A case study of nesting simulation for the Southern Oxidants Study 1999 at Nashville

A regional Eulerian chemical transport model took simulations for a case of Southern Oxidants Study at Nashville from 30 June to 1 July 1999. NCAR/Penn State MM5 provided meteorological information to drive this model with a basic horizontal resolution of 20 km. Both vertical and horizontal nesting techniques were applied to the chemical simulations, and significantly improved the simulations near the ground during nighttime by enhancing the resolution. The multi-scale simulations with 20, 4 and 1.333 km resolutions were performed, and compared to the aircraft and ground-based observations. Generally, the 4 and 1.333 km simulations are similar, but differ much from the 20 km simulation. One important effect of enhancing the resolution is reducing the dilution of NOx emissions at urban sites or near power plants. The 20 km simulation underestimated the pollutant emissions near urban sites and power plants due to its coarse resolution. The high-resolution simulations generally show better agreement with the observations. Simulated results are also compared to the surface observations at Cornelia Fort, a suburban site near Nashville. The associated chemical features, including photostationary state and ozone production efficiency, were discussed through comparisons, which showed that the finest simulation yielded the most consistent results with the observations. The nesting simulations also agree with the measurements in downtown Nashville. Due to the different resolutions, these three simulations showed different aging patterns of the Nashville urban plume.