THE ATMOSPHERIC EXTINCTION AT THE ESO LA SILLA OBSERVATORY

The magnitude of the Earth atmospheric extinction has been determined during about 4'400 nights of astronomical photometric measurements, fr om the Swiss Telescope at the E.S.O. La Silla Observatory (Chile). Thi s has been done at optical wavelengths corresponding to the seven pass bands of the Geneva photometric system between November 1975 and Augus t 1994. The atmospheric extinction during each of these nights can be characterized by a mean value and a standard deviation, since these ni ghts are those of best astronomical quality, the so-called photometric nights. Two telescopes (40 cm and 70 cm), two photoelectric photomete rs and one CCD camera have been used on two different sites at La Sill a. The effects these changes of sites and instrumentation produce on t he long-term stability of the photometric system and, thus, on the det ermination of the atmospheric extinction coefficients, are analysed. T he extinction law at minimum extinction is determined and the relative contributions, at each wavelength, of the Rayleigh-Cabannes molecular diffusion, of the selective absorption by molecular bands (essentiall y ozone) and of the extinction due to aerosols (dust and various conde nsations, including water) are given. The long-term extinction variati on, due to the volcanoes El Chichon (1982, in Mexico) and The Pinatubo (1991, in The Philippines), is described. The maximum mean effect was more than twice larger for The Pinatubo (respectively 0.037 and 0.087 mag in the V band) and the extinction law from The Pinatubo aerosols, at the period of the observed maximum, was hater, or gray. The decant ation of the volcanic aerosols in the atmosphere was very slow, lastin g 1000-1300 days. The extinction law evolved with time, probably becau se of the increase of the mean aerosol size. Apart from these long-ter m effects due to the volcanoes,the minimum value of the atmospheric ex tinction remains relatively stable during the year, while the maximum values are reached during the southern Summer. The extinction law of t he meteorological aerosols is very precisely defined, k(p)(lambda) = b (p) lambda(-1.39), indicating that the distribution of the radii of th e particles, forming the atmospheric hare (mainly water droplets) duri ng the photometric nights, is quite stable and has its mode near the v alue 0.3 mu. There is a tendency to have series of a few (less than ab out five) consecutive nights with similar extinction values. This is t o be related to the typical duration of periods with similar meteorolo gical conditions at La Silla.