- how to estimate base period air quality in a Class I area without monitored data;

- how to handle a base year without data or with a small sample size;

- how to consider a day with missing data for one or several components of PM;

- use of the same days to estimate changes in visibility for the worst days and a different set of

same days to estimate changes in visibility for the best days;

- which predictions to use to derive relative response factors;

- how many and what kind of days to use to develop RRF values

- use of relative response factors which are specific for days with poor vs. days with good


-alternative RRF calculations

Estimating baseline “worst” and “best” visibility in a Class I area without monitors.

There are 156 Class I areas and there are 110 IMPROVE sites in or near Class I areas.

Therefore, approximately 46 Class I areas do not have co-located IMPROVE monitors. EPA’s

“Tracking Guidance” recommends using IMPROVE data from a nearby site to represent the

visibility at each Class I area that does not have ambient data. Table A-2 in Appendix A (of the



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