DIESELEXHAUSTPARTICULATES

Chemical Properties

Diesel exhaust is a complex mixture of combustion products of diesel fuel, and the exact composition of the mixture depends on the nature of the engine, operating conditions, lubricating oil, additives, emission control system, and fuel composition (Obert 1973, Ullman 1989). Diesel engines typically are classified by their service requirements, and the operating conditions for light-and heavy-duty diesel engines differ with respect to engine speed, expected load, fuel composition, and engine emission controls. Light-duty vehicles, such as automobiles and light trucks, typically operate at higher speeds than do heavy-duty vehicles, such as trucks. Depending on operating conditions conditions, fuel composition, and engine-control technology, light- and heavy-duty diesel engines, respectively, can emit 50 to 80 times and 100 to 200 times as much particulate mass as typical catalyst-equipped gasoline engines (McClellan 1986).
Most diesel exhaust particles have aerodynamic diameters falling within a range of 0.1 to 0.25 μm (Groblicki 1979, Dolan et al. 1980, NCR 1982, Williams 1982). The particle size distribution of diesel exhaust is bimodal, with a nuclei mode of 0.0075 to 0.042 μm (for particles formed by nucleation) and an accumulation mode of 0.042 to 1.0 μm (for particles formed by agglomeration of nuclei particles) (Baumgard and Johnson 1996). Approximately 92% of the particles emitted from diesel engines are less than 1.0 μm in diameter (CARB 1997).
Diesel emissions consist of a nonpolar fraction (57%), a moderately polar fraction (9%), and a polar fraction (32%) (Schuetzle and Perez 1983, Schuetzle et al. 1985); the remainder is unrecoverable. The inorganic fraction of the particulate emissions consists primarily of small elemental carbon particles, ranging from 0.01 to 0.08 μm in diameter. Organic and elemental carbon account for approximately 80% of the total particulate matter mass. The remaining 20% is composed of sulfate (mainly sulfuric acid) (Pierson and Brachaczek 1983) and some inorganic additives and components of fuel and motor oil. In general, the organic compounds identified in diesel exhaust emissions contain hydrocarbons, such as alkanes and alkenes, hydrocarbon derivatives, aldehydes, polyaromatic hydrocarbons (PAHs), PAH derivatives, multifunctional derivatives of PAHs, heterocyclic compounds, heterocyclic derivatives, and multifunctional derivatives of heterocyclic compounds (Schuetzle 1988).
Because of their large surface area, diesel exhaust particulates can adsorb relatively large amounts of organic material coming from unburned fuel, lubricating oil, and pyrosynthesis during fuel combustion. A variety of mutagens and carcinogens, such as PAHs and nitro-PAHs, are adsorbed by the particulates (NCR 1982, Tokiwa and Ohnishi 1986, IPCS 1996). The organic-extractable fraction of diesel exhaust particulates is typically in the 20% to 30% range, but it may be as high as 90% (Williams et al. 1989), depending upon vehicle type and operating conditions. In general, incomplete combustion in diesel engines operating under low-load conditions produces relatively low particle concentrations and a higher proportion of organic material associated with the particles (Dutcher et al. 1984). Particulate matter produced at low exhaust-gas temperatures has more adsorbed soluble organics than does particulate matter produced at high exhaust-gas temperatures (Kishi et al. 1992).

Uses

Diesel exhaust particulates have no known uses.

Carcinogenicity

Exposure to diesel exhaust particulates is reasonably anticipated to be a human carcinogen, based on limited evidence of carcinogenicity from studies in humans and supporting evidence from studies in experimental animals and mechanistic studies.

Raw materials

Preparation Products