In past guidance, we have recommended using horizontal grid cell sizes of 2-5 km in

urban scale ozone modeling analyses (U.S. EPA, 1991a). Sensitivity tests performed by Kumar

(1994) in the South Coast Air Basin compared hourly base case predictions obtained with 5 km

versus 10 km versus 20 km grid cells. Results indicate that use of finer grid cells tends to

accentuate higher hourly ozone predictions and increase localized effects of NOx titration during

any given hour. However, statistical comparisons with observed hourly ozone data in this

heavily monitored area appear comparable with the 5 and 20 km grid cells in this study.

Comparisons between hourly ozone predictions obtained with 4 km vs. 12 km grid cells have

also been made in an Atlanta study (Haney, 1996). As in Los Angeles, the use of smaller (i.e., 4

km) grid cells again leads to higher domain wide maximum hourly ozone concentrations.

However, when reviewing concentrations at specific sites, Haney (1996) found that for some

hours concentrations obtained with the 12 km grid cells were higher than those obtained with the

4 km cells. Other studies have shown that model performance does not necessarily improve with

finer resolution modeling. In one study, the model performance at 12 km resolution was equal

to or better than performance at 4 km resolution (Gego, 2005).

Another important aspect in choosing the horizontal grid cell size is the relative response

of the model at various grid cell resolutions. Recent sensitivity tests comparing relative response

factors in predicted 8-hour daily maxima ozone near sites in the eastern United States indicate

relatively small unbiased differences (< .04, in 95% of the comparisons) using a grid with 12 km

vs. 4 km grid cells (LADCO, 1999; Arunachalam, 2006). The largest differences in the relative

response of models at varying resolution is likely to occur in oxidant limited areas. The

horizontal resolution may have a large impact on the spatial distribution and magnitude of NOx

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