STRATOSPHERIC TRENDS OF CFC-12 OVER THE PAST 2 DECADES - RECENT OBSERVATIONAL EVIDENCE OF DECLINING GROWTH-RATES

The long term temporal trend of CCl2F2 (CFC-12) in the lower stratosph ere has been derived from a series of balloon-borne observations of lo ng-lived trace gases made since 1978. In order to remove the effect of dynamical processes in the individual profile observations, nitrous o xide (N2O) is used as a vertical coordinate. The calculated trend in t he lowermost stratosphere follows the observed tropospheric increase v ery closely, however, the observed mixing ratios in the lowermost stra tosphere appear to lag the global mean tropospheric trend by about one year. The most recent observations indicate that the slowing down of the tropospheric increase has propagated into the lowermost stratosphe re. While CFC-12 increased at an average rate of 18.5+/-1.8 ppt per ye ar between 1978 and 1990, the growth rate decreased to 11.9+/-4.2 ppt per year between 1990 and 1997.