Other Gases: Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride

HFCs, PFCs, and SF₆ are three classes of engineered gases that accounted for 2.2 percent of total U.S. greenhouse gas emissions in 2004. At 155.9 MMTCO₂e, their emissions were 9.6 percent higher than in 2003 (142.4 MMTCO₂e). The increase in emissions of the engineered gases from 2003 to 2004 resulted largely from a 12-percent increase in HFC emissions, which more than offset decreases in emissions of PFCs (7.5 percent lower than in 2003) and SF₆ (2.2 percent lower) (Table ES6).

At 124.8 MMTCO₂e, emissions of HFCs made up the majority of U.S. emissions of engineered greenhouse gases, followed by SF₆ at 16.0 MMTCO₂e and PFCs at 6.7 MMTCO₂e. Another group of engineered gases, consisting of other HFCs, other PFCs, and perfluoropolyethers (PFPEs), includes HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee, and a variety of PFCs and PFPEs. They are grouped together in this report to protect confidential data. In 2004, their combined emissions totaled 8.6 MMTCO₂e. Emissions of the engineered gases in this group in 2004 were 13 percent higher than in 2003 and an order of magnitude higher than in 1990, when they totaled less than 0.3 MMTCO₂e. Since 1990, HFC emissions from U.S. sources have increased by 246 percent, PFC emissions have fallen by 66 percent, and SF₆ emissions have fallen by 50 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, and 227ea), which are used as replacements for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), have shorter atmospheric lifetimes, ranging from 1 to 33 years.