Perfluorocarbons

the perfluorocarbons are a group of chemically related greenhouse gases covered by the Kyoto Protocol. Although emissions of perfluorocarbons are low compared to many other pollutants, they are of great concern because the perfluorocarbons are extremely powerful greenhouse gases with very long atmospheric lifetimes. Furthermore, the release of man-made perfluorocarbons is on the rise, due to increasing aluminum and semiconductor chip manufacture. Annual releases of PFM, the most abundant perfluorocarbon, are the global warming equivalent to about 70 megatons of CO2, roughly one two-hundredth of the amount of CO2 released annually.

In the context of climate change, the most important perfluorocarbons are perfluoromethane (PFM) and perfluoroethane (PFE). Also of interest in a wider environmental context are the oxygenated perfluoro-carbons, PFOS and PFOA. These latter compounds are highly soluble in water, and they are found in the ocean environment and in living tissues, but rarely in the atmosphere. Since 1980, the atmospheric concentration of PFM has risen by around 30 percent, despite reductions in emissions per ton from the aluminum industry, and is thought to have risen by around 70 percent since 1960. The atmospheric concentration of PFE has doubled from its concentration in 1980, and is believed to be more than 10 times higher than its 1960 value.

The perfluorocarbon molecules are strong absorbers of infrared radiation, and are therefore powerful greenhouse gases. Although the atmospheric concentration of perfluoromethane is around 100,000 times lower than CO2, the radiative forcing due to this atmospheric loading of PFM is as much as one five-hundredth of the radiative forcing due to CO2. (Radiative forcing is a measure of the global warming effect of a chemical at a given atmospheric concentration.) PFM is a much more powerful greenhouse gas than CO2 as measured by its global warming potential. The global warming potentials of PFM and PFE are 7,390 and 12,200, respectively.

The PFCs are extremely environmentally stable; they are only very slowly destroyed by the action of sunlight and oxygen. The main pathway for removal of PFCs from the environment is via high temperature combustion processes when air is taken into vehicle engines or power station furnaces. This environmental stability arises from the molecules' chemical structure. The PFCs are related to simple hydrocarbons by replacement of all hydrogen atoms by fluorine. For example, the simplest hydrocarbon is methane, CH4. The corresponding PFC is perfluoromethane, CF4. The carbon-fluorine chemical bond is tremendously robust with respect to normal mechanisms by which the atmosphere cleans itself. Consequently, the atmospheric lifetimes of PFM and PFE are around 50,000 years and 10,000 years, respectively.

Natural PFM emissions from soils give rise to a background "clean air" concentration of about 40 pptv (parts per trillion by volume). Its concentration has been increasing throughout the latter half of the 20 th century, to its current value of about 75 pptv due to industrial activity. Both PFM and PFE are produced as a by-product of the electrochemical extraction of aluminum from its ores. Over the past 20 years, the global aluminum industry has significantly improved its performance: currently, an average of 400 grams of PFM and about 40 grams of PFE are released per ton of aluminum produced, down by almost two-thirds since the 1980s. However, this per-ton emissions reduction has been somewhat offset by increases in aluminum production volumes. The semiconductor chip manufacturing industry is a major source of PFE and a secondary source of PFM. Consequently, the increase in chip manufacture is a major influence on the growing emissions of PFE.

Oxygenated perfluorocarbons, PFOS (perfluo-rooctane sulfonate) and PFOA (perfluorooctanoic acid) are thought to be harmful to human health. The U.S. Environmental Protection Agency regards PFOA as a "likely carcinogen"; it has been shown carcinogenic in rodents, as well as causing immune and reproductive system damage. PFOA and PFOS are released to the environment from manufacture and use of nonstick materials, fabric protectors, and fire-fighting foams. Due to their high water solubility and extremely long environmental lifetime, they are found in low concentrations in the blood of humans worldwide, and in many animals, including U.S. dolphins, Chinese pandas, and Arctic polar bears.

see also: Hydrofluorocarbons; Intergovernmental Panel on Climate Change (IPCC); Kyoto Protocol.

A tetrafluoromethane molecule: The most abundant atmospheric perfluorocarbon is tetrafluoromethane.

bibliography. Intergovernmental Panel on Climate Change, www.ipcc.ch (cited November 2007); M.A.K. Khalil, et al., "Atmospheric Perfluorocarbons," Environmental Science and Technology (v.37, 2003).

Christopher J. Ennis Clean Environment Management Centre University of Teesside

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