If it is not feasible to model an entire year, then RRFs are likely to be relatively stable if
at least 10 worst days (and 10 best days) are modeled (similar to the ozone recommendations). If
the worst days occur throughout the year, then more time periods may need to be modeled.
Worst days that occur in the same season are more likely to have similar PM components and are
therefore likely to respond in a similar way.
14.2 What Additional, Secondary Criteria May Be Useful For Selecting
Time Periods to Model?
In Section 14.0, we noted that there may often be conflicts among the four primary
criteria recommended as the basis for choosing episodes to model. Several additional, secondary
selection criteria may be helpful for resolving these conflicts.
Choose time periods which have already been modeled. That is, of course, provided
that past model performance evaluation for such a time period was successful in showing that the
model worked well in replicating observations. Given that the four primary criteria are met
approximately as well by such time periods they are by other candidate time periods, a
State/Tribe could likely save a substantial amount of work in evaluating model performance.
However, large changes in emissions and precursor levels or ratios may make the use of older
time periods undesirable.
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