Two recent studies have confirmed that HFO-1234ze(E), a commonly used ultra-low-GWP replacement for HFCs, breaks down in the atmosphere to produce a small amount of HFC-23, which has one of the greatest global warming impacts.
The two studies are “Fluoroform (CHF3) Production from CF3CHO Photolysis and Implications for the Decomposition of Hydrofluoroolefins and Hydrochlorofluoroolefins in the Atmosphere,” authored primarily by scientists from the University of New South Wales (UNSW)’s School of Chemistry, in Sydney, Australia; and “CHF3 (HFC-23) and CF3CHO Quantum Yields in the Pulsed Laser Photolysis of CF3CHO at 248, 266, 281, and 308 nm,” by scientists at the Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), in Boulder, Colorado.
Both studies demonstrate how trifluoroacetaldehyde (TFE/CF3CHO), the initial by-product of HFO-1234ze(E) and other f-gases via oxidation in the atmosphere, breaks down further via photolysis to create some HFC-23, also known as fluoroform.
In fact, TFE is a decomposition product of other f-gases, including hydrofluoroolefins (HFOs), hydrochlorofluoroolefins (HCFOs), and hydrofluorocarbons (HFCs), noted the UNSW study, adding that HFOs decompose into TFE at a yield up to 100%.
“We have demonstrated comprehensively that some of the most important HFOs do break down into HFCs and have provided the first hard scientific data needed to model and predict the consequences of large-scale emission,” said Christopher Hansen, one of the UNSW researchers and corresponding author of the study, in a post on the UNSW website.
“We are trying to figure out the consequences of large-scale emission before we’ve potentially harmed the environment and human health in an irreversible way.”
Christopher Hansen, University of New South Wales
The NOAA study’s results “show strikingly good agreement with our numbers,” Hansen told NaturalRefrigerants.com. The UNSW study reported a molar yield of HFC-23 of (1.17 ± 0.27) x 10-3 at 1bar atmospheric pressure and 308nm wavelength(at room temperature).; the NOAA study found a molar yield of (1.71 ± 0.70) x 10-3 at 0.87bar and 308nm (Molar yield, which decreases at higher pressures, is the amount of a product formed in moles per mole of reactant consumed.) Quantum yields were also measured by the studies and were very similar at 1bar and 308mm.
The creation of any amount of HFC-23 in the atmosphere is significant given its high 100-year GWP, 14,600 (IPCC, AR6), and its long atmospheric lifetime, 222 years. HFC-23 has historically been primarily generated during the production of HCFC-22, an ozone-depleting gas that has been prohibited globally. HFC-23 destruction is mandatory for parties to the Kigali Amendment to the Montreal Protocol, the 2016 global agreement to phase down HFCs.
But the NOAA study pointed out that there was a significant discrepancy in 2022 between the expected global industrial emissions of HFC-23 (about 1.8kt/yr) and observations of atmospheric HFC-23 (about 13.9kt/yr). Both new studies address whether HFO conversion to HFC-23 could help explain the discrepancy.
Modest results, so far
The NOAA study concluded that the HFO contribution, to date, is small (about 0.215kt/yr of HFC-23), though HFC-23 production in the atmosphere from the degradation of HFOs “will increase in the future if their use and emissions increase.” The NOAA calculated that oxidation of HFOs and HFCs would produce less than 1% of HFC-23, while ozonolysis of HFOs would produce less than 2 x 10-3%.
The UNSW study calculated a small increase in the 100-year GWP of HFO-1234ze(E), from the “direct” level of 0.315 (IPCC, AR6) to the “indirect” level about 6 for its oxidation pathway to HFC-23. In addition, a 2023 study found that HFO-1234ze and other HFOs react with ozone in the atmosphere to produce HFC-23, leading to an indirect GWP of about 12. Thus the combined direct and indirect GWP of HFO-1234ze(E) from the two pathways amounts to about 20, said the UNSW study.
However, UNSW’s GWP estimate “used a factor from a previous study we did,” Hansen told NaturalRefrigerants.com. “We have not run thorough models with our new numbers. We are collaborating with a U.K. group that is doing the proper modelling and meeting in April.” This modelling work, at UNSW and elsewhere, will help determine “the environmental impact of continuing to use HFOs,” said the UNSW post. The modelling would include considering different atmospheric altitudes and pressures, which strongly affects the yield of HFC-23.
The UNSW researchers released a preliminary study linking HFO-1234ze(E) to HFC-23 in 2021.
The American Chemistry Council, an industry group, did not respond to a request for comment on the new studies.
Another HFO, HFO-1234yf, widely used on car air-conditioning and other applications, has separately been shown to oxidize into trifluoroacetic acid (TFA), which mostly comes down in rainfall. Numerous studies have documented TFA’s proliferation in the environment and its potential impact on human health, including one that calls it a “planetary boundary threat.”
A 2024 study by Gabriel Salierno, a Green Chemist at the Toxics Use Reduction Institute and the University of Massachusetts/Lowell, pointed to pathways by which some TFA could convert to HFC-23. It also studied the conversion of TFE to HFC-23 and concluded that a “lower bound” of the “effective [100-year] GWP” of HFOs would be above regulatory thresholds, which in Europe and the U.S. is 150. “If there is at least a 2% chance of HFC-23 formation over the course of 100 years, that would be sufficient for HFO gases to have an effective GWP over regulatory thresholds,” added the study.
“We don’t fully understand the environmental impacts of HFOs at this point,” said Hansen in the UNSW post. “But, unlike previous examples such as the CFCs and leaded petrol, we are trying to figure out the consequences of large-scale emission before we’ve potentially harmed the environment and human health in an irreversible way. We’re trying to try to change the way that science introduces new products.”
New measurement techniques
Hansen and his team used multiple techniques, including two invented just for this study, to measure and evaluate the chemical reaction across the full range of pressures expected in the atmosphere, he noted in the UNSW post. “We used a variety of spectroscopic techniques to observe the reaction,” he said.
For this study, the UNSW researchers performed experiments at a single wavelength (308nm) – the one the used in studies that are presently guiding regulators, industry and governments, Hansen said, adding, “we plan to study this chemistry using other wavelengths of light, where the yield could be higher or lower.”
