(a) This section applies only to a county that borders the United Mexican States and in which a municipality with a population of 500,000 or more is located.
(b) A county that was at any time required, because of the county's designation as a nonattainment area under Section 107(d) of the federal Clean Air Act (42 U.S.C. Section 7407), to participate in the vehicle emissions inspection and maintenance program under this subchapter and Subchapter F, Chapter 548, Transportation Code, shall continue to participate in the program even if the county is designated as an attainment area under the federal Clean Air Act.