Vehicle-based in-situ observations of the water vapor isotopic composition across China: spatial and seasonal distributions and controls

Stable water isotopes are natural tracers in the hydrological cycle and have been applied in hydrology, atmospheric science, ecology, and paleoclimatology. However, the factors controlling the isotopic distribution, both at spatial and temporal scales, are debated in East Asia. For the first time, we made large scale (order 10000 km) continuous observations of near-surface vapor isotopes across China in both pre-monsoon and monsoon seasons, using a newly-designed vehicle-based vapor isotope monitoring system. For both seasons, the observed variations along the sampling route are mainly due to spatial variations, and marginally influenced by synoptic-scale variations. The data thus documents the spatial and seasonal variability of vapor isotopes. The spatial variations of vapor δ18O are mainly controlled by Rayleigh distillation during the pre-monsoon period, but significantly influenced by different moisture sources, continental recycling processes and convection during moisture transport during the monsoon period. The seasonal variation of vapor δ18O reflects the influence of the summer monsoon convective precipitation in southern China, and a dependence on temperature in the North. The spatial and seasonal variations in d-excess reflect the different moisture sources and the influence of continental recycling. The isotope-incorporated global spectral model (Iso-GSM) successfully captures the spatial variation of vapor δ18O during the pre-monsoon period owing to the large latitudinal contrast in humidity and temperature, the overall performance is weaker during the monsoon period. These results provides an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors, and emphasize the need to interpret proxy records in the context of the regional system and moisture sources.