Brown

Originally constructed to develop gaseous emission factors for heavy-duty diesel trucks, the U.S. Environmental Protection Agency’s (EPA) On-Road Diesel Emissions Characterization Facility has been modified to incorporate particle measurement instrumentation. An electrical low-pressure impactor designed 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 University of Minnesota’s air-ejector-based mini-dilution sampler. The second sample was pulled from the plume just above the enclosed trailer, at a point ~11 m from the exhaust discharge. Typical dilution ratios of about 300:1 were obtained for both the dilution and plume sampling systems. Hundreds of particle size distributions were obtained at each sampling location. These were compared both selectively and cumulatively to evaluate the performance of the dilution system in simulating real-world exhaust plumes. The 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 sample. IMPLICATIONS Data gathered during this study suggest that the accumulation mode of diesel exhaust is sensitive to the sample collection procedure. The transfer of laboratory particle size data to ambient models may need to be conducted carefully. Further work is needed to establish the complete particle time history in a truck plume. INTRODUCTION After decades of focusing its attention on visible emissions, mass emissions, and chemical composition, the U.S. Environmental Protection Agency (EPA) has recently focused on another important characteristic of PM emissions: particle size. It has been known for some time that respiratory systems function much like multistage impactors, collecting the largest particles near the inlet (the nose), and leaving the smallest particles to deposit in the deepest parts (the air sacs of the lungs).1 It has also been known for some time that diesel exhaust contains large quantities of small particles.2 Most often, these observations have been primarily qualitative, and the impacts evaluated in terms of how the environment is affected by the emissions. Several advanced analytical tools have become available to quantify and characterize large numbers of very small nanoparticles (typically defined as less than 50 nm), which can number in the millions of particles/cm3 of ambient air.3 These small particles make a negligible contribution to the traditional mass-based PM measurement techniques that dominate most current regulations. However, a number of recent health effects studies indicate that these particles may be considerably more hazardous than those making up most of the mass. Therefore, fine PM (less than 2.5 μm in aerodynamic diameter) is a concern because of its effects not only on the environment in general, but on people in particular.4,5 A number of laboratories across the United States are studying fine PM emissions from a variety of sources, including diesel engines. Many of these laboratories were originally created for the study of gaseous pollutants and other emissions where mass is the quantity of interest. Their sampling equipment and techniques reflect that Brown, Clayton, Harris, and King 1408 Journal of the Air & Waste Management Association Volume 50 August 2000 “collect-and-quantify” approach to emissions measurement. Some laboratories collect samples directly from the source, condition the samples to a constant and defined state, and measure the components of interest. Others collect and dilute samples, simulating what happens to point source emissions after they are released to the atmosphere. It has been repeatedly demonstrated, during annual Relative Accuracy Test Audits of regulated emissions monitoring systems all over the United States, that both raw gas and dilution sampling systems can produce accurate and comparable data when measuring gaseous pollutants. Many studies have shown, however, that diesel PM remains in a state of flux for some time after it is emitted to the atmosphere, in part due to the continuation of instack coagulation and adsorption, but also due to the significant quantities of condensable organics and inorganics usually present in diesel exhaust.6,7 The fate of these condensables is significantly affected by atmospheric aging and dilution of the exhaust stream. As such, the method of exhaust dilution has fallen under a level of scrutiny that was not necessary when dilution systems were first deployed to collect samples for gaseous emissions analysis. The same source sampled by two different dilution systems could yield fundamentally different fine PM measurements. This paper describes a field study in which fine PM samples were collected and analyzed from a heavy-duty diesel vehicle in operation. The purpose of this study was to characterize, under as realistic a condition as possible, the emissions from a tractor trailer operating on an interstate highway in moderately hilly terrain. A secondary purpose was to evaluate a prototype dilution system for its ability to effectively simulate real-world dilution of tractor-trailer exhaust. All of the data for this report were collected using the On-Road Diesel Emissions Characterization (ODEC) facility, constructed and operated by the National Risk Management Research Laboratory’s Air Pollution Prevention and Control Division. BACKGROUND While a detailed description of diesel PM chemistry and morphology is beyond the scope of this paper, a general description is provided. Diesel exhaust PM size distributions are typically trimodal in shape, meaning that the frequency distribution will exhibit three peaks that may or may not overlap one another. Figure 1 shows an exaggerated view of such a distribution. In reality, for the typical exhaust from a modern diesel engine, the coarse mode would be negligible. The accumulation mode would account for most of the mass, as shown here, but would often be dwarfed by the nuclei mode on a distribution graph where the frequency is expressed as a particle number concentration.8 A number of processes occur during atmospheric aging that can alter the size distribution of an aerosol, including nucleation, agglomeration, and adsorption. Homogeneous nucleation is the spontaneous formation of a nanoparticle of volatile material within a locally supersaturated zone. Heterogeneous nucleation involves the same volatile material and driving force (i.e., saturation), but also involves “seed” nuclei that considerably lower the required degree of saturation. Saturation is the condition at which the partial pressure of a volatile material equals its vapor pressure; degree of saturation is usually expressed as a ratio of those pressures. Once a hot exhaust is released into the air, the partial pressure of its volatile components decreases with dilution. The vapor pressure of those components is a function of temperature, which also decreases with dilution. Since the vapor pressure-versus-temperature relationship is nonlinear, it is normal for the saturation ratio to reach a maximum at some dilution level. The organic fraction of diesel exhaust reaches its maximum degree of saturation somewhere between a 5:1 and 50:1 dilution, but does not usually achieve the necessary supersaturation required for homogeneous nucleation.9 H2SO4, on the other hand, may reach the supersaturation levels necessary for homogeneous nucleation at ratios between 10:1 and 50:1, possibly serving as “seed” nuclei for heterogeneous nucleation.10 Therefore, residence times at various dilution levels can be of critical importance to the formation of nuclei mode nanoparticles. The agglomeration of ash particles and carbonaceous products of incomplete combustion forms a majority of the accumulation mode particles. This process begins occurring immediately upon detonation within the engine; those solid particles that were not completely burned begin adsorbing hydrocarbons and sulfates, creating an outer layer that causes any particles that come into contact to stick together. Agglomeration rates are likely characterized by second-order kinetics, meaning that a 1000:1 dilution would decrease the rate by a factor of 1,000,000. Therefore, a majority of the accumulation mode particles that are going to form are expected to do so in the exhaust piping before any dilution takes place. The accumulation mode particles play an important role in determining the characteristics of an aerosol’s size distribution. Adsorption is an alternative path for the volatile material that would otherwise be subject to nucleation. Since the driving force for adsorption is much stronger than for nucleation, the presence of a significant accumulation mode can substantially suppress the formation of nanoparticles, at the expense of having larger particles in the accumulation mode. Ironically, given the current mass-based regulatory climate, most of the industry’s reductions in PM emissions have been taken out of the accumulation mode.11 Brown, Clayton, Harris, and King Volume 50 August 2000 Journal of the Air & Waste Management Association 1409 STUDY DESIGN The goal of the heavy-duty diesel on-road testing program is to characterize emissions under the most realistic conditions attainable. For fine PM emissions, those conditions include the natural processes of ambient dilution and aging as they affect the exhaust of a tractor trailer. This study followed two approaches to including the dilution and aging process in the fine PM sampling. The most realistic approach, of course, is to sample the exhaust plume after it leaves the stack