days are likely to be relatively low, differences in the computed mean visibility for “best” days

are likely to be small. Further, any resulting difference in the progress analysis for “best” days is

likely to be protective of the environment. If our recommended procedure leads to suspected

problems in the outcome of a test, a State may perform a more rigorous version of the analysis

(in which the identity of pre-control and post-control days changes) as part of a weight of

evidence determination.

Selecting predictions to use in deriving RRF. Relative response factors should be

developed for each Class I area. When a Class I area contains a monitoring site, the RRF

estimates should be derived using predictions which are made “near” that site. “Near” is defined

in Section 3.3. For each day, daily average surface predictions of each component of PM made

near a monitor should be estimated. These nearby estimates should then be spatially averaged to

estimate a spatially representative daily concentration. Spatially representative daily

concentrations obtained for each modeled day with monitored data should then be averaged.

This final average should be used to compute the RRF. Thus, component-specific RRF values

for a Class I area with a monitor are the ratio of the temporally averaged spatial mean of nearby

concentrations predicted with future emissions to that predicted with baseline emissions. The

recommended procedure is illustrated in section 3.3 for a grid whose cells are 12 km on a side.

Similar to the PM2.5 NAAQS attainment test, nearby grid cells should be averaged. Note that for

cells larger than 15 km on a side, no spatial averaging is necessary—States should just use the

prediction in the cell containing the monitor.



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