Continuous measurement of water vapor D/H and 18O/16O isotope ratios in the atmosphere

The D/H and 18O/16O ratios in atmospheric water vapor provide rich information on the hydrological cycle and gaseous exchange processes between the terrestrial vegetation and the atmosphere. In this paper, we have demonstrated the feasibility to simultaneously measure both D/H and 18O/16O in atmospheric water vapor using a tunable diode laser absorption spectrometer. Our laboratory tests showed that the 1-h precision (one standard deviation) was 1.1‰ for D/H and 0.07‰ for 18O/16O at the dewpoint temperature of 15 °C. The difference between the laser measurement and cold trap/mass spectrometer analysis was 1.2 ± 6.0‰ (hourly mean ± one standard deviation) for D/H and 1.1 ± 1.3‰ for 18O/16O. Our atmospheric measurement captured the rapidly changing isotopic signals in both D/H and 18O/16O. The measured isotope ratios were highly correlated with the water vapor mixing ratio as expected and followed very closely the Global Meteoric Water Line except during two transitional periods when the deuterium excess of atmospheric vapor deviated from the standard value. In addition, we have refined a method to provide independent, timely performance test of the in situ system.