Transport mechanisms for synoptic, seasonal and interannual SF6 variations and "age" of air in troposphere

We use an atmospheric general circulation model (AGCM) driven chemistry-transport model (ACTM) to sim- ulate the evolution of sulfur hexafluoride (SF 6) in the tro- posphere. The model results are compared with continu- ous measurements at 6 sites over 71 N-90 S. These com- parisons demonstrate that the ACTM simulations lie within the measurement uncertainty over the analysis period (1999- 2006) and capture salient features of synoptic, seasonal and interannual SF6 variability. To understand transport timescales of SF6 within the troposphere, transport times of air parcels from the surface to different regions of the tro- posphere ("age") are estimated from a simulation of an ide- alized tracer. The age estimation error and its sensitivity to the selection of reanalysis meteorology for ACTM nudging or the tracer transport by deep cumulus convection as rep- resented in the model are discussed. Monthly-mean, 2-box model exchange times ( ex ) are calculated from both the ob- served and simulated SF6 time series at the 6 observing sites and show favorable agreement, suggesting that the ACTM adequately represents large-scale interhemispheric transport. The simulated SF6 variability is further investigated through decomposition of the mixing ratio time-tendency into ad- vective, convective, and vertical diffusive components. The transport component analysis illustrates the role of each pro- cess in SF6 synoptic variability at the site level and provides insight into the seasonality of ex .