significantly better agreement is obtained between SAOZ and correlative reference ground-based measurements after applying the new O<sub>3</sub> AMFs. However, systematic seasonal differences between SAOZ and satellite instruments remain. These are shown to mainly originate from (i) a possible problem in the satellite retrieval algorithms in dealing with the temperature dependence of the ozone cross-sections in the UV and the solar zenith angle (SZA) dependence, (ii) zonal modulations and seasonal variations of tropospheric ozone columns not accounted for in the TV8 profile climatology, and (iii) uncertainty on the stratospheric ozone profiles at high latitude in the winter in the TV8 climatology. For those measurements mostly sensitive to stratospheric temperature like TOMS, OMI-TOMS, Dobson and Brewer, or to SZA like SCIAMACHY-TOSOMI, the application of temperature and SZA corrections results in the almost complete removal of the seasonal difference with SAOZ, improving significantly the consistency between all ground-based and satellite total ozone observations.

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