40 C.F.R. § 81.12
The National Capital Interstate Air Quality Control Region (District of Columbia, Maryland, and Virginia) consists of the territorial area encompassed by the boundaries of the following jurisdictions (including the territorial area of all municipalities (as defined in section 302(f) of the Clean Air Act, 42 U.S.C. 1857h(f) ) geographically located within the outermost boundaries of the area so delimited);
District of Columbia
In the State of Maryland: Montgomery County; Prince Georges County.
In the State of Virginia: Arlington County; Fairfax County; Loudoun County; Prince William County.
(As so delimited, the Virginia portion of the region will include the city of Alexandria, the city of Fairfax, and the city of Falls Church.)
40 C.F.R. §81.12