As of revocation of the 1997 ozone NAAQS on April 6, 2015, as set forth in § 50.10(c) , the provisions of subpart AA shall replace the provisions of subpart X, §§ 51.900 to 51.918 , which cease to apply except for § 51.907 for the anti-backsliding purposes of § 51.1105(c)(2) . See subpart X § 51.919 .
40 C.F.R. §51.1119