Potent greenhouse gas more prevalent than assumed

Using new analytical techniques, a research team in California has made the first atmospheric measurements of nitrogen trifluoride, which is thousands of times more effective at warming the atmosphere than an equal mass of carbon dioxide.

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 was in the atmosphere, a quantity that is increasing at 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,” says Ray Weiss of the Scripps Institution of Oceanography, in La Jolla, California, who led the study.

The research will be published on 31 October 2008 in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

Emissions of nitrogen trifluoride were thought to be so low that the gas was not considered to be 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. Current nitrogen trifluoride emissions, however, contribute only about 0.04 percent of the total global warming effect contributed 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 photovoltaic cells and microcircuits. Many industries have used the gas in recent years as an alternative to perfluorocarbons, which are also potent greenhouse gases, because it was believed that no more than 2 percent of the nitrogen trifluoride used in these processes escaped into the atmosphere.

The team at Scripps, a graduate school of the University of California at San Diego, analyzed air samples gathered over the past 30 years. The researchers worked under the auspices of the NASA-funded Advanced Global Atmospheric Gases Experiment (AGAGE) 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 are also 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 in Tasmania, Australia for this study.

The researchers found that 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 say is consistent with its use predominantly in Northern Hemisphere countries. 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 have recently 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 says. “From a climate perspective, there is a need to add nitrogen trifluoride to the suite of greenhouse gases whose production is inventoried and whose emissions are regulated under the Kyoto Protocol, thus providing meaningful incentives for its wise use.”

Michael Prather, an atmospheric chemist at the University of California at Irvine, predicted earlier this year that based on the rapidly increasing use of nitrogen trifluoride, larger amounts of the gas would be found in the atmosphere. Prather says the new Scripps study provides the confirmation needed to establish reporting requirements for production and use of the gas.

“I'd say case closed. It is now shown to be an important greenhouse gas,” says Prather, who was not involved with the Scripps study.

“Now we need to get hard numbers on how much is flowing through the system, from production to disposal.”