Nanoparticle Emissions from Engines

Recently concerns about possible increases in emissions of ultrafine and nanoparticles and their environmental impact have been voiced. A major problem with assessing the significance of such emissions is that, unlike the carbonaceous soot particles associated with older Diesel engines, many of these particles are not formed in the engine but instead by gas-to-particle conversion processes from vapor phase particle precursors as the exhaust dilutes and cools in the atmosphere. These processes are extremely non-linear and difficult to simulate in the laboratory. Recent measurements of such particles and problems associated with their measurement are described. The final measure of merit of a dilution and sampling system is whether it forms exhaust particle size distributions that are similar to those produced by atmospheric dilution. A sampling and dilution scheme capable of simulating atmospheric dilution has not yet been identified. In fact, it may not be possible to come up with a single robust approach that is appropriate to all situations. Current engine particulate emission standards are mass based. Recently, however, interest in other measures, i.e., size, number, or surface area, has increased. This is the result of increased awareness of the influence of particle size on the environmental impact of particulate matter (PM). Several factors have led to this awareness. The EPA has proposed tough new ambient standards on fine particle, particles smaller than 2.5 μm aerodynamic diameter. These standards are based on studies that show an association between adverse health effects and the concentration of fine particles in the atmosphere (Pope, et al., 1995; Dockery, et al., 1993). Ultrafine (Dp < 100 nm) and nanoparticles (Dp < 50 nm) may have even stronger environmental impacts than fine particles. Animal studies have shown that particles that produce little respiratory system response μm diameter range produce much stronger response in the nm diameter range (Seaton, et al., 1995). Concerns about the environmental impacts of tiny particles have been brought into sharper focus by a recent Health Effects Institute study (Bagley, et al., 1996) that suggested that modern low emission engines might actually emit higher concentrations of ultrafine and nanoparticles than older designs. All of these factors have resulted in a greatly increased interest in the measurement of the size of particles emitted by engines, especially in the nm diameter range. For the sake of brevity, this paper focuses on Diesel engines. However, particle emissions from gasoline engines may also be important (Graskow, et al., 1999a; 1999b). Figure 1 illustrates idealized number and mass weighted size distributions (Kittelson, 1998) for diesel particulate matter (DPM). DPM follows a lognormal, trimodal size distribution with the concentration in any size range being proportional to the area under the corresponding curve in that range. Nuclei-mode particles range in diameter from about 5 to 50 nm. They 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.001 0.010 0.100 1.000 10.000 Diameter (μm) N or m al iz ed C on ce nt ra tio n, d C /C to ta l /d lo gD p 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 A lv eo la r D ep os iti on F ra ct io n Mass Weighting Number Weighting Alveolar Deposition Fraction Fine