# The HITRAN2020 molecular spectroscopic database

I.E. Gordon, L.S. Rothman, R.J. Hargreaves, R. Hashemi, E.V. Karlovets, F.M. Skinner, E.K. Conway, C. Hill, R.V. Kochanov, Y. Tan, et al.

► To cite this version:

I.E. Gordon, L.S. Rothman, R.J. Hargreaves, R. Hashemi, E.V. Karlovets, et al. The HITRAN2020 molecular spectroscopic database. Journal of Quantitative Spectroscopy and Radiative Transfer, 2022, 277, pp.107949. 10.1016/j.jqsrt.2021.107949. hal-04568684

HAL Id: hal-04568684

https://hal.science/hal-04568684

Submitted on 6 May 2024

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<figure><img src="image_1.png" /></figure>
# The HITRAN2020 molecular spectroscopic database

I.E. Gordon, L.S. Rothman, R.J. Hargreaves, R. Hashemi, Ekaterina Karlovets, F.M. Skinner, E.K. Conway, C. Hill, R.V. Kochanov, Y. Tan, et al.

► To cite this version:

I.E. Gordon, L.S. Rothman, R.J. Hargreaves, R. Hashemi, Ekaterina Karlovets, et al. The HITRAN2020 molecular spectroscopic database. Journal of Quantitative Spectroscopy and Radiative Transfer, 2022, 277, pp.107949. 10.1016/j.jqsrt.2021.107949. hal-04568684

HAL Id: hal-04568684

https://hal.science/hal-04568684v1

Submitted on 6 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

The HITRAN2020 molecular spectroscopic database

<figure><img src="image_4.png" /></figure>

I.E. Gordon<sup>a, *,</sup> L.S. Rothman<sup>a</sup>, R.J. Hargreaves<sup>a</sup>, R. Hashemi<sup>a</sup>, E.V. Karlovets<sup>a</sup>, F.M. Skinner<sup>a</sup>, E.K. Conway<sup>a</sup>, C. Hill<sup>b</sup>, R.V. Kochanov<sup>a, c, d</sup>, Y. Tan<sup>a, e</sup>, P. Wcisło<sup>f</sup>, A.A. Finenko<sup>a, g</sup>, K. Nelson<sup>a</sup>, P.F. Bernath<sup>h</sup>, M. Birk<sup>i</sup>, V. Boudon<sup>j</sup>, A. Campargue<sup>k</sup>, K.V. Chance<sup>a</sup>, A. Coustenis<sup>l</sup>, B.J. Drouin<sup>m</sup>, J.-M. Flaud<sup>s</sup>, R.R. Gamache<sup>o</sup>, J.T. Hodges<sup>p</sup>, D. Jacquemart<sup>q</sup>, E.J. Mlawer<sup>r</sup>, A.V. Nikitin<sup>c</sup>, V.I. Perevalov<sup>c</sup>, M. Rotger<sup>t</sup>, J. Tennyson<sup>u</sup>, G.C. Toon<sup>m</sup>, H. Tran<sup>L</sup>, V.G. Tyuterev<sup>t, d, c</sup>, E.M. Adkins<sup>p</sup>, A. Baker<sup>n</sup>, A. Barbe<sup>t</sup>, E. Canè<sup>w</sup>, A.G. Császár<sup>y, z</sup>, A. Dudaryonok<sup>c</sup>, O. Egorov<sup>c</sup>, A.J. Fleisher<sup>p</sup>, H. Fleurbaey<sup>k</sup>, A. Foltynowicz<sup>A</sup>, T. Furtenbacher<sup>y</sup>, J.J. Harrison<sup>B, C, D</sup>, J.-M. Hartmann<sup>v</sup>, V.-M. Horneman<sup>E</sup>, X. Huang<sup>F</sup>, T. Karman<sup>a</sup>, J. Karns<sup>a, W, X</sup>, S. Kassi<sup>k</sup>, I. Kleiner<sup>M</sup>, V. Kofman<sup>R</sup>, F. Kwabia-Tchana<sup>M</sup>, N.N. Lavrentieva<sup>c</sup>, T.J. Lee<sup>G</sup>, D.A. Long<sup>p</sup>, A.A. Lukashevskaya<sup>c</sup>, O.M. Lyulin<sup>c</sup>, V.Yu. Makhnev<sup>N</sup>, W. Matt<sup>a, X</sup>, S.T. Massie<sup>H</sup>, M. Melosso<sup>x</sup>, S.N. Mikhailenko<sup>c</sup>, D. Mondelain<sup>k</sup>, H.S.P. Müller<sup>l</sup>, O.V. Naumenko<sup>c</sup>, A. Perrin<sup>L</sup>, O.L. Polyansky<sup>u, N</sup>, E. Raddaoui<sup>q</sup>, P.L. Raston<sup>J, K</sup>, Z.D. Reed<sup>p</sup>, M. Rey<sup>t</sup>, C. Richard<sup>j</sup>, R. Tóbiás<sup>y</sup>, I. Sadiek<sup>A, O</sup>, D.W. Schwenke<sup>G</sup>, E. Starikova<sup>c</sup>, K. Sung<sup>m</sup>, F. Tamassia<sup>w</sup>, S.A. Tashkun<sup>c</sup>, J. Vander Auwera<sup>p</sup>, I.A. Vasilenko<sup>c</sup>, A.A. Vigasin<sup>Q</sup>, G.L. Villanueva<sup>R</sup>, B. Vispoel<sup>T, S, o</sup>, G. Wagner<sup>i</sup>, A. Yachmenev<sup>U, V</sup>, S.N. Yurchenko<sup>u</sup>

<footnote><sup>a</sup>Center for Astrophysics | Harvard & Smithsonian, Atomic and Molecular Physics Division, Cambridge, MA 02138, USA</footnote>

<footnote><sup>b</sup>Nuclear Data Section, International Atomic Energy Agency, Vienna International Centre, PO Box 100, Vienna A-1400, Austria</footnote>

<footnote><sup>c</sup>V.E. Zuev Institute of Atmospheric Optics, Laboratory of Theoretical Spectroscopy, Russian Academy of Sciences, Tomsk 634055, Russia</footnote>

<footnote><sup>d</sup>QUAMER laboratory, Tomsk State University, Tomsk 634050, Russia</footnote>

<footnote><sup>e</sup>Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei, China</footnote>

<footnote><sup>f</sup>Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Torun, Grudziadzka 5, Torun 87-100, Poland</footnote>

<footnote><sup>g</sup>Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia</footnote>

<footnote><sup>h</sup>Department of Chemistry, Old Dominion University, Norfolk VA, USA</footnote>

<footnote><sup>i</sup>German Aerospace Center (DLR), Remote Sensing Technology Institute, Wessling, Germany</footnote>

<footnote><sup>j</sup>Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne Franche-Comté, UMR 6303 CNRS, Dijon Cedex, France</footnote>

<footnote><sup>k</sup>University of Grenoble Alpes, CNRS, LIPhy, Grenoble F-38000, France</footnote>

<footnote><sup>l</sup>Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Paris Observatory, CNRS, PSL University, Sorbonne University, Paris, France</footnote>

<footnote><sup>m</sup>Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote><sup>n</sup>Division of Astronomy, California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote><sup>o</sup>Department of Environmental, Earth & Atmospheric Sciences, University of Massachusetts, Lowell, MA, USA</footnote>

<footnote><sup>p</sup>Chemical Sciences Division, National Institute of Standards and Technology, Gaithersburg, MD, USA</footnote>

<footnote><sup>q</sup>Sorbonne Université, CNRS, De la Molécule aux NANO-objets : Réactivité, Interactions et Spectroscopies, MONARIS, Paris 75005, France</footnote>

<footnote><sup>r</sup>Atmospheric and Environmental Research, Lexington MA, USA</footnote>

<footnote><sup>s</sup>Institut des Sciences Moléculaires d'Orsay, CNRS, Université Paris-Sud, Université Paris-Saclay, Orsay F-91405, France</footnote>

<footnote><sup>t</sup>Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR CNRS 7331, BP 1039, Reims Cedex 2 F-51687, France</footnote>

<footnote><sup>u</sup>Department of Physics and Astronomy, University College London, London WC1E 6BT, UK</footnote>

<footnote><sup>v</sup>Laboratoire de Météorologie Dynamique/IPSL, CNRS, École polytechnique, Sorbonne Université, École normale supérieure, PSL Research University, Palaiseau F-91120, France</footnote>

<footnote><sup>W</sup>Dipartimento di Chimica Industriale "Toso Montanari", Università di Bologna, Viale Risorgimento 4, Bologna 40136, Italy</footnote>

<footnote><sup>X</sup>Dipartimento di Chimica "Giacomo Ciamician", Università di Bologna, Via F. Selmi 2, Bologna 40126, Italy</footnote>

<footnote><sup>Y</sup>MTA-ELTE Complex Chemical Systems Research Group, Budapest, Hungary</footnote>

<footnote><sup>Z</sup>Eötvös Loránd University, Institute of Chemistry, Budapest, Hungary</footnote>

<footnote><sup>A</sup>Department of Physics, Umeå University, Umeå 901 87, Sweden</footnote>

<footnote><sup>B</sup>Department of Physics and Astronomy, University of Leicester, Leicester, UK</footnote>

<footnote><sup>C</sup>University of Leicester, National Centre for Earth Observation, Leicester, UK</footnote>

<footnote><sup>D</sup>University of Leicester, Leicester Institute for Space and Earth Observation, Leicester, UK</footnote>

<footnote><sup>E</sup>Department of Physics, University of Oulu, FIN-90014, Finland</footnote>

<footnote><sup>F</sup>SETI Institute, Mountain View, CA 94043, USA</footnote>

<footnote>* Corresponding author.
E-mail address: igordon@cfa.harvard.edu (I.E. Gordon).</footnote>
The HITRAN2020 molecular spectroscopic database

<figure><img src="image_2.png" /></figure>

I.E. Gordon<sup>a, *,</sup> L.S. Rothman<sup>a</sup>, R.J. Hargreaves<sup>a</sup>, R. Hashemi<sup>a</sup>, E.V. Karlovets<sup>a</sup>, F.M. Skinner<sup>a</sup>, E.K. Conway<sup>a</sup>, C. Hill<sup>b</sup>, R.V. Kochanov<sup>a, c, d</sup>, Y. Tan<sup>a, e</sup>, P. Wcisło<sup>f</sup>, A.A. Finenko<sup>a, g</sup>, K. Nelson<sup>a</sup>, P.F. Bernath<sup>h</sup>, M. Birk<sup>i</sup>, V. Boudon<sup>j</sup>, A. Campargue<sup>k</sup>, K.V. Chance<sup>a</sup>, A. Coustenis<sup>l</sup>, B.J. Drouin<sup>m</sup>, J.-M. Flaud<sup>s</sup>, R.R. Gamache<sup>o</sup>, J.T. Hodges<sup>p</sup>, D. Jacquemart<sup>q</sup>, E.J. Mlawer<sup>r</sup>, A.V. Nikitin<sup>c</sup>, V.I. Perevalov<sup>c</sup>, M. Rotger<sup>t</sup>, J. Tennyson<sup>u</sup>, G.C. Toon<sup>m</sup>, H. Tran<sup>L</sup>, V.G. Tyuterev<sup>t, d, c</sup>, E.M. Adkins<sup>p</sup>, A. Baker<sup>n</sup>, A. Barbe<sup>t</sup>, E. Canè<sup>w</sup>, A.G. Császár<sup>y, z</sup>, A. Dudaryonok<sup>c</sup>, O. Egorov<sup>c</sup>, A.J. Fleisher<sup>p</sup>, H. Fleurbaey<sup>k</sup>, A. Foltynowicz<sup>A</sup>, T. Furtenbacher<sup>y</sup>, J.J. Harrison<sup>B, C, D</sup>, J.-M. Hartmann<sup>v</sup>, V.-M. Horneman<sup>E</sup>, X. Huang<sup>F</sup>, T. Karman<sup>a</sup>, J. Karns<sup>a, W, X</sup>, S. Kassi<sup>k</sup>, I. Kleiner<sup>M</sup>, V. Kofman<sup>R</sup>, F. Kwabia-Tchana<sup>M</sup>, N.N. Lavrentieva<sup>c</sup>, T.J. Lee<sup>G</sup>, D.A. Long<sup>p</sup>, A.A. Lukashevskaya<sup>c</sup>, O.M. Lyulin<sup>c</sup>, V.Yu. Makhnev<sup>N</sup>, W. Matt<sup>a, X</sup>, S.T. Massie<sup>H</sup>, M. Melosso<sup>x</sup>, S.N. Mikhailenko<sup>c</sup>, D. Mondelain<sup>k</sup>, H.S.P. Müller<sup>l</sup>, O.V. Naumenko<sup>c</sup>, A. Perrin<sup>L</sup>, O.L. Polyansky<sup>u, N</sup>, E. Raddaoui<sup>q</sup>, P.L. Raston<sup>J, K</sup>, Z.D. Reed<sup>p</sup>, M. Rey<sup>t</sup>, C. Richard<sup>j</sup>, R. Tóbiás<sup>y</sup>, I. Sadiek<sup>A, O</sup>, D.W. Schwenke<sup>G</sup>, E. Starikova<sup>c</sup>, K. Sung<sup>m</sup>, F. Tamassia<sup>w</sup>, S.A. Tashkun<sup>c</sup>, J. Vander Auwera<sup>p</sup>, I.A. Vasilenko<sup>c</sup>, A.A. Vigasin<sup>Q</sup>, G.L. Villanueva<sup>R</sup>, B. Vispoel<sup>T, S, o</sup>, G. Wagner<sup>i</sup>, A. Yachmenev<sup>U, V</sup>, S.N. Yurchenko<sup>u</sup>

<footnote><sup>a</sup>Center for Astrophysics | Harvard & Smithsonian, Atomic and Molecular Physics Division, Cambridge, MA 02138, USA</footnote>

<footnote><sup>b</sup>Nuclear Data Section, International Atomic Energy Agency, Vienna International Centre, PO Box 100, Vienna A-1400, Austria</footnote>

<footnote><sup>c</sup>V.E. Zuev Institute of Atmospheric Optics, Laboratory of Theoretical Spectroscopy, Russian Academy of Sciences, Tomsk 634055, Russia</footnote>

<footnote><sup>d</sup>QUAMER laboratory, Tomsk State University, Tomsk 634050, Russia</footnote>

<footnote><sup>e</sup>Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei, China</footnote>

<footnote><sup>f</sup>Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University in Torun, Grudziadzka 5, Torun 87-100, Poland</footnote>

<footnote><sup>g</sup>Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia</footnote>

<footnote><sup>h</sup>Department of Chemistry, Old Dominion University, Norfolk VA, USA</footnote>

<footnote><sup>i</sup>German Aerospace Center (DLR), Remote Sensing Technology Institute, Wessling, Germany</footnote>

<footnote><sup>j</sup>Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne Franche-Comté, UMR 6303 CNRS, Dijon Cedex, France</footnote>

<footnote><sup>k</sup>University of Grenoble Alpes, CNRS, LIPhy, Grenoble F-38000, France</footnote>

<footnote><sup>l</sup>Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Paris Observatory, CNRS, PSL University, Sorbonne University, Paris, France</footnote>

<footnote><sup>m</sup>Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote><sup>n</sup>Division of Astronomy, California Institute of Technology, Pasadena, CA, USA</footnote>

<footnote><sup>o</sup>Department of Environmental, Earth & Atmospheric Sciences, University of Massachusetts, Lowell, MA, USA</footnote>

<footnote><sup>p</sup>Chemical Sciences Division, National Institute of Standards and Technology, Gaithersburg, MD, USA</footnote>

<footnote><sup>q</sup>Sorbonne Université, CNRS, De la Molécule aux NANO-objets : Réactivité, Interactions et Spectroscopies, MONARIS, Paris 75005, France</footnote>

<footnote><sup>r</sup>Atmospheric and Environmental Research, Lexington MA, USA</footnote>

<footnote><sup>s</sup>Institut des Sciences Moléculaires d'Orsay, CNRS, Université Paris-Sud, Université Paris-Saclay, Orsay F-91405, France</footnote>

<footnote><sup>t</sup>Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR CNRS 7331, BP 1039, Reims Cedex 2 F-51687, France</footnote>

<footnote><sup>u</sup>Department of Physics and Astronomy, University College London, London WC1E 6BT, UK</footnote>

<footnote><sup>v</sup>Laboratoire de Météorologie Dynamique/IPSL, CNRS, École polytechnique, Sorbonne Université, École normale supérieure, PSL Research University, Palaiseau F-91120, France</footnote>

<footnote><sup>w</sup>Dipartimento di Chimica Industriale "Toso Montanari", Università di Bologna, Viale Risorgimento 4, Bologna 40136, Italy</footnote>

<footnote><sup>x</sup>Dipartimento di Chimica "Giacomo Ciamician", Università di Bologna, Via F. Selmi 2, Bologna 40126, Italy</footnote>

<footnote><sup>y</sup>MTA-ELTE Complex Chemical Systems Research Group, Budapest, Hungary</footnote>

<footnote><sup>z</sup>Eötvös Loránd University, Institute of Chemistry, Budapest, Hungary</footnote>

<footnote><sup>A</sup>Department of Physics, Umeå University, Umeå 901 87, Sweden</footnote>

<footnote><sup>B</sup>Department of Physics and Astronomy, University of Leicester, Leicester, UK</footnote>

<footnote><sup>C</sup>University of Leicester, National Centre for Earth Observation, Leicester, UK</footnote>

<footnote><sup>D</sup>University of Leicester, Leicester Institute for Space and Earth Observation, Leicester, UK</footnote>

<footnote><sup>E</sup>Department of Physics, University of Oulu, FIN-90014, Finland</footnote>

<footnote><sup>F</sup>SETI Institute, Mountain View, CA 94043, USA</footnote>

<footnote>* Corresponding author.
E-mail address: igordon@cfa.harvard.edu (I.E. Gordon).</footnote>
ARTICLE INFO

Article history:

Received 12 March 2021

Revised 15 September 2021

Accepted 17 September 2021

Available online 25 September 2021

Keywords:

HITRAN

Spectroscopic database

Molecular spectroscopy

Spectroscopic line parameters

Absorption cross-sections

Collision-induced absorption

Aerosols

Molecular opacities

A B S T R A C T

The HITRAN database is a compilation of molecular spectroscopic parameters. It was established in the early 1970s and is used by various computer codes to predict and simulate the transmission and emission of light in gaseous media (with an emphasis on terrestrial and planetary atmospheres). The HITRAN compilation is composed of five major components: the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimental infrared absorption cross-sections (for molecules where it is not yet feasible for representation in a line-by-line form), collision-induced absorption data, aerosol indices of refraction, and general tables (including partition sums) that apply globally to the data. This paper describes the contents of the 2020 quadrennial edition of HITRAN. The HITRAN2020 edition takes advantage of recent experimental and theoretical data that were meticulously validated, in particular, against laboratory and atmospheric spectra. The new edition replaces the previous HITRAN edition of 2016 (including its updates during the intervening years).

All five components of HITRAN have undergone major updates. In particular, the extent of the updates in the HITRAN2020 edition range from updating a few lines of specific molecules to complete replacements of the lists, and also the introduction of additional isotopologues and new (to HITRAN) molecules: SO, CH₃F, GeH₄, CS₂, CH₃I and NF₃. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. Also, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often featuring sub-percent uncertainties. Broadening parameters associated with the ambient pressure of water vapor were introduced to HITRAN for the first time and are now available for several molecules.

The HITRAN2020 edition continues to take advantage of the relational structure and efficient interface available at www.hitran.org and the HITRAN Application Programming Interface (HAPI). The functionality of both tools has been extended for the new edition.

© 2021 The Author(s). Published by Elsevier Ltd.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Over the last fifty years, the HITRAN molecular spectroscopic database has provided scientists and engineers with the necessary data to predict and simulate the transmission and emission of electromagnetic radiation in gaseous media. The history of the database was recently reviewed by Rothman [1]. The database is being updated regularly, and official “editions” have been released and described in corresponding papers [2–16]. In the last three decades, the database has been released on a quadrennial basis. This paper describes the new and/or updated data in the HITRAN2020 edition of the database.

There are countless applications of HITRAN in science and industry including, but not limited to, atmospheric, astrophysical, and medical sciences, as well as pollution monitoring. With that being said, the primary goal of HITRAN is to assist interpretation and modeling of spectra in the terrestrial atmosphere. Multiple ongoing (e.g., OCO-2 [17], OCO-3 [18], TES [19], GOSAT [20], ACE [21], TROPOMI [22], GEMS [23]) and upcoming (e.g., FORUM [24], TEMPO [25], MethaneSat [26]) remote-sensing missions rely on the quality of spectroscopic data in the HITRAN database. It is fair to

generalize that remote-sensing missions equipped with spectrometers of any resolution use HITRAN data in the analyses of their retrievals. However, this point is often overlooked since HITRAN data are often being integrated into radiative-transfer codes that are in turn used by atmospheric scientists. Whereas articles describing the HITRAN database are among the most cited articles in geosciences (recent editions have typically been cited over 2000 times each), it is very often not cited when the radiative-transfer codes are being used, despite their heavy reliance on HITRAN. There are many radiative-transfer codes that have HITRAN data directly integrated or are more flexible and allow the user to input HITRAN-formatted files themselves, including LBLRTM [27], MODTRAN [28], GENLN [29], RFM [30], ARTS [31], GARLIC [32], kCARTA [33] and VLIDORT [34], to name a few. These codes are used not only for monitoring the concentrations and atmospheric profiles of gases but also in climate models.

The second most prominent application of HITRAN is the interpretation and modeling of spectra of planetary atmospheres, including those of exoplanets. Many HITRAN-powered radiative-transfer codes listed above are used for both terrestrial and planetary atmospheres. There are also some planetary-designated codes

<footnote>G Planetary Systems Branch, Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USA</footnote>

<footnote>H University of Colorado, Laboratory for Atmospheric and Space Physics, Boulder CO, USA</footnote>

<footnote>I. Physikalisches Institut, Universität zu Köln, Köln 50937, Germany</footnote>

<footnote>J Department of Chemistry and Biochemistry, James Madison University, Harrisonburg, VA 22807, USA</footnote>

<footnote>K Department of Chemistry, University of Adelaide, South Australia 5005, Australia</footnote>

<footnote>L Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, École normale supérieure, PSL Research University, École polytechnique, Paris F-75005, France</footnote>

<footnote>M Université de Paris and Univ Paris Est Creteil, CNRS, LISA, Paris F-75013, France</footnote>

<footnote>N Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia</footnote>

<footnote>O Leibniz Institute for Plasma Science and Technology (INP), Greifswald, Germany</footnote>

<footnote>P Université Libre de Bruxelles, Spectroscopy, Quantum Chemistry and Atmospheric Remote Sensing (SQUARES), C.P. 160/09, Brussels B-1050, Belgium</footnote>

<footnote>Q Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevsky per. 3, Moscow 119017, Russia</footnote>

<footnote>R NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA</footnote>

<footnote>S Research Unit Lasers and Spectroscopies (LLS), Institute of Life, Earth and Environment (ILEE), University of Namur (UNamur), Namur B-5000, Belgique</footnote>

<footnote>T Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels 1180, Belgium</footnote>

<footnote>U Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, Hamburg 22607, Germany</footnote>

<footnote>V Hamburg Center for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany</footnote>

<footnote>W Golisano College of Computing and Information Sciences, Rochester Institute of Technology, Rochester, NY 14623, USA</footnote>

<footnote>X Computer Science Department, State University of New York at Oswego, Oswego, NY 13126, USA</footnote>
ARTICLE INFO

Article history:

Received 12 March 2021

Revised 15 September 2021

Accepted 17 September 2021

Available online 25 September 2021

Keywords:

HITRAN

Spectroscopic database

Molecular spectroscopy

Spectroscopic line parameters

Absorption cross-sections

Collision-induced absorption

Aerosols

Molecular opacities

A B S T R A C T

The HITRAN database is a compilation of molecular spectroscopic parameters. It was established in the early 1970s and is used by various computer codes to predict and simulate the transmission and emission of light in gaseous media (with an emphasis on terrestrial and planetary atmospheres). The HITRAN compilation is composed of five major components: the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimental infrared absorption cross-sections (for molecules where it is not yet feasible for representation in a line-by-line form), collision-induced absorption data, aerosol indices of refraction, and general tables (including partition sums) that apply globally to the data. This paper describes the contents of the 2020 quadrennial edition of HITRAN. The HITRAN2020 edition takes advantage of recent experimental and theoretical data that were meticulously validated, in particular, against laboratory and atmospheric spectra. The new edition replaces the previous HITRAN edition of 2016 (including its updates during the intervening years).

All five components of HITRAN have undergone major updates. In particular, the extent of the updates in the HITRAN2020 edition range from updating a few lines of specific molecules to complete replacements of the lists, and also the introduction of additional isotopologues and new (to HITRAN) molecules: SO, CH₃F, GeH₄, CS₂, CH₃I and NF₃. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. Also, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often featuring sub-percent uncertainties. Broadening parameters associated with the ambient pressure of water vapor were introduced to HITRAN for the first time and are now available for several molecules.

The HITRAN2020 edition continues to take advantage of the relational structure and efficient interface available at www.hitran.org and the HITRAN Application Programming Interface (HAPI). The functionality of both tools has been extended for the new edition.

© 2021 The Author(s). Published by Elsevier Ltd.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Over the last fifty years, the HITRAN molecular spectroscopic database has provided scientists and engineers with the necessary data to predict and simulate the transmission and emission of electromagnetic radiation in gaseous media. The history of the database was recently reviewed by Rothman [1]. The database is being updated regularly, and official “editions” have been released and described in corresponding papers [2–16]. In the last three decades, the database has been released on a quadrennial basis. This paper describes the new and/or updated data in the HITRAN2020 edition of the database.

There are countless applications of HITRAN in science and industry including, but not limited to, atmospheric, astrophysical, and medical sciences, as well as pollution monitoring. With that being said, the primary goal of HITRAN is to assist interpretation and modeling of spectra in the terrestrial atmosphere. Multiple ongoing (e.g., OCO-2 [17], OCO-3 [18], TES [19], GOSAT [20], ACE [21], TROPOMI [22], GEMS [23]) and upcoming (e.g., FORUM [24], TEMPO [25], MethaneSat [26]) remote-sensing missions rely on the quality of spectroscopic data in the HITRAN database. It is fair to

generalize that remote-sensing missions equipped with spectrometers of any resolution use HITRAN data in the analyses of their retrievals. However, this point is often overlooked since HITRAN data are often being integrated into radiative-transfer codes that are in turn used by atmospheric scientists. Whereas articles describing the HITRAN database are among the most cited articles in geosciences (recent editions have typically been cited over 2000 times each), it is very often not cited when the radiative-transfer codes are being used, despite their heavy reliance on HITRAN. There are many radiative-transfer codes that have HITRAN data directly integrated or are more flexible and allow the user to input HITRAN-formatted files themselves, including LBLRTM [27], MODTRAN [28], GENLN [29], RFM [30], ARTS [31], GARLIC [32], kCARTA [33] and VLIDORT [34], to name a few. These codes are used not only for monitoring the concentrations and atmospheric profiles of gases but also in climate models.

The second most prominent application of HITRAN is the interpretation and modeling of spectra of planetary atmospheres, including those of exoplanets. Many HITRAN-powered radiative-transfer codes listed above are used for both terrestrial and planetary atmospheres. There are also some planetary-designated codes

<footnote>G Planetary Systems Branch, Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USA</footnote>

<footnote>H University of Colorado, Laboratory for Atmospheric and Space Physics, Boulder CO, USA</footnote>

<footnote>I. Physikalisches Institut, Universität zu Köln, Köln 50937, Germany</footnote>

<footnote>J Department of Chemistry and Biochemistry, James Madison University, Harrisonburg, VA 22807, USA</footnote>

<footnote>K Department of Chemistry, University of Adelaide, South Australia 5005, Australia</footnote>

<footnote>L Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, École normale supérieure, PSL Research University, École polytechnique, Paris F-75005, France</footnote>

<footnote>M Université de Paris and Univ Paris Est Creteil, CNRS, LISA, Paris F-75013, France</footnote>

<footnote>N Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia</footnote>

<footnote>O Leibniz Institute for Plasma Science and Technology (INP), Greifswald, Germany</footnote>

<footnote>P Université Libre de Bruxelles, Spectroscopy, Quantum Chemistry and Atmospheric Remote Sensing (SQUARES), C.P. 160/09, Brussels B-1050, Belgium</footnote>

<footnote>Q Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevsky per. 3, Moscow 119017, Russia</footnote>

<footnote>R NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA</footnote>

<footnote>S Research Unit Lasers and Spectroscopies (LLS), Institute of Life, Earth and Environment (ILEE), University of Namur (UNamur), Namur B-5000, Belgique</footnote>

<footnote>T Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels 1180, Belgium</footnote>

<footnote>U Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, Hamburg 22607, Germany</footnote>

<footnote>V Hamburg Center for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, Hamburg 22761, Germany</footnote>

<footnote>W Golisano College of Computing and Information Sciences, Rochester Institute of Technology, Rochester, NY 14623, USA</footnote>

<footnote>X Computer Science Department, State University of New York at Oswego, Oswego, NY 13126, USA</footnote>
(including NEMESIS [35], petitRADTRANS [36], PSG [37], Exo-
transmit [38] and HELIOS-K [39]) that employ HITRAN data. One
should keep in mind that not all of these (or terrestrial) codes em-
ploy the most recent versions of HITRAN. Therefore, one needs to
be aware of the particular edition of HITRAN that is implemented
in their chosen radiative-transfer code.

Naturally, the success of previous/current (for instance, Venus
Express [40,41], ExoMars [42,43], Cassini [44], and Hershel [45]) as
well as future (including JWST [46] and ARIEL [47]) space missions
depend on the quality and extent of reference molecular parame-
ters, including spectral parameters in the HITRAN database. In turn,
ground-based telescopes need HITRAN not only to interpret their
observations of astrophysical objects, but also to subtract the ef-
fect of the terrestrial atmosphere [48]. Keeping the aforementioned
applications in mind, HITRAN also plays an integral role in under-
graduate and graduate courses on molecular spectroscopy and/or
radiative transfer.

The greatly improved observational and retrieval capabilities of
terrestrial and planetary remote-sensing missions have thus placed
critical new requirements on HITRAN. Among the needs are: im-
proved accuracy of all spectroscopic parameters, global consistency
of line intensities, improved line-shape parameters (and the means
by which they are represented), the addition of missing molec-
ular bands and trace gas species, representation of phenomena
that are impacting the retrievals including collision-induced ab-
sorption (CIA) bands, advanced line-shape formalisms, line-mixing,
and pressure broadening by gases different than "air" and "self".

The HITRAN project is rising to the challenge through an ex-
tensive scientific collaboration among spectroscopists, atmospheric
scientists, and data scientists. State-of-the-art theoretical and ex-
perimental values have been rigorously evaluated, and semi-
empirical procedures have been developed for where the data were
not available. The data have gone through validation against alter-
native sources, laboratory and field data when available. Fig. 1 in
the HITRAN2012 paper [15] provides an overview of the typical
validation process. In this current paper, we describe the updates
and extension of the database and associated software tools to-
wards meeting the goals of remote sensing and planetary com-
munities, and in parallel, assisting many other applications. For
instance, atmospheric scientists would be interested in improved
quality of spectroscopic parameters of ozone, which, as described
in Section 2.3 will yield better consistency between different spec-
tral regions. They will also appreciate the addition of parameters
associated with the broadening of spectral lines by ambient pres-
sure of water vapor [49] described in multiple subsections. This
will also be welcomed by the exoplanetary community that mod-
els spectra of exoplanets with "steamy" atmospheres. In general,
the planetary community and combustion researchers will be in-
terested in learning about the extension of the number of gases
that now have broadening parameters due to ambient pressure
of H₂, He, CO₂, and H₂O. Medical experts who analyze human
breath for markers of different diseases will appreciate improved
relative intensities of the ¹⁴NO and ¹⁵NO transitions described in
Section 2.8 and the addition of the CS₂ molecule described in
Section 2.53. Cometary scientists will also appreciate the latter.
These are just a few relatively random examples of the gargantuan
extent of the updates in this edition and the rationale for doing
these updates.

Before the release of the HITRAN2016 edition [16], we had re-
structured the database into a relational database format in order
to accommodate the need for additional parameters and flexibility
of their representation [50,51]. Many of these parameters can al-
ready be retrieved from the dynamic and user-friendly web inter-
face HITRANonline (at www.hitran.org), which as of early Septem-
ber, 2021 has over 20700 registered users in the initial six years
of it being made available. This new versatility allows one to re-

quest either the familiar HITRAN-format ASCII files (for those users
that will not require advanced parameters), but also user-defined
formats that can accommodate new features and parameters. The
HITRAN Application Programming Interface (HAPI) [52] that was
released with HITRAN2016 has also been updated for increased ca-
pabilities and speed of calculations.

The HITRAN compilation in its current state comprises five
components that encompass different parametrizations of vari-
ous molecular phenomena required as spectroscopic input into
the radiative-transfer models. The updates to these five portions
of HITRAN, as well as the underlying system of data structure
with accompanying internet user interface and an application pro-
gramming interface (API), will be discussed in the following sec-
tions: (1) Section 2 is dedicated to the line-by-line section, the
original and most popular component, which provides spectro-
scopic parameters for high-resolution molecular absorption and ra-
diance calculations (from the microwave through to the ultravi-
olet region of the spectrum). (2) A second component described
in Section 3 relates to experimental (mostly infrared) absorption
cross-sections. These cross-sections are generally representing ab-
sorption by molecules that have very dense spectra or many low-
lying vibrational modes. (3) Collision-induced absorption datasets
for multiple collisional pairs are described in Section 4. (4) Tables
of aerosol refractive indices are described in Section 5. (5) Global
data that apply in a general manner to the archive is another im-
portant part of the database and are described in Section 6. This
includes its particular component, Total Internal Partition Sums
(TIPS), as well as updates to the HITRAN website, underlying struc-
ture and HAPI, which are also described in Section 6.

The high temperature, HITEMP, database described by Rothman
et al. [53] was established to provide substantially more transitions
(compared to HITRAN), which become necessary for modeling ra-
diative transfer of high-temperature environments [54]. These ad-
ditional transitions are not required for typical atmospheric ap-
plications and are therefore not included in HITRAN. Recently,
the number of molecules available through HITEMP (see www.
hitran.org/hitemp/) has been extended to include nitrogen oxides
[55] and methane [56]. An in-depth description of HITEMP is be-
yond the scope of this work; nevertheless, it is often the case that
updates of HITRAN and HITEMP are performed at the same time
using the same data sources (such as for NO [55]). Therefore, a
brief description of corresponding HITEMP updates are included in
Section 2 for N₂O, CH₄, NO, NO₂, and OH.

In order to better understand the discussion in this paper
it is important to understand the HITRAN definitions of the
parameters and formalisms, which can be found in the docu-
mentation section of the HITRAN website https://hitran.org/docs/
definitions-and-units/. For a complete description of quantum
number identifications of energy levels or states provided for each
molecule in the line-by-line section of the HITRAN database, users
are referred to the Supplementary Material of this work. The global
and local quanta are described in Tables S1 and S2, respectively,
and these supplementary tables supersede those previously de-
scribed in HITRAN2004 (i.e., Tables 3 and 4 of Ref. [13]). New users
of the database should also be aware that all of the HITRAN edi-
tions (including this one) do not strictly adhere to the Système
International (SI) for both historical and application-specific rea-
sons. Thus cm⁻¹ (reciprocal centimeter, the unit of the quantity
wavenumber) is seen throughout, as is atm (atmosphere) for pres-
sure (in SI units of Pascals, 101,325 Pa = 1 atm). Also, the sym-
bol v is used throughout for line position in cm⁻¹, thereby drop-
ping the tilde (ψ) that is the official designation of wavenum-
ber. The HITRAN unit for intensity is traditionally expressed as
cm⁻¹/(molecule cm²) rather than simplifying to the equivalent
cm molecule⁻¹. However, both notations are used throughout this
paper.
(including NEMESIS [35], petitRADTRANS [36], PSG [37], Exo-
transmit [38] and HELIOS-K [39]) that employ HITRAN data. One
should keep in mind that not all of these (or terrestrial) codes em-
ploy the most recent versions of HITRAN. Therefore, one needs to
be aware of the particular edition of HITRAN that is implemented
in their chosen radiative-transfer code.

Naturally, the success of previous/current (for instance, Venus
Express [40,41], ExoMars [42,43], Cassini [44], and Hershel [45]) as
well as future (including JWST [46] and ARIEL [47]) space missions
depend on the quality and extent of reference molecular parame-
ters, including spectral parameters in the HITRAN database. In turn,
ground-based telescopes need HITRAN not only to interpret their
observations of astrophysical objects, but also to subtract the ef-
fect of the terrestrial atmosphere [48]. Keeping the aforementioned
applications in mind, HITRAN also plays an integral role in under-
graduate and graduate courses on molecular spectroscopy and/or
radiative transfer.

The greatly improved observational and retrieval capabilities of
terrestrial and planetary remote-sensing missions have thus placed
critical new requirements on HITRAN. Among the needs are: im-
proved accuracy of all spectroscopic parameters, global consistency
of line intensities, improved line-shape parameters (and the means
by which they are represented), the addition of missing molec-
ular bands and trace gas species, representation of phenomena
that are impacting the retrievals including collision-induced ab-
sorption (CIA) bands, advanced line-shape formalisms, line-mixing,
and pressure broadening by gases different than "air" and "self".

The HITRAN project is rising to the challenge through an ex-
tensive scientific collaboration among spectroscopists, atmospheric
scientists, and data scientists. State-of-the-art theoretical and ex-
perimental values have been rigorously evaluated, and semi-
empirical procedures have been developed for where the data were
not available. The data have gone through validation against alter-
native sources, laboratory and field data when available. Fig. 1 in
the HITRAN2012 paper [15] provides an overview of the typical
validation process. In this current paper, we describe the updates
and extension of the database and associated software tools to-
wards meeting the goals of remote sensing and planetary com-
munities, and in parallel, assisting many other applications. For
instance, atmospheric scientists would be interested in improved
quality of spectroscopic parameters of ozone, which, as described
in Section 2.3 will yield better consistency between different spec-
tral regions. They will also appreciate the addition of parameters
associated with the broadening of spectral lines by ambient pres-
sure of water vapor [49] described in multiple subsections. This
will also be welcomed by the exoplanetary community that mod-
els spectra of exoplanets with "steamy" atmospheres. In general,
the planetary community and combustion researchers will be in-
terested in learning about the extension of the number of gases
that now have broadening parameters due to ambient pressure
of H₂, He, CO₂, and H₂O. Medical experts who analyze human
breath for markers of different diseases will appreciate improved
relative intensities of the ¹⁴NO and ¹⁵NO transitions described in
Section 2.8 and the addition of the CS₂ molecule described in
Section 2.53. Cometary scientists will also appreciate the latter.
These are just a few relatively random examples of the gargantuan
extent of the updates in this edition and the rationale for doing
these updates.

Before the release of the HITRAN2016 edition [16], we had re-
structured the database into a relational database format in order
to accommodate the need for additional parameters and flexibility
of their representation [50,51]. Many of these parameters can al-
ready be retrieved from the dynamic and user-friendly web inter-
face HITRANonline (at www.hitran.org), which as of early Septem-
ber, 2021 has over 20700 registered users in the initial six years
of it being made available. This new versatility allows one to re-

quest either the familiar HITRAN-format ASCII files (for those users
that will not require advanced parameters), but also user-defined
formats that can accommodate new features and parameters. The
HITRAN Application Programming Interface (HAPI) [52] that was
released with HITRAN2016 has also been updated for increased ca-
pabilities and speed of calculations.

The HITRAN compilation in its current state comprises five
components that encompass different parametrizations of vari-
ous molecular phenomena required as spectroscopic input into
the radiative-transfer models. The updates to these five portions
of HITRAN, as well as the underlying system of data structure
with accompanying internet user interface and an application pro-
gramming interface (API), will be discussed in the following sec-
tions: (1) Section 2 is dedicated to the line-by-line section, the
original and most popular component, which provides spectro-
scopic parameters for high-resolution molecular absorption and ra-
diance calculations (from the microwave through to the ultravi-
olet region of the spectrum). (2) A second component described
in Section 3 relates to experimental (mostly infrared) absorption
cross-sections. These cross-sections are generally representing ab-
sorption by molecules that have very dense spectra or many low-
lying vibrational modes. (3) Collision-induced absorption datasets
for multiple collisional pairs are described in Section 4. (4) Tables
of aerosol refractive indices are described in Section 5. (5) Global
data that apply in a general manner to the archive is another im-
portant part of the database and are described in Section 6. This
includes its particular component, Total Internal Partition Sums
(TIPS), as well as updates to the HITRAN website, underlying struc-
ture and HAPI, which are also described in Section 6.

The high temperature, HITEMP, database described by Rothman
et al. [53] was established to provide substantially more transitions
(compared to HITRAN), which become necessary for modeling ra-
diative transfer of high-temperature environments [54]. These ad-
ditional transitions are not required for typical atmospheric ap-
plications and are therefore not included in HITRAN. Recently,
the number of molecules available through HITEMP (see www.
hitran.org/hitemp/) has been extended to include nitrogen oxides
[55] and methane [56]. An in-depth description of HITEMP is be-
yond the scope of this work; nevertheless, it is often the case that
updates of HITRAN and HITEMP are performed at the same time
using the same data sources (such as for NO [55]). Therefore, a
brief description of corresponding HITEMP updates are included in
Section 2 for N₂O, CH₄, NO, NO₂, and OH.

In order to better understand the discussion in this paper
it is important to understand the HITRAN definitions of the
parameters and formalisms, which can be found in the docu-
mentation section of the HITRAN website https://hitran.org/docs/
definitions-and-units/. For a complete description of quantum
number identifications of energy levels or states provided for each
molecule in the line-by-line section of the HITRAN database, users
are referred to the Supplementary Material of this work. The global
and local quanta are described in Tables S1 and S2, respectively,
and these supplementary tables supersede those previously de-
scribed in HITRAN2004 (i.e., Tables 3 and 4 of Ref. [13]). New users
of the database should also be aware that all of the HITRAN edi-
tions (including this one) do not strictly adhere to the Système
International (SI) for both historical and application-specific rea-
sons. Thus cm⁻¹ (reciprocal centimeter, the unit of the quantity
wavenumber) is seen throughout, as is atm (atmosphere) for pres-
sure (in SI units of Pascals, 101,325 Pa = 1 atm). Also, the sym-
bol v is used throughout for line position in cm⁻¹, thereby drop-
ping the tilde (ψ) that is the official designation of wavenum-
ber. The HITRAN unit for intensity is traditionally expressed as
cm⁻¹/(molecule cm²) rather than simplifying to the equivalent
cm molecule⁻¹. However, both notations are used throughout this
paper.
A number of abbreviations have been used throughout this pa-
per when describing data, instruments, and methods that have
been used to update the HITRAN database. These are described in
the text when used, but a list of these abbreviations is also pro-
vided in Appendix A.

## 2. Line-by-line modifications

An overview of changes and additions to the line-by-line sec-
tion for each isotopologue in the database with respect to the HI-
TRAN2016 edition is provided in Table 1. Isotopologues are given in
order of their descending abundance for each individual molecule.
The molecular abundance values in HITRAN are calculated based
on the terrestrial atomic abundances selected from Ref. [57]. It
is important to remember that the intensities in the HITRAN
database are scaled by these abundances. Note that although for
many molecules the amount of lines and spectral ranges have not
changed, many parameters were updated or added. In the subse-
quent subsections dedicated to individual molecules, detailed ac-
counts of those changes are provided.

The definitions of the uncertainty indices used in HITRAN for
spectral parameters in the line-by-line representation are defined
in Table 2. Uncertainty and reference indices are now given for all
parameters in HITRAN except for the Einstein-A coefficients (which
usually share the same source and uncertainty as the intensities),
lower-state energies and quantum numbers. It should be remarked
that the code 0 in Table 2 might lend itself to two different mean-
ings in the case of line position or air pressure-induced shift. It
means that either the uncertainty in the shift reported is greater
than 1 cm⁻¹ or was not reported. The word “default” or “constant”
(code 1 in Table 2) means a constant value, and the word “aver-
age” or “estimate” (code 2 in Table 2) means an average or empir-
ical value. This table will be frequently referred to across different
subsections of Section 2.

2.1. H₂O: water vapor (molecule 1)

Considering that water vapor is the major absorber of light in
the terrestrial atmosphere, it is difficult to overstate the impor-
tance of the quality and extent of spectroscopic parameters for
this molecule in HITRAN. The details surrounding the previous (HI-
TRAN2016) water-vapor compilation can be found in the corre-
sponding paper [16]. To briefly summarize, the wavelength range
has now been extended to approximately 238 nm (42,000 cm⁻¹),
and the study used *ab initio* calculated line lists as its initial
starting point. Whenever possible, line positions were replaced
with accurate experimental data or wavenumbers generated from
a MARVEL-based [58,59] set of empirical energy levels [60]. The
bulk of the intensities were of *ab initio* origin, but in many places
experimental data were used (taken most notably from Refs. [61–
72]). This approach substantially reduces the number of missing
lines (from an atmospheric perspective) in all isotopologues and,
in general, it enhances the quality of the line parameters. Simi-
lar to the two earlier editions (HITRAN2008 [14] and HITRAN2012
[15]), HITRAN2016 [16] continued to utilize the “Diet” algorithm
[73] for broadening parameters, supplemented with the newest
experimental data. Moreover, parameters determined for the ad-
vanced Hartmann-Tran (HT) profile [74,75] were incorporated into
the database, where available from Ref. [66]. Remote-sensing ex-
periments in the IR region have identified that the HITRAN2016
water-vapor compilation results in smaller residuals when com-
pared with previous editions (see Ref. [76] for instance). Neverthe-
less, a number of issues have been found. One of the sources of
discrepancies predominantly manifests itself in the NIR to visible
parts of the spectrum and is associated with erroneous broaden-
ing and shifting parameters of experimental origin. Unfortunately,

one of the “Diet” algorithm features that was designed to elimi-
nate outliers was inadvertently turned off in the HITRAN2016 edi-
tion. This has resulted in some of the issues reported in the visible
region by Baker et al. [77]. Another problem concerned the incor-
rect quantum assignment of certain transitions, which, while not
having adverse affects on atmospheric retrievals, were not accurate
from a spectroscopic perspective. In the new edition, these issues
have been eliminated. Moreover, the water database was extended
into the UV region, and the general quality of the parameters was
improved overall. The details of the HITRAN2020 update are given
below.

2.1.1. H<sub>2</sub><sup>16</sup>O

The water-vapor line lists for HITRAN2020 have received a sig-
nificant update compared to HITRAN2016 [16]. The entire process
of developing the HITRAN2020 line list for the principal isotopo-
logue is presented as a flowchart in Fig. 1. Within the text below,
we will describe the steps in significantly more detail.

In Ref. [78], an *ab initio* dipole moment surface (DMS), under-
pinned by high-level electronic-structure calculations, was devel-
oped and designed to create highly-accurate spectra extending all
the way to the dissociation limit in the near ultraviolet. The mo-
tivation for this work came in part from the forthcoming launch
of NASA's TEMPO (Tropospheric Emissions Monitoring of Pollution)
satellite [25], which carries a short-wavelength instrument (oper-
ating between 290-740 nm) that aims to accurately monitor the
chemical composition in the air across the North American con-
tinent. TEMPO will retrieve water-vapor column densities in the
440-450 nm spectral interval, a region that is often used for wa-
ter retrievals [79-81]; however, the interference of water-vapor ab-
sorption features in the near ultraviolet needs to be accounted for
when targeting trace gases such as formaldehyde.

Atmospheric observations from Lampel et al. [82] indicate that
the “POKAZATEL” [83] line list underestimates the magnitude of
absorption features at near-ultraviolet wavelengths (363 nm) by a
factor of 2.6, a potential source of error for TEMPO retrievals. A
noteworthy point is that the HITRAN2016 line list extends only to
400 nm and the source of transition intensities that underpins a
large portion of the visible transitions are variational in nature and
these use a similar DMS [84] to that was used in the creation of
the POKAZATEL line list, hence the requirement to update the tran-
sition frequency limit and *ab initio* data sources.

Conway et al. [85] calculated H<sub>2</sub><sup>16</sup>O and H<sub>2</sub><sup>18</sup>O line lists that extended to the HITRAN2016 frequency limits and compared the new *ab initio* intensities against a large quantity of experimental sources, most of which feature in the HITRAN2016 line list. It became evident that the new *ab initio* spectra from Conway et al. [85] provide more accurate and reliable transition intensities than other available *ab initio* line lists, particularly at short wavelengths. More recently, Conway et al. [86] created a new near-ultraviolet line list that extends to dissociation and this provides the correct amount of absorption at 363 nm, while also adhering to the upper limit absorption thresholds proposed by Lampel et al. [87] through atmospheric observations, and to that of Wilson et al. [88] through experimental measurements. What cannot be replicated by the *ab initio* spectra are the measured spectra of Du et al. [89] and Pei et al. [90]. The magnitude of the water-vapor cross sections reported both by Du et al. and Pei et al. are significantly larger than the *ab initio* predictions; meaning, for example, that they would adversely perturb atmospheric retrievals of ozone.

To create the HITRAN2020 H<sub>2</sub><sup>16</sup>O line list we started with the
*ab initio* spectra from Conway et al. [86]. While semi-empirical
potential energy surfaces (PES) capable of predicting energy lev-
els to a hundredth [92] and even a thousandth of a wavenum-
ber are gradually appearing [93], they are still far less accurate
than their experimental counterparts, which make use of, for ex-
A number of abbreviations have been used throughout this pa-
per when describing data, instruments, and methods that have
been used to update the HITRAN database. These are described in
the text when used, but a list of these abbreviations is also pro-
vided in Appendix A.

## 2. Line-by-line modifications

An overview of changes and additions to the line-by-line sec-
tion for each isotopologue in the database with respect to the HI-
TRAN2016 edition is provided in Table 1. Isotopologues are given in
order of their descending abundance for each individual molecule.
The molecular abundance values in HITRAN are calculated based
on the terrestrial atomic abundances selected from Ref. [57]. It
is important to remember that the intensities in the HITRAN
database are scaled by these abundances. Note that although for
many molecules the amount of lines and spectral ranges have not
changed, many parameters were updated or added. In the subse-
quent subsections dedicated to individual molecules, detailed ac-
counts of those changes are provided.

The definitions of the uncertainty indices used in HITRAN for
spectral parameters in the line-by-line representation are defined
in Table 2. Uncertainty and reference indices are now given for all
parameters in HITRAN except for the Einstein-A coefficients (which
usually share the same source and uncertainty as the intensities),
lower-state energies and quantum numbers. It should be remarked
that the code 0 in Table 2 might lend itself to two different mean-
ings in the case of line position or air pressure-induced shift. It
means that either the uncertainty in the shift reported is greater
than 1 cm⁻¹ or was not reported. The word “default” or “constant”
(code 1 in Table 2) means a constant value, and the word “aver-
age” or “estimate” (code 2 in Table 2) means an average or empir-
ical value. This table will be frequently referred to across different
subsections of Section 2.

2.1. H₂O: water vapor (molecule 1)

Considering that water vapor is the major absorber of light in
the terrestrial atmosphere, it is difficult to overstate the impor-
tance of the quality and extent of spectroscopic parameters for
this molecule in HITRAN. The details surrounding the previous (HI-
TRAN2016) water-vapor compilation can be found in the corre-
sponding paper [16]. To briefly summarize, the wavelength range
has now been extended to approximately 238 nm (42,000 cm⁻¹),
and the study used *ab initio* calculated line lists as its initial
starting point. Whenever possible, line positions were replaced
with accurate experimental data or wavenumbers generated from
a MARVEL-based [58,59] set of empirical energy levels [60]. The
bulk of the intensities were of *ab initio* origin, but in many places
experimental data were used (taken most notably from Refs. [61–
72]). This approach substantially reduces the number of missing
lines (from an atmospheric perspective) in all isotopologues and,
in general, it enhances the quality of the line parameters. Simi-
lar to the two earlier editions (HITRAN2008 [14] and HITRAN2012
[15]), HITRAN2016 [16] continued to utilize the “Diet” algorithm
[73] for broadening parameters, supplemented with the newest
experimental data. Moreover, parameters determined for the ad-
vanced Hartmann-Tran (HT) profile [74,75] were incorporated into
the database, where available from Ref. [66]. Remote-sensing ex-
periments in the IR region have identified that the HITRAN2016
water-vapor compilation results in smaller residuals when com-
pared with previous editions (see Ref. [76] for instance). Neverthe-
less, a number of issues have been found. One of the sources of
discrepancies predominantly manifests itself in the NIR to visible
parts of the spectrum and is associated with erroneous broaden-
ing and shifting parameters of experimental origin. Unfortunately,

one of the “Diet” algorithm features that was designed to elimi-
nate outliers was inadvertently turned off in the HITRAN2016 edi-
tion. This has resulted in some of the issues reported in the visible
region by Baker et al. [77]. Another problem concerned the incor-
rect quantum assignment of certain transitions, which, while not
having adverse affects on atmospheric retrievals, were not accurate
from a spectroscopic perspective. In the new edition, these issues
have been eliminated. Moreover, the water database was extended
into the UV region, and the general quality of the parameters was
improved overall. The details of the HITRAN2020 update are given
below.

2.1.1. H₂¹⁶O

The water-vapor line lists for HITRAN2020 have received a sig-
nificant update compared to HITRAN2016 [16]. The entire process
of developing the HITRAN2020 line list for the principal isotopo-
logue is presented as a flowchart in Fig. 1. Within the text below,
we will describe the steps in significantly more detail.

In Ref. [78], an *ab initio* dipole moment surface (DMS), under-
pinned by high-level electronic-structure calculations, was devel-
oped and designed to create highly-accurate spectra extending all
the way to the dissociation limit in the near ultraviolet. The mo-
tivation for this work came in part from the forthcoming launch
of NASA's TEMPO (Tropospheric Emissions Monitoring of Pollution)
satellite [25], which carries a short-wavelength instrument (oper-
ating between 290-740 nm) that aims to accurately monitor the
chemical composition in the air across the North American con-
tinent. TEMPO will retrieve water-vapor column densities in the
440-450 nm spectral interval, a region that is often used for wa-
ter retrievals [79-81]; however, the interference of water-vapor ab-
sorption features in the near ultraviolet needs to be accounted for
when targeting trace gases such as formaldehyde.

Atmospheric observations from Lampel et al. [82] indicate that
the “POKAZATEL” [83] line list underestimates the magnitude of
absorption features at near-ultraviolet wavelengths (363 nm) by a
factor of 2.6, a potential source of error for TEMPO retrievals. A
noteworthy point is that the HITRAN2016 line list extends only to
400 nm and the source of transition intensities that underpins a
large portion of the visible transitions are variational in nature and
these use a similar DMS [84] to that was used in the creation of
the POKAZATEL line list, hence the requirement to update the tran-
sition frequency limit and *ab initio* data sources.

Conway et al. [85] calculated H<sub>2</sub><sup>16</sup>O and H<sub>2</sub><sup>18</sup>O line lists that extended to the HITRAN2016 frequency limits and compared the new *ab initio* intensities against a large quantity of experimental sources, most of which feature in the HITRAN2016 line list. It became evident that the new *ab initio* spectra from Conway et al. [85] provide more accurate and reliable transition intensities than other available *ab initio* line lists, particularly at short wavelengths. More recently, Conway et al. [86] created a new near-ultraviolet line list that extends to dissociation and this provides the correct amount of absorption at 363 nm, while also adhering to the upper limit absorption thresholds proposed by Lampel et al. [87] through atmospheric observations, and to that of Wilson et al. [88] through experimental measurements. What cannot be replicated by the *ab initio* spectra are the measured spectra of Du et al. [89] and Pei et al. [90]. The magnitude of the water-vapor cross sections reported both by Du et al. and Pei et al. are significantly larger than the *ab initio* predictions; meaning, for example, that they would adversely perturb atmospheric retrievals of ozone.

To create the HITRAN2020 H<sub>2</sub><sup>16</sup>O line list we started with the
*ab initio* spectra from Conway et al. [86]. While semi-empirical
potential energy surfaces (PES) capable of predicting energy lev-
els to a hundredth [92] and even a thousandth of a wavenum-
ber are gradually appearing [93], they are still far less accurate
than their experimental counterparts, which make use of, for ex-
<table><caption>**Table 1**
Molecules and isotopologues represented in the line-by-line portion of HITRAN.</caption><thead><tr><th rowspan="2">Molecule</th><th rowspan="2">Isotopologue</th><th rowspan="2">Abundance<sup>a</sup></th><th colspan="2">HITRAN2016</th><th colspan="2">HITRAN2020</th></tr><tr><th>Spectral Range<sup>b</sup></th><th># of lines</th><th>Spectral Range<sup>b</sup></th><th># of lines</th></tr></thead><tbody><tr><td rowspan="6">(1) H<sub>2</sub>O</td><td>H<sub>2</sub><sup>16</sup>O</td><td>9.973 × 10<sup>-1</sup></td><td>0-25 711</td><td>146 878</td><td>0-42 000</td><td>319 886</td></tr><tr><td>H<sub>2</sub><sup>18</sup>O</td><td>2.000 × 10<sup>-3</sup></td><td>0-19 918</td><td>39 903</td><td>0-19 992</td><td>42 178</td></tr><tr><td>H<sub>2</sub><sup>17</sup>O</td><td>3.719 × 10<sup>-4</sup></td><td>0-19 946</td><td>27 544</td><td>0-19 946</td><td>27 544<sup>c</sup></td></tr><tr><td>HD<sup>16</sup>O</td><td>3.107 × 10<sup>-4</sup></td><td>0-19 936</td><td>56 430</td><td>0-19 935</td><td>56 430</td></tr><tr><td>HD<sup>18</sup>O</td><td>6.230 × 10<sup>-7</sup></td><td>0-10 729</td><td>10 664</td><td>0-10 729</td><td>10 664</td></tr><tr><td>HD<sup>17</sup>O</td><td>1.159 × 10<sup>-7</sup></td><td>0-10 703</td><td>6360</td><td>0-10 703</td><td>6360</td></tr><tr><td rowspan="10">(2) CO<sub>2</sub></td><td>D<sub>2</sub><sup>16</sup>O</td><td>2.420 × 10<sup>-8</sup></td><td>0-12 797</td><td>23 488</td><td>0-12 797</td><td>23 195<sup>c</sup></td></tr><tr><td><sup>12</sup>C<sup>16</sup>O<sub>2</sub></td><td>9.842 × 10<sup>-1</sup></td><td>158-14 076</td><td>173 024</td><td>158-19 909</td><td>174 446</td></tr><tr><td><sup>13</sup>C<sup>16</sup>O<sub>2</sub></td><td>1.106 × 10<sup>-2</sup></td><td>332-13 735</td><td>70 577</td><td>332-13 735</td><td>69 870</td></tr><tr><td><sup>16</sup>O<sup>12</sup>C<sup>18</sup>O</td><td>3.947 × 10<sup>-3</sup></td><td>1-12 678</td><td>127 850</td><td>1-12 678</td><td>122 140</td></tr><tr><td><sup>16</sup>O<sup>12</sup>C<sup>17</sup>O</td><td>7.340 × 10<sup>-4</sup></td><td>0-12 727</td><td>77 941</td><td>0-12 727</td><td>73 942</td></tr><tr><td><sup>16</sup>O<sup>13</sup>C<sup>18</sup>O</td><td>4.434 × 10<sup>-5</sup></td><td>2-9213</td><td>43 782</td><td>2-9213</td><td>41 058</td></tr><tr><td><sup>16</sup>O<sup>13</sup>C<sup>17</sup>O</td><td>8.246 × 10<sup>-6</sup></td><td>9-8062</td><td>25 175</td><td>9-8062</td><td>23 607</td></tr><tr><td><sup>12</sup>C<sup>18</sup>O<sub>2</sub></td><td>3.957 × 10<sup>-6</sup></td><td>482-8163</td><td>10 522</td><td>482-8163</td><td>10 498</td></tr><tr><td><sup>17</sup>O<sup>12</sup>C<sup>18</sup>O</td><td>1.472 × 10<sup>-6</sup></td><td>491-8194</td><td>15 878</td><td>498-8194</td><td>14 623</td></tr><tr><td><sup>12</sup>C<sup>17</sup>O<sub>2</sub></td><td>1.368 × 10<sup>-7</sup></td><td>535-6933</td><td>6518</td><td>535-6933</td><td>6493</td></tr><tr><td><sup>13</sup>C<sup>18</sup>O<sub>2</sub></td><td>4.446 × 10<sup>-8</sup></td><td>2245-4751</td><td>2916</td><td>539-6687</td><td>2926</td></tr><tr><td><sup>18</sup>O<sup>13</sup>C<sup>17</sup>O</td><td>1.654 × 10<sup>-8</sup></td><td>549-4915</td><td>4190</td><td>549-4915</td><td>3980</td></tr><tr><td><sup>13</sup>C<sup>17</sup>O<sub>2</sub></td><td>1.538 × 10<sup>-9</sup></td><td>575-3615</td><td>1501</td><td>575-3615</td><td>1501</td></tr><tr><td rowspan="5">(3) O<sub>3</sub></td><td><sup>16</sup>O<sub>3</sub></td><td>9.929 × 10<sup>-1</sup></td><td>0-6997</td><td>289 340</td><td>0-6997</td><td>304 262</td></tr><tr><td><sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td>3.982 × 10<sup>-3</sup></td><td>0-2768</td><td>44 302</td><td>0-3165</td><td>57 907</td></tr><tr><td><sup>16</sup>O<sup>18</sup>O<sup>16</sup>O</td><td>1.991 × 10<sup>-3</sup></td><td>1-2740</td><td>18 887</td><td>1-2740</td><td>18 887</td></tr><tr><td><sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td>7.405 × 10<sup>-4</sup></td><td>0-2122</td><td>65 106</td><td>0-2122</td><td>65 467</td></tr><tr><td><sup>16</sup>O<sup>17</sup>O<sup>16</sup>O</td><td>3.702 × 10<sup>-4</sup></td><td>0-2101</td><td>31 935</td><td>0-2102</td><td>31 022</td></tr><tr><td rowspan="5">(4) N<sub>2</sub>O</td><td><sup>14</sup>N<sup>16</sup>O</td><td>9.903 × 10<sup>-1</sup></td><td>0-7797</td><td>33 074</td><td>0-7797</td><td>33 265</td></tr><tr><td><sup>14</sup>N<sup>15</sup>N<sup>16</sup>O</td><td>3.641 × 10<sup>-3</sup></td><td>5-5086</td><td>4222</td><td>5-5086</td><td>4222</td></tr><tr><td><sup>15</sup>N<sup>14</sup>N<sup>16</sup>O</td><td>3.641 × 10<sup>-3</sup></td><td>4-4704</td><td>4592</td><td>4-4704</td><td>4592</td></tr><tr><td><sup>14</sup>N<sup>14</sup>N<sup>18</sup>O</td><td>1.986 × 10<sup>-3</sup></td><td>0-4672</td><td>116 694</td><td>0-10364</td><td>116 694</td></tr><tr><td><sup>14</sup>N<sup>17</sup>O</td><td>3.693 × 10<sup>-4</sup></td><td>550-4430</td><td>1705</td><td>550-4430</td><td>1705</td></tr><tr><td rowspan="5">(5) CO</td><td><sup>12</sup>C<sup>16</sup>O</td><td>9.865 × 10<sup>-1</sup></td><td>3-14 478</td><td>1344</td><td>3-14 478</td><td>1344<sup>c</sup></td></tr><tr><td><sup>13</sup>C<sup>16</sup>O</td><td>1.108 × 10<sup>-2</sup></td><td>3-12 231</td><td>1042</td><td>3-12 231</td><td>1042<sup>c</sup></td></tr><tr><td><sup>12</sup>C<sup>18</sup>O</td><td>1.978 × 10<sup>-3</sup></td><td>3-12 205</td><td>920</td><td>3-12 205</td><td>920<sup>c</sup></td></tr><tr><td><sup>12</sup>C<sup>17</sup>O</td><td>3.679 × 10<sup>-4</sup></td><td>3-10 295</td><td>800</td><td>3-10 295</td><td>800<sup>c</sup></td></tr><tr><td><sup>13</sup>C<sup>18</sup>O</td><td>2.223 × 10<sup>-5</sup></td><td>3-8078</td><td>674</td><td>3-8078</td><td>674<sup>c</sup></td></tr><tr><td rowspan="5">(6) CH<sub>4</sub></td><td><sup>12</sup>CH<sub>4</sub></td><td>9.883 × 10<sup>-1</sup></td><td>0-11 502</td><td>313 943</td><td>0-11 502</td><td>309 863</td></tr><tr><td><sup>13</sup>CH<sub>4</sub></td><td>1.110 × 10<sup>-2</sup></td><td>0-11 319</td><td>77 626</td><td>0-11 319</td><td>77 597</td></tr><tr><td><sup>12</sup>CH<sub>3</sub>D</td><td>6.158 × 10<sup>-4</sup></td><td>7-6511</td><td>54 076</td><td>7-6511</td><td>54 076</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>D</td><td>6.918 × 10<sup>-6</sup></td><td>959-1695</td><td>4213</td><td>959-1695</td><td>4213</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>D</td><td>9.953 × 10<sup>-1</sup></td><td>0-57 028</td><td>15 263</td><td>0-57 028</td><td>15 505</td></tr><tr><td rowspan="3">(7) O<sub>2</sub></td><td><sup>16</sup>O<sub>2</sub></td><td>3.991 × 10<sup>-3</sup></td><td>1-56 670</td><td>2965</td><td>1-56 670</td><td>2975</td></tr><tr><td><sup>16</sup>O<sup>18</sup>O</td><td>7.422 × 10<sup>-4</sup></td><td>0-14 537</td><td>11 313</td><td>0-14 538</td><td>11 313</td></tr><tr><td><sup>16</sup>O<sup>17</sup>O</td><td>9.940 × 10<sup>-1</sup></td><td>0-9273</td><td>103 701</td><td>0-23 727</td><td>251 898</td></tr><tr><td rowspan="3">(8) NO</td><td><sup>14</sup>N<sup>16</sup>O</td><td>3.654 × 10<sup>-3</sup></td><td>1609-2061</td><td>699</td><td>0-15 630</td><td>67 370</td></tr><tr><td><sup>15</sup>N<sup>16</sup>O</td><td>1.993 × 10<sup>-3</sup></td><td>1602-2039</td><td>679</td><td>0-15 503</td><td>65 037</td></tr><tr><td><sup>14</sup>N<sup>18</sup>O</td><td>9.940 × 10<sup>-1</sup></td><td>0-4092</td><td>72 460</td><td>0-4160</td><td>549 425</td></tr><tr><td rowspan="3">(9) SO<sub>2</sub></td><td><sup>32</sup>S<sup>16</sup>O<sub>2</sub></td><td>9.457 × 10<sup>-1</sup></td><td>0-2500</td><td>22 661</td><td>0-3465</td><td>141 665</td></tr><tr><td><sup>34</sup>S<sup>16</sup>O<sub>2</sub></td><td>4.195 × 10<sup>-2</sup></td><td>-</td><td>-</td><td>0-2625</td><td>75 785</td></tr><tr><td><sup>33</sup>S<sup>16</sup>O<sub>2</sub></td><td>7.464 × 10<sup>-3</sup></td><td>-</td><td>-</td><td>0-2793</td><td>208 183</td></tr><tr><td rowspan="2">(10) NO<sub>2</sub></td><td><sup>14</sup>N<sup>16</sup>O<sub>2</sub></td><td>9.916 × 10<sup>-1</sup></td><td>0-3075</td><td>104 223</td><td>0-7978</td><td>171 058</td></tr><tr><td><sup>15</sup>N<sup>16</sup>O<sub>2</sub></td><td>3.646 × 10<sup>-3</sup></td><td>-</td><td>-</td><td>1500-1660</td><td>5860</td></tr><tr><td rowspan="2">(11) NH<sub>3</sub></td><td><sup>14</sup>NH<sub>3</sub></td><td>9.959 × 10<sup>-1</sup></td><td>0-10 349</td><td>65 828</td><td>0-10 349</td><td>76 605</td></tr><tr><td><sup>15</sup>NH<sub>3</sub></td><td>3.661 × 10<sup>-3</sup></td><td>0-5180</td><td>1320</td><td>0-5180</td><td>13 791</td></tr><tr><td rowspan="2">(12) HNO<sub>3</sub></td><td><sup>14</sup>N<sup>16</sup>O<sub>3</sub></td><td>9.891 × 10<sup>-1</sup></td><td>0-1770</td><td>950 864</td><td>0-1770</td><td>950 864</td></tr><tr><td><sup>15</sup>N<sup>16</sup>O<sub>3</sub></td><td>3.636 × 10<sup>-3</sup></td><td>0-923</td><td>58 108</td><td>0-923</td><td>58 108</td></tr><tr><td rowspan="3">(13) OH</td><td><sup>16</sup>OH</td><td>9.975 × 10<sup>-1</sup></td><td>0-19 268</td><td>30 772</td><td>0-43 408</td><td>55 698</td></tr><tr><td><sup>18</sup>OH</td><td>2.000 × 10<sup>-3</sup></td><td>0-329</td><td>295</td><td>0-329</td><td>295</td></tr><tr><td><sup>16</sup>OD</td><td>1.554 × 10<sup>-4</sup></td><td>0-332</td><td>912</td><td>0-332</td><td>912</td></tr><tr><td rowspan="2">(14) HF</td><td><sup>19</sup>F</td><td>9.998 × 10<sup>-1</sup></td><td>24-32 351</td><td>8090</td><td>24-32 351</td><td>8090</td></tr><tr><td><sup>19</sup>F</td><td>1.557 × 10<sup>-4</sup></td><td>13-20 829</td><td>11 920</td><td>13-20 829</td><td>11 920</td></tr><tr><td rowspan="4">(15) HCl</td><td><sup>35</sup>Cl</td><td>7.576 × 10<sup>-1</sup></td><td>8-20 231</td><td>8892</td><td>8-20 231</td><td>8892</td></tr><tr><td><sup>37</sup>Cl</td><td>2.423 × 10<sup>-1</sup></td><td>8-20 218</td><td>8908</td><td>8-20 218</td><td>8908</td></tr><tr><td><sup>35</sup>Cl</td><td>1.180 × 10<sup>-4</sup></td><td>5-15 265</td><td>17 762</td><td>5-15 265</td><td>17 762</td></tr><tr><td><sup>37</sup>Cl</td><td>3.774 × 10<sup>-5</sup></td><td>5-15 246</td><td>17 690</td><td>5-15 246</td><td>17 690</td></tr><tr><td rowspan="4">(16) HBr</td><td><sup>79</sup>Br</td><td>5.068 × 10<sup>-1</sup></td><td>13-16 033</td><td>3039</td><td>13-16 033</td><td>3039</td></tr><tr><td><sup>81</sup>Br</td><td>4.931 × 10<sup>-1</sup></td><td>13-16 031</td><td>3031</td><td>13-16 031</td><td>3031</td></tr><tr><td><sup>79</sup>Br</td><td>7.894 × 10<sup>-5</sup></td><td>7-8780</td><td>1455</td><td>7-8780</td><td>1455</td></tr><tr><td><sup>81</sup>Br</td><td>7.680 × 10<sup>-5</sup></td><td>7-8777</td><td>1455</td><td>7-8777</td><td>1455</td></tr><tr><td rowspan="2">(17) HI</td><td><sup>127</sup>I</td><td>9.998 × 10<sup>-1</sup></td><td>10-13 907</td><td>3161</td><td>10-13 907</td><td>3161</td></tr><tr><td><sup>127</sup>I</td><td>1.557 × 10<sup>-4</sup></td><td>5-7625</td><td>1590</td><td>5-7625</td><td>1590</td></tr><tr><td rowspan="2">(18) ClO</td><td><sup>35</sup>Cl<sup>16</sup>O</td><td>7.559 × 10<sup>-1</sup></td><td>0-1208</td><td>5721</td><td>0-1208</td><td>5721</td></tr><tr><td><sup>37</sup>Cl<sup>16</sup>O</td><td>2.417 × 10<sup>-1</sup></td><td>0-1200</td><td>5780</td><td>0-1200</td><td>5780</td></tr><tr><td rowspan="2">(19) OCS</td><td><sup>16</sup>O<sup>12</sup>C<sup>32</sup>S</td><td>9.374 × 10<sup>-1</sup></td><td>0-7822</td><td>18 264</td><td>0-7822</td><td>21 776</td></tr><tr><td><sup>16</sup>O<sup>12</sup>C<sup>34</sup>S</td><td>4.158 × 10<sup>-2</sup></td><td>0-7796</td><td>6846</td><td>0-7796</td><td>7424</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1**
Molecules and isotopologues represented in the line-by-line portion of HITRAN.</caption><thead><tr><th rowspan="2">Molecule</th><th rowspan="2">Isotopologue</th><th rowspan="2">Abundance<sup>a</sup></th><th colspan="2">HITRAN2016</th><th colspan="2">HITRAN2020</th></tr><tr><th>Spectral Range<sup>b</sup></th><th># of lines</th><th>Spectral Range<sup>b</sup></th><th># of lines</th></tr></thead><tbody><tr><td rowspan="6">(1) H<sub>2</sub>O</td><td>H<sub>2</sub><sup>16</sup>O</td><td>9.973 &times; 10<sup>-1</sup></td><td>0-25 711</td><td>146 878</td><td>0-42 000</td><td>319 886</td></tr><tr><td>H<sub>2</sub><sup>18</sup>O</td><td>2.000 &times; 10<sup>-3</sup></td><td>0-19 918</td><td>39 903</td><td>0-19 992</td><td>42 178</td></tr><tr><td>H<sub>2</sub><sup>17</sup>O</td><td>3.719 &times; 10<sup>-4</sup></td><td>0-19 946</td><td>27 544</td><td>0-19 946</td><td>27 544<sup>c</sup></td></tr><tr><td>HD<sup>16</sup>O</td><td>3.107 &times; 10<sup>-4</sup></td><td>0-19 936</td><td>56 430</td><td>0-19 935</td><td>56 430</td></tr><tr><td>HD<sup>18</sup>O</td><td>6.230 &times; 10<sup>-7</sup></td><td>0-10 729</td><td>10 664</td><td>0-10 729</td><td>10 664</td></tr><tr><td>HD<sup>17</sup>O</td><td>1.159 &times; 10<sup>-7</sup></td><td>0-10 703</td><td>6360</td><td>0-10 703</td><td>6360</td></tr><tr><td rowspan="10">(2) CO<sub>2</sub></td><td>D<sub>2</sub><sup>16</sup>O</td><td>2.420 &times; 10<sup>-8</sup></td><td>0-12 797</td><td>23 488</td><td>0-12 797</td><td>23 195<sup>c</sup></td></tr><tr><td><sup>12</sup>C<sup>16</sup>O<sub>2</sub></td><td>9.842 &times; 10<sup>-1</sup></td><td>158-14 076</td><td>173 024</td><td>158-19 909</td><td>174 446</td></tr><tr><td><sup>13</sup>C<sup>16</sup>O<sub>2</sub></td><td>1.106 &times; 10<sup>-2</sup></td><td>332-13 735</td><td>70 577</td><td>332-13 735</td><td>69 870</td></tr><tr><td><sup>16</sup>O<sup>12</sup>C<sup>18</sup>O</td><td>3.947 &times; 10<sup>-3</sup></td><td>1-12 678</td><td>127 850</td><td>1-12 678</td><td>122 140</td></tr><tr><td><sup>16</sup>O<sup>12</sup>C<sup>17</sup>O</td><td>7.340 &times; 10<sup>-4</sup></td><td>0-12 727</td><td>77 941</td><td>0-12 727</td><td>73 942</td></tr><tr><td><sup>16</sup>O<sup>13</sup>C<sup>18</sup>O</td><td>4.434 &times; 10<sup>-5</sup></td><td>2-9213</td><td>43 782</td><td>2-9213</td><td>41 058</td></tr><tr><td><sup>16</sup>O<sup>13</sup>C<sup>17</sup>O</td><td>8.246 &times; 10<sup>-6</sup></td><td>9-8062</td><td>25 175</td><td>9-8062</td><td>23 607</td></tr><tr><td><sup>12</sup>C<sup>18</sup>O<sub>2</sub></td><td>3.957 &times; 10<sup>-6</sup></td><td>482-8163</td><td>10 522</td><td>482-8163</td><td>10 498</td></tr><tr><td><sup>17</sup>O<sup>12</sup>C<sup>18</sup>O</td><td>1.472 &times; 10<sup>-6</sup></td><td>491-8194</td><td>15 878</td><td>498-8194</td><td>14 623</td></tr><tr><td><sup>12</sup>C<sup>17</sup>O<sub>2</sub></td><td>1.368 &times; 10<sup>-7</sup></td><td>535-6933</td><td>6518</td><td>535-6933</td><td>6493</td></tr><tr><td><sup>13</sup>C<sup>18</sup>O<sub>2</sub></td><td>4.446 &times; 10<sup>-8</sup></td><td>2245-4751</td><td>2916</td><td>539-6687</td><td>2926</td></tr><tr><td><sup>18</sup>O<sup>13</sup>C<sup>17</sup>O</td><td>1.654 &times; 10<sup>-8</sup></td><td>549-4915</td><td>4190</td><td>549-4915</td><td>3980</td></tr><tr><td><sup>13</sup>C<sup>17</sup>O<sub>2</sub></td><td>1.538 &times; 10<sup>-9</sup></td><td>575-3615</td><td>1501</td><td>575-3615</td><td>1501</td></tr><tr><td rowspan="5">(3) O<sub>3</sub></td><td><sup>16</sup>O<sub>3</sub></td><td>9.929 &times; 10<sup>-1</sup></td><td>0-6997</td><td>289 340</td><td>0-6997</td><td>304 262</td></tr><tr><td><sup>16</sup>O<sup>16</sup>O<sup>18</sup>O</td><td>3.982 &times; 10<sup>-3</sup></td><td>0-2768</td><td>44 302</td><td>0-3165</td><td>57 907</td></tr><tr><td><sup>16</sup>O<sup>18</sup>O<sup>16</sup>O</td><td>1.991 &times; 10<sup>-3</sup></td><td>1-2740</td><td>18 887</td><td>1-2740</td><td>18 887</td></tr><tr><td><sup>16</sup>O<sup>16</sup>O<sup>17</sup>O</td><td>7.405 &times; 10<sup>-4</sup></td><td>0-2122</td><td>65 106</td><td>0-2122</td><td>65 467</td></tr><tr><td><sup>16</sup>O<sup>17</sup>O<sup>16</sup>O</td><td>3.702 &times; 10<sup>-4</sup></td><td>0-2101</td><td>31 935</td><td>0-2102</td><td>31 022</td></tr><tr><td rowspan="5">(4) N<sub>2</sub>O</td><td><sup>14</sup>N<sup>16</sup>O</td><td>9.903 &times; 10<sup>-1</sup></td><td>0-7797</td><td>33 074</td><td>0-7797</td><td>33 265</td></tr><tr><td><sup>14</sup>N<sup>15</sup>N<sup>16</sup>O</td><td>3.641 &times; 10<sup>-3</sup></td><td>5-5086</td><td>4222</td><td>5-5086</td><td>4222</td></tr><tr><td><sup>15</sup>N<sup>14</sup>N<sup>16</sup>O</td><td>3.641 &times; 10<sup>-3</sup></td><td>4-4704</td><td>4592</td><td>4-4704</td><td>4592</td></tr><tr><td><sup>14</sup>N<sup>14</sup>N<sup>18</sup>O</td><td>1.986 &times; 10<sup>-3</sup></td><td>0-4672</td><td>116 694</td><td>0-10364</td><td>116 694</td></tr><tr><td><sup>14</sup>N<sup>17</sup>O</td><td>3.693 &times; 10<sup>-4</sup></td><td>550-4430</td><td>1705</td><td>550-4430</td><td>1705</td></tr><tr><td rowspan="5">(5) CO</td><td><sup>12</sup>C<sup>16</sup>O</td><td>9.865 &times; 10<sup>-1</sup></td><td>3-14 478</td><td>1344</td><td>3-14 478</td><td>1344<sup>c</sup></td></tr><tr><td><sup>13</sup>C<sup>16</sup>O</td><td>1.108 &times; 10<sup>-2</sup></td><td>3-12 231</td><td>1042</td><td>3-12 231</td><td>1042<sup>c</sup></td></tr><tr><td><sup>12</sup>C<sup>18</sup>O</td><td>1.978 &times; 10<sup>-3</sup></td><td>3-12 205</td><td>920</td><td>3-12 205</td><td>920<sup>c</sup></td></tr><tr><td><sup>12</sup>C<sup>17</sup>O</td><td>3.679 &times; 10<sup>-4</sup></td><td>3-10 295</td><td>800</td><td>3-10 295</td><td>800<sup>c</sup></td></tr><tr><td><sup>13</sup>C<sup>18</sup>O</td><td>2.223 &times; 10<sup>-5</sup></td><td>3-8078</td><td>674</td><td>3-8078</td><td>674<sup>c</sup></td></tr><tr><td rowspan="5">(6) CH<sub>4</sub></td><td><sup>12</sup>CH<sub>4</sub></td><td>9.883 &times; 10<sup>-1</sup></td><td>0-11 502</td><td>313 943</td><td>0-11 502</td><td>309 863</td></tr><tr><td><sup>13</sup>CH<sub>4</sub></td><td>1.110 &times; 10<sup>-2</sup></td><td>0-11 319</td><td>77 626</td><td>0-11 319</td><td>77 597</td></tr><tr><td><sup>12</sup>CH<sub>3</sub>D</td><td>6.158 &times; 10<sup>-4</sup></td><td>7-6511</td><td>54 076</td><td>7-6511</td><td>54 076</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>D</td><td>6.918 &times; 10<sup>-6</sup></td><td>959-1695</td><td>4213</td><td>959-1695</td><td>4213</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>D</td><td>9.953 &times; 10<sup>-1</sup></td><td>0-57 028</td><td>15 263</td><td>0-57 028</td><td>15 505</td></tr><tr><td rowspan="3">(7) O<sub>2</sub></td><td><sup>16</sup>O<sub>2</sub></td><td>3.991 &times; 10<sup>-3</sup></td><td>1-56 670</td><td>2965</td><td>1-56 670</td><td>2975</td></tr><tr><td><sup>16</sup>O<sup>18</sup>O</td><td>7.422 &times; 10<sup>-4</sup></td><td>0-14 537</td><td>11 313</td><td>0-14 538</td><td>11 313</td></tr><tr><td><sup>16</sup>O<sup>17</sup>O</td><td>9.940 &times; 10<sup>-1</sup></td><td>0-9273</td><td>103 701</td><td>0-23 727</td><td>251 898</td></tr><tr><td rowspan="3">(8) NO</td><td><sup>14</sup>N<sup>16</sup>O</td><td>3.654 &times; 10<sup>-3</sup></td><td>1609-2061</td><td>699</td><td>0-15 630</td><td>67 370</td></tr><tr><td><sup>15</sup>N<sup>16</sup>O</td><td>1.993 &times; 10<sup>-3</sup></td><td>1602-2039</td><td>679</td><td>0-15 503</td><td>65 037</td></tr><tr><td><sup>14</sup>N<sup>18</sup>O</td><td>9.975 &times; 10<sup>-1</sup></td><td>0-4092</td><td>72 460</td><td>0-4160</td><td>549 425</td></tr><tr><td rowspan="3">(9) SO<sub>2</sub></td><td><sup>32</sup>S<sup>16</sup>O<sub>2</sub></td><td>4.195 &times; 10<sup>-2</sup></td><td>0-2500</td><td>22 661</td><td>0-3465</td><td>141 665</td></tr><tr><td><sup>34</sup>S<sup>16</sup>O<sub>2</sub></td><td>7.464 &times; 10<sup>-3</sup></td><td>-</td><td>-</td><td>0-2625</td><td>75 785</td></tr><tr><td><sup>33</sup>S<sup>16</sup>O<sub>2</sub></td><td>3.792 &times; 10<sup>-3</sup></td><td>-</td><td>-</td><td>0-2793</td><td>208 183</td></tr><tr><td rowspan="2">(10) NO<sub>2</sub></td><td><sup>14</sup>N<sup>16</sup>O<sub>2</sub></td><td>9.916 &times; 10<sup>-1</sup></td><td>0-3075</td><td>104 223</td><td>0-7978</td><td>171 058</td></tr><tr><td><sup>15</sup>N<sup>16</sup>O<sub>2</sub></td><td>3.646 &times; 10<sup>-3</sup></td><td>-</td><td>-</td><td>1500-1660</td><td>5860</td></tr><tr><td rowspan="2">(11) NH<sub>3</sub></td><td><sup>14</sup>NH<sub>3</sub></td><td>9.959 &times; 10<sup>-1</sup></td><td>0-10 349</td><td>65 828</td><td>0-10 349</td><td>76 605</td></tr><tr><td><sup>15</sup>NH<sub>3</sub></td><td>3.661 &times; 10<sup>-3</sup></td><td>0-5180</td><td>1320</td><td>0-5180</td><td>13 791</td></tr><tr><td rowspan="2">(12) HNO<sub>3</sub></td><td>H<sup>14</sup>N<sup>16</sup>O<sub>3</sub></td><td>9.891 &times; 10<sup>-1</sup></td><td>0-1770</td><td>950 864</td><td>0-1770</td><td>950 864</td></tr><tr><td>H<sup>15</sup>N<sup>16</sup>O<sub>3</sub></td><td>3.636 &times; 10<sup>-3</sup></td><td>0-923</td><td>58 108</td><td>0-923</td><td>58 108</td></tr><tr><td rowspan="3">(13) OH</td><td><sup>16</sup>OH</td><td>9.975 &times; 10<sup>-1</sup></td><td>0-19 268</td><td>30 772</td><td>0-43 408</td><td>55 698</td></tr><tr><td><sup>18</sup>OH</td><td>2.000 &times; 10<sup>-3</sup></td><td>0-329</td><td>295</td><td>0-329</td><td>295</td></tr><tr><td><sup>16</sup>OD</td><td>1.554 &times; 10<sup>-4</sup></td><td>0-332</td><td>912</td><td>0-332</td><td>912</td></tr><tr><td rowspan="2">(14) HF</td><td>H<sup>19</sup>F</td><td>9.998 &times; 10<sup>-1</sup></td><td>24-32 351</td><td>8090</td><td>24-32 351</td><td>8090</td></tr><tr><td>D<sup>19</sup>F</td><td>1.557 &times; 10<sup>-4</sup></td><td>13-20 829</td><td>11 920</td><td>13-20 829</td><td>11 920</td></tr><tr><td rowspan="4">(15) HCl</td><td>H<sup>35</sup>Cl</td><td>7.576 &times; 10<sup>-1</sup></td><td>8-20 231</td><td>8892</td><td>8-20 231</td><td>8892</td></tr><tr><td>H<sup>37</sup>Cl</td><td>2.423 &times; 10<sup>-1</sup></td><td>8-20 218</td><td>8908</td><td>8-20 218</td><td>8908</td></tr><tr><td>D<sup>35</sup>Cl</td><td>1.180 &times; 10<sup>-4</sup></td><td>5-15 265</td><td>17 762</td><td>5-15 265</td><td>17 762</td></tr><tr><td>D<sup>37</sup>Cl</td><td>3.774 &times; 10<sup>-5</sup></td><td>5-15 246</td><td>17 690</td><td>5-15 246</td><td>17 690</td></tr><tr><td rowspan="4">(16) HBr</td><td>H<sup>79</sup>Br</td><td>5.068 &times; 10<sup>-1</sup></td><td>13-16 033</td><td>3039</td><td>13-16 033</td><td>3039</td></tr><tr><td>H<sup>81</sup>Br</td><td>4.931 &times; 10<sup>-1</sup></td><td>13-16 031</td><td>3031</td><td>13-16 031</td><td>3031</td></tr><tr><td>D<sup>79</sup>Br</td><td>7.894 &times; 10<sup>-5</sup></td><td>7-8780</td><td>1455</td><td>7-8780</td><td>1455</td></tr><tr><td>D<sup>81</sup>Br</td><td>7.680 &times; 10<sup>-5</sup></td><td>7-8777</td><td>1455</td><td>7-8777</td><td>1455</td></tr><tr><td rowspan="2">(17) HI</td><td>H<sup>127</sup>I</td><td>9.998 &times; 10<sup>-1</sup></td><td>10-13 907</td><td>3161</td><td>10-13 907</td><td>3161</td></tr><tr><td>D<sup>127</sup>I</td><td>1.557 &times; 10<sup>-4</sup></td><td>5-7625</td><td>1590</td><td>5-7625</td><td>1590</td></tr><tr><td rowspan="2">(18) ClO</td><td><sup>35</sup>Cl<sup>16</sup>O</td><td>7.559 &times; 10<sup>-1</sup></td><td>0-1208</td><td>5721</td><td>0-1208</td><td>5721</td></tr><tr><td><sup>37</sup>Cl<sup>16</sup>O</td><td>2.417 &times; 10<sup>-1</sup></td><td>0-1200</td><td>5780</td><td>0-1200</td><td>5780</td></tr><tr><td rowspan="2">(19) OCS</td><td><sup>16</sup>O<sup>12</sup>C<sup>32</sup>S</td><td>9.374 &times; 10<sup>-1</sup></td><td>0-7822</td><td>18 264</td><td>0-7822</td><td>21 776</td></tr><tr><td><sup>16</sup>O<sup>12</sup>C<sup>34</sup>S</td><td>4.158 &times; 10<sup>-2</sup></td><td>0-7796</td><td>6846</td><td>0-7796</td><td>7424</td></tr></tbody></table>

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<table><caption>**Table 1 (continued)**</caption><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td><sup>16</sup>O<sup>13</sup>C<sup>32</sup>S</td><td>1.053×10<sup>-2</sup></td><td>0-6660</td><td>3275</td><td>0-6660</td><td>3395</td><td></td></tr><tr><td></td><td><sup>16</sup>O<sup>12</sup>C<sup>33</sup>S</td><td>7.399×10<sup>-3</sup></td><td>0-6631</td><td>3005</td><td>0-6632</td><td>3005</td><td></td></tr><tr><td></td><td><sup>18</sup>O<sup>12</sup>C<sup>32</sup>S</td><td>1.880×10<sup>-3</sup></td><td>0-4046</td><td>1640</td><td>0-4046</td><td>1640</td><td></td></tr><tr><td></td><td><sup>16</sup>O<sup>13</sup>C<sup>34</sup>S</td><td>4.675×10<sup>-4</sup></td><td>-</td><td>-</td><td>1951-2039</td><td>221</td><td></td></tr><tr><td>(20) H<sub>2</sub>CO</td><td>H<sub>2</sub><sup>12</sup>C<sup>16</sup>O</td><td>9.862×10<sup>-1</sup></td><td>0-3100</td><td>40 670</td><td>0-3100</td><td>40 670<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>13</sup>C<sup>16</sup>O</td><td>1.108×10<sup>-2</sup></td><td>0-117</td><td>2309</td><td>0-117</td><td>2309<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>12</sup>C<sup>18</sup>O</td><td>1.978×10<sup>-3</sup></td><td>0-101</td><td>1622</td><td>0-101</td><td>1622<sup>c</sup></td><td></td></tr><tr><td>(21) HOCl</td><td>H<sup>16</sup>O<sup>35</sup>Cl</td><td>7.558×10<sup>-1</sup></td><td>1-3800</td><td>8877</td><td>1-3800</td><td>8877</td><td></td></tr><tr><td></td><td>H<sup>16</sup>O<sup>37</sup>Cl</td><td>2.417×10<sup>-1</sup></td><td>1-3800</td><td>7399</td><td>1-3800</td><td>7399</td><td></td></tr><tr><td>(22) N<sub>2</sub></td><td><sup>14</sup>N<sub>2</sub></td><td>9.927×10<sup>-1</sup></td><td>11-9355</td><td>1107</td><td>11-9355</td><td>1107</td><td></td></tr><tr><td></td><td><sup>14</sup>N<sup>15</sup>N</td><td>7.478×10<sup>-3</sup></td><td>11-2578</td><td>161</td><td>11-2578</td><td>161</td><td></td></tr><tr><td>(23) HCN</td><td>H<sup>12</sup>C<sup>14</sup>N</td><td>9.851×10<sup>-1</sup></td><td>0-17 586</td><td>58 108</td><td>0-17 586</td><td>131 031</td><td></td></tr><tr><td></td><td>H<sup>13</sup>C<sup>14</sup>N</td><td>1.107×10<sup>-2</sup></td><td>2-3405</td><td>652</td><td>0-8000</td><td>22 599</td><td></td></tr><tr><td></td><td>H<sup>12</sup>C<sup>15</sup>N</td><td>3.622×10<sup>-3</sup></td><td>2-3420</td><td>646</td><td>2-3420</td><td>646</td><td></td></tr><tr><td>(24) CH<sub>3</sub>Cl</td><td><sup>12</sup>CH<sub>3</sub><sup>35</sup>Cl</td><td>7.489×10<sup>-1</sup></td><td>0-3198</td><td>110 462</td><td>0-3198</td><td>110 462</td><td></td></tr><tr><td></td><td><sup>12</sup>CH<sub>3</sub><sup>37</sup>Cl</td><td>2.395×10<sup>-1</sup></td><td>0-3198</td><td>109 113</td><td>0-3198</td><td>109 113</td><td></td></tr><tr><td>(25) H<sub>2</sub>O<sub>2</sub></td><td>H<sub>2</sub><sup>16</sup>O<sub>2</sub></td><td>9.950×10<sup>-1</sup></td><td>0-1731</td><td>126 983</td><td>0-1731</td><td>126 983</td><td></td></tr><tr><td>(26) C<sub>2</sub>H<sub>2</sub></td><td>C<sub>2</sub>H<sub>2</sub></td><td>9.776×10<sup>-1</sup></td><td>13-9890</td><td>22 866</td><td>13-10 737</td><td>74 335</td><td></td></tr><tr><td></td><td>H<sup>12</sup>C<sup>13</sup>CH</td><td>2.197×10<sup>-2</sup></td><td>613-6589</td><td>285</td><td>613-9857</td><td>2120</td><td></td></tr><tr><td></td><td>H<sup>12</sup>C<sup>12</sup>CD</td><td>3.046×10<sup>-4</sup></td><td>1-789</td><td>7512</td><td>1-789</td><td>7512</td><td></td></tr><tr><td>(27) C<sub>2</sub>H<sub>6</sub></td><td>C<sub>2</sub>H<sub>6</sub></td><td>9.770×10<sup>-1</sup></td><td>225-3001</td><td>54 460</td><td>225-3071</td><td>63 516</td><td></td></tr><tr><td></td><td>C<sub>2</sub>H<sub>3</sub><sup>13</sup>CH<sub>3</sub></td><td>2.195×10<sup>-2</sup></td><td>285-919</td><td>7107</td><td>285-919</td><td>7107</td><td></td></tr><tr><td></td><td>C<sub>2</sub>H<sub>5</sub>D</td><td>9.131×10<sup>-4</sup></td><td>-</td><td>-</td><td>681-3207</td><td>39 271</td><td></td></tr><tr><td>(28) PH<sub>3</sub></td><td><sup>31</sup>PH<sub>3</sub></td><td>9.995×10<sup>-1</sup></td><td>0-3602</td><td>22 190</td><td>0-3660</td><td>104 759</td><td></td></tr><tr><td>(29) COF<sub>2</sub></td><td><sup>12</sup>C<sup>16</sup>O<sup>19</sup>F<sub>2</sub></td><td>9.865×10<sup>-1</sup></td><td>725-2002</td><td>168 793</td><td>697-2002</td><td>168 793</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>16</sup>O<sup>19</sup>F<sub>2</sub></td><td>1.108×10<sup>-2</sup></td><td>686-815</td><td>15 311</td><td>687-815</td><td>15 311</td><td></td></tr><tr><td>(30) SF<sub>6</sub></td><td><sup>32</sup>S<sup>19</sup>F<sub>6</sub></td><td>9.502×10<sup>-1</sup></td><td>580-996</td><td>2 889 065</td><td>319-965</td><td>336 027</td><td></td></tr><tr><td>(31) H<sub>2</sub>S</td><td>H<sub>2</sub><sup>32</sup>S</td><td>9.499×10<sup>-1</sup></td><td>2-11 330</td><td>36 561</td><td>3-11 330</td><td>36 556<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>34</sup>S</td><td>4.214×10<sup>-2</sup></td><td>5-11 227</td><td>11 352</td><td>6-11 227</td><td>11 452<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>33</sup>S</td><td>7.498×10<sup>-3</sup></td><td>5-11 072</td><td>6322</td><td>6-11 072</td><td>6220<sup>c</sup></td><td></td></tr><tr><td>(32) HCOOH</td><td>H<sup>12</sup>C<sup>16</sup>O<sup>16</sup>OH</td><td>9.839×10<sup>-1</sup></td><td>10-1890</td><td>62 684</td><td>10-1890</td><td>187 596</td><td></td></tr><tr><td>(33) HO<sub>2</sub></td><td>H<sup>16</sup>O<sub>2</sub></td><td>9.951×10<sup>-1</sup></td><td>0-3676</td><td>38 804</td><td>0-3676</td><td>38 804</td><td></td></tr><tr><td>(34) O</td><td><sup>16</sup>O</td><td>9.976×10<sup>-1</sup></td><td>68-159</td><td>2</td><td>69-159</td><td>2</td><td></td></tr><tr><td>(35) ClON<sub>2</sub></td><td><sup>35</sup>Cl<sup>16</sup>O<sup>14</sup>N<sup>16</sup>O<sub>2</sub></td><td>7.496×10<sup>-1</sup></td><td>763-798</td><td>21 988</td><td>763-798</td><td>21 988</td><td></td></tr><tr><td></td><td><sup>37</sup>Cl<sup>16</sup>O<sup>14</sup>N<sup>16</sup>O<sub>2</sub></td><td>2.397×10<sup>-1</sup></td><td>765-791</td><td>10 211</td><td>765-791</td><td>10 211</td><td></td></tr><tr><td>(36) NO<sup>+</sup></td><td><sup>14</sup>N<sup>16</sup>O<sup>+</sup></td><td>9.940×10<sup>-1</sup></td><td>3-2531</td><td>1270</td><td>4-2531</td><td>1270</td><td></td></tr><tr><td>(37) HOBr</td><td>H<sup>16</sup>O<sup>79</sup>Br</td><td>5.056×10<sup>-1</sup></td><td>0-316</td><td>2177</td><td>0-316</td><td>2177</td><td></td></tr><tr><td></td><td>H<sup>16</sup>O<sup>81</sup>Br</td><td>4.919×10<sup>-1</sup></td><td>0-316</td><td>2181</td><td>0-316</td><td>2181</td><td></td></tr><tr><td>(38) C<sub>2</sub>H<sub>4</sub></td><td>C<sub>2</sub>H<sub>4</sub></td><td>9.773×10<sup>-1</sup></td><td>620-3243</td><td>59 536</td><td>620-3243</td><td>59 536</td><td></td></tr><tr><td></td><td>C<sub>2</sub>H<sub>2</sub><sup>13</sup>CH<sub>2</sub></td><td>2.196×10<sup>-2</sup></td><td>614-3181</td><td>18 095</td><td>615-3181</td><td>18 095</td><td></td></tr><tr><td>(39) CH<sub>3</sub>OH</td><td><sup>12</sup>CH<sub>3</sub><sup>16</sup>OH</td><td>9.859×10<sup>-1</sup></td><td>0-1408</td><td>19 897</td><td>0-1408</td><td>19 897</td><td></td></tr><tr><td>(40) CH<sub>3</sub>Br</td><td><sup>12</sup>CH<sub>3</sub><sup>79</sup>Br</td><td>5.010×10<sup>-1</sup></td><td>794-1706</td><td>18 692</td><td>794-1706</td><td>18 692</td><td></td></tr><tr><td></td><td><sup>12</sup>CH<sub>3</sub><sup>81</sup>Br</td><td>4.874×10<sup>-1</sup></td><td>796-1697</td><td>18 219</td><td>795-1697</td><td>18 219</td><td></td></tr><tr><td>(41) CH<sub>3</sub>CN</td><td><sup>12</sup>CH<sub>3</sub><sup>12</sup>C<sup>14</sup>N</td><td>9.739×10<sup>-1</sup></td><td>890-946</td><td>3572</td><td>890-946</td><td>3572</td><td></td></tr><tr><td>(42) CF<sub>4</sub></td><td><sup>12</sup>C<sup>19</sup>F<sub>4</sub></td><td>9.889×10<sup>-1</sup></td><td>582-1519</td><td>842 709</td><td>582-1519</td><td>842 709</td><td></td></tr><tr><td>(43) C<sub>4</sub>H<sub>2</sub></td><td>C<sub>4</sub>H<sub>2</sub></td><td>9.560×10<sup>-1</sup></td><td>0-1303</td><td>251 245</td><td>0-1303</td><td>251 245</td><td></td></tr><tr><td>(44) HC<sub>3</sub>N</td><td>H<sup>12</sup>C<sub>3</sub><sup>14</sup>N</td><td>9.633×10<sup>-1</sup></td><td>0-760</td><td>180 332</td><td>0-3361</td><td>248 273</td><td></td></tr><tr><td>(45) H<sub>2</sub></td><td>H<sub>2</sub></td><td>9.997×10<sup>-1</sup></td><td>15-27 185</td><td>3480</td><td>15-27 185</td><td>3480<sup>c</sup></td><td></td></tr><tr><td></td><td>HD</td><td>3.114×10<sup>-4</sup></td><td>3-36 406</td><td>5129</td><td>3-36 406</td><td>11 575</td><td></td></tr><tr><td>(46) CS</td><td><sup>12</sup>C<sup>32</sup>S</td><td>9.396×10<sup>-1</sup></td><td>1-2586</td><td>1088</td><td>1-2586</td><td>1088</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>34</sup>S</td><td>4.168×10<sup>-2</sup></td><td>1-1359</td><td>396</td><td>1-1359</td><td>396</td><td></td></tr><tr><td></td><td><sup>13</sup>C<sup>32</sup>S</td><td>1.056×10<sup>-2</sup></td><td>1-1331</td><td>396</td><td>1-1331</td><td>396</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>33</sup>S</td><td>7.417×10<sup>-3</sup></td><td>1-156</td><td>198</td><td>1-156</td><td>198</td><td></td></tr><tr><td>(47) SO<sub>3</sub></td><td><sup>32</sup>S<sup>16</sup>O<sub>3</sub></td><td>9.434×10<sup>-1</sup></td><td>0-2825</td><td>14 295</td><td>0-2825</td><td>14 295</td><td></td></tr><tr><td>(48) C<sub>2</sub>N<sub>2</sub></td><td>C<sub>2</sub><sup>14</sup>N<sub>2</sub></td><td>9.708×10<sup>-1</sup></td><td>200-307</td><td>71 775</td><td>200-307</td><td>71 775</td><td></td></tr><tr><td>(49) COCl<sub>2</sub></td><td><sup>12</sup>C<sup>16</sup>O<sup>35</sup>Cl<sub>2</sub></td><td>5.664×10<sup>-1</sup></td><td>793-900</td><td>164 437</td><td>793-900</td><td>164 437</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>16</sup>O<sup>35</sup>Cl<sup>37</sup>Cl</td><td>3.622×10<sup>-1</sup></td><td>800-892</td><td>145 477</td><td>800-892</td><td>145 477</td><td></td></tr><tr><td>(50) SO</td><td><sup>32</sup>S<sup>16</sup>O</td><td>9.479×10<sup>-1</sup></td><td>-</td><td>-</td><td>0-12 631</td><td>42 916</td><td></td></tr><tr><td></td><td><sup>34</sup>S<sup>16</sup>O</td><td>4.205×10<sup>-2</sup></td><td>-</td><td>-</td><td>0-372</td><td>671</td><td></td></tr><tr><td></td><td><sup>32</sup>S<sup>18</sup>O</td><td>1.901×10<sup>-3</sup></td><td>-</td><td>-</td><td>0-363</td><td>677</td><td></td></tr><tr><td>(51) CH<sub>3</sub>F</td><td><sup>12</sup>CH<sub>3</sub><sup>19</sup>F</td><td>9.884×10<sup>-1</sup></td><td>-</td><td>-</td><td>1067-1291</td><td>1499</td><td></td></tr><tr><td>(52) GeH<sub>4</sub></td><td><sup>74</sup>GeH<sub>4</sub></td><td>3.652×10<sup>-1</sup></td><td>-</td><td>-</td><td>648-2270</td><td>12 209</td><td></td></tr><tr><td></td><td><sup>72</sup>GeH<sub>4</sub></td><td>2.741×10<sup>-1</sup></td><td>-</td><td>-</td><td>649-2270</td><td>12 141</td><td></td></tr><tr><td></td><td><sup>70</sup>GeH<sub>4</sub></td><td>2.051×10<sup>-1</sup></td><td>-</td><td>-</td><td>649-2271</td><td>12 092</td><td></td></tr><tr><td></td><td><sup>73</sup>GeH<sub>4</sub></td><td>7.755×10<sup>-2</sup></td><td>-</td><td>-</td><td>649-2270</td><td>12 170</td><td></td></tr><tr><td></td><td><sup>76</sup>GeH<sub>4</sub></td><td>7.755×10<sup>-2</sup></td><td>-</td><td>-</td><td>648-2270</td><td>12 266</td><td></td></tr><tr><td>(53) CS<sub>2</sub></td><td><sup>12</sup>C<sup>32</sup>S<sub>2</sub></td><td>8.928×10<sup>-1</sup></td><td>-</td><td>-</td><td>23-6467</td><td>45 758</td><td></td></tr><tr><td></td><td><sup>32</sup>S<sup>12</sup>C<sup>34</sup>S</td><td>7.921×10<sup>-2</sup></td><td>-</td><td>-</td><td>196-4543</td><td>7237</td><td></td></tr><tr><td></td><td><sup>32</sup>S<sup>12</sup>C<sup>33</sup>S</td><td>1.409×10<sup>-2</sup></td><td>-</td><td>-</td><td>611-4567</td><td>3401</td><td></td></tr><tr><td></td><td><sup>13</sup>C<sup>32</sup>S<sub>2</sub></td><td>1.003×10<sup>-2</sup></td><td>-</td><td>-</td><td>1-4426</td><td>27 024</td><td></td></tr><tr><td>(54) CH<sub>3</sub>I</td><td><sup>12</sup>CH<sub>3</sub><sup>127</sup>I</td><td>9.884×10<sup>-1</sup></td><td>-</td><td>-</td><td>693-3274</td><td>178 247</td><td></td></tr><tr><td>(55) NF<sub>3</sub></td><td><sup>14</sup>N<sup>10</sup>F<sub>3</sub></td><td>9.963×10<sup>-1</sup></td><td>-</td><td>-</td><td>2-2201</td><td>2 717 795</td><td></td></tr></table>

<footnote>a Abundances are calculated from terrestrial atomic abundances in Ref. [57]. Line intensities in the HITRAN database have been scaled by these isotopologue abundances.</footnote>

<footnote>b Spectral ranges are given in cm⁻¹.</footnote>

<footnote>c Although spectral ranges and amount of lines is unchanged with respect to HITRAN2016, there are changes to spectral parameters of lines for these isotopologues.</footnote>
<table><caption>**Table 1 (continued)**</caption><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td><sup>16</sup>O<sup>13</sup>C<sup>32</sup>S</td><td>1.053×10<sup>-2</sup></td><td>0-6660</td><td>3275</td><td>0-6660</td><td>3395</td><td></td></tr><tr><td></td><td><sup>16</sup>O<sup>12</sup>C<sup>33</sup>S</td><td>7.399×10<sup>-3</sup></td><td>0-6631</td><td>3005</td><td>0-6632</td><td>3005</td><td></td></tr><tr><td></td><td><sup>18</sup>O<sup>12</sup>C<sup>32</sup>S</td><td>1.880×10<sup>-3</sup></td><td>0-4046</td><td>1640</td><td>0-4046</td><td>1640</td><td></td></tr><tr><td></td><td><sup>16</sup>O<sup>13</sup>C<sup>34</sup>S</td><td>4.675×10<sup>-4</sup></td><td>-</td><td>-</td><td>1951-2039</td><td>221</td><td></td></tr><tr><td>(20) H<sub>2</sub>CO</td><td>H<sub>2</sub><sup>12</sup>C<sup>16</sup>O</td><td>9.862×10<sup>-1</sup></td><td>0-3100</td><td>40 670</td><td>0-3100</td><td>40 670<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>13</sup>C<sup>16</sup>O</td><td>1.108×10<sup>-2</sup></td><td>0-117</td><td>2309</td><td>0-117</td><td>2309<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>12</sup>C<sup>18</sup>O</td><td>1.978×10<sup>-3</sup></td><td>0-101</td><td>1622</td><td>0-101</td><td>1622<sup>c</sup></td><td></td></tr><tr><td>(21) HOCl</td><td>H<sup>16</sup>O<sup>35</sup>Cl</td><td>7.558×10<sup>-1</sup></td><td>1-3800</td><td>8877</td><td>1-3800</td><td>8877</td><td></td></tr><tr><td></td><td>H<sup>16</sup>O<sup>37</sup>Cl</td><td>2.417×10<sup>-1</sup></td><td>1-3800</td><td>7399</td><td>1-3800</td><td>7399</td><td></td></tr><tr><td>(22) N<sub>2</sub></td><td><sup>14</sup>N<sub>2</sub></td><td>9.927×10<sup>-1</sup></td><td>11-9355</td><td>1107</td><td>11-9355</td><td>1107</td><td></td></tr><tr><td></td><td><sup>14</sup>N<sup>15</sup>N</td><td>7.478×10<sup>-3</sup></td><td>11-2578</td><td>161</td><td>11-2578</td><td>161</td><td></td></tr><tr><td>(23) HCN</td><td>H<sup>12</sup>C<sup>14</sup>N</td><td>9.851×10<sup>-1</sup></td><td>0-17 586</td><td>58 108</td><td>0-17 586</td><td>131 031</td><td></td></tr><tr><td></td><td>H<sup>13</sup>C<sup>14</sup>N</td><td>1.107×10<sup>-2</sup></td><td>2-3405</td><td>652</td><td>0-8000</td><td>22 599</td><td></td></tr><tr><td></td><td>H<sup>12</sup>C<sup>15</sup>N</td><td>3.622×10<sup>-3</sup></td><td>2-3420</td><td>646</td><td>2-3420</td><td>646</td><td></td></tr><tr><td>(24) CH<sub>3</sub>Cl</td><td><sup>12</sup>CH<sub>3</sub><sup>35</sup>Cl</td><td>7.489×10<sup>-1</sup></td><td>0-3198</td><td>110 462</td><td>0-3198</td><td>110 462</td><td></td></tr><tr><td></td><td><sup>12</sup>CH<sub>3</sub><sup>37</sup>Cl</td><td>2.395×10<sup>-1</sup></td><td>0-3198</td><td>109 113</td><td>0-3198</td><td>109 113</td><td></td></tr><tr><td>(25) H<sub>2</sub>O<sub>2</sub></td><td>H<sub>2</sub><sup>16</sup>O<sub>2</sub></td><td>9.950×10<sup>-1</sup></td><td>0-1731</td><td>126 983</td><td>0-1731</td><td>126 983</td><td></td></tr><tr><td>(26) C<sub>2</sub>H<sub>2</sub></td><td>C<sub>2</sub>H<sub>2</sub></td><td>9.776×10<sup>-1</sup></td><td>13-9890</td><td>22 866</td><td>13-10 737</td><td>74 335</td><td></td></tr><tr><td></td><td>H<sup>12</sup>C<sup>13</sup>CH</td><td>2.197×10<sup>-2</sup></td><td>613-6589</td><td>285</td><td>613-9857</td><td>2120</td><td></td></tr><tr><td></td><td>H<sup>12</sup>C<sup>12</sup>CD</td><td>3.046×10<sup>-4</sup></td><td>1-789</td><td>7512</td><td>1-789</td><td>7512</td><td></td></tr><tr><td>(27) C<sub>2</sub>H<sub>6</sub></td><td>C<sub>2</sub>H<sub>6</sub></td><td>9.770×10<sup>-1</sup></td><td>225-3001</td><td>54 460</td><td>225-3071</td><td>63 516</td><td></td></tr><tr><td></td><td>C<sub>2</sub>H<sub>3</sub><sup>13</sup>CH<sub>3</sub></td><td>2.195×10<sup>-2</sup></td><td>285-919</td><td>7107</td><td>285-919</td><td>7107</td><td></td></tr><tr><td></td><td>C<sub>2</sub>H<sub>5</sub>D</td><td>9.131×10<sup>-4</sup></td><td>-</td><td>-</td><td>681-3207</td><td>39 271</td><td></td></tr><tr><td>(28) PH<sub>3</sub></td><td><sup>31</sup>PH<sub>3</sub></td><td>9.995×10<sup>-1</sup></td><td>0-3602</td><td>22 190</td><td>0-3660</td><td>104 759</td><td></td></tr><tr><td>(29) COF<sub>2</sub></td><td><sup>12</sup>C<sup>16</sup>O<sup>19</sup>F<sub>2</sub></td><td>9.865×10<sup>-1</sup></td><td>725-2002</td><td>168 793</td><td>697-2002</td><td>168 793</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>16</sup>O<sup>19</sup>F<sub>2</sub></td><td>1.108×10<sup>-2</sup></td><td>686-815</td><td>15 311</td><td>687-815</td><td>15 311</td><td></td></tr><tr><td>(30) SF<sub>6</sub></td><td><sup>32</sup>S<sup>19</sup>F<sub>6</sub></td><td>9.502×10<sup>-1</sup></td><td>580-996</td><td>2 889 065</td><td>319-965</td><td>336 027</td><td></td></tr><tr><td>(31) H<sub>2</sub>S</td><td>H<sub>2</sub><sup>32</sup>S</td><td>9.499×10<sup>-1</sup></td><td>2-11 330</td><td>36 561</td><td>3-11 330</td><td>36 556<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>34</sup>S</td><td>4.214×10<sup>-2</sup></td><td>5-11 227</td><td>11 352</td><td>6-11 227</td><td>11 452<sup>c</sup></td><td></td></tr><tr><td></td><td>H<sub>2</sub><sup>33</sup>S</td><td>7.498×10<sup>-3</sup></td><td>5-11 072</td><td>6322</td><td>6-11 072</td><td>6220<sup>c</sup></td><td></td></tr><tr><td>(32) HCOOH</td><td>H<sup>12</sup>C<sup>16</sup>O<sup>16</sup>OH</td><td>9.839×10<sup>-1</sup></td><td>10-1890</td><td>62 684</td><td>10-1890</td><td>187 596</td><td></td></tr><tr><td>(33) HO<sub>2</sub></td><td>H<sup>16</sup>O<sub>2</sub></td><td>9.951×10<sup>-1</sup></td><td>0-3676</td><td>38 804</td><td>0-3676</td><td>38 804</td><td></td></tr><tr><td>(34) O</td><td><sup>16</sup>O</td><td>9.976×10<sup>-1</sup></td><td>68-159</td><td>2</td><td>69-159</td><td>2</td><td></td></tr><tr><td>(35) ClON<sub>2</sub></td><td><sup>35</sup>Cl<sup>16</sup>O<sup>14</sup>N<sup>16</sup>O<sub>2</sub></td><td>7.496×10<sup>-1</sup></td><td>763-798</td><td>21 988</td><td>763-798</td><td>21 988</td><td></td></tr><tr><td></td><td><sup>37</sup>Cl<sup>16</sup>O<sup>14</sup>N<sup>16</sup>O<sub>2</sub></td><td>2.397×10<sup>-1</sup></td><td>765-791</td><td>10 211</td><td>765-791</td><td>10 211</td><td></td></tr><tr><td>(36) NO<sup>+</sup></td><td><sup>14</sup>N<sup>16</sup>O<sup>+</sup></td><td>9.940×10<sup>-1</sup></td><td>3-2531</td><td>1270</td><td>4-2531</td><td>1270</td><td></td></tr><tr><td>(37) HOBr</td><td>H<sup>16</sup>O<sup>79</sup>Br</td><td>5.056×10<sup>-1</sup></td><td>0-316</td><td>2177</td><td>0-316</td><td>2177</td><td></td></tr><tr><td></td><td>H<sup>16</sup>O<sup>81</sup>Br</td><td>4.919×10<sup>-1</sup></td><td>0-316</td><td>2181</td><td>0-316</td><td>2181</td><td></td></tr><tr><td>(38) C<sub>2</sub>H<sub>4</sub></td><td>C<sub>2</sub>H<sub>4</sub></td><td>9.773×10<sup>-1</sup></td><td>620-3243</td><td>59 536</td><td>620-3243</td><td>59 536</td><td></td></tr><tr><td></td><td>C<sub>2</sub>H<sub>2</sub><sup>13</sup>CH<sub>2</sub></td><td>2.196×10<sup>-2</sup></td><td>614-3181</td><td>18 095</td><td>615-3181</td><td>18 095</td><td></td></tr><tr><td>(39) CH<sub>3</sub>OH</td><td><sup>12</sup>CH<sub>3</sub><sup>16</sup>OH</td><td>9.859×10<sup>-1</sup></td><td>0-1408</td><td>19 897</td><td>0-1408</td><td>19 897</td><td></td></tr><tr><td>(40) CH<sub>3</sub>Br</td><td><sup>12</sup>CH<sub>3</sub><sup>79</sup>Br</td><td>5.010×10<sup>-1</sup></td><td>794-1706</td><td>18 692</td><td>794-1706</td><td>18 692</td><td></td></tr><tr><td></td><td><sup>12</sup>CH<sub>3</sub><sup>81</sup>Br</td><td>4.874×10<sup>-1</sup></td><td>796-1697</td><td>18 219</td><td>795-1697</td><td>18 219</td><td></td></tr><tr><td>(41) CH<sub>3</sub>CN</td><td><sup>12</sup>CH<sub>3</sub><sup>12</sup>C<sup>14</sup>N</td><td>9.739×10<sup>-1</sup></td><td>890-946</td><td>3572</td><td>890-946</td><td>3572</td><td></td></tr><tr><td>(42) CF<sub>4</sub></td><td><sup>12</sup>C<sup>19</sup>F<sub>4</sub></td><td>9.889×10<sup>-1</sup></td><td>582-1519</td><td>842 709</td><td>582-1519</td><td>842 709</td><td></td></tr><tr><td>(43) C<sub>4</sub>H<sub>2</sub></td><td>C<sub>4</sub>H<sub>2</sub></td><td>9.560×10<sup>-1</sup></td><td>0-1303</td><td>251 245</td><td>0-1303</td><td>251 245</td><td></td></tr><tr><td>(44) HC<sub>3</sub>N</td><td>H<sup>12</sup>C<sub>3</sub><sup>14</sup>N</td><td>9.633×10<sup>-1</sup></td><td>0-760</td><td>180 332</td><td>0-3361</td><td>248 273</td><td></td></tr><tr><td>(45) H<sub>2</sub></td><td>H<sub>2</sub></td><td>9.997×10<sup>-1</sup></td><td>15-27 185</td><td>3480</td><td>15-27 185</td><td>3480<sup>c</sup></td><td></td></tr><tr><td></td><td>HD</td><td>3.114×10<sup>-4</sup></td><td>3-36 406</td><td>5129</td><td>3-36 406</td><td>11 575</td><td></td></tr><tr><td>(46) CS</td><td><sup>12</sup>C<sup>32</sup>S</td><td>9.396×10<sup>-1</sup></td><td>1-2586</td><td>1088</td><td>1-2586</td><td>1088</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>34</sup>S</td><td>4.168×10<sup>-2</sup></td><td>1-1359</td><td>396</td><td>1-1359</td><td>396</td><td></td></tr><tr><td></td><td><sup>13</sup>C<sup>32</sup>S</td><td>1.056×10<sup>-2</sup></td><td>1-1331</td><td>396</td><td>1-1331</td><td>396</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>33</sup>S</td><td>7.417×10<sup>-3</sup></td><td>1-156</td><td>198</td><td>1-156</td><td>198</td><td></td></tr><tr><td>(47) SO<sub>3</sub></td><td><sup>32</sup>S<sup>16</sup>O<sub>3</sub></td><td>9.434×10<sup>-1</sup></td><td>0-2825</td><td>14 295</td><td>0-2825</td><td>14 295</td><td></td></tr><tr><td>(48) C<sub>2</sub>N<sub>2</sub></td><td>C<sub>2</sub><sup>14</sup>N<sub>2</sub></td><td>9.708×10<sup>-1</sup></td><td>200-307</td><td>71 775</td><td>200-307</td><td>71 775</td><td></td></tr><tr><td>(49) COCl<sub>2</sub></td><td><sup>12</sup>C<sup>16</sup>O<sup>35</sup>Cl<sub>2</sub></td><td>5.664×10<sup>-1</sup></td><td>793-900</td><td>164 437</td><td>793-900</td><td>164 437</td><td></td></tr><tr><td></td><td><sup>12</sup>C<sup>16</sup>O<sup>35</sup>Cl<sup>37</sup>Cl</td><td>3.622×10<sup>-1</sup></td><td>800-892</td><td>145 477</td><td>800-892</td><td>145 477</td><td></td></tr><tr><td>(50) SO</td><td><sup>32</sup>S<sup>16</sup>O</td><td>9.479×10<sup>-1</sup></td><td>-</td><td>-</td><td>0-12 631</td><td>42 916</td><td></td></tr><tr><td></td><td><sup>34</sup>S<sup>16</sup>O</td><td>4.205×10<sup>-2</sup></td><td>-</td><td>-</td><td>0-372</td><td>671</td><td></td></tr><tr><td></td><td><sup>32</sup>S<sup>18</sup>O</td><td>1.901×10<sup>-3</sup></td><td>-</td><td>-</td><td>0-363</td><td>677</td><td></td></tr><tr><td>(51) CH<sub>3</sub>F</td><td><sup>12</sup>CH<sub>3</sub><sup>19</sup>F</td><td>9.884×10<sup>-1</sup></td><td>-</td><td>-</td><td>1067-1291</td><td>1499</td><td></td></tr><tr><td>(52) GeH<sub>4</sub></td><td><sup>74</sup>GeH<sub>4</sub></td><td>3.652×10<sup>-1</sup></td><td>-</td><td>-</td><td>648-2270</td><td>12 209</td><td></td></tr><tr><td></td><td><sup>72</sup>GeH<sub>4</sub></td><td>2.741×10<sup>-1</sup></td><td>-</td><td>-</td><td>649-2270</td><td>12 141</td><td></td></tr><tr><td></td><td><sup>70</sup>GeH<sub>4</sub></td><td>2.051×10<sup>-1</sup></td><td>-</td><td>-</td><td>649-2271</td><td>12 092</td><td></td></tr><tr><td></td><td><sup>73</sup>GeH<sub>4</sub></td><td>7.755×10<sup>-2</sup></td><td>-</td><td>-</td><td>649-2270</td><td>12 170</td><td></td></tr><tr><td></td><td><sup>76</sup>GeH<sub>4</sub></td><td>7.755×10<sup>-2</sup></td><td>-</td><td>-</td><td>648-2270</td><td>12 266</td><td></td></tr><tr><td>(53) CS<sub>2</sub></td><td><sup>12</sup>C<sup>32</sup>S<sub>2</sub></td><td>8.928×10<sup>-1</sup></td><td>-</td><td>-</td><td>23-6467</td><td>45 758</td><td></td></tr><tr><td></td><td><sup>32</sup>S<sup>12</sup>C<sup>34</sup>S</td><td>7.921×10<sup>-2</sup></td><td>-</td><td>-</td><td>196-4543</td><td>7237</td><td></td></tr><tr><td></td><td><sup>32</sup>S<sup>12</sup>C<sup>33</sup>S</td><td>1.409×10<sup>-2</sup></td><td>-</td><td>-</td><td>611-4567</td><td>3401</td><td></td></tr><tr><td></td><td><sup>13</sup>C<sup>32</sup>S<sub>2</sub></td><td>1.003×10<sup>-2</sup></td><td>-</td><td>-</td><td>1-4426</td><td>27 024</td><td></td></tr><tr><td>(54) CH<sub>3</sub>I</td><td><sup>12</sup>CH<sub>3</sub><sup>127</sup>I</td><td>9.884×10<sup>-1</sup></td><td>-</td><td>-</td><td>693-3274</td><td>178 247</td><td></td></tr><tr><td>(55) NF<sub>3</sub></td><td><sup>14</sup>N<sup>10</sup>F<sub>3</sub></td><td>9.963×10<sup>-1</sup></td><td>-</td><td>-</td><td>2-2201</td><td>2 717 795</td><td></td></tr></table>

<footnote>a Abundances are calculated from terrestrial atomic abundances in Ref. [57]. Line intensities in the HI-TRAN database have been scaled by these isotopologue abundances.</footnote>

<footnote>b Spectral ranges are given in cm⁻¹.</footnote>

<footnote>c Although spectral ranges and amount of lines is unchanged with respect to HITRAN2016, there are changes to spectral parameters of lines for these isotopologues.</footnote>
<caption>**Table 2**</caption>

The uncertainty codes used by the HITRAN database (as presented in HITRANonline) are based on Table 5 of the HITRAN2004 paper [13]. There are two types of uncertainty code corresponding to absolute uncertainty in cm⁻¹ (used for the line position and pressure-induced line shift parameters) and relative uncertainty in % (used for the line intensity and line-shape parameters).

<table><thead><tr><th>Code</th><th>Absolute uncertainty range</th><th>Code</th><th>Relative uncertainty range</th></tr></thead><tbody><tr><td>0</td><td>≥ 1 or Unreported</td><td>0</td><td>Unreported or unavailable</td></tr><tr><td>1</td><td>≥ 0.1 and &lt; 1</td><td>1</td><td>Default or constant</td></tr><tr><td>2</td><td>≥ 0.01 and &lt; 0.1</td><td>2</td><td>Average or estimate</td></tr><tr><td>3</td><td>≥ 0.001 and &lt; 0.01</td><td>3</td><td>≥ 20%</td></tr><tr><td>4</td><td>≥ 0.0001 and &lt; 0.001</td><td>4</td><td>≥ 10% and &lt; 20%</td></tr><tr><td>5</td><td>≥ 0.00001 and &lt; 0.0001</td><td>5</td><td>≥ 5% and &lt; 10%</td></tr><tr><td>6</td><td>≥ 0.000001 and &lt; 0.00001</td><td>6</td><td>≥ 2% and &lt; 5%</td></tr><tr><td>7</td><td>≥ 0.0000001 and &lt; 0.000001</td><td>7</td><td>≥ 1% and &lt; 2%</td></tr><tr><td>8</td><td>≥ 0.00000001 and &lt; 0.0000001</td><td>8</td><td>&lt; 1%</td></tr><tr><td>9</td><td>≥ 0.000000001 and &lt; 0.00000001</td><td></td><td></td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** A flowchart describing the development of the HITRAN2020 line list for the principal isotopologue of water vapor. The Mikhailenko et al. [91] line list is abbreviated to "20MiKaMoCa" within the chart shown above.</figcaption></figure>
<caption>**Table 2**</caption>

The uncertainty codes used by the HITRAN database (as presented in HITRANonline) are based on Table 5 of the HITRAN2004 paper [13]. There are two types of uncertainty code corresponding to absolute uncertainty in cm⁻¹ (used for the line position and pressure-induced line shift parameters) and relative uncertainty in % (used for the line intensity and line-shape parameters).

<table><thead><tr><th>Code</th><th>Absolute uncertainty range</th><th>Code</th><th>Relative uncertainty range</th></tr></thead><tbody><tr><td>0</td><td>≥ 1 or Unreported</td><td>0</td><td>Unreported or unavailable</td></tr><tr><td>1</td><td>≥ 0.1 and &lt; 1</td><td>1</td><td>Default or constant</td></tr><tr><td>2</td><td>≥ 0.01 and &lt; 0.1</td><td>2</td><td>Average or estimate</td></tr><tr><td>3</td><td>≥ 0.001 and &lt; 0.01</td><td>3</td><td>≥ 20%</td></tr><tr><td>4</td><td>≥ 0.0001 and &lt; 0.001</td><td>4</td><td>≥ 10% and &lt; 20%</td></tr><tr><td>5</td><td>≥ 0.00001 and &lt; 0.0001</td><td>5</td><td>≥ 5% and &lt; 10%</td></tr><tr><td>6</td><td>≥ 0.000001 and &lt; 0.00001</td><td>6</td><td>≥ 2% and &lt; 5%</td></tr><tr><td>7</td><td>≥ 0.0000001 and &lt; 0.000001</td><td>7</td><td>≥ 1% and &lt; 2%</td></tr><tr><td>8</td><td>≥ 0.00000001 and &lt; 0.0000001</td><td>8</td><td>&lt; 1%</td></tr><tr><td>9</td><td>≥ 0.000000001 and &lt; 0.00000001</td><td></td><td></td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** A flowchart describing the development of the HITRAN2020 line list for the principal isotopologue of water vapor. The Mikhailenko et al. [91] line list is abbreviated to "20MiKaMoCa" within the chart shown above.</figcaption></figure>
ample, frequency-comb and Lamb-dip techniques. The MARVEL (Measured Active Rotational-Vibrational Energy Levels) methodology [58,59] utilizes the framework provided by spectroscopic networks [94] and high-quality experimental measurements of line positions. Highly-accurate experimental measurements of line positions [95] were utilized during the latest MARVEL attempts to analyze water spectra. These accurate measurements can simultaneously improve the accuracy of the majority of energy levels [95] involved in connected transitions. Furtenbacher et al. released a much improved set of highly accurate $H_2^{16}O$ energy levels [96,97], named the "W2020" dataset, containing 19,225 empirical energy levels derived from 286,987 non-redundant experimental ro-vibrational transitions. Utilizing the labels and the transition wavenumbers of the W2020 set the information in the *ab initio* line list was updated. Conway et al. [98] recently applied the Hose-Taylor theorem [99] to theoretical spectroscopy and showed that the projection of the total angular momentum (J) onto the body fixed axis (k) can be considered a good quantum number once the square of the projected component's wave-function amplitude is greater than one-half ($\Psi_k^2 > 0.5$). The theory was tested using the DVR3D [100] nuclear motion code on the water and ozone molecules. The asymmetric-top quantum numbers $K_a$ and $K_c$ were determined to a high degree of fidelity up to dissociation. We used these results to label states in the visible and near ultraviolet where MARVEL does not yield this information.

Mikhailenko et al. [91] also released an empirical line list that spans 5690–8340 cm⁻¹, and we compared their results to the *ab initio* calculations of Conway et al. [85]. It was found that high overtones of $\nu_2$, notably $3\nu_2$ and $4\nu_2$, were not accurately represented in the new *ab initio* calculations. Hence, for these bands, we replaced the calculated results of Conway et al. with what was already present in HITRAN2016. We also detected a small number of discrepancies in the transition frequencies created by the latest empirical (MARVEL) energy levels to those present in the empirical line list. In such cases, we chose the source that has the lowest uncertainty associated with it and also considered the type of experiment used to derive the value in the empirical line list.

In the next step, we focused on overwriting the *ab initio* intensities with high-quality experimental measurements wherever it was deemed appropriate. Experimental measurements in the IR which were performed at the Deutsches Zentrum für Luft und Raumfahrt (DLR) [66,102,103] were analyzed by Birk et al. [104] and deficiencies were observed in the *ab initio* data of Lodi et al. [84], particularly in the $\nu_1$ band. The same deficiencies in $\nu_1$ are present in the Conway et al. *ab initio* data but improvements are evident in many other bands [85], especially for those near 1 µm. Differences between the experimental [66] and the new *ab initio* data were found in the relative intensities for $\nu_2$ (ca. 1%) and $2\nu_2$ (ca. 1.5%). The differences occur in the upper wavenumber range of the bands and increase with wavenumber. In the case of the $\nu_3$ fundamental, the average difference changes from 0.18% with the calculation of Lodi et al. [84] to -1.08% with the *ab initio* data of Conway et al. [85]. The $3\nu_3$ band also showed similar behavior, with the mean residuals increasing from -0.7% using the Lodi et al. [84] line list to -3.9%. The latter one is the only band in the 1-µm region where no improvement was observed. Utilizing the high-quality measured intensity data, scaling factors were derived for the *ab initio* data from Conway et al. in the corresponding bands. These factors were then applied to scale the *ab initio* intensities for all lines in these bands, with an intention to improve the accuracy of the transitions (in these bands) where no measurement is available. Further investigations indicated that the underlying electronic structure calculations were the culprit of such irregularities [105] and Conway et al. [85] showed for the $\nu_2$ fundamental how changing the spectrum-fitting profile can result in different transition parameters – in particular, intensities.

All experimental intensity measurements present in the HITRAN2016 line list, with the exception of the SISAM (Spectromètre Interférentiel à Sélection par l'Amplitude de la Modulation) data [61] (excluding any $3\nu_2$ and $4\nu_2$ measurements), are transcribed into HITRAN2020 [62–72]. Comparisons [85] between the new *ab initio* intensities and the intensity measurements within the SISAM data set in the infrared region exhibited a skewed appearance. This is not the first instance of such a structure being attributed to Kitt Peak water-vapor spectra [63,104,106]. If we detected an experimentally determined value for an intensity in the empirical list of Mikhailenko et al. [91], the *ab initio* intensity was overwritten by the experimental datum.

For the first time, electric quadrupole (E2) transitions for the water molecule were identified in experimental spectra, made possible by the availability of an E2 *ab initio* line list [107,108]. The E2 transitions have now been added to the HITRAN2020 line list (only for the principal isotopologue). This room temperature E2 line list for $H_2^{16}O$ ranges from 0 to 10,000 cm⁻¹ and contains 6227 lines with intensities stronger than $10^{-30}$ cm/molecule. It was generated using MARVEL line positions and lower-state energies and theoretical transition intensities computed using a high-level *ab initio* electric quadrupole moment and a state-of-the-art variational approach [109,110]. The character in the last field of lower-state rotational ("local") quanta (i.e. preceding the error code for the line positions) in the traditional 160 character ".par" format will carry a label "q" to denote these transitions (see the Supplementary Material of this paper for the description of the upper- and lower-state quanta in the ".par" format). It should be noted that E2 type transitions are typically 6–8 orders of magnitude weaker than electric dipole transitions [107].

The HITRAN2016 line list included a large number of highly-accurate transition wavenumber measurements from the SISAM data set. Comparisons of these data to derived MARVEL transition frequencies indicated that the transition wavenumbers derived from the most recent version of the MARVEL data [97] match the SISAM values to within their uncertainty. The predicted MARVEL line positions are therefore selected as the preferable source of data because, apart from their validated accuracy, they also would provide consistency throughout the database. Hence, where possible, we overwrite the *ab initio* energy levels and transition frequencies with the MARVEL data.

The accuracy of the HITRAN2020 line list in the visible region is significantly better than that of its predecessor, HITRAN2016. Harder et al. [101] reported water-vapor absorption spectra in the 22,100–22,700 cm⁻¹ interval through atmospheric observations. Using the HITRAN Application Programming Interface (HAPI) [52], we have generated cross sections at a temperature of 288 K using the Voigt profile at a resolution of 0.03 cm⁻¹. In Fig. 2, it becomes clear that the HITRAN2020 line list is significantly better suited for retrieving water in the visible region. It has already been reported that the HITRAN2016 water line list has spectroscopic errors in the visible range [111] and the HITRAN2020 edition offers substantial improvements. These errors were attributed to a collection of irregularities in line shape parameters and inaccurate *ab initio* transition intensities.

To extend this comparison between HITRAN2020 and HITRAN2016 in the visible between 9000 and 20 000 cm⁻¹, we generated two telluric models using the Planetary Spectrum Generator (PSG) [37], each generated with the two respective line lists, and compared these models to a high SNR telluric spectrum extracted from solar observations detailed in Baker et al. [77]. The atmospheric profile used to generate the model was evaluated for the appropriate location and elevation of the observation site in Göttingen, Germany as well as the corresponding observation time of June 17, 2015 UTC 10:50:30.5. The water-vapor abundance and surface pressure were adjusted by fitting the model to a subset
ample, frequency-comb and Lamb-dip techniques. The MARVEL (Measured Active Rotational-Vibrational Energy Levels) methodology [58,59] utilizes the framework provided by spectroscopic networks [94] and high-quality experimental measurements of line positions. Highly-accurate experimental measurements of line positions [95] were utilized during the latest MARVEL attempts to analyze water spectra. These accurate measurements can simultaneously improve the accuracy of the majority of energy levels [95] involved in connected transitions. Furtenbacher et al. released a much improved set of highly accurate $H_2^{16}O$ energy levels [96,97], named the "W2020" dataset, containing 19,225 empirical energy levels derived from 286,987 non-redundant experimental ro-vibrational transitions. Utilizing the labels and the transition wavenumbers of the W2020 set the information in the *ab initio* line list was updated. Conway et al. [98] recently applied the Hose-Taylor theorem [99] to theoretical spectroscopy and showed that the projection of the total angular momentum (J) onto the body fixed axis (k) can be considered a good quantum number once the square of the projected component's wave-function amplitude is greater than one-half ($\Psi_k^2 > 0.5$). The theory was tested using the DVR3D [100] nuclear motion code on the water and ozone molecules. The asymmetric-top quantum numbers $K_a$ and $K_c$ were determined to a high degree of fidelity up to dissociation. We used these results to label states in the visible and near ultraviolet where MARVEL does not yield this information.

Mikhailenko et al. [91] also released an empirical line list that spans 5690–8340 cm⁻¹, and we compared their results to the *ab initio* calculations of Conway et al. [85]. It was found that high overtones of $\nu_2$, notably $3\nu_2$ and $4\nu_2$, were not accurately represented in the new *ab initio* calculations. Hence, for these bands, we replaced the calculated results of Conway et al. with what was already present in HITRAN2016. We also detected a small number of discrepancies in the transition frequencies created by the latest empirical (MARVEL) energy levels to those present in the empirical line list. In such cases, we chose the source that has the lowest uncertainty associated with it and also considered the type of experiment used to derive the value in the empirical line list.

In the next step, we focused on overwriting the *ab initio* intensities with high-quality experimental measurements wherever it was deemed appropriate. Experimental measurements in the IR which were performed at the Deutsches Zentrum für Luft und Raumfahrt (DLR) [66,102,103] were analyzed by Birk et al. [104] and deficiencies were observed in the *ab initio* data of Lodi et al. [84], particularly in the $\nu_1$ band. The same deficiencies in $\nu_1$ are present in the Conway et al. *ab initio* data but improvements are evident in many other bands [85], especially for those near 1 µm. Differences between the experimental [66] and the new *ab initio* data were found in the relative intensities for $\nu_2$ (ca. 1%) and $2\nu_2$ (ca. 1.5%). The differences occur in the upper wavenumber range of the bands and increase with wavenumber. In the case of the $\nu_3$ fundamental, the average difference changes from 0.18% with the calculation of Lodi et al. [84] to -1.08% with the *ab initio* data of Conway et al. [85]. The $3\nu_3$ band also showed similar behavior, with the mean residuals increasing from -0.7% using the Lodi et al. [84] line list to -3.9%. The latter one is the only band in the 1-µm region where no improvement was observed. Utilizing the high-quality measured intensity data, scaling factors were derived for the *ab initio* data from Conway et al. in the corresponding bands. These factors were then applied to scale the *ab initio* intensities for all lines in these bands, with an intention to improve the accuracy of the transitions (in these bands) where no measurement is available. Further investigations indicated that the underlying electronic structure calculations were the culprit of such irregularities [105] and Conway et al. [85] showed for the $\nu_2$ fundamental how changing the spectrum-fitting profile can result in different transition parameters – in particular, intensities.

All experimental intensity measurements present in the HITRAN2016 line list, with the exception of the SISAM (Spectromètre Interférentiel à Sélection par l'Amplitude de la Modulation) data [61] (excluding any $3\nu_2$ and $4\nu_2$ measurements), are transcribed into HITRAN2020 [62–72]. Comparisons [85] between the new *ab initio* intensities and the intensity measurements within the SISAM data set in the infrared region exhibited a skewed appearance. This is not the first instance of such a structure being attributed to Kitt Peak water-vapor spectra [63,104,106]. If we detected an experimentally determined value for an intensity in the empirical list of Mikhailenko et al. [91], the *ab initio* intensity was overwritten by the experimental datum.

For the first time, electric quadrupole (E2) transitions for the water molecule were identified in experimental spectra, made possible by the availability of an E2 *ab initio* line list [107,108]. The E2 transitions have now been added to the HITRAN2020 line list (only for the principal isotopologue). This room temperature E2 line list for $H_2^{16}O$ ranges from 0 to 10,000 cm⁻¹ and contains 6227 lines with intensities stronger than $10^{-30}$ cm/molecule. It was generated using MARVEL line positions and lower-state energies and theoretical transition intensities computed using a high-level *ab initio* electric quadrupole moment and a state-of-the-art variational approach [109,110]. The character in the last field of lower-state rotational ("local") quanta (i.e. preceding the error code for the line positions) in the traditional 160 character ".par" format will carry a label "q" to denote these transitions (see the Supplementary Material of this paper for the description of the upper- and lower-state quanta in the ".par" format). It should be noted that E2 type transitions are typically 6–8 orders of magnitude weaker than electric dipole transitions [107].

The HITRAN2016 line list included a large number of highly-accurate transition wavenumber measurements from the SISAM data set. Comparisons of these data to derived MARVEL transition frequencies indicated that the transition wavenumbers derived from the most recent version of the MARVEL data [97] match the SISAM values to within their uncertainty. The predicted MARVEL line positions are therefore selected as the preferable source of data because, apart from their validated accuracy, they also would provide consistency throughout the database. Hence, where possible, we overwrite the *ab initio* energy levels and transition frequencies with the MARVEL data.

The accuracy of the HITRAN2020 line list in the visible region is significantly better than that of its predecessor, HITRAN2016. Harder et al. [101] reported water-vapor absorption spectra in the 22,100–22,700 cm⁻¹ interval through atmospheric observations. Using the HITRAN Application Programming Interface (HAPI) [52], we have generated cross sections at a temperature of 288 K using the Voigt profile at a resolution of 0.03 cm⁻¹. In Fig. 2, it becomes clear that the HITRAN2020 line list is significantly better suited for retrieving water in the visible region. It has already been reported that the HITRAN2016 water line list has spectroscopic errors in the visible range [111] and the HITRAN2020 edition offers substantial improvements. These errors were attributed to a collection of irregularities in line shape parameters and inaccurate *ab initio* transition intensities.

To extend this comparison between HITRAN2020 and HITRAN2016 in the visible between 9000 and 20 000 cm⁻¹, we generated two telluric models using the Planetary Spectrum Generator (PSG) [37], each generated with the two respective line lists, and compared these models to a high SNR telluric spectrum extracted from solar observations detailed in Baker et al. [77]. The atmospheric profile used to generate the model was evaluated for the appropriate location and elevation of the observation site in Göttingen, Germany as well as the corresponding observation time of June 17, 2015 UTC 10:50:30.5. The water-vapor abundance and surface pressure were adjusted by fitting the model to a subset
<figure><img src="image_6.png" /><figcaption>Fig. 2. Comparison of the HITRAN2020 and HITRAN2016 [16] line lists against the observed water-vapor cross sections reported by Harder et al. [101].</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 3. Comparison of the HITRAN2020 and HITRAN2016 [16] water vapor line lists through modeling the terrestrial atmospheric transmittance. Models were generated with the Planetary Spectrum Generator (PSG) [37] and compared to a telluric spectrum extracted from a high-resolution solar spectrum as described by Baker et al. [77].</figcaption></figure>

of the data between 13,679.89 and 13,698.63 cm⁻¹. The results of this comparison are shown in Fig. 3. The residual root mean square (RMS) for each model comparison shows overall improvements in HITRAN2020. This is partly due to the completeness of the HITRAN2020 line list, which now includes transitions that were missing in HITRAN2016, some of which were identified by Baker et al. [77] and partly due to improvements in the line-shape algorithm described below. Similar results are found by redoing this analysis using a transmission spectrum of the atmosphere at Kitt Peak [112] also derived from solar observations, but extracted using a different method to that used in Baker et al. [77]. The majority of transition intensities within Fig. 3 are from *ab initio* calculations and the line-shape parameters are often estimated, therefore the residuals could be improved with new high quality experimental measurements.

### 2.1.2. H<sub>2</sub><sup>18</sup>O

The HITRAN2020 line list for H<sub>2</sub><sup>18</sup>O also begins with the *ab initio* line list from Conway et al. [85], which extends to 20,000 cm⁻¹. In their comparisons against the experimental transition intensities present in the HITRAN2016 line list, discrepancies were observed that were not evident in any of the H<sub>2</sub><sup>16</sup>O comparisons. A large

amount of scatter was observed in intensity comparisons to measurements in the SISAM data [61], while a large, 20% shift was observed in the Tanaka et al. [113] intensities present in HITRAN2016. No such discrepancies were detected for the principle isotopologue covering the same regions. Considering that the *ab initio* line lists for H<sub>2</sub><sup>16</sup>O and H<sub>2</sub><sup>18</sup>O were calculated using very similar PESs and the same DMS, the source of the discrepancies is more likely to be associated with experiments. Therefore, the Tanaka et al. data is not included in the HITRAN2020 release.

Comparisons against new experimental spectra measured by Mikhailenko et al. [114] between 16,400 and 17,200 cm⁻¹, that was not present in HITRAN2016, were also performed [85]. While Mikhailenko et al. noticed that the HITRAN2016 data missed particularly strong transitions in the region, most from *ab initio* calculations [115], the new *ab initio* calculations were not missing such transitions.

In addition to releasing a much improved set of H<sub>2</sub><sup>16</sup>O energy levels, Furtenbacher et al. [97] also released an updated set of empirical energy levels for both H<sub>2</sub><sup>18</sup>O and H<sub>2</sub><sup>17</sup>O as part of their W2020 dataset. Hence, to develop the H<sub>2</sub><sup>18</sup>O line list for HITRAN2020 we considered the *ab initio* line list from Conway et al. and updated the *ab initio* energy levels with the latest MARVEL
<figure><img src="image_4.png" /><figcaption>Fig. 2. Comparison of the HITRAN2020 and HITRAN2016 [16] line lists against the observed water-vapor cross sections reported by Harder et al. [101].</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Comparison of the HITRAN2020 and HITRAN2016 [16] water vapor line lists through modeling the terrestrial atmospheric transmittance. Models were generated with the Planetary Spectrum Generator (PSG) [37] and compared to a telluric spectrum extracted from a high-resolution solar spectrum as described by Baker et al. [77].</figcaption></figure>

of the data between 13,679.89 and 13,698.63 cm⁻¹. The results of this comparison are shown in Fig. 3. The residual root mean square (RMS) for each model comparison shows overall improvements in HITRAN2020. This is partly due to the completeness of the HITRAN2020 line list, which now includes transitions that were missing in HITRAN2016, some of which were identified by Baker et al. [77] and partly due to improvements in the line-shape algorithm described below. Similar results are found by redoing this analysis using a transmission spectrum of the atmosphere at Kitt Peak [112] also derived from solar observations, but extracted using a different method to that used in Baker et al. [77]. The majority of transition intensities within Fig. 3 are from *ab initio* calculations and the line-shape parameters are often estimated, therefore the residuals could be improved with new high quality experimental measurements.

### 2.1.2. H<sub>2</sub><sup>18</sup>O

The HITRAN2020 line list for H<sub>2</sub><sup>18</sup>O also begins with the *ab initio* line list from Conway et al. [85], which extends to 20,000 cm⁻¹. In their comparisons against the experimental transition intensities present in the HITRAN2016 line list, discrepancies were observed that were not evident in any of the H<sub>2</sub><sup>16</sup>O comparisons. A large

amount of scatter was observed in intensity comparisons to measurements in the SISAM data [61], while a large, 20% shift was observed in the Tanaka et al. [113] intensities present in HITRAN2016. No such discrepancies were detected for the principle isotopologue covering the same regions. Considering that the *ab initio* line lists for H<sub>2</sub><sup>16</sup>O and H<sub>2</sub><sup>18</sup>O were calculated using very similar PESs and the same DMS, the source of the discrepancies is more likely to be associated with experiments. Therefore, the Tanaka et al. data is not included in the HITRAN2020 release.

Comparisons against new experimental spectra measured by Mikhailenko et al. [114] between 16,400 and 17,200 cm⁻¹, that was not present in HITRAN2016, were also performed [85]. While Mikhailenko et al. noticed that the HITRAN2016 data missed particularly strong transitions in the region, most from *ab initio* calculations [115], the new *ab initio* calculations were not missing such transitions.

In addition to releasing a much improved set of H<sub>2</sub><sup>16</sup>O energy levels, Furtenbacher et al. [97] also released an updated set of empirical energy levels for both H<sub>2</sub><sup>18</sup>O and H<sub>2</sub><sup>17</sup>O as part of their W2020 dataset. Hence, to develop the H<sub>2</sub><sup>18</sup>O line list for HITRAN2020 we considered the *ab initio* line list from Conway et al. and updated the *ab initio* energy levels with the latest MARVEL
data. Furtenbacher et al. also utilized the method of Polyansky et al. [116] to generate so-called pseudo-experimental energy levels. Use of these semi-empirical levels for both H<sub>2</sub><sup>18</sup>O and H<sub>2</sub><sup>17</sup>O will be considered as a possible further improvement in a future release.

Following this, we proceeded to update the calculated transition intensities with the measured data present in HITRAN2016 [63,68–71,117], with the exception of the SISAM (aside from the measured 3ν<sub>2</sub> and 4ν<sub>2</sub> bands) and Tanaka et al. data. In addition, we also supplement the line list with newly measured intensities from Mikhailenko et al. [114].

### 2.1.3. H<sub>2</sub><sup>17</sup>O

The HITRAN2016 H<sub>2</sub><sup>17</sup>O line list possessed several small issues [97], mostly related to spectroscopic assignment of states. There were forbidden transitions between ortho-para states, rotational parity was equal for several upper and lower-states, and the list possessed several duplicate transitions. To address these issues, we have updated the energy levels (hence the transition frequencies) with the latest MARVEL data and remedied these transition assignments.

### 2.1.4. HD<sup>16</sup>O, HD<sup>18</sup>O and HD<sup>17</sup>O

The HITRAN2016 HD<sup>16</sup>O, HD<sup>18</sup>O and HD<sup>17</sup>O line lists also possessed several issues related to forbidden transitions. To address these issues, we have updated the transition assignments. Positions and intensities have not been altered from HITRAN2016.

### 2.1.5. D<sub>2</sub><sup>16</sup>O

The D<sub>2</sub><sup>16</sup>O line list in HITRAN2016 was based on a preliminary version of the line list from Kyuberis et al. [118]. This preliminary list was found to contain some incorrect quantum assignments and therefore for HITRAN2020 the published list from Kyuberis et al. [118] was used.

### 2.1.6. Line-shape parameters for water vapor

Line-shape parameters, including the half-width, γ, and the line shift, δ, their associated errors, and the temperature dependence of these parameters have been added to the water-vapor transitions discussed above. The algorithm becomes rather involved due to the fact that the data availability and uncertainties vary greatly with spectral bands and isotopologues but basically follows the “Diet” procedure of Gordon et al. [73]. Line-shape parameters for H<sub>2</sub>O-air and self-collision systems were added for all the isotopologues of water vapor, although for self-collisions only the half-widths are considered. It is important to note that for HITRAN2020, only values associated with the power law of temperature dependencies were considered for water vapor. When the shift does not change sign over the temperature range of the calculations (see below), the power law temperature dependence, m, was also determined. However, when possible, the temperature dependence of the half-width and the line shift were generated using the Gamache-Vispoel double power law (DPL) model [119] and will be considered for the database update in the future. The infrastructure for this is already setup [120] but large effort is required to validate and populate these parameters as mentioned in Section 7.1.1.

The “Diet” procedure takes line-shape data from a number of sources and prioritizes the data for addition to HITRAN. Beginning with an updated version of the measurement database of Gamache and Hartmann [121], which contains data for the seven water-vapor isotopologues in HITRAN, data of known high quality for γ and δ were extracted and put into a “priority” data file. It is important to emphasize that the priority data are from laboratory measurements or line shape parameters determined from the fits of the atmospheric spectra. For instance, Mlawer et al. [122] have identified issues with broadening parameters for a couple of dozen

of lines in the FIR region and have suggested alternative values based on the retrievals. These values form the priority data file for the HITRAN2020 update.

Next, an intercomparison of the measurement data was performed, and the inconsistent references and individual outliers were filtered from the air-broadening database. The intercomparison of H<sub>2</sub>O-air data and H<sub>2</sub>O-H<sub>2</sub>O data were redone and average values from the intercomparison of data were determined and transitions with less than 5% standard deviation for the intercomparison are retained and stored in files for the half-widths and the line shifts. Then, all the lines for which an intercomparison could not be performed, i.e., a single datum for a transition, were written to separate files for the half-widths and the line shifts.

Next in the sequence, theoretical calculations of the line-shape parameters were considered. Taking the Modified Complex Robert-Bonamy (MCRB) calculations of Vispoel et al. [123] for the H<sub>2</sub>O-N<sub>2</sub> collision system and similar calculations for the H<sub>2</sub>O-O<sub>2</sub> collision system, the line-shape information for the H<sub>2</sub>O-air collision system were produced by γ<sub>air</sub> = 0.79γ<sub>N<sub>2</sub></sub> + 0.21γ<sub>O<sub>2</sub></sub> with a similar formula for the line shift. Note the calculations considered 13 temperatures from 200-3000 K; γ and δ were determined for these 13 temperatures so that the temperature dependence could be determined. These MCRB calculations were made for 10,782 rotational transitions for the rotational band and for bands with one to four ν<sub>1</sub>, ν<sub>2</sub>, and ν<sub>3</sub> vibrational quanta exchanged, giving some 140,000 calculated transitions. These data make up the calculated H<sub>2</sub>O-air line-shape files for the H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, H<sub>2</sub><sup>17</sup>O isotopologues. For these three isotopologues, the self-broadening Complex Robert-Bonamy calculations that were added to previous HITRAN databases were used. For the HDO and D<sub>2</sub>O isotopologues self-broadened data are from the calculations in Refs. [124–126].

Next in the algorithm was the use of accurate predicted values of γ, δ, and the temperature dependence of each. Gamache and Hartmann, working from Robert-Bonamy theory, derived a formula that can accurately predict γ and δ [127] at any temperature. Plots showing the agreement of these predictions with the experimental data from the ν<sub>2</sub> band are provided in Supplementary Material. This routine has been successfully applied to H<sub>2</sub>O-air [128], CO<sub>2</sub>-x, where x = N<sub>2</sub>, O<sub>2</sub>, air, CO<sub>2</sub> [129], H<sub>2</sub>O-H<sub>2</sub> [130], and H<sub>2</sub>O-N<sub>2</sub> [131], and the predicted values agree well with the calculated or measured values with a standard deviation of about 5% for the H<sub>2</sub>O studies. A prediction routine was developed based on the H<sub>2</sub>O-air MCRB data (H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, H<sub>2</sub><sup>17</sup>O) and the prediction coefficients determined at the 13 temperatures allowing the temperature dependence to also be determined. These data make the predicted part of the H<sub>2</sub>O-air database.

Lastly, for transitions that are not in the above databases, the half-width is estimated by using the rotation band value if it is available, i.e. neglecting the vibrational dependence. However, there are a large number of H<sub>2</sub>O transitions in the HITRAN database that do not have attributions. These are transitions generally taken from *ab initio* calculations where only the rotational quantum number *J* and parity are “good” quantum numbers. For these transitions, the rotation band calculations were taken and half-widths as a function of *J''* were determined. These data were extrapolated to *J''* = 50 (keeping HITEMP in mind). Note, because of the very strong vibrational dependence of the line shift, no comparable average values can be determined.

This general procedure was done in three groups: (H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, H<sub>2</sub><sup>17</sup>O), (HD<sup>16</sup>O, HD<sup>18</sup>O, HD<sup>17</sup>O) and (D<sub>2</sub><sup>16</sup>O) for air- and self-collisions, producing the isotopologue-dependent files, which are added to HITRAN2020 in the following priority scheme: (1) priority data, (2) intercomparison data, (3) single measurement data, (4) MCRB data, (5) predicted data, and (6) *J*-average data. It was possible to do the temperature dependence of the *J*-average values only for the first group. This procedure assumes that the line-shape
data. Furtenbacher et al. also utilized the method of Polyansky et al. [116] to generate so-called pseudo-experimental energy levels. Use of these semi-empirical levels for both H<sub>2</sub><sup>18</sup>O and H<sub>2</sub><sup>17</sup>O will be considered as a possible further improvement in a future release.

Following this, we proceeded to update the calculated transition intensities with the measured data present in HITRAN2016 [63,68–71,117], with the exception of the SISAM (aside from the measured 3ν<sub>2</sub> and 4ν<sub>2</sub> bands) and Tanaka et al. data. In addition, we also supplement the line list with newly measured intensities from Mikhailenko et al. [114].

### 2.1.3. H<sub>2</sub><sup>17</sup>O

The HITRAN2016 H<sub>2</sub><sup>17</sup>O line list possessed several small issues [97], mostly related to spectroscopic assignment of states. There were forbidden transitions between ortho-para states, rotational parity was equal for several upper and lower-states, and the list possessed several duplicate transitions. To address these issues, we have updated the energy levels (hence the transition frequencies) with the latest MARVEL data and remedied these transition assignments.

### 2.1.4. HD<sup>16</sup>O, HD<sup>18</sup>O and HD<sup>17</sup>O

The HITRAN2016 HD<sup>16</sup>O, HD<sup>18</sup>O and HD<sup>17</sup>O line lists also possessed several issues related to forbidden transitions. To address these issues, we have updated the transition assignments. Positions and intensities have not been altered from HITRAN2016.

### 2.1.5. D<sub>2</sub><sup>16</sup>O

The D<sub>2</sub><sup>16</sup>O line list in HITRAN2016 was based on a preliminary version of the line list from Kyuberis et al. [118]. This preliminary list was found to contain some incorrect quantum assignments and therefore for HITRAN2020 the published list from Kyuberis et al. [118] was used.

### 2.1.6. Line-shape parameters for water vapor

Line-shape parameters, including the half-width, γ, and the line shift, δ, their associated errors, and the temperature dependence of these parameters have been added to the water-vapor transitions discussed above. The algorithm becomes rather involved due to the fact that the data availability and uncertainties vary greatly with spectral bands and isotopologues but basically follows the “Diet” procedure of Gordon et al. [73]. Line-shape parameters for H<sub>2</sub>O-air and self-collision systems were added for all the isotopologues of water vapor, although for self-collisions only the half-widths are considered. It is important to note that for HITRAN2020, only values associated with the power law of temperature dependencies were considered for water vapor. When the shift does not change sign over the temperature range of the calculations (see below), the power law temperature dependence, m, was also determined. However, when possible, the temperature dependence of the half-width and the line shift were generated using the Gamache-Vispoel double power law (DPL) model [119] and will be considered for the database update in the future. The infrastructure for this is already setup [120] but large effort is required to validate and populate these parameters as mentioned in Section 7.1.1.

The “Diet” procedure takes line-shape data from a number of sources and prioritizes the data for addition to HITRAN. Beginning with an updated version of the measurement database of Gamache and Hartmann [121], which contains data for the seven water-vapor isotopologues in HITRAN, data of known high quality for γ and δ were extracted and put into a “priority” data file. It is important to emphasize that the priority data are from laboratory measurements or line shape parameters determined from the fits of the atmospheric spectra. For instance, Mlawer et al. [122] have identified issues with broadening parameters for a couple of dozen

of lines in the FIR region and have suggested alternative values based on the retrievals. These values form the priority data file for the HITRAN2020 update.

Next, an intercomparison of the measurement data was performed, and the inconsistent references and individual outliers were filtered from the air-broadening database. The intercomparison of H<sub>2</sub>O-air data and H<sub>2</sub>O-H<sub>2</sub>O data were redone and average values from the intercomparison of data were determined and transitions with less than 5% standard deviation for the intercomparison are retained and stored in files for the half-widths and the line shifts. Then, all the lines for which an intercomparison could not be performed, i.e., a single datum for a transition, were written to separate files for the half-widths and the line shifts.

Next in the sequence, theoretical calculations of the line-shape parameters were considered. Taking the Modified Complex Robert-Bonamy (MCRB) calculations of Vispoel et al. [123] for the H<sub>2</sub>O-N<sub>2</sub> collision system and similar calculations for the H<sub>2</sub>O-O<sub>2</sub> collision system, the line-shape information for the H<sub>2</sub>O-air collision system were produced by γ<sub>air</sub> = 0.79γ<sub>N<sub>2</sub></sub> + 0.21γ<sub>O<sub>2</sub></sub> with a similar formula for the line shift. Note the calculations considered 13 temperatures from 200-3000 K; γ and δ were determined for these 13 temperatures so that the temperature dependence could be determined. These MCRB calculations were made for 10,782 rotational transitions for the rotational band and for bands with one to four ν<sub>1</sub>, ν<sub>2</sub>, and ν<sub>3</sub> vibrational quanta exchanged, giving some 140,000 calculated transitions. These data make up the calculated H<sub>2</sub>O-air line-shape files for the H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, H<sub>2</sub><sup>17</sup>O isotopologues. For these three isotopologues, the self-broadening Complex Robert-Bonamy calculations that were added to previous HITRAN databases were used. For the HDO and D<sub>2</sub>O isotopologues self-broadened data are from the calculations in Refs. [124–126].

Next in the algorithm was the use of accurate predicted values of γ, δ, and the temperature dependence of each. Gamache and Hartmann, working from Robert-Bonamy theory, derived a formula that can accurately predict γ and δ [127] at any temperature. Plots showing the agreement of these predictions with the experimental data from the ν<sub>2</sub> band are provided in Supplementary Material. This routine has been successfully applied to H<sub>2</sub>O-air [128], CO<sub>2</sub>-x, where x = N<sub>2</sub>, O<sub>2</sub>, air, CO<sub>2</sub> [129], H<sub>2</sub>O-H<sub>2</sub> [130], and H<sub>2</sub>O-N<sub>2</sub> [131], and the predicted values agree well with the calculated or measured values with a standard deviation of about 5% for the H<sub>2</sub>O studies. A prediction routine was developed based on the H<sub>2</sub>O-air MCRB data (H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, H<sub>2</sub><sup>17</sup>O) and the prediction coefficients determined at the 13 temperatures allowing the temperature dependence to also be determined. These data make the predicted part of the H<sub>2</sub>O-air database.

Lastly, for transitions that are not in the above databases, the half-width is estimated by using the rotation band value if it is available, i.e. neglecting the vibrational dependence. However, there are a large number of H<sub>2</sub>O transitions in the HITRAN database that do not have attributions. These are transitions generally taken from *ab initio* calculations where only the rotational quantum number *J* and parity are “good” quantum numbers. For these transitions, the rotation band calculations were taken and half-widths as a function of *J''* were determined. These data were extrapolated to *J''* = 50 (keeping HITEMP in mind). Note, because of the very strong vibrational dependence of the line shift, no comparable average values can be determined.

This general procedure was done in three groups: (H<sub>2</sub><sup>16</sup>O, H<sub>2</sub><sup>18</sup>O, H<sub>2</sub><sup>17</sup>O), (HD<sup>16</sup>O, HD<sup>18</sup>O, HD<sup>17</sup>O) and (D<sub>2</sub><sup>16</sup>O) for air- and self-collisions, producing the isotopologue-dependent files, which are added to HITRAN2020 in the following priority scheme: (1) priority data, (2) intercomparison data, (3) single measurement data, (4) MCRB data, (5) predicted data, and (6) *J*-average data. It was possible to do the temperature dependence of the *J*-average values only for the first group. This procedure assumes that the line-shape
data for the oxygen-16, 18, and 17 species of water are the same, which has been well demonstrated by measurement and calculation [121].

These data sets are summarized in more detail in the Table provided in the Supplementary Material. From these data the Python dictionaries were made using the ro-vibrational quantum numbers as the key. A Python algorithm was written that loads into memory, for all isotopologues of H₂O, the database dictionaries described above and then reads the HITRAN2020 water-vapor line file and selectively adds the line-shape data to each transition in the prioritized scheme discussed above.

It is important to note that all the parameters described above are for the Voigt line shape. However, non-Voigt line shapes and specifically HT profile have also been accommodated. HITRAN2016 already contained many of these parameters, but more have been added for the HITRAN2020 edition, specifically in the 2.3 µm region. The data are based on the new H₂O/HDO database in the spectral range 4190-4340 cm⁻¹ (2.39-2.30 µm) that was generated within the framework of the ESA project SEOM-IAS (Scientific Exploitation of Operational Missions – Improved Atmospheric Spectroscopy Databases), ESA/AO/1-7566/13/I-BG [103,132]. This work has already been partially reported in HITRAN2016 [16], but only line intensities were entered into the database.

In the 4190-4340 cm⁻¹ region, several Fourier-Transform transmittance spectra of pure and air-broadened water vapor at low and high temperatures were measured and analyzed. These measurements were dedicated to water-vapor parameters to be used in TROPOMI/S5-P retrievals. The analysis was based on a multi-spectrum fit using the HT profile. Line positions, intensities, self- and air-broadened line-shape parameters including speed-dependence and Dicke narrowing parameters as well as their temperature dependence were retrieved in the analysis. The line-shape parameters are available in the HITRAN2020 database. The data as well as the measurements can also be downloaded from Zenodo [103,132].

The new 2.3 µm H₂O data together with the new 2.3 µm CH₄ data described in Section 2.6 have been validated by ground-based solar occultation measurements by Frank Hase, KIT, Karlsruhe, Germany. Fig. 4 shows residuals applying HITRAN2012, HITRAN2016, and the new database. The residuals are smallest for the new database. Remaining residuals are caused by imperfect modeling of solar lines.

## 2.2. CO₂: carbon dioxide (molecule 2)

Current and planned atmospheric remote sensing instruments set a very challenging level of 0.3% accuracy on the retrieved CO₂

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Solar occultation spectrum (red) and residuals for different spectroscopic databases, green: new line list, black: HITRAN2012, blue: HITRAN2016, light blue: solar transmission spectrum. Courtesy Frank Hase, KIT, Karlsruhe, Germany.</figcaption></figure>

column [133], which in turn places stringent requirements on the quality of spectroscopic parameters for this molecule.

The HITRAN2016 line list for the twelve stable isotopologues of carbon dioxide has proven to be an overall improvement over the previous editions of the database (see, for instance, Ref. [76]). However, a number of remaining or new issues were identified as described below. For the HITRAN2020 database, these issues were addressed while a number of previously missing bands above 8000 cm⁻¹ were added.

### 2.2.1. CO₂ line positions and intensities

The details of the update for line positions and intensities are provided in a dedicated publication in this special issue [134]. Here we briefly summarize important points. Before describing the new data, it is worth recalling that, below 8000 cm⁻¹, the line positions for CO₂ transitions in HITRAN2016 were predominantly based on the update to the 2015 version of the Carbon Dioxide Spectroscopic Database (CDSD-296) [135]. The line intensities were mostly of *ab initio* origin [136] based on the work of Zak et al. [137–139] except for the bands that were identified as “sensitive” [137], where CDSD intensities were preferred.

### 2.2.2. Improved line positions and “new” bands

The majority of the CO₂ line positions in the HITRAN2020 database were updated using the line positions from the recent 2019 version of CDSD-296 for atmospheric applications [140]. It should be noted that the slightly corrected and updated version of the CDSD-296 database uploaded at ftp.iao.ru as pub/CDSD-296/cdsd-296_version_1.rar was used. The differences with CDSD-296 published in Tashkun et al. [140]: 1. ΔP = 6 region (P = 2V₁ + V₂ + 3V₃ is the polyad number, Vᵢ are the vibrational quantum numbers) of the ¹⁶O¹²C¹⁸O isotopologue was recalculated using the new set of effective dipole moment parameters; 2. A total of 226 lines with Δl₂ = 4 of the ¹²C¹⁶O₂, ¹³C¹⁶O₂, and ¹⁶O¹²C¹⁸O isotopologues were added. The lower-state energies and the uncertainty codes of the line positions were also transferred from CDSD-296 [140] to HITRAN2020.

The line parameters for the principal isotopologue above 8000 cm⁻¹ were extended by including the new bands from the high-temperature line list from Yurchenko et al. [141] (with appropriate intensity cutoff). The assignments for these CO₂ lines were achieved by using the CDSD-296 [140] and NASA Ames [142] databases. The vibrational assignments were replaced with “-2-2-2-20” when the states were not assigned. Uncertainty codes 3 for the line positions and 4 for the line intensities (see Table 2) were used in the case of the newly added CO₂ lines.

Also, the 30022-00001 and 30023-00001 bands of the ¹⁶O¹²C¹⁸O isotopologue, missing in HITRAN2016 [16] and CDSD-296 [140], were included in the HITRAN2020 line list. The line positions for these bands were computed up to J = 34 using the spectroscopic constants obtained by the fit to the measured line positions from Karlovets et al. [143]. In Ref. [143], it was also shown that the R-branch intensities of the 00041-01101 band of the ¹²C¹⁶O₂ isotopologue are in good agreement with the Ames values while HITRAN2016 values are largely overestimated. This band is missing in CDSD2019 [140] and included in HITRAN2016 from an old version of CDSD [144]. The line intensities for the three bands described above were updated using the NASA Ames database [142]. Uncertainty code 4 for line positions and uncertainty code 4 for line intensities (see Table 2) were updated for these bands in the HITRAN2020 CO₂ line list. An overview of the HITRAN2020 line lists for all 12 isotopologues of carbon dioxide in natural abundance is plotted in Fig. 5.
data for the oxygen-16, 18, and 17 species of water are the same, which has been well demonstrated by measurement and calculation [121].

These data sets are summarized in more detail in the Table provided in the Supplementary Material. From these data the Python dictionaries were made using the ro-vibrational quantum numbers as the key. A Python algorithm was written that loads into memory, for all isotopologues of H₂O, the database dictionaries described above and then reads the HITRAN2020 water-vapor line file and selectively adds the line-shape data to each transition in the prioritized scheme discussed above.

It is important to note that all the parameters described above are for the Voigt line shape. However, non-Voigt line shapes and specifically HT profile have also been accommodated. HITRAN2016 already contained many of these parameters, but more have been added for the HITRAN2020 edition, specifically in the 2.3 µm region. The data are based on the new H₂O/HDO database in the spectral range 4190-4340 cm⁻¹ (2.39-2.30 µm) that was generated within the framework of the ESA project SEOM-IAS (Scientific Exploitation of Operational Missions – Improved Atmospheric Spectroscopy Databases), ESA/AO/1-7566/13/I-BG [103,132]. This work has already been partially reported in HITRAN2016 [16], but only line intensities were entered into the database.

In the 4190-4340 cm⁻¹ region, several Fourier-Transform transmittance spectra of pure and air-broadened water vapor at low and high temperatures were measured and analyzed. These measurements were dedicated to water-vapor parameters to be used in TROPOMI/S5-P retrievals. The analysis was based on a multi-spectrum fit using the HT profile. Line positions, intensities, self- and air-broadened line-shape parameters including speed-dependence and Dicke narrowing parameters as well as their temperature dependence were retrieved in the analysis. The line-shape parameters are available in the HITRAN2020 database. The data as well as the measurements can also be downloaded from Zenodo [103,132].

The new 2.3 µm H₂O data together with the new 2.3 µm CH₄ data described in Section 2.6 have been validated by ground-based solar occultation measurements by Frank Hase, KIT, Karlsruhe, Germany. Fig. 4 shows residuals applying HITRAN2012, HITRAN2016, and the new database. The residuals are smallest for the new database. Remaining residuals are caused by imperfect modeling of solar lines.

## 2.2. CO₂: carbon dioxide (molecule 2)

Current and planned atmospheric remote sensing instruments set a very challenging level of 0.3% accuracy on the retrieved CO₂

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Solar occultation spectrum (red) and residuals for different spectroscopic databases, green: new line list, black: HITRAN2012, blue: HITRAN2016, light blue: solar transmission spectrum. Courtesy Frank Hase, KIT, Karlsruhe, Germany.</figcaption></figure>

column [133], which in turn places stringent requirements on the quality of spectroscopic parameters for this molecule.

The HITRAN2016 line list for the twelve stable isotopologues of carbon dioxide has proven to be an overall improvement over the previous editions of the database (see, for instance, Ref. [76]). However, a number of remaining or new issues were identified as described below. For the HITRAN2020 database, these issues were addressed while a number of previously missing bands above 8000 cm⁻¹ were added.

### 2.2.1. CO₂ line positions and intensities

The details of the update for line positions and intensities are provided in a dedicated publication in this special issue [134]. Here we briefly summarize important points. Before describing the new data, it is worth recalling that, below 8000 cm⁻¹, the line positions for CO₂ transitions in HITRAN2016 were predominantly based on the update to the 2015 version of the Carbon Dioxide Spectroscopic Database (CDSD-296) [135]. The line intensities were mostly of *ab initio* origin [136] based on the work of Zak et al. [137–139] except for the bands that were identified as “sensitive” [137], where CDSD intensities were preferred.

### 2.2.2. Improved line positions and “new” bands

The majority of the CO₂ line positions in the HITRAN2020 database were updated using the line positions from the recent 2019 version of CDSD-296 for atmospheric applications [140]. It should be noted that the slightly corrected and updated version of the CDSD-296 database uploaded at ftp.iao.ru as pub/CDSD-296/cdsd-296_version_1.rar was used. The differences with CDSD-296 published in Tashkun et al. [140]: 1. ΔP = 6 region ($P = 2V_1 + V_2 + 3V_3$ is the polyad number, $V_i$ are the vibrational quantum numbers) of the ¹⁶O¹²C¹⁸O isotopologue was recalculated using the new set of effective dipole moment parameters; 2. A total of 226 lines with $\Delta l_2 = 4$ of the ¹²C¹⁶O₂, ¹³C¹⁶O₂, and ¹⁶O¹²C¹⁸O isotopologues were added. The lower-state energies and the uncertainty codes of the line positions were also transferred from CDSD-296 [140] to HITRAN2020.

The line parameters for the principal isotopologue above 8000 cm⁻¹ were extended by including the new bands from the high-temperature line list from Yurchenko et al. [141] (with appropriate intensity cutoff). The assignments for these CO₂ lines were achieved by using the CDSD-296 [140] and NASA Ames [142] databases. The vibrational assignments were replaced with “-2-2-2-20” when the states were not assigned. Uncertainty codes 3 for the line positions and 4 for the line intensities (see Table 2) were used in the case of the newly added CO₂ lines.

Also, the 30022-00001 and 30023-00001 bands of the ¹⁶O¹²C¹⁸O isotopologue, missing in HITRAN2016 [16] and CDSD-296 [140], were included in the HITRAN2020 line list. The line positions for these bands were computed up to $J = 34$ using the spectroscopic constants obtained by the fit to the measured line positions from Karlovets et al. [143]. In Ref. [143], it was also shown that the R-branch intensities of the 00041-01101 band of the ¹²C¹⁶O₂ isotopologue are in good agreement with the Ames values while HITRAN2016 values are largely overestimated. This band is missing in CDSD2019 [140] and included in HITRAN2016 from an old version of CDSD [144]. The line intensities for the three bands described above were updated using the NASA Ames database [142]. Uncertainty code 4 for line positions and uncertainty code 4 for line intensities (see Table 2) were updated for these bands in the HITRAN2020 CO₂ line list. An overview of the HITRAN2020 line lists for all 12 isotopologues of carbon dioxide in natural abundance is plotted in Fig. 5.
<figure><img src="image_9.png" /><figcaption>Fig. 5. Overview of the HITRAN2020 line lists for all 12 naturally abundant isotopologues of carbon dioxide.</figcaption></figure>

### 2.2.3. Addressing the issues found in HITRAN2016

Most of the HITRAN2016 CO₂ issues were identified by comparisons against laboratory and atmospheric spectra obtained with Fourier Transform Spectrometers (FTS) at the Kitt Peak National Observatory, MkIV balloon, and Total Carbon Column Observing Network (TCCON) [145,146]. In particular, it was shown that:

* Comparisons with the Kitt Peak laboratory spectrum demonstrated 10-15% rotationally dependent errors in the *ab initio* intensities of the 40002–01101 band of $^{12}C^{16}O_2$ isotopologue near 4800 cm$^{-1}$. This region is probed by the OCO-2 instrument [147], so it is important, although it is worth pointing out that the intensities of this hot band are about two orders of magnitude weaker than the strongest lines in that region. This comparison helped to identify a systematic issue in the *ab initio* calculations [137] when calculating intensities for the transitions that involve either of the interacting 40002 and 21113 vibrational states which are affected by Coriolis interaction.

* In the 1800–2000 cm$^{-1}$ region, the amount of CO$_2$ retrieved with HITRAN2016 is about 5% larger than that retrieved with previous line lists [14,15]. The biggest deviations in this region were observed for the 11102–00001 band. Two bands 11101–00001 and 11102–00001 borrow the intensities from the strong 00011–00001 band via Coriolis interaction. The CDSD-296 [135] line positions and intensities were used in the previous line lists [14,15] while HITRAN2016 used the UCL *ab initio* line intensities [137]. It was shown in Ref. [140] that the *ab initio* AMES line intensities [142] for the 11101–00001 band deviate considerably from the observations. The same conclusion is valid for the UCL *ab initio* line intensities [137] of this band.

* The TCCON spectra cover the 3950 cm$^{-1}$ to 9500 cm$^{-1}$ region. The HITRAN2016 line lists reduce the CO$_2$ retrieved from the 6220 cm$^{-1}$ and 6338 cm$^{-1}$ windows by 0.5% and 1.5% respectively, raising additional concerns of consistency at the required level of accuracy. This issue is associated with the line intensities in the 30012–00001 band and is discussed below.

Critical validation tests for the spectroscopic data were carried out to find problems due to insufficient accuracy of line parameters in some of the bands in the CO₂ line list using available experimental works and the existing theoretical and semi-empirical databases, including NASA Ames [142], UCL [137–139], and CDSD-296 [140]. All updates for the CO₂ line positions and intensities described above are discussed in Ref. [134].

In the HITRAN2016 line list below 8000 cm$^{-1}$, a number of inconsistencies in the rotational structure in the so-called “sen-

sitive bands” (as defined by Zak et al. [137]) due to the mixing of CDSD [135] and UCL [137–139] line intensities were revealed in Cavity Ring-Down Spectroscopy (CRDS) measurements of $^{18}O$- and $^{13}C$-enriched and “natural” CO$_2$ near 1.74 µm [148–150]. They concern the perpendicular bands of the $\Delta P = 9$ series of transitions. It leads to apparent inconsistency in the rotational structure with strong intensity variation between successive J values or even missing transitions (due to falling below the intensity cutoff) as shown, for example, in Fig. 9 of Ref. [148]. Validation tests were carried out for the bands from Refs. [148–150] and the other bands affected by this problem due to the mixing of CDSD and *ab initio* intensities in the HITRAN line list using literature values and those from the different CO$_2$ databases. Alternative sources of data for each problematic band were identified. See more details in Ref. [134].

### 2.2.4. New experimental data with sub-percent uncertainty

A number of very accurate measurements have become available after the release of HITRAN2016. Here we make use of the most recent CRDS measurements from NIST [151,152] and FTS measurements from DLR (reported in this special issue by Birk et al. [153] with the corresponding measurements and line parameter database made available on Zenodo [154]). The results of these works were used to improve the HITRAN line intensities for several NIR bands of the principal isotopologue of CO$_2$. It is interesting to note that in these works it was found that for these particular bands (discussed below) the rotational distribution of the *ab initio* intensities from Zak et al. [137] used in HITRAN2016 was very accurate; however different band scaling factors were recommended. In Refs. [151,152], the reported intensity uncertainty is better than 0.1%. Fleurbaey et al. [151] showed that a constant scaling of 1.0069±0.0002 of HITRAN2016 values in the 20013–00001 band of CO$_2$ ($\lambda = 2.06$ µm) is consistent with experiment, therefore we have performed this scaling in HITRAN2020. Similarly accurate line intensity measurements for the 3001i–00001 ($i = 2-4$) bands reported by Long et al. [152] were used to improve the CO$_2$ line intensities near 1.6 µm. It was shown in Ref. [152] that their results and the *ab initio* calculations of Zak et al. [137] agree at the 0.06% level for the 30013–00001 (also targeted by the OCO-2 mission) and 30014–00001 bands, but there is a systematic discrepancy of about 1.1% for the 30012–00001 band. Following these results, the HITRAN2016 line intensities were scaled to the experimental band-dependent scaling factors from Long et al. [152]. Uncertainty code 8 (see Table 2) for the line intensities was given for the corresponding bands in the HITRAN2020 line list.

Ambient temperature FTS measurements of pure CO$_2$ have been conducted at the German Aerospace Center (DLR) with a Bruker IFS 125HR in the range 6000–7000 cm$^{-1}$ [153]. Line intensity accuracies of 0.15% have been reported for the strongest bands in that spectral region. They also covered the 3001i–00001 bands, but this time including the 30011–00001 band. The corresponding factor 1.0061 was used to scale the intensities of the 30011–00001 band of the $^{12}C^{16}O_2$ isotopologue according to DLR measurements [153]. Good agreement between measurements of intensities of the 30013–00001 and 30014–00001 bands in Refs. [152,153] were found. Nevertheless, for the 30012–00001 band the differences outside of the stated uncertainties were reported. However, these differences are still small, <0.5%. Further investigations will be carried out for future updates of the database. Although this band is not being targeted by the OCO-2 mission, it is used in LIDAR applications (see Ref. [155], for instance), therefore it is important to minimize possible uncertainties.

The line intensities of the 00031–00001 band of the principal isotopologue near 1.4 µm came from CDSD-296 [135] in the HITRAN2016 edition, because this band was identified as “sensitive” in the *ab initio* calculations [137]. The comparison of the CDSD line
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Overview of the HITRAN2020 line lists for all 12 naturally abundant isotopologues of carbon dioxide.</figcaption></figure>

### 2.2.3. Addressing the issues found in HITRAN2016

Most of the HITRAN2016 CO₂ issues were identified by comparisons against laboratory and atmospheric spectra obtained with Fourier Transform Spectrometers (FTS) at the Kitt Peak National Observatory, MkIV balloon, and Total Carbon Column Observing Network (TCCON) [145,146]. In particular, it was shown that:

* Comparisons with the Kitt Peak laboratory spectrum demonstrated 10-15% rotationally dependent errors in the *ab initio* intensities of the 40002–01101 band of $^{12}C^{16}O_2$ isotopologue near 4800 cm$^{-1}$. This region is probed by the OCO-2 instrument [147], so it is important, although it is worth pointing out that the intensities of this hot band are about two orders of magnitude weaker than the strongest lines in that region. This comparison helped to identify a systematic issue in the *ab initio* calculations [137] when calculating intensities for the transitions that involve either of the interacting 40002 and 21113 vibrational states which are affected by Coriolis interaction.

* In the 1800–2000 cm$^{-1}$ region, the amount of CO$_2$ retrieved with HITRAN2016 is about 5% larger than that retrieved with previous line lists [14,15]. The biggest deviations in this region were observed for the 11102–00001 band. Two bands 11101–00001 and 11102–00001 borrow the intensities from the strong 00011–00001 band via Coriolis interaction. The CDSD-296 [135] line positions and intensities were used in the previous line lists [14,15] while HITRAN2016 used the UCL *ab initio* line intensities [137]. It was shown in Ref. [140] that the *ab initio* AMES line intensities [142] for the 11101–00001 band deviate considerably from the observations. The same conclusion is valid for the UCL *ab initio* line intensities [137] of this band.

* The TCCON spectra cover the 3950 cm$^{-1}$ to 9500 cm$^{-1}$ region. The HITRAN2016 line lists reduce the CO$_2$ retrieved from the 6220 cm$^{-1}$ and 6338 cm$^{-1}$ windows by 0.5% and 1.5% respectively, raising additional concerns of consistency at the required level of accuracy. This issue is associated with the line intensities in the 30012–00001 band and is discussed below.

Critical validation tests for the spectroscopic data were carried out to find problems due to insufficient accuracy of line parameters in some of the bands in the CO₂ line list using available experimental works and the existing theoretical and semi-empirical databases, including NASA Ames [142], UCL [137–139], and CDSD-296 [140]. All updates for the CO₂ line positions and intensities described above are discussed in Ref. [134].

In the HITRAN2016 line list below 8000 cm$^{-1}$, a number of inconsistencies in the rotational structure in the so-called “sen-

sitive bands” (as defined by Zak et al. [137]) due to the mixing of CDSD [135] and UCL [137–139] line intensities were revealed in Cavity Ring-Down Spectroscopy (CRDS) measurements of $^{18}O$- and $^{13}C$-enriched and “natural” CO$_2$ near 1.74 µm [148–150]. They concern the perpendicular bands of the $\Delta P = 9$ series of transitions. It leads to apparent inconsistency in the rotational structure with strong intensity variation between successive J values or even missing transitions (due to falling below the intensity cutoff) as shown, for example, in Fig. 9 of Ref. [148]. Validation tests were carried out for the bands from Refs. [148–150] and the other bands affected by this problem due to the mixing of CDSD and *ab initio* intensities in the HITRAN line list using literature values and those from the different CO$_2$ databases. Alternative sources of data for each problematic band were identified. See more details in Ref. [134].

### 2.2.4. New experimental data with sub-percent uncertainty

A number of very accurate measurements have become available after the release of HITRAN2016. Here we make use of the most recent CRDS measurements from NIST [151,152] and FTS measurements from DLR (reported in this special issue by Birk et al. [153] with the corresponding measurements and line parameter database made available on Zenodo [154]). The results of these works were used to improve the HITRAN line intensities for several NIR bands of the principal isotopologue of CO$_2$. It is interesting to note that in these works it was found that for these particular bands (discussed below) the rotational distribution of the *ab initio* intensities from Zak et al. [137] used in HITRAN2016 was very accurate; however different band scaling factors were recommended. In Refs. [151,152], the reported intensity uncertainty is better than 0.1%. Fleurbaey et al. [151] showed that a constant scaling of 1.0069±0.0002 of HITRAN2016 values in the 20013–00001 band of CO$_2$ ($\lambda = 2.06$ µm) is consistent with experiment, therefore we have performed this scaling in HITRAN2020. Similarly accurate line intensity measurements for the 3001i–00001 ($i = 2-4$) bands reported by Long et al. [152] were used to improve the CO$_2$ line intensities near 1.6 µm. It was shown in Ref. [152] that their results and the *ab initio* calculations of Zak et al. [137] agree at the 0.06% level for the 30013–00001 (also targeted by the OCO-2 mission) and 30014–00001 bands, but there is a systematic discrepancy of about 1.1% for the 30012–00001 band. Following these results, the HITRAN2016 line intensities were scaled to the experimental band-dependent scaling factors from Long et al. [152]. Uncertainty code 8 (see Table 2) for the line intensities was given for the corresponding bands in the HITRAN2020 line list.

Ambient temperature FTS measurements of pure CO$_2$ have been conducted at the German Aerospace Center (DLR) with a Bruker IFS 125HR in the range 6000–7000 cm$^{-1}$ [153]. Line intensity accuracies of 0.15% have been reported for the strongest bands in that spectral region. They also covered the 3001i–00001 bands, but this time including the 30011–00001 band. The corresponding factor 1.0061 was used to scale the intensities of the 30011–00001 band of the $^{12}C^{16}O_2$ isotopologue according to DLR measurements [153]. Good agreement between measurements of intensities of the 30013–00001 and 30014–00001 bands in Refs. [152,153] were found. Nevertheless, for the 30012–00001 band the differences outside of the stated uncertainties were reported. However, these differences are still small, <0.5%. Further investigations will be carried out for future updates of the database. Although this band is not being targeted by the OCO-2 mission, it is used in LIDAR applications (see Ref. [155], for instance), therefore it is important to minimize possible uncertainties.

The line intensities of the 00031–00001 band of the principal isotopologue near 1.4 µm came from CDSD-296 [135] in the HITRAN2016 edition, because this band was identified as “sensitive” in the *ab initio* calculations [137]. The comparison of the CDSD line
intensities with the DLR measurements [153] showed rotationally dependent deviations up to 4% for the 00031–00001 band. In the HITRAN2020 line list, the UCL line intensities [137] of the 00031–00001 band were scaled by the factor of 1.1217 to match the line intensities measured by Birk et al. [153]. Also, the HITRAN2016 line intensities of the 10032–10002 and 01131–01101 relatively weak hot bands located near 6900 cm⁻¹ were compared to the DLR measurements [153]. It was found that the line intensities of the 10032–10002 band in HITRAN2016 should be scaled by a factor of 1.1346 while the line intensities of the 01131–01101 band should be scaled by a factor of 1.0022.

### 2.2.5. Introduction of magnetic dipole transitions

All previous editions of HITRAN provided only electric dipole transitions for CO₂. In this edition, the line parameters of the v₂ + v₃ magnetic dipole band of the ¹²C¹⁶O₂ isotopologue were introduced into HITRAN for the first time. These new data will help spectral studies of CO₂-rich planetary atmospheres. This band is forbidden in electric dipole absorption, but it is allowed in electric quadrupole and in magnetic dipole absorptions. The first observation of the v₂ + v₃ band of ¹²C¹⁶O₂ at 3.3 µm was made in the atmosphere of Mars (Trokhimovskiy et al. [156]) by the Exo-Mars Trace Gas Orbiter ACS instrument (Korablev et al. [42]). This band is located in a CO₂ transparency window and identified as a magnetic dipole band (Perevalov et al. [157]). Detailed spectroscopic studies of this band providing the selection rules for the vibration-rotation transitions, as well as the line position and intensity measurements are given in Refs. [156,157]. The vibrational transition magnetic dipole moment of the v₂ + v₃ band was fit to the line intensities measured with a Bruker IFS 125 HR FTS and a 30 m base multipass gas cell of the V. E. Zuev Institute of Atmospheric Optics SB RAS (Borkov et al. [158]). Using the obtained vibrational transition magnetic dipole moment and the set of the effective Hamiltonian parameters (Majcherova et al. [159]), the line positions and intensities of this band were generated. The maximum line intensities are on the order of 3×10⁻²⁸ cm/molecule. The calculated line intensities for five R-branch lines of this band (R26-R32 and R36) are in a good agreement with the values measured independently by CRDS [160]. In the HITRAN2020 database, the calculated line parameters of this band are presented up to J = 64 corresponding to the intensity cutoff 10⁻³⁰ cm/molecule at 296 K. The line position uncertainty code 4 and line intensity uncertainty code 4 (see Table 2) are used for this band. It should be noted that line intensities of this band retrieved from laboratory spectra [158] are about two times smaller than those recovered from Martian atmosphere spectra [156]. To distinguish these transitions in the HITRAN2020 CO₂ line list, a letter “m” is introduced into the quantum notation of these magnetic dipole CO₂ transitions in the field dedicated to upper state rotational (“local”) quanta (see the Supplementary Material of this paper for the description of the upper- and lower-state quanta in the “.par” format).

### 2.2.6. CO₂ line-shape parameters

The approach we have taken to populate the line-shape parameters of CO₂ broadened by air and CO₂ (self-broadening) is described in the study by Hashemi et al. [161], where different comparisons of the parameters and various validation tests are carried out to demonstrate how the appropriate data sets were chosen for the HITRAN2020 edition. In this section, we highlight these updates and we explain the slight modification (regarding Ref. [161]) based on the new measurements.

The update to the line-shape parameters of CO₂ in the HITRAN2020 edition can be summarized in three components:

* Revising the Voigt profile (VP) [162] parameters that belong into the “.par” format file.

* Addition of the air and self speed-dependent Voigt (SDV) [163–165] parameters for all the transitions of CO₂.

* Updating the already-existing CO₂ line-mixing package developed by Lamouroux et al. [166], and addition of the first-order line-mixing to the database.

These parameters are listed in Table 3 for the Voigt profile (VP) and speed-dependent Voigt (SDV) parameter group in two separate sets.

### 2.2.7. Revising the Voigt profile parameters

The Voigt air- and self-broadened half-widths (γair and γself, respectively) of CO₂ lines and their temperature exponent parameters were re-assessed since the vibrational dependence of the line widths was found to be excessive for some of the CO₂ bands in the HITRAN2016 line list. The concern was that the line widths, calculated for the HITRAN2016 edition, were influenced by some of the less accurate measurements. Additionally, the Lorentzian widths, were retrieved using various line-shape profiles for different bands, and were used in the algorithm [129]. This discrepancy in the line widths using different line-shape models, which can alternate by about 5% [167–169], may have been inadvertently ascribed to a large vibrational dependence of the width parameters. For the bands probed by the OCO-2 mission (1.6 µm and 2.06 µm regions), the HITRAN2016 half-widths belonged to the experimental values of Ref. [170] obtained with the SDV profile without supplying the speed-dependent parameters in the database. To investigate the magnitude of the vibrational-dependence of the broadening parameters in HITRAN2016, the laboratory-measured widths and theoretical values for several bands were collected and examined for CO₂ lines using the VP [161]. Not uncommon for a linear molecule, a relatively weak vibrational dependence was revealed. Accordingly, with regard to updates of the line widths and their temperature dependences for the HITRAN2020 edition, the vibrational dependence of these parameters was ignored, and new values were produced for the air- and self-broadening parameters based on the measured data in Refs. [171,172] using semi-empirical models (the Padé approximants) described in Ref. [161]. In general Padé approximants (Eq. (1)) of the third and fourth-order are used extensively in this edition for many molecules.

$$ \gamma(|m|) = \frac{(a_0 + a_1|m| + a_2|m|^2 + a_3|m|^3 + a_4|m|^4)}{(1 + b_1|m| + b_2|m|^2 + b_3|m|^3 + b_4|m|^4)}, \quad (1) $$

where the rotational running index m was introduced to treat simultaneously the P-, Q- and R- branch transitions with the following relations to the rotational quanta:

$$ \begin{array}{ll} \text{P-branch:} & m = -J'' \\ \text{Q-branch:} & m = J'' \\ \text{R-branch:} & m = J'' + 1 \end{array} \qquad (2) $$

Moreover, the air- and self-shifts (δ₀) in HITRAN2016 (calculated using the semi-classical routine in Ref. [129]) for P- and R-branches were not asymmetric. To produce the rotational and vibrational dependence of shift parameters, the empirical model introduced by Hartmann [173] is implemented to determine the air- and self-shifts of lines for all the vibrational bands of CO₂. With regard to this approach, the shifts of CO₂ lines can be obtained from the available measured shift parameters for one band and, after properly determining the fitting coefficients explained in Refs. [161,173], the shift values can be expanded to the non-measured bands and transitions.

### 2.2.8. Air and self speed-dependent Voigt parameters

To reach the accuracy that is required in atmospheric CO₂ retrievals, it is imperative to include more refined line-shape parameters such as the air and self speed dependence of the line
intensities with the DLR measurements [153] showed rotationally dependent deviations up to 4% for the 00031–00001 band. In the HITRAN2020 line list, the UCL line intensities [137] of the 00031–00001 band were scaled by the factor of 1.1217 to match the line intensities measured by Birk et al. [153]. Also, the HITRAN2016 line intensities of the 10032–10002 and 01131–01101 relatively weak hot bands located near 6900 cm⁻¹ were compared to the DLR measurements [153]. It was found that the line intensities of the 10032–10002 band in HITRAN2016 should be scaled by a factor of 1.1346 while the line intensities of the 01131–01101 band should be scaled by a factor of 1.0022.

### 2.2.5. Introduction of magnetic dipole transitions

All previous editions of HITRAN provided only electric dipole transitions for CO₂. In this edition, the line parameters of the v₂ + v₃ magnetic dipole band of the ¹²C¹⁶O₂ isotopologue were introduced into HITRAN for the first time. These new data will help spectral studies of CO₂-rich planetary atmospheres. This band is forbidden in electric dipole absorption, but it is allowed in electric quadrupole and in magnetic dipole absorptions. The first observation of the v₂ + v₃ band of ¹²C¹⁶O₂ at 3.3 µm was made in the atmosphere of Mars (Trokhimovskiy et al. [156]) by the Exo-Mars Trace Gas Orbiter ACS instrument (Korablev et al. [42]). This band is located in a CO₂ transparency window and identified as a magnetic dipole band (Perevalov et al. [157]). Detailed spectroscopic studies of this band providing the selection rules for the vibration-rotation transitions, as well as the line position and intensity measurements are given in Refs. [156,157]. The vibrational transition magnetic dipole moment of the v₂ + v₃ band was fit to the line intensities measured with a Bruker IFS 125 HR FTS and a 30 m base multipass gas cell of the V. E. Zuev Institute of Atmospheric Optics SB RAS (Borkov et al. [158]). Using the obtained vibrational transition magnetic dipole moment and the set of the effective Hamiltonian parameters (Majcherova et al. [159]), the line positions and intensities of this band were generated. The maximum line intensities are on the order of 3×10⁻²⁸ cm/molecule. The calculated line intensities for five R-branch lines of this band (R26-R32 and R36) are in a good agreement with the values measured independently by CRDS [160]. In the HITRAN2020 database, the calculated line parameters of this band are presented up to J = 64 corresponding to the intensity cutoff 10⁻³⁰ cm/molecule at 296 K. The line position uncertainty code 4 and line intensity uncertainty code 4 (see Table 2) are used for this band. It should be noted that line intensities of this band retrieved from laboratory spectra [158] are about two times smaller than those recovered from Martian atmosphere spectra [156]. To distinguish these transitions in the HITRAN2020 CO₂ line list, a letter “m” is introduced into the quantum notation of these magnetic dipole CO₂ transitions in the field dedicated to upper state rotational (“local”) quanta (see the Supplementary Material of this paper for the description of the upper- and lower-state quanta in the “.par” format).

### 2.2.6. CO₂ line-shape parameters

The approach we have taken to populate the line-shape parameters of CO₂ broadened by air and CO₂ (self-broadening) is described in the study by Hashemi et al. [161], where different comparisons of the parameters and various validation tests are carried out to demonstrate how the appropriate data sets were chosen for the HITRAN2020 edition. In this section, we highlight these updates and we explain the slight modification (regarding Ref. [161]) based on the new measurements.

The update to the line-shape parameters of CO₂ in the HITRAN2020 edition can be summarized in three components:

* Revising the Voigt profile (VP) [162] parameters that belong into the “.par” format file.

* Addition of the air and self speed-dependent Voigt (SDV) [163–165] parameters for all the transitions of CO₂.

* Updating the already-existing CO₂ line-mixing package developed by Lamouroux et al. [166], and addition of the first-order line-mixing to the database.

These parameters are listed in Table 3 for the Voigt profile (VP) and speed-dependent Voigt (SDV) parameter group in two separate sets.

### 2.2.7. Revising the Voigt profile parameters

The Voigt air- and self-broadened half-widths ($γ_{air}$ and $γ_{self}$, respectively) of CO₂ lines and their temperature exponent parameters were re-assessed since the vibrational dependence of the line widths was found to be excessive for some of the CO₂ bands in the HITRAN2016 line list. The concern was that the line widths, calculated for the HITRAN2016 edition, were influenced by some of the less accurate measurements. Additionally, the Lorentzian widths, were retrieved using various line-shape profiles for different bands, and were used in the algorithm [129]. This discrepancy in the line widths using different line-shape models, which can alternate by about 5% [167–169], may have been inadvertently ascribed to a large vibrational dependence of the width parameters. For the bands probed by the OCO-2 mission (1.6 µm and 2.06 µm regions), the HITRAN2016 half-widths belonged to the experimental values of Ref. [170] obtained with the SDV profile without supplying the speed-dependent parameters in the database. To investigate the magnitude of the vibrational-dependence of the broadening parameters in HITRAN2016, the laboratory-measured widths and theoretical values for several bands were collected and examined for CO₂ lines using the VP [161]. Not uncommon for a linear molecule, a relatively weak vibrational dependence was revealed. Accordingly, with regard to updates of the line widths and their temperature dependences for the HITRAN2020 edition, the vibrational dependence of these parameters was ignored, and new values were produced for the air- and self-broadening parameters based on the measured data in Refs. [171,172] using semi-empirical models (the Padé approximants) described in Ref. [161]. In general Padé approximants (Eq. (1)) of the third and fourth-order are used extensively in this edition for many molecules.

$$γ(|m|) = \frac{(a_0 + a_1|m| + a_2|m|^2 + a_3|m|^3 + a_4|m|^4)}{(1 + b_1|m| + b_2|m|^2 + b_3|m|^3 + b_4|m|^4)}, \quad (1)$$

where the rotational running index m was introduced to treat simultaneously the P-, Q- and R- branch transitions with the following relations to the rotational quanta:

$$ \begin{array}{l} \text{P-branch: } m = -J'' \\ \text{Q-branch: } m = J'' \\ \text{R-branch: } m = J'' + 1 \end{array} \qquad (2) $$

Moreover, the air- and self-shifts ($δ₀$) in HITRAN2016 (calculated using the semi-classical routine in Ref. [129]) for P- and R-branches were not asymmetric. To produce the rotational and vibrational dependence of shift parameters, the empirical model introduced by Hartmann [173] is implemented to determine the air- and self-shifts of lines for all the vibrational bands of CO₂. With regard to this approach, the shifts of CO₂ lines can be obtained from the available measured shift parameters for one band and, after properly determining the fitting coefficients explained in Refs. [161,173], the shift values can be expanded to the non-measured bands and transitions.

### 2.2.8. Air and self speed-dependent Voigt parameters

To reach the accuracy that is required in atmospheric CO₂ retrievals, it is imperative to include more refined line-shape parameters such as the air and self speed dependence of the line