TROPOSPHERIC TRENDS FOR CF4 AND C2F6 SINCE 1982 DERIVED FROM SF6 DATED STRATOSPHERIC AIR

The stratosphere can be used as an archive of past tropospheric condit ions. Measurements of the long-lived trace eases CF4, C2F6, and SF6 ha ve been carried out on stratospheric air samples obtained by balloon b orne cryosampling from altitudes of up to 34 km between 1987 and 1995. SF6 observations were used to 'date' the air samples. Above 25 km hei ght the SF6 concentrations are lagging the tropospheric ones by severa l years: 4.5 years for the tropics, 6 years for mid-latitudes and up t o 10 years for the arctic winter vortex. The high ages of stratospheri c air found inside the upper polar vortex may indicate that the lag of the peak halogen burden between the troposphere and the polar stratos phere is longer than previously assumed. The age information from SF6 allows to reconstruct tropospheric trends for CF4 and C2F6 since 1982. Accordingly, for the period 1982 to 1995, concentrations increased fr om 62 to 75 ppt for CF4 and from 1.5 to 2.6 ppt for C2F6. Mean increas e rates of 1.00 ppt yr(-1) for CF4 and 0.084 ppt yr(-1) for C2F6 trans late into annual injection rates of about 14,600 t yr(-1) and 1,900 t yr(-1) respectively. With aluminium production as tile only major sour ce, average emissions per ton of primary aluminium are thus calculated to be 0.77 kg for CF4 and 0.10 kg of C2F6 in 1994.