Emissions of nitrogen oxides and sulfur oxides from the tall stacks of major point sources are important precursors of a variety of photochemical oxidants and secondary aerosol species. Plumes released from point sources exhibit rather limited dimensions and their growth is gradual, being strongly governed by meteorological dispersion processes. However, in the typical regional Eulerian grid modeling framework, a point source plume is actually a subgrid scale feature since grid cell sizes are generally 20 km or greater. On the other hand, plume widths do not generally achieve this horizontal dimension for a considerable distance/time after being emitted. Consequently, point source emissions are subjected to excessive dilution when instantly mixed into the entire grid cell volume when the traditional Eulerian grid modeling approach is applied. Therefore, a plume-in-grid (PinG) technique has been integrated into the state-of-science Community Multiscale Air Quality (CMAQ) chemical transport model to specifically provide a realistic treatment of the dynamic and chemical/aerosol processes impacting pollutant concentrations in major point source plumes.

The CMAQ/PinG model applies a Lagrangian plume technique to simulate the relevant atmospheric dispersion processes governing vertical and horizontal plume expansion. This more realistic treatment of plume growth allows photochemistry and aerosol formation to evolve at the proper spatial and temporal scales. The PinG model utilizes the same chemical mechanisms (i.e., CB-IV, SAPRC-99) and aerosol module used by the Eulerian grid chemical transport model (CTM). During a simulation, the CTM grid concentrations provide boundary conditions at the plume edges in the PinG model. In the PinG model, a continuous plume is simulated by hourly emissions released into a new plume section. The PinG model resolves the detailed horizontal internal structure of each plume section by an attached set of plume cells. Once a plume section reaches the grid size, its subgrid PinG simulation ceases and a feedback of plume material into the CTM grid is performed.

Simulations with the updated CTM/PinG modeling system, which is capable of treating both photochemical gases and aerosol species were conducted with a 36 km grid cell size on a continental U.S domain. Point sources generating emissions greater than 75 tons/day of nitrogen oxides or greater than 150 tons/day of sulfur dioxide were designated for the PinG treatment. Model runs were made for summer periods in July 1999 and 2001. The July 1999 period coincided with the Southern Oxidant Study (SOS) field program and comparisons to available plume data will be highlighted. Results will focus on selected photochemical species and certain aerosol species (i.e. sulfate, nitrate, and ammonium) in various point source plumes during the subgrid scale phase. In addition, results of model runs made without using the PinG technique will be compared to simulation results with the PinG treatment to examine the impact of these different modeling methods on grid concentrations from large point source emissions. Disclaimer - The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.