The partial differential equation that describes the size and composition distribution of atmospheric particles is stated. The equation describes the processes that may influence the particulate size and composition, namely emissions, deposition, advection, turbulent diffusion, condensation, evaporation, coagulation, nucleation, settling and heterogeneous chemical reactions. Each term in the equation is analysed to estimate its influence on the overall distributions under typical urban conditions. Numerical methods are developed to solve the equation in conjunction with an Eulerian gas-phase model.