Stable isotopic composition of near surface atmospheric water vapor and rain–vapor interaction in Taipei, Taiwan

Taiwan is an island located in the western Pacific, and falls under the influence of tropical to sub-tropical climate with two monsoons during summer and winter. This makes it an ideal location to study water vapor dynamics and its influence on the rainfall. The tropical convection/cyclones and south-west monsoon dominate the rainfall in summer while in winter, north-east monsoon overwhelms. To investigate the influence of moisture source variation, recycling and rain–vapor interaction, continuous measurements of the concentration and oxygen and hydrogen isotopic compositions (δ18O and δD) of the near surface atmospheric water vapor have been carried out in Taipei, Taiwan, using a commercial cavity ring down spectrometer. The water vapor mixing ratio varies between 15,000 and 38,000 ppmv, with maximum in the summer months and minimum in the winter. The isotopic composition of water vapor varies between −11‰ to −21‰ for δ18O and −75‰ to −150‰ for δD, but no clear seasonal variation is observed unlike that in the monthly rainwater. Therefore, the usual assumption that the monthly composite samples of rainfall reflect the isotopic variability of near surface water vapor is not valid here. Variation in δ18O, δD and deuterium excess (d-excess) in vapor reflects its interaction with rainwater superimposed by temperature variation and moisture recycling. To study how rainwater interacts with water vapor, two events are selected: a mei-yu event and typhoon Saola, the later affected Taiwan in 2012. Annual rainfall is dominantly contributed by these two types of meteorological phenomena in Taiwan. We find that the vapor and the rainwater are nearly in isotopic equilibrium during typhoon while they are deviated significantly during the mei-yu. A significant contribution of the vapor from recycling of surface water is observed in winter assessed using d-excess in the vapor isotopes.