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

Abstract

table isotope ratio measurements of atmospheric water vapor (δ18Ov and δ2Hv) 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 δ18Ov and δ2Hv. This paper presents a robust calibration procedure for reliable, high-precision δ18Ov and δ2Hv 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 δ18Ov and δ2Hv measurements with an overall accuracy (±0.2? for δ18O, ±0.5? for δ2H) and precision (±0.3? for δ18O, ±3.0? for δ2H) 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.