The Environmental Protection Agency (EPA) is taking final action to determine that the West Pinal County, Arizona nonattainment area (``West Pinal County'' or ``area'') did not attain the 1987 24- hour national ambient air quality standards (NAAQS or ``standards'') for particulate matter with a diameter of 10 micrometers or smaller (PM<INF>10</INF>) by its December 31, 2022 ``Serious'' area attainment date. This action is based on the EPA's calculation of the PM<INF>10</INF> design value for the nonattainment area over the 2020- 2022 period, using complete, quality-assured, and certified PM<INF>10</INF> monitoring data. With this final determination that West Pinal County has failed to attain the PM<INF>10</INF> NAAQS by its attainment date, the State of Arizona is required to submit a revision to the Arizona state implementation plan (SIP) that, among other elements, provides for expeditious attainment of the PM<INF>10</INF> standards and for a five percent annual reduction in the emissions of direct PM<INF>10</INF> or a PM<INF>10</INF> plan precursor pollutant in the nonattainment area.