The effective density and fractal dimension of soot particles from premixed flames and motor vehicle exhaust

A tandem differential mobility analyzer (DMA)—electrical low pressure impactor (ELPI)—is employed to measure the effective density, mass per unit mobility volume, of soot particles. These measurements reveal a sharp decline in soot effective density from 1.2 g/cm3 at 30 nm to <0.3 g/cm3 at 300 nm. This dependence on mobility diameter is well described by a fractal dimension of df=2.15±0.10 for flame-generated soot and df=2.3±0.1 for diesel exhaust particulate matter (PM), with slight deviations suggestive of more compact structures noted for particles at the small end of the size distribution. In the flame, the effective density increases with height above the burner, but the fractal dimension remains constant. Exhaust particle effective densities from two light-duty diesel vehicles and a direct-injection gasoline vehicle are virtually indistinguishable. There is a small, 20%, systematic variation in effective density between idle, 64 km/h, and 112 km/h operation, but this appears to be averaged out over transient vehicle operation. The relative independence from speed, load, and driving mode suggests the possibility that particle-sizing instrumentation may afford an accurate alternative to filter collection for the measurement of PM mass emissions.