New study expands understanding of CF3H formation from HFCs and HFOs/HCFOs

Most HFOs and HCFOs decompose in the troposphere via an intermediate product which then breaks down further. Experimental work, published in 2022, demonstrated that CF₃H (HFC-23) is not formed during the decomposition of HFOs and HCFOs in the troposphere. In this follow-up study [1], the potential formation of CF₃H has been investigated under conditions that occur in the stratosphere, where shorter wavelength more energetic radiation occurs.

The paper concludes that the majority of CF₃CHO will undergo photochemical degradation in the lower troposphere. Hence, photochemical degradation of CF₃CHO will make a negligible contribution to the atmospheric burden of CF₃H. The results suggest that formation of CF₃H from the photolysis of CF₃CHO is restricted to altitudes above 30-40 km, i.e. in the stratosphere. CF₃CHO is primarily a degradation product of CFC-replacement compounds, including third generation replacement compounds such as HFO-1234ze, (CF₃CH=CHCF). The atmospheric lifetimes of HFOs are generally on the order of tens of days, whereas it takes several years for air from the troposphere to be transported up to the stratosphere. Hence, only a small fraction of the HFOs or their degradation products, including CF₃CHO, is expected to reach the stratosphere. Any CF₃CHO either directly emitted or photochemically produced within the lower troposphere, will be a negligible source of CF₃H.

 

Generation of CF₃H from HFC-143a in the stratosphere

The new study explains that long-lived precursors of CF₃CHO, such as HFC-143a (CF₃CH₃), which has an atmospheric lifetime on the order of half a century, can be expected to reach the stratosphere and, through photochemical degradation, deliver CF₃CHO to the stratosphere. For this and similar compounds, the results of the present work suggest that CF₃H may be expected as a secondary degradation product from these compounds in the stratosphere, thus adding a secondary contribution to the radiative forcing of climate. According to the paper the estimated yield of CF₃H from CF₃CHO in the stratosphere is about 2.5%.

Simple Calculation estimating annual CF₃H generation from HFC-143a

Note: This is an EFCTC estimation from published data

A simple calculation allows an estimate of the generation of CF₃H in the stratosphere from HFC-143a. However, a more detailed transport model that accounts for the different physicochemical processes would provide a more accurate estimate. The calculation estimates that emissions of HFC-143a currently generate a maximum of about 20 tonnes of CF₃H annually due to degradation in the stratosphere. The simple calculation uses this data:

• The stratospheric loss of HFC-143a is ∼14% of its total global loss according to Atmospheric Degradation of Ozone Depleting Substances, Their Substitutes, and Related Species, James B. Burkholder, R. A. Cox, and A. R. Ravishankara, Chem. Rev. 2015, 115, 3704−3759, DOI: 10.1021/cr5006759, see Table 6 footnote i.
• 5% yield of CF₃H from CF₃CHO in the stratosphere (from the new study),
• Assumes 100% yield of CF₃CHO from HFC-143a
• A total atmospheric lifetime for HFC-143a of 51.8 years (SAP 2022: World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278), and
• Emissions of HFC-143a reported in SAP 2022 and Global emissions of HFC-143a (CH₃CF₃) and HFC-32 (CH₂F₂) from in situ and air archive atmospheric observations, S. O’Doherty et al., Atmos. Chem. Phys., 14, 9249–9258, 2014, doi:10.5194/acp-14-9249-2014.

 

Reference [1] Photolysis of CF₃CHO at 254 nm and potential contribution to the atmospheric abundance of HFC-23, Mads Peter Sulbaek Andersen, Sasha Madronich, Joanna May Ohide, Morten Frausig, and Ole John Nielsen, Atmospheric Environment Volume 314, 1 December 2023, 120087, https://doi.org/10.1016/j.atmosenv.2023.120087