Air quality Legal Authority

NEPA of 1969: 42 U.S.C. §4321 et seq (Summary of the National Environmental Policy Act), see also 23 CFR §771 FHWA regulations (Environmental Impact and Related Procedures, and 40 CFR §1500-1508 (CEQ regulations)
CAA Amendments (United States Code, Title 42, Chapter 85): 1977 CAA Amendments of 1977, Evolution of the Clean Air Act, CAA Amendments of 1990, Amendments to the Clean Air Act
40 CFR §50, National Primary and Secondary Ambient Air Quality Standards
40 CFR §51, Requirements for Preparation, Adoption, and Submittal of Implementation Plans
40 CFR §93, Determining Conformity of Federal actions to State or Federal Implementation Plans
40 CFR §93.126, Exempt Projects; 40 CFR §93.127, Determining Conformity of Federal Actions to State or Federal Implementation Plans; and 40 CFR §93.128, Determining Conformity of Federal Actions to State or Federal Implementation Plans
FHWA Updated Interim Guidance on MSAT Analysis in NEPA Documents on October 18, 2016

Minnesota Environmental Policy Act: Minnesota Statutes, section 116D, Environmental Policy