The daily distribution of sulfate concentration over the eastern United States during August 1977 is simulated by a Monte Carlo model using quantized emissions, positioned in accordance with the 1973 EPA SO2 emission inventory. Horizontal advection within a single well-mixed vertical layer is driven by observed surface winds, speeded by a factor of 2.5 and veered 20. Direct simulation of regional diffusion is implemented by random perturbation of each quantum's trajectory over each 3 h timestep, corresponding to K = 10 to the 5th power sq m/s. First order kinetics of SO2 to SO4(-2) transformation and deposition of SO2 and SO4 are simulated by probabilistic choice among the chemical and physical pathways over each step. The simulated spatial distributions are compared on a daily basis to measured sulfate concentration from the Sulfate Regional Experiment (SURE) and midday visual range observations, indicating the primary role of airmass residence time over the eastern United States in producing regional pollution. The light extinction coefficient, b(ext), and SURE SO4(-2) show remarkable spatial and temporal agreement throughout the month. The correlations of daily average SO4 concentration and b(ext) over the SURE region with the Monte Carlo simulation are 0.63 and 0.67, respectively. The correlation between daily average b(ext) and measured SO4(-2) is 0.84.