Results of Photochemical Simulations of Subgrid Scale Point Source Emissions with the Models-3 CMAQ Modeling System

Notable aspects of major NOx and SOx point source plumes are their initially small dimensions and their finite growth rate. This diffusion-limited nature of plumes is in sharp contrast to the traditional method applied in Eulerian grid models, which has been to instantly mix point source emissions into an entire grid cell. This instantaneous overdilution of high NOx point emissions into large grid cells can have important implications on the photochemical production of ozone and other species. The subsequent spatial and temporal displacements of chemical processes contribute to model uncertainty. A need for a more realistic subgrid scale plume modeling technique within an Eulerian grid model has been recognized in order to treat the dynamic and chemical processes in plumes emanating from high-emitting point sources.

The plume-in-grid (PinG) approach embedded in the Models-3 Community Multi-scale Air Quality (CMAQ) modeling system has been applied to case study days from the July 1995 Southern Oxidant Study experimental period in the greater Nashville, TN region. Point sources with a range of NOx emission rates were simulated by the PinG model over the entire diurnal period. Modeled plume concentrations of ozone, NOx, SO2, and other species are compared to plume measurements from helicopter and aircraft flights through plumes at different downwind distances during afternoon periods. The CMAQ/PinG runs were made with both the CB4 and RADM2 chemical mechanisms in order to assess differences between these photochemical reaction approaches for high NOx point source emissions. Visualization with color plots of the subgrid plume concentrations and grid concentrations will also be shown.