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Termite killer lingers as potent greenhouse gas

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The concentration of the gas rose at a rate of 4 to 6 percent per year between 1978 and 2007, to a global atmospheric abundance by the end of 2007 of about 1.5 parts per trillion, the researchers report.

These first-ever measurements of sulfuryl dioxide appear in the March 12 Journal of Geophysical Research, a publication of the American Geophysical Union (AGU).

One kilogram of sulfuryl dioxide emitted into the atmosphere has a global warming potential approximately 4,800 times greater than one kilogram of carbon dioxide. However, amounts of sulfuryl fluoride released into the atmosphere (about 2,000 metric tons per year) are far lower than those of carbon dioxide (about 30 billion metric tons per year). So, the absolute effect of present sulfuryl dioxide emissions on global warming is comparably small.

Still, the newfound extended lifetime of the gas "has to be taken into account before large amounts are emitted into the atmosphere," says Jens Muehle, an atmospheric chemist at Scripps Institution of Oceanography, in La Jolla, Calif., and leader of the study. Sulfuryl fluoride became widely used for structural fumigation after a 1987 international treaty to protect the ozone layer required gradual discontinuation of another fumigant-methyl bromide-known to have strong ozone-depleting characteristics.

"Such fumigants are very important for controlling pests in the agricultural and building sectors," says Ron Prinn of the Massachusetts Institute of Technology, in Cambridge, Mass., and a co- author on the new paper. But with methyl bromide being phased out, "industry had to find alternatives, so sulfuryl fluoride has evolved to fill the role." Sulfuryl fluoride is regulated as a toxic substance but not currently as a greenhouse gas.

Muehle said he started detecting an unknown compound in air samples taken at the Scripps pier in early 2004 with a newly developed measurement instrument. He identified the compound as sulfuryl dioxide and concluded that the large fluctuations seen at the pier were likely related to the fumigation of local buildings. The team expanded the analysis to air samples routinely collected around the world at stations of the NASA-funded Advanced Global Atmospheric Gases Experiment (AGAGE) network and to old air samples archived in metal cylinders.

With the help of atmospheric computer models, the research team determined that the most important removal process of sulfuryl fluoride is dissolution into the ocean, where it is decomposed by chemical reactions.

Surprisingly, actual sulfuryl fluoride emissions into the atmosphere over the period studied were about one third less than expected from global industrial production estimates provided by Dow AgroSciences, the chief manufacturer of sulfuryl fluoride in the United States.

"It's extremely important to have independent verification of emissions," says Muehle. "You can't have regulation without verification and you can't have verification without measurements."

The team's report follows closely on the announcement of a similar finding, the greater-than- expected prevalence of nitrogen trifluoride-a gas used as a cleaning agent during the manufacture of thin-film solar cells, flat panel monitors and other electronics. The first measurements of nitrogen trifluoride, reported in October [http://www.agu.org/sci_soc/prrl/2008-35.html], have led to calls for it to be included in the list of greenhouse gases whose emissions are regulated by international treaties. Similarly discussions are underway regarding sulfuryl fluoride.

Peter Weiss | Quelle: American Geophysical Union
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