Emissions of Greenhouse Gases in the United States 2003
Other Gases; Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride
HFCs, PFCs, and SF6 are three classes of engineered gases that accounted for 2.1 percent of total U.S. greenhouse gas emissions in 2003. At 143.4 million metric tons carbon dioxide equivalent, their emissions were 0.3 percent lower than in 2002 (143.7 million metric tons). The decrease in emissions of the engineered gases from 2002 to 2003 resulted largely from an 18-percent reduction in PFC emissions that counteracted increases in emissions of HFCs (0.5 percent) and SF6 (1.6 percent).
At 111.3 million metric tons carbon dioxide equivalent, emissions of HFCs made up the majority of this category, followed by SF6 at 17.3 million metric tons carbon dioxide equivalent and PFCs at 7.3 million metric tons carbon dioxide equivalent. 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 2003, their combined emissions totaled 7.1 million metric tons carbon dioxide equivalent. Emissions in this “other” group in 2003 were 5.9 percent higher than in 2002 and orders of magnitude higher than in 1990, when emissions were less than 0.3 million metric tons carbon dioxide equivalent. Since 1990, HFC emissions from U.S. sources have increased by 208.6 percent, PFC emissions have decreased by 62.9 percent, and SF6 emissions have decreased by 47.1 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.