Reaction of HO2 radical with trifluoroacetaldehyde (CF3CHO) in the atmosphere

Trifluoroacetaldehyde (CF₃CHO) is an intermediate degradation product for some HFOs and HCFOs (not HFO-1234yf) produced by their reaction with hydroxyl radical (OH). Long et. al. [1] report investigations that are mainly focused on obtaining quantitative rate constants for the reactions of aldehydes with HO₂ radical, using computational chemistry. Such rate constants are fundamental parameters for atmospheric modelling. The paper provides new results for the full range of atmospheric temperatures and pressures for the HO₂ reaction with CF₃CHO and concludes that the present findings also have probable implications in understanding the atmospheric lifetimes of perfluorinated aldehydes. The results show that the theoretical methods and computational strategies used in the paper have accuracies comparable to available experiments, where experimental data is available, i.e. for HCHO and CH₃CHO, but no experimental data was reported in the Long, et. al. paper for CF₃CHO reaction with HO₂ radical, and according to Long et. al., there are no reported data elsewhere for HO₂ + CF₃CHO kinetics.

From the calculated rate constants for CF₃CHO with HO₂ radical, the paper concludes that the reaction of CF₃CHO with HO₂ radical is the dominant reaction pathway compared to reaction with OH radical.  The paper comments that in the literature, the major reaction for these aldehydes is considered to be reaction with OH. However, the reported data shows that the reaction of HO₂ with CF₃CHO dominates over OH + CF₃CHO at 0−50 km altitude [the troposphere is up to about 14 km].

According to the paper, the intermediate from the reversible reaction of CF₃CHO with HO₂ radical can further react with NO. Although the paper does not discuss the atmospheric fate of the NO adduct, it may result in efficient removal of the aldehyde, according to a paper [2] referenced by Long et. al.

CF₃CHO + HO₂ CF₃CH(OH)OO

CF₃CH(OH)OO + NO à CF₃CH(OH)O + NO₂

EFCTC comment

CF₃CH(OH)O radical could subsequently break down on the C-C bond which leads to CF₃ radical and HCOOH (formic acid, one of the most abundant organic molecules in Earth’s atmosphere). The CF₃ radical would react further leading to formation of CO₂ and HF via initial reaction with O₂. Further work would be required to determine the different alkoxy radical pathways. Based on the kinetic data presented in the paper, and the average atmospheric concentrations of HO₂ and OH radicals [3], a simple calculation, as very rough estimate, is that 70% of CF₃CHO degradation may potentially occur via the HO₂ reaction pathway, with the remainder via photolysis or reaction with OH radical. However, a more detailed transport model that accounts for the different physicochemical processes would provide a more accurate estimate. Potentially, there are three important pathways for CF₃CHO degradation in the troposphere:

• Photolysis which leads to complete degradation in the troposphere. According to Sulbaek-Andersen, et. al. [4] any CF₃CHO produced within the lower troposphere will be a negligible source of CF₃
• Reaction with OH radical lifetime which leads to minor formation of TFA.
• Reaction with HO₂ radical which may potentially lead to complete degradation.

The photolytic lifetime of trifluoroacetaldehyde (CF₃CHO): A revised atmospheric lifetime for the photolysis reaction of CF₃CHO has been reported by Sulbaek-Andersen, et. al. [4].  The revised lifetime of about 13 days, is longer than the previously determined value of ~ 2 days [5]. The significance of this revision is to (i) reduce the relative contribution of photolysis to the atmospheric removal of CF₃CHO compared to earlier evaluations, and (ii) increase the relative contribution of other removal processes (i.e., reaction of CF₃CHO with HO or HO₂ radicals) compared to earlier evaluations. This Sulbaek-Andersen, et. al. paper was also reported in the November 2023 EFCTC newsletter.

References

[1] Quantitative Kinetics of HO₂ Reactions with Aldehydes in the Atmosphere: High-Order Dynamic Correlation, Anharmonicity, and Falloff Effects Are All Important, Bo Long, Yu Xia, and Donald G. Truhlar, Journal of the American Chemical Society 2022 144 (43), 19910-19920, https://doi.org/10.1021/jacs.2c07994

[2] Kinetics of α-Hydroxy-alkylperoxyl Radicals in Oxidation Processes. HO₂•-Initiated Oxidation of Ketones/Aldehydes near the Tropopause, Ive Hermans, Jean-François Müller, Thanh Lam Nguyen, Pierre A. Jacobs, and Jozef Peeters, J. Phys. Chem. A 2005, 109, 19, 4303–4311, https://doi.org/10.1021/jp044080v

[3] Concentrations of OH and HO₂ radicals during NAMBLEX: measurements and steady state analysis, S. C. Smith, J. D. Lee, W. J. Bloss, G. P. Johnson, T. Ingham, and D. E. Heard, Atmos. Chem. Phys., 6, 1435–1453, 2006, www.atmos-chem-phys.net/6/1435/2006/

[4] Photolysis of CF₃CHO at 254 nm and potential contribution to the atmospheric abundance of HFC-23, Mads Peter Sulbaek Andersen, Sasha Madronich, Joanna May Ohide, Morten Frausig, and Ole John Nielsen, Atmospheric Environment Volume 314, 1 December 2023, 120087, https://doi.org/10.1016/j.atmosenv.2023.120087

[5] Atmospheric Chemistry of Perfluoroaldehydes (CxF2x+1CHO) and Fluorotelomer Aldehydes (C_xF_2x+1CH₂CHO): Quantification of the Important Role of Photolysis, Malisa S. Chiappero, Fabio E. Malanca, Gustavo A. Arguello, Steven T. Wooldridge, Michael D. Hurley, James C. Ball, Timothy J. Wallington, Robert L. Waterland, and Robert C. Buck, J. Phys. Chem. A 2006, 110, 11944.  https://pubs.acs.org/doi/10.1021/jp064262k