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The Chicago Water Isotope Spectrometer (ChiWIS-lab): A tunable diode laser spectrometer for chamber-based measurements of water vapor isotopic evolution during cirrus formation
摘要： We describe a new tunable diode laser (TDL) absorption instrument, the Chicago Water Isotope Spectrometer, designed for measurements of vapor-phase water isotopologues in conditions characteristic of the upper troposphere [190–235 K temperature and 2–500 parts per million volume (ppmv) water vapor]. The instrument is primarily targeted for measuring the evolving ratio of HDO/H2O during experiments in the “Aerosol Interaction and Dynamics in the Atmosphere” (AIDA) cloud chamber. The spectrometer scans absorption lines of both H2O and HDO near the 2.64 μm wavelength in a single current sweep, increasing the accuracy of isotopic ratio measurements. At AIDA, the instrument is configured with a 256-m path length White cell for in situ measurements, and effective sensitivity can be augmented by enhancing the HDO content of chamber water vapor by an order of magnitude. The instrument has participated to date in the 2012–2013 IsoCloud campaigns studying isotopic partitioning during the formation of cirrus clouds and in the AquaVIT-II instrument intercomparison campaign. Realized precisions for 1-s measurements during these campaigns were 22 ppbv for H2O and 16 ppbv for HDO, equivalent to relative precisions of less than 0.5% for each species at 8 ppmv water vapor. The 1-s precision of the [HDO]/[H2O] ratio measurement ranged from 1.6‰ to 5.6‰ over the range of experimental conditions. H2O measurements showed agreement with calculated saturation vapor pressure to within 1% in conditions of sublimating ice and agreement with other AIDA instruments (the AIDA SP-APicT reference TDL instrument and an MBW 373LX chilled mirror hygrometer) to within 2.5% and 3.8%, respectively, over conditions suitable for all instruments (temperatures from 204 K to 234 K and H2O content equivalent to 15–700 ppmv at 200 hPa).
关键词： upper troposphere,isotopic ratio measurements,water isotopologues,AIDA cloud chamber,tunable diode laser
更新于2025-09-23 15:21:01
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Effects of variation in background mixing ratios of N<sub>2</sub>, O<sub>2</sub>, and Ar on the measurement of <i>δ</i><sup>18</sup>O–H<sub>2</sub>O and <i>δ</i><sup>2</sup>H–H<sub>2</sub>O values by cavity ring-down spectroscopy
摘要： Cavity ring-down spectrometers have generally been designed to operate under conditions in which the background gas has a constant composition. However, there are a number of observational and experimental situations of interest in which the background gas has a variable composition. In this study, we examine the effect of background gas composition on a cavity ring-down spectrometer that measures δ18O–H2O and δ2H–H2O values based on the amplitude of water isotopologue absorption features around 7184 cm?1 (L2120-i, Picarro, Inc.). For background mixtures balanced with N2, the apparent δ18O values deviate from true values by ?0.50 ± 0.001 ‰ O2 %?1 and ?0.57 ± 0.001 ‰ Ar %?1, and apparent δ2H values deviate from true values by 0.26 ± 0.004 ‰ O2 %?1 and 0.42 ± 0.004 ‰ Ar %?1. The artifacts are the result of broadening, narrowing, and shifting of both the target absorption lines and strong neighboring lines. While the background-induced isotopic artifacts can largely be corrected with simple empirical or semi-mechanistic models, neither type of model is capable of completely correcting the isotopic artifacts to within the inherent instrument precision. The development of strategies for dynamically detecting and accommodating background variation in N2, O2, and/or Ar would facilitate the application of cavity ring-down spectrometers to a new class of observations and experiments.
关键词： background gas composition,correction models,Cavity ring-down spectroscopy,water isotopologues,isotopic artifacts
更新于2025-09-23 15:19:57
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Precise measurements of 12CH2D2 by tunable infrared laser direct absorption spectroscopy
摘要： We present precise measurements of doubly-deuterated methane (12CH2D2) in natural methane samples using tunable infrared laser direct absorption spectroscopy (TILDAS). Using a 413 m optical path length astigmatic Herriott cell and two quantum cascade lasers (QCLs) scanning the spectral regions of 1090.46 ± 0.1 cm-1 and 1200.23 ± 0.1 cm-1, the instrument simultaneously measures the five main isotopologues of methane. The ratios 13CH3D/12CH4 and 12CH2D2/12CH4 are measured at 0.01‰ and 0.5‰ (1σ) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of δ13C and δD values but with the abundances of all isotopologues driven to thermal equilibrium at 250 °C. The estimated accuracy of Δ12CH2D2 is 1‰ (1σ) based on the results of the heated methane samples. This new TILDAS instrument provides a simple and rapid technique to explore the sources of methane in the environment.
关键词： methane,spectroscopy,isotopologues
更新于2025-09-12 10:27:22
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Laser absorption spectroscopy of rare and doubly-substituted carbon dioxide isotopologues
摘要： Unambiguous detection of the clumped carbon dioxide isotopologue 13C16O18O with isotope ratio mass spectrometry is difficult due to isobaric interference on m/z = 47. We present an analytical technique based on direct absorption laser spectroscopy for precise, direct and simultaneous detection of all isotopologues involved in the isotope exchange reaction 12C16O2 + 13C16O18O ? 12C16O18O +13C16O2 and of 12C16O17O. The achieved precision of 2×10-5 for the 13C16O18O/13C16O2 and 12C16O18O/12C16O2 isotopologue ratios allows to determine the equilibrium constant K of the isotope exchange reaction with an external reproducibility of better than 2×10-5 (1σ) after 9 reference-sample comparisons. Each comparison requires 7 minutes. The isotopic composition of the pure gas can be simultaneously analyzed with a precision of 0.05‰ (1σ) for δ13C and δ18O, and 0.15‰ (1σ) for δ17O. The instrument deploys two interband cascade lasers (ICL) with center wavelengths of 4.3 μm and 4.4 μm. A custom-built optical cell has been designed for single pass and multi pass optical paths (pathlength ratio 1:100); it allows simultaneous detection of rare and abundant isotopologues. The setup is capable to analyze pure CO2 samples of ～100 μmol.
关键词： clumped isotope analysis,mid-IR lasers,tunable lasers,doubly substituted isotopologues,carbon dioxide,laser spectroscopy,laser absorption
更新于2025-09-12 10:27:22