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1. Introduction
Système d'Analyse par Observation Zénithale (SAOZ) UV-visible zenith-sky spectrometers [Pommereau and Goutail, 1988] measure total ozone twice a day at twilight from the ground. Since 1988 when the first instrument was installed at Dumont DâUrville (67¡ S, 123¡ E), a network of 17 instuments - the SAOZ network - has been deployed for global coverage and long term studies in the frame of the Network for Detection of Stratospheric Changes (NDSC). Twilight measurements are particularly adapted for observations at polar circle in winter. Twilight geometry of zenith-sky viewing enhances the path of the sunlight in the stratosphere where resides most ozone. Line-of sight amounts are larger and therefore easier to measure. These amounts are deduced from spectra by differential absorption analysis. Radiative transfer models calculate the air-mass factor (AMF) to convert line-of-sight amounts into vertical columns, the product evidently needed for scientific interpretations and for data bases. These AMFs show relatively small dependence on climatological profiles of air density, ozone and of stratospheric aerosol, so that in the past a single set of AMFs has been used for the whole SAOZ Network for all seasons and locations.
Long-term routine measurements are now available, and method and instruments are validated regularly [Hoffman et al., 1994; Vaughan et al., 1996; Sarkissian et al., 1995b; Roscoe et al., 1999]. But for the detection of trends like those usually observed for ozone, a more accurate approach is needed. As recommended in previous analysis of trends, instrumental changes must be well identified to reduce uncertainties. More, measurement artefacts like those arising from inadequate AMF computation must be corrected when possible.
The goal of this paper is to make a first evaluation of the trend in total ozone measured from the SAOZ network at Sodankyla (Finland), Zhigansk (Siberia), Observatoire de Haute-Provence (OHP, France), Tarawa (Kiribati Island), Kerguelen (Indien Ocean) and Dumont dâUrville (Antarctica), (see table 1 for details) using the long series of data available. Particular attention has been paid to homogeneous set of retrieved data taking into account instrument and spectral analysis changes. Seasonal effect on ozone AMFs will be evaluated using available climatological ozone and temperature profiles. Stratospheric aerosol effect on the AMFs will also be evaluated using SAGE II optical thickness measurements during Mt Pinatubo period as well as Polar Stratospheric Cloud effects. Corrections of these effects will be applied to SAOZ data to produce ozone monthly means.
Table 1: SAOZ series to be used here, instrumental set up and changes.
| Station |
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| Zhigansk | 67 N | 123 E | Dec 91 | UN, 512 | Jul. 99 | ||||||
| Sodankyla | 67 N | 27 E | Feb. 90 | UP, 512 | Dec. 91 | UN, 512 | Jan. 95 | CN, 1024 |
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| Obs. Haute-Provence | 44 N | 6 E | Jun. 92 | CP, 512 | Jun. 95 | CP, 512 | Jul. 95 | CP 1024 |
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| Tarawa | 1 N | 173 E | Aug. 92 | CP, 512 | Dec. 98 | CP, 512 | Dec. 99 | ||||
| Kerguelen | 49 S | 70 E | Dec. 95 | CN, 512 | Dec. 99 | CN, 512 | Dec. 99 | ||||
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Dumont D'Urville |
67 S | 140 E | Jan. 88 | UP, 512 | Jul. 92 | UP, 512 | Dec. 93 | CN, 1024 |
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