EVALUATION OF SOURCE GAS LIFETIMES FROM STRATOSPHERIC OBSERVATIONS

Simultaneous in situ measurements of the long-lived trace species N2O, CH4, CFC-12, CFC-113, CFC-11, CCl4, CH3CCl3, H-1211, and SF6 were mad e in the lower stratosphere and upper troposphere on board the NASA ER -2 high-altitude aircraft during the 1994 campaign Airborne Southern H emisphere Ozone Experiment/Measurements for Assessing the Effects of S tratospheric Aircraft. The observed extratropical tracer abundances ex hibit compact mutual correlations that show little interhemispheric di fference or seasonal variability except at higher altitudes in souther n hemisphere spring. The environmental impact of the measured source g ases depends, among other factors, on the rate at which they release o zone-depleting chemicals in the stratosphere, that is, on their strato spheric lifetimes. We calculate the mean age of the air from the SF6 m easurements and show how stratospheric lifetimes of the other species may be derived semiempirically from their observed gradients with resp ect to mean age at the extratropical tropopause. We also derive indepe ndent stratospheric lifetimes using the CFC-11 lifetime and the slopes of the tracer's correlations with CFC-11. In both cases, we correct f or the influence of tropospheric growth on stratospheric tracer gradie nts using the observed mean age of the air, time series of observed tr opospheric abundances, and model-derived estimates of the width of the stratospheric age spectrum. Lifetime results from the two methods are consistent with each other. Our best estimates for stratospheric life times are 122 +/- 24 years for N2O, 93 +/- 18 years for CH4, 87 +/- 17 years for CFC-12, 100 +/- 32 years for CFC-113, 32 +/- 6 years for CC l4, 34 +/- 7 years for CH3CCl3, and 24 +/- 6 years for H-1211. Most of these estimates are significantly smaller than currently recommended lifetimes, which are based largely on photochemical model calculations . Because the derived stratospheric lifetimes are identical to atmosph eric lifetimes for most of the species considered, the shorter lifetim es would imply a faster recovery of the ozone layer following the phas eout of industrial halocarbons than currently predicted.