Robert Harley
Civil & Environmental Engineering
University of California, Berkeley
510-643-9168
University of California
Transportation Center Year 12 (1999-2000)
The goal of this study is to measure emissions from light-duty passenger vehicles in an on-road setting, in order to assess trends in vehicle emission rates over time.
Many changes to vehicle technology and fuel properties have occurred in the last 10 years, to reduce air pollution from motor vehicles. At the same time, there has been growth in the number of vehicles on the road, the amount of travel per vehicle, and the total amount of gasoline sold.
Emission forecasting models such as MOBILE and EMFAC, and testing of individual vehicles in the laboratory suggest dramatic progress in reducing vehicle emissions. Predictions of the emission models are based in large part on such results of laboratory testing, and the assessments of transportation-sector emissions provided by these models are essential for air quality and transportation planning.
There is concern that the small sample of vehicles used in laboratory testing may not represent accurately the contribution from malfunctioning and high-emitting vehicles that are responsible for the majority of exhaust emissions from the vehicle fleet.
In this study, we compare vehicle emissions measured in a highway tunnel in San Francisco Bay area in summer 1999 with similar measurements made in previous summers (see Kirchstetter et al., Environ. Sci. Technol., vol. 33, pp. 318-328, 1999). Our field sampling site is the Caldecott tunnel, on state highway 24 running inland from Oakland, CA to communities in Contra Costa county. The tunnel has three separate two-lane traffic bores; we measure vehicle emissions in the middle bore where heavy-duty trucks are not allowed. Measurements are made from 4-6 PM when traffic in the middle bore travels eastbound and uphill on a 4% grade. More than 4000 vehicles per hour travel through the tunnel at this time of day.
We measure concentrations of carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), and speciated volatile organic compounds (VOC) at the tunnel entrance and exit. Note that VOC includes methane, non-methane hydrocarbons (NMHC), MTBE, and carbonyls such as formaldehyde. In 1999, we also measured ammonia (NH3) concentrations, though this pollutant was not measured in previous years. The tunnel’s mechanical ventilation system does not operate during our measurements, so air flow through the tunnel is entirely longitudinal and is driven by the flow of vehicles through the tunnel and prevailing winds. We subtract tunnel entrance concentrations from those measured at the tunnel exit, and then divide the measured concentration difference for each pollutant by the increase in total carbon concentrations (dominated by CO2) inside the tunnel. Since the source of carbon measured in the tunnel is the carbon present in gasoline, by carbon balance we compute emission factors in units of mass of pollutant emitted per unit of fuel burned.
Emissions of NOx, NMHC, and CO have been measured at this site since 1994. From 1994 to 1999, emissions decreased by 41 ± 4% for NOx, 59 ± 8% for NMHC, and 54 ± 6% for CO. We report an emission factor of 475 ± 29 mg of ammonia per liter of gasoline burned for the light-duty vehicle fleet observed in the tunnel in summer 1999. This result for ammonia is consistent with another California tunnel study (Fraser and Cass, Environ. Sci. Technol., vol. 32, pp. 1053-1057, 1998), but is much higher than ammonia emission rates observed before vehicles were equipped with three-way catalytic converters. Three-way catalytic converters simultaneously control three pollutant emissions: CO and VOC are oxidized to CO2 and water vapor, while NOx is reduced to N2.
We also report emission factors for 32 individual carbonyls. Carbonyls, a subset of VOC, include oxygenated compounds such as aldehydes and ketones. Some of these compounds may pose direct human health hazards, and some are also highly reactive in the atmosphere and so promote formation of ozone and other constituents of photochemical smog.
Light-duty vehicle emissions of most pollutants declined significantly in the 1990s. The effects of improved emission control technologies more than offset the increases in vehicle travel and fuel consumption that took place over the same time period.
While use of three-way catalytic converters has contributed to decreases in NOx, NMHC, and CO emissions, their use, in combination with fuel-rich engine operation, is the likely cause of ammonia emissions from motor vehicles.
Further research is needed to continue tracking trends in vehicle emissions, and to assess the effects of phasing out MTBE as a gasoline additive.
Kean, A.J.; Harley, R.A.; Littlejohn, D.; Kendall, G.R. (2000). On-road measurement of ammonia and other motor vehicle exhaust emissions. Environ. Sci. Technol., vol. 34, pp. 3535-3539.
Kean, A.J.; Grosjean, E.; Grosjean, D.; Harley, R.A. (2001). On-road measurement of carbonyls in California light-duty vehicle emissions. Accepted for publication in Environ. Sci. Technol.
Conferences Attended
On-Road Measurement of Ammonia and Other Motor Vehicle Exhaust Emissions.
Coordinating Research Council (CRC) Workshop on On-Road Vehicle Emissions, San Diego, March 2000. Presented by Andrew Kean (Ph.D. candidate at UC Berkeley).
On-Road Measurement of Speciated Non-Methane Organic Compound Emissions from Light-Duty Vehicles. Presented at Coordinating Research Council (CRC) Workshop on On-Road Vehicle Emissions, San Diego, March 2001. Presented by Andrew Kean.