This measure provides tax credits to individuals and corporations for certain expenditures they make that reduce emissions of pollutants into the air. For example, a trucking company that modifies the engines on its older, heavy-duty diesel trucks so that the fuel burns more cleanly could be eligible for a tax credit under the measure. A tax credit reduces the amount of taxes paid to the state by an individual or corporation. Under the measure, a maximum total of $218 million in tax credits would be available for award each fiscal year until January 1, 2011. This maximum amount may be reduced under specified circumstances when there is a drop in the state's General Fund revenues. Credits not awarded in one year may be carried over and awarded in a later year.