Development and application of a new air pollution modeling system—II. Aerosol module structure and design

The methods used for simulating aerosol physical and chemical processes in a new air pollution modeling system are discussed and analyzed. Such processes include emissions, nucleation, coagulation, reversible chemistry, condensation, dissolution, evaporation, irreversible chemistry, sedimentation, dry deposition, and radiative scattering and absorption by particles. A new particle size bin structure that nearly eliminates numerical diffusion during growth but still treats nucleation, emissions, coagulation, and transport realistically is discussed. In addition, coagulation is shown to reduce the number and volume concentration of particles less than 0.2 μm in diameter both in the presence and absence of modest rates of particle growth. However, when significant growth occurs, the effect of coagulation is reduced. Further, while sulfate production due to SO2 dissolution and oxidation in cloud drops is confirmed to be important, it is shown here that such production in aerosols is small over time periods simulated in urban air pollution models. Finally, light scattering and absorption coefficient predictions, obtained by applying a Mie code for stratified spheres, are discussed and shown to match data for a given scenario. Remaining processes in the aerosol module are described.