Aldehyde cool-flame chemistry explains a missing source of organic acids.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 41453876.
- Also identified by DOI 10.1038/s41467-025-67986-w and PMC identifier 12864985.
- Licence recorded as CC BY-NC-ND.
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Abstract
Combustion emission is a significant source of organic acids, impacting atmospheric chemistry and climate. Their formation mechanisms, however, remain poorly understood, leading to underestimation in kinetic models. We investigate the cool-flame oxidation of key combustion intermediates-C<sub>1</sub>-C<sub>4</sub> aldehydes and benzaldehyde. Using in-situ synchrotron vacuum ultraviolet photoionization mass spectrometry, we observe the direct conversion of aldehydes to organic acids, a process enhanced by HO<sub>2</sub> radicals. Quantum chemistry calculations reveal that the reaction of RC(O)O<sub>2</sub> with HO<sub>2</sub> on the singlet potential energy surface contributes to organic acids. Incorporating this pathway into a kinetic model significantly improves organic acid prediction. Despite the high-temperature nature of engine combustion, significant spatial and temporal inhomogeneities (e.g., near-wall regions and crevice volumes) lead to localized cool-flame conditions, facilitating organic acid formation and emission. Elucidating the acid formation under cool-flame conditions provides a critical mechanism for accurately modelling anthropogenic organic acid emissions and developing mitigation strategies.