A multiscale air quality model has been developed to follow accurately and efficiently the long‐range transport of pollutants emitted from urban areas. The model employs a two‐dimensional finite element scheme to follow the horizontal transport. The multiscale capability is obtained by using local finite element refinements. The model is applied to a 3‐day intensive measuring period (August 27–29, 1987) over southern California. Uniform and nonuniform grid systems are employed in the simulations. Uniform grid systems used resolutions of 5×5 km2, 10×10 km2, and 20×20 km2, horizontally. Two nonuniform grid systems are used. The first combines the 5×5 km2 grid over urban Los Angeles with the 10×10 km2 grid over the rest of the domain, while the second uses the 5×5 km2 grid followed by the 10×10 km2 grid and 20×20 km2 grid over the ocean and sparsely populated areas. Uniform coarsening of the grid impacted the urban areas by diluting NOx emissions, leading to higher levels of urban ozone predictions. The impact on regional areas was complex. The ozone being transported downwind was not followed as accurately as on the fine grid and the regional levels usually decreased. On the other hand, the artificial dilution of NOx emissions had the adverse effect of increasing regional ozone levels. Therefore grid coarsening sometimes leads to what appears to be satisfactory levels of regional ozone for the wrong reason. By using fine grids over the source areas, more satisfactory predictions of both urban and regional ozone levels were obtained. This is also computationally more efficient than using uniform fine grids over the entire domain.
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.
The BRAVO monitoring network provided an opportunity to conduct a comprehensive evaluation of CMAQ-MADRID over a 4-month period. Tracer simulations revealed uncertainties in the model representation of advection and diffusion processes and the effects of uncertainties in meteorological fields on transport simulations. Results improved with the implementation of a more diffusive horizontal diffusion scheme. The 12-km resolution provided better results than the 4-and 36-km resolution simulations for 15-25 August 1999 and the 36-km resolution provided better results for 5-15 October. Model performance for tracers suggests that as currently formulated, grid-based Eulerian models are not well suited to simulate the impacts of long-range transport of individual point source emissions at specific receptors. Nonetheless, they are suitable for resolving the contributions of source regions that may contain multiple area and point sources. Using a 36-km resolution for a 4-month simulation, the model performance was good in comparison with contemporary models for sulfate (the major PM 2.5 component) in the region of interest (i.e., BBNP), with a low bias and coefficient of determination better than 0.5. However, the model overestimated sulfate and total sulfur significantly in other parts of the modeling domain. For organic particulate matter (OM, the second most prevalent PM 2.5 component at BBNP), the model correctly reproduced the dominance of secondary organic aerosols and explained most of the variance in the OM concentrations; however, it underestimated OM concentrations consistently. Model performance was poor for the less prevalent components of PM 2.5 (i.e., nitrate and black carbon) at BBNP. Diagnostic analyses suggest that the discrepancies between model simulation results and observations are due not only to limitations in the model formulation but also to uncertainties in the model inputs, including emissions, meteorology, and boundary conditions.
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