[1] Since the discovery of the stratospheric ozone depletion in the 1980s [World Meteorological Organization/United Nations Environment Programme (WMO/UNEP ), 1994;Solomon, 1999;Staehelin et al., 2001], the interest of its measurement has substantially increased. Indeed, this gas highly absorbs the dangerous UV radiation and preserves the biosphere. Long series of measurements of total ozone column and of solar UV radiation are needed to define climatologies of both UV and ozone and to study the impact of the ozone depletion at the Earth surface. Total ozone column is determined at Villeneuve d'Ascq, France, by a differential absorption method, using global spectral UV measurements performed with a Jobin Yvon spectroradiometer by the Laboratoire d'Optique Atmospherique. This method is based on the comparison between two irradiance ratios, one simulated beforehand and stored in a lookup table (LUT) and one calculated from the measured spectrum [Stamnes et al., 1991]. This paper presents the method used to improve the choice of the different irradiances used to calculate the ratios and presents the five irradiance ratios that are finally kept. Some sensitivity tests are performed in order to find the validity range of a LUT and the number of LUTs needed. These tests reveal that the aerosol Å ngström coefficient, the aerosol optical depth, and the atmospheric vertical profiles may induce errors in the ozone retrieval. That makes it necessary to calculate 21 LUTs and to select one of them depending on the season and on the aerosol properties. Then an estimation of the uncertainty of the method is performed, enabling us to conclude that the method leads to approximately 3% relative uncertainty. The results for the clear days of the last 3 years at Villeneuve d'Ascq are presented, as well as a comparison with Total Ozone Mapping Spectrometer data. In most of the cases, both values agree to within 5%, but some cases present larger relative differences.

[1] Spectral UV global irradiance has been measured regularly during the year 2000, in the French Alps, using two UV spectroradiometers. The total ozone column amount is retrieved from the ratio of irradiances around 305 and 340 nm, using look up tables based on modeling data. During cloudless days, ozone values from direct sun observations agree to within 5 DU. These ground based values compare reasonably well (to within about 4%, or 12 DU) with TOMS ozone data; this is consistent with the uncertainty of both methods which is estimated to 5 DU for TOMS, and 5 -7 DU for clear sky ground based measurements. Generally the ground based ozone values seem to be slightly higher than the TOMS ozone values. For a few cases, the difference between TOMS and ground based values, can reach 10 to 15%. Different possible explanations of the discrepancies are considered.

[1] A differential absorption method, developed by the Laboratoire d'Optique Atmosphérique (LOA), is applied to retrieve the total ozone column from UV global irradiance spectra under clear and cloudy sky conditions. Contrarily to clear sky, cloudy sky generates a high uncertainty in the retrieved ozone column. This study shows that under cloudy conditions the daily ozone mean is a rather good estimation of the true value. The standard deviation allows us to estimate the relative uncertainty of this mean value, i.e., about 7%. Results for all conditions from 3 years at Villeneuve d'Ascq are presented, as well as a comparison with TOMS (Total Ozone Mapping Spectrometer) data. The results are very similar to those obtained for clear days: In 75% of cases both values agree to within 5%; however, some cases present larger relative differences. Over the 3 years considered, there is a bias less than 3% (LOA > TOMS). This method is applied to five other European sites involved in the EDUCE (European Database for UV Climatology and Evaluation) project. Three of them are Brewer stations. The results of the comparison between the LOA-retrieved ozone and TOMS data show that a relative difference smaller than 5% is obtained in 63-80% of the cases, depending on site and year. Biases smaller than 3% are observed (LOA > TOMS). The comparison between the LOA-retrieved ozone and total ozone data from direct sun and zenith sky observations of the Brewer instruments shows better agreement. For more than about 80% of cases, except at one site, the relative difference is smaller than 5%, and the biases are smaller than 1%. The LOA method allows us to obtain a complementary data set and hence to provide time series of reliable measurements of total ozone column under all sky conditions. Citation: Brogniez, C., M. Houët, A. M. Siani, P. Weihs, M. Allaart, J. Lenoble, T. Cabot, A. de la Casinière, and E. Kyrö (2005), Ozone column retrieval from solar UV measurements at ground level: Effects of clouds and results from six European sites, J. Geophys.

Abstract.A one week field campaign took place in September 2002 at El Arenosillo, Spain. The objective was to compare total ozone column (TOC) and aerosol optical depth (AOD) from near ultraviolet to near infrared, measured by several Spanish and French instruments. Three spectroradiometers, Brewer, SPUV02, and LICOR, and a CIMEL photometer, have been used simultaneously and the results are presented for four clear days. TOC values are given by the Brewer instrument, and by SPUV02, using two different methods. The ground instruments compare satisfactorily (within 5 DU) and the values are consistent with TOMS data (within 10 DU).AOD from the various instruments are compared at seven different wavelengths between 320 nm and 1020 nm: the agreement is very good at 350, 380, and 870 nm; at the four other wavelengths the difference is smaller than 0.03, which can be explained by a relative difference of 4% only between the calibrations of the various instruments. Larger AOD diurnal variations were observed at short wavelengths than in the visible and near infrared; this is most likely due to changes in aerosol size along the day, during the campaign.