Review of diesel particulate matter sampling - Department of ...

Figure 14. - Illustration of factors affecting diesel aerosol dilution at the
tailpipe
Data shown in figure 15 came from two different field studies (Kittelson, et al.,1988). In field
study 2 the sampling distance was determined by towing a mobile laboratory on a flatbed
trailer behind the tractor at speeds of 50 to 55 mi/hr (22-25 m/s). Atmospheric residence
times were very short, ranging from 0.01 to 0.4 s. Dilution ratios ranged from 3 to 90:1. In
field study 5 the mobile laboratory followed the tractor down the roadway at the same speeds.
Dilution ratios of 1,000:1 were obtained in approximately 1 s at a distance of about 25 m.
The relationship between the saturation ratio and dilution ratio and impact on the extractable
fraction is complex. The saturation ratio and extractable fraction peak at intermediate dilution
ratios (approximately 5 to 50:1). The opportunity for homogeneous and heterogeneous
nucleation is greatest when the saturation ratio is highest. However, the amount of time spent
at high saturation ratio is very short for on-highway conditions, typically less than 1 sec. The
importance of the extractable fraction to the nuclei mode was suggested in the discussion of
figures 11 and 12 where size distributions for engines with high and low extractable fractions
are shown. The VN/VT ratio (volume fraction in the nuclei mode) for the engine with high
extractable fraction is 53,000 part/μm 3 compared to 5,100 part/μm 3 for the engine with low
extractable fraction.
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