On-Site Calibration for High Precision Measurements of Water Vapor Isotope Ratios Using Off-Axis Cavity-Enhanced Absorption Spectroscopy

Stable isotope ratio measurements of atmospheric water vapor (delta O-18(v) and delta H-2(v)) are scarce relative to those in precipitation. This limitation is rapidly changing due to advances in absorption spectroscopy technology and the development of automatically calibrated field-deployable instrument systems. These systems allow high throughput, in situ monitoring of the temporal variability in delta O-18(v) and delta H-2(v). This paper presents a robust calibration procedure for reliable, high-precision delta O-18(v) and delta H-2(v) measurements at less than hourly intervals in this study. The method described here was developed and tested using a coupled system consisting of a commercial water vapor isotopic source device and a commercial water vapor isotope analyzer [Los Gatos Research (LGR) model WVIA-24] based on the off-axis integrated cavity output spectroscopy (off-axis ICOS) technique. The isotope analyzer shows a time-dependent response that varies with water vapor mixing ratio, suggesting the need of regular (hourly) calibration achievable by a single reference water source evaluated at a range of mixing ratios. By using a three-point calibration procedure with a range of user-specified water vapor mixing ratios, the authors were able to produce hourly delta O-18(v) and delta H-2(v) measurements with an overall accuracy (+/- 0.2 parts per thousand for delta O-8, +/- 0.5 parts per thousand for delta H-2) and precision (+/- 0.3 parts per thousand for delta O-18, +/- 3.0 parts per thousand for delta H-2) in the laboratory. The calibration procedure reliably produced data that were consistent with those collected by the conventional cryogenic method in an old-growth forest.