Measuring variations of δ<sup>18</sup>O and δ<sup>2</sup>H in atmospheric water vapour using two commercial laser-based spectrometers: an instrument characterisation study

Abstract

Abstract. Variations of stable water isotopes in water vapourhave become measurable at a measurement frequency of about 1 Hz in recent years using novel laser spectroscopic techniques. This enables us to perform continuous measurements for process-based investigations of the atmospheric water cycle at the time scales relevant for synoptic and mesoscale meteorology. An important prerequisite for the interpretation of data from automated field measurements lasting for several weeks or months is a detailed knowledge about instrument properties and the sources of measurement uncertainty. We present here a comprehensive characterisation and comparison study of two commercial laser spectroscopic systems based on cavity ring-down spectroscopy (Picarro) and off-axis integrated cavity output spectroscopy (Los Gatos Research). The uncertainty components of the measurements were first assessed in laboratory experiments, focussing on the effects of (i) water vapour mixing ratio, (ii) measurement stability, (iii) uncertainties due to calibration and (iv) response times of the isotope measurements due to adsorption-desorption processes on the tubing and measurement cavity walls. Based on the experience from our laboratory experiments, we set up a one-week field campaign for comparing measurements of the ambient isotope signals from the two laser spectroscopic systems. The optimal calibration strategy determined for both instruments was applied as well as the correction functions for water vapour mixing ratio effects. The root mean square difference between the isotope signals from the two instruments during the field deployment was 2.3 ‰ for δ 2 H, 0.5 ‰ for δ 18 O and 3.1 ‰ for deuterium excess. These uncertainty estimates from field measurements compare well to those found in the laboratory experiments. The present quality of measurements from laser spectroscopic instruments combined with a calibration system opens new possibilities for investigating the atmospheric water cycle and the land-atmosphere moisture fluxes.

Water vapor δ2H, δ18O and δ17O measurements using an off‐axis integrated cavity output spectrometer – sensitivity to water vapor concentration, delta value and averaging‐time

“…These results are comparable, or represent an improvement to, results from a previous study using WVIA [26] and TIWA-45EP [9] instruments.…”

“…However, in practice, the precision has been found to depend on delta values and the performance varied using different techniques and instruments [9,26] . Therefore, based on the above reasons, in order to completely understand an instrument performance, the tests under different water vapor concentrations and delta values are required.…”

“…In order to utilize δ [23,24] , delta range [9,26] and averaging-time [10,27] ) on instrument performance will improve our capacity to interpret observations of δ 2 H, δ 18 O and δ 17 O from complex environments.…”

“…However, before One limitation of our calibration method is the difficulty of distinguishing the error source between the testing instrument and the calibration unit. This is a standard issue for most calibration studies relying on a range of calibration units [24][25][26][27] . The issue could be reduced by comparing …”

Empirical validation and proof of added value of MUSICA's tropospheric δD remote sensing products

“…It consists of two syringe pumps and allows injecting two different standards into the vaporizer where a constant dry air flow sustains immediate evaporation of the liquid in the air stream (for more details see also Aemisegger et al, 2012). The calibration is made every 8 h. It uses two different δD standards (at −142.2 and −245.3 ‰) that are analysed by the instrument at three different humidity levels covering the typical humidity range at Izaña and at Teide (during summer).…”