Quantitative kinetics reveal that reactions of HO2 are a significant sink for aldehydes in the atmosphere and may initiate the formation of highly oxygenated molecules via autoxidation

Abstract

Large aldehydes are widespread in the atmosphere and their oxidation leads to secondary organic aerosols. The current understanding of their chemical transformation processes is limited to hydroxyl radical (OH) oxidation during daytime and nitrate radical (NO₃) oxidation during nighttime. Here, we report quantitative kinetics calculations of the reactions of hexanal (C₅H₁₁CHO), pentanal (C₄H₉CHO), and butanal (C₃H₇CHO) with hydroperoxyl radical (HO₂) at atmospheric temperatures and pressures. We find that neither tunneling nor multistructural torsion anharmonicity should be neglected in computing these rate constants; strong anharmonicity at the transition states is also important. We find rate constants for the three reactions in the range 3.2–7.7 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 298 K and 1 atm, showing that the HO₂ reactions can be competitive with OH and NO₃ oxidation under some conditions relevant to the atmosphere. Our findings reveal that HO₂-initiated oxidation of large aldehydes may be responsible for the formation of highly oxygenated molecules via autoxidation. We augment the theoretic studies with laboratory flow-tube experiments using an iodide-adduct time-of-flight chemical ionization mass spectrometer to confirm the theoretical predictions of peroxy radicals and the autoxidation pathway. We find that the adduct from HO₂ + C₅H₁₁CHO undergoes a fast unimolecular 1,7-hydrogen shift with a rate constant of 0.45 s⁻¹. We suggest that the HO₂ reactions make significant contributions to the sink of aldehydes.