Abstract
Abstract. Variations of stable water isotopes in water vapour have 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 δ2H, 0.5‰ for δ18O 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.
Highlights
The atmospheric transport patterns of water vapour significantly influence local climates and Earth’s surface hydrology
When measuring isotopic composition of water samples, the delta values have to be normalised according to the International Atomic Energy Agency (IAEA) VSMOW2-SLAP2 scale as described in IAEA (2009), which corresponds to a two point calibration with a fixed zero point (VSMOW) and a second reference point (SLAP, Standard Light Antarctic Precipitation)
As L1115-i, WVIA and WVIA-EP were connected to the WVISS in parallel, the large change in the δ2H signal calibration factors of the L1115-i must come from the instrument itself. This might indicate that regular twopoint calibration is necessary for L1115-i and that one standard might be sufficient for WVIA calibration, but further investigation with more calibration runs would be needed to confirm this preliminary finding
Summary
The atmospheric transport patterns of water vapour significantly influence local climates and Earth’s surface hydrology. As naturally available tracers of phase transitions of water, stable isotopes provide useful information on the atmospheric water cycle, in particular on conditions during phase changes such as evaporation from the sea surface (Pfahl and Wernli, 2008; Sodemann et al, 2008; Uemura et al, 2008), plant transpiration (Farquhar et al, 2007), cloud formation (Federer et al, 1982; Ciais and Jouzel, 1994) and post-condensation exchange with below cloud vapour (Field et al, 2010) To investigate these processes and their impact on stable water isotopes in atmospheric waters at the temporal scale of significant weather events, high frequency measurements of stable water isotopes are essential. At the end a short case study is presented of comparative ambient air measurements in field conditions with the two laser systems operated in a way that was found optimal during the laboratory tests
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