Update on the atmospheric degradation of HFOs and HCFOs

Update on the atmospheric degradation of HFOs and HCFOs

Three recent academic papers have further enhanced our understanding of the atmospheric degradation of HFOs and HCFOs.

Reaction of Ozone with HFOs/HCFOs. The minor reaction of HFOs and HCFOs with ozone in the atmosphere was already known, but a recent paper by McGillen et. al. [1] has identified the formation of extremely small yields of HFC-23 for some HFOs/HCFOs. The yields vary depending on the specific HFO/HCFO and according to the paper are related to the molecular structure. No HFC-23 was formed from HFO-1234yf, the calculated overall yield of HFC-23 from data reported in the paper for HFO-1336mzz(Z) is 0.0005%, and from HFO-1234ze(E), 0.092%.  The overall yield is calculated from the percentage of HFO/HCFO that reacts with ozone compared to reaction with hydroxyl radical (the main reaction pathway) and the yield of HFC-23 following reaction with ozone. This paper, discussed in more detail in the next newsletter item, was reported in Science with this comment:

“Still, the actual implications of the work—published in the Proceedings of the National Academy of Sciences—are unclear, says Stephen Montzka, an atmospheric scientist at the U.S. National Oceanic and Atmospheric Administration who was not involved in the research. The amount of fluoroform [HFC-23] produced by HFOs is minute, he notes, so they still have a lower global warming potential than chemicals used in the past.”

Potential emissions of HFC-23 from reaction with ozone (ozonolysis) in the context of reported global HFC-23 emissions are discussed in the third newsletter item.

Reaction of HO₂ radical with trifluoroacetaldehyde. A second paper by Long et. al. [2] discussed in the fourth newsletter item, describes a theoretical investigation of the reaction of trifluoroacetaldehyde (CF₃CHO), an intermediate degradation product from some HFOs/HCFOs (not HFO-1234yf), with HO₂ radical in the atmosphere, and concludes that that the reaction of CF₃CHO with HO₂ radical is the dominant reaction pathway compared to the reaction with OH radical. This may influence the yields of important degradation products from the degradation of CF₃CHO.

Photolytic Lifetime of Trifluoroacetaldehyde. The third paper by Sulbaek Andersen et. al. [3], also reported in the November 2023 EFCTC newsletter, provided a revised atmospheric lifetime for the photolysis reaction of CF₃CHO, which is discussed, together with the HO₂ radical reaction pathway.

References

[1] Ozonolysis can produce long-lived greenhouse gases from commercial refrigerants, Max R. McGillen, Zachary T. P. Fried, M. Anwar H. Khan, Keith T. Kuwata, Connor M. Martin, Simon O’Doherty , Francesco Pecere, Dudley E. Shallcross, Kieran M. Stanley , and Kexin Zhang, PNAS 2023 Vol. 120 No. 51 e2312714120, https://doi.org/10.1073/pnas.2312714120

[2] 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

[3] 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