40 C.F.R. § 81.162
The Northeast Plateau Intrastate Air Quality Control Region (California) consists of the territorial area encompassed by the boundaries of the following jurisdictions or described area (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: Lassen County, Modoc County, Siskiyou County.
Shasta County-that portion of Shasta County which lies east and north of a line described as follows: Beginning at the Shasta-Siskiyou County boundary and running south along the range line common to R. 2 E. and R. 1 E., Mt. Diablo Base and Meridian, to the southwest corner of T. 35 N., R. 2 E; then east along the township line common to T. 35 N. and T. 34 N. to the northwest corner of T. 34 N., R. 3 E.; then south along the range line common to R. 3 E. and R. 2 E. to the southwest corner of T. 33 N., R. 3 E.; then east along the township line common to T. 33 N. and T. 32 N. to the northwest corner of T. 32 N., R. 4 E.; then south along the range line common to R. 4 E. and R. 3 E. to the point of intersection with the northwest corner of the Lassen Volcanic National Park boundary; then east along the north boundary of Lassen Volcanic National Park to the point of intersection with the Lassen-Shasta County boundary.
40 C.F.R. §81.162