Relation between particle mass and number for submicrometer airborne particles

The relationship between particle mass and the number of ambient air particles for the submicrometer size range was examined using a tapered element oscillating microbalance (TEOM) to determine the mass concentration, and a scan-ning mobility particle sizer (SMPS) to determine the volume concentration and total number of particles. The techniques were validated through their application to the estimation of submicrometer particle density for two laboratory generated aerosols of known bulk density (sodium chloride and di-2-ethylhexyl-sebacate). Further evaluation was done with the submicrometer fraction of laboratory generated environmental tobacco smoke (ETS), for which the estimated density of 1.18±0.06 g/cm3 was very close to the previously reported literature value of 1.12 g cm3. Finally, ambient air particles were examined and an estimate of the average submicrometer particle densities for these aerosols was found to vary from 1.2 to 1.8 g cm-3 depending on the time of day. This high variation in the density of the ambient air submicrometer particles, makes it hard to estimate the mass concentration from the SMPS number concentration with better than 60% uncertainty, based on an assumed density value.