The refrigerants and industrial gases meant to replace the most damaging greenhouse gases may themselves be breaking down into persistent environmental toxins and new climate pollutants. Researchers at the University of Bristol are building a photochemical reactor to measure exactly how much trifluoroacetic acid (TFA)—a plant-damaging compound that accumulates in water—and potent long-lived greenhouse gases like HFC-23 and carbon tetrafluoride are produced when hydrofluoroolefins (HFOs) and related compounds react with ozone and hydroxyl radicals in the atmosphere. Current international treaties do not control these fourth-generation compounds because their direct global warming potential is low and they last only days to months. But their breakdown products can persist for decades or centuries. No comprehensive study has yet quantified the yields of these byproducts across the full range of HFOs, hydrobromofluoroolefins, and hydrochlorofluoroolefins now entering commercial use. If the team succeeds, their data will feed directly into the World Meteorological Organization’s Quadrennial Ozone Assessment and inform national and international policy on which replacement compounds to approve. The results could also resolve mismatches between estimated emissions and actual atmospheric measurements of long-lived greenhouse gases and ozone-depleting substances.
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Concerns are growing over the environmental impacts of the halogenated organic compounds currently being developed and deployed as next-generation replacements for hydrofluorocarbons (HFCs). HFCs are widely used industrial gases with many applications, but they are recognised to be long-lived greenhouse gases (GHGs). Hence, they are being phased out under various international environmental agreements and replaced by the shorter-lived hydrofluoroolefins (HFOs), hydrobromofluoroolefins (HBFOs) and hydrochlorofluoroolefins (HCFOs). Current concerns arise from the breakdown products of these replacement compounds in the atmosphere, which include trifluoroacetic acid (TFA), a persistent environmental contaminant known to be a phytotoxin, and trifluoromethane (HFC-23), a potent and long-lived GHG. Recent measurements from our laboratory also show carbon tetrafluoride (PFC-14), another GHG with high global warming potential (GWP) and long lifetime, is a product of ozonolysis of some HFOs, and suggest that similar reaction pathways may produce the ozone-depleting chlorofluorocarbon CFC-13 from the oxidation of certain HCFOs. To date, no study has comprehensively quantified the yields of TFA, long-lived GHGs, and ozone-depleting substances (ODSs) from the oxidation of HFOs, HBFOs or HCFOs by ozone or hydroxyl radicals (OH), despite these reactions being their major loss pathways in the atmosphere. Here, we propose such a study using instrumentation unique to our laboratory. The outcomes will inform the design of future HFOs, HBFOs and HCFOs to minimise the environmental impact of their use. Environmental treaties such as the Montreal Protocol, which came into force in 1987, have been successful in limiting the use of many GHGs and ODSs. HFOs, HBFOs and HCFOs are now being detected in the atmosphere at low abundances. These fourth-generation refrigerants, propellants and foam-blowing agents have atmospheric lifetimes of days to months and GWPs = 1 over a 100-year period. Consequently, these compounds are not controlled under the Kigali Amendment to the Montreal Protocol, nor are they included in the Paris Agreement (successor to the Kyoto Protocol). As such, their production, use and subsequent emission to the atmosphere are expected to increase rapidly over the coming years. The true environmental consequences of growing, and potentially widespread use of these three classes of compounds therefore require urgent study.? We propose to quantify the yields of TFA, long-lived GHGs and ODSs from the oxidation of HFOs/HBFOs/HCFOs via ozonolysis and reaction with the hydroxyl radical, and their impact on global and regional scales. This will be achieved by:?? Building a new photochemical reactor to complement the EXTreme RAnge (EXTRA) ozonolysis chamber already located in our laboratory, with each reactor coupled to a Medusa preconcentration unit and gas chromatograph-mass spectrometer (GCMS) to quantify long-lived GHGs and ODSs from OH reactions and ozonolysis of HFOs, HCFOs, HBFOs.? Quantifying yields of TFA and its precursors, such as trifluoroacetyl fluoride (TFF) and trifluoroacetaldehyde, from HFO breakdown pathways initiated by ozonolysis and OH reactions.?? Computationally modelling the reaction pathways and kinetics to understand the mechanisms that produce long-lived GHGs, ODSs, TFA and TFF.?? Examining the impacts of removal of atmospheric HFOs, HCFOs and HBFOs by ozonolysis and OH reactions on a global scale using 3-D chemistry transport modelling.? The results will inform national and international policy on the use of HFOs/HBFOs/HCFOs, feed into the World Meteorological Organization Quadrennial Ozone Assessment, and help resolve the discrepancies between accountancy derived (bottom-up) and observation inferred (top-down) emissions estimates for long-lived GHGs and ODSs.
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