Respondent South Coast Air Quality Management District (District)--the California subdivision responsible for air pollution control in the Los Angeles metropolitan area--enacted six Fleet Rules prohibiting the purchase or lease by various public and private fleet operators of vehicles that do not comply with requirements in the Rules. Petitioner Engine Manufacturers Association sued the District and its officials, claiming that the Fleet Rules were pre-empted by §209 of the federal Clean Air Act (CAA), which prohibits the adoption or attempted enforcement of any state or local "standard relating to the control of emissions from new motor vehicles or new motor vehicle engines," 42 U. S. C. §7543(a). In upholding the Rules, the District Court found that they were not "standard[s]" under §209 because they regulate only the purchase of vehicles that are otherwise certified for sale in California, and distinguished decisions of the First and Second Circuits pre-empting similar state laws as involving a restriction on vehicle sales rather than vehicle purchases. The Ninth Circuit affirmed.

Held:

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