Health & Saf. Code, § 40914

(a) Each district plan shall be designed to achieve a reduction in districtwide emissions of 5 percent or more per year for each nonattainment pollutant or its precursors, averaged every consecutive three-year period, unless an alternative measure of progress is approved pursuant to Section 39607.

(b) A district may use an alternative emission reduction strategy which achieves less than an average of 5 percent per year reduction in districtwide emissions if the district demonstrates to the state board, and the state board concurs in, either of the following:

(1) That the alternative emission reduction strategy is equal to or more effective than districtwide emission reductions in improving air quality.

(2) That despite the inclusion of every feasible measure in the plan, and an expeditious adoption schedule, the district is unable to achieve at least a 5-percent annual reduction in districtwide emissions.

(c) For purposes of this section and Section 41503.1, for each district that is designated nonattainment for a state ambient air quality standard but is designated attainment for the federal air quality standard for the same pollutant, reductions in emissions shall be calculated with respect to the actual level of emissions that exist in each district during 1990, as determined by the state board. All reductions in emissions occurring after December 31, 1990, including, but not limited to, reductions in emissions resulting from measures adopted prior to December 31, 1990, shall be included in this calculation. For each district that is designated nonattainment for both state and federal ambient air quality standards for a single pollutant, reductions in emissions shall be calculated with respect to the actual level of emissions that exist in each district during the baseline year used in the state implementation plan required by the federal Clean Air Act. All reductions in emissions occurring after December 31 of the baseline year, including, but not necessarily limited to, reductions in emissions resulting from measures adopted prior to December 31 of the baseline year, shall be included in this calculation.