The Advanced Statistical Trajectory Regional Air Pollution (ASTRAP) model simulates long-term transport and deposition of oxides of sulfur and nitrogen. It is a potential screening tool for assessing long-term effects on regional visibility from sulfur emission sources. However, a rigorous evaluation is required before the model can be recommended for the particular application. As a first step, the authors modified the original 1985 IBM-3033 version of the model to create the ASTRAP-EPA version for applications on the EPA VAX-8600 computer using existing EPA preprocessed meteorological and emissions data files. Additional modifications improved the model design by eliminating several model assumptions and replacing some modeling approaches. The cumulative effect of the model modifications was assessed by comparing the quarterly 1980 calculations of sulfur wet deposition of both versions with screened measurements. The seasonal correlation coefficients and standard errors of each model version are insignificantly different at the 0.05 level, demonstrating that the two model versions indeed produce similar results. In general, the improvements in model design only slightly enhance model performance. Sensitivity of ASTRAP-EPA calculations of sulfur wet deposition was also assessed for several model assumptions and values of model parameters. ASTRAP-EPA model predictions are most sensitive to three parameters - the model time step, the truncation of trajectories near the border of wind-data-void regions, and the temporal aggregation of ensemble trajectory statistics. The maximum quarterly predictions of sulfur wet deposition, across southwestern Pennsylvania and northern West Virginia, decrease by as much as 30% when either the model time step changes from 3 hours to 6 hours, or when trajectories are not truncated, or when trajectory statistics are not temporally aggregated.