Transport of atmospheric aerosol by air masses is considered on the basis of a realistic model taking into account almost all factors influencing the particle size distribution: turbulent diffusion, energy-momentum thermo-hydrodynamic balance, the relief of the substrate surface, location of the sources of aerosols and the particle growth due to coagulation process. After formulating the set of basic equations the procedure of their numerical solution is described. The model is applied for reproducing the aerosol distribution over the town of Bratsk in the summer period. The results are compared with the concentration levels of submicron and highly disperse aerosols measured at several points over Bratsk during a two-week summer expedition in 1990. The model is shown to give reasonable information on the aerosol size spectra in the lower layer of the atmosphere at heights up to 500 m.