15.3.1 Horizontal Resolution Recommendations for Ozone

For coarse portions of regional grids, we recommend a grid cell size of 12 km if feasible,

but not larger than 36 km. For urban and fine scale portions of nested regional grids, it may be

desirable to use grid cells about 4 km, but not larger than 12 km. States/Tribes should examine

past model applications and data analyses for their area when choosing the fine grid resolution.

Past model applications and data analyses may help determine whether a grid cell size as small

as 4 km (or smaller) is necessary for a particular area. Model performance and the relative

response to emissions controls should be considered in the decision. States/Tribes should

consider diagnostic tests to assess the difference in model performance and response from

varying model grid resolution, particularly in oxidant-limited areas.

All ozone monitor locations within a nonattainment area should ordinarily be placed

within the fine scale portion of a nested regional grid if nested models are used. States/Tribes

choosing an urban grid or fine portion of a nested grid with cells 12 km or larger should consider

applying plume-in-grid algorithms to major point sources of NOx. The use of plume-in-grid

should be discussed with the appropriate EPA Regional Office.

15.3.2 Horizontal Resolution Recommendations for the PM2.5 NAAQS

The formation of secondary particulate matter shares the same basic photochemical

processes as ozone. In fact, they are closely interconnected in many respects. Since there

appears to be little difference in RRF estimates for 8-hour ozone with 12 km vs. 4 km grid cells,

we would expect this finding to hold for secondary components of particulate matter. But what



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