California Code, Health and Safety Code - HSC § 39608

(a) The state board, in consultation with the districts, shall identify, pursuant to subdivision (e) of Section 39607, and classify each air basin which is in attainment and each air basin which is in nonattainment for any state ambient air quality standard. This identification and classification shall be made on a pollutant-by-pollutant basis. Where the state board finds that data is not sufficient to determine the attainment or nonattainment status for an air basin, the state board shall identify the air basin as unclassified.

(b) The state board may assign an attainment, nonattainment, or unclassified designation to one or more areas within any air basin unless the state board finds and determines that the pollutant for which the designation applies affects the entire region or is produced by emission sources throughout the region.

(c) Designations made by the state board shall be reviewed annually and updated as new information becomes available.