Measured Spectra of Solar Ultraviolet Irradiances at Athens Basin, Greece

From a data archive of spectral energy distribution of global and diffuse solar irradiances measured in Athens during a field experiment, the impacts of changes in solar zenith angle, site altitude, and gaseous-aerosol pollutants loading, on spectral composition of ultraviolet (UV) radiation reaching the ground, has been determined for cloudless conditions. Measurements of spectral energy distribution of ultraviolet irradiance showed that in urban atmospheres there is a significant altitude effect on spectral UV irradiances which is more pronounced on shorter wavelength UVB than on longer wavelength UVA. In particular the largest attenuation in the UVB band, produced by the altitude effect, between the non-urban site of Mt. Hymettus and the urban site of Athens Museum, was about 27%; while the corresponding attenuation in the UVA band reached 20%. Correspondingly, the respective attenuation caused by altitude effect in the diffuse UVB band was 12%; whereas the altitude effect increases UVA band to as much as 9% between the summit of Mt. Hymettus and the urban site of Athens Museum. Depletion of UV irradiances by the urban atmosphere of downtown Athens was strongly related to aerosol-gaseous pollutants loading. The spectral UV measurements were found to be sensitive to both, changes in solar zenith angle and atmospheric turbidity. The spectral ratio of diffuse-to-direct irradiance critically depends on both solar zenith angle and aerosol-gaseous pollutants loading, increasing rapidly toward the shorter wavelengths. Finally, the hypothesis that the increased levels of aerosol-gaseous pollutants may act as a filter to the transfer of UV energy to the ground is supported, by the limited set of spectral measurements used in the present work.