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This proposed rulemaking would update the regulations governing implementation of the Ozone National Ambient Air Quality Standards (NAAQS) listed in part 51 of the Code of Federal Regulations. These updates would reconcile regulatory provisions that were vacated as part of the decision in South Coast Air Quality Management District v. EPA , 882 F.3d 1138 (D.C. Cir. 2018) and Sierra Club v. Wheeler, 956 F.3d 612 (D.C. Cir. 2020). The regulatory provisions would also be updated to reflect additional developments in implementation of the ozone NAAQS since the promulgation of the last ozone implementation rule in 2018—Implementation of the 2015 NAAQS for Ozone: Nonattainment Area State Implementation Plan (SIP) Requirements (2015 SIP Requirements Rule) (83 FR 62998, December 6, 2018). The 2015 SIP Requirements Rule and the SIP Requirements Rule for the 2008 Ozone NAAQS (80 FR 12264, March 6, 2015, 2008) govern attainment planning requirements that apply to areas designated nonattainment for the applicable ozone NAAQS, and states in the Ozone Transport Region, as well as anti-backsliding requirements for areas once designated nonattainment for the revoked ozone NAAQS. This proposed action would clarify national policy by updating affected provisions in the ozone SIP Requirements Rules.