The Missing Greenhouse Gas
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NEWS FEATURE The missing greenhouse gas Growth of the electronics industry will boost emissions of a ‘hidden’ — but extremely potent — greenhouse gas. Hannah Hoag reports. 1 ur insatiable appetite for equipment cleaning, NF3 is also marketed potential for global warming , and one that gadgets — mobile phones, MP3 as a plasma etchant for thin-fi lm liquid author Michael Prather says we should be Oplayers and fl at-screen TVs — may crystal displays (LCDs) and thin-fi lm keeping an eye on. be adding a hidden greenhouse gas to the photovoltaics, which convert the sun’s Prather, an atmospheric scientist at the Earth’s atmosphere. Countries that ratifi ed energy into electricity. University of California, Irvine, became the Kyoto Protocol committed to reducing Increasing demand for plasma products curious about the potential of NF3 to their emissions of carbon dioxide and saw worldwide production of the gas surge contribute to global warming during the fi ve other heat-trapping gases: methane, in 2007. Air Products and Chemicals, Inc., draft ing of the 2001 Intergovernmental nitrous oxide, hydrofl uorocarbons, a global supplier of industrial gases based Panel on Climate Change report2. “We perfl uorocarbons and sulfur hexafl uoride. in Hometown, Pennsylvania, bumped up said we’d look into it since we knew But these aren’t the only climate-altering its production of NF3 from 2,000 tonnes to almost nothing,” he says. “We need to try chemicals being produced by human 3,000 tonnes. Th is increase looks likely to to fi gure out what is going on with the activity. In the 13 years since the Protocol become a trend, as the company’s planned production of the gas and see if it is in was fi rst drawn up, scientists have expansion in the United States and Korea the atmosphere.” discovered that other gases, such as will see a further boost to 3,200 tonnes by Using a state-of-the-art model that nitrogen trifl uoride (NF3), could become 2009. Other companies, including Kanto follows the fl ow of the gas through the an increasing part of the climate problem. Denka, DuPont, Formosa Plastics and atmosphere in three dimensions, Prather Th e dominant use of NF3 is in the Mitsui Chemicals, are following suit, each and Juno Hsu calculate a new NF3 lifetime manufacturing of semiconductors — an producing or planning to produce about of 550 years, about 25 per cent shorter than industry worth more than US$250 2,000 tonnes of the gas annually. previous calculations, but “still beyond billion annually — where the gas was any societal time frames,” they write. “Th e fi rst introduced as an alternative to the POTENTIAL HEAT lifetime is shorter, which is good, but it is perfl uorocarbon C2F6, which has an still long,” says Prather. atmospheric lifetime of 10,000 years and Th ough the gas goes unmonitored, Using the new lifetime, the 100-year is nearly 9,000 times more powerful a a new study in the 27 June issue of global warming potential (GWP) of NF3 greenhouse gas than carbon dioxide. Geophysical Research Letters shows that NF3 also adjusts slightly, dropping from 17,200 Although it is primarily employed in is a ‘missing greenhouse gas’ with a high to 16,800. Th e GWP is an index used to estimate the potential future impacts of a gas on the climate system. It represents how much a given mass of greenhouse gas is estimated to contribute to global warming, compared to an equal amount of carbon dioxide, which carries a GWP of 1. By this comparison, NF3 is quite potent. “Fluorinated gases can absorb a lot of the Earth’s outgoing radiation,” says Keith Shine, an atmospheric scientist at the University of Reading and lead author of the 1995 IPCC report on climate change. “It has all the warning signs: large production, large lifetime and large global warming potential,” says Prather. If all the NF3 produced this year were to be released into the atmosphere, it would be on par to adding 67 million tonnes of carbon dioxide to the air. In 2005, carbon dioxide emissions exceeded 15 billion tonnes globally. ANDRES BALCAZAR / ISTOCKPHOTO However, most of the NF3 produced is destroyed and never reaches the atmosphere. Two earlier studies found A powerful but unmonitored greenhouse gas, nitrogen trifl uoride, is used in the manufacture of LCD screens for that only two to three per cent of the gas many electronic gadgets. escapes destruction aft er its use3,4. 99 nature reports climate change | VOL 2 | AUGUST 2008 | www.nature.com/reports/climatechange NEWS FEATURE John Langan, director of electronics the CO2 emitted by things that we think Th ere are a whole host of gases technology at Air Products, says their are big,” he says. For comparison, the in the atmosphere, from known and experience with NF3 indicates that the 3,600-megawatt Scherer coal-fi red power unknown sources, that have the potential gas is destroyed with high effi ciency. “Th e plant in Georgia, among the largest in the to contribute to global warming, but 97 percent destruction effi ciency is at the United States, generates 25 million tonnes only a handful were included in the lower end. We’ve measured 98 percent or of carbon dioxide annually. “All the little Kyoto Protocol when it was fi rst draft ed in 99 percent. To put it in perspective, the other pieces add up. Th at’s why we’re looking at 1995. “It’s rather strange that it has stuck chamber cleaning gases have destruction the pieces.” to those gases and that we don’t have a effi ciencies from 30 to 70 percent,” he says. mechanism for bringing in new gases,” Nitrogen trifl uoride “has says Shine. KEEPING WATCH “I don’t think it should be included in all the warning signs: Kyoto yet, to be honest — there is no point But Prather is sceptical about these in adding gases to a treaty that we are still numbers. He says the industry has a poor large production, large trying to get enforced,” says Prather. track record for self-reporting — a 1982 lifetime and large global study found that atmospheric levels of the warming potential.” Published online: 10 July 2008 ozone-depleting gas CFC-12 didn’t match doi:10.1038/climate.2008.72 estimates of the Chemical Manufacturers Michael Prather Association5 — and that atmospheric levels References 1. Prather, M. J. & Hsu, J. Geophys. Res. Lett. 35, L12810, of NF3 should be measured and monitored. However, Shine says the study is Although it is diffi cult to measure the gas, overstated and uses extreme numbers. “It doi:10.1029/2008GL034542 (2008). 2. Climate Change 2001: Th e Scientifi c Basis (eds Houghton, J. T. et al.) he aims to do just that. considers a scenario where all of it is emitted (Cambridge Univ. Press, Cambridge, UK, 2001). Even if emissions are low, Prather into the atmosphere. Th at is unlikely to 3. Lee, J-Y. et al. Bull. Korean Chem. Soc. 28, 1383–1388 (2007). 4. Robson, J. I., Gohar, L. K., Hurley, M. D., Shine, K. P. & believes caution is warranted because, happen,” he says. But he does agree that NF3 Wallington, T. J. Geophys. Res. Lett. 33, L10817, like the ozone-destroying chemical should be watched closely. “If there continues doi:10.1029/2006GL026210 (2006). CFC-12, NF3 is long-lived and will take to be a massive growth in production, even if 3. Rowland,F. S. et al. Geophys. Res. Lett 9, 481–484 (1982) centuries to be removed. “Compared to all emissions are only 1 to 3 percent … it could global CO2, [the amount being produced] become important. It could become a big Hannah Hoag is a freelance science writer is small. But it is not small compared to part of national emissions,” he says. based in Montreal. 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