recognizing some leeway in the precision of the predicted location of daily average PM2.5 and

daily maximum ozone concentrations. Finally, standard practice in defining a gridded modeling

domain is to start in the southwest corner of the domain, and determine grid cell location from

there. Considering several cells “near” a monitor rather than the single cell containing the

monitor diminishes the likelihood of inappropriate results which may occur from the geometry of

the superimposed grid system.

Earlier ozone modeling guidance (U.S. EPA,1996a) has identified 15 km as being “near”

a site. This is also consistent with the broad range of intended representativeness for urban scale

ozone monitors identified in 40CFR Part 58, Appendix D. As noted in section 1.3, secondary

particulate matter is likely to constitute an important fraction of PM2.5. Further, a major purpose

of urban monitoring performed to determine attainment of the NAAQS for PM2.5 is to estimate

likely exposure to PM2.5 concentrations over 24-hour periods. For PM2.5, sites having

“neighborhood” or “urban” scales of representativeness are generally most suitable for

estimating exposure or compliance with the NAAQS. Because of the rather long sampling time

(i.e., 24 hours) inherent in the PM2.5 NAAQS, locations within the lower range of “urban” scale

(4-50 km) would seem consistent with a definition of “near” a site. Therefore, we recommend

following the same ground rules used for defining “near” as we originally defined for ozone

modeling (U.S.EPA, 1999a).



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