The update of the line positions and intensities in the line list of carbon dioxide for the HITRAN2020 spectroscopic database

https://doi.org/10.1016/j.jqsrt.2021.107896Get rights and content

Highlights

  • The new HITRAN2020 line list for all 12 naturally abundant isotopologues of carbon dioxide replaces the previous one from the HITRAN2016 edition.

  • Evaluation of the HITRAN CO2 line lists by comparison to laboratory and atmospheric spectra below 8310 cm−1 was carried out.

  • The updates made for the bands having mixed CDSD and UCL-IAO line intensities in the HITRAN2016 line list below 8000 cm−1 are presented.

  • The CO2 bands in the 1.4- to 2.1-µm region were improved.

  • New CO2 bands above 8000 cm−1 were added to the HITRAN2020 edition from the new high-temperature UCL-4000 12C16O2 line list (ExoMol database) and the CRDS spectra for the 16O12C18O isotopologue.

  • The magnetic dipole 01111–00001 band of the 12C16O2 isotopologue in the 3.3-µm region was introduced into HITRAN for the first time.

Abstract

This paper describes the updates of the line positions and intensities for the carbon dioxide transitions in the 2020 edition of the HITRAN spectroscopic database. The new line list for all 12 naturally abundant isotopologues of carbon dioxide replaces the previous one from the HITRAN2016 edition. This update is primarily motivated by several issues related to deficient HITRAN2016 line positions and intensities that have been identified from laboratory and atmospheric spectra. Critical validation tests for the spectroscopic data were carried out to find problems caused by inaccuracies in CO2 line parameters. New sources of data were selected for the bands that were deemed problematic in the HITRAN2016 edition. Extra care was taken to retain the consistency in the data sources within the bands. The comparisons with the existing theoretical and semi-empirical databases (including ExoMol, NASA Ames, and CDSD-296) and with available experimental works were carried out. The HITRAN2020 database has been extended by including additional CO2 bands above 8000 cm−1, and magnetic dipole lines of CO2 were introduced in HITRAN for the first time by including the ν23 band in the 3.3-µm region. Although the main topic of this article is line positions and intensities, for consistency a recent algorithm for the line-shape parameters proposed in Hashemi et al. JQSRT (2020) was reapplied (after minor revisions) to the line list.

Introduction

A plethora of critical applications drives a constant demand for ever higher-quality spectroscopic parameters of carbon dioxide (CO2) transitions. Atmospheric CO2 is the key gaseous contributor to the greenhouse effect in the terrestrial atmosphere. Its continuous increase in the atmosphere over the last two centuries from anthropogenic sources is considered the principal driver of climate change. The Martian and Venusian (and those of many rocky exoplanets) atmospheres consist mostly of carbon dioxide with mole fractions of over 95%. Monitoring CO2 is important in combustion. There are several satellite-based observatories, including GOSAT [1], [2], [3], GOSAT-2 [4], OCO-2 [5,6], OCO-3 [7], MIPAS [8], and ACE [9], as well as ground-based networks TCCON [10] and NDACC [11], monitoring atmospheric CO2. Accurate interpretation of the data from these missions is necessary to make informed decisions regarding controlling the anthropogenic contribution to the greenhouse effect and, ultimately, climate change. All aforementioned CO2 remote sensing activities depend on the reliability of the reference CO2 spectroscopy.

The HITRAN molecular spectroscopic database is the widely recognized standard providing the spectroscopic data for atmospheric applications. Among many other molecules, it includes the necessary calculated and experimental parameters for all naturally abundant isotopologues of carbon dioxide. The HITRAN2016 [12] CO2 line list for 12 stable isotopologues contains 559,874 transitions. Most of the issues related to spectral completeness (at least below 8000 cm−1) and insufficient accuracy of line parameters were resolved in HITRAN2016 [12]. However, there is still a lot of room for improvement to meet the ever-increasing demands of the scientific community. Since the release of the HITRAN2016 database [12], many new experimental and theoretical data have become available, and they can be used as sources for improving line positions, line intensities, and line-shape parameters of CO2. The HITRAN-related updates, including the CO2-air and CO2single bondCO2 line-shape parameters together with the speed dependence of the broadening parameters, their temperature-dependent exponents, the full and first-order line mixing, as well as their temperature dependencies have been published in Hashemi et al. [13].

In the present work, we will mainly focus on the issues associated with deficiencies in the HITRAN2016 line positions and intensities. Most of these problematic cases have been identified in laboratory and atmospheric spectra, mainly from the Kitt Peak National Observatory, MkIV balloon, cavity ring down spectroscopy (CRDS) measurements, and the Total Carbon Column Observing Network (TCCON) [[14], [15], [16]]. A general review of the CO2 bands affected by these issues was performed, and alternative sources of data for each CO2 problematic band were considered. Apart from new experimental data, the additional resources for improvements were provided by two recent comprehensive CO2 line lists, namely the 2019 version of the semi-empirical CDSD-296 [17] list and the ExoMol variational line list [18], hereafter referred to as UCL-4000. Using these spectroscopic data, we improved and extended the CO2 line lists for all 12 stable isotopologues of carbon dioxide.

The updates of the line positions, intensities, and line-shape parameters were incorporated into the carbon dioxide line list of the 2020 edition of the HITRAN spectroscopic database. The remainder of this paper is organized as follows. In Section 2, we present an overview of CO2 line lists in the HITRAN2016, ExoMol, NASA Ames, and CDSD-296 spectroscopic databases. In Section 3, we describe evaluations of the HITRAN CO2 line list based on laboratory and atmospheric measurements between 670 and 8310 cm−1. For wavenumbers below 8000 cm−1, we discuss line intensity updates to the HITRAN2016 line list for bands provided by the Carbon Dioxide Spectroscopic Databank (CDSD) based on global fits of semi-empirical models to measurements and those obtained by the ab initio calculations of University College London (UCL). In Section 4, we discuss new measurements with sub-percent relative uncertainty [19], [20], [21], [22] that were used to improve some CO2 bands in the 1.4- to 2.1-µm region. In Section 5, we present new CO2 bands above 8000 cm−1 that have been included in the HITRAN2020 edition from the latest high-temperature line list of the ExoMol [18] database (with appropriate cut-off) for the main CO2 isotopologue and the CRDS spectra [14] for the 16O12C18O isotopologue. The line parameters of the ν23 magnetic dipole band of the 12C16O2 isotopologue, which have been introduced into HITRAN for the first time, will be presented in Section 6. The revision, which updates the air- and self-broadened line-shape parameters of CO2, described in Ref. [13], will be discussed in Section 7. The presentation of the final version of the HITRAN2020 CO2 line list will be given in Section 8, followed by a closing discussion and conclusions in Section 9.

Section snippets

Review of the current carbon dioxide line lists

The present section describes the status of the most currently used spectroscopic databases of carbon dioxide: HITRAN2016 [12], NASA Ames [23], and the recent ExoMol [18] and CDSD-296 [17] line lists. The laboratory and atmospheric spectra helped to expose several problems in the HITRAN2016 CO2 line list occurring in various spectral regions (for more details, see Section 3). The CDSD-296, ExoMol, and NASA Ames line lists were used to update, extend or improve the line positions and

Evaluation of the HITRAN CO2 line lists by laboratory and atmospheric spectra

In this section, we present the general review of problematic bands identified in laboratory and atmospheric spectra, mainly those obtained from the Kitt Peak National Observatory, MkIV balloon, CRDS, and TCCON [14], [15], [16], [38], [39], [40]. Most of the issues have been associated with deficient line positions and intensities for which critical validation tests were performed, and alternative sources of data for each problematic band were suggested.

Several CO2 line lists, including

New experimental data with sub-percent uncertainty

Accurate line parameters of the spectral lines are generally required for the most demanding atmospheric applications. In particular, the accurate values of the line intensities with uncertainties as low as 0.3%−0.5% [49] are necessary for forward models used in retrievals of CO2 concentrations from some remote sensing missions. A number of very accurate measurements have become available after the release of HITRAN2016 in the NIR region: the most recent CRDS measurements from NIST [19,20] and

New bands above 8000 cm−1

The recent high-temperature ExoMol UCL-4000 line list containing almost 2.5 × 109 transitions at T = 4000 K for the 12C16O2 isotopologue was published in Ref. [18]. To compare this line list with HITRAN2016, we converted UCL-4000 to HITRAN format at 296 K and applied an intensity cut-off of 10−30 cm/molecule using the ExoMol_to_HITRAN.py program downloaded from http://exomol.com/software/ [59]. This code transfers the position, intensity, Einstein A, lower state energy, and upper/lower level

Addition of magnetic dipole band of 12C16O2 at 3.3 µm

The HITRAN2020 database has been extended by including the 01111-00001 (v2+v3) magnetic dipole band of the principal isotopologue of carbon dioxide. These line parameters were introduced into HITRAN for the first time; all previous editions of HITRAN provided only CO2 electric dipole transitions. Interestingly, the first observation of the v2+v3 band has been reported at 3.3 µm in the atmosphere of Mars [61] by the ExoMars Trace Gas Orbiter ACS instrument [62]. This band is forbidden as an

The line-shape parameters for the HITRAN2020 CO2 line list

A major update of the line-shape parameters of CO2 broadened by air and CO2 is described in Ref. [13]; this involved both enhancing the HITRAN database and improving completeness. A systematic extrapolation method was introduced for producing the air- and self-broadened half-width Voigt profile parameters for unmeasured transitions [69], based on the measurements reported in Ref. [70]. The temperature-dependent exponents of the air- and self-broadening parameters were generated based on

Overview summary of the HITRAN2020 CO2 line list

The HITRAN2020 line list for the twelve stable isotopic species of carbon dioxide contains 545,084 transitions. It covers the spectral range from 0.757 cm−1 to 19,908.186 cm−1 with J ≤ 128 and with the lower-state energies up to 6533.030 cm−1. Most of the line positions and their corresponding lower state energies were replaced by using the CDSD-296 database [17]. For the problematic line intensities identified in new laboratory and atmospheric spectra, critical validation tests were performed

Conclusion

This work presents an improved and extended version of the HITRAN2020 spectroscopic database for the carbon dioxide molecule. The database includes updates of the line positions and intensities described in this work, and line shape parameters (described in Hashemi et al. [13]) for 12 stable CO2 isotopologues. Critical validation tests for the spectroscopic data, including the comparisons with the most advanced theoretical and semi-empirical databases [17,18,23,27] and accurate experimental

CRediT authorship contribution statement

E.V. Karlovets: Formal analysis, Methodology, Software, Writing – original draft. I.E. Gordon: Supervision, Conceptualization, Methodology, Writing – review & editing. L.S. Rothman: Writing – review & editing. R. Hashemi: Data curation, Writing – review & editing. R.J. Hargreaves: Software. G.C. Toon: Data curation, Software. A. Campargue: Data curation, Writing – review & editing. V.I. Perevalov: Data curation, Writing – review & editing. P. Čermák: Software. M. Birk: Data curation. G. Wagner:

Declaration of Competing Interest

None.

Acknowledgments

This work was supported by NASA grant funding from AURA NNX17AI78G. The National Institute of Standards and Technology (NIST) received support from the NASA Science Team for the OCO Missions (NRA) NNH17ZDA001N‐OCO2 and the NIST Greenhouse Gas and Climate Science Measurements Program. This work was also supported by the Ministry of Science and Higher Education of the Russian Federation. The work performed at UCL was supported by the STFC Projects No. ST/M001334/1 and ST/R000476/1. Part of this

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