The radiative transfer model
To interpret the observations carried out at Lampedusa a modified version the UVSPEC radiation transfer model [Mayer, B., G. Seckmayer, and A. Kylling, Systematic long-term comparison of spectral UV measurements and UVSPEC modelling results, J. Geophys. Res., 102, 8755– 8767, 1997] is used. The UVSPEC model allows the choice of one among three methods for the solution of the radiative transfer equation: the two stream, the discrete ordinate method (DISORT) for plane-parallel atmosphere, and the pseudospherical approximation of DISORT (SDISORT) which accounts for the sphericity of the atmosphere. We generally operate the model in six-stream mode for the calculation of the irradiance, and in 16 stream mode for the calculation of fluxes, and use the SDISORT solver. The model can simulate radiation quantities such as radiance, direct and diffuse irradiances, and actinic fluxes between 200 and 800 nm assuming an extraterrestrial solar spectrum and taking into account the atmospheric structure and composition, and the surface characteristics.
The atmosphere is divided into layers with constant gas concentration and aerosol optical properties. These properties may vary from layer to layer. Ozone is assumed to be the only absorbing gas. Thus large differences between the model and the observations are expected in the spectral regions where absorption bands by other species (mainly water vapor and molecular oxygen) are present.
The model inputs are solar zenith angle, total ozone, ground albedo, vertical profiles of pressure, temperature, air density, and ozone; aerosols are characterized by single scattering albedo, asymmetry factor, and optical depth at each layer. This study is limited to clear sky conditions, and clouds are not taken into account.
Two main modifications to the original model by Mayer et al.  have been implemented: an increase of the atmospheric vertical resolution, to allow a more detailed description of the tropospheric structure, and a new parameterization of the aerosol to include in the model the observed aerosol properties. Lidar observations show that large differences in the aerosol vertical distribution may occurr on different days, with maximum altitudes variable from 1 to 8 km [di Sarra, A., T. Di Iorio, M. Cacciani, G. Fiocco, and D. Fuà, Saharan dust profiles measured by lidar at Lampedusa, J. Geophys. Res., 106, 10,335-10,348, 2001]. Thus the atmosphere was divided in 64 homogeneous layers from 0 to 120 km with a vertical resolution of 150 m in the lowest 2.1 km, 300 m between 2.1 and 8.4 km, 1 km up to 25 km, and growing thickness up to 120 km.
The original version of UVSPEC includes four prescribed aerosol types (urban, rural, maritime, and tropospheric) for the lowest 2 km of the atmosphere. In our version of the model a specific aerosol size distribution, refractive index, and vertical profile can be defined.
The MODTRAN radiative transfer model version 4.2 [Berk, A., Bernstein, L.S., Anderson, G.P., Acharya, P.K., Robertson, D.C., Chetwynd J.H. & Adler-Golden, S.M. (1998).
MODTRAN Cloud and Multiple Scattering Upgrades with Application to AVIRIS. Remote Sens. Environ. 65, 367-375] has been used for the simulation of broadband shortwave
and longwave irradiances.
The model computes radiance and irradiances in the spectral range 0-50000 cm-1 with a resolution of 1 cm-1, including spherical refractive geometry, solar and lunar source functions, thermal and surface radiation, and scattering (Rayleigh, Mie, single and multiple), and default profiles (gases, aerosols, clouds, fogs, and rain).
In particular, 6 climatological atmospheric profiles (tropical, mid-latitude summer/winter, subarctic summer/winter, and US Standard for 6 atmospheric gases (H2O, CO2, O3, N2O, CO, CH4), plus single profiles for HNO3, NO, NO2, SO2, O2, N2, NH3 and the heavy molecules (CFC’s) are included.
Concerning aerosol profiles, both tropospheric (rural, urban, desert, Navy, fogs) and stratospheric (background, aged, high, fresh, and extreme) particles types can be selected. User-defined aerosol properties (extinction, absorption and asymmetry factor) can be defined up to 4 different atmospheric layers.
Prescribed cloud types are cumulus, altostratus, stratus, stratocumulus, nimbostratus, and cirrus (standard, subvisual and NOAA). User-defined cloud properties can be selected.
Spectral ground emittance and reflectance (Lambertian or BRDF) are accounted for.
Finally, spectral quantities can be weighted with filter function with the aim of reproducing specific instrument measurements.