High-accuracy 12C16O2 line intensities in the 2 μm wavelength region measured by frequency-stabilized cavity ring-down spectroscopy

Ames-2021 CO2 Dipole Moment Surface and IR Line Lists: Toward 0.1% Uncertainty for CO2 IR Intensities

A highly accurate CO₂ ab initio dipole moment surface (DMS), Ames-2021, is reported along with ¹²C¹⁶O₂ infrared (IR) intensity comparisons approaching a 1-4‰ level of agreement and uncertainty. The Ames-2021 DMS was accurately fitted from CCSD(T) finite-field dipoles computed with the aug-cc-pVXZ (X = T, Q, 5) basis for C atom and the d-aug-cc-pVXZ (X = T, Q, 5) basis for O atoms, and extrapolated to the one particle basis set limit. Fitting σ_rms is 3.8 × 10^-7 au for 4443 geometries below 15 000 cm^-1. The corresponding IR intensity, S_Ames-2021, are computed using the Ames-2 potential energy surface (PES), which is the best PES available for CO₂. Compared to high accuracy IR studies for 2001i-00001 and 3001i-00001 bands, S_Ames-2021 matches NIST experiment-based intensities [S_NIST-HIT16 or S_HIT20] to -1.0 ± 1.3‰, or matches DLR experiment-based intensities [S_{DLR-HIT16/UCL/Ames}] to 1.9 ± 3.7‰. This indicates the systematic deviations and uncertainties have been significantly reduced in S_Ames-2021. The S_UCL2015 (or S_HITRAN2016) have larger deviations (vs S_DLR) and uncertainties (vs S_DLR, S_NIST) which are attributed to the less accurate Ames-1 PES adopted in UCL-296 line list calculation. The S_Ames-2021 intensity of ¹²C¹⁶O₂ and ¹³C¹⁶O₂ is utilized to derive new absolute ¹³C/¹²C ratios for Vienna PeeDee Belemnite (VPDB) with uncertainty reduced by ¹/₃ or ²/₃. Further evaluation of S_Ames-2021 intensities are carried out on those CO₂ bands discussed in the HITRAN2020 update paper. Consistent improvements and better accuracies are found in band-by-band analysis, except for those bands strongly affected by Coriolis couplings, or very weak bands measured with relatively larger experimental uncertainties. The Ames-2021 296 K IR line lists are generated for 13 CO₂ isotopologues, with 18 000 cm^-1 and S_296 K > 1 × 10^-31 cm/molecule cutoff and then combined with CDSD line positions (except ¹⁴C¹⁶O₂). The Ames-2021 DMS and 296 K IR line lists represent a major improvement over previous CO₂ theoretical IR intensity studies, including Ames-2016, UCL-296, and recent UCL DMS 2021 update. A real 1 permille level of agreement and uncertainty will definitely require both more accurate PES and more accurate DMS.

Absolute 13C/12C Isotope Amount Ratio for Vienna Pee Dee Belemnite from Infrared Absorption Spectroscopy

Measurements of isotope ratios are predominantly made with reference to standard specimens that have been characterized in the past. In the 1950s, the carbon isotope ratio was referenced to a belemnite sample collected by Heinz Lowenstam and Harold Urey¹ in South Carolina's Pee Dee region. Due to the exhaustion of the sample since then, reference materials that are traceable to the original artefact are used to define the Vienna Pee Dee Belemnite (VPDB) scale for stable carbon isotope analysis². However, these reference materials have also become exhausted or proven to exhibit unstable composition over time³, mirroring issues with the international prototype of the kilogram that led to a revised International System of Units⁴. A campaign to elucidate the stable carbon isotope ratio of VPDB is underway⁵, but independent measurement techniques are required to support it. Here we report an accurate value for the stable carbon isotope ratio inferred from infrared absorption spectroscopy, fulfilling the promise of this fundamentally accurate approach⁶. Our results agree with a value recently derived from mass spectrometry⁵, and therefore advance the prospects of SI-traceable isotope analysis. Further, our calibration-free method could improve mass balance calculations and enhance isotopic tracer studies in CO₂ source apportionment.

A well-isolated vibrational state of CO2verified by near-infrared saturated spectroscopy with kHz accuracy

Quantitative determination of atmospheric CO₂concentration by remote sensing relies on accurate line parameters.

Cavity ring-down spectroscopy of CO2 near λ = 2.06 μm: Accurate transition intensities for the Orbiting Carbon Observatory-2 (OCO-2) "strong band"

The λ = 2.06 μm absorption band of CO₂ is widely used for the remote sensing of atmospheric carbon dioxide, making it relevant to many important top-down measurements of carbon flux. The forward models used in the retrieval algorithms employed in these measurements require increasingly accurate line intensity and line shape data from which absorption cross-sections can be computed. To overcome accuracy limitations of existing line lists, we used frequency-stabilized cavity ring-down spectroscopy to measure 39 transitions in the ¹²C¹⁶O₂ absorption band. The line intensities were measured with an estimated relative combined standard uncertainty of u _r = 0.08 %. We predicted the J-dependence of the measured intensities using two theoretical models: a one-dimensional spectroscopic model with Herman-Wallis rotation-vibration corrections, and a line-by-line ab initio dipole moment surface model [Zak et al. JQSRT 2016;177:31-42]. For the second approach, we fit only a single factor to rescale the theoretical integrated band intensity to be consistent with the measured intensities. We find that the latter approach yields an equally adequate representation of the fitted J-dependent intensity data and provides the most physically general representation of the results. Our recommended value for the integrated band intensity equal to 7.183 × 10^-21 cm molecule^-1 ± 6 × 10^-24 cm molecule^-1 is based on the rescaled ab initio model and corresponds to a fitted scale factor of 1.0069 ± 0.0002. Comparisons of literature intensity values to our results reveal systematic deviations ranging from -1.16 % to +0.33 %.

Twenty-Five-Fold Reduction in Measurement Uncertainty for a Molecular Line Intensity

To accurately attribute sources and sinks of molecules like CO_{2}, remote sensing missions require line intensities (S) with relative uncertainties u_{r}(S)<0.1%. However, discrepancies in S of ≈1% are common when comparing different experiments, thus limiting their potential impact. Here we report a cavity ring-down spectroscopy multi-instrument comparison which revealed that the hardware used to digitize analog ring-down signals caused variability in spectral integrals which yield S. Our refined approach improved measurement accuracy 25-fold, resulting in u_{r}(S)=0.06%.

Optical Measurement of Radiocarbon below Unity Fraction Modern by Linear Absorption Spectroscopy

High-precision measurements of radiocarbon (¹⁴C) near or below a fraction modern ¹⁴C of 1 (F¹⁴C ≤ 1) are challenging and costly. An accurate, ultrasensitive linear absorption approach to detecting ¹⁴C would provide a simple and robust benchtop alternative to off-site accelerator mass spectrometry facilities. Here we report the quantitative measurement of ¹⁴C in gas-phase samples of CO₂ with F¹⁴C < 1 using cavity ring-down spectroscopy in the linear absorption regime. Repeated analysis of CO₂ derived from the combustion of either biogenic or petrogenic sources revealed a robust ability to differentiate samples with F¹⁴C < 1. With a combined uncertainty of ¹⁴C/¹²C = 130 fmol/mol (F¹⁴C = 0.11), initial performance of the calibration-free instrument is sufficient to investigate a variety of applications in radiocarbon measurement science including the study of biofuels and bioplastics, illicitly traded specimens, bomb dating, and atmospheric transport.