Currently, states that are out of compliance with the National Ambient Air Quality Standards must, according to the Clean Air Act Amendments of 1990 (CAAA), develop and implement control strategies that demonstrate specific degrees of reduction in emissions-with the degree of reduction depending upon the severity of the problem. One tool that has been developed to aid regulators in both deciding an appropriate course of action and to demonstrate the desired reductions in mobile emissions is EPA's Mobile 5a emission estimation model. In our study, Mobile 5a has been used to examine the effects of regulatory strategies, as applied to the Northeast United States, on vehicle emissions under worst-case ozone-forming conditions.
We examined the effects of the following mandatory and discretionary measures: inspection and maintenance (I/M), enhanced I/M, anti-tampering, reformulated gasolines (RFG I & 11), Federal Tier I & II vehicle standards, California's program of emission standards (LEV, ULEV, ZEV), Stage II refueling, and on-board vapor recovery (VRS). The rank order of effectiveness, for pollutant reduction, of these measures depends on the pollutant and the time period chosen. Two time periods are of interest, near-term (1993-2000) and far-term (2000-2010). In terms of non-methane organic gas (NMOG) emission reductions in the near-term, RFG I is the most effective measure followed by enhanced I/M and then regular I/M with less effectiveness attributable to the remaining programs. In the long-term the benefits from RFG I & II on NMOG emissions are still present; however, they are overshadowed by those from I/M programs and Tier I vehicles. The NOx situation is different due to the smaller benefit of reformulated gasolines on NOx emissions. In the short-term the largest benefits come from I/M programs with those from Tier I vehicles emerging by the year 2000. In the long-term regular I/M and Tier I vehicles are the most significant measures with fewer benefits from enhanced I/M. The results we obtain indicate that the adoption of the California vehicles in the Northeast instead of Tier I vehicles, with the other requirements of the 1990 CAAA, would not provide any emissions benefits before the year 2005. This is due, primarily to the reduced benefits of I/M assumed in Mobile 5a for California cars. In accordance with this result, the sensitivity of the aggregate fleet emissions to the possible phase-in date of the California cars is small.
A separate finding is that the in-use aggregate-fleet emissions of light-duty trucks are not attenuated by fleet rollover as substantially as those of the light-duty passenger cars. Additionally, the relative growth rate in vehicle miles traveled for light-duty trucks is much higher. Thus, the relative contribution of light-duty trucks to vehicle emissions is predicted to increase.