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Discussion papers
https://doi.org/10.5194/hess-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-1
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 16 Jan 2020

Submitted as: research article | 16 Jan 2020

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This preprint is currently under review for the journal HESS.

Novel Keeling plot based methods to estimate the isotopic composition of ambient water vapor

Yusen Yuan1,2, Taisheng Du1, Honglang Wang3, and Lixin Wang2 Yusen Yuan et al.
  • 1Center for Agricultural Water Research in China, China Agricultural University, Beijing 100083, China
  • 2Department of Earth Sciences, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202, USA
  • 3Department of Mathematical Sciences, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202, USA

Abstract. Keeling plot approach, a general method to identify the isotopic composition of source atmospheric CO2 and water vapor (i.e., evapotranspiration), has been widely used in terrestrial ecosystems. The isotopic composition of ambient water vapor (δa), another important source of atmospheric water vapor, is not able to be estimated to date using the Keeling plot approach. Here we proposed two new methods to estimate δa using the Keeling plot curves: one using intersection point method and another replying on intermediate value theorem. As actual δa value was difficult to measure directly, we used two indirect approaches to validate our new methods. First, we made an external vapor tracking using Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to facilitate explaining the variation of δa. The trajectory vapor origin results were consistent with the expectations of the δa values estimated by these two methods. Second, regression analysis was used to evaluate the relationship between δa values estimated from these two independent methods and they are in strong agreement. This study provides an analytical framework to estimate δa using existing facilities, and provides important insights into the traditional Keeling plot approach by showing: (a) an evidence that δa was constant in a certain moment among different heights, a key assumption of the Keeling plot approach, (b) a possibility to calculate the proportion of evapotranspiration fluxes to total atmospheric vapor using the same instrumental setup for the traditional Keeling plot investigations, and c) perspectives on estimation of isotope composition of ambient CO2 (δa13C).

Yusen Yuan et al.

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Short summary
The isotopic composition of ambient water vapor is an important source of atmospheric water vapor and is not able to be estimated to date using the Keeling plot approach. Here we proposed two new methods to estimate the isotopic composition of ambient water vapor: one using intersection point method and another replying on intermediate value theorem.
The isotopic composition of ambient water vapor is an important source of atmospheric water...
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