Coordinated interlaboratory comparisons of CO line intensities: towards primary spectroscopic measurements of the amount of gas and spectroscopic reference data for high accuracy remote sensing applications

Abstract

The accuracy of atmospheric retrievals depends on instrumental accuracy and the accuracy of the spectroscopic reference data underlying the models used in atmospheric remote sensing. It depends on full sets of line-shape parameters and especially on the line intensities. Due to continuously growing demands regarding the accuracy in atmospheric studies, the WMO compatibility goals for some molecules can be set even at the promille level. For the first time, such accuracy has been shown in recent work [1], where measurements performed by some of us agreed with ab initio calculations of Polyansky and coworkers at the subpromille level. Such a low uncertainty in line intensity determination had not been consistently demonstrated before, either by experiment or theory. In particular, reference line intensities archived in spectral databases such as HITRAN [2] typically have standard uncertainties at the few percent level. A scatter between previous data sets obtained in different laboratories was comparable to these uncertainties [3]. Following recommendation 1A.5 of the 2022 workshop "Metrology for Climate Action", the CCQM Task Group on Advanced Spectroscopy performed the first pilot study on the comparison of absolute line intensities for the 12C16O isotopologue of carbon monoxide from the (3-0) band located in the spectral range near 1.6 µm [4]. These line intensities can be utilized for primary measurements of the amount of was based on linear absorption spectroscopy. Coordinated measurements were performed by participating laboratories from DLR, NCU, NIM, NIST and PTB. Three experimental techniques, which are based on the measurement of different physical quantities, were involved in the study: Fourier transform spectroscopy (FTS), cavity ring-down spectroscopy (CRDS), and cavity mode-dispersion spectroscopy (CMDS). All measurements were done using a sample of nominally pure CO at near natural isotopic abundance and temperature near 296 K.A set of lines was chosen so that the same transitions were probed by all three experimental techniques, spanning the intensity ratio of 240:1. The weighted mean experimental results have subpromille relative standard uncertainties and agree with ab initio calculations by Polyansky and coworkes at 0.3‰ level, whereas the experimental band intensity agrees with theoretical predictions at 0.5‰ level. We show that such coordinated experiments may reduce statistical and systematic uncertainty. Due to the fact that several substantially different measurement techniques were involved, the lack or presence of otherwise unaccounted sources of systematic uncertainty can be verified. References 1. K. Bielska et al., Phys. Rev. Lett. 129, 043002 (2022). 2. I. E. Gordon et al., J. Quant. Spectrosc. Radiat. Transf. 277, 107949 (2022). 3. V. V. Meshkov et al., J. Quant. Spectrosc. Radiat. Transf. 280, 108090 (2022). 4. J.T. Hodges et al., International Comparison CCQM-P229: Pilot Study to Measure Absolute Line Intensities of Selected 12C16O Transitions, BIPM, 2024 (in review).