<p><strong>Abstract.</strong> 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

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... 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<span class="thinspace"></span>% 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 ~<span class="thinspace"></span>5<span class="thinspace"></span>% 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.</p>