40 C.F.R. § 81.21
The San Francisco Bay Area Intrastate Air Quality Control Region 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):
In the State of California: Alameda County, Contra Costa County, Marin County, Napa County; San Francisco County, San Mateo County, Santa Clara County.
Solano County-that portion of Solano County which lies south and west of a line described as follows: Beginning at the intersection of the westerly boundary of Solano County and the 1/4 section line running east and west through the center of Section 34, T. 6 N., R. 2 W., M.D.B. & M., thence east along said 1/4 section line to the east boundary of Section 36, T. 6 N., R. 2 W., thence south 1/2 mile and east 2.0 miles, more or less, along the west and south boundary of Los Putos Rancho to the northwest corner of Section 4, T. 5 N., R. 1 W., thence east along a line common to T. 5 N. and T. 6 N. to the northeast corner of Section 3, T. 5 N., R. 1 E., thence south along section lines to the southeast corner of Section 10, T 3 N., R. 1 E., thence east along section lines to the south 1/4 corner of Section 8, T. 3 N., R. 2 E., thence east to the boundary between Solano and Sacramento Counties.
Sonoma County-that portion of Sonoma County which lies south and east of a line described as follows:Beginning at the southeasterly corner of the Rancho Estero Americano, being on the boundary line between Marin and Sonoma Counties, California; thence running northerly along the easterly boundary line of said Rancho Estero Americano to the northeasterly corner thereof, being an angle corner in the westerly boundary line of Rancho Canada de Jonive; thence running along said boundary of Rancho Canada de Jonive westerly, northerly and easterly to its intersection with the easterly line of Graton Road; thence running along the easterly and southerly line of Graton Road, northerly and easterly to its intersection with the easterly line of Sullivan Road; thence running northerly along said easterly line of Sullivan Road to the southerly line of Green Valley Road; thence running easterly along the said southerly line of Green Valley Road and easterly along the southerly line of State Highway 116, to the westerly line of Vine Hill Road; thence running along the westerly and northerly line of Vine Hill Road, northerly and easterly to its intersection with the westerly line of Laguna Road; thence running northerly along the westerly line of Laguna Road and the northerly projection thereof to the northerly line of Trenton Road; thence running westerly along the northerly line of said Trenton Road to the easterly line of Trenton-Healdsburg Road; thence running northerly along said easterly line of Trenton-Healdsburg Road to the easterly line of Eastside Road; thence running northerly along said easterly line of Eastside Road to its intersection with the southerly line of Rancho Sotoyome; thence running easterly along said southerly line of Rancho Sotoyome to its intersection with the township line common to Townships 8 and 9 North, Mt. Diablo Base and Meridian; thence running easterly along said township line to its intersection with the boundary line between Sonoma and Napa Counties, State of California.
40 C.F.R. §81.21