The Regional Acid Deposition Model (RADM) has been applied to several of the field experiments that were part of the Acid Models Operational and Diagnostic Evaluation Study (Acid MODES) to assess the model's ability to simulate photochemical production of ozone in regions dominated by point source emissions. The comparison of model simulations at different grid resolutions suggests that increased resolution improves the simulation of ozone photochemistry in such regions. Further analysis of NOx, and HOx concentrations and photochemical production rates of ozone, however, show that the model's response to large point source emissions is very unsystematic both spatially and temporally. This is due to the models inability to realistically simulate the small scale (subgrid) gradients in precursor concentrations in and around large point source plumes. Because of the inherently nonlinear nature of ozone photochemistry with respect to concentrations of NOx and VOC, ozone formation rates in model grid cells depend enormously on grid resolution, dispersion rates (primarily wind speed and mixed layer height), chemical background (VOCs and radicals) and NOx emission rates.