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https://doi.org/10.5194/acp-2020-380
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-380
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 May 2020

Submitted as: research article | 07 May 2020

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This preprint is currently under review for the journal ACP.

Transport model diagnosis of the mean age of air derived from stratospheric samples in the tropics

Hanh T. Nguyen1, Kentaro Ishijima2,a, Satoshi Sugawara3, and Fumio Hasebe4 Hanh T. Nguyen et al.
  • 1Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan
  • 2Japan Agency for Marine-Earth Science and Technology, Yokohama 236-0001, Japan
  • 3Miyagi University of Education, Sendai 980-0845, Japan
  • 4Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
  • apresent address: Meteorological Research Institute, Tsukuba 305-0052, Japan

Abstract. Stratospheric profiles of the mean age of air estimated from cryogenic air samples acquired during the CUBE/Biak field campaign over Indonesia are investigated with the aid of an atmospheric chemistry transport model nudged to ERA-Interim meteorological fields. Application of the boundary impulse response (BIR) method and Lagrangian backward trajectories to the transport field simulated by a single model prove useful in interpreting the observational results, which include discrepancies between CO2- and SF6-derived mean ages. This may be because the BIR method takes unresolved diffusive processes into account while the Lagrangian method distinguishes the pathways the air parcels have taken before reaching the sample site. The capability to estimate the vertical profiles of the clock tracer concentrations and the water vapor “tape recorder” is another advantage of the Lagrangian method, confirming the reality of the trajectory calculations. The profile of CO2-mean age is reproduced reasonably well by trajectory-derived mean age, while BIR-derived mean age is much greater than CO2 age at 28 and 29 km, possibly due to high diffusivity in the transport model. On the other hand, SF6 age is reproducible only in the lower stratosphere, but far exceeds the trajectory-derived mean age above 25 km. As air parcels of mesospheric origin are missing in the Lagrangian age estimation, this discrepancy, together with the fact that the observed SF6 concentrations are much lower than the trajectory-derived values in this height region, is consistent with the idea that the stratospheric air samples are mixed with SF6-depleted mesospheric air, leading to overestimation of the mean age.

Hanh T. Nguyen et al.

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Latest update: 24 May 2020
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Short summary
The velocity of stratospheric circulation is often measured by the time since the air entered the stratosphere. This study tries to understand its vertical profile in the tropics by comparing observational data and model simulations. Our interpretation mutually consistent among them is encouraging, while some limitations such as the treatment of seasonal variation of CO2 and mesospheric loss of SF6 are reconfirmed stressing a need of using multiple variables in the future.
The velocity of stratospheric circulation is often measured by the time since the air entered...
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