Reducing emissions of halogenated gases

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% remains in the troposphere, 8% in the stratosphere, and less than 0.4% 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% per year. Emissions of other HCFCs and HFCs used as replacement for CFCs are also increasing rapidly, some by as much as 30% per year. The radiative importance of these gases varies significantly, ranging from more than 50% for that of CFC-11 (per unit weight of emission) for substances such as HCFC-125 and HCFC-141b to less than 4% 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 kt 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% globally). Other polyfluorocarbons (PFCs) now being produced for insulation purposes include PFC-4 and 5.