Impact of California reformulated gasoline on motor vehicle emissions. I. Mass emission rates

This paper addresses the impact of California phase 2 reformulated gasoline (RFG) on motor vehicle emissions. Phase 2 RFG was introduced in the San Fr ancisco Bay Area in the first half of 1996, resulting in large changes to g asoline composition. Oxygen content increased from 0.2 to 2.0 wt%; and alke ne, aromatic, benzene, and sulfur contents decreased. Gasoline density and T-50 and T-90 distillation temperatures also decreased. Light-duty vehicle emission rates were measured in a Bay Area roadway tunnel in summers 1994-1 997. Vehicle speeds and driving conditions inside the tunnel were similar e ach year. The average model year of the vehicle fleet was about one year ne wer each successive summer. Large reductions in pollutant emissions were me asured in the tunnel over the course of this study, due to a combination of RFG and fleet turnover effects. Between summers 1994 and 1997, emissions o f carbon monoxide decreased by 31 +/- 5%, non-methane volatile organic comp ounds (VOC) decreased by 43 +/- 8%, and nitrogen oxides (NOx) decreased by 18 +/- 4%. It was difficult to separate clearly the fleet turnover and RFG contributions to these changes. Nevertheless, it was clear that the effect of RFG was greater for VOC than for NO, The RFG effect on vehicle emissions of benzene was estimated to be a 30-40% reduction. Use of RFG increased fo rmaldehyde emissions by about 10%, while acetaldehyde emissions did not cha nge significantly. RFG effects reported here may not be the same for other driving conditions or for other vehicle fleets. RFG effects on evaporative emissions are also important. The combined effect of phases 1 and 2 of Cali fornia's RFG program was a 20% reduction in gasoline vapor pressure, about one-fifth of which occurred following the introduction of phase 2 RFG.