To establish a basis for validation of an urban airshed model based on a macroscale grid, a sub-grid, or microscale, model was undertaken capable of predicting the elevation in concentration at the monitoring station, above background levels, that is contributed by local automotive emissions. The development and validation of a simple diffusion model for CO to investigate the feasibility of modeling at two spatial scales is described. Limited results are reported for four LA County stations. Previous efforts in measuring and predicting local variations in concentrations due to nearby sources were reviewed. A general description of the model is given, details of its formulation for application in the vicinity of low structures (one to two story buildings) are discussed and graphs are presented which display the relationship between the magnitude of concentration elevation and the distance of the sampling probe from the street, the speed and direction of the local wind, and the magnitude of the local turbulent eddy diffusivity.