Work

Aldehyde cool-flame chemistry explains a missing source of organic acids

Year: 2025

Type: article

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—C1–C4 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 HO2 radicals. Quantum chemistry calculations reveal that the reaction of RC(O)O2 with HO2 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. Using synchrotron vacuum ultraviolet photoionization mass spectrometry and quantum chemistry, this study uncovers a previously missing process by which aldehydes in engine combustion form organic acids, a finding critical for accurate emissions modelling and mitigation strategies.

Source: Nature Communications

DOI: 10.1038/s41467-025-67986-w