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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 04 Jul 2019

Submitted as: research article | 04 Jul 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Simulating age of air and distribution of SF6 in the stratosphere with SILAM model

Rostislav Kouznetsov1,2, Mikhail Sofiev1, Julius Vira1,a, and Gabriele Stiller3 Rostislav Kouznetsov et al.
  • 1Finnish Meteorological Institute, Helsinki, Finland
  • 2Obukhov Institute for Atmospheric Physics, Moscow, Russia
  • 3Karlsruhe Institute of Technology, Karlsruhe, Germany
  • acurrently at: Cornell University, Ithaca, NY, USA

Abstract. The paper presents a comparative study of age of air (AoA) derived with several approaches: a widely used passive tracer accumulation method, the SF6 accumulation, and a direct calculation of an "ideal age" tracer. The simulations have been performed with the Eulerian chemistry transport model SILAM driven with the ERA-Interim reanalysis for 1980–2018.

The Eulerian environment allowed for simultaneous application of several approaches within the same simulation, and interpretation of the obtained differences. A series of sensitivity simulations revealed the role of the vertical profile of turbulent diffusion in the stratosphere, destruction of SF6 in the mesosphere, as well as the effect of gravitational separation of gases with strongly different molar masses.

The simulations reproduced well the main features of the SF6 distribution in the atmosphere retrieved from the MIPAS satellite instrument. It was shown that the apparent very old air in the upper stratosphere derived from the SF6 profile observations is a result of destruction and gravitational separation of this gas in the upper stratosphere and mesosphere. The effect of these processes add over 4 years to the actual AoA, which, according to our calculations, does not exceed 6–6.5 years.

The destruction of SF6 and varying rate of emission make it unsuitable to reliably derive AoA or its trends. However, observations of SF6 provide a very useful means for validation of stratospheric circulation in a model with properly implemented SF6 loss.

Rostislav Kouznetsov et al.
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Rostislav Kouznetsov et al.
Rostislav Kouznetsov et al.
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Short summary
Estimates of the age of stratospheric air (AoA), its distribution, and trends, obtained by different experimental methods differ among each other. The AoA derived form MIPAS satellite observations, the richest observational dataset on sulphur hexafluoride SF6 in the stratosphere, are a clear outlier. With multi-decade simulations of AoA and SF6 in the stratosphere we show that the origin of discrepancy is in a methodology of deriving AoA from observations rather than in observational data.
Estimates of the age of stratospheric air (AoA), its distribution, and trends, obtained by...