Model simulations with mesospheric SF₆ sink included resolve the discrepancy between model age of air becoming younger in the Northern hemisphere versus observations of age of air with positive trend

The figure shows modelled age of air for 30 km and 30 - 50 N from several EMAC model runs, and observed time series of age of air. The observations are displayed in red (balloon sampling data) and blue (MIPAS data); the model runs with the mesospheric SF₆ sink and the realistic increase of tropospheric SF₆ included are presented in black (free running model, REF calculation within the CCMI framework) and pink (EMAC with specified dynamics); all these data sets show a positive trend of age of air with time; the model run without mesospheric SF₆ sink and an idealized linear SF₆ increase in the troposphere is the only model run (green) with decreasing age of air.

The study by Loeffel et al. (2021) demonstrates that the mesospheric sink of SF₆ plays a dominant role in understanding the measurement of the age of air on basis of this tracer. SF₆ is destroyed in the mesosphere via several chemical reactions, with electron attachment as the most prominent one. SF₆ poor air, re-entering the stratosphere after its path through the mesosphere is erroneously assigned to a very high age. Since the SF₆ sink reactions, as all chemical reactions, scale with the available amount of SF₆, the amount of destroyed SF₆ becomes larger with increasing lower-atmosphere SF₆ levels, and finally outweighs the expected increase of SF₆ due to age of air becoming younger in an accelerated Brewer-Dobson circulation under climate change.

Detail can be found under https://acp.copernicus.org/preprints/acp-2021-232/