¹⁷O‐excess and d‐excess of atmospheric water vapor measured by cavity ring‐down spectrometry: Evidence of a matrix effect and implication for the calibration procedure

Abstract

Rationale: Producing robust high-frequency time series of raw atmospheric water vapor isotope data using laser spectrometry requires accurate calibration. In particular, the chemical composition of the analyzed sample gas can cause isotope bias. This study assesses the matrix effect on calibrated δ 17 O, δ 18 O, δ 2 H, 17 O-excess, and d-excess values of atmospheric water vapor. Methods: A Picarro L2140-i cavity ring-down spectrometer with an autosampler and a vaporizer is used to analyze δ 17 O, δ 18 O, δ 2 H, 17 O-excess, and d-excess of two water standards. Isotope data obtained using synthetic air and dry ambient air as carrier gas at water mixing ratios ranging from 2000 to 30 000 ppmv are compared. Based on the results, atmospheric water vapor measurements are calibrated. The expected precision is estimated by Monte Carlo simulation. Results: The dry air source strongly impacts raw isotope values of the two water standards but has no effect on the mixing ratio dependency functions. When synthetic air is used, δ 17 O, δ 18 O, and 17 O-excess of calibrated atmospheric water vapor are overestimated by 0.6‰, 0.7‰, and 217 per meg, respectively, whereas δ 2 H and d-excess are underestimated by 1.5‰ and 7.3‰. Optimum precisions for the calibrated δ 17 O, δ 18 O, δ 2 H, 17 O-excess, and d-excess values and 12 min integration time are 0.02‰, 0.03‰, 0.4‰, 14 per meg, and 0.4‰, respectively. Conclusions: Regarding the obtained results, recommendations for the calibration of atmospheric water vapor isotope measurements are presented. The necessity to use dry ambient air as dry air source when running the standards for calibration is pointed out as a prerequisite for accurate atmospheric water vapor 17 O-excess and d-excess measurements. 1 | INTRODUCTION Stable isotope ratios of atmospheric water vapor, commonly expressed by δ 2 H, δ 18 O, and d-excess (=δ 2 H-8δ 18 O), provide key information on processes in Earth's hydrologic cycle, for example, cloud formation, precipitation, evaporation, and plant transpiration, and can serve as tracers for moisture sources and atmospheric transport patterns (e.g., 1-8 ). Recent analytical developments enabled the measurement of δ 17 O of atmospheric water vapor in addition to δ 18 O, allowing the determination of 17 O-excess (=δ 017 O-0.528δ 018 O with δ 0 = 1000 ln(δ/1000 + 1)). Compared to d-excess, the 17 O-excess parameter is less sensitive to temperature and equilibrium isotope fractionation effects that accompany phase transitions and isotope exchange between different water reservoirs. 9 The 17 O-excess is a powerful indicator of water evaporation 10-12 and can serve to identify mixing between evaporated and unevaporated waters, occurring, for example, due to periodic flooding of lakes in evaporative environments, during groundwater formation, or in plant

A passive method for sampling water in the soil–plant–atmosphere continuum for stable hydrogen and oxygen isotope analyses

“…earlier studies 24,29 ). In addition, a rigorous calibration protocol, including multiple corrections for the mixing ratio, the matrix effect and the drift effect, must be implemented to normalize the raw isotope data 24 …”

“…The recovery of water from the SPAC requires logistical arrangements in the field and implementation of different time-and cost-consuming techniques in both the laboratory and the field. 6,7,[19][20][21][22][23][24] For instance, four techniques have been used for the discrete sampling of atmospheric WV. 25 These techniques are (1) the standard cryogenic trap (e.g.…”

“…In addition, a rigorous calibration protocol, including multiple corrections for the mixing ratio, the matrix effect and the drift effect, must be implemented to normalize the raw isotope data. 24 In the last two decades, soil water has been extracted by destructive sampling techniques which are cryogenic vacuum extraction, centrifugation, mechanical squeezing, microwave extraction and direct vapor equilibration. 25 Cryogenic vacuum extraction has been regarded as a routine extraction technique.…”

“…Picarro CRDS or Los Gatos ICOS) presuppose special arrangements in the field such as a stable power supply, LN supply and drying units (e.g. earlier studies 24,29 ). In addition, a rigorous calibration protocol, including multiple corrections for the mixing ratio, the matrix effect and the drift effect, must be implemented to normalize the raw isotope data 24 …”

Examination of the parameters controlling the triple oxygen isotope composition of grass leaf water and phytoliths at a Mediterranean site: a model–data approach

“…The mean of four measurement runs of liquid water standards was used to normalize the atmospheric water vapor isotope data to VSMOW-SLAP scale. The calibration protocol is described in detail by Voigt et al (2022). In brief, three liquid water standards that covered the expected isotope range of atmospheric water vapor at the study site were analyzed at a water mixing ratio of 11000 ppmv using a Picarro autosampler system (A0325, Picarro Inc., California, US) coupled to a high-precision vaporizer (A0211, Picarro Inc., California, US).…”