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The Clean Air Act (CAA) section 111(d) directs the EPA to promulgate a procedure “similar” to that provided by CAA section 110, under which states submit 111(d) plans for regulatory implementation to the EPA. In 1975, EPA addressed this requirement by promulgating “implementing regulations” under 40 CFR part 60, subpart B. These implementing regulations contain, among other things, deadlines for the submission of, and for EPA’s action on, “state plans”, as well as deadlines for the promulgation of related “federal plans.” In 2019 the EPA finalized 40 CFR part 60, subpart Ba, a new subpart that updated the implementing regulations for prospective emission guidelines. However, the United States Court of Appeals for the District of Columbia Circuit (in American Lung Ass’n v. EPA, No. 19-1140) vacated the Subpart Ba state and federal plan timelines due to a finding of inadequate justification. Amendments to the timelines in subpart Ba, consistent with the court vacatur, and additional updates and tools to aid in implementation of emission guidelines were proposed in the Federal Register on Dec. 23, 2022 (87 FR 79176). The public comment period closed on Feb. 27, 2023. This action will respond to comments and finalize amendments. The docket number for this rulemaking is EPA-HQ-OAR-2021-0527.