GROWTH OF FLUOROFORM (CHF3, HFC-23) IN THE BACKGROUND ATMOSPHERE

There is growing concern over the emission and accumulation of very lo ng-lived fluorinated trace gases in the atmosphere, due to their large global warming potentials (GWPs). Unlike CFCs and other ozone-depleti ng, chlorinated and brominated chemicals, consumption of these fluorin ated compounds is not controlled by the Montreal Protocol or any other international agreement. Of all the known and potential trace 'greenh ouse' gases, the two with the highest GWPs are sulfur hexafluoride (SF 6) and fluoroform (CHF3, HFC-23). Whereas several studies have reporte d the detection and accumulation of SF6 in the atmosphere, the presenc e of HFC-23 has remained unreported. We have found that present-day HF C-23 concentrations (c. 11 pptv in late 1995) exceed those of SF6 by a factor of three. Concentrations have steadily increased in the atmosp here since at least 1978, and are continuing to do so at a present rat e of 5% per year. Furthermore, HFC-23 appears to be long-lived in the atmosphere, with a stratospheric lifetime of at least 1000 years, and a modelled tropospheric lifetime of 230 years. In terms of global warm ing, the cumulative emissions of HFC-23 up to, and including, 1995 are equivalent to 1.6 billion tonnes of CO2.