Development of a calibration system for stable water vapor isotope measurements using Cavity Ring-Down Spectroscope

Abstract

Recent development of laser-based spectroscopy introduces continuous measurement method for water vapor isotopic compositions. The measurement precision and accuracy should precede to obtain high temporal resolution data. Here, we developed a calibration system for water vapor isotopic measurement for applications to field studies. Each standard water vapor, evaporated from two isotopically distinct standard waters, was constantly introduced to the Cavity Ring-Down Spectroscope (CRDS). The theoretical isotopic compositions of standard water vapor, calculated using an equilibrium fractionation accompanied by constant temperature, were used as references to calculate a measurement error. This study was conducted in various environment, such as on-board measurements and in Antarctica, to validate the stability of the system when the humidity was lower than the analyzer recommendations. The cavity humidity was calibrated using local Automatic Weather Station (AWS), and then the calibration equation was determined in each different humidity level using enriched (δ18O: -7.98‰, δD: -56.1‰) and depleted (δ18O: -37.44‰, δD: -294.8‰) standards. The calibration system successfully reproduced normalized errors(Δ) within reported values, less than 0.1±0.21‰ (1±1.4‰) for δ18O (δD), without humidity dependency. As a result, the method is widely applicable for long-term high resolution observations at extremely dry environment like polar regions and high altitude regions.