# Sensitivity of age of air trends to the derivation method for non-linear increasing inert SF6

Fritsch, F.; Garny, H.; Engel, A.; Bönisch, Harald; Eichinger, R.

##### Abstract:
Mean age of air (AoA) is a diagnostic of transport along the stratospheric Brewer–Dobson circulation. While models consistently show negative trends, long-term time series (1975–2016) of AoA derived from observations show non-significant positive trends in mean AoA in the Northern Hemisphere. This discrepancy between observed and modelled mean AoA trends is still not resolved. There are uncertainties and assumptions required when deriving AoA from trace gas observations. At the same time, AoA from climate models is subject to uncertainties, too.

In this paper, we focus on the uncertainties due to the parameter selection in the method that is used to derive mean AoA from SF$_{6}$ measurements in Engel et al. (2009, 2017). To correct for the non-linear increase in SF$_{6}$ concentrations, a quadratic fit to the time series at the reference location, i.e. the tropical surface, is used. For this derivation, the width of the AoA distribution (age spectrum) has to be assumed. In addition, to choose the number of years the quadratic fit is performed for, the fraction of the age spectrum to be considered has to be assumed. Even though the uncertainty range due to all different aspects has already been taken into account for the total errors in the AoA values, the systematic influence of the parameter selection on AoA trends is described for the first time in the present study.
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 Zugehörige Institution(en) am KIT Institut für Meteorologie und Klimaforschung - Atmosphärische Spurenstoffe und Fernerkundung (IMK-ASF)KIT-Zentrum Klima und Umwelt (ZKU) Publikationstyp Zeitschriftenaufsatz Publikationsjahr 2020 Sprache Englisch Identifikator ISSN: 1680-7316, 1680-7324 KITopen-ID: 1000123392 Erschienen in Atmospheric chemistry and physics Verlag European Geosciences Union (EGU) Band 20 Heft 14 Seiten 8709-8725 Nachgewiesen in DimensionsWeb of ScienceScopus
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