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The Environmental Protection Agency is enforcing stricter limits on hydrofluorocarbons, highly potent greenhouse gases used in refrigerators and air conditioners that contribute to global warming
hydrofluorocarbon (HFC)
, any of several
organic compounds
composed of
hydrogen
,
fluorine
, and
carbon
. HFCs are produced synthetically and are used primarily as refrigerants. They became widely used for this purpose beginning in the late 1980s, with the introduction of the
Montreal Protocol
, which phased out the use of chemicals such as
halons
and
chlorofluorocarbons
(CFCs) that contribute to the depletion of Earth’s
ozone layer
. However, while HFCs have an
ozone depletion
potential of zero, they are
potent
greenhouse gases
, and thus their manufacture and use became increasingly regulated in the 21st century.
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In general, HFCs are relatively nonflammable, chemically stable, and nonreactive. Many are colourless, odourless
gases
, but some—such as HFC-365mfc (1,1,1,3,3-pentafluorobutane)—are
liquids
at room temperature. As refrigerants, HFCs are used in a wide variety of cooling systems, from refrigerators and freezers to automotive air-conditioning units. HFCs are also used as blowing agents in the production of
polymer
foams; as firefighting agents (having replaced halons); as
solvents
in cleaning products for
plastics
and
metals
and in plasma etching for
semiconductor
technology; and as
propellants
in metred-dose inhalers prescribed for the
treatment
of
asthma
.
There are different routes to the synthesis of HFCs. For example, HFC-134a (1,1,1,2-tetrafluoroethane, or R134a), one of the most widely used HFCs, can be prepared from
trichloroethylene
or
tetrachloroethylene
through
halogen
exchange and hydrofluorination, in which
chlorine
is replaced by hydrogen and fluorine, or through
isomerization
followed by hydrogenolysis, in which hydrogen is used to split an isomer into the desired reaction products. Other HFCs may be prepared through the fluorination of
olefins
(unsaturated
hydrocarbons
containing at least one carbon-carbon double bond) with hydrogen fluoride.
Once released into the
atmosphere
, HFCs decompose relatively quickly; for example, the atmospheric lifetime for HFC-134a is about 14 years. (CFCs, by comparison, can remain in the atmosphere for 100 years.) The breakdown of HFCs occurs in the
troposphere
(the lowest portion of the atmosphere), where they are split by reactions with hydroxyl
radicals
(∙OH). Within the troposphere, the carbon-fluorine bonds in HFCs are highly effective at trapping
solar radiation
(specifically,
infrared radiation
) and redirecting that
radiant energy
toward Earth’s surface. This so-called positive
radiative forcing
effect contributes to
global warming
.
In 2007 the average 100-year global warming potential (GWP) of HFCs was estimated to be 3,770 times that of
carbon dioxide
(the standard reference chemical for GWP calculations); weighted averaging (based on HFC consumption) predicted a 100-year GWP of 2,400 by 2040. Warming potential, however, varies widely for the individual HFCs. For instance, HFC-23 (trifluoromethane), which is generated as a by-product in the production of the hydrochlorofluorocarbon HCFC-22 (chlorodifluoromethane), has an atmospheric lifetime of 270 years and a 100-year GWP of 11,700, which surpasses known GWPs for some of the most environmentally harmful CFCs.
HFCs have become increasingly abundant in Earth’s atmosphere. For example, between 1978 and 2005, atmospheric concentrations of HFC-23 increased from about 3 to around 18 parts per trillion (ppt). Likewise, concentrations of HFC-134a increased from levels that were undetectable prior to the 1990s to about 35 ppt in 2005. Because they are
anthropogenic
(human-generated) sources of positive radiative forcing, HFC emissions have been targeted for reduction by the
Kyoto Protocol
.
