In-situ measurement of water vapor isotopes for atmospheric and ecological applications

In this paper we describe a system for in-situ measurement of H216O/H218O in air based on tunable diode laser (TDL) absorption spectroscopy. Laboratory tests showed that its 60-min precision (one standard deviation) was 0.21 per mil at a water vapor volume mixing ratio of 2.67 mmol/mol (dewpoint temperature -10.8oC at sea level) and improved to 0.09 at 15.3 mmol/mol (dewpoint temperature 13.4oC). The TDL measurement of the vapor generated by a dewpoint generator differed from the equilibrium prediction by -0.11 +- 0.43 per mil (mean +- one standard deviation). Its measurement of the ambient water vapor differed from the cold-trap/mass spectrometer method by -0.36 +- 1.43 per mil. The larger noise of the latter comparison was caused primarily by the difficulty in extracting vapor from air without altering its isotope content. In a one-week test in Logan, Utah in August 2003, the isotope ratio of water vapor in ambient air was positively correlated with the water vapor mixing ratio and also responded to wetting events (rain and irrigation) in an expected manner.

This system has been in continuous operation in New Haven, Connecticut since December 2003. We suggest that such uninterrupted measurement may open a new window on the hydrologic cycle, particularly processes involving phase changes of water, and can increase the power of the isotope method in ecological applications.