Bipolar Diffusion Charging of Soot Aggregates

Abstract

Experimental measurements of bipolar diffusion charging efficiency over the range of 15–400 nm are compared for oil droplets, flame generated soot aggregates, and diesel engine exhaust particulate matter to explore possible effects of particle morphology. Charging efficiency is recorded using a tandem differential mobility analyzer (DMA) approach; the first DMA selects a monodisperse aerosol and the second compares the flux of these particles through a neutralizer versus an identical blank housing. Electrostatic precipitation of mobility selected soot particles onto TEM grids provides comparative data on changes in soot particle morphology with mobility diameter. The measurements yield soot charging efficiencies that slightly, but systematically, differ from those of equal mobility diameter oil droplets. Single positive charging of soot climbs to ∼ 10% higher than oil droplets at 50 nm and then decreases to 15% lower by 400 nm as the soot develops a progressively more fractal-like structure. Negative charging exhibits the same pattern, except the variations are +15% and –10%, respectively. These trends, as well as those for double and triple charging, fall intermediate between Fuchs predictions and the model of Wen et al. (1984a) for charging of fibrous particles.

Acknowledgments

I would like to thank Mike Loos and Adolfo Mauti for their generous help with the diesel vehicle tests, Thanasis Mamakos (Aristotle University) for help with the Fuchs calculations, and Yi Liu (Wayne State University) for his expert help creating the TEM images.

Notes

¹For ± 1, fits are to combined present PAO oil and Wiedensoholer et al. (1986) data. For ± 2 and ± 3, fits are to present data only.

²The TSI and Wiedensohler (1988) values for a _{+ 1,4} differ: It is −0.1553 in the former, and −0.1544 in the latter case.