Stratospheric Age-Of-Air and SF6 Simulations with Silam

Abstract

The spatial distribution of Age of Air (AoA) in the stratosphere has been extensively used to evaluate and compare general circulation and chemical transport models. We performed multi-decade simulations of SF6 and several AoA tracers in the atmosphere with Silam chemistry-transport model driven with ERA-Interim reanalysis. The resulting distributions of AoA agree well with each other and with simulations made earlier, however disagree with AoA derived from observed SF6 concentrations in polar areas. The simulations of SF6 were made in two variants: fully passive passive, and with account for mesospheric depletion and for gravitational separation. The results indicate a good agreement with observations by MIPAS satellite instrument for non-passive SF6, and have substantial discrepancies in polar areas for passive SF6. Thus we conclude that the discrepancy between SF6 and modelled AoA originates from the violation of the assumption of the SF6 passiveness.