University of Hawaii Scientists Uncover Critical Atmospheric Chemistry Behind Global Spread of Refrigerant Byproduct
HONOLULU, HAWAII — In a first-of-its-kind study published in Chemical Science, University of Hawaii at Manoa researchers uncovered the molecular mechanisms that determine how trifluoroacetic acid, a persistent environmental pollutant produced by common refrigerants, becomes incorporated into ice crystals and transported to the most remote regions of Earth-including the Arctic.
The research, conducted by scientists in the Department of Chemistry and funded by the National Science Foundation’s Engineering Research Center for Environmentally Applied Refrigerant Technology Hub, or EARTH, marks the first comprehensive experimental study of TFA’s behavior in atmospheric freezing processes and yields critical information about why this “forever chemical” is accumulating in pristine polar environments.
The Growing TFA Crisis
Trifluoroacetic acid is the most abundant per- and polyfluoroalkyl substance present in Earth’s atmosphere and represents one of the most pressing environmental challenges facing the refrigeration industry. TFA primarily enters the atmosphere through the degradation of hydrofluorocarbons (HFCs) like 1,1,1,2-tetrafluoroethane and hydrofluoroolefins (HFOs) like 2,3,3,3-tetrafluoropropene-compounds widely used in air conditioning, refrigeration systems, and automotive cooling.
Global monitoring data from NOAA indicate an alarming trend: atmospheric concentrations of 1,1,1,2-tetrafluoroethane increased more than an order of magnitude from 1998 to 2021, from 8.7 parts per trillion to 121 parts per trillion. Directly linked to that, TFA concentrations in the hydrosphere have increased sixfold in this time period, reaching median levels of 180 ng/L.
“What makes TFA particularly concerning is its extreme persistence,” explained lead author Dr. Koushik Mondal. “From a chemistry standpoint, TFA is exceptionally unreactive to atmospheric removal processes such as photolysis and reactions with hydroxyl radicals or atomic oxygen. This is why it’s often termed a ‘forever chemical’-once it’s in the environment, it stays there.”
Next-Generation Refrigerants May Worsen the Problem
While the most commonly used HFC refrigerants are being phased out in favor of HFO refrigerants due to their low global warming potential, recent studies suggest that these next-generation alternatives could contribute even more significantly to the formation of TFAs compared to their HFC predecessors. This finding has raised serious questions about the long-term sustainability of current refrigerant replacement strategies.
The increase in the concentration of TFA poses serious threats to ecosystems due to its bioaccumulation in aquatic organisms and terrestrial plants, as well as causing mammalian toxicity that includes reproductive and liver damage. Similarly, the chemical atmospheric lifetime through wet deposition processes is in the range of 10 to 15 days, although due to extreme stability, it accumulates irreversibly in water bodies over time.
Breakthrough Experimental Approach
In order to learn about TFA’s behavior under atmospheric conditions, especially at subzero temperatures typical of the upper troposphere and the Arctic regions, the research team created an advanced experimental system. The setup brings together ultrasonic levitation at cryogenic temperatures with ultra-sensitive time-dependent Raman spectroscopy and high-speed imaging, thereby enabling the study of low-temperature atmospheric chemistry and freezing processes in carefully simulated terrestrial environments.
“Our cryogenically cooled ultrasonic levitator allows us to study single water droplets as they freeze, completely free from any surface interference,” said co-author Souvick Biswas. “This contactless approach is crucial because it replicates what actually happens when TFA-containing water droplets freeze in the atmosphere.”
The experimental chamber can reach temperatures as low as 262 K (around -11°C), representative of conditions in the Earth’s upper troposphere, under which TFA-containing water droplets freeze. Using Raman spectroscopy, the team tracked changes on a molecular level in real time while levitated droplets turned from liquid into ice.
Key Findings: Chemical Change Upon Freezing
The most striking result of this study is the dramatic change in the structure of TFA during freezing. At room temperature and when dissolved in liquid water, TFA is present mainly as the dissociated anion, CF₃COO⁻, due to its strong acid nature (it is about 30,000 times stronger than acetic acid and has easily released its proton).
The research team observed, however, that a surprising transformation occurs as the water droplet freezes into hexagonal ice. With the development of the crystal structure of the ice, the dissociated anionic form of TFA is substantially converted to its undissociated neutral form, CF₃COOH. This transformation is evidenced through distinct changes in the Raman spectral features, notably in the C-C and C-O stretching regions.
“We obtained quantitative kinetic data about this transformation,” said Dr. Rui Sun, co-corresponding author of the study. “The rate constant for ice formation was 0.025 ± 0.001 s-1 and the rate constant of TFA conversion was 0.020 ± 0.005 s-1. The slightly larger value for ice formation suggests that the growth of the hexagonal crystal structure of ice is the critical step that triggers the neutral TFA formation inside the ice lattice.”
Computation for Molecular-Level Understanding
Supporting their experimental observations, the investigators conducted state-of-the-art quantum chemistry calculations using DFT. These revealed that undissociated neutral TFA exhibits remarkable stability within the hexagonal ice environment compared with amorphous ice structures, supporting the experimentally observed preference for neutral TFA formation as freezing progresses.
The optimized structures revealed large distortions in the ordered hexagonal lattice of the ice crystal due to TFA incorporation. Furthermore, the calculations unraveled different types of secondary hydrogen-fluorine interactions between TFA and the ice crystal: anionic forms interact directly with the lattice via F-H contact, while the undissociated neutral form exhibits long-range interactions through its terminal O-H group.
“These secondary interactions are crucial,” explained co-author Nils Melbourne, who performed the theoretical analysis. “They not only stabilize the TFA within the ice but also distort the crystal structure in ways that could affect ice properties and potentially contribute to ice melting.”
Presence of TFA in Remote Arctic Regions: Explanation
Another very important implication of the study is the unexpected abundance of TFA in remote polar regions, far from industrial sources. This work offers a plausible explanation at a molecular level for such a phenomenon through the role of neutral TFA in ice nucleation.
This observation aligns with previous ones that undissociated perfluorinated acids initiate nucleation of ice more easily compared to their dissociated forms. Such preferred neutral TFA formation upon atmospheric freezing indicates that TFA is likely involved in the glaciation process of clouds, allowing TFA to be incorporated into ice crystals and be transported over long ranges in the atmosphere.
“Our experimental study yields clear evidence that TFA is an active player in the formation of both ice-nucleating particles and new particles,” said Professor Ralf I. Kaiser, the corresponding author and principal investigator. “This explains how TFA can travel thousands of miles from urban and industrial sources into pristine Arctic environments–it’s literally being carried there in clouds through glaciation processes.”
The study also documented significant freezing point depression caused by TFA. At a 10% TFA aqueous solution, the freezing point was depressed by 10 ± 1 K relative to that of pure water. This observation, taken with the anionic secondary interactions with ice crystals, suggests that accumulation of TFA may contribute, in part, to the melting of polar ice-a finding of profound implication for understanding climate feedback mechanisms.
Implications for Refrigerant Policy
The research has urgent implications for policy in refrigerants and the ongoing phase-out of substances with high global warming potential. The HFO refrigerants were introduced as environmentally benign replacements for HFCs because their direct greenhouse gas effects are much lower; however, this study contributes to the growing evidence that indirect environmental costs through TFA formation may be as problematic.
“The transition from HFCs to HFOs was intended to address climate change by reducing direct greenhouse gas emissions,” Professor Kaiser said. “However, our research shows we need to consider the full lifecycle environmental impact of these compounds, including their persistent degradation products like TFA.”
Henry’s law constant data show that at 298 K, TFA is ten times more soluble in water than acetic acid. More interestingly, partition coefficients predict a TFA adsorption probability of over fifty times that of acetic acid at temperatures as low as 240 K. These are just some of the physical properties of TFA that help describe its effective atmospheric scavenging into waters and ice, thus concentrating it within an environmental medium.
A Call for an All-Encompassing Solution
The researchers underline that their findings point toward the need to adopt a more holistic approach in the development and deployment of refrigerants. The accumulation of TFA in water raises profound concerns about long-term environmental impact, considering its resistance to both natural degradation channels and biodegradation processes.
“This is the first systematic molecular-level experimental investigation of droplet freezing chemistry involving the simplest perfluorinated acid,” Professor Kaiser noted. “Further experimental investigations are earnestly required for a better understanding of the atmospheric implications of these ‘forever chemicals’ globally.”
The kinetic data from the present study can be implemented in models of cloud glaciation and long-range transport; comprehensive simulations of atmospheric concentrations of TFAs should now be possible for all atmospheric regimes. This will be exceptionally useful to policymakers, environmental scientists, and the refrigeration industry in their work toward truly sustainable cooling solutions.
The Path Forward
The study, carried out within the NSF EARTH Engineering Research Center, makes a key contribution toward understanding the unintended impacts of today’s refrigerant technologies. As the world increasingly moves to eliminate high-GWP refrigerants per the Kigali Amendment to the Montreal Protocol, studies like this one reinforce the need for full environmental evaluation before large-scale deployment of replacement compounds.
“Our work demonstrates that the environmental story of refrigerants doesn’t end when they stop being greenhouse gases,” Dr. Mondal concluded. “We need to understand the complete environmental fate of these chemicals from their release through their ultimate degradation products. Only then can we develop truly sustainable solutions for the cooling and refrigeration systems that modern society depends upon.”
The research team continues to investigate the atmospheric chemistry of fluorinated compounds; more studies are planned, looking into other perfluorinated acids and their roles in atmospheric processes. Results from these ongoing investigations will further inform the next generation of refrigeration technologies that balance performance, efficiency, and genuine environmental sustainability.
Citation: Mondal, K., Biswas, S., Melbourne, N., Sun, R., and Kaiser, R. I. “Probing the freezing chemistry of singly levitated aqueous trifluoroacetic acid droplets in a cryogenically cooled simulation chamber relevant to Earth’s upper troposphere.” Chemical Science, 2025, 16, 11039-11048.
Funding: This study was funded through NSF award 2330175, Center: NSF Engineering Research Center for Environmentally Applied Refrigerant Technology Hub (EARTH).
