Global warming
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Study Links HFOs to Formation of Super Greenhouse Gas HFC-23

Atmospheric HFO degradation products TFA and TFE, the study says, have pathways to generating HFC-23, which has a GWP of 14,600.

A new study points to a link between the long-term fate of HFO degradation products in the atmosphere and the formation of HFC-23, a potent global warming gas with a 100-year GWP of 14,600 (IPCC, AR6).

The study – “On the Chemical Pathways Influencing the Effective Global Warming Potential of Commercial Hydrofluoroolefin Gases” – was published April 4 in the journal ChemSusChem. Its author is Gabriel Salierno, a Green Chemist at the Toxics Use Reduction Institute at the University of Massachusetts/Lowell.

Widely used HFOs such as HFO-1234yf and HFO-1234ze(E) have a very low GWP (less than 1), and HFO-1336mzz(Z) has a GWP of 2; but if their atmospheric degradation products ultimately produce HFC-23, their effective GWP would be much higher, the study explains. These degradation products include trifluoroacetic acid (TFA), trifluoroacetaldehyde (TFE) and trifluoroacetyl fluoride (TFF), all considered per- and polyfluoroalkyl substances (PFAS), or “forever chemicals,” under a generally accepted definition set by the Organisation for Economic Co-operation and Development (OECD).

TFA results from the total atmospheric decomposition of HFO-1234yf and other f-gases and is known to be captured in the atmosphere by rainwater, which distributes it throughout the environment with possible health impacts. TFE is formed from the complete atmospheric breakdown of HFO-1234ze(Z) and HFO-1336mzz(Z). (TFF is rapidly transformed into TFA via hydrolysis.) “Although the atmospheric reaction networks of TFE, TFF, and TFA have a fair level of complexity, the relevant atmospheric chemical pathways are well characterized in the literature, enabling a comprehensive hazard assessment of HFC-23 formation as a secondary HFO breakdown product in diverse scenarios,” the study said.

Given the link to HFC-23 formation, 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. “While further research is crucial to refine climate risk assessments, the existing evidence suggests a non-negligible climate hazard associated with HFOs,” the study said.

The definition of GWP considers the average atmospheric lifetime of a gas but only includes the GWP of degradation products “after high confidence confirmation,” the study said. But “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.”

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.

Current atmospheric levels of HFC-23 have experienced a significant increase, and 55% of the excess “is not explained” by the failure of countries to meet reduction targets of HFC-23 emissions and HCFC-22 production, the study said. “There is a gap in the understanding of current atmospheric HFC-23 concentration trends that motivates the present review as a comprehensive global warming hazard assessment of HFOs.”

Since TFA is the one of the most likely breakdown products of HFOs with three or four carbon atoms and many other PFAS, conversion to HFC-23 “is a hazard that cannot be overlooked and might explain unexpected but certain atmospheric HFC-23 surges,” the study said.

The study recommended a reevaluation of the low-GWP designation of commonly used HFOs. “While these substances do have a relatively short atmospheric lifespan, their tendency to break down into HFC-23 can have significant, yet often overlooked, implications for the global efforts to address climate change.”

Leading to HFC-23

While HFOs are acknowledged to degrade in the atmosphere primarily to TFE and TFA, many secondary breakdown pathways “are notably leading to HFC-23” within 100 years, the study concluded.

For example, it was recently found that HFC-23 and carbon monoxide are the primary product of the fast, spontaneous and unimolecular ultraviolet (UV) B-facilitated decarbonylation of TFE. Thus, sunlight operating about half the day can lead to an effective 100-year GWP of 1,920 ± 900 for HFOs like HFO-1234ze(E) and HFO-1336mzz(Z) that break into TFE, according to the study. The study described decarbonylation of TFE by UVB radiation as having “medium environmental likelihood,” adding, “This pathway alone suggests that these HFOs could be significant global warming agents.”

The study noted that research funded by fluorocarbon gas manufacturers suggests that TFE would not produce HFC-23. However, the study found that “the methods employed to disprove the formation of HFC-23 from TFE via photolytic decarbonylation” to be insufficient. The American Chemistry Council did not immediately respond to a request for comment on the study.

For TFA, the study points to several “decarboxylation pathways” that produce HFC-23 and CO2, both in the atmosphere and on or near Earth.

For example, visible sunlight offers a decarboxylation route for TFA with “high environmental likelihood” and a 3,500–14,800 effective 100-year GWP contribution. In particular, TFA present in water aerosol is “susceptible to reacting with widely available visible sunlight,” and “HFC-23 production is thermodynamically favored.” Moreover, the increase of TFA in surface waters seen in a number of studies “exacerbate the concentration of TFA in re-evaporated water aerosols.”

Another pathway to HFC-23 from TFA, hydrothermal decarboxylation from ocean heat, is unlikely considering typical ocean temperature ranges – though it may become more likely as ocean heat content increases to record-breaking levels, the study said. This process has “medium-to-low environmental likelihood” but would be “substantial” at temperatures exceeding 35°C (95°F), with a 111–800 effective 100-year GWP contribution.

In addition, certain aerobic soil microorganisms and fungi – such as mycorrhizal fungi, particularly Pisolithus tinctorius – are able to convert TFA into HFC-23 via decarboxylation, with “medium- to-low environmental likelihood,” the study said.

Finally the study noted that hydroxyl (OH) radicals – which are the catalyst for converting HFO-1234yf into TFA – can convert up to 20% of gaseous TFA into HFC-23. But given that this competes with rainwater for the attention of TFA (wet deposition), the study said the formation of HFC-23 in this case has “low environmental likelihood” with a 0–3,000 effective 100-year GWP contribution.

The Office of Air and Radiation at the U.S. Environmental Protection Agency (EPA) has calculated the net benefit of $265–$270 billion from phasing down HFCs, considering both climate benefits and compliance costs. However, the study noted that models estimating the social costs of greenhouse gases (SC-GHG) do not incorporate “environmental fate reactions” and are limited to describing climate impact using GWP based on gases’ atmospheric lifetimes.

Thus, incorporating secondary HFC-23 generation into the analysis of HFOs “reveals the possibility of a substantially higher SC-GHG relative to their market price and expected profits,” the study said. “This signifies that the environmental impact of HFOs may exceed their economic feasibility, thereby warranting careful consideration of the trade-offs between commercial interests and climate change mitigation efforts.”

Other studies

Another recent study has determined that three HFOs react with ozone when they are released into the atmosphere to produce HFC-23. The study, “Ozonolysis can produce long-lived greenhouse gases from commercial refrigerants,” was published on December 11, 2023, in the Proceedings of the National Academy of Sciences (PNAS).

The study found in experiments that three HFOsHFO-1234ze(E), HFO-1336mzz(Z) and HFO-1243zf – generated HFC-23 by reacting with ozone (ozonolysis), with HFO-1234ze(E) producing more than eight times as much R23 as the other two. But two other HFOs – HFO-1234yf and HCFO-1233xf – did not produce R23 from ozone in the experiments.

In addition, a 2021 study by researchers at the University of New South Wales (UNSW) in Sydney, Australia, linked production of HFC-23 in the atmosphere to HFO-1234ze(E) as a result of oxidation by OH radicals and photolysis (action of light) of degradation product TFE.

“While these substances do have a relatively short atmospheric lifespan, their tendency to break down into HFC-23 can have significant, yet often overlooked, implications for the global efforts to address climate change.”

– Gabriel Salierno, author of “On the Chemical Pathways Influencing the Effective Global Warming Potential of Commercial Hydrofluoroolefin Gases”

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