A subgrid-scale plume model has been developed for a better treatment of dynamics of the emissions from concentrated emission sources in air quality models. The model is based on the Gaussian model description of the dispersion of NOx emissions from the power plants. Detailed inorganic and, if desired, organic atmospheric chemistry is included in the model. The plume model has been interfaced with the Urban and Regional Multiscale (URM) model and has been used to quantify the impact of subgrid-scale plume modeling on evolution of ozone and other species in the Northeast. Various simulations were performed in the northeastern United States where there are a large number of power plants over the domain. First, a case was chosen where only one large point source was followed using the subgrid plume model to better identify the local impacts of subgrid treatment. Two simulations were performed for this case; the first included a detailed inorganic chemistry in the plume, whereas the second included only simple NO-NO2-O3 chemistry. A second case was chosen where all point sources emitting more than 25 tons NOx per day were followed independently using the plume model to study the regionwide impact on predicted ozone. In addition to the two plume treatments used in the test above, a third was added that followed the plume chemistry using a full organic plus inorganic mechanism. In this case, the limited-NOx, detailed inorganic and detailed inorganic plus organic chemistry, are compared with the base case where the power plant emissions were injected directly into the airshed grid. Using the subgrid-scale plume model for large point sources had significant local impacts on predicted ozone concentrations, but regionwide impact was very small (less than 2%). Results from the simulation with the detailed organic chemistry found little difference than the detailed inorganic chemistry, though significant local differences were found between those and the simplified NOx chemistry or the calculation without the subgrid-scale treatment. ¿ American Geophysical Union 1996