The APRAC-1A diffusion model was developed as a versatile and practical model for computing the concentrations of pollutants at any point within a city. The model calculates pollutant contributions from diffusion on various scales, including: extra-urban diffusion, mainly from sources in upwind cities; intra-urban diffusion from freeway, arterial, and feeder street sources; local diffusion of emissions within a street canyon. The model treats only carbon monoxide (CO), a relatively inert gas in the atmosphere but an important pollutant in terms of health. Motor vehicles are the major source of this gas. (Author/JR)