The best known halocarbons are CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and the newer substitutes, HFCs (hydroflurocarbons). Halocarbons are used for, for example, for propellants, refrigeration devices, air conditioning, for certain types of heat pumps, and for foam plastic.
Halocarbons are long lasting and powerful greenhouse gases. For example, CFC12 can remain in the atmosphere for over 102 years and has 3800 times more powerful warming effect that CO2 molecule. However, international regulation, such as the Montreal Protocol, has effectively limited the emissions these gases, and the concentrations of most of these gases are now declining or at least levelling off.
Ozone plays two different roles in the atmosphere. At ground level, tropospheric ozone can act both as a direct, warming greenhouse gas, and as an indirect controller of greenhouse gas lifetimes. In the stratosphere, ozone has a cooling effect because it is acting as a shield which filters out most of the ultraviolet light from the Sun.
Ozone is created and destroyed by ultraviolet light from the Sun. It is created from oxygen by high energy rays, while low energy rays destroy it. Some ozone is man-made by various kinds of air pollution (traffic emissions, biomass combustion), which then reacts in sunlight.
Tropospheric ozone is estimated to have caused around one third of all the direct greenhouse gas related warming seen since the industrial revolution. Tropospheric ozone is a particularly difficult greenhouse gas to keep track of because of its short lifespan in the atmosphere and the vast variations in its regional concentrations. The current estimate for global tropospheric ozone is 370 million tonnes, equivalent to about 50 parts per billion.
Tropospheric ozone can effect the lifetime of some greenhouse gases. The break down of tropospheric ozone in sunlight leads to the production of hydroxyl (OH) radicals. These radicals help to diminish some other greenhouse gases, like methane, and hence lessen their global warming potential.
New synthetic greenhouse gases
The new synthetic greenhouse gases are long lasting and absorb the radiation extremely efficiently. The synthetic greenhouse gases include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6).
HFCs are used in the refrigeration and air-conditioning, aerosol, fire protection and foam-blowing industries. PFCs are created in the making of aluminium and magnesium and in enriching uranium, and they are being used also in eye surgery. SF6 is used in power plants as an insulation gas, gas-insulated switchgear and circuit breaker equipment and in scientific applications.
The emissions of these new greenhouse gases are relatively small and their joint heating effect is approximately 1% of the carbon dioxide’s heating effect.