Effects of aerosol optical depth and single scattering albedo on surface UV irradiance

Spectral measurements of solar global and direct UV irradiance are routinely conducted at Thessaloniki, Greece with a double monochromator Brewer spectroradiometer. From absolutely calibrated direct and global irradiance spectra, aerosol optical depth and diffuse irradiance spectra between 300 nm and 365 nm are derived. The modification of the global and diffuse irradiance and of the relationship between direct and diffuse irradiance by the aerosol single scattering albedo (SSA) is investigated with respect to aerosol optical depth and solar zenith angle using radiative transfer model calculations. Model calculations were used also to investigate the effect of SSA on the radiance distribution in the ultraviolet. The model-derived relations are compared with measurements, with the aim to establish an indirect method of deriving an effective single scattering albedo from spectral measurements of the direct and global irradiance. The uncertainties introduced by different sources into the SSA estimates are discussed. Finally the effective SSA is determined for two days with different aerosol amounts and composition. The overall accuracy in determining indirectly the SSA depends strongly on the amount of aerosols varying between 0.1 and 0.2 units of SSA.