No abstract

Light-duty vehicle emissions were measured at the Caldecott Tunnel in August and October 1994. In the interval between these two periods, the average oxygen content of gasoline sold in the San Francisco Bay area increased from 0.3 to 2.0% by weight. Compared to the August (low-oxygenate) sampling period, measured pollutant emission rates during the October (high-oxygenate) sampling period for CO and VOC decreased by 21 ( 7 and 18 ( 10%, respectively, while NO x emissions showed no significant change. Formaldehyde emissions increased by 13 ( 6%, acetaldehyde emissions did not change significantly, and benzene emissions decreased by 25 ( 17%. Speciated VOC emission profiles show that the use of oxygenated gasoline resulted in higher MTBE and lower aromatic hydrocarbon emissions, higher isobutene, and lower aromatic aldehydes. The normalized reactivity of NMOG emissions did not change significantly between the low-oxygenate and highoxygenate sampling periods. VOC exhaust speciation profiles for vehicles operating in the hot-stabilized mode at the Caldecott Tunnel match the speciation profile for cold-start emissions from new vehicles as measured in the Auto/Oil program. California's motor vehicle emission factor model, EMFAC7F, accurately predicts the VOC/NO x ratio measured at the Caldecott Tunnel in August, but underpredicts the observed CO/NO x ratio by a factor of 1.5-2.2 over the range of vehicle speeds observed at the tunnel.

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 onroad 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.

Ammonia is the primary alkaline gas in the atmosphere and contributes to fine particle mass, visibility problems, and dry and wet deposition. The objective of this research was to measure ammonia and other exhaust emissions from a large sample of on-road vehicles using California phase 2 reformulated gasoline with low sulfur content (∼10 ppm by weight). Vehicle emissions of ammonia, NO x , CO, and CO 2 were measured in the center bore of a San Francisco Bay area highway tunnel on eight 2-h afternoon sampling periods during summer 1999. Ammonia concentrations were divided by total carbon (mainly CO 2 ) concentrations to compute an emission factor of 475 ( 29 mg L -1 (95% C.I.). The molar ratio of nitrogen emitted in the tunnel in the form of ammonia to that emitted in the form of NO x was 0.27 ( 0.01. Emissions of NO x and CO have been measured at this tunnel sampling location since 1994. From 1994 to 1999, emissions decreased by 41 ( 4% for NO x and 54 ( 6% for CO. These reductions include the impacts of turnover in the vehicle fleet and the use of reformulated gasoline. Between 1997 and 1999, when fuel properties did not change significantly, emissions of NO x and CO decreased by 26 ( 2% and 31 ( 3%, respectively. While use of three-way catalytic converters has contributed to decreases in NO x and CO emissions, their use, in combination with fuel-rich engine operation, is the likely cause of the ammonia emissions from motor vehicles observed during this study.

This paper addresses the impact of California phase 2 reformulated gasoline (RFG) on motor vehicle emissions. Phase 2 RFG was introduced in the San Francisco Bay Area in the first half of 1996, resulting in large changes to gasoline composition. Oxygen content increased from 0.2 to 2.0 wt%; and alkene, 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 1994−1997. Vehicle speeds and driving conditions inside the tunnel were similar each year. The average model year of the vehicle fleet was about one year newer each successive summer. Large reductions in pollutant emissions were measured 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 of carbon monoxide decreased by 31 ± 5%, non-methane volatile organic compounds (VOC) decreased by 43 ± 8%, and nitrogen oxides (NO x ) 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 x . The RFG effect on vehicle emissions of benzene was estimated to be a 30−40% reduction. Use of RFG increased formaldehyde emissions by about 10%, while acetaldehyde emissions did not change 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 California's RFG program was a 20% reduction in gasoline vapor pressure, about one-fifth of which occurred following the introduction of phase 2 RFG.

Laboratory studies have provided a foundation of knowledge regarding vehicle emissions, but questions remain regarding the relationship between on-road vehicle emissions and changes in vehicle speed and engine load that occur as driving conditions change. Light-duty vehicle emissions of CO, NO x , and NMHC were quantified as functions of vehicle speed and engine load in a California highway tunnel for downhill and uphill traffic on a ∼4% grade. Emissions were measured throughout the day; average speed decreased inside the tunnel as traffic volume increased. Emissions of CO were typically 16-34 g L -1 (i.e., grams of CO emitted per liter of gasoline consumed) during downhill driving and ranged from 27 to 75 g L -1 during uphill driving. Downhill driving and moderate-speed uphill driving resulted in similar CO emission factors. The factor of 2 increase in CO emissions observed during higherspeed uphill driving is likely evidence of enriched engine fuel/ air ratios; this was unexpected because uphill driving observed in this study occurred at moderate engine loads within the range experienced during the city driving cycle of the U.S. emissions certification test. Emissions of NO x (as NO 2 ) were typically 1.1-3.3 g L -1 for downhill driving and varied between 3.8 and 5.3 g L -1 for uphill driving. Unlike observations for CO, all uphill driving conditions resulted in higher NO x emission factors as compared to downhill driving. NO x emissions increased with vehicle speed for uphill driving but not as strongly as CO emissions. Emissions of CO and NO x are functions of both vehicle speed and specific power; neither parameter alone captures all the relevant effects on emissions. In contrast to results for CO and NO x reported here and results for NMHC reported previously by Pierson et al. (Atmos. Environ. 1996, 30, 2233-2256, emissions of NMHC per unit of fuel burned for downhill driving were over 3 times greater than NMHC emissions for uphill driving. Emission rates of CO and NO x varied more with driving conditions when expressed per unit distance traveled rather than per unit fuel burned while NMHC emission rates normalized to distance traveled were approximately constant for uphill versus downhill driving during peak traffic periods.

Gasoline-powered motor vehicles are a major source of toxic air contaminants such as benzene. Emissions from light-duty vehicles were measured in a San Francisco area highway tunnel during summers 1991, 1994-1997, 1999, 2001, and 2004. Benzene emission rates decreased over this time period, with a large (54 +/- 5%) decrease observed between 1995 and 1996 when California phase 2 reformulated gasoline (RFG) was introduced. We attribute this one-year change in benzene mainly to RFG effects: 36% from lower aromatics in gasoline that led to a lower benzene mass fraction in vehicle emissions, 14% due to RFG effects on total nonmethane organic compound mass emissions, and the remaining 4% due to fleet turnover. Fleet turnover effects accumulate over longer time periods: between 1995 and 2004, fleet turnover led to a 32% reduction in the benzene emission rate. A approximately 4 microg m(-3) decrease in benzene concentrations was observed at a network of ambient air sampling sites in the San Francisco Bay area between the late 1980s and 2004. The largest decrease in annual average ambient benzene concentrations (1.5 +/- 0.7 microg m(-3) or 42 +/- 19%) was observed between 1995 and 1996. The reduction in ambient benzene between spring/summer months of 1995 and 1996 due to phase 2 RFG was larger (60 +/- 20%). Effects of fuel changes on benzene during fall/winter months are difficult to quantify because some wintertime fuel changes had already occurred prior to 1995.

The temporary ineffectiveness of motor vehicle emission controls at startup causes emission rates to be much higher for a short period after starting than during fully warmed, or stabilized, vehicle operation. Official motor vehicle emission inventories estimate that excess emissions during cold-start operation contribute a significant fraction of all hydrocarbon, carbon monoxide (CO), and nitrogen oxide (NO x ) emissions from California vehicles. In an effort to verify these estimates under real-world conditions, vehicle emissions were measured in an underground parking garage in Oakland, CA, during March 1997. Hot stabilized emissions were measured as vehicles arrived at the garage in the morning, and cold-start emissions were measured as vehicles exited in the afternoon; the incremental, or excess, emissions associated with vehicle starting were calculated by difference. Composite emissions from ~135 vehicles were sampled during each of six morning and six afternoon periods. Measured stabilized exhaust emissions were 19 ± 2 g nonmethane hydrocarbons (NMHC), 223 ± 17 g CO, and 8.6 ± 1.3 g NO x per gal of gasoline consumed. Cold-start emissions of 69 ± 2 g IMPLICATIONS Cold starts are thought to contribute a large fraction of total emissions from California's motor vehicle fleet. This study suggests that the importance of cold-start emissions may be overstated, and that control strategies that focus exclusively on reducing cold-start emissions may not achieve projected improvements in air quality. Instead, greater emphasis should be placed on reducing warm running emissions from in-use vehicles. Transit-oriented strategies such as park-and-ride lots may also provide greater air quality benefits than previous assessments have indicated, especially if travel by older, high-emitting vehicles can be reduced.NMHC/gal, 660 ± 15 g CO/gal, and 27.8 ± 1.2 g NO x /gal were measured for vehicles spending an average of ~60 sec in the garage after starting in the afternoon. Using second-by-second emissions data from California's lightduty vehicle surveillance program, average fuel use during cold start was estimated to be ~0.07 gal, and the coldstart period was estimated to last for ~200 sec. When coldstart emission factors measured in the garage were scaled to represent the full 200-sec cold-start period, incremental start emission factors of 2.1 g NMHC, 16 g CO, and 2.1 g NO x per vehicle start were calculated. These emission factors are lower than those used by California's motor vehicle emission inventory model (MVEI 7G) by 45% for NMHC, 65% for CO, and 12% for NO x . This suggests that the importance of cold-start emissions may be overstated in current emission inventories. Overall, the composition of volatile organic compound (VOC) emissions measured during cold start was similar to that of hot stabilized VOC emissions. However, the weight fractions of unburned fuel and acetylene were higher during cold start than during hot stabilized driving. INTRODUCTIONMotor vehicle tailpipe emissions of carbon monoxide (C...