A long-range transport model of SO2 and sulfate has been developed. The model consists of two major programs: (1) program that calculates the trajectories of plumes leaving from any chosen source area (s), dispersion parameters, and the parameters necessary for concentration calculations along trajectory segments using observed wind data. (2) A program that calculates the concentrations of SO2 and sulfate along trajectory segments and interpolates the plume concentrations to grid intersections. In the model the effect of the diurnal change of mixing layer height is taken into consideration. Trajectories of mean winds of the nighttime and daytime mixing layer, and trajectories of the mean wind of the layer between daytime mixing height and nighttime mixing height, are included. Removals, owing to dry deposition and precipitation, are also included. The model is applied to calculate the distributions of 24-hour average concentrations of SO2 and sulfate over the eastern United States. The distribution patterns of calculated concentrations of SO2 and sulfate were statistically related to those of observed concentrations.