On January 5, 2021, the United States Court of Appeals for the Tenth Circuit granted the Environmental Protection Agency's (EPA) motion for a voluntary remand without vacatur of two parts of an EPA 2020 final rule approving Colorado's infrastructure state implementation plan (SIP) submission for the 2015 8-hour ozone national ambient air quality standards (NAAQS) (2020 final rule). In this document, EPA is taking final action to approve those two remanded parts of the 2020 final rule. First, EPA is finalizing our conclusion that Colorado's infrastructure SIP submission meets the State's good neighbor obligations under Clean Air Act (CAA) section 110(a)(2)(D)(i)(I). Lastly, EPA is also finalizing our conclusion that Colorado's infrastructure SIP submission provided ``necessary assurances'' of the State's authority to regulate agricultural sources under CAA section 110(a)(2)(E)(i). EPA is taking this action pursuant to the CAA.