in the past. Also, there may be ways of dealing with potential problems posed by using larger

than desired grid cells. For example, use of plume-in-grid algorithms for large point sources of

NOx should be considered as an alternative with coarser than desired grid cells. And dispersion

modeling of primary PM can be used to offset the need of fine spatial resolution in a grid model.

The relative importance of using a domain with grid cells as small as 4 km should be

weighed on a case by case basis by those implementing the modeling/analysis protocol. The

most important factor to consider is model response to emissions controls. Analysis of ambient

data, sensitivity modeling, and past modeling results can be used to evaluate the expected

response to emissions controls at various horizontal resolutions for both ozone and PM2.5. If

model response is expected to be different (and more accurate) at higher resolution, then higher

resolution modeling should be considered. If model response is expected to be similar at both

high and lowe(er) resolution, then high resolution modeling may not be necessary.

In this guidance, we identify upper limits for horizontal grid cell size which may be

larger than desired for some applications. This is intended to provide flexibility for considering

competing factors (e.g., number of modeled days versus grid cell size) in performing a modeling

analysis within the limits of time and resources.



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