Cite as: 540 U. S. 461 (2004)
Opinion of the Court
to section 7411 or 7412 of this title [emission standards for new and existing stationary sources]." § 7479(3).
40 CFR § 51.166(b)(12) (2002) (repeating statutory definition). Alaska's SIP contains provisions that track the statutory BACT requirement and definition. 18 Alaska Admin. Code §§ 50.310(d)(3) and 50.990(13) (2003). The State, with slightly variant terminology, defines BACT as "the emission limitation that represents the maximum reduction achievable for each regulated air contaminant, taking into account energy, environmental and economic impacts, and other costs." § 50.990(13). Under the federal Act, a limited class of sources must gain advance EPA approval for the BACT prescribed in the permit. 42 U. S. C. § 7475(a)(8).
CAA also provides that a PSD permit may issue only if a source "will not cause, or contribute to, air pollution in excess of any . . . maximum allowable increase or maximum allowable concentration for any pollutant" or any NAAQS. § 7475(a)(3). Congress left to the Agency the determination of most maximum allowable increases, or "increments," in pollutants. EPA regulations have defined increments for nitrogen oxides. 40 CFR § 51.166(c) (2002). Typically, to demonstrate that increments will not be exceeded, applicants use mathematical models of pollutant plumes, their behavior, and their dispersion. Westbrook, Air Dispersion Models: Tools to Assess Impacts from Pollution Sources, 13 Natural Resources & Env. 546, 547-548 (1999).
Among measures EPA may take to ensure compliance with the PSD program, two have special relevance here. The first prescription, § 113(a)(5) of the Act, provides that "[w]henever, on the basis of any available information, [EPA] finds that a State is not acting in compliance with any requirement or prohibition of the chapter relating to the construction of new sources or the modification of existing sources," 42 U. S. C. § 7413(a)(5), EPA may "issue an order prohibiting the construction or modification of any major stationary source in any area to which such requirement ap-
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