The radiative effects of tropospheric aerosols at the island of Lampedusa, in the Mediterranean, have been investigated by comparing measurements and results from a radiative transfer model. The model was used to reproduce the measured ultraviolet irradiance spectra (286.5--363 nm) in three cases of cloud-free conditions in May 1999, allowing the estimation of the aerosol properties and of the direct radiative forcing. Observations show very different aerosol loading and distribution, connected to the different origins of the air masses: low aerosol optical depths are associated with air masses from North Atlantic/Europe (25 and 27 May), and larger particles up to 7 km altitude and larger optical depths are observed for mineral dust coming from the Saharan region (18 May). The detailed description of the atmospheric structure and composition was used to initialize the radiative transfer model. The estimated single-scattering albedo and asymmetry parameter at 500 nm for the desert dust are in the range 0.73--0.84 and in the range 0.75--0.79, respectively. Radiative transfer calculations show that differences of the surface ultraviolet irradiance larger than 10% may arise from the lack of a detailed knowledge of the aerosol size distribution. Aerosol may also increase or reduce the absorption effectiveness of tropospheric ozone, depending on the characteristics of the particles. Estimates of the direct aerosol radiative forcing in the spectral range 300--800 nm at the surface and at the top of the atmosphere (TOA) were also derived. At the surface, aerosols produce a decrease of the instantaneous downward irradiance with respect to an aerosol-free atmosphere by 70.8, 37.2, and 39.1 W m-2, for 18 May (the aerosol optical depth is 0.511 at 415 nm), 25 (0.165) and 27 (0.224), respectively. The radiative forcing per unit optical depth at the surface is largest for aerosol of continental/marine origin, transported from North. The forcing at the TOA is negative, thus producing a cooling, in the desert dust case, and close to zero or positive for aerosol originating from North.