Formation of Nanoparticles during Exhaust Dilution

A recent Health Effects Institute (HEI) study (Bagley, et al., 1996) showed dramatic increases in nanoparticle emissions for a 1991 engine compared to 1988 engine, both running on very low sulfur fuel. This has raised great concerns about very high nanoparticle emissions from modern, low particle mass emission engines. However it is important to recognize that the 1988 engine also produced high nanoparticle emissions when it was run on a 0.3% fuel similar to what it would have been operated on when it was new. Furthermore, diesel exhaust size distribution measurements made during 70’s and 80’s often showed high nanoparticle emissions and roadside measurements have nearly always shown a large nuclei (nanoparticle) mode (Whitby and Cantrell, 1975; Whitby, et al., 1975; Wilson, et al., 1977; Kittelson, et al., 1988; Weingartner, et al., 1997). Figures 1 and 2 show size distributions from the HEI study, and from a roadway measurement made in the early 80’s in the plume of an engine of the same make as that used in the HEI study. The size distributions are surprisingly similar. Thus, high nanoparticle emissions are not a new development. What is new, and unexpected, is that nanoparticle emissions may not decrease along with mass emissions, but may actually increase. Nanoparticles are mostly volatile, except those formed from metallic additives. Most of these volatile nanoparticles are formed during exhaust dilution. The number formed is very sensitive to the nature of the dilution process.