SAP Report: Photo-chemical production of HFC-23 in the atmosphere
The Report of the Scientific Assessment Panel (SAP) in response to Decision XXXV/7: Emissions of HFC-23 [1], published September 2024, discusses in detail the yields and formation (flux) of HFC-23 from the photolysis of CF3CHO (and intermediate breakdown product of some HFCs, HFOs and HCFOs) and ozonolysis of some HFOs.
According to the SAP report, “HFC-23 is produced in the atmosphere from reactions that oxidize fluorinated gases present in the atmosphere. An upper limit to this source in 2022 is estimated to be 0.43 kt HFC-23 yr-1, which accounts for less than 3.1% of global HFC-23 emissions in that year. This estimate is an upper limit, meaning that the actual value could be substantially smaller. The magnitude of this HFC-23 source is estimated using measured atmospheric abundances of relevant fluorinated gases, where available, and laboratory kinetic studies of the reactions that lead to HFC-23 production.”
The SAP report notes that “Perhaps the largest uncertainty in the derived HFC-23 fluxes stems from use of an upper limit to the CF3CHO UV photolysis product yield of 0.003. If updated work were able to directly measure the product yield of HFC-23 in the photolysis of CF3CHO, it likely would reduce the estimates of total HFC-23 flux, while the absolute contribution from the ozonolysis reaction pathway of the unsaturated compounds would be unchanged.”
Unsaturated source gases (haloolefins) containing a CF3CH= moiety react with the OH radical to lead to the formation of CF3CHO. These same unsaturated source gases can also react with ozone to produce HFC-23 directly (McGillen et al., 2023 [2]), but with small yields and at much slower rates than reaction with OH given typical ambient OH and ozone concentrations. The McGillen et al. 2023 paper is discussed in the EFCTC newsletter [3].
The SAP report states it is important to note that the Sulbaek Andersen and Nielsen study [4] for photolysis of CF3CHO did not detect HFC-23 as a product in the UV photolysis of CF3CHO at wavelengths relevant for the troposphere; they were only able to report an upper-limit on the product yield of HFC-23 based on their measurement sensitivity. Because it was performed in a manner that was more sensitive to detecting production of HFC-23, the Sulbaek Andersen and Nielsen study provides the best upper-limit to the product yield for use in interpreting the atmospheric production of HFC-23. There have been no additional peer-reviewed experimental studies of CF3CHO photolysis under conditions relevant to the troposphere published since the 2022 Scientific Assessment Report.
The SAP report adds that “Furthermore, the annual atmospheric production of HFC-23 from the oxidation of HFOs is likely to be lower on a global scale than the values used to derive HFC-23 flux, given 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.”
The SAP report contains a table [5] reporting the yields of CF3CHO and fluxes of HFC-23 from relevant HFCs, HFOs and HCFOs for photolysis of CF3CHO and ozonolysis of HFOs where data is available.
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] 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
[3] 13 February 2024 EFCTC Newsletter Reaction of Ozone with HFOs and HCFOs
[4] Tropospheric photolysis of CF3CHO, Sulbaek Andersen, M. P., and O. J. Nielsen, Atmos. Environ., 272, 118935 (2022) doi:10.1016/j.atmosenv.2021.118935. Reported in EFCTC News item , January 2022
[5] Report of the Scientific Assessment Panel in response to Decision XXXV/7: Emissions of HFC-23, Table 2 Table 2. Photochemical sources of CF3CHO and HFC-23 (CHF3) for species listed in the 2022 SAR Annex under conditions relevant for the global troposphere.