Spectral measurements of solar ultraviolet irradiance carried out near Thessaloniki, Greece, are analyzed with respect to very strong variations of aerosol optical depth while the other influencing parameters are about constant. When aerosol optical depth is increased about twofold, global UV irradiance (from sun and sky) decreases less than 10%. In contrast, direct irradiance (from sun only) is reduced about twofold. If spectral measurements of global irradiance only are made, it is not possible to distinguish between situations with highly different aerosol contents. Measurements of both, global and direct irradiance, appreciate additional information to adapt radiation transfer models to actual atmospheric aerosol conditions.