Cal. Health & Safety Code § 40912

The plans for districts responsible for or affected by air pollutant transport shall provide for attainment and maintenance of the state and federal standards in both the upwind and downwind district. Each upwind district’s plan shall contain, at a minimum, all mitigation requirements established by the state board pursuant to subdivision (b) of Section 39610. Each downwind district’s plan shall contain sufficient measures to reduce emissions originating in the district below the level at which violations of state ambient air quality standards would occur in the absence of the transport contribution.