How aromatic carbonyls autoxidize in the atmosphere?

Aromatic carbonyls, emitted either directly in the atmosphere or secondarily formed through hydrocarbon oxidations, represent one of the key members in the family of volatile organic compounds (VOCs). They are common constituents of natural and polluted atmospheres, and their gas-phase oxidation yields highly oxygenated organic molecules (HOM), which are key to the formation of atmospheric aerosol. Although, there are investigations in explaining the autoxidation chemistry of aliphatic carbonyls (Barua et al., 2023; Castañeda et al., 2012, Wang et al., 2015), insights underpinning the molecular level mechanism for the aromatic carbonyl autoxidation, on the other hand, have remained scarce (Iuga et al., 2008). The present work is an attempt to start filling this gap.

Herein, we conducted a combined theoretical-experimental analyses in atmospheric conditions for the OH radical initiated autoxidation of aromatic carbonyls, namely, benzaldehyde (PhCHO), acetophenone (PhCOCH₃), and phenylethanal (PhCH₂CHO). The energetics of the species in the proposed mechanism were obtained using high-level quantum chemical calculations. Subsequently, master equation simulations and multiconformer transition state theory (MC-TST) were used to estimate the rate coefficients and branching ratios for the autoxidation pathways.

A nitrate-based time-of-flight chemical ionization mass spectrometer (nitrate-CIMS) was used to detect the products in these oxidation reactions. Chemical ionization was achieved by supplying synthetic air (sheath flow) containing nitric acid (HNO₃) under exposure to X-rays. This produces nitrate (NO₃⁻) ions which are mixed with the sample flow and ionizes HOMs as NO₃⁻ adducts. The precursors are mixed in a quartz flow tube reactor where the oxidant OH is produced in-situ by the ozonolysis reaction of tetramethylethylene (TME).

The study indicates that autoxidation in aromatic carbonyls proceeds via a bicyclic peroxy radical (BPR) intermediate similar to that observed in case of toluene autoxidation (Iyer et al., 2023). The mechanism involves opening of the BPR ring to produce ring-broken intermediates having high excess energy. These nascent intermediates can then lead to several autoxidation pathways resulting in the HOM formation. Our flow reactor measurements for PhCHO oxidation at variable reaction times show the ample formation of HOM monomers and dimers, well in-line with the proposed mechanism. 

 

REFERENCES

Barua, S. et al. (2023). Atmos. Chem. Phys., 23, 10517–10532.

Castañeda, R. et al. (2012). J. Mex. Chem. Soc., 56, 316–324.

Iuga, C. et al. (2008). Chem. Phys. Chem., 9, 1453–1459.

Iyer, S. et al. (2023). Nat. Commun., 14, 4984.

Wang, S. et al. (2015). Proc. Combust. Inst., 35, 473–480.