The effect of variable tropospheric aerosol conditions on UV radiation is analyzed using both model simulations and UV radiation measurements. A sensitivity study of the aerosol effect on UV radiation is performed by numerical modeling with a focus on the dependence on wavelength, solar zenith angle, and detector geometry. Potential day to day variability in atmospheric aerosols produces changes of spectral integrated radiation quantities of the order of 20% to 45%. Equivalent effects are induced by total ozone variations between 40 Dobson units (DU) and 90 DU for those spectral integrals having a high sensitivity in the UV-B wavelength range. At least 80% of the aerosol effect is caused by typical variations in aerosol optical depth and single scattering albedo. The variation of other aerosol optical properties such as phase function, spectral extinction, and extinction profile is of minor importance. Our results indicate that information on both aerosol optical depth and single scattering albedo is necessary in most cases for an adequate description of aerosol impact on UV radiation. The sensitivity study supplies a tool for the comparison of aerosol effects derived from various UV radiation measurements with respect to spectral weighting and/or detector geometry. The results of the sensitivity study contribute to the discussion of a local aerosol study performed using measurements we made at Hoher Peissenberg, Germany. Two years of ozone and nitrogen dioxide photolysis frequency measurements are analyzed with respect to aerosol-induced radiation changes in the UV-B and UV-A wavelength region. A procedure is introduced to extract the aerosol influence from the impact of other atmospheric parameters, particularly from the ozone amount for investigations in the UV-B. A comparison of the derived aerosol effect with model simulations shows good agreement. ¿ 1999 American Geophysical Union