Note that if the episode is drawn from a recent time period (especially the three years

upon which the nonattainment designation is based), days which are chosen are likely to have

monitored observations very close to the baseline design value. In the absence of such

information, we suggest “+ 10 ppb” as a default recommendation for purposes of prioritizing

choice of episodes94. If the base and baseline/current periods do not coincide, “close to” is

within + 10 ppb of the design value during the base period straddling the episode. If it is not

feasible to meet this default criterion for all monitoring sites, meeting it at sites with

baseline/current design values > 85 ppb should receive greatest priority.

Choose days with intensive data bases.

Preference should be given to days with measurements aloft, available measurements of

indicator species (see Section 18) and/or precursor measurements. These preferences result from

a desire to incorporate a rigorous model performance evaluation as a part of the attainment

demonstration. This reduces the likelihood of “getting the right answer for the wrong reason”.

Thus, the likelihood of mischaracterizing ozone/precursor sensitivity is reduced.

94The analysis in section 4.1 showed that relative model response may differ on “low”

ozone days compared to days that are at or above the level of the NAAQS. Therefore, ambient

(and modeled) concentrations that are more than 10 ppb above the design value are preferable to

episodes with ambient concentrations that are more than 10 ppb below the design value.


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