Comparison of the particle size distribution of heavy-duty diesel exhaust using a dilution tailpipe sampler and an in-plume sampler during on-road operation

Originally constructed to develop gaseous emission factors for heavy-duty d iesel trucks, the U.S. Environmental Protection Agency's (EPA) On-Road Dies el Emissions Characterization Facility has been modified to incorporate par ticle measurement instrumentation. An electrical low-pressure impactor desi gned to continuously measure and record size distribution data was used to monitor the particle size distribution of heavy-duty diesel truck exhaust. For this study, which involved a high-mileage (900,000 mi) truck running at full load, samples were collected by two different methods. One sample was obtained directly from the exhaust stack using an adaptation of the Univer sity of Minnesota's air-ejector-based mini-dilution sampler. The second sam ple was pulled from the plume just above the enclosed trailer, at a point s imilar to 11 m from the exhaust discharge. Typical dilution ratios of about 300:1 were obtained for both the dilution and plume sampling systems. Hund reds of particle size distributions were obtained at each sampling location . These were compared both selectively and cumulatively to evaluate the per formance of the dilution system in simulating real-world exhaust plumes. Th e data show that, in its current residence-time configuration, the dilution system imposes a statistically significant bias toward smaller particles, with substantially more nanoparticles being collected than from the plume s ample.