Ground-based spectral measurements of actinic flux density (300--660 nm wavelength) and downward UV irradiance (300--324 nm) under cloudless conditions have been compared with the results of one-dimensional radiative transfer calculations employing concurrent airborne vertical profile measurements of aerosol particle size distributions. Good agreement (within ¿10%) between measured and calculated spectra was found. The remaining differences were explained by uncertainties inherent in the aerosol particle microphysical input data and the column ozone content. A respective sensitivity analysis of the calculated spectra, which was based on the observed variability of microphysical properties, has shown that the particle number concentration is the most crucial input uncertainty for both the actinic flux density and the UV irradiance. For the wavelength range investigated, the uncertainty of the column ozone content is of minor importance for both spectral quantities.