Potential emissions of HFC-23 due to reaction of ozone with HFOs/HCFOs

The potential emissions of HFC-23 due to reaction of ozone (ozonolysis) with HFOs/HCFOs reported by McGillen et. al. [1] are extremely small and have not materially contributed to the increase in atmospheric HFC-23 emissions observed in recent years. The SAP 2022 Assessment provides an estimate of HFO emissions and assumes that 50% of the HFO emissions are HFO-1234yf. For 2020, it estimates HFO 1234yf emissions at 30,000 tonnes [2]. Therefore, the other HFO emissions are also estimated to be 30,000 tonnes, and this may have resulted in less than 30 tonnes of HFC-23 emissions in 2020, based on the reported yields of HFC-23 from HFOs by reaction with ozone (ozonolysis).  This is less than 0.2% of the reported emissions of HFC-23 in 2020. According to McGillen et. al., no HFC-23 is formed from HFO-1234yf by reaction with ozone.

The SAP 2022 Assessment reports a rise in HFC-23 emissions derived from atmospheric monitoring to 17.2 ± 0.8 Gg yr–1 (17,200 tonnes) in 2019, and a similar value of 16.5 ± 0.8 Gg yr^–1 (16,500 tonnes) in 2020. Up until 2013, global bottom-up emissions, derived from reported data, track (within ± 2 Gg yr^–1, 2,000 tonnes) the global emissions derived from atmospheric measurements. Similarly, up until 2013, the ratio of HFC-23 emissions to HCFC-22 production (E23/P22) derived from atmospheric data closely matched that derived from bottom-up estimates. Between 2015 and 2019, as reported abatement increased dramatically in China and India, bottom-up emissions (derived from reported data) and E23/P22 declined substantially. However, emissions and E23/P22, derived from atmospheric data, increased. By 2019, the difference between top-down and bottom-up emissions and E23/P22 was the largest since atmospheric records began [3]

The SAP 2022 Assessment provides a forecast of the reduction in emissions as CO₂e from the replacement of HFCs by low-GWP HFOs and non-halogenated alternatives, assuming full compliance with the provisions of the Montreal Protocol, including the Kigali Amendment. Figure 7-5 from the SAP 2022 Assessment, reproduced here, shows historical and projected emissions of ODSs, HFCs and alternatives. The explanation notes that “The contributions of the low-GWP HFOs in panels b and c, are smaller than the thickness of the green curves.” Any potential emissions of HFC-23 due to ozonolysis of some HFOs would not change this observation.

SAP 2022 Assessment Figure 7-5

The use of HFOs and HCFOs with a good balance of safety, technical and environmental properties (low or no flammability, ultra-low GWP) has greatly contributed to the reduction in use of HFCs to meet the requirements of the F-gas Regulation and the Kigali Amendment. The SAP 2022 Assessment [4] states that “In short, the replacement of old equipment containing HFCs with high Global Warming Potentials (GWPs) by new installations and low-GWP alternatives, as well as not-in-kind solutions, has the potential for multiple positive effects on climate change. For example, the emissions of low-GWP alternatives will directly reduce projected radiative forcing of climate. Also, and thought to have greater potential climate benefit, the transition to new refrigerants is an opportunity to implement design changes for achieving higher energy efficiency and therefore lowering greenhouse gas emissions from energy use.”

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] World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, 509 pp.; WMO: Geneva, 2022. Section 7.2.5.1

[3] TEAP Report, September 2023: Volume 6: Response to Decision XXXIV/7: Strengthening institutional processes with respect to information on HFC-23 by-product emissions: 3.3.1 HFC-23 emissions derived from atmospheric monitoring

[4] Scientific Assessment of Ozone Depletion: 2022 Section 7.2.2.3 Energy Efficiency