Climate Change 2001: Working Group I: The Scientific Basis

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4.2.1.3 Hydrofluorocarbons (HFCs)

The HFCs with the largest measured atmospheric abundances are (in order), HFC-23 (CHF₃), HFC-134a (CF₃CH₂F), and HFC-152a (CH₃CHF₂). The recent rises in these HFCs are shown in Figure 4.3 along with some major HCFCs, the latter being controlled under the Montreal Protocol and its Amendments. HFC-23 is a by-product of HCFC-22 production. It has a long atmospheric lifetime of 260 years, so that most emissions, which have occurred over the past two decades, will have accumulated in the atmosphere. Between 1978 and 1995, HFC-23 increased from about 3 to 10 ppt; and it continues to rise even more rapidly (Oram et al., 1996). HFC-134a is used primarily as a refrigerant, especially in car air conditioners. It has an atmospheric lifetime of 13.8 years, and its annual emissions have grown from near zero in 1990 to an estimated 0.032 Tg/yr in 1996. The abundance continues to rise almost exponentially as the use of this HFC increases (Montzka et al., 1996b; Oram et al., 1996; Simmonds et al., 1998). HFC-152a is a short-lived gas with a mean atmospheric lifetime of 1.4 years. Its rise has been steady, but its low emissions and a short lifetime have kept its abundance below 1 ppt.

4.2.1.4 Perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆)

PFCs, in particular CF₄ and C₂F₆, as well as SF₆ have sources predominantly in the Northern Hemisphere, atmospheric lifetimes longer than 1,000 years, and large absorption cross-sections for terrestrial infra-red radiation. These compounds are far from a steady state between sources and sinks, and even small emissions will contribute to radiative forcing over the next several millennia. Current emissions of C₂F₆ and SF₆ are clearly anthropogenic and well quantified by the accumulating atmospheric burden. Harnisch and Eisenhauer (1998) have shown that CF₄ and SF₆ are naturally present in fluorites, and out-gassing from these materials leads to natural background abundances of 40 ppt for CF₄ and 0.01 ppt for SF₆. However, at present the anthropogenic emissions of CF₄ exceed the natural ones by a factor of 1,000 or more and are responsible for the rapid rise in atmospheric abundance. Atmospheric burdens of CF₄ and SF₆ are increasing as shown in Figures 4.4 and 4.5, respectively. Surface measurements show that SF₆ has increased by about 7%/yr during the 1980s and 1990s (Geller et al., 1997; Maiss and Brenninkmeijer, 1998). Recent relative rates of increase are 1.3%/yr for CF₄ and 3.2%/yr for C₂F₆ (Harnisch et al., 1996). The only important sinks for PFCs and SF₆ are photolysis or ion reactions in the mesosphere. These gases provide useful tracers of atmospheric transport in both troposphere and stratosphere.

A new, long-lived, anthropogenic greenhouse gas has recently been found in the atmosphere (Sturges et al., 2000). Trifluoromethyl sulphur pentafluoride (SF₅CF₃) – a hybrid of PFCs and SF₆ not specifically addressed in Annex A of the Kyoto Protocol – has the largest radiative forcing, on a per molecule basis, of any gas found in the atmosphere to date. Its abundance has grown from near zero in the late 1960s to about 0.12 ppt in 1999.