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

Saturation Ratio
3
2.5
2
1.5
1
0.5
0
Influence of Exhaust Soot on Saturation Ratio
Adiabatic Dilution, Equilibrium Adsorption
High saturation ratios may lead to nucleation of new particles
1 10 100 1000
Dilution Ratio
Low Soot High Soot
Figure 8.- Saturation vs. dilution ratio for engines emitting different amounts
of DPM.
Residence Time
The kinetics of adsorption become important when the exhaust is going through rapid changes
in temperature or adsorbate concentration. For laboratory measurements, filters used to
measure DPM are usually in the exhaust stream long enough that it is assumed that the SOF
will reach equilibrium with the solids on the filter. For laboratory studies attempting to
simulate on-highway dilution conditions using near real-time instruments, the rate at which
adsorption and condensation occurs must be considered.
It is expected that the tendency to produce nuclei during dilution will be maximized at an
intermediate rate of dilution. Higher saturation ratios will be produced and nucleation will be
more likely if dilution takes place on a time scale faster than characteristic times for
adsorption. Under these circumstances, there is little time for adsorption to take place and the
exhaust behaves as if there is a much smaller surface area available for adsorption. On the
other hand, if the rate of dilution is too high, the exhaust will spend little time in the
intermediate dilution ratio range where saturation ratios and nucleation rates are highest and
nucleation will be minimized.
Calculations were made for adsorption of a representative organic at dilution conditions that
led to the highest saturation ratio. A typical SOF of 10% on DPM was assumed and an
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