The unmonitored area analysis should identify areas where future year design values are

predicted to be greater than the NAAQS. The unmonitored area analysis for a particular

nonattainment area is intended to address potential problems within or near that nonattainment

area. The analysis should include, at a minimum, all nonattainment counties and counties

surrounding the nonattainment area (located within the State). In large States, it is possible that

unmonitored area violations may appear in counties far upwind or downwind of the local area of

interest. In those cases, the distance to the nonattainment area and ability of the modeling to

represent far downwind areas should be evaluated on a case by case basis. In order to examine

unmonitored areas in all portions of the domain, it is recommended to use interpolated spatial

fields of ambient data combined with gridded modeled outputs.

3.4.1 Why does the unmonitored area analysis need to use both ambient

data and model output?

Ambient data can be interpolated to provide a set of spatial fields. The spatial fields will

provide an indication of concentrations in monitored and unmonitored areas. But a simple

interpolation of the ambient data cannot identify unmonitored areas with higher concentrations

than those measured at monitors. The interpolated concentration between monitors will

generally be the same or lower than the measured concentration at the monitors (assuming that

more sophisticated statistical techniques are not used, such as adding a nugget effect or a trend

surface). The interpolation technique does not account for emissions or chemistry information

that may be needed to identify potential unmonitored violations.


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