Improved sizing of soot primary particles using mass-mobility measurements

Abstract

The properties and impacts of aggregated aerosol particles (i.e., soot, metal oxide fumes) depend on their morphology, as characterized by fractal dimension, prefactor, and primary particle diameter. The morphology may be measured directly by time-consuming ex situ microscopy or rapid but indirect in situ methods. Previously, it was found that particle mass and mobility measurements could be used for the estimation of the primary particle diameter of zirconia aggregates, using plausible assumptions related to the fractal structure (specifically, prefactor k a and exponent D a ). Since the formation and growth of zirconia aggregates are different from carbon soot, here we compare primary particle diameters measured directly from transmission electron microscopy analysis of soot particles with the diameters estimated from mass-mobility measurements. Performing extensive measurements on soot emissions from two reciprocating engines over a range of operating conditions, we found that there are no universal values of k a and D a that can be used for all conditions. However, new optimized values of k a and D a are estimated here for soot particles. The variation of the primary particle diameter with particle size is also taken into consideration and is shown to be essential to obtain physically realistic results. Using optimized values of k a and D a , the average primary particle sizing error is reduced for all soot types. This suggests that with some calibration, in situ sizing of the primary particle diameter, using mass and mobility measurements, can provide useful accuracy.

Universal relations between soot effective density and primary particle size for common combustion sources

“…Measurements of the effective density can then be used to estimate the average primary particle size using the method of Eggersdorfer et al (2012), a method that was later optimized by Dastanpour et al (2016) for soot. The approach estimates the Sauter mean diameters of the primary particles, d va ; as…”

“…The form of Equation (3) arises from consideration of the degree of "shielding" of primary particles as m increases. This model was calibrated using TEM images from two types of internal combustion engines to obtain D a ¼ 1.1 and k a ¼ 1.13 (Dastanpour et al 2016), which are the parameter values adopted throughout this review. Using the mass-mobility exponent (Equation (1)), this can be rewritten as…”

Effective density and volatility of particles sampled from a helicopter gas turbine engine

“…Figure 4 shows the primary particle diameters calculated using these relationships which have an uncertainty of at least 20% (Dastanpour et al 2016). The figure shows that the primary particle size of the soot from the 22,000 RPM condition has a larger primary particle diameter and this difference increases with an increase in aggregate mobility diameter.…”

“…The surface-area mean (Sauter) primary particle diameter was calculated using the denuded effective density functions given in Figure 3 using the scaling relationship given by Eggersdorfer et al (2012) and using scaling exponents given by Dastanpour et al (2016) for soot ( D  =1.1; a k =1.13). Figure 4 shows the primary particle diameters calculated using these relationships which have an uncertainty of at least 20% (Dastanpour et al 2016).…”

“…primary particle size; Dastanpour et al 2016). The mass-mobility relationship is often expressed in term of the particle effective density, defined as the mass divided by the mobility-equivalent volume, which is often measured using a differential mobility analyzer (DMA) with a centrifugal particle mass analyzer (CPMA) or an aerosol particle mass analyzer (APM), and condensation particle counter (CPC).…”