New NDACC recommendations for the retrieval of total ozone columns from ground-based zenith-sky UV-visible observations

Total ozone column measurements are routinely performed at sunrise and sunset using groundbased zenith-sky UV-visible spectrometers deployed all over the world in about 35 stations, most of them belonging to the NDACC network. Despite several cross evaluation exercises, it has been recognized that the O3 data sets still suffer from residual inconsistencies mainly due to (1) differences in the DOAS settings, in particular the ozone absorption cross sections used for the various instruments and (2) a lack of homogeneity in the air mass factors (AMFs) applied to O3 slant columns for their conversion into vertical columns. Recently, the NDACC UV-visible Working Group has formulated new recommendations aiming at improving the homogeneity of the UV-visible total ozone measurements. Regarding the spectral analysis, a list of recommended settings has been established including fitting interval, cross sections data sets, as well as wavelength calibration and residual amount in the reference spectrum determination methods. In case of AMFs, look-up tables (LUTs) based on the columnresolved TOMS version 8 (V8) O3 profile climatology are now used, which allows accounting for the dependence of the AMF on the latitudinal, seasonal, and daily variations of the ozone vertical profile. The calculated LUTs, only suitable for background aerosols conditions, depend on latitude, day of year, O3 column, wavelength, SZA, surface albedo, and station altitude. The error budget on the AMFs as well as on the spectral analysis has been evaluated. In a companion paper (Goutail et al.), the impact of the recommendations is investigated through their application to measurements from the NDACC/SAOZ (Système d'Analyse par Observation Zénithale) network. The consistency of the new SAOZ data sets with correlative ground-based and satellite UV observations is also discussed there