Still an issue: stratospheric chlorine
This figure shows the temporal development of CFC-12 at 20 km altitude, as measured by MIPAS. A slight decline of mixing ratios is seen at tropical latitudes, which is caused by decreasing emissions in response to the Montreal protocol. The pronounced annual cycle at polar latitudes reflects the role of fluorochlorocarbons as tracer of transport and the age of stratospheric air.
The use of age tracers of finite lifetime is complicated by the fact that knowledge of the lifetimes is a precondition to use these gases as age tracers, while the lifetime itself depends on atmospheric circulation which is to be analyzed by means of age analysis.
An overview over the role of chlorine in the stratosphere has recently been given in a review paper which appeared in the 25th anniversary issue of Atmósfera. See http://www.journals.unam.mx/index.php/atm/article/view/38656