| Inadequate understanding of transport rates for stratospheric trace gases introduces major uncertainties into models of stratospheric ozone. Stratospheric age spectra and mean ages have recently emerged as valuable theoretical tools for characterizing transport. For an air parcel at a particular point in the stratosphere, the age spectrum is the probability distribution function for transit times from the tropical tropopause for each fluid element comprising the parcel. The mean age is the average transit time, corresponding to the first moment of the age spectrum. Previous studies derived mean ages from observations of conserved tracers that increase with time at a nearly uniform rate, e.g. CO2 and SF6. Most models significantly underestimate mean age compared to these observations, raising concern that the calculated residence time for stratospheric pollutants may be too short and thus simulated pollutant concentrations may be too low.;Here, we show that CO2 is especially useful for studying stratospheric transport because of its unique temporal variations (regular seasonal and interannual cycles and long-term increase). Complete age spectra are derived by observing the propagation of this signal into the stratosphere, providing new insight into transport mechanisms and rates. Extensive CO 2 measurements for 1992–1998, obtained from NASA's ER-2 aircraft, along with simultaneous observations of N2O and CH4, are analyzed to derive empirical age spectra for both tropical and midlatitude regions of the lower stratosphere. Tropical age spectra are narrow, with seasonal variation indicating faster ascent during northern winter, consistent with a circulation driven by breaking of extratropical waves. Midlatitude age spectra are bimodal, with a young (<1 yr) peak representing transport from the tropics along isentropes and an older (4–6 yr) peak representing diabatic descent from above in the mean meridional circulation.;At altitudes where the seasonal cycle has damped out, mean ages are inferred from CO2 data obtained using both the ER-2 and stratospheric balloons. A steep meridional gradient is observed for mean age at 20 km, with midlatitude air at least 2 years older than tropical air. This difference persists up to 32 km, the maximum altitude sampled. At midlatitudes a region of nearly constant mean age (4–6 yr) was observed in the middle stratosphere that appears to be the source region for the older peak in the age spectra obtained for lower altitudes. |
