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Preprints
https://doi.org/10.5194/acp-2020-95
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-95
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  24 Feb 2020

24 Feb 2020

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A revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Gravitational separation of Ar/N2 and age of air in the lowermost stratosphere in airborne observations and a chemical transport model

Benjamin Birner1, Martyn P. Chipperfield2,3, Eric J. Morgan1, Britton B. Stephens4, Marianna Linz5, Wuhu Feng2,6, Chris Wilson2,3, Jonathan D. Bent1,a, Steven C. Wofsy5, Jeffrey Severinghaus1, and Ralph F. Keeling1 Benjamin Birner et al.
  • 1Scripps Institution of Oceanography, UC San Diego, La Jolla, CA 92093, USA
  • 2School of Earth and Environment, University of Leeds, Leeds, LS2 9JT,UK
  • 3National Centre for Earth Observation, University of Leeds, Leeds, LS2 9JT, UK
  • 4National Center for Atmospheric Research, Boulder, CO 80301, USA
  • 5Department of Earth and Planetary Sciences, and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
  • 6National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
  • anow at: Picarro, Inc., Santa Clara, CA 95054, USA

Abstract. Accurate simulation of atmospheric circulation, particularly in the lower stratosphere, is challenging due to unresolved wave-mean flow interactions and limited high-resolution observations for validation. Gravity-induced pressure gradients lead to a small but measurable separation of heavy and light gases by molecular diffusion in the stratosphere. Because the relative abundance of Ar to N2 is exclusively controlled by physical transport, the argon-to-nitrogen ratio (Ar/N2) provides an additional constraint on circulation and the age of air (AoA), i.e. the time elapsed since entry of an air parcel into the stratosphere. Here we use airborne measurements of N2O and Ar/N2 from nine campaigns with global coverage spanning 2008–2018 to calculate AoA and to quantify gravitational separation in the lowermost stratosphere. To this end, we develop a new N2O-AoA relationship using a Markov Chain Monte Carlo algorithm. We observe that gravitational separation increases systematically with increasing AoA for samples with AoA between 0 to 3 years. These observations are compared to a simulation of the TOMCAT/SLIMCAT 3-D chemical transport model, which has been updated to include gravitational fractionation of gases. We demonstrate that although AoA at old ages is slightly underestimated in the model, the relationship between Ar/N2 and AoA is robust and agrees with the observations. This highlights the potential of Ar/N2 to become a new AoA tracer that is subject only to physical transport phenomena and can supplement the suite of available AoA indicators.

Benjamin Birner et al.

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Benjamin Birner et al.

Benjamin Birner et al.

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Short summary
With new high precision observations from 9 aircraft campaigns and 3D chemical transport modeling, we show that the argon-to-nitrogen ratio (Ar/N2) in the lowermost stratosphere provides a useful constraint on the “age of air” (the time elapsed since entry of an air parcel into the stratosphere). Therefore, Ar/N2 in combination with traditional age of air indicators, such as CO2 and N2O, could provide new insights into atmospheric mixing and transport.
With new high precision observations from 9 aircraft campaigns and 3D chemical transport...
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