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. The recent development and improvement of commercial laser-basedspectrometers 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 lackcontinuous observations in the Amazon, a region that significantlyinfluences atmospheric and hydrological cycles on local to global scales. Inorder to achieve accurate on-site observations, commercial water isotopeanalysers require regular in situ calibration, which includes the correctionof H2O concentration dependence ([H2O] dependence) of isotopicmeasurements. Past studies have assessed the [H2O] dependence for airwith H2O concentrations of up to 35 000 ppm, a value that is frequentlysurpassed in tropical rainforest settings like the central Amazon where weplan continuous observations. Here we investigated the performance of twocommercial analysers (L1102i and L2130i models, Picarro, Inc., USA) formeasuring δ18O and δ2H in atmospheric moisture atfour different H2O levels from 21 500 to 41 000 ppm. These H2Olevels were created by a custom-built calibration unit designed for regularin situ calibration. Measurements on the newer analyser model (L2130i) hadbetter precision for δ18O and δ2H and demonstratedless influence of H2O concentration on the measurement accuracy at eachconcentration level compared to the older L1102i. Based on our findings, weidentified the most appropriate calibration strategy for[H2O] dependence, adapted to our calibration system. The best strategyrequired conducting a two-point calibration with four different H2Oconcentration levels, carried out at the beginning and end of thecalibration interval. The smallest uncertainties in calibrating[H2O] dependence of isotopic accuracy of the two analysers wereachieved using a linear surface fitting method and a 28 h calibrationinterval, except for the δ18O accuracy of the L1102i analyserfor which the cubic fitting method gave the best results. The uncertainties in[H2O] dependence calibration did not show any significant differenceusing calibration intervals from 28 up to 196 h; this suggested that one[H2O] dependence calibration per week for the L2130i and L1102ianalysers is sufficient. This study shows that the cavity ring-down spectroscopy (CRDS) analysers,appropriately calibrated for [H2O] dependence, allow the detection ofnatural signals of stable water vapour isotopes at very high humiditylevels, which has promising implications for water cycle studies in areaslike the central Amazon rainforest and other tropical regions.

atmospheric measurement techniques

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