40 CFR §51.1002

1. (a)Initial classification as Moderate PM2.5 nonattainment area. Any area designated nonattainment for a PM2.5 NAAQS shall be classified at the time of such designation, by operation of law, as a Moderate PM2.5 nonattainment area.
2. (b)Reclassification as Serious PM2.5 nonattainment area. A Moderate nonattainment area shall be reclassified to Serious under the following circumstances:
   1. (1)The EPA shall reclassify as Serious through notice-and-comment rulemaking any Moderate PM2.5 nonattainment area that the EPA determines cannot practicably attain a particular PM2.5 NAAQS by the applicable Moderate area attainment date.
   2. (2)A Moderate PM2.5 nonattainment area shall be reclassified by operation of law as a Serious nonattainment area if the EPA finds through notice-and-comment rulemaking that the area failed to attain a particular PM2.5 NAAQS by the applicable Moderate area attainment date.