Diesel Exhaust Particle Size
Since the mid-1990s, particle size distributions from internal combustion engines have been receiving increased attention due to possible adverse health effects of fine and ultrafine particulates. Diesel emission control strategies, based on both engine design and aftertreatment, are being examined and re-evaluated for their effectiveness in the control of the finest fractions of diesel particulates and particle number (PN) emissions. However, a fair performance assessment of various control technologies can be possible only if the research community reaches a consensus on the definition and the measurement techniques of the smallest fractions of diesel particulates. The determination of particle sizes and numbers is much more sensitive to the measuring techniques and parameters than the quantification of particulate mass emissions. Dilution and sampling methods are key variables that must be taken into consideration to ensure accurate and repeatable results. On the other hand, particle sizing instruments exist that have significantly better sensitivities than the gravimetric measurement, thus presenting an attractive alternative for the PM emission measurement in future engines, provided standardized measuring methods are developed.
Ambient particulate matter is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):
- PM10—particulates of an aerodynamic diameter of less than 10 µm
- Fine particles of diameters below 2.5 µm
- Ultrafine particles of diameters below 0.1 µm or 100 nm
- Nanoparticles, characterized by diameters of less than 50 nm.
A typical size distribution of diesel exhaust particulates is shown in Figure 1 (note that a logarithmic scale is used for particle aerodynamic diameter). Nearly all diesel particulates have sizes of significantly less than 1 µm. As such, they represent a mixture of fine, ultrafine, and nanoparticles. Due to the current PM sampling techniques (diluted exhaust, temperature <52°C), diesel particulate matter includes both solids, such as elemental carbon and ash, and liquids, such as condensed hydrocarbons, water, and sulfuric acid. Formation of particulates starts with nucleation, which is followed by subsequent agglomeration of the nuclei particles. The nucleation occurs both in the engine cylinder (carbon, ash) and in the dilution tunnel (hydrocarbons, sulfuric acid, water), through homogeneous and heterogeneous nucleation mechanisms.
Figure 1. Diesel Particulate Size Distribution
Size distributions of diesel particulates have a well established bimodal character which corresponds to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode. Size distributions are usually presented using either particle mass or particle number weighting. In each representation normal-logarithmic distribution curves are produced, as shown in Figure 1. Both the maximum particle concentration and the position of the nuclei and accumulation mode peaks, however, depend on which representation is chosen. In mass distributions, the majority of the particulates (i.e., the particulate mass) is found in the accumulation mode. In number distributions, on the other hand, most particles are found in the nuclei mode. In other words, diesel particulate matter is composed of numerous small particles holding very little mass, mixed with relatively few larger particles which contain most of the total mass. A small fraction of diesel particulates reside in a third, coarse mode (Figure 1). These three particle modes can be characterized as follows:
- Nuclei mode: The diameter of the original nucleus, such as formed during sulfuric acid nucleation, is about 1 nm [Abdul-Khalek 1999]. Today’s measuring techniques are capable of detecting a minimum particle size of approximately 3 nm. According to various definitions, the diameters of nuclei mode particles were generally less than 40-50 nm (0.04-0.05 µm). Based on particle size research in the 1990s technology heavy-duty diesel engines, it has been postulated that the nuclei mode extends through sizes from 3 to 30 nm (0.003-0.03 µm) [Kittelson 2002][Hall 2001]. All of the above size ranges place nuclei mode particles entirely within the nanoparticle range. The maximum concentration of nuclei mode particles occurs at 10-20 nm. The nuclei mode, depending on the engine technology and particle sampling technique, typically contains only 0.1-10% of the total PM mass, but it often includes more than 90% of the total particle count. Sometimes the nuclei mode particles present as much as 99% of the total particulate number. Nuclei mode particles are composed mostly of volatile condensates (hydrocarbons, sulfuric acid) and contain little solid material.
- Accumulation mode: The accumulation mode is made of sub-micron particles of diameters ranging most often from 30 to 500 nm (0.03-0.5 µm nm) [Kittelson 2002], with a maximum concentration between some 100-200 nm (0.1-0.2 µm). As shown in Figure 1, the accumulation mode extends through the fine, ultrafine, and the upper end of the nanoparticle range. Accumulation mode particles are made of solids (carbon, metallic ash) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species).
- Coarse mode: These particles with aerodynamic diameters above 1 µm (1000 nm) contain 5-20% of the total PM mass and practically no contribution to particle numbers [Kittelson 2002]. The coarse particles are not generated in the diesel combustion process. Rather, they are formed through deposition and subsequent re-entrainment of particulate material from walls of the engine cylinder, exhaust system, or the particulate sampling system.
Other particle weightings that may be used include particle surface (which would produce a curve located between the mass and number weightings in Figure 1) and particle volume weighting, which is proportional to the mass weighting.