California Code, Health and Safety Code - HSC § 40002

(a) There is continued in existence and shall be, in every county, a county district, unless the entire county is included within the Antelope Valley district, the bay district, the Mojave Desert district, the south coast district, the Sacramento Metropolitan Air Quality Management District, the San Joaquin Valley Air Quality Management District, if that district is created, a regional district, or a unified district.

(b) If only a part of the county is included within the Antelope Valley district, the bay district, the south coast district, the Mojave Desert district, the San Joaquin Valley Air Quality Management District, if that district is created, a regional district, or a unified district, there is in that part of the county not included within any of those districts a county district, for which different air quality rules and regulations may be required.