A long-term analysis of turbidity data, obtained by operating a multiwavelength sun-photometer at a mountain station in northern Italy, has been carried out to investigate the influence of the El Chich¿n volcanic eruption on the optical characteristics of the atmosphere. An estimate of the vertical load of aerosol particles indicates that it exceeds 2--12 times the normal background value for the station. Aerosol size distributions, obtained by inversion of spectral optical thickness measurements, show a clearly biomodal nature during the period of greater volcanic influence, with the appearance of a distinct large-particle mode centered 0.4 and 0.5 μm, revealing the presence of sulphate aerosol particles of volcanic origin in the stratosphere above the station. Sun-photometric measurements prove to be an effective technique for the analysis of the stratospheric aerosol characteristics and their influence on the Earth's radiation budget. The eight wavelengths of the instrument provide a whole series of spectral information which is not ordinarily obtainable from lidar measurements and leads to specific considerations on the suspended aerosol and its variations in time. ¿ American Geophysical Union 1988