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 EPA's 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 proposes to address those two remanded parts of the 2020 final rule: EPA's conclusion that Colorado's infrastructure SIP submission met the State's good neighbor obligation under Clean Air Act (CAA) section 110(a)(2)(D)(i)(I); and EPA's 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 proposing to approve Colorado's infrastructure SIP submission pursuant to CAA section 110.