Because of their ozone and climate-damaging effects, some refrigerants based on long-lasting CFCs are banned. Alternative refrigerants that only have a short lifespan in the atmosphere and quickly disappear are therefore increasingly being used in refrigerators: hydrofluoroolefins. But these chemicals also appear to have unintended consequences for the environment: Some of them can form the long-lasting greenhouse gas fluoroform when they react with ozone, a new study shows. This calls into question their classification as climate-friendly chemicals.
So-called chlorofluorocarbons (CFCs) remain in the atmosphere for hundreds of years, damaging the ozone layer and have therefore been banned since the United Nations Montreal Protocol of 1987. Since then, the industry has used so-called hydrofluoroolefins as alternative refrigerants for refrigerators etc. These fluorine-containing chemical compounds are believed to have a significantly shorter lifespan in the atmosphere than CFCs because they react easily with hydroxyl radicals in the air and are therefore broken down before they can harm the ozone layer. But there is also apparently a worrying chemical side reaction, as a research team has now discovered.
Some refrigerants produce fluoroform
Researchers led by Max McGillen from the French National Center for Scientific Research (CNRS) in Orléans determined in the laboratory how quickly five substances from the group of hydrofluoroolefin refrigerants change chemically and which products are formed as a result. They found that three of the five hydrofluoroolefins (CH2=CHCF3, trans-CHF=CHCF3 and cis-CF3CH=CHCF3) formed the product fluoroform (CHF3, trifluoromethane) within a few days when there was a lot of ozone (O3) in their environment. was present. To determine whether this reaction also occurs outside the laboratory, the researchers then examined the chemical composition of the atmosphere and the distribution of the gases it contains. This analysis found that hydrofluoroolefins in the atmosphere are much more likely to react with hydroxyl radicals than with ozone, but that up to three percent of all hydrofluoroolefin molecules could also react with ozone.
Although these side reactions occur only rarely and very slowly, they are not harmless, as the scientists report. Fluoroform is a very long-lived gas that remains in the atmosphere for a long time. According to the analyses, it can still act as a greenhouse gas there in 500 years and thus contributes to global warming. The reaction with hydroxyl radicals, on the other hand, actually occurs very quickly, so that most hydrofluoroolefins are converted into harmless substances after just ten days, as McGillen and his colleagues discovered.
Not all hydrofluoroolefins are harmful to the climate
Overall, the study shows that modern refrigerants made from hydrofluoroolefins are significantly more environmentally friendly than previous CFC refrigerants, but they also pose serious environmental risks. Because of their byproduct fluoroform, some hydrofluoroolefins exceed the limits set by the World Weather Organization for the acceptable effect of such chemicals on the atmosphere. McGillen and his colleagues suspect that other, as yet unexplored, hydrofluoroolefins can form long-lived greenhouse gases containing fluorine similar to fluoroform when they come into contact with ozone. However, within the group of hydrofluoroolefins there are also substances that are chemically structured in such a way that they cannot form such products. The researchers hope that this knowledge will enable industry to better assess the long-term environmental impact of the chemicals it uses in the future.
Source: Max McGillen (CNRS) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2312714120