Cal. HSC § 40300

(a) The boards of supervisors of two or more counties within an air basin may hold a public hearing to determine whether the counties under their jurisdiction should become part of a regional district. (b) Such boards of supervisors shall hold a public hearing to resolve such a question, if a petition is submitted to each such board of supervisors. A petition submitted to a board of supervisors shall be signed by not less than 10 percent of the qualified electors of the county under its jurisdiction.