Diffusional isotope fractionation of singly and doubly substituted isotopologues of H2, N2 and O2 during air-water gas transfer

Air-water gas transfer largely influences the geochemical and biogeochemical cycles of essential atmospheric components (e.g. O-2 and CO2), in which gas molecular diffusion in water is recognized as the rate limiting step. Isotope compositions in these gas molecules are useful tools to quantify this mass transfer process, in which diffusional isotope fractionation factors (i.e. alpha(diff)) are the key intrinsic parameters. These alpha(diff)s are often determined by gas transfer experiments with large uncertainties because the roughness of water surface can affect the interpretation of experimental data. In this study, molecular dynamic simulations were employed to investigate directly the diffusional isotope fractionation for singly and doubly substituted isotopologues of H-2, N-2, and O-2. The results show that diffusional isotope fractionation factors are dependent on both the molecular mass and moment of inertia, which is consistent with previous findings for polyatomic molecules rather than for monoatomic ones. When comparing with the kinetic isotope fractionation (i.e. alpha(k)) determined by gas transfer experiments, I found that alpha k is likely close to (alpha(diff))(1/2) within errors (i.e. alpha(k) = (alpha(diff))(1/2)), rather than to (alpha(diff))(2/3) that has often been employed to calculate alpha(diff )using alpha(k) in literature (i.e. alpha(k) = (alpha(diff))(2/3)). If this is the case, the results further indicate that the nuclear quantum effect is not significant when alpha(diff) is of interest. With these findings, I determined the isotope fractionation relationship theta for different O-2 isotopologues to be 0.5100 +/- 0.0002 and 1.9535 +/- 0.0013 respectively for (17)theta(diff) (equivalent to ln(17)alpha(diff)/ln(diff)(18 alpha)) and (36)theta(diff) (equivalent to ln(36)theta(diff)/ln(18)alpha(diff)) as an example.(C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Air-water gas transfer plays a significant role in the geochemical and biogeochemical cycles of atmospheric components. This study used molecular dynamic simulations to investigate the diffusional isotope fractionation for H-2, N-2, and O-2. The results revealed that the fractionation factors depend on molecular mass and moment of inertia, and the kinetic isotope fractionation is likely proportional to the square root of the diffusional isotope fractionation. These findings suggest that the nuclear quantum effect is not significant in the studied isotope fractionation process.