Estimate of HFC-23 production from HFO and HCFO emissions

The Scientific Assessment Panel (SAP) published estimates of photo-chemical production of HFC-23 in the atmosphere, from the photolysis of CF₃CHO (an intermediate breakdown product of some HFCs, HFOs and HCFOs) and ozonolysis of some HFOs [1]. An estimate is now provided by EFCTC based on new information for HFC-23 yield and a review of HFO emissions. The objective of the SAP report [1] was to define an upper limit for this source in 2022 as part of the evaluation of the HFC-23 emissions gap [2] and not an estimate of emissions. The upper limit was estimated from:

• The CF₃CHO UV photolysis product yield upper limit of 0.003, based on measurement sensitivity although HFC-23 was not detected, as there was no additional peer-reviewed publication at that time; and
• Global HFC-23 fluxes (emissions) derived using concentrations of relevant HFOs/HCFOs measured over Europe. The SAP report explains that global mean concentrations of these short-lived gases are very likely substantially smaller than those measured over Europe and used to derive the upper limits fluxes of HFC-23.

HFC-23 yield: Thomson et al. [3] reports the overall HFC-23 yield as 0.00064 from atmospheric CF₃CHO. Compared to the upper limit of 0.003, used in the SAP report, this would reduce the HFC-23 production by a factor of about 4.7.

Global emissions of HFOs and HCFOs: European atmospheric abundances of HFO-1234ze(E), HFO- 1336mzz(Z) and HCFO-1233zd(E) were used to derive global emissions for these substances. The SAP report [1] estimates global emissions of about 109,000 tonnes for HFO-1234ze(E), 52,000 tonnes for HFO-1336mzz(Z) and 75,000 tonnes for HCFO-1233zd(E), a total of about 235,000 tonnes. See explanatory note for calculation. Clearly these are a considerable overestimate of global emissions as commercial scale production is reported to have started in the period 2012 to 2017 [4] at a much smaller scale than these emissions. These three substances are used mainly as refrigerants and foam blowing agents, which means that production must exceed emissions for recently introduced substances. For example, 7 years after HFC-134a was introduced annual emissions were about 40% of annual production [5]. According to one 2020 report [4], Europe was the biggest market for HFO-1234ze(E), with Europe and Japan being the major markets for HFO- 1336mzz(Z) and HCFO-1233zd(E).  The total supply of all three substances reported for Europe in the years 2019 – 2022 is around 8000 tonnes, with about 1000 tonnes in 2016 [6]. One publication [7] estimated EU emissions of these substances as about 4500 tonnes in 2020. Emissions from Switzerland were reported for HFO-1234ze(E) and HCFO-1233zd(E) at about 41 tonnes/year for 2019/2020 [8]. While an estimate of global emissions is not possible, it is likely that annual emissions of these three substances are considerably less than 40,000 tonnes in 2022, a reduction of at least a factor of 5 compared to upper limit emissions reported in the SAP [1].

Estimate of HFC-23 production from HFO and HCFO emissions: The SAP report estimated an upper limit of 340 tonnes/year HFC-23 production from CF₃CHO due to emissions of HFO-1234ze(E), HFO- 1336mzz(Z) and HCFO-1233zd(E). Considering the recently published HFC-23 yield and a lower emission estimate results in an estimated annual HFC-23 production of less than 15 tonnes in 2022.  Production of HFC-23 from reaction of the HFO or HCFO with ozone is estimated at less than 2 tonnes annually in 2022, compared to about 9 tonnes in the SAP report. Globally, emissions of HFC-23 were 14,000 ± 900 tonnes/year in 2023 [2].

Explanatory note, example of HFC-23 emissions calculation: The SAP report [1] estimated an upper limit production of < 150 tonnes/year of HFC-23 due to emissions of HFO-1234ze(E). This equates to approximately 108,500 tonnes of HFO-1234ze(E) emissions. HFO-1234ze(E) gives 100 % yield of CF₃CHO. The SAP report states that 75% of CF₃CHO undergoes photolysis with a maximum yield of 0.003 HFC-23. The calculation is then: 108,500*0.75*0.003*70 (mol. wt. of HFC-23)/ 114 (mol. wt. HFO-1234ze(E)) = 149.9 tonnes HFC-23.  A similar calculation applies to HCFO-1233zd(E), but HFO- 1336mzz(Z) gives 200% yield of CF₃CHO.

References

[1] Report of the Scientific Assessment Panel in response to Decision XXXV/7: Emissions of HFC-23, 15 September 2024, Lead Authors: S. A. Montzka, NOAA Global Monitoring Laboratory, USA; J. B. Burkholder, NOAA Chemical Sciences Laboratory, USA, available here, or from or from  Scientific Assessment Panel (SAP) | Ozone Secretariat (unep.org).

[2] Adam, B., Western, L.M., Mühle, J. et al. Emissions of HFC-23 do not reflect commitments made under the Kigali Amendment. Commun Earth Environ 5, 783 (2024). https://doi.org/10.1038/s43247-024-01946-y

[3] Thomson, Joshua D., Jyoti S. Campbell, Ethan B. Edwards, Christopher Medcraft, Klaas Nauta, Maria Paula Pérez-Peña, Jenny A. Fisher, David L. Osborn, Scott H. Kable, and Christopher S. Hansen. "Fluoroform (CHF₃) Production from CF₃CHO Photolysis and Implications for the Decomposition of Hydrofluoroolefins and Hydrochlorofluoroolefins in the Atmosphere." Journal of the American Chemical Society (2024).

[4] Booten, Charles W., Scott R. Nicholson, Margaret K. Mann, and Omar Abdelaziz. Refrigerants: market trends and supply chain assessment. No. NREL/TP-5500-70207. National Renewable Energy Lab. (NREL), Golden, CO (United States), 2020. [5] AFEAS HFC-134a production and release data, https://unfccc.int/sites/default/files/hcfc134a00.pdf.

[6] European Environment Agency (EEA) Report, Fluorinated greenhouse gases 2024, https://www.eionet.europa.eu/etcs/etc-cm/products/etc-cm-report-2024-05.

[7] Behringer, D., Heydel, F., Gschrey, B., and others, for Umweltbundesamt (UBA), Dessau-Roßlau (Germany). (2021). Persistent degradation products of halogenated refrigerants and blowing agents in the environment. Type, environmental concentrations, and fate with particular regard to new halogenated substitutes with low global warming potential. Final report. https://www.umweltbundesamt.de/publikationen/persistent-degradation-products-of-halogenated

[8] Rust, Dominique, Ioannis Katharopoulos, Martin K. Vollmer, Stephan Henne, Simon O'doherty, Daniel Say, Lukas Emmenegger, Renato Zenobi, and Stefan Reimann. "Swiss halocarbon emissions for 2019 to 2020 assessed from regional atmospheric observations." Atmospheric Chemistry and Physics 22, no. 4 (2022): 2447-2466. https://doi.org/10.5194/acp-22-2447-2022