Opinion ID: 187435
Heading Depth: 3
Heading Rank: 3

Heading: Second Factor: Use of Design Values

Text: Under the second of the nine factors, which looks to air quality, EPA uses monitoring data to compute a design value to describe the concentration of ambient PM2.5 in a county. See supra at 30. The agency then compares the design value to the annual NAAQS to help determine whether the county is violating PM2.5 standards or contributing to violations nearby. See id.; EPA Br. 14. State petitioners argue that EPA arbitrarily designated some counties as nonattainment despite their relatively low design values and other counties as attainment despite their higher design values. For example, EPA designated Hardin County, Kentucky, as attainment even though its design value of 14.1 was higher than the 12.5 design value for Westchester County, New York, which was designated as nonattainment. See Technical Support Document § 6.4.3.2 (Factor 2); id. § 6.2.2 (Factor 2). As EPA explained, however, design values alone do not determine designations based on contribution. Indeed, they are merely one component, albeit an important one, of a complex process that ultimately yields designations. Petitioners' argument ignores the multiple factors EPA uses in making case-by-case assessments of counties' contributions to nearby violations. See Holmstead Memo Guidance at 4. Some areas, like Hardin County, may have relatively high design values but still fall short of violating the annual PM2.5 standard of 15 micrograms per cubic meter. And many such counties are not contributing areas because they have low rankings for other factors like population, traffic, and emissions levels. See, e.g., Technical Support Document § 6.4.3.2 (explaining Hardin County's attainment designation). The three areas state petitioners use as examples of inconsistent treatment  Nassau, Suffolk, and Westchester Counties in New York  may have had lower design values when compared to other attainment areas, but each ranked high for emissions levels, population, and number of commuters, all of which support the determination that they contributed to nearby violations. See id. § 6.2.2. In short, EPA had ample evidence upon which it based its designation of these counties as nonattainment, despite their relatively low design values.