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

Abstract

Abstract. The paper presents a comparative study of age of air (AoA) derived from several approaches: a widely used passive-tracer accumulation method, the SF6 accumulation, and a direct calculation of an ideal-age tracer. The simulations were 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, and 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 observed by the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) 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 the mesosphere. These processes make the apparent SF6 AoA in the stratosphere several years older than the ideal-age AoA, which, according to our calculations, does not exceed 6–6.5 years. The destruction of SF6 and the varying rate of emission make SF6 unsuitable for reliably deriving AoA or its trends. However, observations of SF6 provide a very useful dataset for validation of the stratospheric circulation in a model with the properly implemented SF6 loss.