Retrieval of downwelling UV actinic flux density spectra from spectral measurements of global and direct solar UV irradiance

In the present study, a method is proposed for retrieving UV actinic flux d ensity spectra at ground level from global and direct UV spectral irradianc e measurements. The most important parameters to establish the relation bet ween downwelling actinic flux density and global UV irradiance are the rati o of direct to global irradiance and the description of the angular distrib ution of the diffuse radiation field. Under clear skies, retrieved spectral actinic flux densities agreed with model calculations to within +/-5% for a variety of aerosol conditions. The assumption that the diffuse radiation is isotropic leads to a wavelength-dependent overestimation of actinic flux densities from 10 to 15%, depending on solar zenith angle, especially in a tmospheres with high aerosol content. Various sources of uncertainties, whi ch are involved in the proposed methodology, are discussed and quantified.