Indirect GWPs for HFCs, HFOs and HCFOs due to formation of HFC-23

A recent paper by Adam et al. [1] models the yields of HFC-23 and calculates the indirect GWPs from atmospheric degradation of HFCs, HFOs and HCFOs. The paper estimates that the production of HFC-23 from their breakdown processes is likely in the range 13 to 35 tonnes/year, which is negligible compared to the global discrepancy between HFC-23 emissions inferred from atmospheric measurements and reported emissions (accounting for <0.5% of the discrepancy).

The paper used a 3D chemistry and transport model using kinetic rate constants, photolysis rates and deposition parameters, as well as considering the uncertainties in these values. Two pathways for the in-atmosphere production of HFC-23 from HFC and HFO source gases have been studied. Firstly, HFC-23 is produced as a minor product in the UV photolysis of trifluoroacetaldehyde, CF₃CHO, which is an intermediate degradation product formed from some F-gases. Studies have shown that the yield of HFC-23 from CF₃CHO UV photolysis at tropospherically relevant wavelengths is small but potentially significant. Secondly, direct formation of HFC-23 has been detected and quantified during the reactions of HFO-1234ze(E), HFO-1336mzz(Z) and HCFO-1233zd(E) with ozone (O₃) [note this is a minor reaction pathway].

HFCs containing CF₃CH₂- groups can form CF₃CHO and include HFC-143a (CF₃CH₃), HFC-236fa (CF₃CH₂CF₃), HFC-245fa (CF₃CH₂CHF₂) and HFC-365mfc (CF₃CH₂CF₂CH₃). These HFCs are monitored in the global atmosphere.  HFOs and HCFOs that form CF₃CHO include HFO-1234ze(E), HFO-1336mzz(Z), HCFO-1233zd(E) and these can also react directly with ozone.

The reported weight yields of HFC-23 from reaction with ozone and photolysis of CF₃CHO for widely used HFOs and HCFOs are shown in table 1, together with the indirect GWPs calculated in the paper.

Table 1.  Median yield of HFC-23 per mass of emissions of each of the three HFOs/HCFOs considered in the model, and indirect GWPs

	Weight yield of HFC-23 from HFO or HCFO kg generated/1000 tonnes emitted & weight % yield		Indirect GWP due to HFC-23 formation
	from reaction with ozone	from photolysis of CF3CHO
HFO-1234ze(E)	150 ± 30 (0.015%)	370 ± 30 (0.037%)	8.5 ± 0.4
HFO-1336mzz(Z)	1.7 ± 0.3 (0.00017%)	260 ± 20 (0.026%)	7.2 ± 0.3
HCFO-1233zd(E)	100 ± 15 (0.01%)	320 ± 20 (0.032%)	7.1 ± 0.5

For completeness, another paper by Garavagno et al. [2] has reported that the reaction of HFO-1234yf with ozone results in the formation of a very low yield of CF₄. The reaction with ozone is a minor route for the degradation of HFO-1234yf. The calculated indirect GWP due to formation of CF₄ is 0.03. HFO-1234yf does not form CF₃CHO as an intermediate degradation product. This paper was discussed in the January 2026 EFCTC Newsletter (Recent paper on reaction of HFOs and HCFOs with ozone)

EFCTC comment: The extremely small indirect GWPs, from formation of HFC-23 or CF₄ means that they do not change the conclusion that these HFO and HCFOs will not make any significant contribution to the radiative forcing of climate change.

For the HFCs, the indirect GWPs, reported in the paper, are negligible compared to their direct GWPs. The indirect GWPs are shown in Table 2

Table 2. Indirect GWPs for relevant HFCs

	Indirect GWP
HFC-143a	4.3 ± 0.3
HFC-236fa	0.81 ± 0.06
HFC-245fa	2.3 ± 0.2
HFC-365mfc	2.8 ± 0.2

Total HFC-23 generated in the atmosphere from HFCs, HFOs and HCFOs. The paper estimates that the contribution of production of HFC-23 from breakdown processes of HFCs, HFOs and HCFOs is likely in the range 13 to 35 tonnes/year, which is far lower than previous “upper bound” calculations, and compares this quantity to reported total emissions of HFC-23 to the atmosphere. These findings suggest that global emissions of HFC-23 are dominated by direct emissions into the atmosphere, and that the contribution of in-atmosphere production to the global discrepancy between inferred from atmospheric measurements and reported emissions is negligible (accounting for <0.5% of the discrepancy). The paper notes that further work to better characterize the key parameters for the generation of HFC-23 from atmospheric degradation of HFCs, HFOs and HCFOs would lead to more accurate estimates of HFC-23 production.

Explanatory note. Papers reporting generation of HFC-23 from HFOs and HCFOs has been discussed in several previous EFCTC newsletters, and slightly different results are reported depending on the atmospheric models used and available experimental data.

References

1. Adam, B., Holland, R., Van Hoomissen, D., Burkholder, J. B., Khan, M. A. H., Griffiths, P., Mühle, J., Shallcross, D., and Rigby, M.: Confirming the minimal role of in-atmosphere production on the global HFC-23 budget using a 3D chemical and transport model, Atmos. Chem. Phys., 26, 10979–10996, https://doi.org/10.5194/acp-26-10979-2026, 2026.
2. Garavagno, M. d. l. A., Wenger. A., Holland, R. E. T., Fena, B. R., Goldstein, S. D., Hicks, D. E., et al., Atmospheric Oxidation of Hydrofluoroolefins and Hydrochlorofluoroolefins by Ozone Produces HFC-23, PFC-14, and CFC-13. Environmental Science & Technology 2025 59 (48), 26031-26040. https://doi.org/10.1021/acs.est.5c11383