Swapping one greenhouse gas for another

by Carolyn Gramling
Thursday, January 5, 2012

There are no miracle cures, it would seem. Nitrogen trifluoride is a colorless, nontoxic gas that has helped make the semiconductor industry greener by replacing a well-known greenhouse gas threat. But nitrogen trifluoride is a powerful greenhouse gas in its own right, with a potential impact on climate 17,000 times greater than carbon dioxide — and it is currently unregulated. As industrial production of the gas continues to grow — used in the manufacture of LCD flat screens — scientists, manufacturers and environmental regulators ought to be keeping a much closer eye on this “missing” greenhouse gas, a new study says.

In the late 1990s, the specialty commercial gas manufacturer Air Products began to develop nitrogen trifluoride as a substitute for perfluorocarbons (PFCs), greenhouse gases that were widely used in the semiconductor industry. Nitrogen trifluoride is also a greenhouse gas, but far less escapes into the atmosphere during manufacturing (2 to 3 percent, compared to 60 to 70 percent for PFCs). In 2002, the U.S. Environmental Protection Agency recognized Air Products with a Climate Protection Award for its innovation with nitrogen trifluoride, which helped the semiconductor industry achieve a reduction of PFC emissions to 10 percent below 1995 levels — far ahead of its 2010 deadline.

But nitrogen trifluoride has its own set of problems, says Michael Prather, an atmospheric chemist at the University of California at Irvine. It’s an improvement over PFCs, but it’s still a cause for concern, he says in a study published June 26 in Geophysical Research Letters.

One reason that nitrogen trifluoride is such a potent greenhouse gas is that it lingers in the atmosphere for 550 years (far longer than carbon dioxide, which lasts from 50 to 200 years), according to new calculations by Prather and colleague Juno Hsu, also of the University of California at Irvine. Nitrogen trifluoride is also a highly efficient heat-absorber, soaking up large amounts of radiation from the sun.

Scientists assess the climate-impact dangers of various gases based on a measure called the global warming potential, a relative number that compares the gases' ability to warm the atmosphere with that of carbon dioxide. For PFCs, the global warming potential is as much as 6,500 times greater than that of carbon dioxide. For nitrogen trifluoride, it’s as much as 17,000 times greater.

Although scientists have long been aware that nitrogen trifluoride could be a powerful greenhouse gas, PFCs were much more immediately worrisome because of their greater abundance; the large-scale production and high emissions of PFCs made them a bigger concern than the lesser production, and assumed lesser emissions, of nitrogen trifluoride. Nitrogen trifluoride was, for many years, a rare specialty gas, initially used by groups such as the military in lasers and in rocket fuel. But in the 1990s, the gas began to be used in the semiconductor industry to etch computer microchips; now, it is increasingly used to manufacture LCD flat screens for televisions, computer monitors and cell phones. And as the popularity of those products continues to increase, the production of nitrogen trifluoride is rising accordingly.

Given its minimal usage in the mid-1990s, nitrogen trifluoride was not covered by the Kyoto protocol, the 1997 global warming treaty. But by 2001, its rapidly increasing potential in industry prompted scientists to include it in the 2001 Intergovernmental Panel on Climate Change (IPCC) report, says Charles E. Kolb Jr., an atmospheric chemist with the Massachusetts-based research firm Aerodyne Research, Inc.

That increasing production is an issue, simply because it’s not entirely clear how much of the gas escapes not only during its usage, but also during production, storage and transport, Prather says. Studies sponsored by Air Products itself and conducted at both MIT and Aerodyne assessed the gas' greenhouse potential; other researchers, meanwhile, have assessed how much of it was likely to escape. Under ideal conditions of use, they found, most of the gas would be destroyed during use, so that only 2 to 3 percent of the gas might be lost to the atmosphere.

But those are ideal conditions, and the studies only consider what escapes during industrial use, Prather says. “All of this production is potentially releasable unless you know it’s not. If they can get it down to 3 percent escaping, that’s pretty small, but it’s still a 500-year gas. My point is that we don’t know, and we should be monitoring it.” The trouble, he says, is that there are currently no reported measurements of atmospheric nitrogen trifluoride in peer-reviewed studies — so no one can actually say whether it’s increasing in the atmosphere or not.

Kolb agrees. “The obvious scientific point is that we haven’t yet detected nitrogen trifluoride in the atmosphere,” he says, adding that the real question is whether enough of the gas is being released that satellite- and ground-based instruments can even detect it starting to build up. “Because if it’s not there, then it’s not a problem. But if it is, people will have to pay attention to how they use it. We don’t have those data and it would be smart to launch a more concerted effort to look for it.”

Air Products agrees that measuring nitrogen trifluoride in the atmosphere would be an important step; in fact, the semiconductor industry already voluntarily reports its use of the gas to EPA, says Corning Painter, vice president of electronics at Air Products. “If it became compulsory, the industry would support that,” Painter says. “We’re interested in measuring it; we feel global warming is an important issue,” he adds, noting that his company has taken great pride in its award from EPA. But Air Products maintains that the risk of significant gas escaping is extremely low. Most of the gas is consumed during its use; of the gas that isn’t consumed, chemical “scrubbing” of the chamber containing the nitrogen trifluoride removes another 97 percent, Painter says. “The deal is, if we’re right, very, very little nitrogen trifluoride is being released. [Similarly], if all the natural gas that went into peoples' houses went into the atmosphere, that would be a big problem — but most is burned off during use. It’s the same analogy — the stuff doesn’t work if you don’t destroy it.”

The atmospheric measurements are where it gets tricky: Unlike some other greenhouse gases, nitrogen trifluoride is very difficult to measure, and no systems are currently set up to catch it, Prather says. Although he plans to try to take his own atmospheric measurements this year, “it’s slippery,” he adds. “You need to freeze it with liquid nitrogen and trap it and then get rid of other stuff — it’s hard, so even people [already] doing fluorinated gases can’t do this easily. That’s why it hasn’t been done."
Indeed, it’s too simplistic to say that the gas itself is doing harm, or that LCD screens are helping to cause climate change, Prather says, adding that he’s frustrated by media headlines citing “killer LCD screens” in response to the findings of his study. He says he isn’t picking on nitrogen trifluoride in particular, nor is he trying to pillory Air Products. “My feeling is [nitrogen trifluoride] is a great improvement” over the previous PFC-based technology used in the industry, he says. “I’m just saying that for all such environmental compounds, there needs to be an evaluation of what the threat is.”

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