|
Other Gases: Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride
HFCs, PFCs, and SF6 are three classes of engineered gases that account for 1.8 percent of U.S. GWP-weighted emissions of greenhouse gases. At 120.6 million metric tons carbon dioxide equivalent in 2002, their emissions were 6.2 percent higher than in 2001. The increase in emissions of the engineered gases from 2001 to 2002 resulted largely from increases in HFC emissions (8.4 percent) that counteracted decreases in emissions of PFCs (-2.2 percent) and SF6 (-4.7 percent).
At 83.1 million metric tons carbon dioxide equivalent, emissions of HFCs make up the majority of this category, followed by SF6 at 16.7 million metric tons and PFCs at 7.6 million metric tons. Another group of engineered gases, consisting of other HFCs, other PFCs, and perfluoropolyethers (PFPEs), includes HFC-152a, HFC-227ea, HFC-4310mee, and a variety of PFCs and PFPEs. They are grouped together in this report to protect confidential data. In 2002, their combined emissions totaled 13.2 million metric tons carbon dioxide equivalent. Emissions in this “other” group in 2002 were 13.5 percent higher than in 2001 and orders of magnitude higher than in 1990, when emissions were less than 180,000 metric tons carbon dioxide equivalent. Since 1990, HFC emissions from U.S. sources have increased by 126.0 percent, PFC emissions have decreased by 60.8 percent, and SF6 emissions have decreased by 58.7 percent.
Emissions of the high-GWP gases specified in the Kyoto Protocol are very small (at most a few thousand metric tons). On the other hand, some of the gases (including PFCs and SF6) have atmospheric lifetimes measured in the thousands of years, and consequently they are potent greenhouse gases with GWPs thousands of times higher than that of carbon dioxide per unit of molecular weight. Some of the commercially produced HFCs (134a, 152a, 4310mee, 227ea), which are used as replacements for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), have shorter atmospheric lifetimes, ranging from 1 to 33 years.
Released: October 2003 |