If the Administrator determines, based on the testing under paragraph (1), that urban buses subject to the particulate matter (PM) standard applicable pursuant to subsection (b) do not comply with such standard in use over their full useful life, he shall revise the standards applicable to such buses to require (in addition to compliance with the PM standard applicable pursuant to subsection (b)) that all new urban buses purchased or placed into service by owners or operators of urban buses in all metropolitan statistical areas or consolidated metropolitan statistical areas with a 1980 population of 750,000 or more shall be capable of operating, and shall be exclusively operated, on low-polluting fuels. The Administrator shall establish the pass-fail rate for purposes of testing under this subparagraph.

The Administrator shall promulgate a schedule phasing in any low-polluting fuel requirement established pursuant to this paragraph to an increasing percentage of new urban buses purchased or placed into service in each of the first 5 model years commencing 3 years after the determination under subparagraph (A). Under such schedule 100 percent of new urban buses placed into service in the fifth model year commencing 3 years after the determination under subparagraph (A) shall comply with the low-polluting fuel requirement established pursuant to this paragraph.

The Administrator may extend the requirements of this paragraph to metropolitan statistical areas or consolidated metropolitan statistical areas with a 1980 population of less than 750,000, if the Administrator determines that a significant benefit to public health could be expected to result from such extension.