Other Gases: Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride
HFCs, PFCs, and SF6 are three classes of high-GWP gases that accounted
for 2.2 percent of total U.S. greenhouse gas emissions in 2005. At 160.2
MMTCO2e, their emissions were 7.2 percent higher than in 2004 (149.5 MMTCO2e).
The increase in emissions of the high-GWP gases from 2004 to 2005 resulted
largely from an 8.9-percent increase in HFC emissions, which more than
offset decreases in emissions of PFCs (5.2 percent lower than in 2004)
and SF6 (2.1 percent lower) (Table ES6).
At 131.8 MMTCO2e, emissions of HFCs made up the majority of U.S. emissions
of high-GWP greenhouse gases, followed by SF6 at 15.7 MMTCO2e and PFCs
at 6.7 MMTCO2e. Another group of high-GWP 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 2005, their combined
emissions totaled 6.1 MMTCO2e. Emissions of the gases in this group in
2005 were 13 percent higher than in 2004 and an order of magnitude higher
than in 1990, when they totaled less than 0.4 MMTCO2e. Since 1990, HFC
emissions from U.S. sources have increased by 265 percent, PFC emissions
have fallen by 67 percent, and SF6 emissions have fallen by 49 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.
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