The Environmental Protection Agency (EPA) is taking final action to determine that the Plumas County nonattainment area failed to attain the 2012 annual fine particulate matter (``PM<INF>2.5</INF>'') national ambient air quality standard (NAAQS or ``standard'') by the December 31, 2021 ``Moderate'' area attainment date. This determination is based on ambient air quality monitoring data from 2019 through 2021. With this final determination, Clean Air Act (CAA or ``Act'') section 188(b)(2) requires that the nonattainment area be reclassified to Serious by operation of law. Within 18 months from the effective date of the reclassification to Serious, the State must submit a revision to its State Implementation Plan (SIP) that complies with the statutory and regulatory requirements for Serious PM<INF>2.5</INF> nonattainment areas.