industry is at least four times more abundant in the atmosphere than
previously thought, scientists have found. To better control its use,
nitrogen trifluoride (NF3) should be added to the list of gases
regulated under future climate-change agreements, they recommend.
NF3 is 12,000-20,000 times more efficient at trapping heat than carbon
dioxide, the best-known of six greenhouse gases regulated by the 1997
Kyoto protocol on climate change.
In the past ten years, NF3 has become an environmentally preferable
alternative to more volatile perfluorocarbons. It is now commonly used
by manufacturers of plasma TVs and other flat-panel displays as a
source of reactive fluorine atoms, used to etch the silicon chips in
the devices.
Because only very small amounts of the gas were thought to escape to
the atmosphere in these processes - about 2% of all NF3 produced - it
was long assumed that its contribution to man-made global warming was
negligible.
Screen burn
This notion was first challenged earlier this year when Michael
Prather, an atmospheric chemist at the University of California in
Irvine, questioned the commonly assumed emission rates of the gas1.
Now, analyses of air samples taken at two coastal clean-air stations
in California and Tasmania, Australia, have for the first time
confirmed that a significantly higher percentage of overall NF3
production escapes to the atmosphere.
The team, led by Ray Weiss of the Scripps Institution of Oceanography
in La Jolla, California, used a combined gas-chromatography and mass-
spectrometry system to measure NF3 levels in their samples.
They found that over the past three decades, the atmospheric
concentration of the gas has increased more than 20-fold, from 0.02 to
0.454 parts per trillion, with most emissions occurring in the
Northern Hemisphere. The overall amount of the gas in the atmosphere,
estimated in 2006 at less than 1,200 tonnes, was then actually 4,200
tonnes and has since risen to 5,400 tonnes, they report in Geophysical
Research Letters2.
Given its strong global-warming potential and estimated atmospheric
lifetime of 740 years, this is equivalent to the effect of about 67
million tonnes of carbon dioxide – roughly the total annual CO2
emissions of Finland.
"I'd say case closed. It is now shown to be an important greenhouse
gas," says Prather, who was not involved with the second study. "Now
we need to get hard numbers on how much is flowing through the system,
from production to disposal."
Early catch
"Industries were quite dismissive of Michael Prather's original paper
as pure speculation," says Piers Forster, an atmospheric chemist at
the University of Leeds, UK. "This new paper shows that NF3 is there
in significant quantities, and it's increasing."
The two papers have caught the problem in good time for industries to
clean up their act, he adds. Liquid crystal display (LCD) screens, for
example, can be produced in a more environmentally friendly way, and
may soon begin to replace plasma screens.
"The problem may die away naturally," agrees Jim Haywood, an
atmospheric scientist with the UK Meteorological Office. "But in the
meantime, it may well be worth including NF3 in the list of regulated
greenhouse gases."
*
References
1. Prather, M. J . & Hsu, J. Geophys. Res. Lett. 35, L12810
(2008).
2. Weiss, R. F., Mühle, J., Salameh, P. K. & Harth, C. M.
Geophys. Res. Lett. (2008) doi:10.1029/2008GL035913.
Original article available at http://www.nature.com/news/2008/081024/full/news.2008.1189.html
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