California Code, Health and Safety Code - HSC § 39610

(a) Not later than December 31, 1989, the state board shall identify each air basin, or subregion thereof, in which transported air pollutants from upwind areas outside the air basin, or subregion thereof, cause or contribute to a violation of the state ambient air quality standard for ozone, and shall identify the district of origin of the transported air pollutants based upon the preponderance of available evidence. The state board shall identify and determine the priorities of information and studies needed to make a more accurate determination, including, but not limited to, emission inventories, pollutant characterization, ambient air monitoring, and air quality models.

(b) The state board shall, in cooperation with the districts, assess the relative contribution of upwind emissions to downwind ozone ambient air pollutant levels to the extent permitted by available data, and shall establish mitigation requirements commensurate with the level of contribution. In assessing the relative contribution of upwind emissions to downwind ozone ambient air pollutant levels, the state board shall determine if the contribution level of transported air pollutants is overwhelming, significant, inconsequential, or some combination thereof. Any determination by the state board shall be based upon a preponderance of the available evidence.

(c) The state board shall make every reasonable effort to supply air pollutant transport information to heavily impacted districts prior to the development of plans to attain the state ambient air quality standards, shall consult with affected upwind and downwind districts, and shall adopt its findings at a public hearing.

(d) The state board shall review and update its transport analysis at least once every three years.

(e) The state board shall conduct appropriate studies to carry out its responsibilities under this section.