The Relationship between Mass and Mobility for Atmospheric Particles: A New Technique for Measuring Particle Density

摘要

We describe a new technique for measuring the relationship between electrical mibility and mass. For sphericla pargicles, the mass-mobility relationship can be used to determine particle density. For nonspherical particles, this relationship is ffected by both the density and the dynamic shape factor; additional information would be required to determine either one. However, combinations of shape factors and densities that are consistent with measurements can be obtaiend. We show that the density of spherical particles of known composition can be measured to within approx5 with this approach. We applied the technique to urban atmospheric aerosols of approx0.1 and approx0.3 #mu#m in Atlanta, GA, during August 1999. The Atlanta data show that particles of a given mobility often have several distinct masses. Based on complementary measuremtns, we argue that the most abundant mass consits of spherical hydroscopic paticles. The measured mass for these paricles 9assuming that they are spherical) fell into the range of 1.54 to 1.77 g cm~(-3) at 3-6 relative humidity, which agreed to within about 5 of values calculated based on the measured size-resolved composition. Particles that were more and less massive than these were also observed. The less masive particles had "effective densities" of 0.25-0.64 g cm~(-3) and the more massive particles had "effective densities" of 1.7-2.2 g cm(~-3). We hypothesize that the less massive particles consist of chain agglomerate soot.