Stratospheric trends of CFC-12 over the past two decades: Recent observational evidence of declining growth rates

The long term temporal trend of CCl2F2 (CFC-12) in the lower stratosphere has been derived from a series of balloon-borne observations of long-lived trace gases made since 1978. In order to remove the effect of dynamical processes in the individual profile observations, nitrous oxide (N2O) is used as a vertical coordinate. The calculated trend in the lowermost stratosphere follows the observed tropospheric increase very closely, however, the observed mixing ratios in the lowermost stratosphere 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 stratosphere. While CFC-12 increased at an average rate of 18.5+/-1.8 ppt per year between 1978 and 1990, the growth rate decreased to 11.9+/-4.2 ppt per year between 1990 and 1997.