05 Sep 2023
 | 05 Sep 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Correction of stratospheric age-of-air derived from SF6 for the effect of chemical sinks

Hella Garny, Roland Eichinger, Johannes C. Laube, Eric A. Ray, Gabriele P. Stiller, Harald Bönisch, and Laura Saunders

Abstract. Observational monitoring of the stratospheric transport circulation, the Brewer-Dobson-Circulation (BDC), is crucial to estimate any decadal to long-term changes therein, a prerequisite to interpret trends in stratospheric composition and to constrain the consequential impacts on climate. The transport time along the BDC (i.e., the mean age of stratospheric air, AoA) can best be deduced from trace gas measurements of tracers which increase linearly in time and are chemically passive. The gas SF6 is often used to deduce AoA, because it has been increasing monotonically since the ~1950s, and previously its chemical sinks in the mesosphere have been assumed to be negligible for AoA estimates. However, recent studies have shown that the chemical sinks of SF6 are stronger than assumed, and become increasingly relevant with rising SF6 concentrations.

To adjust biases in AoA that result from the chemical SF6 sinks, we here propose a simple correction scheme for SF6-based AoA estimates accounting for the time-dependent effects of chemical sinks. The correction scheme is based on theoretical considerations with idealized assumptions, resulting in a relation between ideal AoA and apparent AoA which is a function of the tropospheric reference time-series of SF6 and of the AoA-dependent effective lifetime of SF6. The correction method is thoroughly tested within a self-consistent data set from a climate model that includes explicit calculation of chemical SF6 sinks. It is shown within the model that the correction successfully reduces biases in SF6-based AoA to less than 5 % for mean ages below 5 years. Tests with using only sub-sampled data for deriving the fit coefficients show that applying the correction scheme even with imperfect knowledge of the sink is far superior to not applying a sink correction.

Further, we show that based on currently available measurements, we are not able to constrain the fit parameters of the correction scheme based on observational data alone. However, the model-based correction curve lies within the observational uncertainty, and we thus recommend to use the model-derived fit coefficients until more high-quality measurements will be able to further constrain the correction scheme. The application of the correction scheme to AoA from satellites and in-situ data suggests that it is highly beneficial to reconcile different observational estimates of mean AoA.

Hella Garny et al.

Status: open (until 20 Oct 2023)

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Hella Garny et al.


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Short summary
The transport circulation in the stratosphere is important for the distribution of tracers but its strength is difficult to measure. Mean transport times can be inferred from observations of trace gases with certain properties, such as SF6. However, this gas does have a chemical sink in the high atmosphere which can lead to substantial biases in inferred transport times. In this paper we present a method to correct mean transport times derived from SF6 for the effects of chemical sinks.