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2. SAOZ intrument

The SAOZ instrument is a broad-band (300-600 nm), medium resolution (1.0 nm) diode-array spectrometer [Pommereau and Goutail, 1988]. Spectra are recorded every 5 minutes during twilight up to 94¡ solar zenith angle (SZA). Spectra are analysed by differential absorption spectroscopy relatively to a single reference spectrum taken with the same instrument on a clear day at low SZA. Absorptions by O3, NO2, H2O and O4 are derived by an iterative technique, as presented by Sarkissian [1992] but with updated cross-sections and improved wavelength calibration. Line-of-sight ozone, usually identified as slant ozone column (O3slant) is derived from a band 100 nm wide centred at 510 nm. Several versions of the instrument have been developed since the first in 1988 corresponding to different instrumental set up using several combinations of gratings (UFS200 and CP 200, identified as U or C in the set up), detectors (512 or 1024 pixels, PCD or NMOS, all made by Hamamatsu identified as P or M in the set up), and computers (HP or PC for new versions, only since 1999 at OHP in this study), due to progressive improvements since 1988. Table 1 gives information on the main instrumental changes for each station. For each period, main instrumental settings and data analysis are identical. Previous intercomparison campaign [Hoffman et al., 1994; Vaughan et al., 1996; Roscoe et al., 1999] have shown that the spread in ozone line-of sight amounts retrieved from different instruments is better than 1%, indicating the global agreement in the community between instruments and softwares for ozone measurements. Two software versions have been used here for the spectral analysis: one, ANA5V2 for 512 pixels instruments and the other, SAM, for the 1024 pixels version. Note that data sent to NDSC up to now are those analysed with ANA5V2 software.

Total vertical ozone columns O3vert are obtained from the line-of-sight (slant) columns O3slant, derived from the differential spectra, using the folowing relationship:

where O3ref is the amount of ozone in the reference spectrum. SAOZ data are vertical columns iderived from this equation and averaged between 86¡ and 91¡ SZA to minimise errors (Pommereau et al. [1991] and Van Roozendael et al. [1994]). The number of spectra at each twilight is usually about 10.

The colour index, which will be used later in this paper to evaluate the effect of aerosol on AMF, is the ratio of fluxes at 550 and 350 nm corrected from measured absorptions and normalized to 1 at 90¡ SZA to remove tropospheric contribution [Sarkissian et al., 1991; 1994]. The colour index depends on the altitude and on the optical thickness of the stratospheric scatterers: a reddening (a colour index increasing at twilight) corresponds to high altitude (24 km) geometricaly thin (1 - 3 km) and opticaly thick (>0.001) cloud. A blueing (decrease of the colour index at twilight) corresponds to a geometricaly thick (5 - 7 km) and optically thick (>0.001) low altitude cloud.

Real-time and re-analysis SAOZ programs use standard SAOZ AMF described in Sarkissian, [1992], the same set of AMF for all latitudes and seasons but calculated for 60¡N in winter.

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