SULFUR-HEXAFLUORIDE - A POWERFUL NEW ATMOSPHERIC TRACER

Long-term observations of the atmospheric trace gas sulfur hexafluorid e (SF6) at four background monitoring stations, Neumayer, Antarctica ( 1986-1994), Cape Grim, Tasmania (1978-1994), Izana, Canary Islands(199 1-1994) and Alert, Canada (1993-1994) are presented. These data sets a re supplemented by two meridional profiles collected over the Atlantic Ocean (1990 and 1993) and occasional observations at the regional sit e Fraserdale, Canada (1994). The analytical system and the method of S F6 calibration are described. Compared with data from Neumayer and Iza na reported earlier, measurements are updated for all sites until the end of 1994 and the precision has improved by more than a factor of 2, With the Cape Grim archived air samples, the atmospheric SF6 chronolo gy is extended by 8 more years back to 1978. For the period from Janua ry 1978 to December 1994 the data confirm a stable and unbroken quadra tic rise in tropospheric SF6 from 0.50 to 3.11 ppt in the southern hem isphere and for July 1991 to December 1994 from 2.69 to 3.44 ppt in th e northern hemisphere. The global mean tropospheric increase rate in l ate 1994 was 0.225 ppt yr(-1) (6.9% yr(-1)). The long term trend and i nterhemispheric gradients are due to industrial production and emissio n, rising approximately linearly with time and located predominantly ( 94%) in the northern hemisphere. The interhemispheric exchange time (1 .7 +/- 0.2 yr) derived from SF6 ground level observations when using a two-box model of the atmosphere is considerably larger if compared to the exchange time derived from two- and three-dimensional models (1.1 yr). The chemical and biological inertness of SF6 up to stratospheric conditions results in an atmospheric lifetime of more than 800 years and makes SF6 a powerful tool for modelling transport processes in the atmosphere. Moreover, the tropospheric SF6 chronology is a very valua ble input function for mixing studies in linked compartments like the stratosphere, the hydrosphere and the cryosphere.