Structural Properties of Diesel Exhaust Particles Measured by Transmission Electron Microscopy (TEM): Relationships to Particle Mass and Mobility

Abstract

Structural properties of diesel particles preclassified by particle mobility and mass are measured using transmission electron microscopy (TEM). These measurements enable us to determine the dynamic shape factor and inherent material density of diesel particles. We also compare fractal dimensions obtained independently using two different approaches.

We show that the projected area equivalent diameter of mobility-classified diesel particles nearly equals the mobility diameter in the size range reported here (50 to 220 nm). Evidence for doubly charged particles and possible “fragments” are observed for DMA-classified particles on TEM substrates. The fractal dimension was obtained using two independent approaches. Images obtained by TEM were analyzed to determine the maximum length and the number of primary particles. The fractal dimension obtained from these measurements, D _fL, was 1.75. The fractal dimension obtained from the mass-mobility relationship, D _fm was 2.35. We found that these values are in reasonable agreement after accounting for the relationship between the projected area diameter and maximum length. The size-dependent dynamic shape factor and inherent material density of diesel particles are obtained from independent measurements of mobility (DMA), mass (APM), and volume (TEM). We found that the dynamic shape factor increased from 1.11 to 2.21, and that the inherent material density increased from 1.27 to 1.78 g/cm³ as particle mobility size increased from 50 to 220 nm. The increase in dynamic shape factor with size occurs because large particles are more irregular than smaller ones. The increase in density occurs because the ratio of elemental carbon to condensed organics increases with increasing size.