Reaction of Ozone with HFOs and HCFOs
The presence of an olefinic bond (C=C bond) in HFOs and HCFOs results in their relatively rapid reaction with hydroxyl (OH) radicals present in the atmosphere, leading to short atmospheric lifetimes measured in days or weeks and small direct global warming potentials. However, the olefinic bond also allows minor reaction with ozone (ozonolysis), and whilst these reactions are slow, McGillen et. al. [1] report that HFC-23 may be produced in very small yields from some HFOs/HCFOs. According to McGillen et. al. no HFC-23 is formed from HFO-1234yf. The paper accounts for this process in atmospheric chemical and transport modelling simulations and reports the indirect global warming potentials due to the formation of HFC-23. The results reported in the paper are summarised in the table below. The calculated indirect GWPs due to HFC-23 formation are also shown in the table. It is worthwhile noting that some hydrocarbon refrigerants have indirect GWPs [2] of similar magnitude (propane, 9.50; n-butane, 6.5). All of these refrigerants (HFOs, HCFOs and HCs) are classified as ultralow GWP refrigerants (0-30 GWP) [3].
The ozonolysis experiments for HFO-1234yf also showed no formation of CF4 which might, potentially, have been formed by a similar mechanism to HFC-23 (CF3H) formation for some HFOs.
| Substance | Structure | Degradation via ozonolysisa | Yield of HFC-23 from ozonolysis routeb | Overall yield of HFC-23 from HFO/HCFO | Indirect GWP due to HFC-23 | Direct GWP AR6 |
| HFO-1234yf | CF3CF=CH2 | 0% | 0% | 0 | 0.50 | |
| HFO-1234ze(E) | CF3CH=CHF | 2.96% | 3.11% | 0.092% | ~ 12 | 1.37 |
| HFO-1336mzz(Z) | CF3CH=CHCF3 | 0.13% | 0.42% | 0.0005% | <0.1 | 2.08 |
| HCFO-1233xf | CF3CCl=CH2 | 0% | 0% | 0 | 4 | |
| HFO-1243zf | CF3CH=CH2 | 1.25% | 0.37% | 0.0046% | <1 | 0.26 |
a: atmospheric modelling results.
b: experimental measurements.
The yields of HFC-23 following reaction with ozone (ozonolysis) are experimental measurements. The percentage of degradation by reaction with ozone route is calculated from atmospheric modelling simulations, with the main degradation route being the reaction with hydroxyl (OH) radical. The indirect GWP due to HFC-23 formation in this table is taken from Fig.4 of the paper. The supporting information for the paper explains that the GWP was calculated for each HFO under consideration and also for CHF3. The calculated direct GWPs for the HFOs in Fig.4 of the paper are slightly different from the 100 year AR6 GWP values, which are included in the table for completeness. HCFO-1233xf and HFO-1243zf are not used commercially as refrigerants but are used in the study due to their molecular structure to improve understanding of the mechanism and structural effects.
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] Lifetimes, direct and indirect radiative forcing, and global warming potentials of ethane (C2H6), propane (C3H8), and butane (C4H10), Atmos. Sci. Lett. 2018;19:e804. wileyonlinelibrary.com/journal/asl2 https://doi.org/10.1002/asl.804 Øivind Hodnebrog, Stig B. Dalsøren, Gunnar Myhre, Center for International Climate and Environmental Research-Oslo (CICERO), Oslo, Norway
[3] Montreal Protocol on Substances that Deplete the Ozone Layer, UNEP 2022 Report of the Refrigeration, Air-conditioning and Heat Pumps Technical Options Committee 2022 Assessment. Table 3-1: Classification of 100-year GWP levels, available at Technology and Economic Assessment Panel (TEAP) | Ozone Secretariat (unep.org)