Sulfur Hexafluoride (SF6)

Introduction

Sulfur hexafluoride (chemical formula SF₆) is a compound of sulfur and fluorine and the most potent greenhouse gas known. Several classes of compounds containing fluorine—including the chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydro-fluorocarbons (HFCs), perfluorocarbons (PFCs), and SF₆—are potent greenhouse gases. HCFCs and CFCs are better-known as destroyers of atmospheric ozone, and are regulated under the 1987 Montreal Protocol that has been widely hailed as successful at halting depletion of the ozone layer.

Although SF₆ is a potent greenhouse gas and its concentration in the atmosphere is increasing rapidly, its contribution to global warming is much smaller than that of the major greenhouse gases, especially carbon dioxide and methane, because it is present in smaller quantities. SF₆ is one of the six greenhouse gases regulated under the Kyoto Protocol.

Historical Background and Scientific Foundations

SF₆ engages in few chemical reactions. Because of its stability, it persists in the atmosphere for an extremely long time once released. According to the Intergovern-mental Panel on Climate Change (IPCC), SF₆ has a lifetime in the atmosphere of 3,200 years. For a 20-year time horizon, each pound of SF₆ contributes as much to global warming as 16,300 lb (7,400 kg) of carbon dioxide (CO₂); for a 100-year horizon, 22,800 times as much; and for a 500-year horizon, 32,600 times as much.

Figures are often named for SF₆ that simply state its 100-year global warming potential without explaining what it is. For example, the U.S. Environmental Protection Agency (EPA) says simply that SF₆ has “a global warming potential 23,900 times greater than CO₂” (EPA, 2006), naming its efficacy over the 100-year time horizon. (Here, the difference between 22,800 years [IPCC] and 23,900 [EPA] arises from uncertainties in the precise physical properties of SF₆, but the discrepancy is only about 5%.) Another way of stating the greenhouse potency of SF₆ is that 1 lb (0.45 kg) of SF₆ will cause as much greenhouse warming over 100 years as 11 tons (10 metric tons) of CO₂.

WORDS TO KNOW

OZONE LAYER: The layer of ozone that begins approximately 9.3 mi (15 km) above Earth and thins to an almost negligible amount at about 31 mi (50 km) and shields Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 15.5 mi (25 km) above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration, but man-made changes are of the greatest concern.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).

SF₆ is gaseous at normal temperatures and pressures. Because of its non-toxicity and low reactivity, it is used for certain medical purposes such as a contrast agent in ultrasonic imaging. Another application is as a tracer chemical to track oceanic and atmospheric circulations. However, its primary use is as an electrical insulator in high-voltage equipment. SF₆ is highly resistant to the passage of electrical current, so it serves to keep current flowing between metal parts at very different voltages. It is therefore used in gas-insulated transformer substations, switching gear on the power grid, and high-voltage circuit breakers. About 80% of SF₆ produced worldwide is used by the electric power industry.

Atmospheric concentrations of the fluorine-containing compounds HFCs, PFCs, and SF₆ have increased rapidly from 1998 to 2005 (and no doubt since). In particular, the SF₆ concentration increased from less than 1 part per trillion in 1980 to 4.2 parts per trillion in 1998 and to 5.6 parts per trillion in 2005, about a 50% increase for 1998 to 2005. The radiative forcing from this gas increased over that time by 36% for 1998 to 2005, to 0.0029 watts per square meter (W/m²). (An atmospheric gas's radiative forcing is the amount of power per square meter that it causes Earth to retain that it would otherwise radiate out into space.) This should be compared to the radiative forcing from CO₂, 1.66 W/m² in 2005, some 572 times greater. The total radiative forcing of the HFCs, PFCs, and SF₆ as of 2005 was 0.017 W/m², about 1% of the radiative forcing from carbon dioxide. However, as the IPCC noted, the concentrations of these gases were increasingly rapidly, and because of their extremely long lifetimes, some up to 50,000 years, their affect on Earth's climate is essentially permanent.

Impacts and Issues

In the U.S. political setting of the early 2000s, mandatory national regulation of greenhouse gases, including SF₆, was not an option. The EPA did, however, maintain a voluntary program called the SF₆ Emission Reduction Partnership for Electric Power Systems. In this program, the EPA develops information on best practices for managing SF₆ to reduce emissions from the electric power industry and sponsors conferences on SF₆ with industry partners to spread such information.

According to the EPA, the most cost-effective options for reducing SF₆ emissions in the electricity industry are leak detection and repair, which could reduce SF₆ emissions by 20%; recycling of SF₆ from retired high-voltage equipment, which might reduce emissions by 10%; and employee education and training, so that employees understand the environmental impact of SF₆ releases and are aware of policies for managing SF₆. Partner corporations made a nonbinding agreement to inventory their annual emissions, establish a strategy for replacing old, leaky equipment, recycle SF₆, and submit annual progress reports to the EPA.

See Also Chlorofluorocarbons and Related Compounds.

BIBLIOGRAPHY

Books

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Fuyuno, Ichiko. “Scheme to Track Greenhouse Gases Takes to the Air.” Nature 438 (2005): 268-269.

Lee, Jennifer B. “Voluntary Pacts to Curb Greenhouse Gases.” The New York Times (October 18, 2007).

Web Sites

“HFCs, PFCs, and Sulfur Hexafluoride.” U.S. Environmental Protection Agency, 2001. <http:// http://www.eia.doe.gov/oiaf/1605/vr99data/pdf/chap6.pdf> (accessed October 30, 2007).

“SF₆ Emission Reduction Program for Power Systems.” U.S. Environmental Protection Agency, September 16, 2006. <http://www.epa.gov/electricpowersf6/overview.html> (accessed October 30, 2007).

Larry Gilman