Classifications and reclassifications.

(a) Initial classification as Moderate PM 2.5 nonattainment area. Any area designated nonattainment for a PM 2.5 NAAQS shall be classified at the time of such designation, by operation of law, as a Moderate PM 2.5 nonattainment area. (b) Reclassification as Serious PM 2.5 nonattainment area. A Moderate nonattainment area shall be reclassified to Serious under the following circumstances: (1) The EPA shall reclassify as Serious through notice-and-comment rulemaking any Moderate PM 2.5 nonattainment area that the EPA determines cannot practicably attain a particular PM 2.5 NAAQS by the applicable Moderate area attainment date. (2) A Moderate PM 2.5 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 PM 2.5 NAAQS by the applicable Moderate area attainment date.