States will be able to use the EPA-provided software or are free to develop alternative techniques
that may be appropriate for their areas or situations.
The first step in the analysis is to interpolate ambient data. Ideally, design values should
be interpolated. The same 5 year weighted average design values that are used in the monitor
based model attainment test can be used in the development of ambient spatial fields. Care
should be taken so that the interpolated fields are not unduly influenced by monitoring sites that
do not have complete data. Since the design values can vary significantly from year to year, it is
important to use a consistent set of data. In some cases, it may be preferable to interpolate
individual years of data or individual design values, and then average those up to get the 5 year
We are not recommending a single interpolation technique. EPA has provided example
analyses in the past using the Kriging interpolation technique (U.S.EPA, 2004c). Alternatively,
EPA’s BenMAP software, which was used to create interpolated fields for the Clean Air
Interstate Rule (CAIR) uses the Voronoi Neighbor Averaging (VNA) technique (Abt, 2003)22.
For ozone analyses, a single spatial field of ozone values is needed. But for the PM2.5
NAAQS, we recommend creating interpolated spatial fields for PM2.5 (FRM) and for each
component of PM2.5. For the annual PM2.5 NAAQS, a set of quarterly average spatial fields can
be created. The 4 quarters are averaged to get an annual average set of fields.
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