Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass-burning plume were performed on board a research aircraft during a profile descent over a ground-based site in northeastern Greece (40¿24'N, 23¿57'E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm-1, and the particle absorption coefficient was around 15 Mm-1, giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6--9% of the total accumulation mode particle mass (<1 ¿m diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude-dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short-wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground-based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short-wave radiative forcing (daytime average) between 280 nm and 4 ¿m is up to -64 W m-2 at the surface and up to -22 W m-2 at the TOA.