Chlorofluorocarbons as an environmental hazard

Chlorofluorocarbons are used in industry for cleaning electronic components, filling refrigerators and air-conditioning systems, producing plastic foams, and propelling aerosol sprays. CFC's slowly diffuse upward into the stratosphere, where intense ultraviolet radiation causes the molecules to release free chlorine atoms. These free chlorine atoms break down ozone. Since the chlorine atoms are regenerated in the reaction, they are free to repeat the cycle significantly depleting the ozone layer, which protects the earth from the harmful effects of excess ultraviolet rays from the sun which can cause skin cancer and damage crops. CFCs are also potent greenhouse gases in the lower tropospheric layer of the atmosphere.

The most common CFCs are called CFC-11 and -12. CFC-12 has 20,000 times carbon dioxide's capacity to trap heat in the atmosphere. It is widely used in automobile air-conditioning units. Its pre-industrial level was 0 and the 1986 level was 400 parts per trillion. In 1986 the annual increase was 5 percent. CFC-11 has 17,500 carbon dioxide's capacity to trap heat in the atmosphere. It is widely used in commercial refrigerators, and in insulating foam in which it is trapped. Its pre-industrial level was 0 and the 1986 level was 230 parts per trillion. In 1986, its annual increase was 5 percent. Other major chlorofluorocarbons are CFC-113, used mainly as a cleaning agent in the electronics industry, CFC-114 and CFC-115.

Growth rates in the concentrations of chlorofluorcarbons 11 and 12 (CFC-11 and 12) in the atmosphere have been declining since the late 1980s, consistent with reductions in industrial emissions under the Montreal Protocol. In less than a decade, rates of annual increase have decreased from a high of 11 parts per thousand volume (pptv) and 19 pptv for CFC-11 and 12, respectively, to 4 pptv and 14 pptv in 1994. Of the 9 million tonnes of CFC-11 released to date, about 60 percent remains in the troposphere, 8 percent in the stratosphere, and less than 0.4 percent in the oceans. Continued reductions under the Montreal Protocol will likely see CFC-11 concentrations peaking at about 275 pptv within 2-3 years, decreasing thereafter.

Meanwhile the production and release of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) continue to grow. In 1992, HCFC-22 concentrations were about 100 pptv and rising at 7-9 percent per year. Emissions of other HCFCs and HFCs used as replacement for CFCs are also increasing rapidly, some by as much as 30 percent per year. The radiative importance of these gases varies significantly, ranging from more than 50 percent for that of CFC-11 (per unit weight of emission) for substances such as HCFC-125 and HCFC-141b to less than 4 percent for HCFC-123, HFC-152a and HCFC-225ca. Estimates for atmospheric lifetimes of these gases, although generally much lower than those of the CFCs, are also still under revision.

Fully fluorinated gases, while still not released in large quantities, are becoming of increasing concern because of their very long atmospheric lifetimes. Sulphur hexafluoride, used for electrical equipment insulation, magnesium production and science laboratories, has an estimate lifetimes of about 1,000 years. Although its Global Warming Potential (GWP) is estimated at about 20,000, low production (currently about 4 kilo tonnes per year) will likely keep direct contribution to global warming from becoming significant. Similarly, carbon tetrafluoride (tetrafluoromethane) and dicarbon hexafluoride (hexafluoromethane), produced by the aluminium industry, have lifetimes of up to 50,000 years and GWP values between 5,000 and 10,000, but net contribution to global warming are as yet small (0.5 percent globally). Other polyfluorocarbons (PFCs) now being produced for insulation purposes include PFC-4 and PFC-5.

In 1990, Canada emitted to the atmosphere approximately 10.6 kilotonnes of CFCs and 8.4 kilotonnes of HCFCs (altered CFCs which break down in the lower atmosphere).