Mean ages of stratospheric air derived from in situ observations of CO₂, CH4, and N₂O

Accurate mean ages for stratospheric air have been derived from a spatially and temporally comprehensive set of in situ observations of CO₂, CH₄, and N₂O obtained from 1992 to 1998 from the NASA ER-2 aircraft and balloon flights. Errors associated with the tropospheric CO₂ seasonal cycle and interannual variations in the CO₂ growth rate are < 0.5 year throughout the stratosphere and < 0.3 year for air older than 2 years (N₂O < 275 ppbv), indicating that the age spectra are broad enough to attenuate these influences over the time period covered by these observations. The distribution of mean age with latitude and altitude provides detailed, quantitative information about the general circulation of the stratosphere. At 20 km, sharp meridional gradients in the mean age are observed across the subtropics. Between 20 and 30 km, the average difference in mean age between the tropics and midlatitudes is ∼2 years, with slightly smaller differences at higher and lower altitudes. The mean age in the midlatitude middle stratosphere (∼25-32 km) is relatively constant with respect to altitude at 5 ± 0.5 years. Comparison with earlier balloon observations of CO₂ dating back to the 1970s indicates that the mean age of air in this region has remained within ±1 year of its current value over the last 25 years. A climatology of mean age is derived from the observed compact relationship between mean age and N₂O. These characteristics of the distribution of mean age in the stratosphere will serve as critically needed diagnostics for models of stratospheric transport.