Effects of volatile coatings on the morphology and optical detection of combustion-generated black carbon particles.

Abstract

We have measured time-resolved laser-induced incandescence (LII) from combustion-generated mature soot extracted from a burner and (1) coated with oleic acid or (2) coated with oleic acid and then thermally denuded using a thermodenuder. The soot samples were size selected using a differential mobility analyser and characterized with a scanning mobility particle sizer, centrifugal particle mass analyser, and transmission electron microscope. The results demonstrate a strong influence of coatings particle morphology and on the magnitude and temporal evolution of the LII signal. For coated particles higher laser fluences are required to reach LII signal levels comparable to those of uncoated particles. This effect is predominantly attributable to the additional energy needed to vaporize the coating while heating the particle. LII signals are higher and signal decay rates are significantly slower for thermally denuded particles relative to coated or uncoated particles, particularly at low and intermediate laser fluences. 4 ACKNOWLEDGMENTSWe thank Daniel Strong for the rendition of the experimental setup shown in Fig. 1. We are very grateful to Chris Sorensen for his sage advice on fractal analysis. We also appreciate Alexei Khalizov's insightful comments about soot restructuring and Jeff Headrick's assistance with the TEM image analysis.

Effects of volatile coatings on the laser-induced incandescence of soot

“…Numerous studies have shown that this aggregate morphology can become substantially compressed when the particles are coated with oxygenated hydrocarbons [48,[51][52][53][54][55][56][57][58][59][60][61] or sulfuric acid [21,56,[62][63][64]. This morphology restructuring may persist even when the coating is removed in a thermodenuder [51,53,55,56,60,63,64]. We recently showed that, for oleic acid coatings, the restructuring is largely reversible if the coating is removed by laser irradiation but is irreversible if the coating is removed in a thermodenuder [51].…”

“…Mature uncoated soot aggregates have a dendritic, branched-chain morphology described by a fractal dimension of *1.7 to 1.9 [40][41][42][43][44][45][46][47][48][49][50][51]. Numerous studies have shown that this aggregate morphology can become substantially compressed when the particles are coated with oxygenated hydrocarbons [48,[51][52][53][54][55][56][57][58][59][60][61] or sulfuric acid [21,56,[62][63][64].…”

“…This morphology restructuring may persist even when the coating is removed in a thermodenuder [51,53,55,56,60,63,64]. We recently showed that, for oleic acid coatings, the restructuring is largely reversible if the coating is removed by laser irradiation but is irreversible if the coating is removed in a thermodenuder [51]. The particle maintains its restructured morphology after the coating is removed in a thermodenuder, even if the particle is laser heated after the thermodenuder.…”

Effect of Thermodenuding on the Structure of Nascent Flame Soot Aggregates

“…Here, the term "nascent" is used to refer to freshly emitted soot particles that have a negligible coating on the monomers. The diameter of these monomers varies in a range from 10 nm to more than 50 nm, depending on the fuel source and combustion conditions [4][5][6][7]. During the atmospheric processing, soot particles interact with organic and inorganic materials (in the form of aerosol or condensable vapors).…”

“…Similarly, China et al [8,43], using numerical simulations, predicted small changes in the absorption cross-section (a few percent), but a much more substantial change in the total scattering cross section (up to~300%) upon soot compaction. In addition to affecting the optical properties, changes in the soot structure can also affect the results of laser induced incandescence measurements [5]. Finally, the condensation of secondary organic matter preferentially takes place in empty pores on soot particles, and therefore, it is possible that compaction will affect secondary organic condensation on soot [44].…”