A discussion of deuterium excess production in rain water
Study on the characteristic of hydrogen and oxygen isotopes in precipitation is an important aspect of water cycle research. However, the production process of deuterium excess (D-excess) in rain water has not been fully explained. From the perspective of characteristics of air-liquid interface, the differences in isotopic fractionation between the processes of free water evaporation and rain water condensation are analyzed. The result shows that the two processes follow the kinetic fractionation and the equilibrium fractionation, respectively. This directly indicates the generation of D-excess in rain water, however, the interface effect of evaporation-condensation processes controls whether the diffusion process controls the fractionation or not. The free water evaporation process under natural conditions has a smaller specific interface area, the interface effect of isotopic fractionation is large, and the molecule diffusion is apparent. Thus, the free water evaporation is a non-equilibrium fractionation process; on the other hand, the process of rain water condensation has a larger specific interface area, the interface effect is ignorable, and the molecule diffusion does not affect the isotopic fractionation. This process is characterized as the equilibrium fractionation. Using the above result as the assumed conditions, the isotopic fractionation process of water cycle from seawater evaporation to precipitation condensation is simulated under different temperature conditions ( 10 - 40℃ ) . The simulated result and the Global Meteoric Water Line (GMWL) coincide with each other, which supports the conclusion that the difference of the interface effects in the free water evaporation and the rain water condensation processes causes the D-excess in precipitation.
hydrogen and oxygen stable isotopesD-excessspecific interface areainterface effect
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