Short-term variations of vapor isotope ratios reveal the influence of atmospheric processes

Stable isotopes of atmospheric water vapor reveal rich information on water movement and phase changes in the atmosphere. Here we presented two nearly continuous time-series of δ D and δ 18 O of atmospheric water vapor ( δ v ) measured at hourly intervals in surface air in Beijing and above a winter wheat canopy in Shijiazhuang using in-situ measurement technique. During the precipitation events, the δ v values in both Beijing and Shijiazhuang were in the state of equilibrium with precipitation water, revealing the influence of precipitation processes. However, the δ v departures from the equilibrium state were positively correlated with local relative humidity. Note that the δ v tended to enrich in Beijing, but deplete in Shijiazhuang during the precipitation events, which mainly resulted from the influence of transpiration processes that enriched the δ v in Shijiazhuang. On seasonal time-scale, the δ v values were log-linear functions of water vapor mixing ratios in both Beijing and Shijiazhuang. The water vapor mixing ratio was an excellent predictor of the δ v by the Rayleigh distillation mechanisms, indicating that air mass advection could also play an important role in determining the δ v . On a diurnal time-scale, the δ v reached the minimum in the early afternoon hours in Beijing which was closely related to the atmospheric processes of boundary layer entrainment. During the peak of growing season of winter wheat, however, the δ v reached the minimum in the early morning, and increased gradually through the daytime, and reached the maximum in the late afternoon, which was responsible by the interaction between boundary layer entrainment and the local atmospheric processes, such as transpiration and dew formation. This study has the implications for the important role of vegetation in determining the surface δ v and highlights the need to conduct δ v measurement on short-term (e.g. diurnal) time scales.