Age of air from ACE-FTS measurements of sulfur hexafluoride

Abstract. Climate models predict that the Brewer-Dobson Circulation (BDC) will accelerate due to tropospheric warming. This would increase trace gas transport from the tropics to higher latitudes and alter the spatial distribution of greenhouse gases and therefore impact the radiative properties of the atmosphere, resulting in a feedback effect. The stratospheric “age of air”, representing the time since air in the stratosphere exited the troposphere, serves as a diagnostic tool for assessing stratospheric transport. Changes in age of air can therefore indicate changes in the BDC, but detecting these changes requires a long-term observation-based record of age of air. The long-lived trace gas sulfur hexafluoride (SF₆) has an increasing concentration in the troposphere and can serve as a clock to derive age of air. However, it is difficult to measure due to its small concentrations, so historically, the availability of age of air datasets derived from SF₆ has been limited. Existing datasets include age of air derived from balloon- and aircraft-based measurements from the 1970s to the present and using satellite-based measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) for the 2002–2012 period. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) provides the longest available continuous time series of vertically-resolved SF₆ measurements, spanning 2004 to the present. In this study, a new age of air product is derived from the ACE-FTS SF₆ dataset. The method is also applied to the MIPAS SF₆ dataset. The ACE-FTS product is in good agreement with other observation-based age of air datasets and shows the expected global distribution of age of air values. Two applications of the dataset are then demonstrated: evaluating age of air in a chemistry climate model and calculating the linear trend in age of air in twelve regions within the lower stratospheric midlatitudes (14–20 km, 40–70°) in each hemisphere. All trends are negative and significant to two standard deviations. This is therefore the first observation-based age of air trend study to suggest an acceleration of the shallow branch of the BDC, which transports air poleward in the lower stratosphere, in both hemispheres.