A new two-layer reactive plume model is developed, in which arbitrary crosswind resolution of the emission field of each precursor is preserved, and dynamic plume-plume and plume-background interactions are explictly accomodated. The model has a hybrid formulation, having Lagrangian downwind transport and Wulerian crosswind spread. It is applied in a diagnostic mode to simulate the observed behavior of plumes of the metropolitan St. Louis area and the Labadie power plant. The RAPS emissions inventory gave detailed spatial resolution of the emission field, numerous stationary and mobile upper air wind soundings provided the basis for transport simulation, and aircraft data provided detailed crosswind profiles of pollutant concentrations across the plumes at downwind sections. Model simulations of ozone were generally good, even in crosswind detail, given an appropriate background characterization. Simulated values of the rate of SO2 oxidation were quantitatively not as satisfying.