In 1984 the U.S. EPA initiated a Mesoscale Acid Deposition Study. The study was organized to assess and understand the interactions and importance of local emissions in mesoscale precipitation scavenging. One important component of the project was the simulation of the field events using a detailed meteorological and chemical modeling framework (called MesoSTEM). A dynamical meteorological model, Mesoscale Atmospheric Simulation System (MASS) was used to predict the meteorological fields, including precipitation rates. These fields are used as inputs to a comprehensive chemical model Sulfur Transport Eulerian Model (STEM-II). The three-dimensional models are used to quantify the relationships between emissions, chemical production, and wet deposition on the mesoscale. The linkage of the models, and the application of MesoSTEM to the May 2nd and 3rd, 1985 Philadelphia Mesoscale Field Study data set is the subject of the report.