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On April 30, 2018, the United States Environmental Protection Agency (EPA) signed the final notice to designate all remaining counties that were not previously designated in the November 16, 2017 final rulemaking as non-attainment, attainment/unclassifiable, or unclassifiable in regards to the 2015 ozone NAAQS of 70 parts per billion (ppb). This standard was reduced from the previously enforceable 2008 ozone NAAQS of 75 ppb.

The following areas in Arizona were designated as marginal non-attainment:

  • Phoenix-Mesa area, including the following:
    • Gila County (partial);
    • Maricopa County (partial);
    • Pinal County (partial); and
    • Various Indian Reservations.
  • Yuma area, including the following:
    • Yuma County (partial) and
    • Various Indian Reservations.

All other counties in Arizona were designated as attainment/unclassifiable. The non-attainment designations are expected to become final 60 days after the final rulemaking is published in the federal register.