The increasing amount of atmospheric health hazards is a matter of great concern. Severe accidents in nuclear reactors may be source terms for radioactive species-into the atmosphere. Radioactive fusion products (Cs, I, Te, Ba, etc.) may be released together with structural materials (Fe, Ni; Sn, etc.) and control materials (Ag, In, Cd). These materials will then be transported in the, form of aerosols from the reactor vessel possibly throughout the primary system and the containment into the atmosphere. Knowledge of size and chemical composition of the aerosol particles is necessary for the estimation of the released radioactivity: Existing descriptions assume uniform or very simplified compositions, but we introduce a Monte Carlo method to provide a real multicomponent description of the general dynamic equation (GDE). The evolution of size and composition distributions from coagulation, condensation and removal mechanisms is described in the development of the code MOSAIC. Calculations are performed for code validation and to compare with the simplified descriptions of the chemical composition. Large differences from the simplified descriptions occur for growth by condensation for gravitational deposition.