modeled with a dispersion model or a combination of grid and dispersion models (see Section


We expect that modeling analyses for nonattainment areas will use grid cell sizes of 12

km or less. If a regional scale model is applied, most of the domain will likely cover

rural/remote areas or locations which are not out of compliance with the NAAQS. For the

regional outer nest of the domain, grid cells as large as 36 km may be used.

15.3.3 Horizontal Resolution Recommendations for Regional Haze

The glidepath analysis for regional haze focuses on Class I areas. Most of these are in

remote or rural locations. Regional haze is not likely to be dominated by local sources. Further,

light extinction is more likely to be dominated by secondary particulate matter, due to the

efficiency of light scattering by secondary particles. All these attributes indicate that it will be

necessary to model a regional scale domain for regional haze related applications. Because of

the remoteness of Class I areas, grid cell sizes up to 36 km on a side should suffice for regional

haze-related modeling. States may wish to perform diagnostic tests using plume-in- grid

analyses, as well as finer horizontal resolution to determine if results may differ using more

finely resolved emissions and meteorology. Guidance on vertical resolution presented in Section

15..4 is also applicable for regional haze-related applications.



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