A chronology of tropospheric CF4 mixing ratios is reported for the northern hemisphere. A decline of global emissions of CF4 from 16,000 to 11,000 metric tons yr-1 is found for the periods 1978--1990 and 1992--1998, respectively. The atmospheric chronology of CF4 is applied to determine the age of air of cryogenic air samples collected on rocket and balloon platforms. Age values of 7 years are found inside the polar vortex between 25--44 km altitude and of 11 years between 56 and 61 km altitude, which is significantly higher than predicted by most models currently used to assess the effects of high-flying aircraft. The results suggest that the mesosphere is more isolated from below than previously believed. A comparison of age values derived from CF4 and SF6 relative to predicted deviations constrains the atmospheric lifetime of SF6 to >5,000 years. ¿ 1999 American Geophysical Union