Variations of Stable Isotopic Composition in Atmospheric Water Vapor and their Controlling Factors-A 6-Year Continuous Sampling Study in Nanjing,Eastern China

摘要

Long-term continuous monitoring of water vapor isotopes has great potential to improve our understanding of mechanisms that control stable isotope variations in hydrological processes, which is essential for accurate interpretation of isotopic proxy records. Here we presented a 6-year daily data set of stable isotopes in atmospheric water vapor (δ~(18)O_v, δD_v, and d-excess_v) and precipitation (δ~(18)O_p, δD_p, and d-excessp) at Nanjing in Eastern China, an area under the influence of the East Asian Monsoon. Based on isotopic variations, we divided a year into three seasons, that is, summer monsoon season (June-September), spring (March-May), and fall-winter (October-February). During summer monsoon season, the low δ~(18)O_v values were mainly caused by the upstream convective rainout process during the large-scale water vapor transport, and the d-excess_v values were the lowest due to the weak kinetic isotopic fractionation over its oceanic moisture source. Spring had the highest δ~(18)O_v values, which was related to less distillation of local moisture under relatively high air temperatures. The relatively high d-excess_v values resulted from the kinetic isotopic fractionation due to drier condition. The fall-winter season has the lowest δ~(18)O_v values because the local distillation was the strongest with low air temperature. The d-excess_v values were highest during this season because of the strongest kinetic isotopic fractionation during the continental moisture recycling under dry climate condition. Using the isotopic composition of both precipitation and water vapor, we found that the annual mean subcloud raindrop evaporation is ~11 of precipitation. These findings improve our understanding of hydrological cycle on the East Asian Monsoon region and will potentially improve our interpretation of numerous isotopic proxy records from this region.