Airborne measurements of the spectrally resolved actinic flux (280--420 nm) between the ground and 12 km altitude have been made using a new calibrated dual-channel spectroradiometer. The measurements were made as part of the Photochemical Activity and Ultraviolet Radiation/Altitude Dependence of the Tropospheric Ozone Photolysis (PAUR/ATOP) measurement campaign in Greece during June 1996. Flights were made over the Agean Sea under cloudless conditions for various aerosol loads and solar zenith angles. The spectral actinic flux measurements are compared with radiative transfer model simulations based on the multistream discrete ordinate radiative transfer (DISORT) algorithm. All input to the radiative transfer model was provided by independent measurements performed simultaneously on the nearby island of Agios Efstratios. For altitudes between 3000--12,000 m the agreement between the measurements and the model simulations is within ¿5% for wavelengths larger than 310 nm and within ¿10% at 295--310 nm. For the lowest flight altitude, 108 m, the model underestimates the measured actinic flux systematically by about 12%. This may be partly explained by uncertainties in the aerosol optical properties and the surface albedo. Flights on days with small and large amounts of aerosols showed that under otherwise identical conditions the actinic flux increased by up to 10% when the aerosol amount was larger.