conditions which correspond to high measured concentrations.

Choose episodes having days with monitored 8-hour daily maxima close to observed

average 4th high daily maximum ozone concentrations.

We want to use episodes whose severity is comparable to that implied by the form of the

NAAQS (i.e., an episode whose severity is exceeded, on average, about 3 times/year at the time

of the selected episode). The objective is to choose episodes with days which are approximately

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.

143

as severe as the average 4th high 8-hour daily maximum concentration specified in the NAAQS.

As such, even if a full summer is being modeled, it is important to analyze the ambient data to

ensure that an adequate number of ozone conducive days are modeled.

Air quality measurements recorded during the baseline/current period can be used to

characterize episode severity. This is done by selecting a 5-year period which “straddles” a

modeled episode. For example, if an episode from 2002 were modeled, we recommend looking

at measured 8-hour daily maxima at each site in the nonattainment area during 2000-2004.

Using this information it should be possible to assess the relative severity of the days chosen for

modeling at each site. Limiting this characterization to the five years straddling an episode

avoids problems posed by long term trends in emissions in assessing episode severity. However,

it leaves unanswered the question of whether the 5-year period selected to assess severity of a

modeled day is typical or atypical. If there is an underlying long term trend in ambient ozone

attributable to meteorological cycles or emissions changes, it may not be appropriate to compare

different periods with one another using air quality observations. Thus, if one uses a 10-year old

episode with an exceptional data base, there is greater uncertainty in ranking its severity relative

to the current period of interest than if the episode were drawn from the current period.

 

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