In Situ Measurement of the Water Vapor 18O/16O Isotope Ratio for Atmospheric and Ecological Applications

Abstract

In this paper a system for in situ measurement of H216O/H218O in air based on tunable diode laser (TDL) absorption spectroscopy is described. Laboratory tests showed that its 60-min precision (one standard deviation) was 0.21‰ at a water vapor volume mixing ratio of 2.67 mmol mol−1 (dewpoint temperature −10.8°C at sea level) and improved to 0.09 at 15.3 mmol mol−1 (dewpoint temperature 13.4°C). The TDL measurement of the vapor generated by a dewpoint generator differed from the equilibrium prediction by −0.11 ± 0.43‰ (mean ± one standard deviation). Its measurement of the ambient water vapor differed from the cold-trap/mass spectrometer method by −0.36 ± 1.43‰. 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 1-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. It is suggested 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.

Mixing ratio and carbon isotopic composition investigation of atmospheric CO 2 in Beijing, China

“…Here, the single point mixing ratio and delta calibration method (Wen et al, 2013(Wen et al, , 2008Lee et al, 2005) was used to calibrate Std1 with Std2 and vice versa.…”

Measuring variations of δ<sup>18</sup>O and δ<sup>2</sup>H in atmospheric water vapour using two commercial laser-based spectrometers: an instrument characterisation study

“…Lis et al, 2008), are not optimal for calibrating vapour instruments as the produced calibration vapour quantity is very limited, making extended calibration runs of more than 5 min impracticable. Fractionation effects have to be accounted for if partial evaporation methods are used (Lee et al, 2005;Wen et al, 2008;Wang et al, 2009). For example, with a dew point generator as a calibration system, dry air is bubbled through a water reservoir at a controlled temperature and the liquid water is continuously enriched in heavy isotopes, following a Rayleigh distillation process.…”

Water vapor δ<sup>2</sup>H and δ<sup>18</sup>O measurements using off-axis integrated cavity output spectroscopy

“…Therefore directly adding liquid water to a dry air stream with complete evaporation (to avoid isotopic fractionation) seems to be a more promising approach to generate water vapor standards. One potential implementation would be to use a dew point generator where dry air is bubbled through water held at constant temperature (Wen et al, 2008;Lee et al, 2005;Wang et al, 2009). The water vapor mixing ratio of the saturated air can be determined if water temperature and air pressure are precisely measured.…”

“…In addition to dedicated research instruments (Griffith et al, 2006;Kerstel et al, 2006;Iannone et al, 2009a;Sayres et al, 2009) several commercial laser based instruments are now available (Lee et al, 2005;Lis et al, 2008;Gupta et al, 2009). In this study, we use the commercially available water vapor isotope analyzer from Los Gatos Research, Inc., which is based on off-axis integrated cavity output spectroscopy (Baer et al, 2002).…”