Impact of California reformulated gasoline on motor vehicle emissions: 2. VOC speciation and reactivity

This paper addresses the impact of California phase 2 reformulated gasoline (RFG) on the composition and reactivity of motor vehicle exhaust and evaporative emissions. Significant changes to gasoline properties that occurred in the first half of 1996 included an increase in oxygen content; decreases in alkene, aromatic, benzene, and sulfur contents; and modified distillation properties. Vehicle emissions were measured in a San Francisco Bay Area roadway tunnel in summers 1994−1997; gasoline samples were collected from local service stations in summers 1995 and 1996. Equilibrium gasoline headspace vapor composition was calculated from measured liquid gasoline composition. Addition of methyl tert-butyl ether (MTBE) and reduction of alkenes and aromatics in gasoline between summers 1995 and 1996 led to corresponding changes in the composition of gasoline headspace vapors. Normalized reactivity of liquid gasoline and headspace vapors decreased by 23 and 19%, respectively. Ozone formation should be reduced because of both lower gasoline vapor pressure, which leads to lower mass emissions, and reduced reactivity of gasoline vapors. The reactivity of on-road emissions measured in the tunnel decreased by 8% or less. The reduction in reactivity of on-road emissions was less than that of evaporative emissions because of increased weight fractions of highly-reactive isobutene and formaldehyde in vehicle exhaust, which resulted from the increased use of MTBE in gasoline. On-road vehicle emissions of volatile organic compounds in the tunnel appear to be dominated by vehicles that have reduced catalytic converter activity.