1st CIPM STG-CENV • Stakeholder meeting • 16-18 September 2024 • BIPM • Sèvres (France)

1A: Atmosphere Physics and Chemistry

The NIST Greenhouse Gas Urban Testbed System

by Dr. Anna Karion


At present accuracies of reference spectroscopic parameters, especially those of line intensities, do not satisfy WMO compatibility goals, which for some molecules are at the permille level, nor are they sufficient for the most demanding physical models used in atmospheric remote sensing and radiative transfer. Until recently such accuracy in line intensities has not been consistently demonstrated by laboratory measurements nor by theoretical calculations. In particular, reference intensities of absorption lines in spectral databases such as HITRAN [1] have uncertainties of a few percent with similar scatter between data sets obtained in different laboratories [2]. Recent advances in theoretical calculations [3] and new experimental methods [4-7] have led to the realization of more accurate line intensity measurements. Permille-level uncertainties in line intensity were reported in a CO2 study performed by some of us [8], where there was an average agreement between theory and one experiment of 3 ‰. In the present work [9], we achieved unprecedented agreement of 1 ‰ between theory and three experiments for the CO molecule in the (3-0) band. This result corresponds to a twenty-fold reduction in uncertainty compared to literature data. Here, the underlying theoretical part relies on state-of-the-art quantum calculations of the dipole moment curve and empirical potential energy curve. The experimental part involves measurements that were independently performed in three laboratories with substantially different spectroscopic techniques – relying on complementary linear measurements of absorption, resonant dispersion and transmission, respectively. This multi-pronged theoretical and experimental approach improves the ability to identify and bound otherwise unresolved sources of systematic uncertainty and can be applied to other molecules that are important in atmospheric studies. This work is part of an interinstitutional effort to establish primary spectroscopic methods and reference data for amount of substance measurements. To extend these results, a new task Group on Advanced Spectroscopy within Consultative Committee for Amount of Substance (CCQM) meeting at the International Bureau of Weights and Measures (BIPM) has been initiated. In this fashion we intend to bridge the gas metrology and molecular spectroscopy communities. We recommend that more international, coordinated efforts leveraging the expertise of various theoretical and experimental groups be pursued. These types of coordinated yet interactive comparisons are expected to yield more accurate spectroscopic reference data. References [1] I. E. Gordon et al., J. Quant. Spectrosc. Radiat. Transf. 277, 107949 (2022) [2] V. V. Meshkov et al., J. Quant. Spectrosc. Radiat. Transf. 280, 108090 (2022) [3] E. Zak et al., J. Quant. Spectrosc. Radiat. Transf. 177, 31 (2016) [4] A. J. Fleisher et al., Phys. Rev. Lett. 123, 043001 (2019) [5] A. Cygan et al., Opt. Express 27, 21810 (2019) [6] M. Birk et al., J. Quant. Spectrosc. Radiat. Transf. 272, 107791 (2021) [7] V. Werwein et al., Appl. Optics 56, E99 (2017). [8] O. L. Polyansky et al., Phys. Rev. Lett. 114, 243001 (2015) [9] K. Bielska et al., Phys. Rev. Lett. (2022), accepted for publication