Figure 4.1 - Daily relative response factors as a function of daily maximum base modeled

concentrations for monitors in the Baltimore nonattainment area.

The analysis examined daily RRFs, but in practice, the RRFs are not calculated on a daily

basis30. A mean RRF is calculated based on the mean base case concentration (across model

days) divided by the mean future case concentration (across the same model days). As such, we

also calculated mean RRFs using various minimum concentration thresholds. The minimum

concentration thresholds examined ranged from 70-85 ppb in 5 ppb increments31. The

30Although it is recommended to calculate mean RRFs, examination of daily RRFs can

provide important information. Information gleaned from daily RRFs can be used to examine

model response under different meteorological regimes. Additionally, days that are particularly

unresponsive to emissions controls can indicate problems in the model inputs and/or formulation

and may also provide a focus for the development of control strategies. For example, controlling

specific upwind sources may provide benefits on particular days that appear to be otherwise


31It was clear from the plots that a minimum threshold value of less than 70 ppb would

not be appropriate. An upper threshold of 85 ppb was examined because it is equal to the

NAAQS. We are generally concerned about the model response on days that exceed the



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