Halocarbons
The best known in this group of gases are CFCs (chlorofluorocarbons),
HCFCs (hydrochlorofluorocarbons) and the newer substitutes HFCs (hydroflurocarbons).
While the concentration of halocarbons are much lower than those of the
other greenhouse gases, the warming effect that they produce ranges from
3000 to 13000 times that of carbon dioxide. These gases very rarely occur naturally.
 CFCs
were used as spray can propellents, solvents, cleaners and coolants until
the mid 1970s. Many of the world's nations agreed to control the
use of CFCs in 1987 when they signed the Montreal Protocol on Substances
that depleted the ozone layer. The substitute HFCs, while less damaging
to the ozone layer, still trap heat in the atmosphere and are adding to
the greenhouse effect.
Once these gases are in the atmosphere, they resist breakdown and don't
disappear for many decades. They can remain in the atmosphere for up to
400 years. While the concentration of CFCs is stabilizing due to the emission controls mentioned above, levels of the longer lasting gases are increasing.
 Some
halocarbons that are effective in trapping heat are not restricted under
the Montreal Protocol neither are the CFC substitutes. Due to their long
atmospheric lifetimes they will continue trapping heat for centuries to
come.
Greenhouse gases:
Carbon dioxide
Methane
Nitrous oxide
Ozone
Water vapour
|
