Transport mechanisms for synoptic, seasonal and interannual SF6 variations in troposphere.

We use an atmospheric general circulation model (AGCM) driven Chemistry-Transport Model (ACTM) to simulate the evolution of sulfur hexafluoride (SF₆) in the atmosphere. The model results are compared with continuous measurements at 6 sites over 71° N- 90° S. These comparisons 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 troposphere ("age") are estimated from a simulation of an idealized tracer. Monthly-mean, 2-box model exchange times (τ_ex) are calculated from both the observed and simulated SF₆ time series at the 6 observing sites and show favorable agreement, suggesting that the model adequately represents large-scale interhemispheric transport. The simulated SF₆ variability is further investigated through decomposition of the mixing ratio time-tendency into advective, convective, and vertical diffusive components. The transport component analysis illustrates the role of each process in SF₆ synoptic variability at the site level and provides insight into the seasonality of τ_ex. [ABSTRACT FROM AUTHOR]