about primary components of particulate matter? Because there is no chemistry, we expect

spatial gradients in concentrations to be much higher for primary particulate matter than for

secondary particulate matter. This follows, because it takes some time (less than 24 hours) for

the chemistry to occur. Although it is clear that spatial resolution of primary PM will impact the

predicted concentrations, it is not clear how it will impact the relative change in concentrations

due to controls (RRFs). Areas without strong gradients in primary PM will likely have little

benefit from fine scale resolution. Areas that have large gradients in primary PM may need to

use finer resolution grid models or may need to supplement the grid modeling with dispersion

modeling (see Section 5.3) . This particularly true if violating monitors are strongly impacted by

one or more large sources of primary PM.

The bottom line of the preceding discussion is that we feel comfortable recommending

that States may use grid cell sizes as large as 12 km for urban scale applications addressing

secondary components of particulate matter. We are less sure about an acceptable upper limit

for cell size in applications addressing primary components. We believe it is prudent to assume

that, in some cases, cells as small as 4 km (or possibly smaller) are needed. Those implementing

the modeling/analysis protocol may wish to perform a diagnostic test using a grid model without

chemistry to see whether estimated RRF’s for primary components are affected if one decreases

the grid cell size from 12 km to 4km. Alternatively, large sources of primary PM can be


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