Halocarbons Reassessed

When chemist Thomas Midgley Jr. and his research team at the Frigidaire division of General Motors were charged with coming up with a new refrigerant gas in the late 1920s, they were given two criteria: The gas had to be nontoxic and nonflammable. Air-conditioning and refrigeration systems at the time typically used ammonia, methyl chloride, propane, or sulfur dioxide, which could be dangerous if they leaked.

Fast-forward to today, and Midgley would have to add one more item to the list of criteria: environmentally benign. Midgley’s first commercial refrigerant gas was the chlorofluorocarbon CCl₂F₂. The compound, labeled CFC-12, was produced by DuPont under the Freon brand.

CFC-12 and a succession of related compounds such as CFC-11 (CCl₃F) and CFC-113 (CCl₂FCClF₂) became widely used as refrigerants in home and automotive applications and in other uses such as propellants in aerosol spray cans, foam blowing agents, and fire suppressants. They also became the center of one of history’s great unintended consequences, as some 50 years later they faced scientific scrutiny and regulatory control for destroying stratospheric ozone and for being potent greenhouse gases.

Newer refrigerants are environmentally friendlier. They typically include hydrogen or fluorine in place of chlorine and bromine and don’t deplete the ozone layer. But some of these replacement compounds still have a high global-warming potential (GWP). Scientists studying climate change and decisionmakers in the business and regulatory communities could use a comprehensive database of information so it’s easier to make one-to-one comparisons of the potential climate impacts of the compounds.

Thanks to a new report by an international multidisciplinary research team, they now have one. Øivind Hodnebrog and Jan S. Fuglestvedt of the Center for International Climate & Environmental Research-Oslo, in Norway; Timothy J. Wallington of Ford Motor Co.’s Research & Advanced Engineering division in Dearborn, Mich.; and coworkers have calculated the GWPs of more than 200 halocarbons and related compounds (Rev. Geophys. 2013, DOI: 10.1002/rog.20013). The list includes more than 100 compounds that were not included in the Intergovernmental Panel on Climate Change’s most recent assessment report, which was issued in 2007.

The researchers systematically scoured published literature to find infrared spectra for all types of halocarbons. They used the IR data to calculate the radiative efficiency of the compounds, which is a measure of how well a compound traps heat. They then coupled the radiative efficiencies with information on the atmospheric lifetimes of the compounds to calculate GWPs.

“The novel aspect of our study is that this is the first time the IR spectral data for all these compounds have been systematically compiled and the global-warming potential calculated consistently,” Wallington says.

GWP is defined as the warming potential over a chosen amount of time of 1 kg of a gas relative to 1 kg of CO₂, which is given a GWP value of 1. Among the study’s findings is that, using a 100-year time period, nearly half of the compounds have GWPs of less than 150, which is the limit set for vehicle refrigerants in Europe. Several of the compounds have GWPs of less than 1. Another notable result is that the radiative efficiencies for about one-fourth of the compounds are different—typically lower—by more than 5% from those in IPCC’s report.

“It’s a very thorough study of GWPs for these various gases,” says Drew T. Shindell, a climatologist at NASA’s Goddard Institute for Space Studies at Columbia University. “For climate, the lower the GWP the better, so the lower numbers on some of the new replacement gases are clearly good news.”

Explaining the discrepancies with previous data is straightforward, Wallington says. The team used new data to update parameters describing removal of CO₂ from the atmosphere, which lowered values of earlier assessed compounds by about 6% on average. The team also considered the nonuniform mixing of the gases in the atmosphere. Typically in GWP calculations scientists make the assumption that a gas is uniformly distributed throughout the atmosphere, Wallington explains. It’s a good approximation for long-lived gases that have time to disperse, he says. But it’s not so good for short-lived ones because, in the days to decades they last, they don’t have time to reach the upper troposphere, where the radiative effects of the gases are greatest.

“It does make a difference.” Wallington says. “Many of the short-lived compounds have lower GWPs than before.”

CFCs 11, 12, and 113, with GWPs of 4,660, 10,200, and 5,820, were among the most commonly used gases in home refrigerators and in automobiles until the early 1990s. A replacement compound, the hydrochlorofluorocarbon HCFC-22, CHClF₂, weighs in with a GWP of 1,760. It is also used as a refrigerant but more importantly as a precursor to polytetrafluoroethylene, commonly known as Teflon. Under the Montreal protocol, these gases are already phased out or in the last stages of being phased out. For refrigerant applications, manufacturers have primarily switched to the hydrofluorocarbon HFC-134a, CH₂FCF₃, which has a GWP of 1,300.

The new study should make the work of policymakers easier, but interpreting GWP values remains tricky, points out atmospheric chemist Mario J. Molina of the University of California, San Diego. Molina shared the 1995 Nobel Prize in Chemistry for his work on predicting the impact of CFCs on stratospheric ozone.

“The amount of halocarbons used as refrigerants is tiny compared with CO₂ emissions,” Molina says. “So a gas with a GWP of less than 1 is not very meaningful. CO₂ from breathing probably has more of an effect on climate than that.”

The real concern, Molina explains, starts only if the GWP is more than 100, or really even more than 1,000, and the compound is produced in a large amount. “But we need to pay attention to any new compounds that are long-lived,” he says. “Should society do nothing to control them and their demand takes off in the future, particularly in emerging markets in developing regions, they will become important in climate change.”

Among the latest replacement compounds, the hydrofluoroolefin HFO-1234yf, CF₃CF=CH₂, which is being commercialized jointly by DuPont and Honeywell, is slated to replace HFC-134a in automobile air conditioners. In the new study, its GWP value dropped from 4 to less than 1.

“There isn’t anything particularly magical about how a company chooses a refrigerant,” Wallington says. “We look at all the options available to us and consider safety, health impact, environmental burden, and performance characteristics—it has to be effective, durable, a good value for the money, and ready to implement.”

For Ford, HFO-1234yf is the only compound that meets those criteria and government regulations. General Motors also is adopting HFO-1234yf. But Daimler, the parent company of Mercedes-Benz, still has safety concerns over the slightly flammable gas and has decided to go with CO₂ instead—CO₂ has long been used as a refrigerant gas, and it is cheap and available.

The new study was a true international collaboration, Wallington emphasizes. “At Ford, we think that is the way forward toward achieving consensus on the environmental impacts of these and other compounds of possible concern.”