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

Area of Science:

• Atmospheric Chemistry
• Combustion Science
• Chemical Kinetics

Background:

• Combustion emissions significantly contribute to atmospheric organic acids.
• Current kinetic models underestimate organic acid concentrations due to poorly understood formation mechanisms.
• Aldehydes are key intermediates in combustion processes.

Purpose of the Study:

• Investigate the formation mechanisms of organic acids from C1-C4 aldehydes and benzaldehyde under cool-flame conditions.
• Identify the role of hydroperoxyl (HO2) radicals in aldehyde conversion to organic acids.
• Improve the accuracy of kinetic models for predicting organic acid emissions.

Main Methods:

• In-situ synchrotron vacuum ultraviolet photoionization mass spectrometry (VUV-PI-MS) to observe real-time reactions.
• Quantum chemistry calculations to elucidate reaction pathways on potential energy surfaces.
• Integration of newly identified pathways into existing kinetic models.

Main Results:

• Direct conversion of aldehydes to organic acids was observed.
• Hydroperoxyl (HO2) radicals were found to enhance this conversion process.
• The reaction RC(O)O2 + HO2 on the singlet potential energy surface was identified as a key pathway.

Conclusions:

• A novel mechanism for organic acid formation from aldehydes via HO2 radical reactions was elucidated.
• Incorporating this pathway significantly improved organic acid predictions in kinetic models.
• Understanding cool-flame chemistry is crucial for modeling anthropogenic organic acid emissions and developing mitigation strategies.