The aerosol ultraviolet optical depth observed at a station in the French Southern Alps

Ultraviolet spectral irradiances are systematically recorded in Briancon (at an altitude of 1310 m), in the French Southern Alps. Up to now, aerosols optical depth (AOD) had never been routinely measured in Briancon, so it has appeared of great interest to take advantage of the many spectra recorded to learn more about aerosols over the spectral range: 320 nm - 448 nm. To retrieve AOD, one only has to remove the contribution of ozone and molecule from the total optical depth, obtained from the direct irradiance. The method has been implemented for every spectra acquired during clear sky days selected among the period from July 2004 to March 2005. Data recorded show that at a given wavelength, AOD is roughly constant during the day. Lowest values for AOD at 12:00 U. T. are found to be around 0.05 at 440 nm and 0.15 at 320 nm, and may increase threefold for some days. AOD range between 320 and 448 nm is larger in summer than in winter. This agrees with the way α, the Angstrom parameter, varies from one season to the other. Unfortunately, one can only make a restricted use of α, as its determination over the ultraviolet range is subject to high AOD uncertainties. The main source of AOD error rests in the 10% of uncertainty on direct irradiance measured. This gives rises to relatively high uncertainties on AOD, which increase all the more when aerosols load in the atmosphere is low. In Briancon, renowned for its air goodness, true AOD values are likely to be reached within 45% - 50% as the best in a majority of cases.