The UV-VIS-NIR spectral optical properties of soot and soot containing aerosols were investigated in detail during the AIDA Soot Aerosol Campaign 1999. One aim of the campaign was a comprehensive comparison of the microphysical properties of Diesel and spark generator soot. The mass specific extinction cross section at lambda = 450 nm of Diesel soot is 10.6 +/- 0.5 m(2) g(-1) which is almost a factor of two larger than the corresponding value of 5.7 +/- 0.3 m(2) g(-1) measured for spark generator soot. Coagulation-induced particle growth does not affect the soot extinction cross section and has a weak influence on the scattering properties of the soot aggregates. Atmospheric processing of freshly emitted soot was simulated in mixing experiments. The formation of mixed Diesel soot and dry ammonium sulfate particles by coagulation has only a minor effect on the soot absorption cross section. The coating of spark generated soot with organic material results in a strong increase of the single scattering albedo. A significant increase of the absorption coefficient at lambda = 473 nm during the coating process can be attributed to an enhancement of the specific soot absorption cross section by more than 30%. (C) 2003 Elsevier Ltd. All rights reserved.