On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: II. Comparison of measurements and theory

Differential mobility analyzers (DMAs) are sometimes used to characterize aerosols that contain aggregates of low fractal dimension. However, these instruments are normally calibrated for spherical particles and the calibrations are not directly applicable to aggregates. A method proposed by Lall and Friedlander [(2006). On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis, I: Theoretical analysis. Journal of Aerosol Science, in press] for characterizing ultrafine aggregate number, surface area and volume distributions by electrical mobility measurements was tested experimentally. The method is best applied to idealized aggregates composed of uniform primary particles smaller than the mean free path of the gas. It relates the number and size of the primary particles that compose the aggregate to the mobility diameter of a spherical particle. Aggregate number distributions were obtained by calculations based on aggregate drag and aggregate charging efficiency; surface area and volume were obtained by summing over the primary particles that compose the aggregate.The theory was tested experimentally using silver aggregates generated by an evaporation–condensation method. Primary particle diameter was 18.5±3.5nm. To obtain distributions with respect to particle volume, aggregates were sintered to form spheres. It was assumed that the aggregate volume does not change upon sintering and coagulation was neglected. Thus the number of aggregates in a given volume   range (number distribution, dN/dlogv vs. vv) should not change after sintering. Agreement between aggregate number distribution based on idealized aggregates and the values measured for spheres of sintered aggregates was good. The agreement also indicates that the aggregate volumes based on idealized aggregates were accurate. The aggregate number distribution and volume based on the conventional calibration for spheres were significantly overpredicted. A separate experimental test of the theory was made using literature data for diesel aggregates. Primary particle diameter was 31.9±7.2nm. Aggregate volumes calculated from theory agreed well with aggregate volumes measured by transmission electron microscope analysis.