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High-accuracy Laboratory Spectroscopic Measurements of Atmospheric Gases for Monitoring Greenhouse Gases


Climate science increasingly relies on satellite- and ground-based spectrometers for the top-down inventories of atmospheric greenhouse gases, most notably carbon dioxide. This inventory is critical to improving our understanding of the variations in carbon dioxide concentrations, which arise from the various sources, sinks, and atmospheric circulation patterns. These field measurements are arguably the most demanding ever attempted using absorption spectroscopy. To ensure the integrity and long-term continuity of these measurements, spectrometer operation relies on reference data of the highest metrological quality and traceability to the International System of Units (SI). 

NIST is providing the spectroscopic parameters required for these spectrometers to operate at the targeted precision and accuracy. 


Schematic figure showing sample introduction into an instrument and resulting response on computer screen

Schematic figure illustrating the technique and observables involved in determining the intensity of an absorption transition.  A sample of known mole fraction of a light-absorbing analyte such as CO2 is introduced into an optical cavity formed by two highly reflective mirrors, in which the cavity is used to achieve effective pathlengths exceeding tens of kilometers. A tunable laser probes optical frequencies about the transition to yield an absorption spectrum, yielding an integrated area that corresponds to the line intensity. 

Credit: Figure adapted from Rich Press

The remote sensing of atmospheric gases from satellite- or ground-based spectrometers needs predictive models that describe the spectrally resolved absorption of photons throughout the atmospheric column. These models must accurately account for the dependence of photon absorption on wavelength, pressure, temperature, and chemical composition. Key to these models are line-by-line spectroscopic reference data describing transition frequencies, intensities, and various line shape parameters required to simulate absorption spectra.   

We focus on providing high-accuracy laboratory measurements of spectroscopic reference data for greenhouse gases (GHGs) such as CO2, CH4, H2O, and N2O, to enable quantitative retrievals in remote sensing of these and other species.  Our reported uncertainties also serve as benchmarks for quantum-based calculations of these quantities, thereby extending the impact of this work to include a broad range of species and conditions. Notably, the most accurate GHG measurements also require simultaneous and collocated observations of O2 because its concentration in the atmosphere is well known and does not change with time. 

An important application of our reference data includes measurements of CO2 concentrations by orbiting spacecraft, [e.g., NASA's Orbiting Carbon Observatory Missions, (OCO-2 and OCO-3)] in which CO2 mixing ratios are determined with an uncertainty of 0.3 %, using the relatively weak O2 spectrum at 760 nm (A-band) as a reference. These measurements demand that the line parameters be measured with unprecedented precision and with accuracy established by traceability to the SI. Reference data for the O2 A-band also support meteorological observations and weather modeling and have the potential to determine global surface pressure in remote locations where current measurements are inadequate or non-existent. Similarly, NIST provides reference data for the 1.27 mm band of O2 used by the Total Carbon Column Observatory Network (TCCON) an international network of ground-based high-resolution spectrometers that measure atmospheric CO2 levels. These measurements are also critical to the following generation of satellite-borne instruments, which will rely on laser technology to measure diurnal and seasonal variations in atmospheric greenhouse. 

Major Accomplishments

We have substantially reduced (tenfold or more in many cases) measurement uncertainties for line shape parameters in CO2 and O2 bands. Line intensities in the weak (1.6 mm) and strong (2.06 mm) CO2 bands as well as the A-band (760 nm) and singlet-delta band (1.27 mm) of O2 were measured with uncertainties of from 0.1% to 0.3%, and transition frequencies for these two CO2 bands were measured with uncertainties at kHz levels. 

Associated Publications

  1. Adkins, E.M, Karman, T., Campargue, A., Mondelain, D. and Hodges, J.T., “Parameterized Model to Approximate Theoretical Collision-Induced Absorption Band Shapes for O2-O2 and N2-N2,” Journal of Quantitative Spectroscopy and Radiative Transfer, 310, 108732, (2023).  

  2. Reed, Z.D, Tran, H., Ngo, H.N., Hartmann, J.-M. and Hodges, J.T., “Effect of non-Markovian Collisions on Measured Integrated Line-Shapes of CO,” Physical Review Letters, 130, 143001 (2023). 

  3. Bielska, K., Kyuberis, A.A., Reed, Z.D.,  Li, G., Cygan, A.,  Ciuryło, R.,  Adkins, E.M.,  Lodi, L., Zobov, N.F., Ebert V., Lisak, D.,  Hodges, J.T.,  Tennyson, J. and Polyansky, O.L., “Sub-promille Measurements and Calculations of CO (3-0) Overtone Line Intensities,” Physical Review Letters, 129, 043002 (2022). 

  4. Long, D.A., Adkins, E.M., Mendonca J., Roche, S. and Hodges, J.T., “The Effects of Advanced Spectral Line Shapes on Atmospheric Carbon Dioxide Retrievals,” Journal of Quantitative Spectroscopy and Radiative Transfer, 291, 108324 (2022). 

  5. Gordon, I.E., et al., “The HITRAN2020 Molecular Spectroscopic Database,” Journal of Quantitative Spectroscopy and Radiative Transfer, 277, 107949, (2022). 

  6. Hashemi, R.,  Gordon, I.E.,  Adkins, E.M.,  Hodges, J.T.,  Long, D.A., Birk, M., Boone, C.D.,  Fleisher, A.J., Predoi-Cross, A. and Rothman, L.S., “Improvement of the Spectroscopic Parameters of the Air- and Self-Broadened N2O and CO Lines for the HITRAN2020 Database Applications,” Journal of Quantitative Spectroscopy and Radiative Transfer. 271, 107735 (2021). 

  7. Fleurbaey, H., Reed, Z.D., Adkins, E.M., Long, D.A. and Hodges, J.T., “High Accuracy Spectroscopic Parameters of the 1.27 mm band of O2 measured with Comb-Referenced, Cavity Ring-Down Spectroscopy,” Journal of Quantitative Spectroscopy and Radiative Transfer, 270, 107684 (2021). 

  8. Adkins, E.M., Long, D.A. and Hodges, J.T., “Air-Broadening in Near-infrared Carbon Dioxide Line Shapes: Quantifying Contributions from O2, N2 and Ar,” Journal of Quantitative Spectroscopy and Radiative Transfer, 270, 107669, (2021). 

  9. Reed, Z.D., Drouin, B.J., Long, D.A. and Hodges, J.T., “Molecular Transition Frequencies of CO2 Near 1.6 µm with kHz-level Uncertainties,” Journal of Quantitative Spectroscopy and Radiative Transfer, 271, 107681, (2021). 

  10. Tran, D.D., Delahaye, T., Armante, R., Hartmann, J.-M., Mondelain, D., Fleurbaey, H., Hodges, J.T. and Tran, H., “Validation of Spectroscopic Data in the 1.27 mm Spectral Region by Comparisons with Ground-Based Atmospheric Measurements,” Journal of Quantitative Spectroscopy and Radiative Transfer. 261, 107495, (2021). 

  11. Fleisher, A.J., Yi, H., Srivastava, A., Polyansky, O.L., Zobov, N.F. and Hodges, J.T., “Absolute 13C/12C Isotope Amount Ratio for Vienna Pee Dee Belemnite from Infrared Absorption Spectroscopy”, Nature Physics Letters, April 26, (2021). 

  12. Reed, Z.D., Fleurbaey, H., Long, D.A. and Hodges, J.T., “Molecular transition Frequency Measurements at the 10-12 Relative Uncertainty Level,” Optica 7, 1209-1219, (2020). 

  13. Bailey, D. M., Zhao, G., and Fleisher, A. J., "Precision Spectroscopy of Nitrous Oxide Isotopocules with a Cross-Dispersed Spectrometer and a Mid-Infrared Frequency Comb," Analytical Chemistry, 92, 13759-13766 (2020).  

  14. Long, D. A., Reed, Z. D., Fleisher, A. J., Mendonca, J., Roche, S., and Hodges, J. T., "High-Accuracy Near-Infrared Carbon Dioxide Intensity Measurements to Support Remote Sensing," Geophysical Research Letters, 47, (2020).  

  15. Mendonca, J., Strong, K., Wunch, D., Toon, G. C., Long, D. A., Hodges, J. T., Sironneau, V. T., and Franklin, J. E., "Using a speed-dependent Voigt line shape to retrieve O-2 from Total Carbon Column Observing Network solar spectra to improve measurements of XCO2," Atmospheric Measurement Techniques, 12, 35-50 (2019).  

  16. Ghysels, M., Liu, Q. N., Fleisher, A. J., and Hodges, J. T., "A variable-temperature cavity ring-down spectrometer with application to line shape analysis of CO2 spectra in the 1600 nm region," Applied Physics B-Lasers and Optics, 123 , (2017).  

  17. Lin, H., Reed, Z. D., Sironneau, V. T., and Hodges, J. T., "Cavity ring-down spectrometer for high-fidelity molecular absorption measurements," Journal of Quantitative Spectroscopy & Radiative Transfer, 161, 11-20 (2015).  

  18. Long, D. A., Wojtewicz, S., Miller, C. E., and Hodges, J. T., "Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)<-(00001) near-infrared carbon dioxide band," Journal of Quantitative Spectroscopy & Radiative Transfer, 161, 35-40 (2015).  

  19. Wagner, G., Maxwell, S., Douglass, K., Long, D. A., Hodges, J. T., Fleisher, A. J., and Plusquellic, D. F., Low Power Integrated Path Differential Absorption Lidar Detection of CO2, CH4 and H2O over a 5.5 km Path using a Waveform Driven EO Sideband Spectrometer 2015.  

  20. Reed, Z. D., Sperling, B., van Zee, R. D., Whetstone, J. R., Gillis, K. A., and Hodges, J. T., "Photoacoustic spectrometer for accurate, continuous measurements of atmospheric carbon dioxide concentration," Applied Physics B-Lasers and Optics, 117, 645-657 (2014). 

Created March 26, 2009, Updated November 20, 2023