PriSpecTemp report: literature study on experimental line intensity of CO, CO2 and O2

PriSpecTemp report: literature study on experimental line intensity of CO, CO2 and O2 Lüttschwager Alexandra1; Berezkin Kirill1; Li, Gang1; Castrillo Antonio2; Lisak Daniel3; Vainio Markku4; Uotila Touko4; Seppä Jeremias5 1 PTB, Braunschweig, Germany. https://orcid.org/0000-0002-3934-6805 https://orcid.org/0009-0000-8873-521X https://orcid.org/0000-0002-5605-7896 2 University of Campania “Luigi Vanvitelli”, Caserta, Italy. https://orcid.org/0000-0002-1253-9172 3 Nicolaus Copernicus University, Torun, Poland. https://orcid.org/0000-0001-5545-7989 4 University of Helsinki, Helsinki, Finland. https://orcid.org/0000-0002-9962-3827 KPH-4522-2024 3 VTT, Espoo, Finland. https://orcid.org/0000-0002-6781-8443 DOI: 10.7795/120.20250221 Veröffentlichungsjahr: 2025 Acknowledgement: The project (22IEM03 PriSpecTemp) has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. Angaben zum Urheberrecht: open access Zitierform: 22IEM03 PriSpecTemp, Activity 1.1.1: Intensities in CO, CO2 and O2 spectra, stand 2024”. A. Lüttschwager, K. Berezkin, G. Li A. Castrillo D. Lisak M. Vainio, T. Uotila, J. Seppä, DOI: 10.7795/120.20250221 22IEM03 PriSpecTemp Report A1.1.1 Confidentiality: public Licence: CC-BY-4.0 1 of 24 22IEM03 PriSpecTemp A1.1.1. Literature study on experimental line intensity of CO, CO2 and O2 Author names of the participants for the activity: A.V. Lüttschwager, K. Berezkin, G. Li (PTB) A. Castrillo (SUN) D. Lisak (UMK) M. Vainio, T. Uotila (UH) J. Seppä (VTT) Publication date: 24.01.2025 https://www.prispectemp.ptb.de The project (22IEM03 PriSpecTemp) has received funding from the European Partnership on Metrology, cofinanced from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. 2 Glossary ABSCO Calculated molecular absorption coefficients for a range of pressures, temperatures and H2O volume mixing ratios and stored in look-up tables AMES The NASA Ames PAH IR Spectroscopic Database AMTS Advanced Moisture and Temperature Sounder CA-CRDS Comb Assisted Cavity Ring-Down Spectroscopy CDSD-296 High-resolution Carbon Dioxide Spectroscopic Databank CMDS Cavity Mode Dispersion Spectroscopy CRD Cavity Ring-Down CRDS Cavity Ring-Down Spectroscopy DFB Distributed-Feedback Laser DLR Deutsches Zentrum für Luftund Raumfahrt (German Aerospace Center) DMS Dipole Moment Surface DMF Dipole Moment Function FS-CRDS Frequency Stabilized Cavity Ring-Down Spectroscopy FTIR Fourier-Transform Infrared Spectroscopy FTS Fourier-Transform Spectroscopy ILS technique Intracavity Laser Spectroscopy (not to be confused with ILS – instrumental line shape) GEISA Gestion et Etude des Informations Spectroscopiques Atmosphériques (Database) HITRAN High-resolution transmission molecular absorption database (Spectroscopic database) HR High Resolution HTP Hartmann-Tran Profile (spectral profile named after its inventors) IUPAC International Union of Pure and Applied Chemistry MIR Middle Infrared Region NICE-OHMS Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy NIR Near Infrared Region NIST National Institute of Standards and Technology RVSGT Rotational–Vibrational Spectroscopic Gas Thermometry TDLAS Tuneable Diode Laser Absorption Spectroscopy 3 TABLE OF CONTENTS A1.1.1. Literature study on experimental line intensity of CO, CO2 and O2 ..................................... 1 1 Summary ................................................................................................................................. 4 2 Introduction .............................................................................................................................. 4 3 Carbon monoxide, CO ............................................................................................................. 4 3.1 Line intensity of CO at 4.5 µm ............................................................................................ 5 3.2 Line intensity of CO at 2.3 µm ............................................................................................ 6 3.3 Line intensity of CO at 1.57 µm .......................................................................................... 7 Carbon dioxide, CO2 ....................................................................................................................... 9 3.4 Line intensity of CO2 at 4.3 µm ........................................................................................... 9 3.5 Line intensity of CO2 at 2.8 µm ......................................................................................... 10 3.6 Line intensity of CO2 near 2 µm ........................................................................................ 12 3.7 Line intensity of CO2 at 1.60 µm and 1.57 µm .................................................................. 14 4 Oxygen, O2 ............................................................................................................................ 16 4.1 A-band of O2 (0.76 µm) .................................................................................................... 16 5 References ............................................................................................................................ 19 4 1 Summary This report is an overview of the existing literature sources for the atmospheric “sensor" molecules (CO, CO2, O2). Existing accurate measurements of the line intensity in different frequency ranges (molecular bands), suitable for both highly accurate ab initio calculations, measurements, and specification of the reference temperatures are targeted. The document is partitioned as follows: after a brief introduction in the second section, 3rd, 4th and 5th sections are dedicated to carbon monoxide, carbon dioxide and molecular oxygen, respectively. Information about the bands is collected in subsections. In case of CO, CO2, vibrational excitation determines the partition within a subsection: the bands with low excitation are mentioned first. For the oxygen molecule only electronic A-band is considered. 2 Introduction Small molecules containing oxygen are ever-present atmospheric constituents, reflecting functioning of the planet. An equilibrium between CO2 and O2 tells primarily about the efficiency of photosynthesis counterweighting the breath, and CO reflects an incomplete burn of carbon-containing substances (fuels or forests). In the gas phase, absorption spectra of light molecules such as CO2, O2 and CO consist of groups of spectral features (bands) with regular structure (see. Figure 3-4, Figure 0-1 and Figure 4-1). Integrated intensities of different bands decrease with increasing excitation. Knowing the intensities of low frequency fundamental transitions, which absorb in the range of the Earth’s infrared emission interval is helpful for determining the impact of a molecule on the global warming. These are normally highly absorbing transitions which require small absorption path and concentrations. High overtones (or weak forbidden bands as it is the case with O2) are useful for the atmospheric sensing. Knowledge of the structure or/and intensities of spectral bands is a necessary input for theoretical calculations, which yield essential molecular parameters, such as dipole moment function, Hermann-Wallis coefficients, Coriolis coupling constants, broadening and shifting coefficients, etc. Molecular constants, in turn, make possible the modelling of any band at different temperatures, regardless of its accessibility by an experiment. Whereas molecular constants determine the probability of a transition between energy levels, the temperature influences the population of the levels of the initial state, causing intensity redistribution between different lines within an absorption band. Therefore, the absolute value of the intensities is of a secondary importance for the temperature determination, but the accurate knowledge of the ratios between different lines is essential. 3 Carbon monoxide, CO Carbon monoxide is one of the most studied molecules because of its small size (hence accessibility of extensive theoretical treatment) and the involvement in many anthropogenic and natural processes on Earth. The CO molecule was included in the HITRAN database since its very first version to aid the remote sensing of the terrestrial atmospheric CO. A major update was made in HITRAN 2016 [ 1 ], largely based on the work of Li et al. [ 2 ]. In this work, a semi-empirical dipole moment function (DMF) was derived from appropriately weighted experimental line intensities in literature and two CRDS measurements of the 40 [2] and 60 band [ 3 ]. Combined with the potential energy surface (PEC) from Coxon and Hajigeorgiou [ 4 ], ro-vibrational line intensities of the 00 to 70 bands were derived. In the latest version of HITRAN (HITRAN 2020) [ 5 ], the line intensities of the 10 and 30 bands have been updated [ 6 ] using scaling factors of 1.02 and 1.026, respectively, derived from the measurements and calculations [ 7 , 8 ,27]. CO is also a molecule routinely monitored in many industrial processes, some of them involving high temperatures. For this reason, the HITEMP database [ 9 ] (high-temperature analogue of HITRAN) includes Figure 3-1. Dipole moment function of the ground state of CO, taken fom Ref. [15] (Figure 1). 5 higher vibrational bands and rotational lines, whose intensities become significant with increasing temperature. The current CO line list in the HITEMP2010 database [9] is largely based on Refs. [ 10 , 11 ], supplemented with the values from Cologne Database for Molecular Spectroscopy (CDMS) [ 12 ] in the MW region and experimental data for the overtone bands 20 and 30 from [ 13 ] and [ 14 ], respectively. It consists of 113631 lines, with 𝑣𝑚𝑎𝑥 = 20 and 𝐽𝑚𝑎𝑥 = 150 , where 𝑣, 𝐽 are vibrational and rotational quantum numbers, respectively. The dipole moment function is a key physical property to access the infrared intensities (in dipole approximation). For carbon monoxide it is unusual: it does not stay positive for all bond lengths like in hydrogen halides family, but changes sign in the vicinity of the equilibrium bond length (1.128 Å), see Figure 3-1 (the latter is taken from Ref. 15 ). Measurements in the microwave and IR regions allow reconstruction of the dipole moment function in an interval of c.a. 1 Å, centered at the equilibrium bond length. A polynomial expansion, obtained from experimental values in Ref. [2] and used in HITRAN16 [1] is shown in green in Figure 3-1. One can see that the polynomial representation exhibits non-physical behavior with increasing or decreasing bond length. Adding information on high overtone, 70 (Ref. [ 16 ], red curve on Figure 3-1) expands the interval. Significant effort is put into the generation of an empirical dipole moment function, which not only reproduces the experimental values, but has correct asymptotes [ 17 , 18 ]. Regardless of all the efforts invested by both theoreticians and experimentalists, new unresolved issues pop-up [15]. In the following sub-sections, a part of relatively recent articles is listed where the experiments are performed on a similar technical (and modern) level. Most or earlier works can be found in the references of the articles cited here. 3.1 Line intensity of CO at 4.5 µm CO fundamental band at 4.5 m is the strongest absorption feature this molecule has. High intensity makes it difficult to measure accurately. The measurement requires strong control over small partial pressures (high dilutions) and absorption paths. Nevertheless, the band is successfully investigated. A very extensive study on the line parameters of both, fundamental and first overtone bands were performed by Zou and Varanasi back in 2002 [19]. Several broadband spectra at different pressures and temperatures (down to 174 K) were recorded. Spectral parameters were extracted by multi-spectrum fitting procedure. Voigt profile was accepted as a line shape model, the instrument function was considered as well. Work by A. Predoi-Cross et al. [21] was performed using a similar instrumentation as in Ref. [19], but more advance fitting functions (line profiles) were applied, which lead to a significant lowering of the uncertainties. Ref. [19] clearly demonstrates, that accounting for the speed dependence and line mixing is necessary, and a simple symmetric Voigt function does not provide an accurate description of the line shape. This approach was later expanded to a larger J-interval as well as to three CO isotopologues in Ref. [7]. Table 1. Summary of the experimental works, focused on the intensity measurements in the fundamental band of CO. Relative uncertainties in the last column correspond to a standard deviation (𝑘 = 1). Reference Year Technique Transitions/Bands Isotopologue u(S)/S (%) [19] 2002 FTIR P23-R25 12C16O ~ 0.5 for strong lines [20] 2007 TDLAS R9, R10, R17, R18 12C16O 4 (only for R9, R10) [21] 2016 FTIR P22-R22 12C16O Fit uncertainties below 0.1 [7] 2018 FTIR P33-R38 12C16O 13C16O, 12C18O, 13C18O 0.04 at m = 20 to 0.07 at m = 30, less than 0.15% for m = 40 1-2% [22] 2023 TDLAS P20 12C16O 3 Laser-based measurements of P20 in [22] and R9, R10 [20] yielded intensity with 3% and 4% uncertainty, respectively. It is a much larger value than a typical multi-spectrum fit of a full band [7, 19, 21] can provide, but the measurements remain an important cross-check between different methods. It is worth noting, that Table 1 in Ref. [22] is a good reference source of the experimental works dedicated to the development of a laser-based CO sensor. 6 3.2 Line intensity of CO at 2.3 µm First overtone of CO was measured essentially by the broadband FT-based techniques. Relatively recent publications, reporting experimental intensities, are listed in Table 2. Table 2. Summary of the experimental works, focused on the intensity measurements in the first overtone of CO. Relative uncertainties in the last column correspond to a standard deviation (𝑘 = 1). Reference Year Technique Transitions/Bands Isotopologue u(S)/S (%) [19] 2002 FTIR P24-R23 12C16O ~ 0.7 for strong lines [13] 2003 FTIR P23-R22 12C16O 0.026 for integrated intensity at 296 K* [23] 2012 FTIR P29-R29 P29-R29 12C16O 13C16O, 12C18O, 13C18O 0.05 for |𝑚|<20** Announced to be published later [24] 2017 FTIR P24-R23 2C16O 0.05 for strong lines * Ref [13] reports only standard deviations of transition dipole moments, which are, possibly, do not contain cross-corelations with other parameters and appear underestimated (order of magnitude of 0.005%). Ref. [19] reports not only a large set of spectroscopic parameters for the fundamental band, but for the first overtone as well. Work by Brault et al. [13] on self-broadening and shifting of CO, which followed Ref. [19] in quick succession, reported a new set of HR spectra at room temperatures, and an observation that the line asymmetries (deviations from Voight profiles) cannot be ignored if high accuracies are targeted. A long list of line profiles was used, stating the importance of speed dependence and line mixing for the modelling. Intensities in Ref. [19] are 4.2% smaller than in the contemporary version of HITRAN, which lead to the 2004 update to CO line list in HITRAN 2004. All following studies use asymmetric profiles. Figure 3-2 Transition moments for the second overtone; open circles retrieved from multispectrum fit , line from Herman–Wallis factor fit. The figure is copied from Ref. [[13]] Recently Reed et al. had shown [29] that there is an effect of intensity depletion for the low J lines in the second overtone due to the intermolecular interaction (inelastic collisions). Figure 1 in Ref. [28] is another representation of the same difference between the calculations of a free molecule and experimental results. The same effect should be detectable for all other bands, and it is: upon an inspection of Figure 5 in [13], one can see that the dipole matrix elements in the band centre deviate very slightly, but systematically for the Herman-Wallis parabolic fit. Such behaviour is typical for many molecules, for example, for hydrogen halides, but it remains devoid of attention for the most systems 7 A very extensive study by Devi et al. [23] uses two spectrometers, different cells, pressures and temperatures, as well as sample compositions (neat gas and air-broadened CO, different isotopic enrichment). Total of 26 spectra were fitted simultaneously. The authors state that the usage of all data set was essential for the low uncertainty of the derived parameters. The manuscript by Esteki et al. [24] focuses primarily on shifting and broadening coefficients at 298 K in the first overtone, but it reports the line strengths as well. It is shown that speed-dependent Voigt profile yields smaller uncertainties. The authors, similar to Refs. [13] and [23], claim achieving intensities below ~ 0.05 %. One should note that all works, mentioned here (Table 2), report deviations among them and major databases, which go above uncertainty margins. In Ref. [24] the deviations for the reported intensities amount to c.a. 1.01 for HITRAN 2012 and 0.94 for GEISA 2015 [ 25 ]. It shows that regardless of the efforts, there are still some unresolved issues. 3.3 Line intensity of CO at 1.57 µm Second overtone (Figure 3-4) is often subjected to experimental investigations. A selection of recent studies involving intensity analysis is listed in Table 3. Older works can be found in the reference sections of articles in Table 3. The overtone band is still in the range of frequencies, where conventional FT spectrometers are efficient. Due to the band’s weakness, long absorption paths and relatively high pressures are needed, both of which are easier to control or determine than those parameters, necessary for the absorption measurements in the fundamental region. It means that smaller uncertainties can be reached. In the broadband study of the main isotopologue by Sung et al. [14] the path of 12.8 m is used whereas a recent article by Borkov et al. [27] a multi-pass cell with 30 m base tuned up to 1.057 km. As a results Ref. [27] reports the results for six isotopologues of CO as well as the hot transition (41). The vibrational frequency is high enough that the laser-based methods can be applied [8, 26, 28, 29, 30]. It gives a possibility of cross-checking the results, obtained by different techniques, which helps to pinpoint technique-related biases. An impressive example of such intercomparison, enriched by ab initio calculations is Ref. [28], where the results of experimental studies as well as the calculations agree on the level of 0.1% In all cases the measurements were performed at the temperatures, close to 298 K. Experimental values can be found in the supplemental material to this article [28]. High accuracy of the spectral data in 1.57 m region allowed Reed et al. [29] to investigate the impact of intermolecular interactions on intensities of free molecules even at the sub atmospheric pressures. Line intensities (area under the lines) were shown to decrease with rising pressure of nitrogen and remain constant in case helium was used as a buffer gas (see Figure 3-3, which is also Figure 1 in [29]). It complies with the stronger interaction of CO with nitrogen due to the presence of quadrupole moment and a larger polarizability than those of He. The lines with small rotational quantum numbers affected in a larger extent, since the anisotropy of the interactions is more pronounced. Similar effects were observed during the past decades for many absorbers and perturbers, but at significantly higher Figure 3-3. Pressure dependence of the relative differences in the integrated line shapes for (30) band R-branch transitions of CO, taken from Ref. [29]. 8 pressures. Now the technology allows to see it (and subsequently take account) even at typical atmospheric conditions. Table 3. Summary of the experimental works, focused on the intensity measurements in the second overtone of CO. Relative uncertainties in the last column correspond to a standard deviation (𝑘 = 1). Reference Year Technique Transitions/Bands Isotopologue u(S)/S (%) [14] 2004 FTIR P25 to R25 12C16O 0.1-0.7 (at 298 K) [26] 2013 FS-CRDS P27, P28 P14, P15 of (41) R0-R2 P15, P14 P1, P2 12C16O 12C16O 12C18O 12C17O 13C16O 0.6 in most cases [8] 2019 CMDS R23 12C16O 0.07 [27] 2020 FTIR Up to 𝐽=41 for 12C16O 12C16O, 13C16O, 12C18O, 12C17O, 13C18O, 13C17O (in natural abundance) 2-2.5 for strong unsaturated lines of 12C16O [28] 2022 CMDS CRDS FTIR P27, R23, R26-R29 P27, R23, R26-R29 P22-R22 12C16O 0.1-0.12 NCU 0.09-0.18 NIST 0.13 PTB [29] 2023 CRDS R1, R3, R5, R7 12C16O <0.1 [30] 2024 CRDS CMDS R23, R25-R31 12C16O 0.07 for R27, 0.08 for R27 Figure 3-4. Spectrum of the second overtone of CO at 300 K, measured at PTB during PriSpecTemp project. 15 The line intensities in the ABSCO 5.1 database [ 77 ] for the 30013 band are data from Ref. [ 78 ] rescaled by 1.4% and they are 0.4%-0.9% larger than DLR intensities. These differences are systematic with rotational quantum number J – they increase with J as shown in Fig. 18 of Ref. [75]. On the other hand, NIST and DLR data there is no clear trend of the intensity ratios with J. Figure 0-3. Ratios of DLR [75] to NIST [71] line intensities and their weighted averages for CO2 bands 30014, 30013, and 30012. Adopted from Fig. 17 of Ref. [75]. Ab initio dipole moment surface reported in Ames-2021 [ 79 ] provides CO2 line intensities with a few permille level agreement with NIST and DLR results. This comparison is shown in Fig. 3 (adopted from Fig. 4 of Ref. [79]. The mean relative differences between NIST and Ames are 0.2 ‰ for (30012) and 2.39 ‰ for (30013) bands and corresponding differences between DLR and Ames are -5.03 ‰ and 0.5 ‰. Figure 0-4. Relative differences between experimental ( NIST [71] and DLR [75]) and theoretical Ames-2021 [79] line intensities of 12C16O2 for different IR bands. Adopted from Fig. 4 of Ref. [79]. The line-shape parameters are available for the Voigt and quadratic speed-dependent Voigt profiles. The linemixing parameters Y are also available together with its temperature-dependence exponent. Most of the airbroadened and some self-broadened parameters (γ_air, n_air, n_self, y_self, y_air, γ_SDV_0_air_296, n_SDV_air_296, γ_SDV_2_air_296, δ_SDV_0_air_296, n_SDV_self_296, δ_SDV_0_self_296, Y_SDV_air_296, Y_SDV_self_296, n_ γ_SDV_2_air_296, n_Y_SDV_air_296) are estimated as explained in Ref. [ 80 ]. In Ref. [80], the airand self-broadening and their temperature dependencies were evaluated based on available experimental data. The pressure shift parameters were calculated with a semi-empirical approach based on available experimental data. These line-shape parameters were used for calculating the first-order line-mixing. The remaining self-broadening parameters are: γ_self from Pade approximation to the data available in [ 81 , 82 , 75, 80] as described in Ref. [ 83 ], δ_air from Ref. [ 84 ], δ_self, γ_SDV_0_self_296, γ_SDV_2_self_296 from Ref. [75]. Comparison of self-broadening and shifting between [75], [80] and [78] is shown in [75]. The broadening γ_0_self agree within 1% - 2%, and the speed-dependent broadening γ_2_self differ by up to 30% - 40% between these three sources. 16 4 Oxygen, O2 4.1 A-band of O2 (0.76 µm) The A-band of molecular oxygen (O2) is one of the bands arising from the 𝑏1Σ𝑔 + ← 𝑋3Σ𝑔 − electronic transition. The transition is electric dipole and spin forbidden, but magnetic dipole allowed. For the A-band, the vibrational state remains unchanged, leading to an absorption band centered at 762 nm (13 122 cm–1), see Figure 4-1. The practical importance of the oxygen A-band stems from the needs of atmospheric remote sensing. Owing to its well-known and uniform mixing ratio over the full range of atmospheric conditions, O2 is used to calibrate intensities of atmospheric spectra taken by satellite and ground-based instruments [ 85 ]. The A-band is particularly important because it is largely free of spectral interferences from other atmospheric species, and many of the A-band transitions are relatively weak and do not saturate for long pathlengths [ 86 ]. The stringent precision requirements of CO2 monitoring have motivated the work towards ever-more accurate O2 A-band intensity measurements [85, 87 ]. Figure 4-1. Overview of the absorption spectrum of oxygen in the A-band (measured at PTB). The rotational energy levels of molecular oxygen are described by the quantum numbers 𝐽 and 𝑁, where 𝑁 is the rotational angular momentum and 𝐽 is the total angular momentum: 𝐽 = 𝑁 + 𝑆, with 𝑆 denoting the spin angular momentum. The triplet (𝑆 = 1) ground state has rotational levels 𝐽´´ = 𝑁´´, 𝑁´´ ± 1, while the excited electronic state 𝑏 is a singlet state (𝑆 = 0) with rotational levels 𝐽´ = 𝑁´. As a result, four types of transitions (branches) appear, labeled Δ𝑁Δ𝐽 : 𝑃𝑃 , 𝑃𝑄 , 𝑅𝑄 and 𝑅𝑅 , where 𝑃, 𝑄, and 𝑅 denote angular momentum quantum number changes of -1, 0, and +1, respectively. Transitions 𝑃𝑃(𝑁´´, 𝐽´´ = 𝑁´´ + 1) and 𝑃𝑄(𝑁´´, 𝐽´´ = 𝑁´´ − 1) form the 𝑃 branch with about 2 cm−1 line separation. Transitions 𝑅𝑅 and 𝑅𝑄 form the 𝑅 branch with a decreasing line separation for higher 𝐽, leading to a bandhead at about 13 156 cm1. Individual transitions are labeled Δ𝑁 𝑁´´Δ𝐽 𝐽´´. For example, P7Q6 denotes a transition that starts from state 𝑁´´ = 7, 𝐽´´ = 6 and for which the angular momentum changes are Δ𝑁 = −1, Δ𝐽 = 0. Angular momentum index 𝑚 is often used when plotting the line intensities as a function of the initial state – as an example, see Figure 4-2. For the PP and PQ branches the index is defined 𝑚 = −𝐽´´ and for the RR and RQ branches it is 𝑚 = 𝐽´´ + 1 [86]. Key papers reporting O2 A-band line intensities and other parameters are listed in Table 7. Significantly reduced uncertainties of the line intensities have been obtained after the HITRAN 2008 update. The most recent (2008 onwards) experimental data are largely from the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) measurements of the Hodges group (NIST) [ 88 , 89 ,86, 90 ]. The first FS-CRDS measurements focused on the P branch [88,90], including rare O2 isotopologues [89]. In 2010-2011 the group extended their study to the R branch, measuring several lines of not only the main isotopologue [86] but also of rare isotopologues [ 91 ]. Some of the newer line lists include Dicke narrowing parameters, which allow for 13000 13050 13100 13150 0 5 10 15 Absorption cross section [cm-/atm]/10-4 Wavenumber [cm-] A-band of O2 17 calculation of the Galatry line profile and corrected a calculation error in the HITRAN database which led the O2 line intensities to have an incorrect frequency dependence [ 92 ]. Recent contributions have highlighted the importance of collision-induced absorption (CIA) and line mixing effects, with an effort to upgrade the Absorption coefficient (ABSCO) look-up tables used for Orbiting Carbon Observatory (OCO) missions [85,87,77]. These effects have been better characterized via measurements at higher pressures and by applying new multispectrum fitting software with sophisticated line-shape models such as Rautian and speed-dependent Voigt; see Drouin et al. [85], which also includes discussion about Galatry vs. speed-depend line shape models and mentions ongoing efforts on applying Hartmann-Tran profile. Detailed comparisons of different line shape models (Galatry, speed-dependent Voigt & Rautian) can also be found in the paper of Predoi-Cross et al., which reports line intensities and other parameters obtained from multispectrum fits of high-resolution FTS measurements [ 93 ]. The 2020 contribution of Payne et al. combines earlier FS-CRDS measurements with new FT-CRDS and Fourier Transform Spectroscopy (FTS) measurements [77]. These have been included to provide full band coverage and more comprehensive information at various temperatures and pressures. It is worth pointing out the discussion in section 2.3.2. which states “The inspection revealed and overlooked inconsistency between the FTRS and CRDS in prior work of several percent, as well as a similar inconsistency between the ‘new’ [77] CRDS and the prior CRDS data in this same region…” thus suggesting that there is need for further scrutiny of the oxygen A-band line intensities, the P-branch in particular. Nevertheless, the article [77] nicely summarizes the status of oxygen spectroscopy in the context of Orbiting Carbon Observatories and is given as the main reference in HITRAN 2020 for the data of oxygen A-band. As for the HITRAN 2020 update [1], oxygen A-band line intensities were “changed up to 5% at higher J values due to a re-assessment of the high-J data [88] to determine Herman-Wallis terms utilized in the last two HITRAN editions [86] ”; see Figure 4-2. Figure 4-2. Ratios of intensities from HITRAN 2020 (ABSCO 5.1) [77] compared with HITRAN 2016 [1] and HITRAN 2012 [74]. The HITRAN 2016 update, which utilized a prior ABSCO release (5.0) kept the HermanWallis factors from HITRAN 2012 fixed. The changes are due to band scaling and the application of new Herman-Wallis factors. Weaker O2 absorption lines that overlap with the magnetic dipole A-band include the A-band electric quadrupole transitions [ 94 ] and the (magnetic dipole) hot band 𝑣´ = 1 ← 𝑣´´ = 1 [ 95 ]. Although the electric quadrupole transitions are about 105 times weaker than the respective magnetic dipole allowed transitions, they can be observed in atmospheric spectra with up to 1 % absorbance. Some of the strongest electric quadrupole transitions (with line intensities of about 10-29 cm molec-1) have been measured with FS-CRDS [94]. As for the hot band, the line intensities are in room temperature about three orders of magnitude weaker than those of the A-band; several lines have nevertheless been measured [95]. 18 Table 7. Summary of the experimental works reporting the intensity data for the A-band of O2. Uncertainties and accuracies reflect the relative quality of the datasets. For a detailed information about uncertainty evaluation, one should address original sources. Ref. Published Technique Number of transitions Isotopologues Notes on the measured line intensity [96] 1987 TDLAS 54 16O2 ~1% variation in linestrengths determined in different pressures [97] 1999 FTS+LPAC Not specified 16O18O Accuracy: 10% [98] 1999 FTS+LPAC Not specified 16O2 Accuracy: 0.3% [99] 2000 FTS 44 16O2 Measurement precision:1% Absolute accuracy 2% [95] 2004 NICE-OHMS 12 (16O2), 17 (16O18O) 16O2 16O18O Experimental precision: Stronger lines: ~10% Weaker lines: errors ~90% [100] 2007 TDLAS ~40 16O2 Average uncertainty: ±3% Deviation from the database: -8% [93] 2008 FTS 56 16O2 Accuracy: ~1% [88] 2008 FS-CRDS 32 16O2 ~1% lower results than in the databases [89] 2009 FS-CRDS 65 (16O18O), 54 (16O17O) 16O17O, 16O18O, 17O18O, 18O2 16O18O: ~2% deviation of the database values, 16O17O: systematic offset compared to database [90] 2009 FS-CRDS 20 16O2 High J-transitions [86] 2010 FS-CRDS 34(R-branch) 16O2 Relative st. dev. 0.3% [91] 2011 FS-CRDS 100 (R-branch) 16O17O, 16O18O, 17O2, 17O18O, 18O2 [85] 2017 FS-CRDS, FTS Not specified 16O2 19 5 References 1 I.E. 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