Characterizing water vapour concentration dependence of commercial cavity ring-down spectrometers for continuous on-site atmospheric water vapour isotope measurements in the tropics

Abstract

The recent development and improvement of commercial laser-based spectrometers have expanded in situ continuous observations of water vapour (H2O) stable isotope compositions (e.g. δ18O and δ2H) in a variety of sites worldwide. However, we still lack continuous observations in the Amazon, a region that significantly influences atmospheric and hydrological cycles on local to global scales. In order to achieve accurate on-site observations, commercial water isotope analysers require regular in situ calibration, which includes the correction of H2O concentration dependence ([H2O] dependence) of isotopic measurements. Past studies have assessed the [H2O] dependence for air with H2O concentrations of up to 35 000 ppm, a value that is frequently surpassed in tropical rainforest settings like the central Amazon where we plan continuous observations. Here we investigated the performance of two commercial analysers (L1102i and L2130i models, Picarro, Inc., USA) for measuring δ18O and δ2H in atmospheric moisture at four different H2O levels from 21 500 to 41 000 ppm. These H2O levels were created by a custom-built calibration unit designed for regular in situ calibration. Measurements on the newer analyser model (L2130i) had better precision for δ18O and δ2H and demonstrated less influence of H2O concentration on the measurement accuracy at each concentration level compared to the older L1102i. Based on our findings, we identified the most appropriate calibration strategy for [H2O] dependence, adapted to our calibration system. The best strategy required conducting a two-point calibration with four different H2O concentration levels, carried out at the beginning and end of the calibration interval. The smallest uncertainties in calibrating [H2O] dependence of isotopic accuracy of the two analysers were achieved using a linear surface fitting method and a 28 h calibration interval, except for the δ18O accuracy of the L1102i analyser for which the cubic fitting method gave the best results. The uncertainties in [H2O] dependence calibration did not show any significant difference using calibration intervals from 28 up to 196 h; this suggested that one [H2O] dependence calibration per week for the L2130i and L1102i analysers is sufficient. This study shows that the cavity ring-down spectroscopy (CRDS) analysers, appropriately calibrated for [H2O] dependence, allow the detection of natural signals of stable water vapour isotopes at very high humidity levels, which has promising implications for water cycle studies in areas like the central Amazon rainforest and other tropical regions.