REFRIGERANTS
Ozone depletion 50 years on
Professor Dick Powell of Refrigerant Solutions discusses the Ozone Depletion hypothesis which was published in Nature 50 years ago this April.
O
n 9th April 1974 F. Sherwood Rowland (1927-2012) and Mario Molina (1943 – 2020) shook the world of refrigerants when the journal ‘Nature’ published their
hypothesis that stratospheric ozone could be destroyed by CFCs. These compounds had been introduced in the 1930s as non-fl ammable, low toxicity, effi cient refrigerants as alternatives to hydrocarbons, methyl chloride, ammonia and sulfur dioxide whose hazards were restricting the development of the refrigeration industry. Following World War 2 the usage the CFCs grew rapidly contributing to the increase in living standards in the developed nations and fi nding new outlets, particularly as aerosol propellants. In the mid ‘50s James Lovelock (1919-2022), renowned independent scientist, developed the electron capture detector (ECD) for gas liquid chromatography, a technique widely used by chemists for analysing mixtures of volatile mixtures, including refrigerants.
The ECD is especially superior to other commonly used methods for detecting CFCs, with a sensitivity better than the fl ame ionisation detector (FID) by 10 to 1000 and the thermal conductivity detector (TCD) by a million times. In 1970 Lovelock used his ECD to study the atmospheric concentrations of fl uorinated gases, publishing his results in Nature. At a site in south-west Ireland, he measured the concentrations of CFC-11 (CCl3
F) and sulfur hexafl uoride (SF6 ), a gas used to suppress
arcing in electrical switch gear. Signifi cantly, the concentrations were signifi cantly higher when the wind was blowing easterly from Europe, where the F-gas sources were located, than north- westerly from locations where sources were absent. In his 1971 paper Lovelock also commented that: ‘The
presence of stable sulphur and carbon fl uorides in the atmosphere is not in any sense a hazard, and their existence has only been detected by the very sensitive technique of gas phase electron absorption.’ This was a remark that was to be strongly questioned just three years later. Being a chemist, Lovelock was aware that the fl uorine- containing gases could only be man-made and from his measurements was able to estimate how much was likely to be present in the atmosphere. At a chance meeting with Ray McCarthy (1920 – 2008), the head of Du Pont’s Freon laboratory, Lovelock was able to compare his fi gure with McCarthy’s estimate of the total CFCs and related compounds produced since the 1930s based on his industrial knowledge. Surprisingly, the two values, one from measurement and the other from the commercial source, were in agreement. The implication was obvious – CFCs degraded only slowly in the environment. In 1972 McCarthy initiated a meeting of the major CFC
24 March 2024 •
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manufacturers in Washington DC called the Fluorocarbon Panel under the umbrella of the Manufacturing Chemists Association to discuss the possible environmental impact of the CFCs. He defi ned the objective of the group thus: ‘Fluorocarbons are intentionally or accidentally vented to the atmosphere world-wide at a rate approaching one billion pounds per year. These compounds may either be accumulating in the atmosphere or returning to the surface, land or sea, in pure form or as decomposition products. Under any of these alternatives it is prudent that we investigate any eff ects which the compounds may produce on plants or animals now or in the future.’ Having heard by chance about McCarthy’s concern over the
environmental fate of CFCs, F Sherwood Rowland a chemistry professor at the University of California, Irvine, and his co- worker Mario Molina decided to investigate. Rowland had been surprised that CFCs persisted in the troposphere (lower atmosphere) where he expected them to be destroyed by ultra- violet (UV) light, but Molina’s lab experiments confi rmed this lack of reactivity. However, they realised that small amounts
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