Molecular-Weight Growth: Ozone-Assisted Low-Temperature Oxidation of Crotonaldehyde

Alec C. DeCecco,¹ Alan R. Conrad,¹ Nath-Eddy Moddy,¹ Ahren W. Jasper,² Nils Hansen,³ Philippe Dagaut,⁴ and Denisia M. Popolan-Vaida^1,*

1Department of Chemistry, University of Central Florida, Orlando, FL 32816, USA
2Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA
3Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA
4Centre National de la Recherche Scientifique (CNRS), INSIS, ICARE, 1C Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France

The reaction of unsaturated carbonyls with ozone is recognized to lead to the formation of Criegee intermediates (CIs), which play a key role in controlling the atmospheric budget of hydroxyl radicals and secondary organic aerosols. The reaction network of two CIs (CH₃CHOO and CHOCHOO) formed in the ozone-assisted oxidation reaction of crotanaldehyde is investigated over a temperature range between 390 K and 800 K in an atmospheric pressure jet stirred reactor (JSR) at a residence time of 1.3 s, stoichiometry of 0.5, and 89% Ar dilution. Molecular-beam mass spectrometry in conjunction with single photon tunable synchrotron vacuum ultraviolet radiation is used to identify elusive intermediates by means of experimental photoionization energy scans and ab initio threshold energy calculation for isomer identification. Addition of ozone (1000 ppm) is observed to trigger the oxidation of crotonaldehyde already at 390 K, which is below the temperature where the oxidation reaction of crotonaldehyde in the absence of ozone was observed. The observed crotonaldehyde + O₃ product, C₄H₆O₄, is found to be linked to the ketohydroperoxide resulting from the isomerization of the primary ozonide. A network of CI reactions is identified in the temperature region below 600 K, characterized by CIs addition to species like alkenes, aldehydes, ketones, and carboxylic acids. The region below 600 K is also characterized by the formation of important amounts of typical low-temperature oxidation products, such as H₂O₂, CH₃OOH, and C₂H₅OOH. Detection of additional oxygenated species such as methanol, ethanol, ketene, and aldehydes indicate multiple active oxidation routes. The results of these studies provide new mechanistic insights into ozone-assisted oxidation reactions of aldehydes, which is critical for the development of improved kinetics models.