The Role of Ozone Vibrational Resonances in the Isotope Exchange Reaction 16 O 16 O + 18 O → 18 O 16 O + 16 O: The Time-Dependent Picture

Yuen, Chi Hong; Lapierre, David; Gatti, Fabien; Kokoouline, Viatcheslav; Tyuterev, Vladimir G.

doi:10.1021/acs.jpca.9b06139

Abstract

Ocean isotopic evaporation models, such as the Craig‐Gordon model, rely on the description of nonequilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient‐diffusion type measurements have been performed in ocean settings to test the validity of the commonly used parametrization of nonequilibrium isotopic fractionation during ocean evaporation. In this work, we present 6 months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating nonequilibrium fractionation factors (k, ‰) for ocean evaporation and their wind speed dependency. The Keeling Plot method and Craig‐Gordon model combination were sensitive enough to resolve nonequilibrium fractionation factors during evaporation resulting into mean values ofk₁₈ = 5.2 ± 0.6‰ andk₂ = 4.3 ± 3.4‰. Furthermore, we evaluate the relationship betweenkand 10‐m wind speed over the ocean. Such a relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (a) in the observed wind speed range [0–10 m s⁻¹], the sensitivity ofkto wind speed is small, in the order of −0.2‰ m⁻¹ s fork₁₈, and (b) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. The water vapord‐excess variability predicted under the closure assumption using thekvalues estimated in this study is in agreement with observations over the Atlantic Ocean.

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