Chemical kinetics and atmospheric transport processes are combined in this finite difference formulation to compute concentration histories for photochemical smog. The kinetics are expressed as a lumped parameter reaction mechanism which was derived from laboratory data in the literature. Diffusion coefficients, which depend on height and on time, come from atmospheric measurements. Inputs consist of source inventories for the Los Angeles basin and the solar irradiation curves for the appropriate days. Predicted time histories of reactive hydrocarbons, oxides of nitrogen and ozone are consistent with the variations observed at an air monitoring station. With refined descriptions of advection, the mathematical model will serve as a tool in planning legislation and guiding urban planning in the future. (Author)