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[1] The objective of this study is to investigate the characteristics of dD, d18 O, and deuterium excess (d) of precipitation and water vapor in surface air in Beijing, China. The dD, d18 O, and d of atmospheric water vapor in surface air were measured continuously with an in situ technique. Much less day-to-day and diurnal variations in the vapor isotopic contents were observed in the summer monsoon season (June-August) than in the rest of the year. Outside the monsoon season, the vapor dD and d18 O showed a log linear dependence on the vapor mixing ratio, and d showed a negative correlation with the local relative humidity (RH). Both relationships were statistically significant. The vapor mixing ratio and RH were poor predictors of the vapor isotopic temporal variability during the peak summer monsoon activities. In addition, an analysis was presented of the interaction of the isotopic exchange between the vapor and the condensed phase. The dD and d18 O departure from the equilibrium state was positively correlated with RH, and the d departure from the equilibrium state was negatively correlated with RH.

Leaf water 18 O enrichment is an important factor controlling the H 2 18 O, C 18 OO, and O 18 O exchanges between the biosphere and the atmosphere. At present, there is limited capacity to explain the enrichment mechanisms in field conditions. In this study, three models of varying complexity were used to simulate the leaf water 18 O enrichment at the canopy scale. Comparisons were made among the models and with high-frequency isotopic measurements of ecosystem water pools in wheat and corn. The results show that the steady state assumption was a better approximation for ecosystems with lower canopy resistance, that it is important to consider the effect of leaf water turnover in modeling the enrichment and not necessary to deal with time changes in leaf water content, and that the leaf-scale Péclet effect was incompatible with the big-leaf modeling framework for canopy-air interactions. After turbulent diffusion has been accounted for in an apparent kinetic factor parameterization, the mean 18 O composition of the canopy foliage water was a well-behaved property predictable according to the principles established by leaf-scale studies, despite substantial variations in the leaf water enrichment with leaf and canopy positions. In the online supplement we provided a discussion on the observed variability of leaf water 18 O composition with leaf and canopy positions and on the procedure for correcting isotopic measurements for organic contamination.