The ozonolysis of alpha-pinene in a large aerosol chamber was used to generate secondary organic aerosol (SOA) mass by homogeneous nucleation, or by heterogeneous nucleation, either on soot, or on (NH4)(2)SO4 seed aerosols. The rate of the alpha-pinene + ozone reaction and the aerosol yield of similar to 19% are in good agreement with literature data. The organic coating of soot particles leads to a compaction of the fractal agglomerates expressed by an increase in fractal dimension from 1.9 to 2.1 for Diesel soot, and from 2.0 to 2.3 for spark generated "Palas" soot. The dielectric coating of the soot particles with SOA layers between 2 to I I nm gives rise to a substantial enhancement of their single scattering albedo, from about 0.2 to 0.5, and increases the effective absorption coefficients of both soot types by ca. 30%. The coating of both soot types increases the hygroscopic growth factors (HGF) to values close below the HGF measured for pure SOA material d/d(0) similar to 1.12 at 90% RH. (C) 2003 Elsevier Ltd. All rights reserved.