Scientists at the University of New South Wales have discovered that when a popular new refrigerant (HFO) breaks down, it produces trace amounts of fluoride - a greenhouse gas with strong global warming potential. The discovery, made using innovative atmospheric modeling techniques, calls for a new look at the environmental impact of these chemicals.
Scientists at the University of New South Wales have discovered that hydrofluoroolefins marketed as environmentally friendly refrigerants degrade into harmful greenhouse gases, raising concerns about their long-term environmental impact. A team of scientists has found that some of the most important new refrigerants partially break down into persistent greenhouse gas pollutants, including compounds that have been internationally banned.
Refrigerant is a chemical that changes between liquid and gaseous states, transferring heat in the process. They are widely used in refrigeration as well as room heating and cooling. In addition, these chemicals are used as aerosol propellants and flame retardants, and in the production of foamed plastics.
Hydrofluoroolefins (HFOs), which react rapidly in the lower atmosphere, have become the main synthetic chemicals for refrigerants and are considered a more environmentally friendly alternative than their predecessors.
Hydrofluoroolefins are known to break down into chemicals such as trifluoroacetaldehyde, but there has been discussion about whether the compound breaks down further into fluoromethanes - the most environmentally damaging hydrofluorocarbons (HFCs) they are used to replace.
A paper published in the Journal of the American Chemical Society, led by Dr. Christopher Hansen from the Department of Chemistry at the University of New South Wales, demonstrates that hydrofluoroolefins do decompose into small amounts of fluorides. This new study shows that we need to look more closely at the environmental impact of HFOs and raises questions about their long-term safety.
Dr Hansen said: "We do not yet fully understand the environmental impact of HFOs. However, unlike previous examples such as chlorofluorocarbons and leaded gasoline, we are trying to understand the consequences of large-scale emissions before they cause potential irreversible harm to the environment and human health. We are trying to change the way science introduces new products."
The ozone hole is caused by human activity releasing ozone-depleting chemicals, including chlorofluorocarbons (CFCs) - some of the earliest synthetic chemicals used in refrigerants and aerosol cans.
Thanks to the Montreal Protocol, the international community began to phase out CFCs and replaced them with hydrofluorocarbons (HFCs) globally starting in the mid-1990s.
Although HFCs do not contribute to ozone depletion, they are potent greenhouse gases. "In the end, scientists found that 1 kilogram of fluoroform - a once commonly used HFC - emitted today would have the equivalent of more than 14,000 kilograms of carbon dioxide in warming the Earth's surface over the next century," Dr Hansen said.
The global phase-out of HFCs began in 2016 after recognizing that they contribute highly to greenhouse warming.
Hydrofluoroolefins, which have short atmospheric lifetimes, have become the leading synthetic alternatives and are rapidly proliferating as refrigerants, foam blowing agents (such as those used in insulating foam) and aerosol propellants.
While scientists know something about the chemical pathways by which HFOs break down, there has been debate over whether they actually break down into some of the least environmentally friendly HFCs.
Hydrofluoroolefins are made up of chemical units that are more reactive than their previous counterparts, so they do not rise to the upper atmosphere and become long-lasting greenhouse gases.
"But, as chemists, we look at the structures of these molecules and start to try to imagine what they become," Hansen said. "So we can't just say, oh, this thing only has a two-week lifespan, it can't be a greenhouse gas, we have to see what it becomes. Most chemists look at these structures and they can work out the reactions that actually make hydrofluorocarbons."
However, confirming whether HFO breaks down into HFCs in low yields requires difficult experiments, and most existing techniques and instrumentation lack the sensitivity and specificity to do so. Hansen and his team used a variety of techniques, including two invented specifically for this study, to measure and evaluate chemical reactions in the atmosphere over the range of expected pressures.
"We used a variety of spectroscopic techniques to observe the reaction. We also created a mixture of gases at different pressures to simulate an atmosphere contaminated by trace amounts of direct decomposition products of hydrofluoroolefins," Hansen said. "We then used lasers to simulate photons from the sun to drive the reaction."
We know that the yield of hydrofluoroolefins decomposed into fluorinated carbonyl compounds such as trifluoroacetaldehyde can reach or exceed 100%. This means that all HFO molecules become the first product, and for some HFOs, for every HFO molecule decomposed, it is possible to get two molecules of the product. This study shows that the next step in the reaction produces a small amount of fluoroform from the decomposition of trifluoroacetaldehyde in the presence of light. Fluoromethane is the hydrofluorocarbon with the greatest global warming potential.
"We have comprehensively demonstrated that some of the most important HFOs do break down into HFCs and have provided the first conclusive scientific data for modeling and predicting the consequences of large-scale emissions," Hansen said. "While the reaction produces only small amounts of fluoromethane, this chemical can persist in the atmosphere for up to 200 years and has a global warming potential more than 14,000 times that of CO2. Small amounts produced can still have a significant impact."
Many atmospheric crises catch us unawares. "Think of leaded gasoline, the deadly smog events of the 20th century, the ozone hole crisis, but it's not because our models aren't good enough, it's because important chemical components are missing from the models," he said.
This research now resolves a long-standing controversy and provides the conclusive scientific data needed to model and predict the impacts of large-scale emissions of HFOs before policymakers may need to respond to emerging environmental crises.
The climate modeling group at UNSW, along with scientists around the world, is now ready to feed this data into models to help calculate the environmental impacts of continued use of HFOs.
While questions remain, this paper provides important evidence for the next steps in addressing the environmental impact of the chemicals we emit into the atmosphere. The team is planning further new experimental work. "In this paper, we performed experiments using a single wavelength, the wavelength currently used in studies that guide regulators, industry and government," Hansen said. "We plan to use other wavelengths of light to study this chemical reaction because the yields may be higher or lower at other wavelengths of light." "
Compiled from /ScitechDaily