Mobile Source Contribution to Ambient 1,3-Butadiene, Benzene and Particle-Bound PAH at a Tollbooth

摘要

Mobile source emissions account for as much as 60% (34 megatons) and 77% (271 megatons) of the total national emissions of 1,3-butadiene (BD) and benzene, respectively (NTI 1996), both "known human carcinogens." The same figure for particle phase polyaromatic organic matter (POM) expressed as 16-PAH is 6%) (14 tons). This study was conducted to characterize the mobile source contribution to the ambient concentrations of these three human carcinogens: BD, benzene and particle phase PAHs at a busy tollbooth. Three-hour measurements of traffic and BD concentration were taken over seven weekdays at the Baltimore City Harbor Tunnel tollbooth during the period of July 18-29, 2001. Traffic volume and vehicle class were concurrently monitored. The 3-hour outdoor BD levels varied according to time of day and traffic volume from a mean of 2.0 mug/m~3 (SD=1.3, n=7) to 11.9 mug/m~3 (SD=4.6, n=7) with corresponding 3-hour mean traffic volumes of 1413 (SD=144) and 16893 (SD=692), respectively. For the same period, benzene concentration varied from 3.0 mug/m~3 (SD=3.1, n=7) to 22.3 mug/m~3 (SD=7.6, n=7). The mdian PAH concentration ranged from 21 ng/m~3 to 199 ng/m~3. BD and benzene minimum and maximum concentrations coincided with minimum and maximum traffic counts, respectively, during time intervals of 12 a.m.- 3 a.m. and 6 a.m. - 9 a.m. Much of the variability in ambient BD (62%), benzene (77%), and PAH (85%) was explained by vehicle counts, vehicle class, temperature and wind speed. From this tollbooth facility, BD mobile source strength of 0.32 ng/m~3/vehicle is estimated for 2-axle traffic. The source strength of >2-axle vehicles (10.4 ng/m~3/vehicle) exceeds the 2-axle source strength by a factor of 33. Similarly, a nine fold increase in the benzene mobile source strength was observed for >2-axle vehicles compared to 2-axle vehicles. For PAH, the mobile source strength for >2-axle vehicle was higher than that for 2-axle vehicle by a factor of 60. We believe that the difference in source strength between the >2- and 2-axle classes is primarily attributable to fuel type (i.e., diesel vs. gasoline). Since the number of >2-axle vehicles outnumbered 2-axle vehicles 29:1, the overall contribution by the two vehicle class was comparable for ambient BD, but not for benzene and PAH.