Modeling aerosol formation and photochemistry in point source plumes with the CMAQ plume-in-grid system

The nitrogen oxides (NO_x) and sulfur oxides (SO₂) emitted from point source stacks are important precursors of a variety of photochemical oxidants (e.g., O₃) and secondary aerosol species. A pollutant plume emanating from a major point source exhibits limited dimensions after release and its growth is gradual, being strongly governed by meteorological processes. In contrast, a point source plume is often a subgrid scale feature within a typical Eulerian regional grid modeling domain which exhibits rather large grid cell sizes ( 20 km or greater). Consequently, point source emissions are subjected to excessive dilution when instantly mixed into an entire grid cell volume when applying the traditional Eulerian grid modeling approach. 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 aerosol/chemical 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 which lead to horizontal and vertical plume expansion. This realistic treatment of plume growth and mixing processes allows photochemistry and aerosol formation to evolve at the proper temporal and spatial scales. The PinG model utilizes the same chemical mechanisms (CB-IV, SAPRC99) and aerosol module applied in the CMAQ chemical transport model (CTM). Since the PinG algorithm is integrated into the Eulerian grid framework of the CMAQ/CTM model, boundary conditions are provided by the CTM grid concentrations adjacent to each plume edge. A contiguous plume is simulated with hourly emissions released into a new plume section. The PinG module resolves the detailed horizontal internal structure of each plume section by an attached set of plume cells. Once the plume section width reaches the grid cell size, the PinG simulation ceases and a feedback of plume material into the CTM grid is performed.

Simulations with the current CTM/PinG modeling system were conducted on a regional modeling domain with a 36 km horizontal grid spacing during summer periods in 1995 and 1999, which coincided with the Southern Oxidant Study (SOS) field program. Point sources generating emissions of NO_x ≥ 75 tons/day or SO₂ ≥ 150 tons/day were designated for the PinG treatment. Thus, a variety of emission mixtures of NO_x and SO₂ existed among the 77 point sources targeted for the PinG treatment. Results to be presented will focus on the evolution of selected aerosol species (sulfate, nitrate and ammonium)and photochemical gas species (e.g., O₃, HNO₃, hydroxyl radical (OH)) in various point source plumes. Model results demonstrating the influences of release time and the SO₂/NO_x emission ratio on aerosol formation and photochemical production will be highlighted. Initial results for sources with comparable SO₂ emissions reveal that aerosol sulfate (SO₄) formation appears to be inhibited in the highest NO_x-rich plumes compared to low NO_x plumes, which is supported by experimental plume data. Available SOS plume study data will also be compared to PinG modeled species concentrations. (Disclaimer - Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.)