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.

Step 1

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

weighted average.

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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