The concept of the age of stratospheric air describes the time scales involved in transport and exchange of air from the troposphere into the stratosphere. Any air parcel observed in the stratosphere is composed of a mixture of so called irreducible air parcels with different transport pathways and times. This distribution of transit times is called the age spectrum and its mean value is called the mean age of stratospheric air. The mean age can be derived from the temporal delay between the occurrence of a certain mixing ratio of a long-lived tracer in the stratosphere and the troposphere, if the tracer has no sinks or sources in the stratosphere and the temporal increase or decrease of its tropospheric concentration is monotonic.

CO2 is a tracer which shows a well documented temporal increase in the atmosphere and has no sinks in the stratosphere. The stratospheric source of CO2 from the oxidation of methane is well known and can be well quantified. Therefore, CO2 is a suitable tracer to derive the mean age of stratospheric air, certainly at higher altitudes where the seasonal cycle in tropospheric CO2 is damped out.

Measurements of CO2 performed on whole air samples collected cryogenically in the stratosphere at altitudes up to 35 km for the time period between about 1976 and 1990 (Schmidt and Khedim, 1991) were reanalysed and combined with another series of observations performed over the last decade employing the same technique. A total of about 40 vertical profiles of CO2 relying on consistent calibration scales is now available. This new data base is used to derive the temporal evolution of the mean age of the air and to investigate possible long term changes of this important dynamical parameter in the stratosphere. Changes in the general circulation, which could be driven by the increased concentrations of greenhouse gases in the atmosphere as well as by diminishing ozone concentrations in the stratosphere, should influence the observed mean age values. The observation of changes in mean age is therefore suggested as a sensitive indicator for possible changes in the stratospheric circulation.