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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Sep 2018

Research article | 11 Sep 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Beyond Craig and Gordon: A model of water vapor isotopologues in the marine boundary layer

Xiahong Feng1, Eric S. Posmentier1, Leslie J. Sonder1, and Naixin Fan1,a Xiahong Feng et al.
  • 1Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA
  • acurrent address: Department Biogeochemical Integration, Max Planck Institute for Biogeochemistry, Hans-Knöll-Straße 10 07745, Jena, Germany

Abstract. We develop a one-dimensional (1D) steady state isotope marine boundary layer (MBL) model that includes meteorologically important features missing in Craig and Gordon type models, namely height-dependent diffusion/mixing, lifting to deliver air to the free troposphere, and convergence of subsiding air. Kinetic isotopic fractionation results from this height-dependent diffusion that starts as pure molecular diffusion at the air-water interface and increases with height due to turbulent eddies. Convergence causes mixing of dry, isotopically depleted air with ambient air. Model results fill a quadrilateral in δD-δ18O space, of which three boundaries are respectively defined by 1) vapor in equilibrium with various sea surface temperatures (SSTs); 2) mixing of vapor in equilibrium with seawater and vapor in subsiding air; and 3) vapor that has experienced maximum possible kinetic fractionation. Model processes also cause variations in d-excess of MBL vapor. In particular, mixing of relatively high d-excess descending/converging air into the MBL increases d-excess, even without kinetic isotope fractionation. The model is tested by comparison with seven datasets of marine vapor isotopic ratios, with excellent correspondence. About 95 % of observational data fall within the quadrilateral predicted by the model. The distribution of observations also highlights the significant influence of vapor from nearby converging descending air on isotopic variations within the MBL. At least three factors may explain the ~ 5 % of observations that fall slightly outside of the predicted regions in δD-δ18O and d-excess-δ18O space: 1) variations in seawater isotopic ratios, 2) variations in isotopic composition of subsiding air, and 3) influence of sea spray.

Xiahong Feng et al.
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Xiahong Feng et al.
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Publications Copernicus
Short summary
We created a new model simulating vapor isotopic variations within ~ 1000 meters above the sea surface. The model incorporates two processes previously not accounted for in simple isotopic evaporation models, 1) height-dependent diffusivity and 2) lifting of the air to deliver vapor to precipitating clouds. Calculated isotopic ratios agree well with data from seven marine cruises. The model enables better understanding of both modern climate, and ancient climate interpreted from ice core data.
We created a new model simulating vapor isotopic variations within ~ 1000 meters above the sea...