Experts’ reflections on the Perez-Peña et al. paper titled ‘Assessing the atmospheric fate of trifluoroacetaldehyde (CF3CHO) and its potential as a new source of fluoroform (HFC-23) [1].’

Photolysis yields of HFC-23, general principle

The output from any atmospheric model depends on the input data. In this paper the use of 0.3% and 1% HFC-23 quantum yields directly results in the conclusion that HFC-23 yields are significant. the values were selected as Perez-Peña et al. tested two quantum yields for the HFC-23 channel, 0.3% was chosen to reflect the values in the Andersen paper [2] reported as an upper limit at 1 bar. 1% was the extrapolated value by Campbell [4] and chosen to reflect a realistic upper bound.” The following sections go into more detail on these choices.

The 0.3% quantum yield used by Perez-Peña et al.

The Andersen et al. paper [2] states that no formation of CF₃H (HFC-23) was observed under any of the experimental conditions and an estimated upper limit for the yield of HFC-23 of 0.3 % was established. EFCTC experts note that this is not the quantum yield, but the molar yield.

Andersen et al. further mention that the total quantum yield for CF₃CHO at 308 nm is estimated to be 0.16. This value is near identical to that measured by Chiappero et al. [5] who used a 308-nm pulsed laser radiation source and reported 0.17 for the 308 nm quantum yield. The paper finds, experimentally, an upper limit molar yield of 0.3% CF₃H from photolysis, which means that the upper limit quantum yield for CF₃H formation is 0.3% x 0.16 = 0.05% and not 0.3%. Therefore, the Perez-Peña et al. paper has a higher upper limit for the quantum yield of CF₃H by a factor of about 6, compared to the Andersen et al. paper.

Andersen et al. [2] comment that their results are in contradiction to those published most recently [6], which claimed to have observed HFC-23, and which has created a lot of debate in the scientific community. This is the first study of broadband UV photolysis of CF₃CHO, combined with FTIR detection of CF₃H.” They add that their results show no detectable CF₃H produced during the tropospheric photolysis of CF₃CHO (<0.3%). Diluents (air, N₂, and O₂), pressures (100-700 Torr), and broadband UV (wavelengths of 290-400 nm) used in their work are experimental conditions directly relevant for the tropospheric photolysis of CF₃CHO, i.e. for photochemical processes occurring in the atmosphere from surface to below the tropopause (0-15 km altitude).

EFCTC experts note that the troposphere is where the HFOs and degrade in a matter of days or weeks (see also EFCTC Newsletter item January 2022).

The 1% quantum yield used by Perez-Peña et al.

The 1% value was extrapolated from experiments at low pressure: At zero pressure, the quantum yield of HFC-23 at 308 nm photolysis was measured to be (0 bar) 15%. At 4 Torr of helium, it dropped to 7% and further to 1.5% at 33 Torr pressure of NO radical scavenger. The extrapolated quantum yield at 1 bar pressure was estimated to be ~1%.

Taking into account that the standard atmospheric pressure is 760 Torr, our experts note that it is unclear why this value would have been selected as:

• An extrapolation of the experimental observations to higher pressures may lead to erroneous conclusions and large uncertainties; and
• Results of experiments at higher pressures relevant to the troposphere are available in the Andersen et al. paper [2].

Contribution of the HFC-23 photolysis channel to the current HFC-23 growth rate

The Perez-Peña et al. paper estimates the contribution of HFC-23 due to the photolysis of CF₃CHO formed as an intermediate in the atmospheric degradation of HFO-1234ze: We used HFO-1234ze emissions projections from Wang et al. [3] of 12.6 Gg per year for 2015 and 124.4 Gg per year for 2050, with the former the only data currently available for present-day HFO-1234ze emission estimates. Using 2015 HFO-1234ze emissions, we estimated final HFC-23 mixing ratios after 30 days of simulation of 2.3–8.7 ppq in the global background and 0.14–0.31 ppt over an area representative of China. Recent atmospheric observations show that the annual growth rate of HFC-23 is ∼1 ppt per year. Using four 2015 global scenarios, we find that the HFC-23 photolysis channel of CF₃CHO produced from HFO-1234ze could account for ∼4–15% of the current HFC-23 global growth rate, with the most likely scenarios at the lower end of this range.

It should be noted that the Wang et al. paper [3] mentions a current emission substitute scenario assuming that HCFC-141b is completely substituted by HFO-1234ze, which means the emissions of HFO-1234ze in year 2015 would be 12.6 Gg.” EFCTC experts note that this is a scenario of the effect of HFO-1234ze emissions, and not an emission estimate for 2015. The estimated emissions of HCFC-141b in China in 2015, result from its use in polyurethane foam since about 2000. According to the FTOC 2022 Assessment report [7] there is essentially little if any use of HFOs in foams in Article 5 Parties (including China) at that time (2022) and less than 20,000 tonnes use (not emissions) in foams in non-Article 5 Parties (eg USA, Europe). In fact, it is likely that global emissions of HFO-1234ze were negligible before 2015. This means that degradation of HFO-1234ze can be discounted as contributing significantly to the current global emissions of HFC-23.

 

References and Notes

[1] Assessing the atmospheric fate of trifluoroacetaldehyde (CF3CHO) and its potential as a new source of fluoroform (HFC-23) using the AtChem2 box model, Maria Paula Perez-Peña, Jenny A. Fisher, Christopher Hansen and Scott H. Kable, Environ. Sci.: Atmos., 2023, 3, 1767, https://doi.org/10.1039/D3EA00120B

[2] Tropospheric photolysis of CF₃CHO, M. P. Andersen and O. J. Nielsen, Atmos. Environ., 2022, 272, 118935. Tropospheric photolysis of CF3CHO - ScienceDirect

[3] Emissions, degradation and impact of HFO-1234ze from China PU foam industry, Y. Wang, Z. Wang, M. Sun, J. Guo and J. Zhang, Sci. Total Environ., 2021, 780, 146631.

[4] J. Campbell, PhD thesis, UNSW, Sydney, 2022.

[5] Chiappero,M.S.; Malanca, F.E.; Arguello, G.A.; Wooldridge, S.T.; Hurley, M.D.; Ball, J.C.; Wallington, T.J.; Waterland, R.L.; Buck, R.C. Atmospheric chemistry of perfluoroaldehydes (CxF2x+1CHO) and fluorotelomer aldehydes (CxF2x+1CH2CHO): Quantification of the important role of photolysis. Journal of Physical Chemistry A, 2006, 110, 11944–11953.

[6] https://assets.researchsquare.com/files/rs-199769/v1_covered.pdf?c=1631852903

[7] Montreal Protocol on Substances that Deplete the Ozone Layer United Nations Environment Programme (UNEP) Report of the Flexible and Rigid Foams Technical Options Committee 2022 Assessment Report Figure 4.4 page 77. Available at https://ozone.unep.org/science/assessment/teap