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The impact of sulfur hexafluoride (SF6) sinks on age of air climatologies and trends

Löffel, Sheena and Eichinger, Roland and Garny, Hella and Reddmann, Thomas and Fritsch, Frauke and Versick, Stefan and Stiller, Gabriele and Haenel, Florian (2022) The impact of sulfur hexafluoride (SF6) sinks on age of air climatologies and trends. Atmospheric Chemistry and Physics (22), pp. 1175-1193. Copernicus Publications. doi: 10.5194/acp-22-1175-2022. ISSN 1680-7316.

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Official URL: https://acp.copernicus.org/articles/22/1175/2022/acp-22-1175-2022.html


Mean age of air (AoA) is a common diagnostic for the strength of the stratospheric overturning circulation in both climate models and observations. AoA climatologies and AoA trends over the recent decades of model simulations and proxies derived from observations of long-lived tracers do not agree. Satellite observations show much older air than climate models, and while most models compute a clear decrease in AoA over the last decades, a 30-year time series from measurements shows a statistically nonsignificant positive trend in the Northern Hemisphere extratropical middle stratosphere. Measurement-based AoA derivations are often founded on observations of the trace gas sulfur hexafluoride (SF6), a fairly long-lived gas with a near-linear increase in emissions during recent decades. However, SF6 has chemical sinks in the mesosphere that are not considered in most model studies. In this study, we explicitly compute the chemical SF6 sinks based on chemical processes in the global chemistry climate model EMAC (ECHAM/MESSy Atmospheric Chemistry). We show that good agreement between stratospheric AoA in EMAC and MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is reached through the inclusion of chemical SF6 sinks, as these sinks lead to a strong increase in the stratospheric AoA and, therefore, to a better agreement with MIPAS satellite observations. Remaining larger differences at high latitudes are addressed, and possible reasons for these differences are discussed. Subsequently, we demonstrate that the AoA trends are also strongly influenced by the chemical SF6 sinks. Under consideration of the SF6 sinks, the AoA trends over the recent decades reverse sign from negative to positive. We conduct sensitivity simulations which reveal that this sign reversal does not result from trends in the stratospheric circulation strength nor from changes in the strength of the SF6 sinks. We illustrate that even a constant SF6 destruction rate causes a positive trend in the derived AoA, as the amount of depleted SF6 scales with increasing SF6 abundance itself. In our simulations, this effect overcompensates for the impact of the accelerating stratospheric circulation which naturally decreases AoA. Although various sources of uncertainties cannot be quantified in detail in this study, our results suggest that the inclusion of SF6 depletion in models has the potential to reconcile the AoA trends of models and observations. We conclude the study with a first approach towards a correction to account for SF6 loss and deduce that a linear correction might be applicable to values of AoA of up to 4 years.

Item URL in elib:https://elib.dlr.de/148732/
Document Type:Article
Title:The impact of sulfur hexafluoride (SF6) sinks on age of air climatologies and trends
AuthorsInstitution or Email of AuthorsAuthor's ORCID iD
Eichinger, RolandDLR, IPAhttps://orcid.org/0000-0001-6872-5700
Fritsch, FraukeDLR, IPAhttps://orcid.org/0000-0001-6414-9726
Date:24 January 2022
Journal or Publication Title:Atmospheric Chemistry and Physics
Refereed publication:Yes
Open Access:Yes
Gold Open Access:Yes
In ISI Web of Science:Yes
Page Range:pp. 1175-1193
Publisher:Copernicus Publications
Keywords:Stratospheric circulation, tracer transport, age of air
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:Earth Observation
DLR - Research area:Raumfahrt
DLR - Program:R EO - Earth Observation
DLR - Research theme (Project):R - Atmospheric and climate research
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Earth System Modelling
Deposited By: Garny, Hella
Deposited On:10 Feb 2022 08:17
Last Modified:14 Feb 2022 11:28

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