motor vehicles. Although the Complex and Predictive models are distinct from models used to estimate the mobile source inventory, their capability of reflecting actual emissions needs to be improved. In some cases, algorithms used to develop the regression equations for the models ignore important parameters that can influence emissions. For example, the Complex Model, developed by EPA, does not account for temperature variations when calculating evaporative emissions. The Predictive Model, developed by the California Air Resources Board, excludes consideration of evaporative emissions. Another potential source of error in both models arises from their treatment of high-emitting vehicles. As noted above, a large portion of motor-vehicle emissions come from high-emitting vehicles. However, the emissions from these vehicles are likely to be quite variable and thus difficult to characterize through sampling a small subset of the total population.
The scheduled implementation of Phase II of the federal RFG program in 2000 offers a unique opportunity to track and document the impact of a new ozone-mitigation program. Plans should be made and implemented for an atmospheric measurements program to assess the impact of Phase II RFG on (1) emissions of ozone precursors from the on-road and non-road motor vehicle fleet, as well as ozone-forming potential of those emissions; and (2) the impact of these changes, if any, on ambient concentrations of ozone and its precursors.