New Study Finds Atmospheric Levels of GHG NF3 At Least 4x Higher Than Previously Estimated

Jose Michael

Measured and modeled atmospheric NF3 concentrations and trends from 1978 to 2008. Northern Hemisphere NF3 measurements are shown as filled circles, together with the spline curve Northern Hemisphere trend (solid line) fitted to these measurements. The modeled Southern Hemisphere trend and modeled global mean trend (dotted line) are shown as dashed and dotted lines, respectively. Southern Hemisphere measurements are plotted as filled squares. Click to enlarge. Source: Weiss (2008)

Measuring background atmospheric abundances and trends of the greenhouse gas nitrogen trifluoride (NF3) for the first time, a team of researchers at Scripps Institution of Oceanography at UC San Diego found that NF3 levels are at least four times higher than previously estimated.

The team, led by Scripps geochemistry professor Ray Weiss, found that the mean global tropospheric concentration of NF3 has risen quasi-exponentially from about 0.02 ppt (parts-per-trillion) at the beginning of their measured record in 1978, to a 1 July 2008 value of 0.454 ppt, with a rate of increase of 0.053 ppt yr-1, or about 11% per year. The research will be published 31 October in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

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.

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% of the NF3 used in these processes escaped into the atmosphere.

This rise rate corresponds to about 620 metric tons of current NF3 emissions globally per year, or about 16% of the poorly-constrained global NF3 production estimate of 4,000 metric tons yr-1. This is a significantly higher percentage than has been estimated by industry, and thus strengthens the case for inventorying NF3 production and for regulating its emissions.

—Weiss et al. (2008)

Emissions of NF3 were thought to be so low that the gas was not considered to be a significant potential contributor to global warming, and it is not covered by the Kyoto Protocol. The gas is 17,000 times more potent on a 100-year time horizon as a global warming agent than a similar mass of carbon dioxide, and survives in the atmosphere about five times longer than carbon dioxide. Current NF3 emissions, however, contribute only about 0.15% of the total global warming effect contributed by current human-produced carbon dioxide emissions.

Accurately measuring small amounts of NF3 in air has proven to be a very difficult experimental problem, and we are very pleased to have succeeded in this effort.

—Ray Weiss

The Scripps team analyzed air samples gathered over the past 30 years, working 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 Tasmania for this research.

The team analyzed NF3 using a modified version of the Medusa cryogenic preconcentration and gas chromatograph/mass spectrometer (GC/MS) system. The Scripps team devised a procedure to separate NF3 in the GC/MS from more abundant volatile atmospheric gases, especially krypton (Kr) and carbon dioxide (CO2) that would otherwise quench the mass spectrometer.

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 NF3 in the Northern Hemisphere than in the Southern Hemisphere, which the researchers said is consistent with its use predominantly in Northern Hemisphere countries. The current observed rate of increase of NF3 in the atmosphere corresponds to emissions of about 16% of the amount of the gas produced globally.

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. From a climate perspective, there is a need to add NF3 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.

—Ray Weiss

Michael Prather is a UC Irvine atmospheric chemist who predicted earlier this year that based on the rapidly increasing use of NF3, larger amounts of the gas would be found in the atmosphere. Prather said the new Scripps study provides the confirmation needed to establish reporting requirements for production and use of the gas.

This result reinforces the critical importance of basic research in determining the overall impact of the information technology industry on global climate change, which has already been estimated to be equal to that of the aviation industry,” added Larry Smarr, director of the California Institute for Telecommunications at UCSD, who was not involved in the Scripps study.

Co-authors of the paper are Scripps researchers Jens Mühle, Peter Salameh and Christina Harth.

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