Soot is formed during the incomplete combustion and pyrolysis of hydrocarbons and has an enormous impact on human health and the environment. It is responsible for a majority of the more than 5 million deaths attributed to air pollution worldwide annually, and it is the second leading cause of global warming. Remarkably, despite its ubiquity, the chemical pathways leading to its formation are still a mystery. There is a large body of evidence linking polycyclic aromatic hydrocarbon (PAH) species to soot formation, but the chemical pathway driving the transition of these gas-phase precursors to condensed phase particles (i.e., particle inception) is not known. Another gap in the understanding of soot formation involves the rapid growth of these incipient particles or hydrocarbon clusters by hydrocarbon addition. At temperatures where soot inception occurs (≥~1200 K), the PAHs typically present in appreciable quantity are in the range of two to five aromatic rings and are far too small and volatile to form stable clusters bound by dispersive or van der Waals forces. A long history of research indicates a need for a chemical mechanism that rapidly bonds hydrocarbon precursors covalently at these temperatures. Current proposed chemical mechanisms for cluster formation are simply too slow to explain particle formation. Most PAH-radical reactions require activation of highly stable PAHs through H abstraction via significant barriers. In addition, reactions between radicals quickly consume the radical pool and yield very stable species that again require activation via appreciable barriers. We have combined electronic structure calculations of species of potential importance in soot inception with aerosol mass spectrometry measurements of species observed through extraction of samples from hydrocarbon flames. This study provides clues to the high-temperature production of carbonaceous particles via a free-radical chain-reaction mechanism. This mechanism could be critical to soot formation during combustion. It may also be relevant to the formation of stardust in the outflow from carbon-rich stars in interstellar space.

Finding the smoking gun: Solving the mystery of soot formation

2019 Fall National Meeting

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