The sectional aerosol behavior code COSIMA simulates the time evolution of the structural, dynamical, and optical properties of airborne agglomerate particles as well as their heterogeneous chemical interactions with reactive trace gases utilizing a formalism based on fractal scaling laws. The modeled processes include diffusion to the walls and sedimentational deposition, Brownian and gravitational coagulation, molecular transport from the gas phase to the accessible particle surface, surface adsorption and reactions, gas phase reactions, and dilution effects due to sampling (e.g. during aerosol chamber experiments). The effect of hydrodynamic interactions and shielding on particle mobility is considered within the framework of the Kirkwood-Riseman theory. Rayleigh-Debye-Gans theory is used to deal with light absorption and scattering. The code is validated against new experimental data on the dynamics of Diesel and graphite spark soot as well as recent theoretical and simulation results. Applying the Kirkwood-Riseman formalism to compute the mobility of fractal like agglomerates significantly enhances coagulation rates as well as wall and depositional loss but does not affect the form of the self preserving size distributions attained in the long time regime if Brownian coagulation dominates the aerosol dynamics. (C) 2003 Elsevier Ltd. All rights reserved.