Climatic and environmental controls on stable isotopes in atmospheric water vapor near the surface observed in Changsha, China

Continuous measurements were made of isotopic compositions in atmospheric water vapor (δv) and in atmospheric precipitation (δp) near the surface in the subtropical monsoon region (Changsha, China) from October 2014 to March 2017 to investigate the change characteristics of vapor isotopes and their interactions with precipitation isotopes. On a diurnal time scale, the δ18Ov values were more negative in the daytime and more positive at night, and the change in dv was reversed during non-rainy days; however, the δ18Ov values were more positive in the daytime and more negative at night, and the variation in dv was not significant during rainy days. On a seasonal time scale, affected by distinctive moisture sources, the vapor isotopes had obvious seasonal variations, with higher δ18Ov and dv values occurring in cold seasons than in warm seasons. There were generally consistent and significant negative linear relationships of dv with temperature and absolute humidity, but the linear relationships between δ18Ov and meteorological factors were variable on the different time scales, which was closely associated with the local weather conditions on the short time scales from diurnal to intra-seasonal and movements of air masses with distinct thermodynamic properties on seasonal and sometimes intra-seasonal time scales. Although the change trend of daily δ18Ov closely approximated that of δ18Op, δ18Op experienced significant relative enrichment compared to δ18Ov, with an average enrichment of 9.18‰. Overall, the isotopic compositions of atmospheric water vapor were in equilibrium with those of precipitation while rainfall events occurred, except for those in the cold season in 2014, and when tracing short-duration weather events, δ18Ov provided more details and information related to weather processes than δ18Op.