INTRODUCTION This report describes a study conducted by TRC for the American Petroleum Institute (API) Task Force AQ-7@ Non-Reactive Pollutant Assessment. The performance of the Gaussian dispersion model has been evaluated for predicting the transport and diffusion of gases and fine particles in the atmosphere. Dispersion characteristics predicted by a number of alternative Gaussian modeling schemes have been compared with observed tracer dispersion for six different rural field programs. Each of these tracer programs provides measured concentration values for monitors deployed in a series of arcs at fixed distances from a nonbuoyant point source release. The results have been analyzed to characterize the uncertainties inherent in standard model predictions. Alternative modeling schemes have also been evaluated to identify possible improvements in models and to assess basic limitations to Gaussian model performance.