In Situ Monitoring of Water Isotopic Composition for Vapor and Precipitation Near-Surface Ground in East Asia Subtropical Monsoon Region

Hydrogen and oxygen isotopes in atmospheric water vapor (δv) and precipitation (δp or δr) were continuously measured using a laser-based water isotope spectrometer in Guangzhou, southeastern China, from March 2016 to February 2018. The measurements were conducted to investigate the variations in water isotopes in the hydrological cycle under the subtropical monsoon climate. The isotopic composition ranged from −24.4‰ to −11.1‰ for δ18O in water vapor (δ18Ov) and from −11.5‰ to 2.3‰ for δ18O in precipitation (δ18Or). The values of δv and δr were enriched during the dry season and depleted during the wet season, exhibiting systematic seasonal variation. A negative correlation was observed between monthly δv and precipitation amount, indicating that the values of δv exhibits an ‘amount effect’. However, a corresponding amount effect was not observed in the values of δr. The mean difference between δv and δr was −9.7‰ for δ18O and −76‰ for δD, suggesting that equilibrium fractionation is the dominant process during precipitation. The local meteoric vapor line (LMVL) for Guangzhou (δD = 6.6δ18O − 6.4) exhibited a slope similar to that of the equilibrium local meteoric vapor line (ELMVL) but with an intercept difference of 8.6. This difference in intercepts can be attributed to the vertical profile of δv. The δD-q (q refers to water vapor concentration) relationship is useful for identifying water vapor sources and tracking isotopic changes during atmospheric transport and precipitation. The local water vapor was found to originate primarily from the mixing of oceanic air masses. Data points falling between the oceanic source mixing line and the Rayleigh curve likely reflect post-condensation processes, such as raindrop re-evaporation or mixing with surrounding ambient vapor. Short periods of heavy precipitation were observed to cause severe depletion in δv, resulting in values falling below the Rayleigh curve.