Promotion of Particle Formation by Resonance-Stabilized Radicals During Hydrocarbon Pyrolysis

We measured aerosol mass spectra and mobility size distributions of particles formed during the pyrolysis of ethylene, indene, and a mixture of ethylene with a small amount of indene. Measurements were recorded with a scanning mobility particle sizer and an aerosol mass spectrometer employing vacuum ultraviolet photoionization using tunable radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. The results demonstrate that particle formation occurs at a temperature of similar to 1123 +/- 50 K for ethylene alone, at similar to 923 +/- 50 K for indene alone, and at a comparable temperature to that of indene for ethylene seeded with a small amount of indene. Our results demonstrate that indene and indenyl, a resonance-stabilized radical (RSR) formed from indene pyrolysis, promote particle formation at lower temperatures and in the absence of acetylene, supporting the hypothesis that RSRs promote soot-particle inception. Even a small amount of indene added to ethylene promotes particle formation while circumventing the traditional pathways for mass growth by acetylene-addition reactions. Further support is provided by the appearance in the aerosol mass spectra of prominent peaks for masses corresponding to other RSRs from particles formed at lower pyrolysis temperatures. In addition, odd-numbered carbon species, associated with RSRs, appear in higher concentrations at low temperatures and are closely tied to particle inception. In indene pyrolysis, mass signals indicative of covalently bound indenyl dimers and trimers are prominent at pyrolysis temperatures near soot onset. Photoionization efficiency (PIE) curves demonstrate that, while the isomeric composition for some of these peaks may differ between ethylene and indene, the addition of indene to ethylene does not notably alter the isomers formed at these peaks between ethylene and the ethylene-indene mixture. PIE curves also show that m/z 202, which is commonly assumed to be pyrene, is composed principally of fluoranthene under these conditions. (c) 2021 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)