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Stratospheric Ozone

HCFOs and Stratospheric Ozone

HCFOs contain chlorine and as a result their potential impact on the ozone layer has been studied. HCFO-1233zd(E) is oxidised rapidly in the lower atmosphere with atmospheric lifetimes of 42 days (AR6 value), hence it is a very short lived substance (VSLS) [see explanatory note] that, in view of its very minimal effect on stratospheric ozone, is not listed as an Ozone Depleting Substance in Annex II of Regulation (EU) 2024/590 on substances that deplete the ozone layer. Annex II of Regulation 2024/590 lists Ozone Depleting Substances not controlled under the Montreal Protocol.  It takes several months for a substance released in northern temperate regions of the world to be transported through the lower atmosphere before it is injected into the stratosphere. Consequently, very little of these HCFOs can be transported to the ozone layer.

In the case of HCFO-1233zd(E), for material emitted between 30° and 60°N, an average Ozone Depletion Potential (ODP) has been calculated to be 0.00034 [1][2]. A more recent paper reported essentially the same ODP of HCFO-1233zd(E) at 0.0003 [3][4]. The authors of the papers which reported these data stated that “The short lifetime, low ODP, and low GWP indicate that [these substances] should have minimal effects on ozone and climate.” The SAP 2022 Assessment Report [1 quote Annex, 3] states the ODP of HCFO-1233zd(E) is <0.0004.
Explanatory note: Very short lived substances (VSLSs) are defined as trace gases whose local lifetimes are shorter than 0.5 years and consist of organic and inorganic halogenated source gases (SGs) and product gases (PGs). The SGs include all VSLSs present in the atmosphere in the form they were emitted [5].

HFCs and stratospheric ozone

Chemical Reaction Effects

In 1993, preliminary work was presented suggesting that CF3O radicals formed in the degradation of many HFCs could deplete stratospheric ozone [6]. Later the same year it was shown that CF3O radicals react only slowly with ozone and that HFCs do not destroy ozone through a catalytic cycle involving CF3O [7][8]. Another paper in 1995 [9] considered the possible depletion of stratospheric ozone by other species formed in the atmospheric degradation of HFCs. This paper concluded that of all the known fluorine-containing radical species, only fluorine atoms themselves react at an appreciable rate with ozone, and the concentration of fluorine atoms is sharply limited by the irreversible formation of HF. Fluorine radical species do not participate in direct catalytic ozone-depletion cycles of any significant length and do not impact stratospheric ozone concentrations and that the available scientific data strongly suggest that HFCs pose no threat to stratospheric ozone. These results were briefly discussed in a 2015 chemical review “Atmospheric Degradation of Ozone Depleting Substances, Their Substitutes, and Related Species” [10]. These conclusions would also be valid for HFOs.

Temperature Effects

The potential effect of HFCs on stratospheric temperatures may augment the reactions that deplete stratospheric ozone. Calculations were performed by computer modelling using scenarios of future HFC production and release up to the year 2050. A business-as-usual (without the Kigali Amendment) HFC scenario was used to calculate the total projected impact on globally averaged total ozone from HFCs and concluded that it remains about 0.1 DU (Dobson unit) by 2050 [11]. To put this in context, the ozone layer’s average thickness is about 300 Dobson Units [12], and the natural variability is larger [13]. The Kigali Amendment to the Montreal Protocol will reduce emissions of HFCs compared to the business-as-usual scenario.

  • SAP 2022:  World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, 509 pp.; WMO: Geneva, 2022. Section 1.3 Halogenated very short-lived substances. Available at https://ozone.unep.org/science/assessment/sap ↩
  • Patten, K. O. and Wuebbles, D. J.: Atmospheric lifetimes and Ozone Depletion Potentials of trans-1-chloro-3,3,3-trifluoropropylene and trans-1,2-dichloroethylene in a three-dimensional model, Atmos. Chem. Phys., 10, 10867-10874, https://doi.org/10.5194/acp-10-10867-2010, 2010. ↩
  • SAP 2022:  World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, 509 pp.; WMO: Geneva, 2022. Section 1.3 Halogenated very short-lived substances. Available at https://ozone.unep.org/science/assessment/sap ↩
  • Sulback Andersen, M.P., J.A. Schmidt, A. Volkova, and D.J. Wuebbles, A three-dimensional model of the atmospheric chemistry of E and Z-CF3CH=CHCl (HCFO-1233(zd) (E/Z)), Atmos. Environ., 179, 250−259, doi:10.1016/j.atmosenv.2018.02.018, 2018. ↩
  • SAP 2022:  World Meteorological Organization (WMO). Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, 509 pp.; WMO: Geneva, 2022. Section 1.3 Halogenated very short-lived substances. Available at https://ozone.unep.org/science/assessment/sap ↩
  • Biggs, C. E. Canosa-Mas, D. E. Shallcross, R. P. Wayne, C. Kelly and H. W. Sidebottom, Proceedings of the STEP-HALOCSIDEIAFEAS Workshop, University College Dublin, Ireland, March 1993, p. 177. ↩
  • Upper limits for the rate constants of the reactions of CF3O2 and CF3O radicals with ozone at 295 K, 0. J. Nielsen and J. Sehested, Chem. Phys. Lett., 1993, 213,433. https://doi.org/10.1016/0009-2614(93)89139-9. ↩
  • Do Hydrofluorocarbons Destroy Stratospheric Ozone? A. R. Ravishankara, A. A. Turnipseed, N. R. Jensen, S. Barone, M. Mills, C. J. Howard and S. Solomon, Science, 1994, 263, 75. https://doi.org/10.1126/science.263.5143.71. ↩
  • Hydrofluorocarbons and stratospheric ozone, Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. J. Chem. Soc., Faraday Discuss. 1995, 100, 55.1995, DOI:10.1039/fd9950000055  http://dx.doi.org/10.1039/fd9950000055. ↩
  • Atmospheric Degradation of Ozone Depleting Substances, Their Substitutes, and Related Species, J. B. Burkholder, R. A. Cox, and A. R. Ravishankara, Chem. Rev. 2015, 115, 3704−3759, https://doi.org/10.1021/cr5006759 ↩
  • Hurwitz, M.M., E.L. Fleming, P.A. Newman, F. Li, and Q. Liang, Early action on HFCs mitigates future atmospheric change, Environ. Res. Lett., 11, doi:10.1008/1748-9326/11/11/114019, 2016. ↩
  • From https://ozonewatch.gsfc.nasa.gov/facts/dobson_SH.html. The Dobson Unit is the most common unit for measuring ozone concentration. One Dobson Unit is the number of molecules of ozone that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the Earth). Expressed another way, a column of air with an ozone concentration of 1 Dobson Unit would contain about 2.69x1016ozone molecules for every square centimeter of area at the base of the column. Over the Earth’s surface, the ozone layer’s average thickness is about 300 Dobson Units or a layer that is 3 millimeters thick. ↩
  • Natural Variability of Stratospheric Ozone, Lead Authors: Elisa Manzini & Katja Matthes  http://www.atmosp.physics.utoronto.ca/people/sparc/ccmval_final/PDFs_CCMVal%20June%2015/ch8.pdf. ↩