Characteristics of Seasonal Variation of Near-Surface Water Vapor D/H Isotope Ratio Revealed by Continuous in situ Measurement in Sapporo, Japan

High-temporal measurements of the stable isotope ratio (delta D) of near-surface atmospheric water vapor by an Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS)-type water vapor isotope analyzer (WVIA) were performed in Sapporo from April 2009 to June 2011. The data obtained from the WVIA was corrected by comparing the dD values with those obtained from the cold trap method and subsequent cavity ring-down spectroscopy isotopic water analysis. The dD values showed a marked seasonal cycle but showed a different seasonal cycle from that of the surface air temperature. The dD values simulated by the isotopic-incorporated Global Spectral Model showed almost the same seasonal cycle as that of observed dD values, although simulated values showed about 10 parts per thousand difference from observed values in 10-month average. It is found that the monthly mean SSTmon of the nearest sea was a better predictor for delta D-mon than the monthly mean air temperature. Based on the data measured by the WVIA deployed on the Oshoro-Maru of Hokkaido University, from the Sea of Okhotsk to the Pacific Ocean, we confirmed that dD of water vapor generally increased with increasing SST and the highest values of dD were almost the same as those of vapor in equilibrium with ocean liquid.