(b) On or before December 31, 1994, and once every three years thereafter, the district shall assess its progress toward attainment of the state ambient air quality standards. Each triennial assessment shall be incorporated into the district’s triennial plan revision prepared pursuant to Section 40925. Each triennial assessment shall contain, at a minimum, both of the following:

1. (1) The extent of air quality improvement achieved during the preceding three years, based upon ambient pollutant measurements, best available modeling techniques, and air quality indicators identified by the state board for that purpose under subdivision (f) of Section 39607.
2. (2) The expected and revised emission reductions for each measure scheduled for adoption in the preceding three-year period.