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In order to investigate the influence of the atmospheric aerosol on the ultraviolet radiation on earth, the measurement campaign Photochemical Activity and Ultraviolet Radiation (PAUR II) Modulation was carried out in the central Mediterranean Sea during the period May-June 1999. Two sites were chosen for measurements: the island of Crete (Greece), and the island of Lampedusa (Italy). The aerosol features over the Lampedusa island, as well as the dust coming from Sahara desert, were investigated by measurements of direct and diffuse solar irradiance carried out with an aureolemeter. The columnar volume size distributions of the aerosol showed a four-modal shape in a less turbid atmosphere when the aerosol optical depth was less than 0.2 at lambda, = 500 nm, and a tri-modal shape in a turbid atmosphere when the aerosol optical depth at the same wavelength was greater than 0.5; the background aerosol turned out to be mainly composed of sea salt. The increase of the aerosol optical depth and of the particles density with radius about 1 mum has been found to be strictly related to the passage of Saharan dust in the time periods 14-22 May and 1-3 June, 1999. The columnar volume of particles obtained by the aureolemeter has been compared with the columnar volume of particles retrieved by in situ measurements carried out with a forward scattering spectrometer probe (FSSP) aboard a light aircraft flying over the island. Although the above two techniques refer to aerosol columns of different height and operate with different resolutions, their relevant results are in good agreement, especially during days with lower aerosol content. The two volume radius distributions have been also compared and their behaviours show a satisfactory agreement, mainly for particles with radius greater than 1 mum. (C) 2001 Elsevier Science Ltd. All rights reserved.