8-Hour Ozone NAAQS
Given that a model application produces a time series of estimated 1-hour ozone
concentrations (which can be used to calculate running 8-hour averages), what values should be
chosen from within the time series? We recommend choosing predicted 8-hour daily maximum
concentrations from each modeled day (excluding “ramp-up” days) for consideration in the
modeled attainment test. The 8-hour daily maxima should be used, because they are closest to
the form of concentration specified in the NAAQS.
The second decision that needs to be made is, “which one(s) of the 8-hour daily maxima
predicted in cells near a monitor should we use to calculate the RRF?” We recommend choosing
the nearby grid cell with the highest predicted 8-hour daily maximum concentration with
baseline emissions for each day considered in the test, and the grid cell with the highest
predicted 8-hour daily maximum concentration with the future emissions for each day in the test.
Note that, on any given day, the grid cell chosen with the future emissions need not be the same
as the one chosen with baseline emissions.
We believe selecting the maximum (i.e., peak) 8-hour daily maxima on each day for
subsequently calculating the relative response factor (RRF) is preferable for several reasons.
First, it is likely to reflect any phenomenon which causes peak concentrations within a plume to
migrate as a result of implementing controls. Second, it is likely to take better advantage of data
produced by a finely resolved modeling analysis.
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