Observations of a Correlation Between Primary Particle and Aggregate Size for Soot Particles

Abstract

For decades, soot has been modeled as fractal-like aggregates of nearly equiaxed spherules. Cluster-cluster aggregation simulations, starting from a population of primary particles, give rise to structures that closely match real aerosols of solid particles produced in flames. In such simulations, primary particle size is uncorrelated with aggregate size, as all aggregates contain primary particles drawn from the same population. Aerosol measurements have been interpreted with this geometric model. Examination of transmission electron micrographs of soot samples from various sources shows that primary particle sizes are not well mixed within an aerosol population. Larger aggregates tend to contain larger primary particles and the variation in size is much larger between aggregates than within aggregates. The soot sources considered here are all substantially not well-mixed (aircraft jet engine, inverted diffusion flame, gasoline direct injection engine, heavy-duty compression ignition engine). The observed variations in primary particle size can be explained if soot aggregates are formed and grew by coagulation in small zones of the combustion chamber, prior to dilution and transport (with minimal coagulation) to the sampling system.

Effective Density and Volatility of Particles Emitted from Gasoline Direct Injection Vehicles and Implications for Particle Mass Measurement

“…Ghazi et al (2013) have shown that the mass-mobility exponent will increase if the primary particle size increases with the soot aggregate size. Although transmission electron microscopy (TEM) images were not collected in this study, Barone et al (2012), Lee et al (2013), , and Dastanpour and Rogak (2014) have all observed that primary particle size increases with soot aggregate size in emissions from GDI engines.…”

Improved sizing of soot primary particles using mass-mobility measurements

“…Details of the experimental methods are further described in Dastanpour and Rogak (2014) and Graves et al (2015).…”

“…Even if 10% of the particle mass was composed of volatile material, primary particle diameter would have approximately decreased by only 5% under the vacuum condition of the TEM (considering densities of the particle core and coating to be 1800 kg=m 3 and 1200 kg=m 3 , respectively). A TSI 379020A two-stage rotary disk diluter was also used upstream of the TD in GDI campaign (Dastanpour and Rogak 2014). The first and second stages of the diluter were heated to 80 C and 300 C, respectively.…”

“…Dastanpour and Rogak (2014) have recently shown that the average diameter of the primary particles changes with the aggregate size in many combustion sources. They have also shown that the primary particles are more uniform in individual aggregates compared to ensembles of aggregates.…”

“…Transmission Electron Microscopy (TEM) is an ex situ method for direct characterization of soot morphology (Medalia and Heckman 1969;Samson et al 1987;Brasil et al 1999;Gaddam and Vander Wal 2013;Kholghy et al 2013;Dastanpour and Rogak 2014;Johnson et al 2015;Thajudeen et al 2015). Statistically reliable primary particle sizes can only be acquired by time-consuming analysis of a great number of TEM images.…”