Measurement of black carbon and particle number emission factors from individual heavy-duty trucks.
Emission factors for black carbon (BC) and particle number (PN) were measured from 226 individual heavy-duty (HD) diesel trucks driving through a 1-km-long California highway tunnel in August 2006. Emission factors were based on concurrent increases in BC, PN, and CO2 concentrations (measured at 1 Hz) that corresponded to the passage of individual HD trucks. The distributions of BC and PN emission factors from individual HD trucks are skewed, meaning that a large fraction of pollution comes from a small fraction of the in-use vehicle fleet. The highest-emitting 10% of trucks were responsible for approximately 40% of total BC and PN emissions from all HD trucks. BC emissions were log-normally distributed with a mean emission factor of 1.7 g kg(-1) and maximum values of approximately 10 g kg(-1). Corresponding values for PN emission factors were 4.7 x 10(15) and 4 x 10(16) # kg(-1). There was minimal overlap among high-emitters of these two pollutants: only 1 of the 226 HD trucks measured was found to be among the highest 10% for both BC and PN. Monte Carlo resampling of the distribution of BC emission factors observed in this study revealed that uncertainties (1sigma) in extrapolating from a random sample of n HD trucks to a population mean emission factor ranged from +/- 43% for n=10 to +/- 8% for n=300, illustrating the importance of vehicle sample sizes in emissions studies. When n=10, sample means are more likely to be biased due to misrepresentation of high-emitters. As vehicles become cleaner on average in the future, skewness of the emissions distributions will increase, and thus sample sizes needed to extrapolate reliably from a subset of vehicles to the entire in-use vehicle fleet will become more of a challenge.
Ban-Weiss
George A
GA
Department of Mechanical Engineering, University of California, Berkeley, California 94720-1740, USA.
Lunden
Melissa M
MM
Kirchstetter
Thomas W
TW
Harley
Robert A
RA
D001336:Automobiles; D002140:California; D002245:Carbon Dioxide / Q000032:analysis; D003198:Computer Simulation; D008956:Models, Chemical; D052638:Particulate Matter / Q000032:analysis*; D018401:Sample Size; D012621:Seasons; D053260:Soot / Q000032:analysis*; D035501:Uncertainty; D001335:Vehicle Emissions / Q000032:analysis*
D052638:Particulate Matter; D053260:Soot; D001335:Vehicle Emissions; D002245:Carbon Dioxide