Potent Greenhouse Gas More Common in Atmosphere Than Estimated
WASHINGTON -- New research indicates a powerful greenhouse gas is at least four times more prevalent in the atmosphere than previously estimated. The research, based on data from a NASA-funded measurement network, examined nitrogen trifluoride, which is thousands of times more effective at warming the atmosphere than an equal mass of carbon dioxide.
Using new analytical techniques, Ray Weiss of the Scripps Institution of Oceanography in La Jolla, Calif., led a team of researchers in making the first atmospheric measurements of nitrogen trifluoride. The amount of the gas in the atmosphere, which could not be detected using previous techniques, had been estimated at less than 1,200 metric tons in 2006. The new research shows the actual amount was 4,200 metric tons. In 2008, about 5,400 metric tons of the gas are in the atmosphere, a quantity that is increasing at a rate of about 11 percent per year.
"Accurately measuring small amounts of nitrogen trifluoride in air has proven to be a very difficult experimental problem, and we are very pleased to have succeeded in this effort," Weiss said. The research will be published Oct. 31 in the American Geophysical Union's Geophysical Research Letters.
Emissions of nitrogen trifluoride were thought to be so low that the gas was not considered a significant potential contributor to global warming. It was not covered by the Kyoto Protocol, the 1997 agreement to reduce greenhouse gas emissions signed by 182 countries. The gas is 17,000 times more potent as a global warming agent than a similar mass of carbon dioxide. It survives in the atmosphere about five times longer than carbon dioxide. However, current nitrogen trifluoride emissions contribute only about 0.04 percent of the total global warming effect caused by current human-produced carbon dioxide emissions.
Nitrogen trifluoride is one of several gases used during the manufacture of liquid crystal flat-panel displays, thin-film solar cells and microcircuits. Many industries have used the gas in recent years as an alternative to perfluorocarbons, which also are potent greenhouse gases, because it was believed that no more than two percent of the nitrogen trifluoride used in these processes escaped into the atmosphere.
The Scripps team analyzed air samples gathered during the past 30 years, including samples from the NASA-funded Advanced Global Atmospheric Gases Experiment network of ground-based stations. The network was created in the 1970s in response to international concerns about chemicals depleting the ozone layer. It is supported by NASA as part of its congressional mandate to monitor ozone-depleting trace gases, many of which also are greenhouse gases. Air samples are collected at several stations around the world. The Scripps team analyzed samples from coastal clean-air stations in California and Tasmania for this research.
The researchers found concentrations of the gas rose from about 0.02 parts per trillion in 1978 to 0.454 parts per trillion in 2008. The samples also showed significantly higher concentrations of nitrogen trifluoride in the Northern Hemisphere than in the Southern Hemisphere, which the researchers said is consistent with its use predominantly in that hemisphere. The current observed rate of increase of nitrogen trifluoride in the atmosphere corresponds to emissions of about 16 percent of the amount of the gas produced globally.
In response to the growing use of the gas and concerns that its emissions are not well known, scientists recently have recommended adding it to the list of greenhouse gases regulated by Kyoto.
"As is often the case in studying atmospheric emissions, this study shows a significant disagreement between 'bottom-up' emissions estimates and the actual emissions as determined by measuring their accumulation in the atmosphere," Weiss said.
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