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
(H 2 O) stable isotope compositions (e.g. δ18 O and δ2 H) 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 H 2 O concentration dependence ([H 2 O] dependence) of isotopic
measurements. Past studies have assessed the [H 2 O] dependence for air
with H 2 O 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 δ18 O and δ2 H in atmospheric moisture at
four different H 2 O levels from 21 500 to 41 000 ppm. These H 2 O
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 δ18 O and δ2 H and demonstrated
less influence of H 2 O 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
[H 2 O] dependence, adapted to our calibration system. The best strategy
required conducting a two-point calibration with four different H 2 O
concentration levels, carried out at the beginning and end of the
calibration interval. The smallest uncertainties in calibrating
[H 2 O] 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 δ18 O accuracy of the L1102i analyser
for which the cubic fitting method gave the best results. The uncertainties in
[H 2 O] dependence calibration did not show any significant difference
using calibration intervals from 28 up to 196 h; this suggested that one
[H 2 O] 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 [H 2 O] 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.