# The HITRAN2016 molecular spectroscopic database

I. E. Gordon, L. S. Rothman, C. Hill, Roman Kochanov, Y. Tan, P. Bernath, M. Birk, V. Boudon, A. Campargue, K. Chance, et al.

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## ► To cite this version:

I. E. Gordon, L. S. Rothman, C. Hill, Roman Kochanov, Y. Tan, et al. The HITRAN2016 molecular spectroscopic database. Journal of Quantitative Spectroscopy and Radiative Transfer, 2017, 203, pp.3 - 69. 10.1016/j.jqsrt.2017.06.038. hal-01765945

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Submitted on 3 Sep 2020

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Distributed under a Creative Commons Attribution 4.0 International License
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The HITRAN2016 molecular spectroscopic database

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I.E. Gordon<sup>a,*</sup>, L.S. Rothman<sup>a</sup>, C. Hill<sup>a,b</sup>, R.V. Kochanov<sup>a,c</sup>, Y. Tan<sup>a</sup>, P.F. Bernath<sup>d</sup>, M. Birk<sup>e</sup>, V. Boudon<sup>f</sup>, A. Campargue<sup>g</sup>, K.V. Chance<sup>a</sup>, B.J. Drouin<sup>h</sup>, J.-M. Flaud<sup>i</sup>, R.R. Gamache<sup>j</sup>, J.T. Hodges<sup>k</sup>, D. Jacquemart<sup>l</sup>, V.I. Perevalov<sup>m</sup>, A. Perrin<sup>n</sup>, K.P. Shine<sup>o</sup>, M.-A.H. Smith<sup>p</sup>, J. Tennyson<sup>b</sup>, G.C. Toon<sup>h</sup>, H. Tran<sup>n</sup>, V.G. Tyuterev<sup>q</sup>, A. Barbe<sup>q</sup>, A.G. Császár<sup>r,rr</sup>, V.M. Devi<sup>s</sup>, T. Furtenbacher<sup>r</sup>, J.J. Harrison<sup>t,tt,ttt</sup>, J.-M. Hartmann<sup>n</sup>, A. Jolly<sup>i</sup>, T.J. Johnson<sup>u</sup>, T. Karman<sup>a,v</sup>, I. Kleiner<sup>i</sup>, A.A. Kyuberis<sup>a,w</sup>, J. Loos<sup>e</sup>, O.M. Lyulin<sup>m</sup>, S.T. Massie<sup>x</sup>, S.N. Mikhailenko<sup>m</sup>, N. Moazzen-Ahmadi<sup>y</sup>, H.S.P. Müller<sup>z</sup>, O.V. Naumenko<sup>m</sup>, A.V. Nikitin<sup>m</sup>, O.L. Polyansky<sup>b,w</sup>, M. Rey<sup>q</sup>, M. Rotger<sup>q</sup>, S.W. Sharpe<sup>u</sup>, K. Sung<sup>h</sup>, E. Starikova<sup>m</sup>, S.A. Tashkun<sup>m</sup>, J. Vander Auwera<sup>aa</sup>, G. Wagner<sup>e</sup>, J. Wilzewski<sup>a,e</sup>, P. Wcisło<sup>bb</sup>, S. Yu<sup>h</sup>, E.J. Zak<sup>b</sup>

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

<footnote><sup>b</sup> University College London, Dept. of Physics and Astronomy, London WC1E 6BT, UK</footnote>

<footnote><sup>c</sup> Tomsk State University, Laboratory of Quantum Mechanics of Molecules and Radiative Processes, Tomsk, Russia</footnote>

<footnote><sup>d</sup> Old Dominion University, Dept. of Chemistry & Biochemistry, Norfolk, VA, USA</footnote>

<footnote><sup>e</sup> DLR, Institute for Remote Sensing Technology, Wessling, Germany</footnote>

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

<footnote><sup>g</sup> Université Grenoble, Grenoble, France</footnote>

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

<footnote><sup>i</sup> Université Paris Est, LISA, Créteil, France</footnote>

<footnote><sup>j</sup> University of Massachusetts, Dept. of Environmental, Earth & Atmospheric Sciences, Lowell, MA, USA</footnote>

<footnote><sup>k</sup> NIST, Chemical Sciences Division, Gaithersburg, MD, USA</footnote>

<footnote><sup>l</sup> Université Pierre et Marie Curie, LADIR, Paris, France</footnote>

<footnote><sup>m</sup> Institute of Atmospheric Optics, Laboratory of Theoretical Spectroscopy, Tomsk, Russia</footnote>

<footnote><sup>n</sup> CNRS, Ecole Polytechnique, Université Paris-Saclay, Laboratoire de Météorologie Dynamique/IPSL, 91128 Palaiseau, France</footnote>

<footnote><sup>o</sup> University of Reading, Dept. of Meteorology, Reading, UK</footnote>

<footnote><sup>p</sup> NASA Langley Research Center, Science Directorate, Hampton, VA, USA</footnote>

<footnote><sup>q</sup> Université de Reims, GSMA, Reims, France</footnote>

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

<footnote><sup>s</sup> The College of William and Mary, Dept. of Physics, Williamsburg, VA, USA</footnote>

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

<footnote><sup>u</sup> Battelle Pacific Northwest National Laboratory, Richland, WA, USA</footnote>

<footnote><sup>v</sup> Radboud University, Institute for Molecules and Materials, Nijmegen, The Netherlands</footnote>

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

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

<footnote><sup>y</sup> The University of Calgary, Dept. of Physics and Astronomy, Calgary, AB, Canada</footnote>

<footnote><sup>z</sup> University of Cologne, I. Physikalisches Institut, Cologne, Germany</footnote>

<footnote><sup>aa</sup> Université Libre de Bruxelles, Service de Chimie Quantique et Photophysique, C.P. 160/09, B-1050 Brussels, Belgium</footnote>

<footnote><sup>bb</sup> Nicolaus Copernicus University, Institute of Physics, Torun, Poland</footnote>

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

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

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

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

Article history:

Received 23 February 2017

Received in revised form

29 June 2017

Accepted 29 June 2017

Available online 5 July 2017

Keywords:

HITRAN

Spectroscopic database

Molecular spectroscopy

Molecular absorption

Spectroscopic line parameters

Absorption cross-sections

Collision-induced absorption

Aerosols

ABSTRACT

This paper describes the contents of the 2016 edition of the HITRAN molecular spectroscopic compilation. The new edition replaces the previous HITRAN edition of 2012 and its updates during the intervening years. The HITRAN molecular absorption compilation is composed of five major components: the traditional line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, infrared absorption cross-sections for molecules not yet amenable to representation in a line-by-line form, collision-induced absorption data, aerosol indices of refraction, and general tables such as partition sums that apply globally to the data. The new HITRAN is greatly extended in terms of accuracy, spectral coverage, additional absorption phenomena, added line-shape formalisms, and validity. Moreover, molecules, isotopologues, and perturbing gases have been added that address the issues of atmospheres beyond the Earth. Of considerable note, experimental IR cross-sections for almost 300 additional molecules important in different areas of atmospheric science have been added to the database. The compilation can be accessed through www.hitran.org. Most of the HITRAN data have now been cast into an underlying relational database structure that offers many advantages over the long-standing sequential text-based structure. The new structure empowers the user in many ways. It enables the incorporation of an extended set of fundamental parameters per transition, sophisticated line-shape formalisms, easy user-defined output formats, and very convenient searching, filtering, and plotting of data. A powerful application programming interface making use of structured query language (SQL) features for higher-level applications of HITRAN is also provided.

Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

This article describes the data and software that have been added, modified, or enhanced in the HITRAN (High Resolution Transmission) compilation since the previous edition of 2012 [1] (hereafter called HITRAN2012 in the text). The line-by-line portion and the absorption cross-sections of the new edition, hereafter called HITRAN2016, have now been cast into an underlying relational database structure as opposed to the long-standing fixed-length ASCII record format. A user-friendly powerful internet tool, HITRANonline (accessible through www.hitran.org), is provided to the user to query, filter, and plot sections of the data and to retrieve outputs in a host of convenient formats (see Hill et al. [2]).

The HITRAN compilation is composed of several components that include (1) line-by-line spectroscopic parameters for high resolution molecular absorption and radiance calculations (from the microwave through the ultraviolet region of the spectrum); (2) infrared absorption cross-sections (generally representing absorption by molecules that have very dense spectra or many low-lying vibrational modes); (3) collision-induced absorption datasets; (4) tables of aerosol refractive indices; and (5) global data that apply in a general manner to the archive. The updates to these five portions of HITRAN as well as the new underlying system of data structure with accompanying internet user interface will be discussed in the following sections.

The new edition of the HITRAN database substantially increases the database's potential to accurately model radiative processes in the atmosphere of the Earth and other planets. Apart from improving the accuracy of the existing parameters, more parameters have been introduced, and new bands and isotopologues added. Two gases ($C_2N_2$ and $COCl_2$) are introduced to the database for the first time. Finally, a very dramatic expansion of the cross-sectional part of the database is featured in this new edition.

It is necessary to call attention to some specifics of the HITRAN database. The units used throughout HITRAN editions including this one do not strictly adhere to the SI system for both historical and application-specific reasons. Thus $cm^{-1}$ (reciprocal centimeter) is seen throughout, as is atm (atmosphere) for pressure (in SI units of Pascals, 101,325 Pa = 1 atm). Also, the symbol $\nu$ is used throughout for line position in $cm^{-1}$, thereby dropping the tilde ($\tilde{\nu}$) that is the official designation of wavenumber. The HITRAN unit for intensity is traditionally expressed as $cm^{-1}$/

(molecule $cm^{-2}$) rather than simplifying to the equivalent cm molecule$^{-1}$.

1.1. Overview of parameters (including new ones)

The traditional and probably most applied portion of HITRAN is the line list of high-resolution spectral parameters. These line-by-line parameters reflect values of individual transitions between energy levels of rovibronic states that are required by high-resolution radiative-transfer codes. These parameters are shown in Tables 1 and 2. Table 1 gives an overview of parameters that are traditionally provided in the “.par” format as per HITRAN2004 [3] formalism as well as broadening and shift parameters due to the pressure of $H_2$, He and $CO_2$ (see Wilzewski et al. [4], Hill et al. [2] and Kochanov et al. [5] for details). Table 2 provides information on the parameters required for non-Voigt line-shape representations (see Wcisło et al. [6], Hill et al. [2] and Kochanov et al. [5] for details). Ideally, they would arise from theoretically-consistent calculations; however, since the emphasis in HITRAN is to provide the user with the most accurate values available, they are often the values obtained from controlled laboratory measurements when quantum-mechanical calculations are not yet of comparable accuracy. Nevertheless, theoretical calculations and semi-empirical formulae are widely used in HITRAN to both interpolate and extrapolate needed parameters for transitions missing from the limited observed set.

Section 2 presents a description of changes made to the line-by-line portion of HITRAN. It is organized in the order of the molecules in HITRAN (a sequential number related to the entry of the molecule into HITRAN). The discussions in the molecular subsections give an overview of the addition of new bands, replacement of line positions and/or intensities, and new or modified line-shape parameters where applicable. There are also citations to more detailed studies of the modifications.

With the recent advances in both laboratory spectrometers and the power of theoretical treatments such as ab initio calculations, the accuracy and the access to weak, yet important, transitions have had a big impact on this edition of HITRAN.

Table 3 provides an overview of changes and additions to the database with respect to the HITRAN2012 edition.

The line-by-line parameters (as well as the portion of HITRAN devoted to cross-sections) have now been cast into a relational
<table><caption>**Table 1**

Parameter identifiers in HITRANonline.</caption><thead><tr><th colspan="4">Parameters from the HITRAN2004 format in HITRANonline</th></tr><tr><th>Parameter</th><th>Units</th><th>Identifier<sup>a</sup></th><th>Description</th></tr></thead><tbody><tr><td>Molecule ID</td><td>–</td><td>molec_id</td><td>HITRAN integer identifying the molecule</td></tr><tr><td>Isotopologue ID</td><td>–</td><td>local_iso_id</td><td>HITRAN integer identifying the isotopologue of a particular molecule</td></tr><tr><td>ν</td><td>cm<sup>-1</sup></td><td>nu</td><td>Transition wavenumber</td></tr><tr><td>S</td><td>cm<sup>-1</sup>/(molecule cm<sup>-2</sup>)</td><td>sw</td><td>Transition intensity, weighted by isotopologue abundance</td></tr><tr><td>A</td><td>s<sup>-1</sup></td><td>a</td><td>Einstein A-coefficient</td></tr><tr><td>γ<sub>air</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma-air</td><td>Air-broadened Lorentzian half width at half maximum (HWHM) coefficient (for Voigt lineshape)</td></tr><tr><td>γ<sub>self</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma- self</td><td>Self-broadened Lorentzian HWHM coefficient (for Voigt lineshape)</td></tr><tr><td>E''</td><td>cm<sup>-1</sup></td><td>elower</td><td>Lower-state energy</td></tr><tr><td>n<sub>air</sub></td><td>–</td><td>n-air</td><td>Temperature-dependence exponent for γ<sub>air</sub></td></tr><tr><td>δ<sub>air</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>delta-air</td><td>Air pressure-induced line shift</td></tr><tr><td>V<sup>b</sup></td><td>–</td><td>global upper quanta</td><td>Upper-state "global" quanta in HITRAN2004 format</td></tr><tr><td>V<sup>b</sup></td><td>–</td><td>global lower quanta</td><td>Lower-state "global" quanta in HITRAN2004 format</td></tr><tr><td>Q<sup>b</sup></td><td>–</td><td>local upper quanta</td><td>Upper-state "local" quanta in HITRAN2004 format</td></tr><tr><td>Q<sup>b</sup></td><td>–</td><td>local lower quanta</td><td>Lower-state "local" quanta in HITRAN2004 format</td></tr><tr><td>I<sub>err</sub><sup>b</sup></td><td>–</td><td>ierr</td><td>Ordered list of indices corresponding to uncertainty estimates of transition parameters</td></tr><tr><td>I<sub>ref</sub><sup>b</sup></td><td>–</td><td>iref</td><td>Ordered list of reference identifiers for transition parameters</td></tr><tr><td>g'</td><td>–</td><td>gp</td><td>Upper-state statistical weight</td></tr><tr><td>g''</td><td>–</td><td>gpp</td><td>Lower-state statistical weight</td></tr><tr><td colspan="4"><strong>Metadata and other special parameters</strong></td></tr><tr><td>Transition ID</td><td>–</td><td>trans_id</td><td>Unique integer identifying the transition</td></tr><tr><td>Global Isotopologue ID</td><td>–</td><td>global_iso_id</td><td>Global integer ID identifying the isotopologue (unique across the whole database)</td></tr><tr><td>qns'</td><td>–</td><td>statep</td><td>Upper-state quantum numbers in HITRANonline format</td></tr><tr><td>qns''</td><td>–</td><td>statepp</td><td>Lower-state quantum numbers in HITRANonline format</td></tr><tr><td>.par line</td><td>–</td><td>par_line</td><td>Complete representation of the line in the HITRAN2004 160-character format</td></tr><tr><td colspan="4"><strong>Parameters for broadening by new perturbing species, X<sup>c</sup>, and line-mixing parameters</strong></td></tr><tr><td>γ<sub>X</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma_X</td><td>Lorentzian HWHM coefficient (for Voigt lineshape) for broadening by perturber X</td></tr><tr><td>n<sub>X</sub></td><td>–</td><td>n_X</td><td>Temperature-dependence exponent for γ<sub>X</sub></td></tr><tr><td>n<sub>self</sub></td><td>–</td><td>n self</td><td>Temperature-dependence exponent for γ<sub>self</sub></td></tr><tr><td>δ<sub>self</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>delta self</td><td>Self-induced pressure line shift</td></tr><tr><td>δ<sup>air</sup></td><td>cm<sup>-1</sup> atm<sup>-1</sup> K<sup>-1</sup></td><td>deltap-air</td><td>Linear temperature dependence coefficient for δ<sub>air</sub></td></tr><tr><td>δ<sup>self</sup></td><td>cm<sup>-1</sup> atm<sup>-1</sup> K<sup>-1</sup></td><td>deltap-self</td><td>Linear temperature dependence coefficient for δ<sub>self</sub></td></tr><tr><td>δ<sub>H2</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup> K<sup>-1</sup></td><td>deltap-H2</td><td>Linear temperature dependence coefficient for δ<sub>H2</sub></td></tr><tr><td>δ<sub>X</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>delta_X</td><td>Pressure-induced line shift due to perturber X</td></tr><tr><td>Y<sub>air</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>y-air</td><td>First-order (Rosenkranz) line coupling coefficient within Voigt profile; air-broadened environment</td></tr><tr><td>Y<sub>self</sub></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>y-self</td><td>First-order (Rosenkranz) line coupling coefficient within Voigt profile; air-broadened environment</td></tr></tbody></table>

database structure, described in Section 6.1. This underlying structure has many advantages that were not available in the previous fixed-length format of previous HITRAN editions. The database is now able to add many more parameters (becoming necessary not only for planetary atmosphere applications, but also to satisfy sophisticated remote-sensing requirements), is not limited to constraints of field length, and can accommodate functional forms for parameters. It also allows for a much more robust way for the managers to check for errors, whether they be by transcription, violated selection rules, incorrect quantum assignments, etc.

## 1.2. New line-shape representations

Shapes of individual transitions in molecular spectra at atmospheric conditions are mainly determined by collisions and the Doppler effect. In the simplest case these two effects can be handled with the Voigt profile, where the collisions are described just as a single-parameter exponential relaxation. In the previous

editions of HITRAN, the line shapes, with a very few exceptions, were represented with the Voigt profile. It turns out, however, that this model is too simple to reproduce atmospheric spectra with accuracy at the sub-percent level (see for instance Refs. [9,10]). For some light molecules, in particular for molecular hydrogen (important for research on dense atmospheres of gas giants), the non-Voigt effects are much more pronounced [6]. At this point the database provides Voigt (for every transition), and speed-dependent Voigt, Galatry [11] and Hartmann-Tran [7,8] profiles parameters where they had been measured and validated.

Ab initio line-shape models, originating from first principles, are far too complex to be simply implemented in a line-by-line spectroscopic database. Therefore, a compromise between simplicity of the model and the fidelity of experimental spectra representation had to be found. In recent decades, a number of phenomenological models that take into account both speed-dependent effects and velocity-changing collisions had been advanced. Recently, Tran et al. developed a computationally efficient algorithm [7,8] for calculating one of them, the partially-correlated

<footnote><sup>a</sup> The third column header, identifier, denotes the names of the variables used in the new relational database structure. <sup>b</sup> These parameters are given in Table 1 in the way they appear in the ".par" file. However, one should note that they are stored in a flexible and substantially more detailed way in the database. The names containing X correspond to the foreign broadeners, where X is the name of the perturbing species (CO<sub>2</sub>, H<sub>2</sub>, He). Unless explicitly given, the parameters imply transition between lower and upper states; double primes (") and primes (') are for lower and upper energy states, respectively.</footnote>
<table><caption>**Table 2**
Parameter identifiers in HITRANonline for non-Voigt parameters.</caption><thead><tr><th colspan="3">Parameters for non-Voigt parameters</th></tr><tr><th>Parameter</th><th>Units</th><th>Identifier<sup>a</sup></th><th>Description</th></tr></thead><tbody><tr><td>γ<sub>0</sub><sup>SDV</sup>(X; 296)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma_SDV_0_X_296</td><td>Air-(Self-) broadened Lorentzian HWHM coefficient in 296 K (for SDV profile)</td></tr><tr><td>γ<sub>2</sub><sup>SDV</sup>(X; 296)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma_SDV_2_X_296</td><td>Speed-dependence of the SDV halfwidth in 296 K; air-(self-) broadened case</td></tr><tr><td>δ<sub>0</sub><sup>SDV</sup>(X; 296)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>delta_SDV_0_X_296</td><td>Air-(Self-) induced pressure line shift in 296 K (for SDV profile)</td></tr><tr><td>δ<sup>SDV</sup>(X; 296)</td><td>cm<sup>-1</sup> atm<sup>-1</sup> K<sup>-1</sup></td><td>deltap_SDV_X_296</td><td>Linear temperature dependence coefficient for δ<sub>0</sub><sup>SDV</sup>(X; 296)</td></tr><tr><td>n<sup>SDV</sup>(X; 296)</td><td>–</td><td>n_SDV_X_296</td><td>Temperature dependence exponent around 296 K for γ<sub>0</sub><sup>SDV</sup>(X; 296)</td></tr><tr><td>γ<sub>X</sub><sup>SDV</sup>(296)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>Y_SDV_X_296</td><td>First-order (Rosenkranz) line coupling coefficient within SDV<sup>b</sup> profile; air-(self-) broadened case</td></tr><tr><td>γ<sub>X</sub><sup>G</sup></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma_g_X</td><td>Air-(Self-) broadened Lorentzian HWHM coefficient in 296 K (for Galatry profile)</td></tr><tr><td>μ<sub>X</sub><sup>G</sup></td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>beta_g_X</td><td>Dicke narrowing parameter for the air-(self-) broadened Galatry line profile</td></tr><tr><td>γ<sub>0</sub><sup>HT</sup>(X; T<sub>ref</sub>)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma-HT_0_X_T</td><td>Speed-averaged HTP halfwidth in temperature range around T = T<sub>ref</sub> <sup>d</sup> due to perturber X</td></tr><tr><td>n<sup>HT</sup>(X; T<sub>ref</sub>)</td><td>–</td><td>n-HT_X_T</td><td>Temperature dependence exponent in the range corresponding to T = T<sub>ref</sub> for γ<sub>0</sub><sup>HT</sup>(X; T<sub>ref</sub>)</td></tr><tr><td>γ<sub>2</sub><sup>HT</sup>(X; T<sub>ref</sub>)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>gamma-HT_2_X_T</td><td>Speed-dependence of the HTP halfwidth in temperature range around T = T<sub>ref</sub> due to perturber X.</td></tr><tr><td>δ<sub>0</sub><sup>HT</sup>(X; T<sub>ref</sub>)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>delta-HT_0_X_T</td><td>Speed-averaged line shift of the HTP in temperature range around T = T<sub>ref</sub> due to perturber X</td></tr><tr><td>δ<sup>HT</sup>(X; T<sub>ref</sub>)</td><td>cm<sup>-1</sup> atm<sup>-1</sup> K<sup>-1</sup></td><td>deltap-HT_0_X_T</td><td>Linear temperature dependence coefficient for δ<sub>0</sub><sup>HT</sup>(X; T<sub>ref</sub>) in temperature range around T = T<sub>ref</sub></td></tr><tr><td>δ<sub>2</sub><sup>HT</sup>(X; T<sub>ref</sub>)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>delta-HT_2_X_T</td><td>Speed-dependence of the HTP line shift in temperature range around T = T<sub>ref</sub> due to perturber X</td></tr><tr><td>ν<sup>HT</sup>(X)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>nu-HT_X</td><td>Frequency of velocity changing collisions in the HT profile formalism</td></tr><tr><td>κ<sup>HT</sup>(X)</td><td>–</td><td>kappa-HT_X</td><td>Temperature dependence of ν<sup>HT</sup>(X)</td></tr><tr><td>η<sup>HT</sup>(X)</td><td>–</td><td>eta-HT_X</td><td>Correlation parameter in HT profile formalism</td></tr><tr><td>Y<sup>HT</sup>(X; 296)</td><td>cm<sup>-1</sup> atm<sup>-1</sup></td><td>Y-HT_X_296</td><td>First-order (Rosenkranz) line coupling coefficient within HT profile; air-(self-) broadened case</td></tr></tbody></table>

speed-dependent hard-collision profile [12] with quadratic speed dependence [9]. An IUPAC (International Union of Pure and Applied Chemistry) task group recommended the use of this profile for high-resolution spectra representation [13] and call it the Hartmann-Tran (HT) profile. Ngo et al. [14] demonstrated that the HT profile well reconstructs the line shapes of the most important atmospheric molecules. Therefore, the HT profile has been adopted as the default beyond-Voigt profile in HITRAN. For measurements with sufficiently high accuracy, we encourage spectroscopists to report not only the basic Voigt-profile parameters, but also the HT parameters. The details about the HT parametrization adopted in HITRAN are given in Wcisło et al. [6]. Apart from new parameters introduced to represent this profile, one should note that these parameters are also provided at four different temperature ranges: 0–100 K, 100–200 K, 200–400 K and T> 400 K. The reference parameters are given at the following reference temperatures: 50 K, 150 K, 296 K and 700 K respectively.

It should be noted that even if the parameters of some more advanced model (preferably HT) are now provided in HITRAN for a growing number of transitions, the Voigt-profile parameters are still given for every transition in HITRAN. Therefore, for users not interested in making use of sophisticated line shapes, the present version of HITRAN remains compatible with the previous ones.

## 2. Line-by-line modifications

### 2.1. H<sub>2</sub>O (molecule 1)

The HITRAN2016 edition has undergone a substantial revision and expansion of the database of water vapor.

In HITRAN2012 [1] a very large expansion of the dynamic range of the line intensities for non-deuterated isotopologues of water vapor was implemented, thanks to the ab initio calculations from

the BT2 line list [15] and, in selected spectral intervals, Lodi et al. [16] for the principal isotopologue and from Lodi and Tennyson for H<sub>2</sub><sup>18</sup>O and H<sub>2</sub><sup>17</sup>O [17]. The ab initio results were replaced with high-quality experimental or-semi-empirical data wherever possible. In HITRAN2016, we have done a similar expansion of the dynamic (and spectral) range of the singly-deuterated isotopologues. In addition, the D<sub>2</sub><sup>16</sup>O isotopologue makes its debut in the database. D<sub>2</sub>O has a very low natural abundance on Earth but is measurable in the atmospheres of other planets, notably Venus which has an enhanced deuterium content [18] raising the importance of D<sub>2</sub>O. Also, for fitting D-enriched HDO laboratory spectra, a good D<sub>2</sub>O line list is essential. For this reason, D<sub>2</sub>O transitions have been included using a lower intensity cutoff of 10<sup>-32</sup> cm<sup>-1</sup>/(molecule·cm<sup>-2</sup>) once isotopic abundance is accounted for.

A dedicated paper detailing the update of the water-vapor dataset in HITRAN2016 along with atmospheric and laboratory validations is planned. Here we summarize only the most important details.

#### 2.1.1. Line positions and intensities

This update is informed by the work of an International Union of Pure and Applied Chemistry (IUPAC) task group which produced systematic sets of empirical energy levels (and hence transition frequencies) for all the stable isotopologues of water [19–22]. In HITRAN2012 non-deuterated isotopologues already benefited from the availability of the IUPAC dataset of empirically-derived energy levels (and transition wavenumbers derived from them), and we extend this to the deuterated species here. However, some important caveats of that compilation have to be noted.

a) The accuracy of some of the transitions generated from the IUPAC set may be inferior to individual high-accuracy experiments. Therefore, just like in HITRAN2012, we have given

<footnote><sup>a</sup> The third column header, identifier, denotes the names of the variables used in the new relational database structure. The names containing the X literal correspond to air- or self- broadening</footnote>

<footnote><sup>b</sup> SDV stands for speed-dependent Voigt.</footnote>

<footnote><sup>c</sup> HT stands for the Hartmann-Tran profile [7,8].</footnote>

<footnote><sup>d</sup> For the four temperature ranges and corresponding T<sub>ref</sub> values consult Wcisło et al. [6] and Sections 1.2 and 2.45.</footnote>
<table><caption>**Table 3**
Molecules and isotopologues represented in line-by-line portion of HITRAN.</caption><thead><tr><th>Molecule</th><th>Isotopologue<sup>a</sup></th><th>HITRAN2012 Spectral Coverage (cm<sup>-1</sup>)</th><th>HITRAN2016 Spectral Coverage (cm<sup>-1</sup>)</th><th>HITRAN2012 Number of Transitions</th><th>HITRAN2016 Number of Transitions</th><th>Non-Voigt line shape for at least some transitions?</th><th>Broadening by H<sub>2</sub>, He and CO<sub>2</sub></th></tr></thead><tbody><tr><td rowspan="6">(1) H<sub>2</sub>O</td><td>161</td><td>0–25,711</td><td>0–25,711</td><td>142 045</td><td>140 751<sup>d</sup></td><td rowspan="6">Yes</td><td rowspan="6"></td></tr><tr><td>181<sup>b</sup></td><td>0–19,918</td><td>0–19,918</td><td>39 903</td><td>39 901</td></tr><tr><td>171<sup>b</sup></td><td>0–19,946</td><td>0–19,946</td><td>27 544</td><td>27 544</td></tr><tr><td>162</td><td>0–22,708</td><td>0–22,708</td><td>13 237</td><td>56 430</td></tr><tr><td>182</td><td>0–3825</td><td>0–10,729</td><td>1611</td><td>10 664</td></tr><tr><td>172</td><td>1234–1599</td><td>0–10,703</td><td>175</td><td>6313</td></tr><tr><td>262</td><td>c</td><td>0–12,812</td><td>c</td><td>32 184</td><td></td><td></td></tr><tr><td rowspan="10">(2) CO<sub>2</sub></td><td>626</td><td>345–12,785</td><td>158–14,076</td><td>169 292</td><td>173 024</td><td></td><td></td></tr><tr><td>636</td><td>406–12,463</td><td>332–13,735</td><td>70 611</td><td>70 577</td><td></td><td></td></tr><tr><td>628</td><td>0–9558</td><td>1–12,678</td><td>116 482</td><td>127 850</td><td></td><td></td></tr><tr><td>627</td><td>0–9600</td><td>0–12,727</td><td>72 525</td><td>77 941</td><td></td><td></td></tr><tr><td>638</td><td>489–6745</td><td>2–9213</td><td>26 737</td><td>43 782</td><td></td><td></td></tr><tr><td>637</td><td>583–6769</td><td>9–8062</td><td>2953</td><td>25 175</td><td></td><td></td></tr><tr><td>828</td><td>491–8161</td><td>482–8163</td><td>7118</td><td>10 522</td><td></td><td></td></tr><tr><td>827</td><td>626–5047</td><td>491–8194</td><td>821</td><td>15 878</td><td></td><td></td></tr><tr><td>727</td><td>535–6933</td><td>535–6933</td><td>5187</td><td>6518</td><td></td><td></td></tr><tr><td>838</td><td>4599–4888</td><td>2245–4751</td><td>121</td><td>2916</td><td></td><td></td></tr><tr><td>837</td><td>c</td><td>549–4915</td><td>c</td><td>4190</td><td></td><td></td></tr><tr><td>737</td><td>c</td><td>575–3615</td><td>c</td><td>1501</td><td></td><td></td></tr><tr><td rowspan="5">(3) O<sub>3</sub></td><td>666</td><td>0–6997</td><td>0–6997</td><td>261 886</td><td>289 340</td><td></td><td></td></tr><tr><td>668</td><td>0–2768</td><td>0–2768</td><td>44 302</td><td>44 302</td><td></td><td></td></tr><tr><td>686</td><td>1–2740</td><td>1–2740</td><td>18 887</td><td>18 887</td><td></td><td></td></tr><tr><td>667</td><td>0–2122</td><td>0–2122</td><td>65 106</td><td>65 106</td><td></td><td></td></tr><tr><td>676</td><td>0–2101</td><td>0–2101</td><td>31 935</td><td>31 935</td><td></td><td></td></tr><tr><td rowspan="5">(4) N<sub>2</sub>O</td><td>446<sup>b</sup></td><td>0–7797</td><td>0–7797</td><td>33 074</td><td>33 074</td><td rowspan="5">Yes</td><td rowspan="5"></td></tr><tr><td>456</td><td>5–5086</td><td>5–5086</td><td>4222</td><td>4222</td></tr><tr><td>546</td><td>4–4704</td><td>4–4704</td><td>4592</td><td>4592</td></tr><tr><td>448</td><td>542–4672</td><td>0–4672</td><td>10 364</td><td>116 694</td></tr><tr><td>447</td><td>550–4430</td><td>550–4430</td><td>1705</td><td>1705</td></tr><tr><td rowspan="5">(5) CO</td><td>26</td><td>3–8465</td><td>3–14,478</td><td>1019</td><td>1344</td><td rowspan="5">Yes</td><td rowspan="5">Yes</td></tr><tr><td>36</td><td>3–6279</td><td>3–12,231</td><td>797</td><td>1042</td></tr><tr><td>28</td><td>3–6267</td><td>3–12,205</td><td>770</td><td>920</td></tr><tr><td>27</td><td>3–6339</td><td>3–10,295</td><td>728</td><td>800</td></tr><tr><td>38</td><td>3–6124</td><td>3–8078</td><td>712</td><td>674</td></tr><tr><td>37</td><td>1807–6197</td><td>3–8168</td><td>580</td><td>601</td><td></td><td></td></tr><tr><td rowspan="4">(6) CH<sub>4</sub></td><td>211</td><td>0–11,502</td><td>0–11,502</td><td>336 829</td><td>313 943</td><td></td><td></td></tr><tr><td>311</td><td>0–11,319</td><td>0–11,319</td><td>72 420</td><td>77 626</td><td></td><td></td></tr><tr><td>212</td><td>15–6511</td><td>7–6511</td><td>54 550</td><td>54 550</td><td></td><td></td></tr><tr><td>312</td><td>959–1695</td><td>959–1695</td><td>4213</td><td>4213</td><td></td><td></td></tr><tr><td rowspan="3">(7) O<sub>2</sub></td><td>66</td><td>0–15,928</td><td>0–57,028</td><td>1787</td><td>15 263</td><td rowspan="3">Yes</td><td rowspan="3">Yes</td></tr><tr><td>68</td><td>1–15,853</td><td>1–56,670</td><td>875</td><td>2965</td></tr><tr><td>67<sup>b</sup></td><td>0–14,538</td><td>0–14,537</td><td>11 313</td><td>11 313</td></tr><tr><td rowspan="3">(8) NO</td><td>46</td><td>0–9274</td><td>0–9274</td><td>103 701</td><td>103 701<sup>d</sup></td><td></td><td></td></tr><tr><td>56</td><td>1609–2061</td><td>1609–2061</td><td>699</td><td>699</td><td></td><td></td></tr><tr><td>48</td><td>1602–2039</td><td>1602–2039</td><td>679</td><td>679</td><td></td><td></td></tr><tr><td rowspan="2">(9) SO<sub>2</sub></td><td>626<sup>b</sup></td><td>0–4093</td><td>0–4093</td><td>72 460</td><td>72 460</td><td></td><td rowspan="2">Yes</td></tr><tr><td>646<sup>b</sup></td><td>0–2501</td><td>0–2501</td><td>22 661</td><td>22 661</td><td></td></tr><tr><td>(10) NO<sub>2</sub></td><td>646</td><td>0–3075</td><td>0–3075</td><td>104 223</td><td>104 223</td><td></td><td></td></tr><tr><td rowspan="2">(11) NH<sub>3</sub></td><td>446</td><td>0–7000</td><td>0–10,349</td><td>45 302</td><td>65 828</td><td rowspan="2">Yes</td><td rowspan="2">Yes</td></tr><tr><td>456</td><td>0–5180</td><td>0–5180</td><td>1090</td><td>1320</td></tr><tr><td rowspan="2">(12) HNO<sub>3</sub></td><td>146</td><td>0–1770</td><td>0–1770</td><td>903 854</td><td>950 864</td><td></td><td></td></tr><tr><td>156</td><td>0–923</td><td>0–923</td><td>58 108</td><td>58 108</td><td></td><td></td></tr><tr><td rowspan="3">(13) OH</td><td>61</td><td>0–19,268</td><td>0–19,268</td><td>30 772</td><td>30 772</td><td></td><td></td></tr><tr><td>81</td><td>0–329</td><td>0–329</td><td>295</td><td>295</td><td></td><td></td></tr><tr><td>62</td><td>0–332</td><td>0–332</td><td>912</td><td>912</td><td></td><td></td></tr><tr><td rowspan="2">(14) HF</td><td>19</td><td>24–46,985</td><td>24–32,352</td><td>10 073</td><td>8090</td><td rowspan="2">Yes</td><td rowspan="2">Yes</td></tr><tr><td>29</td><td>13–47,365</td><td>13–20,830</td><td>24 303</td><td>11 920</td></tr><tr><td rowspan="5">(15) HCl</td><td>15</td><td>8–34,250</td><td>8–20,321</td><td>11 879</td><td>8892</td><td rowspan="5">Yes</td><td rowspan="5">Yes</td></tr><tr><td>17</td><td>8–34,240</td><td>8–20,219</td><td>11 907</td><td>8908</td></tr><tr><td>25</td><td>5–33,284</td><td>5–15,266</td><td>29 994</td><td>17 762</td></tr><tr><td>27</td><td>5–33,258</td><td>5–15,247</td><td>29 911</td><td>17 690</td></tr><tr><td>19</td><td>13–16,034</td><td>13–16,034</td><td>3039</td><td>3039</td></tr><tr><td rowspan="3">(16) HBr</td><td>11</td><td>13–16,032</td><td>13–16,032</td><td>3031</td><td>3031</td><td></td><td></td></tr><tr><td>29</td><td>7–8781</td><td>7–8781</td><td>1455</td><td>1455</td><td></td><td></td></tr><tr><td>21</td><td>7–8778</td><td>7–8778</td><td>1455</td><td>1455</td><td></td><td></td></tr><tr><td rowspan="2">(17) HI</td><td>17</td><td>10–13,908</td><td>10–13,908</td><td>3161</td><td>3161</td><td></td><td></td></tr><tr><td>27</td><td>5–7625</td><td>5–7625</td><td>1590</td><td>1590</td><td></td><td></td></tr><tr><td rowspan="2">(18) ClO</td><td>56</td><td>0–1208</td><td>0–1208</td><td>5721</td><td>5721</td><td></td><td></td></tr><tr><td>76</td><td>0–1200</td><td>0–1200</td><td>5780</td><td>5780</td><td></td><td></td></tr><tr><td rowspan="4">(19) OCS</td><td>622</td><td>0–4200</td><td>0–7822</td><td>15 618</td><td>18 264</td><td rowspan="4">Yes</td><td rowspan="4">Yes</td></tr><tr><td>624</td><td>0–4166</td><td>0–7796</td><td>6087</td><td>6846</td></tr><tr><td>632</td><td>0–4056</td><td>0–6660</td><td>3129</td><td>3275</td></tr><tr><td>623</td><td>0–4164</td><td>0–6631</td><td>2886</td><td>3005</td></tr></tbody></table>
<table><caption>**Table 3 (continued)**</caption><thead><tr><th>Molecule</th><th>Isotopologue<sup>a</sup></th><th>HITRAN2012 Spectral Coverage (cm<sup>-1</sup>)</th><th>HITRAN2016 Spectral Coverage (cm<sup>-1</sup>)</th><th>HITRAN2012 Number of Transitions</th><th>HITRAN2016 Number of Transitions</th><th>Non-Voigt line shape for at least some transitions?</th><th>Broadening by H<sub>2</sub>, He and CO<sub>2</sub></th></tr></thead><tbody><tr><td rowspan="3">(20) H<sub>2</sub>CO</td><td>822</td><td>0–4046</td><td>0–4046</td><td>1641</td><td>1640</td><td rowspan="3">Yes</td><td rowspan="3"></td></tr><tr><td>126</td><td>0–3100</td><td>0–3100</td><td>40 670</td><td>40 670</td></tr><tr><td>136</td><td>0–117</td><td>0–117</td><td>2309</td><td>2309</td></tr><tr><td rowspan="2">(21) HOCl</td><td>128</td><td>0–101</td><td>0–101</td><td>1622</td><td>1622</td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>165<sup>b</sup></td><td>1–3800</td><td>1–3800</td><td>8877</td><td>8877</td></tr><tr><td rowspan="2">(22) N<sub>2</sub></td><td>44</td><td>11–9355</td><td>11–9355</td><td>1107</td><td>1107</td><td rowspan="2"></td><td rowspan="2"></td></tr><tr><td>45<sup>b</sup></td><td>11–2578</td><td>11–2578</td><td>161</td><td>161</td></tr><tr><td rowspan="3">(23) HCN</td><td>124</td><td>0–3424</td><td>0–17,586</td><td>2955</td><td>58 108</td><td rowspan="3"></td><td rowspan="3">Yes</td></tr><tr><td>134</td><td>2–3405</td><td>2–3405</td><td>652</td><td>652</td></tr><tr><td>125</td><td>2–3420</td><td>2–3420</td><td>646</td><td>646</td></tr><tr><td rowspan="2">(24) CH<sub>3</sub>Cl</td><td>215</td><td>0–3198</td><td>0–3198</td><td>107 642</td><td>110 462</td><td rowspan="2"></td><td rowspan="2">Yes</td></tr><tr><td>217</td><td>0–3198</td><td>0–3198</td><td>104 854</td><td>109 113</td></tr><tr><td>(25) H<sub>2</sub>O<sub>2</sub></td><td>1661<sup>b</sup></td><td>0–1731</td><td>0–1731</td><td>126 983</td><td>126 983</td><td></td><td></td></tr><tr><td rowspan="3">(26) C<sub>2</sub>H<sub>2</sub></td><td>1221</td><td>604–9890</td><td>13–9890</td><td>12 613</td><td>22 866</td><td rowspan="3">Yes</td><td rowspan="3">Yes</td></tr><tr><td>1231</td><td>613–6589</td><td>613–6589</td><td>285</td><td>285</td></tr><tr><td>1222</td><td>1–789</td><td>1–789</td><td>7512</td><td>7512</td></tr><tr><td rowspan="2">(27) C<sub>2</sub>H<sub>6</sub></td><td>1221</td><td>706–3001</td><td>225–3001</td><td>43 592</td><td>54 460</td><td rowspan="2"></td><td rowspan="2">Yes</td></tr><tr><td>1231</td><td>725–919</td><td>285–919</td><td>6037</td><td>7107</td></tr><tr><td>(28) PH<sub>3</sub></td><td>1111</td><td>0–3602</td><td>0–3602</td><td>22 190</td><td>22 190</td><td></td><td></td></tr><tr><td rowspan="2">(29) COF<sub>2</sub></td><td>269</td><td>696–2002</td><td>725–2002</td><td>168 793</td><td>168 793</td><td></td><td></td></tr><tr><td>369</td><td>686–815</td><td>686–815</td><td>15 311</td><td>15 311</td><td></td><td></td></tr><tr><td rowspan="2">(30) SF<sub>6</sub></td><td>29</td><td>580–996</td><td>580–996</td><td>2 889 065</td><td>2 889 065</td><td></td><td></td></tr><tr><td>121<sup>b</sup></td><td>2–11,330</td><td>2–11,330</td><td>36 561</td><td>36 561</td><td></td><td></td></tr><tr><td rowspan="2">(31) H<sub>2</sub>S</td><td>141<sup>b</sup></td><td>5–11,227</td><td>5–11,227</td><td>11 352</td><td>11 352</td><td></td><td></td></tr><tr><td>131<sup>b</sup></td><td>5–11,072</td><td>5–11,072</td><td>6322</td><td>6322</td><td></td><td></td></tr><tr><td>(32) HCOOH</td><td>126</td><td>10–1890</td><td>10–1890</td><td>62 684</td><td>62 684</td><td></td><td></td></tr><tr><td>(33) HO<sub>2</sub></td><td>166<sup>b</sup></td><td>0–3676</td><td>0–3676</td><td>38 804</td><td>38 804</td><td></td><td></td></tr><tr><td rowspan="2">(34) O</td><td>6<sup>b</sup></td><td>68–159</td><td>68–159</td><td>2</td><td>2</td><td></td><td></td></tr><tr><td>5646</td><td>763–798</td><td>763–798</td><td>21 988</td><td>21 988</td><td></td><td></td></tr><tr><td rowspan="2">(35) ClONO<sub>2</sub></td><td>7646</td><td>765–791</td><td>765–791</td><td>10 211</td><td>10 211</td><td></td><td></td></tr><tr><td>1214</td><td>794–1706</td><td>794–1706</td><td>18 692</td><td>18 692</td><td></td><td></td></tr><tr><td rowspan="2">(36) NO<sup>+</sup></td><td>46</td><td>1634–2531</td><td>3–2531</td><td>1206</td><td>1270</td><td></td><td></td></tr><tr><td>169</td><td>0–316</td><td>0–316</td><td>2177</td><td>2177</td><td></td><td></td></tr><tr><td rowspan="2">(37) HOBr</td><td>161</td><td>0–316</td><td>0–316</td><td>2181</td><td>2181</td><td></td><td></td></tr><tr><td>221</td><td>701–3243</td><td>620–3243</td><td>18 097</td><td>59 536</td><td></td><td></td></tr><tr><td rowspan="2">(38) C<sub>2</sub>H<sub>4</sub></td><td>231</td><td>2947–3181</td><td>614–3181</td><td>281</td><td>18 095</td><td></td><td></td></tr><tr><td>219</td><td>794–1706</td><td>794–1706</td><td>19 897</td><td>19 897</td><td></td><td></td></tr><tr><td rowspan="2">(39) CH<sub>3</sub>OH</td><td>2161<sup>b</sup></td><td>0–1408</td><td>0–1408</td><td>18 692</td><td>18 692</td><td></td><td></td></tr><tr><td>211</td><td>796–1697</td><td>796–1697</td><td>18 219</td><td>18 219</td><td></td><td></td></tr><tr><td>(40) CH<sub>3</sub>Br</td><td>2124</td><td>890–946</td><td>890–946<sup>d</sup></td><td>3572</td><td>3572<sup>d</sup></td><td></td><td></td></tr><tr><td>(41) CH<sub>3</sub>CN</td><td>29</td><td>594–1313</td><td>582–1519</td><td>60 033</td><td>842 709</td><td></td><td></td></tr><tr><td>(42) CF<sub>4</sub></td><td>29</td><td>594–1313</td><td>582–1519</td><td>124 126</td><td>251 245</td><td></td><td></td></tr><tr><td>(43) C<sub>4</sub>H<sub>2</sub></td><td>2211</td><td>0–758</td><td>0–1303</td><td>180 332</td><td>180 332</td><td></td><td></td></tr><tr><td rowspan="2">(44) HC<sub>3</sub>N</td><td>1224</td><td>0–760</td><td>0–760</td><td></td><td></td><td></td><td></td></tr><tr><td>11</td><td>15–36,024</td><td>15–27,185</td><td>4017</td><td>3480</td><td>Yes</td><td></td></tr><tr><td rowspan="5">(45) H<sub>2</sub></td><td>12<sup>b</sup></td><td>3–36,406</td><td>3–36,406</td><td>5129</td><td>5129</td><td>Yes</td><td></td></tr><tr><td>22</td><td>1–2586</td><td>1–2586</td><td>1088</td><td>1088</td><td></td><td></td></tr><tr><td>24</td><td>1–1359</td><td>1–1359</td><td>396</td><td>396</td><td></td><td></td></tr><tr><td>32</td><td>1–1331</td><td>1–1331</td><td>396</td><td>396</td><td></td><td></td></tr><tr><td>23</td><td>1–156</td><td>1–156</td><td>198</td><td>198</td><td></td><td></td></tr><tr><td>(47) SO<sub>3</sub></td><td>26</td><td>0–2825</td><td>0–2825</td><td>10 881</td><td>14 295</td><td></td><td></td></tr><tr><td>(48) C<sub>2</sub>N<sub>2</sub></td><td>4224</td><td>c</td><td>200–307</td><td>c</td><td>71 775</td><td></td><td></td></tr><tr><td rowspan="2">(49) COCl<sub>2</sub></td><td>2655</td><td>c</td><td>793–900</td><td>c</td><td>164 436</td><td></td><td></td></tr><tr><td>2657</td><td>c</td><td>800–892</td><td>c</td><td>145 477</td><td></td><td></td></tr></tbody></table>

<sup>a</sup> Abbreviated code for isotopologues.

<sup>b</sup> Although spectral ranges and amount of lines is unchanged with respect to HITRAN2012, there are changes to spectral parameters of lines for these isotopologues.

<sup>c</sup> Not included in HITRAN2012.

<sup>d</sup> These numbers will change eventually once changes discussed in sections 2.8 and 2.41 will be finalized.

preference to the experimental line position data from previous HITRAN editions if the uncertainty code was 5 (0.00001–0.0001 cm⁻¹) or higher. Comparisons with atmospheric spectra have shown that this was a correct choice.

b) The datasets constructed in Refs. [19–22] do not include experimental data from the papers published after them. Quite a few new levels have become available over the years and some reassignments were in order (see for instance Mikhailenko et al. [23] and Liu et al. [24]). We partially updated the IUPAC datasets here. In particular, for the HD¹⁷O and HD¹⁸O species, levels from Kyuberis et al. [25] were used. For H₂¹⁸O and H₂¹⁷O, and to a lesser extent for H₂¹⁶O, levels

from Mikhailenko et al. [23, 26] and Liu et al. [24] were used. Unfortunately, the IUPAC and Mikhailenko et al. quantum assignments often differ and complete matching would require a substantial amount of time in the future. An update of the IUPAC energy levels is in progress which will endeavor to resolve these assignment issues.

Many of the updated water vapor lines use line positions generated from the IUPAC energy levels and theoretical transition intensities, based on a high-accuracy *ab initio* dipole moment surface [16]. The methodology developed by Lodi and Tennyson [17] involves using several calculated line lists to identify reliable
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Percentage *ab initio*-experimental vs. intensity for bands in the 1850–2280 cm⁻¹ and 2390–4000 cm⁻¹ regions of H₂¹⁶O. The error limit of the experimental data is 1% [31].</figcaption></figure>

theoretical predictions. Lodi and Tennyson's intensity data were used in their entirety for H₂¹⁷O and H₂¹⁸O in HITRAN2012. Subsequent measurements and analysis by Regalia et al. [27,28] gave good agreement with these intensities. The present update relies heavily on Lodi-Tennyson style calculations for all isotopologues, supplemented with high-quality experimental data where available. This approach has proven to work well in application to atmospheric and laboratory spectra (see for instance Campargue et al. [29] and Ponomarev et al. [30]).

Given the reliance on these theoretical intensities, it is important to understand the systematic errors which *ab initio* calculations may show. In the same issue of this journal, Birk et al. [31] give an extensive intercomparison of *ab initio* calculations with high-quality experimental data. The experimental intensities used in the intercomparison are all included in the HITRAN2016 update. Much of the data show agreement between *ab initio* and experiment within 2%. However, for some bands, notably those involving excitation of the stretching modes, there are larger offsets of up to 8% which can be attributed to systematic errors in the *ab initio* dipole moment calculations. Furthermore, in the $v_1$ fundamental, differences show a systematic dependence on wavenumber, $\Delta K_a$ and $\Delta J$. Other vibrational bands involving the changes in $v_1$ quanta also showed larger systematic errors. The theoretical calculations also have problems in predicting local resonances accurately, although these resonances start to appear

more frequently in the NIR part of the spectrum. The information on systematic differences between the *ab initio* theory and experiment for H₂¹⁶O can be used to estimate uncertainties for minor isotopologues for which no high-accuracy experimental intensities are available.

An important finding of this work is that for an individual vibrational band an intensity-independent offset appears in graphical representations of differences between experiment and theory vs. intensity. This information can be used to validate experiment as well as theory. The constant offset can also be used to predict weak line intensities where experimental data are unavailable by scaling *ab initio* values.

Fig. 1 compares theory and experiment for the 1850–2280 cm⁻¹ and 2390–4000 cm⁻¹ regions. The agreements for the fundamental bands (001)–(000), (010)–(000), the overtone (020)–(000), the hot bands (030)–(010), (020)–(010), (011)–(010), (001–010), (100)–(010) are all excellent, mainly within 2%. The bands involving changes in the $v_1$ quanta, (100)–(000) and (110)–(010), however, show large scatter and an offset around –2%. The scatter includes the systematic differences of +5% to –13%, which becomes obvious when plotting the differences against wavenumber or lower-state energy and color coding $\Delta K_a$ and/or $\Delta J$ [31].

The 2016 update provides comprehensive line lists for the six main isotopologues of water: H₂¹⁶O, H₂¹⁸O, H₂¹⁷O, HD¹⁶O, HD¹⁸O and HD¹⁷O, as well as the newly-added isotopologue D₂¹⁶O. The

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Spectral fits to the high-H₂O Darwin (Australia) spectrum using the HITRAN2012 line list (left panel) and the new line list (right panel). The improvement in the residuals is due to the addition of HDO lines (three in this plot) denoted by the orange trace. Note the slight change of scale in the residuals on the left and right sides.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Flow diagram for the construction of line positions and intensities for H₂¹⁶O below 8340 cm⁻¹. See text for details.</figcaption></figure>

use of variational calculations to provide the underlying line lists guarantees the completeness of the lists for the intensity cutoff employed. This completeness leads to a significant expansion of the number of important transitions of the deuterated isotopologues into the NIR (near-infrared) region. These line lists are presented and analyzed in articles published in this issue [25]. Fig. 2 shows an example where atmospheric retrieval from a Total Carbon Column Observing Network (TCCON) [32] spectrum

benefits from inclusion of HDO lines in the NIR. It is also important to note a significant increase in coverage for HD¹⁷O and HD¹⁸O which were poorly represented in previous HITRAN releases.

The variational (calculated) intensities described above were then replaced with intensities from available high-quality experiments, many of which were already in the HITRAN2012 database. The details will be given in a dedicated paper, but some of the experiments that represent a substantial bulk of new experimental
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Flow diagram for the construction of line positions and intensities for H₂¹⁸O and H₂¹⁷O. The asterisks indicate that the IUPAC levels were slightly updated in this work. LED refers to the experimental data from Ref. [26]. See text for details.</figcaption></figure>

intensity data are described below.

2.1.1.1. *DLR experiments.* An extensive work in the spectral ranges 1850–2280 cm⁻¹ and 2390–4000 cm⁻¹ was carried out by Loos et al. [33,34] and in the range 4190–4340 cm⁻¹ by Birk et al. [31]. In Fig. 3 we will refer to these, and the experiments from the same authors that are already in HITRAN, as DLR. In Loos et al. [33,34] line positions, intensities, self- and air-broadened line-shape parameters, their temperature dependence as well as Rosenkranz line coupling coefficients were retrieved from numerous Fourier-Transform transmittance measurements of self- and air-broadened water vapor at 296 K as well as air-broadened water vapor measurements at low and high temperatures. During the analysis, a large effort was undertaken to give consolidated error bars. In the analysis, a multispectrum fitting approach applying a quadratic speed-dependent hard collision model based on the Hartmann-Tran profile [7,8] and extended to account for line mixing in the Rosenkranz first order perturbation approximation [35] was used. Line positions, intensities and self-broadening, self-speed-

dependence and self-shift parameters, as well as in some cases self-line coupling coefficients, were retrieved from pure water-vapor measurements of total pressures up to 20 mbar. Air-broadening, speed-dependence, pressure shift parameters, Dicke narrowing and line-mixing coefficients as well as temperature-dependence parameters were obtained from air-broadened measurements at ambient temperature and total pressures from 30 to 1000 mbar as well as low and high-temperature measurements at 100 mbar total pressure. The intensities of lines with retrieved line parameters range from 3 × 10⁻²⁶ to 3 × 10⁻¹⁹ cm⁻¹/(molecule cm⁻²). In the 4190–4340 cm⁻¹ region, several Fourier-Transform transmittance spectra of pure and air-broadened water vapor at 296 K as well as low and high temperatures were measured and analyzed by Birk et al. [31]. These measurements were dedicated to water vapor parameters to be used in TROPOMI/S5-P [36] retrievals. The analysis was also based on a multispectrum 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.

2.1.1.2. *MiKaMo16* experimental set. Recently, Mikhailenko et al. [23] have compiled a database of measurements in 5850–8340 cm⁻¹ region from the Grenoble-Tomsk collaboration reported in Refs. [37–41] supplemented with measurements from Sironneau and Hodges [42] and for strong lines from Toth [43] and for very weak lines from ab initio work of Partridge and Schwenke [44,45]. This database is here referred to as MiKaMo16. Unfortunately, due to some differences in assignments with the variational Lodi-Tennyson-like line lists described above, it was hard to take full advantage of this line list. However, tests against TCCON spectra have shown that this line list is superior to any other line list in the region of 7000–8340 cm⁻¹. It was therefore used in HITRAN2016 in that region as is almost everywhere in this spectral region. This, however, creates some consistency issues with assignments of energy levels throughout the database, but substantial improvement of atmospheric retrievals justifies this approach.

As examples of the complexity of the update, Figs. 3 and 4 show the flow diagrams of the construction of position-intensity line lists of the principal isotopologue (only up to 8340 cm⁻¹ for simplicity) and for H₂¹⁸O/H₂¹⁷O respectively. Note that the line list from Lodi et al. [16] for the principal isotopologue is limited to 10⁻²⁸ cm⁻¹/(molecule cm⁻²); therefore intensities for weaker lines still originate from HITRAN2012/BT2.

Whenever a rotational quantum number could not be determined unambiguously, the index of symmetry (1, 2, 3, and 4 as defined in the BT2 work [15]) accompanied with a negative sign was used. Note that 1 and 2 indicate para states, whereas 3 and 4 indicate ortho states. For the case of unassigned vibrational quanta, a “-2” label has been adopted.

2.1.2. Line-shape parameters for the H₂¹⁶O, H₂¹⁸O and H₂¹⁷O line lists

The air-broadened half widths (in Voigt profile formalism) for the first three isotopologues of water in HITRAN2008 and, with some modifications, in HITRAN2012 were derived using the procedure “Diet” described by Gordon et al. [46]. This procedure continues to be used in this edition with experimental outliers identified and removed. One notable example is the removal of a large portion of the half widths measured in Jenouvrier et al. [47] from the experimental data that feeds the “Diet”. Indeed, some of the recent evaluations of atmospheric spectra showed that many data in Jenouvrier et al. are questionable (see for instance Armante et al. [48]). However, although the removal of this extensive dataset from the updated Gordon et al. [46] procedure, as was done in the GEISA database [49] and the MiKaMo16 linelist, have improved the residuals in some of the lines, some of the residuals became worse than in HITRAN2012. The main problem here is due to the fact that, while there are definite outliers in the values reported in Jenouvrier et al. [47], it is the most extensive study of the air-broadened line widths in the 4200–6600 cm⁻¹ region. In fact, for many transitions no other measurements exist. Removal of these measurements from the experimental dataset has invoked the use of the approximations of different levels (some are very crude) from the work of Jacquemart et al. [50] for many of the transitions. The only solution in this case was to filter the Jenouvrier et al. dataset. This was done through direct comparisons with the TCCON spectra for some of the transitions and cross-comparison with similar rotational transitions but from other bands, accounting for vibrational dependence from Eq. (1) of Jacquemart et al. [50]. After this filtering procedure, about 1400 measurements (out of almost 5000) from Jenouvrier et al. [47] were retained.

It is also important to emphasize that measurements by Birk
and Wagner [51] in the v₂ band region were given a priority and

were written into the database directly as was done in
HITRAN2012.

Since the modeling of water vapor absorption lines using a
Voigt model is no longer sufficient in many applications and the
HT profile provides considerably higher accuracy, experimental HT
profile parameters of Birk et al. and Loos et al. [31,33,34] in the
spectral ranges 1850–2280 cm⁻¹, 2390–4000 cm⁻¹ and 4190–
4340 cm⁻¹ were accommodated wherever possible. The experi-
ments and analysis procedures applied are described briefly in
Section 2.1.1.

2.1.3. Line-shape parameters for the deuterated isotopologues

The addition of the line-shape parameters for D₂O follows a
similar procedure as the other isotopologues; however, the avail-
able data for air-broadening of D₂O are considerably less complete
than for the other isotopologues. There are some measurements
for air-broadening of D₂O [52–55] as well as Complex Robert-Bo-
namy calculations [56]. These data amount to several hundred
lines from the measured data and just over 550 lines from the
theoretical calculations. Using these data, average values as a
function of *J*'' were determined and extrapolation to *J*'' = 50 was
done using a third-order polynomial fit.

Using these data, the half widths and some line shifts have
been added to the D₂O lines in the database using a priority
scheme. First, the measured values were added; if the measure-
ment did not exist for the transition in question, the theoretical
value was used. If neither exists, the *J*-averaged value was used.
This algorithm allowed half widths to be added to all D₂O lines in
the HITRAN database.

For self-broadening parameters of D₂O, the work of Gamache
et al. [57] was used. These data are an extensive set of calculations
for D₂O rotational band transitions extended to 6536 lines using
the partner transition rule [58]. Using these data, a set of *J*-aver-
aged half widths was determined for *J* = 0–50 [57]. These data
have been added to the algorithm that adds line-shape data to the
water lines in the HITRAN database.

2.1.4. Future plans

Recently, Lampel et al. [59] have identified important absorp-
tion features in atmospheric spectra due to water vibration-rota-
tion transitions in the near ultraviolet around 363 nm. Laboratory
measurements of water vibration-rotation transition intensities
only extend to 25,470 cm⁻¹ (393 nm) [60] although multiphoton
spectra do provide some information on energy levels in the re-
gion of question [61,62]. Future updates will look to extend cov-
erage to water absorption in the near ultraviolet; these data are
important for upcoming satellite missions, including TEMPO [63]
which will record spectra at these wavelengths.

At the intensity limit assumed for D₂¹⁶O, the even rarer iso-
topologues D₂¹⁷O and D₂¹⁸O should also be visible. Line lists for
these two species have also been prepared as part of the work on
deuterated water in Ref. [25]; these data will be added in the
forthcoming updates to HITRAN2016.

We will be populating the database of the HT line-shape
parameters after evaluating existing literature values and new
measurements as they become available.

2.2. CO₂ (molecule 2)

Accurate and comprehensive line lists for all naturally abun-
dant isotopologues of carbon dioxide are required by remote-
sensing missions dedicated to monitor the concentration of carbon
dioxide in Earth's atmosphere. The recently launched OCO-2
mission [64–66], together with several other space and ground
based projects (GOSAT [67], AIRS [68], ASCENDS [69], TCCON [32],
NDACC [70]) are dedicated to explicitly monitor the atmospheric
<table><caption>**Table 4**
Comparison of HITRAN2016 and HITRAN2012 line lists for isotopologues of carbon dioxide.</caption><thead><tr><th rowspan="2">ISO/abundance</th><th colspan="3">HITRAN2012</th><th colspan="3">HITRAN2016</th></tr><tr><th>Number of lines</th><th>Spectral region (cm<sup>-1</sup>)</th><th>Q(296 K)</th><th>Number of lines</th><th>Spectral region (cm<sup>-1</sup>)</th><th>Q(296 K)</th></tr></thead><tbody><tr><td>626/0.984204</td><td>169,292</td><td>345.936–12,784.056</td><td>286.94</td><td>173,024</td><td>158.301–14,075.298</td><td>286.094</td></tr><tr><td>636/1.1057 × 10<sup>-2</sup></td><td>70,611</td><td>406.834–12,462.046</td><td>578.41</td><td>70,577</td><td>332.649–13,734.963</td><td>576.644</td></tr><tr><td>628/3.9470 × 10<sup>-3</sup></td><td>116,482</td><td>0.736–9557.398</td><td>609.48</td><td>127,850</td><td>1.472–12,677.181</td><td>607.713</td></tr><tr><td>627/7.3399 × 10<sup>-4</sup></td><td>72,525</td><td>0.757–9599.317</td><td>3552.70</td><td>77,941</td><td>0.757–12,726.562</td><td>3542.610</td></tr><tr><td>638/4.4345 × 10<sup>-5</sup></td><td>26,737</td><td>489.678–6744.158</td><td>1229.10</td><td>43,782</td><td>2.945–9212.609</td><td>1225.270</td></tr><tr><td>637/8.2462 × 10<sup>-6</sup></td><td>2953</td><td>583.593–6768.643</td><td>7162.90</td><td>25,175</td><td>9.086–8061.741</td><td>7140.024</td></tr><tr><td>828/3.9556 × 10<sup>-6</sup></td><td>7118</td><td>491.688–8160.439</td><td>324.21</td><td>10,522</td><td>482.813–8162.743</td><td>323.424</td></tr><tr><td>728/1.4718 × 10<sup>-6</sup></td><td>821</td><td>626.438–5046.875</td><td>3776.40</td><td>15,878</td><td>491.181–8193.172</td><td>3766.044</td></tr><tr><td>727/1.3685 × 10<sup>-7</sup></td><td>5187</td><td>535.384–6932.980</td><td>11,002.00</td><td>6518</td><td>535.383–6932.693</td><td>10,971.91</td></tr><tr><td>838/4.4440 × 10<sup>-8</sup></td><td>121</td><td>4599.239–4887.290</td><td>653.50</td><td>2916</td><td>2245.898–4750.068</td><td>652.242</td></tr><tr><td>837/1.653 × 10<sup>-8</sup></td><td>N/A</td><td>N/A</td><td>7615.20</td><td>4190</td><td>549.472–4914.496</td><td>7593.900</td></tr><tr><td>737/1.5375 × 10<sup>-9</sup></td><td>N/A</td><td>N/A</td><td>N/A</td><td>1501</td><td>575.852–3614.084</td><td>22,129.96</td></tr><tr><td>646/radioactive</td><td>N/A</td><td>N/A</td><td>N/A</td><td>41,610</td><td>426.445–7928.788</td><td>2033.353</td></tr></tbody></table>

Note: ISO is the AFGL shorthand notation for the isotopologue, abundance is the terrestrial value assumed by HITRAN, and Q(296) is the partition sum at 296 K.

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Overview of the line lists of stable symmetric isotopologues of carbon dioxide in HITRAN2012 and HITRAN2016.</figcaption></figure>

CO₂ content. These experiments aim not only to look at overall CO₂ concentration and its variation, but also wish to pinpoint where CO₂ is being produced (sources) and where it is absorbed (sinks). This activity is clearly vital to monitoring and essential for eventually controlling the CO₂ content of the atmosphere [71]. A successful retrieval of CO₂ concentration requires validated line lists with transition intensities given at sub-percent accuracy, line positions accurate to 0.0001 cm⁻¹ or better, and beyond-Voigt-profile line-shape models [65,72,73].

Determination of isotopic ratios of carbon in Earth's samples and astrophysical objects remains crucial for modeling geophysical processes. For example, quantification of ¹⁴C in fossil fuels can provide information about the sources of human-related contribution to the total CO₂ concentration in the terrestrial atmosphere. This can be done with recently developed cavity-enhanced laser spectroscopic techniques in the IR [74–76]. However, these measurements require a priori simultaneous knowledge of reliable

line intensities of many isotopologues. Precise determination of ¹³C/¹²C and ¹⁶O/¹⁷O/¹⁸O ratios is also vital, for instance, in understanding processes of formation of radiation fields in the Martian atmosphere, which is 96% rich in carbon dioxide [77].

A summary of the carbon dioxide line list in the HITRAN2012 database and comparison to HITRAN2016 is given in Table 4. The HITRAN2012 database was considerably improved with respect to its previous 2008 edition. However, several issues related to spectral completeness, inconsistency of multiple data sources, and insufficient accuracy of line intensities, still remained unsolved. The majority of entries in the 2012 version of the HITRAN database were taken from the effective Hamiltonian calculations included in the 2008 edition of the CDSD-296 database [78].

For less abundant isotopologues, obtaining high-quality experimental data is not trivial. Therefore fits of the effective Hamiltonian or the effective dipole moment [78], were based on only four major isotopologues ¹²C¹⁶O₂, ¹³C¹⁶O₂, ¹⁶O¹²C¹⁸O and
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Overview of the line lists of stable asymmetric isotopologues of carbon dioxide in HITRAN2012 and HITRAN2016.</figcaption></figure>

$^{16}$O$^{12}$C$^{17}$O, for which measured spectroscopic parameters were available. As a result, several spectral gaps were present in HITRAN2012 (see for instance discussion in Refs. [79,80]) which represent regions where experimental data were unavailable. For similar reasons, no entries were included in the database for the $^{18}$O$^{13}$C$^{17}$O, $^{17}$O$^{13}$C$^{17}$O, and $^{16}$O$^{14}$C$^{16}$O isotopologues (837, 737, and 646 in old AFGL notation). An overview of this problem is displayed in Figs. 5 and 6, where CO$_2$ ro-vibrational spectra from HITRAN2012 and HITRAN2016 are compared for different isotopologues.

Wherever possible, the effective Hamiltonian fits were extrapolated to the trace isotopologues, using a method of isotopic substitution [81]. In the 2012 edition, multiple data sources caused sporadic discontinuities in intensity patterns of ro-vibrational lines [82–84]. Furthermore, a high percentage of line intensities in HITRAN2012 have stated uncertainty of 20% or worse (HITRAN uncertainty index equal to 3). Although this assessment has been proven to be overly pessimistic in many cases [82,83,85–87], the uncertainty budget, especially for the Effective Hamiltonian calculations, was still too high for precise measurements of atmospheric CO$_2$ concentration. The most accurate entries in HITRAN2012 were taken from NASA JPL measurements by Toth et al. [88–90] and covered the 1.6-µm and 2.06-µm spectral regions, which are used in remote-sensing measurements. The stated 1–5% accuracy of these experimental line intensities (HITRAN uncertainty index equal to 7 and 6), has been confirmed by a number of comparisons [82,83,91]; nonetheless the rigorous requirements for part-per-million resolution in measurements of CO$_2$ atmospheric concentration were not achieved.

Since the 2012 release of the HITRAN database, a large number of experimental and theoretical studies have been devoted to improve the knowledge of line positions, line intensities and line shapes of CO$_2$ isotopologues. For a comprehensive review of measurements and theoretical models see Ref [92], and references therein.

Theoretical line lists (denoted as "Ames") for 12 stable and one radioactive (646) isotopologue of CO$_2$ were published by Huang

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Comparison of line intensities between HITRAN2016, HITRAN2012 (Toth et al. [88]), and other accurate experimental and theoretical sources for the 20012–00001 band (2-µm band) of $^{12}$C$^{16}$O$_2$: measurements Wübbeler et al. [102], NIST [104] and UniNA2 [103], and theory Ames-1 [79]. The zero relative deviation line corresponds to HITRAN2016 line intensities (in this case from Zak et al. [82]). The running index *m* equals -J, *J*, and *J* + 1 for the P, Q, and R branches, respectively.</figcaption></figure>

et al. in the 0–20,000 cm$^{-1}$ spectral region and for temperatures below 1500 K [79,93,94]. Room-temperature line lists (denoted as "UCL-IAO") for 13 CO$_2$ isotopologues were also calculated by Zak et al. [82,83,85,91] in the 0–8000 cm$^{-1}$ spectral region. Both of these latter studies contained intensities computed with *ab initio* dipole moment surfaces and semi-empirical line positions, based on a fitted potential energy surface for the Ames effort and on the effective Hamiltonian calculations for UCL-IAO. A major advantage of the variational approach used in the Ames and UCL-IAO line lists is that it should give similar accuracy for all isotopologues. This allows coverage of spectral regions currently not probed by experiments for rare isotopologues. UCL-IAO also provides uncertainty estimates of line intensities, based on a purely theoretical methodology [17]. Such a reliable analysis allows for the detection of ro-vibrational resonance interactions, which significantly lower the accuracy of theoretical line positions and intensities. Using this method, the lines identified as unreliable have been replaced with the data from CDSD-296 [92] and, in several cases of interpolyad resonance interactions (asymmetric isotopologues), with the
experimental data from Lyulin et al. [95], Karlovets et al. [96,97] and Campargue et al. [98].

Recently, there have been a number of high-precision near-IR spectroscopic measurements which provide rigorous tests of theoretical line intensities based on effective dipole moment surface and ab initio calculations [84–87,99]. Particularly, in the 1.6-µm and 2.0-µm spectral regions, the UCL-IAO line lists have been experimentally verified as accurate to the sub-percent level. Fig. 7 compares the UCL-IAO and Ames line lists to HITRAN2012 (Toth et al. blue open circles [100, 101]) for the 20012 – 00001 band and to state-of-the-art experiments including advanced high-resolution laser measurements [101–104]. A number of comparisons here suggest that the UCL-IAO study models line intensities more accurately than the Ames study. Note that more recent results from Ames, which are available from their website (www.huang.seti.org), give closer agreement with UCL-IAO. From Fig. 7 it is clear that there is a 1–3% average difference in line intensities between the new and the previous version of HITRAN for this band. The independent experiments from the National Institute of Standards and Technology [104], and the University of Naples II [103] confirm, however, a conservative 0.5% accuracy of line intensities for this band in HITRAN2016. This level of accuracy could potentially satisfy even the most stringent requirements of current remote-sensing missions. Interestingly, although line intensities for this band and the 20013 – 00001 and 30013 – 00001 bands probed by OCO-2 originate from the same source (Toth et al. [100]), the agreement between UCL-IAO and HITRAN2012 is substantially better for the OCO-2 bands.

For wavenumbers greater than 8000 cm⁻¹, the majority of the line parameters were taken from CDSD-296 [92]. As we have already mentioned above, both HITRAN2012 and CDSD-296 have several spectral gaps, in particular in the wavenumber region greater than 8000 cm⁻¹. Recently, several experimental studies of the carbon dioxide spectra in the high-frequency region have been performed [105–108]. The measured line intensities allowed determining the absent effective dipole moment parameters for several series of transitions. Using these effective dipole moment parameters and an effective Hamiltonian from Ref. [109], the line positions and intensities for the principal isotopologue were generated and included into HITRAN2016 covering the 9800–10,500 cm⁻¹ and 11,600–12,400 cm⁻¹ wavenumber gaps.

Line positions were updated with respect to the previous version of the database. The majority of lines come from the effective Hamiltonian calculations included in UCL-IAO line lists, which are based on the fits to the observed line positions collected from the literature and published in the latest, 2015 release of the CDSD-296 database [92]. These fits were completed and updated with recent, accurate measurements performed on isotopically-enriched samples of CO₂. Uncertainties in the fitted line positions depend on the quality of the experimental data and vary from 0.001 cm⁻¹ to 10⁻⁹ cm⁻¹. For asymmetric isotopologues, a number of bands are affected by strong interpolyad anharmonic resonance interactions. The effective Hamiltonian model does not include this type of interaction for the asymmetric isotopologues. Hence in such cases, line positions were taken directly from measurements [80,95–98,110,111].

The uncertainty codes for the line positions were transferred from CDSD-296 to HITRAN2016. The uncertainty code 3 (0.001–0.01 cm⁻¹) was given for the line positions in the 9800–10,500 cm⁻¹ and 11,600–12,400 cm⁻¹ wavenumber regions. Partition functions in the current release of the database are based on the direct summations taken from the variational calculations of Huang et al. [79]. On average, the new partition functions agree excellently with those of CDSD-296 [83]; however they do not agree perfectly with those in HITRAN2012 (from TIPS [112]) and three previous editions of the database and differ at 296 K by

about –0.3%. Although this difference is marginal, it could have an effect in the applications where sub-percent accuracy is required.

As stated above, the radioactive isotopologue ¹⁴CO₂, 646, has been added to the database. This is the first edition of HITRAN where radioactive species have been incorporated (also for CO, see Section 2.5). All lines of the 646 isotopologue were taken from the UCL line lists given in Ref. [83]. Due to issues with what constitutes a so-called natural terrestrial abundance of radioactive species (which is part of the traditional definition of intensities in HITRAN, see Eq. (1) of the Definitions and Units documentation in HITRANonline), line intensities for these type of species are given for unit abundance; a 10⁻²⁷ cm⁻¹/(molecule·cm⁻²) cut-off value for the intensity has been applied. This cut-off produced 41,610 lines in the J range 0 to 114. Vibrational assignments for the 646 isotopologue were based on isotopic shifts of energy levels and respective assignments for the 626 and 636 isotopologues, and hence should be regarded as provisional. An abundance-scaled intensity cut-off of 10⁻³⁰ cm⁻¹/(molecule·cm⁻²) is used for all stable isotopologues. Note that, for the time being, data for the radioactive isotopologues are provided as static files rather than through the HITRANonline interface.

Uncertainties of line intensities were informed by theoretical error analysis, which classified lines as reliable, intermediate, or unreliable. Bands with reliable lines stronger than 10⁻²³ cm⁻¹/(molecule·cm⁻²) (for unit abundance) were assigned HITRAN uncertainty code 8 (i.e. accuracy of 1% or better). Line intensities of reliable parallel bands weaker than 10⁻²³ cm⁻¹/(molecule·cm⁻²) were given an uncertainty code 7 (i.e. accuracy 1-2%). Reliable perpendicular bands weaker than 10⁻²³ cm⁻¹/(molecule·cm⁻²) and intermediate lines were marked with HITRAN uncertainty code 6 (i.e. accuracy 2-5%). So-called unreliable lines were taken from the effective dipole moment calculations [92] and experiments. Typical intensity uncertainties for these entries range between 5 and 20%.

It is important to point out that an intensive study of the 1.6-µm and 2.06-µm bands that includes non-Voigt lineshapes and line mixing has been published by the OCO-2 spectroscopy support group ABSCO (ABsorption COefficient tables for the OCO-2 mission) [84,86]. The data were fit using a multi spectrum fit procedure which, among other things, enables retrieval of the line-shape parameters using the speed-dependent Voigt (SDV) profile as well as line mixing. These are very good experiments and it is debatable whether to use them for the strong and weak bands in place of UCL-IAO parameters described above. Indeed the ABSCO team have validated (using TCCON spectra) the cross-sections generated using results of Refs. [84,86] and found them to be the most efficient [113]. However, achieving high-precision results in nuanced correlations, with line mixing and model assumptions that can create discontinuities in inter-band comparisons, is difficult. At the moment, HITRAN cannot provide users with tools that can be used to generate cross-sections from the works of Devi et al. [84] and Benner et al. [86]. The usable products of the ABSCO effort are absorption coefficients (available upon request from the ABSCO group) rather than spectral parameters, which are available in the publications. Moreover, these absorption coefficients are empirically scaled by the factors of 0.6% and 1.4% for the 1.6-µm and 2.06-µm bands respectively, due to lingering data and/or model biases (the use of partition function HITRAN 2012/TIPS is up to 0.3% of this factor). After these studies, an update of the multi spectrum fitting code with CDSD partition functions was done. Additional methods to adjust the intensity distribution closer to the UCL list by scaling experimental conditions within the uncertainties are under evaluation. The intensities of the band at 2.06 µm are already within 0.7% of Zak et al. [82], indicating that the additional 0.7% scaling of ABSCO cross-sections may be unrelated to intensities. These issues will be considered for future
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Log intensity diagram for the $^{16}$O$_{3}$ lines in the 200–5000 cm$^{-1}$ range in HITRAN2016: every point corresponds to a single transition. Newly included lines are colored green, updated lines blue and unchanged HITRAN2012 lines grey (intensities are given in cm$^{-1}$/molecule·cm$^{-2}$ at 296 K).</figcaption></figure>

editions of the database.

For wavenumbers greater than 8000 cm$^{-1}$, two sources of the line intensities are used: CDSD-296 [92] and the newly-generated line list in the 9800–10,500 cm$^{-1}$ and 11,600–12,400 cm$^{-1}$ regions based on the new measurements [105–108]. The uncertainty codes of the CDSD-296 line intensities were transferred to HITRAN2016. Based on the uncertainties of the line intensity measurements in the 10,700–10,860 cm$^{-1}$ region [106], we use uncertainty code 5 (5%–10%) for the line intensities of the 3003i-00001 (i= 1,2,3,4) series of bands and based on the uncertainties of the line intensity measurements in the 10,000–10,300 cm$^{-1}$ and 11,600–12,400 cm$^{-1}$ [107] wavenumber regions we use uncertainty code 3 (> 20%) for the line intensities of the 4003i-00001 (i= 1,2,3,4,5) and of the 6001i-00001 (i= 1,2,3,4,5,6,7) series of bands.

The Voigt line-shape parameters throughout the entire database were calculated using the predictive routine of Gamache and Lamouroux explained in Refs. [114–116]. For the line mixing, we now provide a code from Lamouroux et al. [117] which has been updated to operate with HITRAN2016. We note that Lamouroux et al. [117] line mixing coupled with the HITRAN2012 data has worked really well and in fact produced residuals hardly exceeding 1% when applied to the TCCON data in Ref. [113], although slightly inferior to the ABSCO cross-sections in the 2.06-µm region.

## 2.3. O$_{3}$ (molecule 3)

The spectroscopic database for ozone is important for atmospheric and environmental applications due to its critical roles in UV shielding, radiative transfer and health. It is also considered a possible bio-marker for future observations of exoplanets [118]. In order to understand the impacts of ozone on the atmosphere and to monitor ozone a precise, knowledge of its line parameters at all wavelengths is required.

The ozone parameters for this edition of HITRAN have been updated in two steps. First, the parameters of spectroscopic models, line positions, intensities, and lower-state energy levels resulting from analyses of experimental spectra have been introduced into the S&MPO (Spectroscopy and Molecular Properties of Ozone) information system [119]. The S&MPO offers various tools for spectral simulations and for comparisons with experimental records. At the initial stage of the work, line lists were

generated using empirically-fitted model parameters of the effective Hamiltonians (EH) which considered nearby strongly interacting levels and of the corresponding effective dipole transition moments (ETM). This procedure was necessary to obtain complete sets of transitions as only a limited number of unblended lines could be accurately measured in congested ozone spectra. State-of-the-art studies of ozone laboratory spectra in the infrared have been recently reviewed by Barbe et al. [120] and Mikhailenko [121]. Note that most of the compilations of ozone data in the infrared accessible via the VAMDC (Virtual Atomic and Molecular Data Center) portal [122,123] are also based on the S&MPO line lists. This is the case of recent improvements in HITRAN and also in GEISA [49] for which the last release essentially contained the earlier 2015 update from the S&MPO.

In the second step, the line lists were validated by simulating atmospheric spectra as discussed in Section 2.3.3.

The new or completely recalculated bands of the principal isotopologue of ozone implemented in HITRAN2016 are summarized in Table 5. The present release of the ozone data contains 29,006 new lines and 46,688 updated lines of $^{16}$O$_{3}$ (a graphical overview in the 200–5000 cm$^{-1}$ region can be seen in Fig. 8).

### 2.3.1. Cold bands

#### 2.3.1.1. 5-µm region: tetrad {(002),(101),(200),(030)} – (000).
The 5-µm range is of particular importance for the retrieval of atmospheric ozone from ground-based and satellite observations, as this is the second strongest absorbing range in the ozone spectrum after the 10-µm bands. For this reason, the consistency of ozone intensities between the 5-µm and 10-µm regions as well as with UV ozone absorption is a major issue that has been a subject of many discussions ([124,125] and references therein). The line parameters in previous HITRAN versions for the strongest bands, i.e. the first triad $\nu_{1}+\nu_{3}$, $2\nu_{1}$, $2\nu_{3}$, in the 5-µm interval were from the work by Flaud et al. [126]. Subsequently, the FTS spectra were re-visited by Barbe et al. [127] though the corresponding data had not been included in HITRAN. Recently new measurements and analyses have been performed by the GSMA (Groupe de Spectrométrie Moléculaire et Atmosphérique) team from Reims University [120,128]. In 2015, a major update of line positions and intensities based on the list generated together with the Laboratory of Theoretical Spectroscopy of the Institute of Atmospheric Optics (Tomsk) was introduced in the S&MPO [119]. For the $2\nu_{1}$ and
<table><caption>**Table 5**
HITRAN2016 ozone update summary: principal isotopologue $^{16}$O$_{3}$.</caption><thead><tr><th>Band v'-v''</th><th>N</th><th>Region (cm<sup>-1</sup>)</th><th>S<sub>v</sub></th></tr></thead><tbody><tr><td>001-010</td><td>1781</td><td>281.14-377.51</td><td>1.255 &times; 10<sup>-21</sup></td></tr><tr><td>100-010</td><td>619</td><td>295.42-446.63</td><td>2.646 &times; 10<sup>-23</sup></td></tr><tr><td>030-020</td><td>2558</td><td>598.26-804.86</td><td>1.942 &times; 10<sup>-21</sup></td></tr><tr><td>101-020</td><td>6</td><td>656.12-725.68</td><td>4.937 &times; 10<sup>-25</sup></td></tr><tr><td>002-100</td><td>1779</td><td>899.45-1176.42</td><td>3.233 &times; 10<sup>-21</sup></td></tr><tr><td>101-100</td><td>3604</td><td>911.32-1215.49</td><td>6.100 &times; 10<sup>-20</sup></td></tr><tr><td>101-001</td><td>3457</td><td>921.37-1211.67</td><td>5.148 &times; 10<sup>-21</sup></td></tr><tr><td>002-001</td><td>4379</td><td>926.56-1193.79</td><td>1.651 &times; 10<sup>-19</sup></td></tr><tr><td>200-100</td><td>3966</td><td>940.22-1229.80</td><td>3.141 &times; 10<sup>-21</sup></td></tr><tr><td>030-100</td><td>20</td><td>956.13-1170.40</td><td>1.916 &times; 10<sup>-23</sup></td></tr><tr><td>200-001</td><td>2964</td><td>956.68-1235.44</td><td>6.852 &times; 10<sup>-21</sup></td></tr><tr><td>030-001</td><td>15</td><td>957.33-1201.66</td><td>2.254 &times; 10<sup>-24</sup></td></tr><tr><td>002-010</td><td>467</td><td>1302.52-1435.34</td><td>3.750 &times; 10<sup>-23</sup></td></tr><tr><td>101-010</td><td>1637</td><td>1347.52-1438.73</td><td>9.393 &times; 10<sup>-22</sup></td></tr><tr><td>030-010</td><td>369</td><td>1366.12-1475.95</td><td>2.268 &times; 10<sup>-23</sup></td></tr><tr><td>200-010</td><td>102</td><td>1366.62-1541.76</td><td>5.305 &times; 10<sup>-24</sup></td></tr><tr><td>002-000</td><td>6486</td><td>1856.83-2280.54</td><td>1.019 &times; 10<sup>-19</sup></td></tr><tr><td>200-000</td><td>6122</td><td>1909.98-2340.81</td><td>3.020 &times; 10<sup>-20</sup></td></tr><tr><td>101-000</td><td>7067</td><td>1921.44-2328.94</td><td>1.194 &times; 10<sup>-18</sup></td></tr><tr><td>030-000</td><td>105</td><td>2022.87-2272.37</td><td>4.092 &times; 10<sup>-22</sup></td></tr><tr><td>021-000</td><td>1482</td><td>2335.99-2440.14</td><td>4.360 &times; 10<sup>-22</sup></td></tr><tr><td>120-000</td><td>2095</td><td>2395.22-2616.83</td><td>3.063 &times; 10<sup>-22</sup></td></tr><tr><td>013-010</td><td>2459</td><td>2889.23-3090.28</td><td>4.735 &times; 10<sup>-21</sup></td></tr><tr><td>112-010</td><td>2046</td><td>2910.00-3121.94</td><td>3.816 &times; 10<sup>-22</sup></td></tr><tr><td>023-010</td><td>1355</td><td>3564.72-3657.05</td><td>3.676 &times; 10<sup>-22</sup></td></tr><tr><td>013-000</td><td>2673</td><td>3584.62-3791.77</td><td>5.713 &times; 10<sup>-21</sup></td></tr><tr><td>122-010</td><td>620</td><td>3584.75-3751.75</td><td>3.530 &times; 10<sup>-23</sup></td></tr><tr><td>112-000</td><td>3056</td><td>3600.25-3831.27</td><td>8.306 &times; 10<sup>-22</sup></td></tr><tr><td>400-010</td><td>4</td><td>3623.87-3653.87</td><td>6.743 &times; 10<sup>-25</sup></td></tr><tr><td>311-100</td><td>729</td><td>3739.97-3826.22</td><td>2.398 &times; 10<sup>-23</sup></td></tr><tr><td>005-100</td><td>508</td><td>3742.91-3726.13</td><td>1.660 &times; 10<sup>-23</sup></td></tr><tr><td>104-100</td><td>51</td><td>3752.69-3863.55</td><td>8.192 &times; 10<sup>-25</sup></td></tr><tr><td>005-001</td><td>278</td><td>3807.31-3917.54</td><td>9.849 &times; 10<sup>-24</sup></td></tr><tr><td>311-001</td><td>436</td><td>3810.30-3946.73</td><td>9.846 &times; 10<sup>-24</sup></td></tr><tr><td>104-001</td><td>950</td><td>3820.17-3894.94</td><td>2.218 &times; 10<sup>-22</sup></td></tr><tr><td>005-010</td><td>848</td><td>4128.33-4228.24</td><td>2.371 &times; 10<sup>-23</sup></td></tr><tr><td>311-010</td><td>1018</td><td>4129.14-4228.62</td><td>3.444 &times; 10<sup>-23</sup></td></tr><tr><td>104-010</td><td>66</td><td>4146.09-4228.28</td><td>6.310 &times; 10<sup>-25</sup></td></tr><tr><td>023-000</td><td>1192</td><td>4268.37-4357.21</td><td>1.928 &times; 10<sup>-22</sup></td></tr><tr><td>122-000</td><td>724</td><td>4284.15-4454.93</td><td>4.359 &times; 10<sup>-23</sup></td></tr><tr><td>400-000</td><td>8</td><td>4325.56-4398.58</td><td>5.896 &times; 10<sup>-25</sup></td></tr><tr><td>213-100</td><td>63</td><td>4384.72-4423.50</td><td>1.417 &times; 10<sup>-24</sup></td></tr><tr><td>321-100</td><td>126</td><td>4429.78-4466.82</td><td>5.750 &times; 10<sup>-24</sup></td></tr><tr><td>114-001</td><td>306</td><td>4452.15-4508.72</td><td>1.313 &times; 10<sup>-23</sup></td></tr><tr><td>213-010</td><td>503</td><td>4757.08-4825.80</td><td>2.346 &times; 10<sup>-23</sup></td></tr><tr><td>104-000</td><td>1093</td><td>4802.98-4978.61</td><td>7.789 &times; 10<sup>-23</sup></td></tr><tr><td>005-000</td><td>1514</td><td>4806.33-4938.21</td><td>5.300 &times; 10<sup>-22</sup></td></tr><tr><td>311-000</td><td>1053</td><td>4827.65-4928.49</td><td>3.450 &times; 10<sup>-22</sup></td></tr></tbody></table>

Note: N is the number of transitions, S<sub>v</sub> is the integrated intensity in units of cm<sup>1</sup>/(molecule·cm<sup>2</sup>).

2ν<sub>3</sub> bands, the difference in the integrated intensities, S<sub>v</sub>, exceeded 6% between S&MPO and HITRAN2012. The analysis [128] covering the 1856–2273 cm<sup>-1</sup> region also included the 3ν<sub>2</sub> band, which is coupled with ν<sub>1</sub>+ν<sub>3</sub> via an accidental Coriolis interaction. Another resonance accounted for in this list explains the anomalously strong ΔK<sub>a</sub> = 5 lines that have been detected in atmospheric observations by Goldman et al. [129]. Janssen et al. [125] reported a comparative study of the sensitivity of FTIR ground-based measurements of the atmospheric ozone concentration with respect to line parameters included in the HITRAN2012, GEISA2015 and S&MPO databases in 10- and 5-μm regions. They have concluded that “only S&MPO gives an entirely consistent result at the ± 1% level”. Consequently, we include the S&MPO line positions and intensities in the present HITRAN release. This should improve both intensity distribution in the ro-vibrational lines of the 5-μm bands and their consistency with the 10-μm range. The line-shape parameters for all the lines were estimated using the semi-empirical procedure described in the HITRAN2008 paper. This model largely relies on a slightly revised empirical model for the

fundamental bands from Ref. [130].

2.3.1.2. 2335–2617 cm<sup>-1</sup>: bands [(120),(021)–(000)]. Another spectral interval where S&MPO and HITRAN2012 ozone data differ significantly is for the 2ν<sub>2</sub>+ν<sub>3</sub>, ν<sub>1</sub>+2ν<sub>2</sub> bands in the 2335–2617 cm<sup>-1</sup> range. Previous HITRAN versions included the MIPAS list [131] for 2ν<sub>2</sub>+ν<sub>3</sub> and older data from Flaud et al. [126] for ν<sub>1</sub>+2ν<sub>2</sub>. S&MPO data in this range were initially based on the analyses of Barbe et al. [132]. Subsequently, new Reims FTS measurements with a better signal-to-noise ratio [120] suggested an empirical intensity scaling factor of 0.86 for these bands. The resulting line list is provided for this release of HITRAN. Recently, accurate *ab initio* dipole moment surfaces of ozone were constructed by Tyuterev et al. [133] that allowed theoretical intensities to be obtained from variational nuclear-motion calculations which used the potential energy surface of Ref [134]. The shape of the ν<sub>1</sub>+2ν<sub>2</sub> band given by these calculations is qualitatively more consistent with the S&MPO data incorporated in the
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Comparison of HITRAN2012 and HITRAN2016 ozone line lists in the range of $2\nu_2+\nu_3$ and $\nu_1+2\nu_2$ bands with recent ab initio calculations using the dipole-moment surface of Tyuterev et al. [133] that provides qualitative confirmation of the rotational distribution of intensities in the $\nu_1+2\nu_2$ band given by HITRAN2016.</figcaption></figure>

new list of HITRAN2016 than HITRAN2012 (see Fig. 9).

2.3.1.3. **3584–3831 cm⁻¹ range: [(013),(112)]–(000) bands.** The S&MPO contains data for the $\nu_2 + 3\nu_3$ and $\nu_1 + \nu_2 + 2\nu_3$ bands in the range 3584–3831 cm⁻¹ based on a larger sample of assigned transitions [135] than the previous releases of HITRAN (see Table 4 of Babikov et al. [119]). The parameters can be found in the corresponding sections of the online S&MPO site [119], and 5729 transitions are now included that makes the integrated intensity of the $\nu_1 + \nu_2 + 2\nu_3$ band nearly 5 times stronger (Table 5) than in the previous release.

2.3.1.4. **4268–4399 cm⁻¹ range: [(023),(122),(400)]–(000) bands.** In the range 4200–4525 cm⁻¹, only the $3\nu_1 + \nu_3$ [136] and $2\nu_1 + 2\nu_2 + \nu_3$ [137] cold bands were included in the previous releases of HITRAN, whereas the $2\nu_2 + 3\nu_3$, $\nu_1 + 2\nu_2 + 2\nu_3$ and $4\nu_1$ bands in the 4268–4399 cm⁻¹ range were missing, although they

correspond to a total integrated intensity of $2.4 \times 10^{-22}$ cm⁻¹/molecule cm⁻². The corresponding analyses [121,138] are as yet unpublished, but the line parameters can be found at the online S&MPO site [119] and are now incorporated in HITRAN2016.

2.3.1.5. **4802–4929 cm⁻¹ range: [(005),(311),(104)]–(000) bands.** Line parameters for the $5\nu_3$, $3\nu_1 + \nu_2 + \nu_3$ and $\nu_1 + 4\nu_3$ bands around 4900 cm⁻¹ are calculated for the HITRAN database using the spectroscopic parameters of Flaud et al. [139]. These bands were then re-analyzed using new spectra recorded in Reims [120] resulting in a new set of the Hamiltonian parameters for the strongly coupled [(005),(104),(311)] vibration states. The corresponding line lists have been generated using these improved spectroscopic parameters. Unpublished Hamiltonian parameters can be found in the S&MPO system at http://smpo.iao.ru/1408x675/en/lev/par/1/19/. The (S&MPO–HITRAN2012) differences in the line positions are up to 0.065 cm⁻¹. The intensity ratios for three cold bands are shown in Fig. 10.

### 2.3.2. Hot bands

2.3.2.1. **281–447 cm⁻¹ (dyad - $\nu_2$).** The ozone spectrum in the range 281–447 cm⁻¹ is dominated by hot $\nu_3-\nu_2$ and $\nu_1-\nu_2$ bands whose intensities were underestimated in the previous HITRAN releases compared to the S&MPO line list [120] which was generated using the empirical transition moments parameters of Birk et al. [140]. The S&MPO parameters are confirmed by the recent ab initio intensity calculations [133] and the corresponding update is thus included in the present HITRAN release.

2.3.2.2. **598–1541 cm⁻¹: hot bands towards the tetrad region.** The unpublished version of the newly determined tetrad [(002),(101), (200),(030)] parameters by Tyuterev et al. [128] can be found at the S&MPO online site (http://smpo.iao.ru/1396x659/en/lev/par/1/6/). Using this set, 14 hot bands in the range 598–1541 cm⁻¹ that go from the (010), (020), (001) and (100) states to the tetrad were re-calculated and are now included in the present HITRAN release with a significantly augmented number of transitions. The transition moment parameters of Ref. [128] for [(101),(030)]–(010) bands and those extrapolated from cold bands [141] were used.

2.3.2.3. **2889–3122 cm⁻¹ range: [(013),(112)]–(010) bands; 3564–3752 cm⁻¹ range: [(023),(122)]–(010) bands.** In the ranges 2889–3122 cm⁻¹ and 3564–3752 cm⁻¹ the [(013),(112)]–(010) and [(023),(122)]–(010) hot bands are updated using the same upper-state EH parameters as for the corresponding cold bands and the ETM parameters of Mikhailenko et al. [142].

2.3.2.4. **3623–4229 cm⁻¹: hot bands to the V=5 triad [(005),(311), (104)].** New line lists for 14 hot bands in the range 3623–4229 cm⁻¹ corresponding to the transitions from the (010, (001) and (100) states to the updated [120] triad [(005),(311),(104)] of five vibrational quanta are also included.

2.3.2.5. **4384–4826 cm⁻¹: missing hot bands to (213),(321) and (114) upper states.** The coupled vibrational states [(213),(321),(114)] have been re-analyzed in Ref [143] with the FTS spectra recorded in Reims. Using these upper-state parameters we now provide four associated hot bands [119] in the range 4384–4826 cm⁻¹ that were missing in previous HITRAN versions. The ETM parameters were extrapolated from the corresponding cold bands: for $2\nu_2 + \nu_2 + 3\nu_3 - \nu_1$ and $\nu_2 + \nu_2 + 4\nu_3 - \nu_3$ from Ref [144], for $2\nu_2 + \nu_2 + 3\nu_3 - \nu_2$ from Ref [145], and for $3\nu_2 + 2\nu_2 + \nu_3 - \nu_1$ from Ref [137].

### 2.3.3. Validations and empirical corrections

As mentioned in the introduction to Section 2.3, the second
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Intensity ratios for the 5$v_3$ (red circles), 3$v_1+v_2+v_3$ (blue triangles) and $v_1 + 4v_3$ (olive triangles) bands between the previous HITRAN2012 line list and S&MPO data that are incorporated in the new HITRAN2016 release (color can be seen in online version of paper).</figcaption></figure>

step was a validation of effective model calculations by a comparison of simulated absorption cross-sections with observed spectra. To this end, high-resolution laboratory FTS spectra of Reims and Kitt-Peak (up to $5700 \text{ cm}^{-1}$) were used alongside MklV balloon atmospheric spectra [146], using similar procedures to those described by Toon et al. in [147]. The MklV balloon spectra cover 650–5650 $\text{cm}^{-1}$ simultaneously in every spectrum, so every window is fitted in every spectrum, making it easier to quantify window-to-window biases.

No major problems were seen with the initial $O_3$ data set prepared for HITRAN2016, at least for strong and medium-strength bands. The new line list provides improved spectral fits in the 2100 $\text{cm}^{-1}$ region as compared with HITRAN2012 and also better consistency with other bands in terms of the retrieved $O_3$ amounts in the 2000–2200 $\text{cm}^{-1}$ region. But it was found that empirical corrections were needed to some line positions to reduce residuals between observed and simulated cross-sections. These corrections concern a limited sample of about 0.1% transitions and are essentially of two types. First, small line position corrections are typically of the order of ~$10^{-3} \text{ cm}^{-1}$ up to $10^{-2} \text{ cm}^{-1}$ in a few cases. The most extreme deviations were corrected already for HITRAN2016, but additional corrections will be done in the near future. Note that these corrections are not reflected in Table 5 where only updates for entire bands rather than individual transitions were carried out. A total of 75,694 lines for $^{16}O_3$ have been added or updated (this includes corrections from Table 5 and the empirical corrections discussed in this subsection). Note that there are 289,340 lines of the principal isotopologue of ozone in total in the HITRAN2016 edition. Secondly, there is an indication that pressure-induced line shift parameters may need to be added or extended in the future.

It is known [120] that for the fundamental bands the effective model fits reproduce observed spectra within experimental accuracy (~$10^{-4} \text{ cm}^{-1}$ for line positions), but the precision of the calculations gradually decreases for higher wavenumber ranges. The RMS of available line position fits increases on average to ~0.001 $\text{cm}^{-1}$ in the 3000 $\text{cm}^{-1}$ region, to ~0.002 $\text{cm}^{-1}$ around 4000 $\text{cm}^{-1}$ and to ~0.003 $\text{cm}^{-1}$ in the 5000 $\text{cm}^{-1}$ range, primarily because of accidental resonances. A typical example is a sharp resonance of the (111) state interacting with the “dark” (040) state

for $[J,K_a] = [44, 4]$ that was not included in the effective model. This resonance produces a perturbation of $0.048 \text{ cm}^{-1}$ in line positions and mixing of the line intensities between coupled transitions. The upper-state energy level and six corresponding P, Q, R- transitions have been empirically corrected both for the cold $v_1+v_2+v_3$ and hot $v_1+v_2+v_3-v_2$ bands. The major part of the relatively large line position corrections (~0.02–0.12 $\text{cm}^{-1}$ for about ten transitions) to previous HITRAN line lists occurs near 2.5-µm and corresponds to the analysis of {(103),(004),(310)}–(000) bands by Perrin et al. [148]. This region is extremely complicated to model because of the strong coupling of 9 bands including some “exotic” resonances, for instance{(103),(004)} with (310) and {(211),(202),(032)}. Analyses of new spectra recorded in Reims for these bands are in progress as outlined in Refs. [120,121], but are not yet finalized. In the meantime, empirical corrections have been applied to corresponding series of levels, mostly around $4000 \text{ cm}^{-1}$, and to associated transitions. This process removes some noticeable residuals between simulated and observed absorption cross-sections.

Relative intensities of various bands play a key role in terms of the consistency of retrieved $O_3$ amount from different spectral windows. From this point of view, the HITRAN2016 line list is certainly much better than its predecessors. For example, the HITRAN2016/S&MPO linelist produces 8% larger retrieved amounts around $2182 \text{ cm}^{-1}$ than previous line lists, bringing this region into better consistency with others. This improvement can be attributed to the major update of the tetrad {(101),(200),(002),(030)} bands [120,128] in the 5-µm region and confirms the conclusion of Janssen et al. [125] concerning the internal consistency of the S&MPO list that is now incorporated in HITRAN2016. The $3000 \text{ cm}^{-1}$ region yields the lowest retrieved $O_3$ amounts, about 5% lower than average, in fits to MklV limb spectra and Kitt Peak lab spectra [146]. This indicates possible errors in line widths or intensities, since the errant line positions were corrected.

### 2.3.4. Future work

There remain several issues for future work. The problem of absolute ozone intensities is still an important challenge [120,124] requiring very precise measurements and ab initio calculations with absolute uncertainties better than 1–3%. This concerns
particularly the isotopic species. New data on ¹⁷O- and ¹⁸O- containing isotopologues will be produced in the near future. Modeling of the complicated 2.5 µm region [120,121,148] must be significantly improved. In general, for the spectral intervals beyond the fundamental bands, the effective models do not provide experimental accuracy for all lines. In order to obtain line position accuracy that is better than 0.001 cm⁻¹ in the FTS range, which is important for atmospheric applications, we could extend experimental determination of ro-vibrational levels and systematically replace transition frequencies by available empirical (E-E')/ħ values as described in Refs. [149,150] where this approach was used for the analysis and line lists of CRDS spectra. Further extension of ozone spectral data to highly-excited states near the dissociation threshold (Ref. [151] and references therein) would also be important for the understanding of the dynamics of ozone formation and depletion [152] and for interpretation of satellite observations involving non-LTE effects in the upper atmosphere.

## 2.4. N₂O (molecule 4)

The N₂O line list has been updated using the recently-published line list for the ¹⁴N₂¹⁸O (¹⁴N¹⁴N¹⁸O) isotopologue [153]. In the immediate future, we will also add (for the first time) the line list that has already been constructed for the ¹⁵N₂¹⁶O (¹⁵N¹⁵N¹⁶O) isotopologue based on Ref. [154]. Both line lists were generated using the results of the global modeling of the line positions and intensities within the framework of the method of effective operators. The polyad model of effective Hamiltonian (EH) [155] was used in both cases. The parameters of the effective Hamiltonians were fit to the line positions collected from the literature. The global RMS of the line position fit for ¹⁵N₂¹⁶O is 0.0011 cm⁻¹ and for ¹⁴N₂¹⁸O is 0.0016 cm⁻¹. The uncertainties in the fitted line positions depend on the quality of experimental data and vary from 0.0046 cm⁻¹ to 2 × 10⁻⁶ cm⁻¹.

The line intensities were calculated using fitted sets of the effective dipole moment parameters for a particular isotopologue where possible. Note that the line intensity measurements for rare isotopologues of N₂O are very scarce. Therefore, based on the theoretical results of Ref. [156] (in which contributions due to isotopic substitution to the values of the effective dipole moment parameters were shown to be negligible) the majority of the line intensities were calculated using the effective dipole moment parameters of the principal isotopologue [157]. The partition sums were taken from Ref. [112] for ¹⁴N₂¹⁸O and calculated using direct summation for ¹⁵N₂¹⁶O [154].

It is well known that the nitrous oxide EH polyad model does not work in a totally satisfactory manner because of the existence of a number of Coriolis and anharmonic interpolyad resonance interactions (see, for example, Refs. [158,159]). But usually these kinds of interactions take place in the high-wavenumber region for the weak bands. Adopting an intensity cutoff of 10⁻²⁹ cm⁻¹/molecule cm⁻² at 296 K, including the isotopic abundance factors 1.986 × 10⁻³ for ¹⁴N₂¹⁸O and 1.32 × 10⁻⁵ for ¹⁵N₂¹⁶O, overcomes the problem of the interpolyad resonance perturbations in practice because the most affected transitions are omitted. Nevertheless, line parameters for the very weak bands have to be used with caution.

For the ¹⁴N₂¹⁸O isotopologue, Toth's [160] lines given in HITRAN2012 are retained and only new line parameters from the line list of Ref. [153] are added to HITRAN2016. Line positions from the line lists of Refs. [153,154] are assigned uncertainty code 6 (0.000001–0.00001 cm⁻¹) in the MW region and 3 (0.001–0.01 cm⁻¹) in the IR region. For the line intensities from these line lists, the uncertainty code 3 (> 20%) is adopted.

Line-shape parameters as well as Rosenkranz [35] line-mixing coefficients for the main isotopologue lines belonging to the ν₃ ro-

vibrational band are taken from the work of Loos et al. [161]. These parameters originate from a multispectrum fit of air broadened Fourier-Transform transmittance measurements at ambient temperature. The experimental data of Loos et al. was fitted using a quadratic speed-dependent Voigt model based on the Hartmann-Tran profile [7,8,14], extended to account for line mixing in the Rosenkranz first-order perturbation approximation [35]. As a result, air-broadening, air-speed-dependence, air pressure shift and line-mixing parameters were retrieved in the spectral range 2184.8 to 2251.6 cm⁻¹ for P- and R-branch lines up to J'' = 40 and 39, respectively.

## 2.5. CO (molecule 5)

In order to improve the quality and consistency (between bands and isotopologues) of the intensities of the lines of carbon monoxide in HITRAN, a new piece-wise dipole moment function (DMF) was created [162] using the direct fit approach that was developed in Ref. [163], supplemented with new ab initio calculations [162]. In order to ensure the best results, new experiments were carried out in Ref. [162] for the 4–0 band and the first measurements of the 6–0 band were performed to add into the direct fit.

The line intensity and position calculations were performed using this new DMF in conjunction with the wavefunctions calculated from the experimentally-determined potential energy function (PEF) from Coxon and Hajigeorgiou [164]. Calculations were carried out using version 8.0 of LEVEL [165].

The new dipole moment function has proven to be a substantial improvement over the one used previously. In this edition of the database it was used to calculate HITRAN intensity values in all the bands of all isotopologues. Fig. 6 of Li et al. [162] shows a comparison of the available experimental data in different bands of the principal isotopologue, with new calculations, HITRAN2012, and ab initio calculations. It is clear that intensities from Li et al. agree much better with experimental values than previous data. This also becomes important for the consistency of intensities between the isotopologues (due to the use of the same dipole moment function for all of them); previously a lot of lines for the principal isotopologues had intensities from accurate laboratory measurements whereas minor isotopologue intensities were still calculated with outdated dipole moment functions. Finally, the new work allows one to add a few more bands and also line lists for radioactive isotopologues of carbon monoxide, which, as we mentioned in the CO₂ section, are provided in static files rather than through the HITRANonline interface.

Apart from the intensities, the Li et al. line list features: a) line positions calculated based on the potential energy function from Ref. [164]; b) air- and self-broadening parameters from a sophisticated semi-empirical approach, c) air-induced shift obtained for all the lines by extrapolating measurements carried out in the 2–0 band by Devi et al. [166], where the Hartmann and Boulet [167] approach was used for extrapolating; d) widths (and their temperature dependencies) and shift due to pressure of H₂ and CO₂ important for planetary and combustion research were also provided employing semi-empirical approaches similar to the ones used for air and self pressure-induced values.

We have adapted the Li et al. line list for this edition of the HITRAN database with a few notable changes:

1) Since the original Li et al. line list was aimed at high-temperature applications, it was truncated here with an intensity cutoff of 10⁻³¹ cm⁻¹/(molecule ●cm⁻²) (in natural abundance of the isotopologues) for all the lines except those in the MW region where a cutoff of 10⁻⁴² cm⁻¹/(molecule ●cm⁻²) was used to satisfy some astrophysical applications.
<table><caption>**Table 6**
Scheme of HITRAN2016 update of ¹²CH₄ and ¹³CH₄. All regions not given in the table remain identical to the HITRAN2012 edition.</caption><thead><tr><th>Range (cm<sup>-1</sup>)</th><th>Positions</th><th>Intensities</th><th>Line shapes</th></tr></thead><tbody><tr><td>0–1370</td><td>HIT12, with many hot bands removed. P1–P0 (ν<sub>4</sub> band only), P1–P1, P2–P2, P3–P3, P2–P1 and P3–P2 from MeCaSDa [186], ¹³CH₄ ν<sub>2</sub> band restored from HIT08</td><td>P1–P0 (ν<sub>4</sub> band only), P1–P1, P2–P2, P3–P3 and P3–P2 from MeCaSDa [186], P2–P1 from HIT08</td><td>HIT12 when available or algorithm from Ref. [176]</td></tr><tr><td>1370–4000</td><td>HIT12 with many line positions fixed using HIT08,S Tyuterev et al. [187] and empirical adjustments of HIT12 to properly fit KP spectra. Several missing lines restored from HIT08.</td><td>HIT12</td><td>HIT12</td></tr><tr><td>4000–4315</td><td>HIT08 with many lines improved using some of the preliminary results from Birk et al. [188], and occasionally HIT12</td><td>HIT08 with many lines improved using some of the preliminary results from DLR Birk et al. [188]</td><td>HIT08 and in some cases HIT12</td></tr><tr><td>4499–4630</td><td>HIT12 mixed with Devi et al. [189]</td><td>HIT12 mixed with Devi et al. [189]</td><td></td></tr><tr><td>4670–5300</td><td>Nikitin et al. [190] with 10<sup>-27</sup> cm<sup>-1</sup>/(molecule·cm<sup>-2</sup>) cutoff applied in 4800–5000 cm<sup>-1</sup> region</td><td></td><td></td></tr><tr><td>5300–5550</td><td>Nikitin et al. [192]</td><td></td><td></td></tr><tr><td>5550–5855</td><td>GOSAT list [193] for ¹²CH₄. Other isotopologues HIT12</td><td></td><td></td></tr><tr><td>5855–6250</td><td>GOSAT list [193] with some lines from Nikitin et al. [194] and HIT12 based on validation for ¹²CH₄. For ¹³CH₄ Starikova et al. [195].</td><td></td><td></td></tr><tr><td>7920–8250</td><td>CRDS measurements Béguier et al. [196]</td><td></td><td></td></tr><tr><td>9028–10,435</td><td>FTS measurements Béguier et al. [197]</td><td></td><td></td></tr></tbody></table>

Note: HIT16 = new HITRAN edition, HIT08 = HITRAN2008 edition [198], HIT12 = HITRAN2012 edition [1], KP = Kitt Peak FTS lab spectra.

2) It was found that there was a minor compatibility issue between LEVEL and PEF from Coxon and Hajigeorgiou [164]. This resulted in a growing (with rotational quantum number) deviation of the calculated line positions from their real values. This has now been fixed.

3) The line positions from the high-accuracy experiments that employ best calibration standards, including frequency combs, were used wherever available. In the first overtone, data from Pollock et al. [168] were used, in the second overtone data from Refs. [169,170] were employed, while in the fifth overtone Tan et al. line positions were taken when available [171].

4) When high-quality experimental measurements of the air- and self-broadening and shift parameters were available in HITRAN2012, they were taken in place of the semi-empirical values from Li et al. This includes the values for the speed-dependent Voigt (with line mixing) profile for the first overtone from Devi et al. [166,172].

5) Helium-broadening parameters, their temperature dependence and pressure shift due to He were added. The broadening parameters were taken from Sinclair et al. [173] who provide both experimental values and values obtained from a fit to a semi-empirical model. We used experimental values where available and semi-empirical values for all other transitions. Although these measurements were carried out only in the fundamental band, they were used here for all lines ignoring vibrational dependence. The temperature dependence of this broadening was taken to be 0.6 for all the lines based on the average value of the measurements by Picard-Bersellini et al. [174]. Finally, the pressure induced shifts are based on measurements of Luo et al. [175] in the fundamental band and extrapolated wherever possible to other bands using the Hartmann and Boulet [167] approach.

## 2.6. CH₄ (molecule 6)

The state-of-the art of methane data in the previous HITRAN releases has been described by Brown et al. [176]. Since then, several new studies have been in progress or have been published both for cold and room-temperature spectra [177–183] and hot spectra measurements [184,185], although many experimental spectra have not yet been fully analyzed. Toon et al. [147] have reported some deficiencies in available databases including

HITRAN2012 [1] by simulations of solar occultation spectra, acquired by the JPL MklV Fourier transform spectrometer from balloon, covering the 650–5650 cm⁻¹ region. These deficiencies have been addressed in this new edition, which includes superior spectral parameters and new lines and bands for ¹²CH₄ and ¹³CH₄. At this time no update was made to CH₃D. An overall brief picture of the update is given in Table 6, while the details are given in the subsequent sections. Note that only regions where revision to HITRAN2012 was carried out are shown. In the regions not mentioned in the table, HITRAN2012 line parameters have been retained.

### 2.6.1. Hot bands in the spectral range below 1370 cm⁻¹

The HITRAN2012 database exhibited some noticeable problems, including those with hot band lines of ¹²CH₄ in the low wavenumber range. The line list in this spectral region relies on calculations resulting from the effective Hamiltonian and dipole moment parameter fits, since extensive experimental line assignments have not been performed for many years. However, extrapolation errors have been introduced in some small spectral regions that were insufficiently well characterized. Recently, a new global fit of methane lines including high-temperature emission data in the 1100–1500 cm⁻¹ region was published [185]. This study included a huge number of new assignments and allowed a much more reliable modeling of hot-band lines. It has resulted in a determination of effective Hamiltonian parameters. The results were first used to update the MeCaSDa database [186] of calculated methane lines. Fig. 11 compares the resulting computed cross-sections with the HITRAN2012 and ExoMol [199] databases. This plot, in logarithmic scale, clearly displays that there was likely a problem in HITRAN2012 but better agreement of the new calculated line list with Exomol.

It is well known that measurement and modeling of line intensities in high-temperature spectra are quite difficult, particularly in non-Boltzmann conditions. In order to avoid large uncertainties in extrapolated intensities, as was the case for some hot bands in the previous HITRAN editions, we have checked new lists against ab initio calculations. The line lists for hot bands resulting from new experimental spectra analyses by Amyay et al. [185], were compared to first-principle variational calculations of line intensities by Rey et al. [200,201], based on the ab initio dipole moment and potential energy surfaces of Nikitin et al. [202–204]
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Comparison of cross-sections calculated at 1 cm⁻¹ resolution between different databases: MeCaSDa [186] (used for the present HITRAN update), HITRAN2012, and ExoMol [199].</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Comparison of line intensities for hot bands of ¹²CH₄ in the 7 to 8 µm range using the HITRAN2016 updated list based on experimental spectra analyses, Amyay et al. [185] (upper panels in black) with variational calculations, Rey et al. [201] based on ab initio dipole moment [202] and potential energy [203,204] surfaces using the line list included in the TheoReTS database [205] (lower panels upside down in red).</figcaption></figure>

<caption>**Table 7**</caption>

Number of calculated ¹²CH₄ lines taken from the MeCaSDa database [186] for some transitions, up to a maximum *J* value. The difference for the number of lines in the two databases is due to a different intensity cut-off with *I*<sub>min</sub> down to 10⁻⁴⁰ cm/molecule in MeCaSDa. The weakest transitions were not retained for the present HITRAN update that includes *I*<sub>min</sub> = 10⁻²⁵ for P3-P2 and 10⁻³⁰ cm/molecule for other hot bands.

<table><thead><tr><th>Transition</th><th>P1-P0 (ν<sub>4</sub>)</th><th>P1-P1</th><th>P2-P2 &lt; 300 cm<sup>-1</sup></th><th>P3-P2 &lt; 1370 cm<sup>-1</sup></th></tr></thead><tbody><tr><td>J<sub>max</sub></td><td>30</td><td>30</td><td>30</td><td>30</td></tr><tr><td># lines (MeCaSDa)</td><td>13,315</td><td>54,677</td><td>457,620</td><td>2,175,406</td></tr><tr><td># lines(HIT16)</td><td>9662</td><td>9721</td><td>10,701</td><td>950</td></tr></tbody></table>

as reported in the TheoReTS database [205]. An example of comparison is given in Fig. 12. Those bands that were in good intensity agreement were considered sufficiently reliable to be incorporated in this new HITRAN release. Some other bands require further investigation and were not included. Note that, at this stage, these

qualitative comparisons did not involve line assignment or line position validations.

This new calculated line list was thus used to replace some of the HITRAN2012 lines in order to generate the HITRAN2016 update, for the P1-P0 (Dyad-GS, for the ν₄ band only), P1-P1 (Dyad-
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Fits to TCCON spectra measured from Park Falls, WI, in March 2005 at a solar zenith angle of 58 deg., in windows used by TCCON to retrieve CO. The left panels show fits to the region occupied by the CO P-branch. The right panels show fits in the R-branch region. The upper panels show fits using HITRAN 2012 CH₄; the lower panels with the HITRAN2016 CH₄ linelist. Note different scales.</figcaption></figure>

Dyad), P2–P2 (Pentad–Pentad), P3–P3 (Octad–Pentad) and P4–P3 (Tetradecad–Octad) transitions (a schematic of the polyads can be seen in Fig. 2 of Ref. [5]). Table 7 shows the spectral regions and the number of lines that exist in MeCaSDa and what proportion of them was adapted for HITRAN2016. It is important to mention that validations against theoretical [205] and laboratory [185] spectra suggested adapting the P2-P1 line list from MeCaSDa, with intensities adapted from HITRAN2008 wherever possible.

It should also be noted that, recently, new highly-accurate measurements of pure rotation lines were performed in the Dunkerque group [206]. These lines have been included in the global fit leading to the effective Hamiltonian parameters used for the present update.

### 2.6.2. 1370–4000 cm⁻¹ region

This region mainly includes the $ν_2$ band, the Pentad bands and relatively weak bands at the lower part of the Octad. As discussed by Toon et al. [147], HITRAN2012 data were plagued with erroneous line positions for lines of intermediate strength, many of which had been substantially superior in the HITRAN2008 edition of the database. It was found that a large number of these lines had been assigned line position uncertainty code 1 (i.e. between 0.1 and 1 cm⁻¹) and originate from the model used in Daumont et al. [207]. For HITRAN2016, wherever possible in the Pentad region, these transitions were reverted to HITRAN2008 and, where this was not possible, line positions from Tyuterev et al. [187] were used. Unfortunately not all problematic line positions were easy to identify on a global scale by just the uncertainty index. Validations against Kitt Peak FT spectra recorded at multiple thermodynamic conditions have shown that more errors associated with line positions remained although the offending lines were not labeled with a low uncertainty code. The most obvious outliers have been

identified and fixed either by employing Ref. [187] or adjusting line positions to fit the low-pressure KP experimental spectra. This provided a significant improvement of the residuals, including those discussed in Toon et al. [147].

### 2.6.3. 4000–4630 cm⁻¹ (correspond to strongest octad bands)

Just as for the Pentad region, HITRAN2012 data had substantial errors in line positions for lines of intermediate strength, while better values actually existed in the HITRAN2008 edition of the database. However, unlike the Pentad region, the intensities of lines in the Octad region were also often inferior to those from HITRAN2008. Also, unlike the Pentad region, reverting line positions of HITRAN2012 lines with uncertainty code 1 to the HITRAN2008 values did not always work. One should also keep in mind that HITRAN2008 was not better than HITRAN2012 everywhere in this region. In addition to lacking ¹³CH₄ lines in that region, HITRAN2008 contained many duplicated transitions. Often a transition would appear as an empirical unassigned line as well as predictions from Hilico et al. [208]. With that being said, some observable transitions were also missing in HITRAN2008.

The following procedure was therefore developed:

1) In the 4000–4315 cm⁻¹ region, the HITRAN2008 line list was used as the starting point. Many unassigned empirical lines were then given assignments and lower-state energies from corresponding predicted Hilico et al. [208] lines, while predicted lines (usually with inaccurate line position and intensity) were removed. In the region 4190–4315 cm⁻¹, parameters of lines with the worst remaining residuals (with respect to low-pressure Kitt Peak FTS spectra) were then replaced with the preliminary results of Birk et al. [188], which used quantum assignments of yet unpublished theoretical work [209] that was not always the same as in HITRAN2012.
<figure><img src="image_18.png" /><figcaption>**Fig. 14.** Overview comparison of the HITRAN line list of $^{12}$CH$_{4}$ above 7920 cm$^{-1}$ in the 2012 and 2016 editions. In this region, line parameters all have an empirical origin: CRDS between 7920 and 8250 cm$^{-1}$ [196], FTS by Brown in the 8250–9028 cm$^{-1}$ interval [218] by Béguier et al. in the 9028–10,923 cm$^{-1}$ interval [197], and ICLAS between 11,000 and 11,500 cm$^{-1}$ [219]. The different polyad and corresponding quantum numbers, P, are indicated. (The polyad number P is equal to 2(V$_{1}$ + V$_{3}$) + V$_{2}$ + V$_{4}$, where V$_{i}$ are the normal mode vibrational quantum numbers).</figcaption></figure>

2) In the 4315–4499 cm$^{-1}$ region, HITRAN2012 parameters were retained.

3) In the 4499–4630 cm$^{-1}$ region, a mixture of HITRAN2012 with parameters from Devi et al. (where available) was used [189].

Fig. 13 shows fits to TCCON spectra measured from Park Falls, WI, in March 2005 at a solar zenith angle of 58 deg., in windows used by TCCON to retrieve CO. The left panels show fits to the region occupied by the CO P-branch. The right panels show fits in the R-branch region. The CO lines are less than 10% deep and so are mainly hidden by stronger CH$_{4}$ and solar lines. The upper panels show fits using HITRAN2012 CH$_{4}$; the lower panels with the HITRAN2016 CH$_{4}$ linelist. The rms spectral fits and the peak residuals are significantly reduced in both windows with the HITRAN2016 linelist. The retrieved CH$_{4}$ amounts (written over the top right of each panel) are much more consistent between the

two windows using the HITRAN2016 linelist (0.1%), than when using HITRAN2012 (2%). It implies that the 2016 CH$_{4}$ intensities are much more self-consistent across these two regions than in HITRAN2012. With that being said, there is still large room for improvements. A more consistent Octad list in terms of line parameters and assignments is planned to be worked out for the next HITRAN update.

### 2.6.4. Tetradecad range 4800–6250 cm$^{-1}$

Contrary to the low-energy range, in the Tetradecad range and above, positions and intensities are mostly experimental values.

The Tetradecad of methane contains 14 bands with 60 strongly coupled sub-bands with very congested spectra difficult to analyze because of numerous resonance perturbations. The previous HITRAN2012 line list was mainly based on the analysis of Refs. [210,211], which included about 3000 assigned lines. This was only

<figure><img src="image_19.png" /><figcaption>**Fig. 15.** Comparison of unassigned experimental line list of $^{12}$CH$_{4}$ in the Tetracontad range (Béguier et al. [197]) included in HITRAN2016 (upper panels in black) with variational calculations, Rey et al. [205] based on ab initio dipole moment [202], and potential energy [203,204] surfaces using the line list included in the TheoReTS database [205] (lower panel upside down in red).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. 16.** Overview of assigned lines [195] in the experimental ¹³CH₄ DAS list [223] included in HITRAN2016 in the 1.66-µm range.</figcaption></figure>

a partial assignment with relatively large uncertainties for about 70% of weak sub-bands. In HITRAN2016, major updates have been included in four spectral intervals. Long-path Fourier Transform spectra (L = 1600 m) in the lower Tetradecad interval 4800–5300 cm⁻¹ recorded in Reims have been re-analyzed by Nikitin et al. [190]. The list includes transitions with line positions adjusted according to their measured values and transitions with calculated line positions, all intensities being derived from the model fit. For HITRAN2016 in the 4800–5000 cm⁻¹ region, transitions with intensities greater than 10⁻²⁷ cm⁻¹/(molecule·cm⁻²) were retained.

In the next interval, 5300–5550 cm⁻¹, the list contains mostly observed positions and intensities. The analysis is in progress [192], but a preliminary empirical line list is included in the present release because the previous versions were quite poor in this range.

In the interval 5550–6250 cm⁻¹, a recent version of the GOSAT line list [193] is included. In the 2v₃ region (5855–6250 cm⁻¹), the GOSAT line list [193] was supplemented with predominantly experimental line parameters of the WKLMC list (Wang-Kassi-Leshchishina-Mondelain-Campargue) obtained from high-sensitivity laser measurements in Grenoble [212]. The previous GOSAT assignments [211] included in the HITRAN2012 list have been considerably extended in the recent work [194] based on new analyses in the 5855–6250 cm⁻¹ range. The modeling of these complicated spectra was accomplished using an approach combining *ab initio* calculations with effective Hamiltonian (EH) fits. Initial values of the EH parameters were first computed from the molecular potential energy surface of Nikitin et al. [203] using irreducible tensor techniques [213,214] via the high-order contact transformation method as described by Tyuterev et al. [187]. This information has been only partially adapted for HITRAN2016. Indeed, the validations against experimental and atmospheric spectra (similar to those done in Toon et al. [147]) showed that the GOSAT line list [193] produces better residuals than the one from Ref. [194], although the latter one is substantially more complete in terms of both amount of lines and proportion of assigned lines. In the end, the GOSAT line list [193] was supplemented with some lines from Ref. [194] and occasionally with HITRAN2012. For instance, very accurate line positions measured with frequency comb in Zolot et al. [215] were retained. In the future, a more sophisticated combination of the GOSAT line list [193] and the one from Ref. [194] will be performed. For broadening, a combination of parameters from Lyulin et al. [191],

GOSAT line list and HITRAN2012 was used.

### 2.6.5. Higher wavenumber ranges

In the Icosad range (6300–7500 cm⁻¹), the WKLMC list provided empirical values of the lower-state energy levels. Recently, on the basis of global variational *ab initio* spectra predictions [180], about 13,000 transitions of 108 new bands have been assigned [216] in the WKLMC list at 80 K and 296 K. The assigned experimental list at 80 K as well as calculated ones are provided as Supplementary Material of Rey et al. [216]. These assignments will be transferred and extended in the WKLMC list at 296 K and included in the next update of HITRAN.

Fig. 14 gives an overview comparison between the HITRAN2012 and HITRAN2016 lists of ¹²CH₄ above 7920 cm⁻¹. New room-temperature measurements by Cavity Ring Down Spectroscopy (CRDS) in the 7920–8250 cm⁻¹ interval have allowed better characterizing of the 1.25-µm methane transparency window [21]. In the Triacontad region (8250–9028 cm⁻¹), line parameters relying on FTS measurements by Brown [42] remain unaltered with respect to HITRAN2012. Above 9028 cm⁻¹, new FTS line parameters are adopted in the region of the 3v₃ band (9028–9200 cm⁻¹) [22]. From the same FTS spectra, the first empirical line list was retrieved in the Triacontad region (9520–10,435 cm⁻¹) [22]. It is worth mentioning that between 7920 cm⁻¹ and 10,923 cm⁻¹ no assignments or empirical lower-state energy values are provided except for a few tens of transitions in the region of the 3v₃ band near 9046 cm⁻¹. Complementary information in this case can be provided by variational *ab initio* predictions [31]. The estimated error for line positions in the best first-principle calculation in the 1-µm range is on average ~0.3–2 cm⁻¹; that is not sufficient for high-resolution applications. However, intensities are in qualitatively good agreement with observations, see Fig. 15. Recent *ab initio* intensity results [217] for the strongest ¹²CH₄ lines below 7600 cm⁻¹ have confirmed accurately measured intensity values to 1–3% on average for the “stable” transitions non perturbed by accidental resonances. Predicted spectra [31] include lower-state energies for all lines and can be recalculated for various temperature conditions that make it suitable for low- and medium-resolution modeling of observations in planetary atmospheres similar to those reported for Titan in Ref [45].
<figure><img src="image_21.png" /><figcaption>**Fig. 17.** Comparisons of the updated positions versus the HITRAN2012 compilation. The panels show differences between listed frequencies labeled by the quantum number *J*'' plotted vs. line intensities for the transitions for the atmospheric IR *d*¹Δg – *χ*³*ς*⁻g system of ¹⁶O¹⁶O and the atmospheric *b*¹*ς*⁺g – *χ*³*ς*⁻g system of ¹⁶O¹⁶O, ¹⁶O¹⁸O and ¹⁶O¹⁷O. Note that half of the *b*¹*ς*⁺g – *χ*³*ς*⁻g magnetic dipole transitions were removed from the plots for the purpose of clarity because the spin pairs were on top of each other.</figcaption></figure>

Finally, in the 10,923–11,502 cm⁻¹ spectral range, the HITRAN lists reproduce line parameters derived by Intracavity Laser Absorption Spectroscopy (ICLAS) [220]. For the ICLAS data [176,220], empirical lower-state energy values were derived by the two-temperature-method [221].

### 2.6.6. ¹³CH₄ isotopologue

In HITRAN2012, the entire $ν_2$ band of ¹³CH₄ (centered at about 1550 cm⁻¹) was accidentally dropped from the compilation. This band is now restored back from HITRAN2008 [198].

For the ¹³CH₄ isotopologue, new measurements and analyses in the 3750–4700 cm⁻¹ octad range have been reported by Brown et al. [222] for temperatures between 80 and 296 K. This included over 4700 positions and 3300 intensities for assigned observed transitions. The line list based on the analysis and spectra fits of this range contains 9500 calculated lines. It is available as supplementary material attached to Ref. [222] and will be incorporated in the next HITRAN update.

First assignments of the strongest bands at the high-wave-number boundary of the ¹³CH₄ Tetradecad (5852–6200 cm⁻¹ region dominated by 2$ν_3$) have been reported by Starikova et al. [195]. Differential Absorption spectra (DAS) recorded in Grenoble [223] at 80 K and 296 K have been used for this analysis. For the 80 K spectra, about 2900 lines of 3717 observed lines were assigned. The theoretical model in both spectral intervals, 3750–4700 cm⁻¹ and 5852–6200 cm⁻¹, was based on the combined approach [187] using ab initio predictions for line positions and intensities [224] with subsequent EH fits, which is similar to that for the main ¹²CH₄ isotopologue as outlined in Section 2.6.4 Supplementary materials of Ref. [195] contain a partially-assigned experimental DAS list at 80 K. For the present HITRAN2016 release, the assignments were extended for the 296 K list. Note that many

observed absorption features in the congested spectrum could have contributions from various overlapping transitions. As the assignment criteria, the ratio $R = I_{re-calc}/\{I_{obs}(296 \text{ K})\}$ was used as well as the coincidence of $J_{low}$ obtained theoretically and via the empirical 2T-method. Here $I_{re-calc}$ stands for intensities re-calculated from the 80 K DAS list via the Boltzmann factor. In doubtful cases, but with consistent $J_{low}$ values, we have kept empirical line parameters. For big *R* deviations and inconsistent $J_{low}$ values, a default value $E_{low} = 333.3333 \text{ cm}^{-1}$ was applied. The self- and airline broadening were calculated following the method described in the GOSAT paper [211].

Fig. 16, giving the overview of ¹³CH₄ lines in the 2$ν_3$ range, shows that most of the strong and medium lines in the HITRAN2016 release have consistent assignments.

An extension of this work using the approach of Ref. [216] is in progress for the Icosad range of ¹³CH₄ [225].

### 2.6.7. CH₃D and other isotopologues

The parameters for this singly-deuterated methane molecule have not been updated for the 2016 edition. We note a recent paper by Bray et al. [226] which indicated the need for updating the intensities of CH₃D in the pure rotational band. We are considering this work for a future update of HITRAN.

### 2.6.8. Line-shape parameters for methane

Our knowledge of pressure broadening of methane by air, N₂, H₂, He, CH₄, and other gases remains incomplete, particularly for the near-infrared and for weaker transitions in all spectral regions. Basic coefficients of Lorentz broadening (widths, shifts, temperature dependences) all vary as a function of the transition quantum numbers, and values obtained for the fundamental bands are not easily applicable to the very complicated polyads having multiple
<figure><img src="image_22.png" /><figcaption>**Fig. 18.** High-air mass ground-based Kitt Peak spectrum fitted without (a) and with (b) $b^1\Sigma_g^+(v=3)-X^3\Sigma_g^-(v=0)$ band of $O_2$.</figcaption></figure>

underlying vibrational states. Available measured values for Lor-
entz pressure broadened line-shape parameters from the previous
edition of HITRAN were transferred into the new compilation, but
most of the lines have been given crudely-estimated air- and self-
broadened half widths, as described by Brown et al. [176]. Good
theoretical models, confirmed by measurements, must be im-
plemented in order to have accurate values, not the estimates used
here for 99% of the present database. Since the release of the 2012
database, additional methane line-shape measurements have be-
come available in several spectral regions for $^{12}$CH$_4$
[179,181,183,189,227–232], and CH$_3$D [226,233,234], and these
will be added in the future updates to HITRAN2016.

The recent study of Mendonca et al. [235] illustrates the
growing evidence that for remote-sensing applications requiring
the highest accuracies of line parameters to model the observed
methane spectrum, Voigt lineshapes are inadequate. It is well
known that collisional line mixing can significantly affect ab-
sorption spectral shapes, and line mixing cannot be neglected for
accurate retrievals of atmospheric CH₄ abundance. The HI-
TRAN2012 database [1] had included the line-mixing parameters
for CO₂, CO and O₂, however without providing guidance or tools
for users to implement these parameters in their calculations. The
new structure of the HITRAN database [236] and the HITRAN
Application Programming Interface (HAPI) [5] provide the poten-
tial for the inclusion of line-mixing parameters in the database and
provide tools for their implementation.

A theoretical approach proposed to model line-mixing effects
for CH₄ by Tran et al. [[237-239] has been successfully applied to
calculate absorption in the ν₃, ν₄ and 2ν₃ bands. The same model
has been used to simulate the spectra in the ν₂ + ν₃ band as a test
to demonstrate whether it can be applied to other vibrational
bands. Comparisons with spectra measured under different pres-
sure conditions [189] show that line mixing only weakly affects

the absorption spectral line shape of this band under these pres-
sure conditions. Therefore, more calculations for other vibrational
bands under large pressure conditions are needed to demonstrate
if this model can be applied to various vibrational bands. While the
work of Tran et al. ([239] and references therein) represents sig-
nificant progress in understanding non-Voigt line shapes in me-
thane, additional laboratory studies of line mixing, speed depen-
dence and narrowing are required to provide the basic parameters
for future methane compilations in the spectral regions most
utilized for remote sensing.

2.7. $O_2$ (molecule 7)

An extensive update of the HITRAN oxygen line list for the new edition makes use of the following significant progress on: (1) a self-consistent set of energy levels and line positions for the entire HITRAN oxygen line list; (2) lineshape, line mixing, collision induced absorption (CIA), and new speed-dependent Voigt (SDV) formalism for the A-band; (3) HT profile parameters for the B-bands. We also added the $b^1\Sigma_g^+(v=3)-X^3\Sigma_g^-(v=0)$ and $b^1\Sigma_g^+(v=2)-X^3\Sigma_g^-(v=1)$ bands.

2.7.1. *Line positions and energy levels*

A self-consistent set of spectroscopic constants from an up-
dated isotopically invariant Dunham fit by Yu et al. in 2014 [240]
were used to update line positions and energy levels throughout
the entire HITRAN2012 oxygen line list, except for the so-called
zero-frequency lines (due to Zeeman splitting of the ground state),
see HITRAN1986 paper for details [241]. This updated Dunham fit
was obtained by adding new microwave and infrared measure-
ments [242,243] to the first global analysis of O₂ by Yu et al. in
2012 [244] that simultaneously fits all available experimental line
positions for all the bands and isotopologues from the microwave
to UV region. The new microwave measurements [242] include 324 rotational transitions in the $a^1Δ_g$ $ν = 0$ and 1 states of the six $O_2$ isotopologues and helped determine two more hyperfine parameters, the electric quadrupole interaction $eQq$ and the nuclear spin-rotation interaction, $C_l$. The new infrared work [243] reported 1644 $b^1Σ_g^+ - X^3Σ_g^-$ transition frequencies for the six $O_2$ isotopologues and revealed a $0.2 \text{ cm}^{-1}$ calibration error in the $^{17}O^{18}O$ $v' - v'' = 1 - 0$ Raman data of Edwards et al. [245]. It resolved discrepancies in the Raman data for $^{16}O^{17}O$, $^{17}O^{17}O$, and $^{17}O^{18}O$, and improved the vibrational parameterization of the ground electronic state.

A quantum number assignment error has been fixed in HITRAN2016 for 17 lines in the $ΔNΔJ = PO$ branch of the $^{16}O^{16}O - X(v', v'') = (0, 0)$ band. These lines were incorrectly labeled with a shift of two in $N''$ and $J''$ in HITRAN2012, as was pointed out in Yu et al. [240].

Band-by-band comparisons of the updated line positions and lower state energies versus the HITRAN2012 compilation were performed. For all the bands in the $a^1Δ_g - X^3Σ_g^-$ system (for which the data in HITRAN originate from Refs. [246–249]), systematic differences, smaller than experimental uncertainties, were found for measured lines and expected deviations were found for extrapolated lines. As depicted in the upper left panel of Fig. 17, systematic differences of $0.0007 \text{ cm}^{-1}$ were found for the $^{16}O^{16}O - X(v', v'') = (0, 0)$ band. Similar deviations were found for all other bands except the A-band ($b - X(v', v'') = (0, 0)$) of $^{16}O^{16}O$, $^{16}O^{17}O$ and $^{16}O^{18}O$, where the data originated from Refs. [250,251] (see discussion below).

As shown in upper right, lower left and lower right panels of Fig. 17, deviations are very small for one sub-group of the A-band transitions while they are large for another sub-group. The sub-group of transitions with small deviations belongs to the $^{1}P_0$, $^{1}P_1$, $^{1}Q$ branches while the other sub-group with large deviations belongs to the $^{1}N_0$, $^{1}R_R$, $^{1}Q$, $^{1}S$ and $^{1}T_S$ branches.

### 2.7.2. A-band region near 762 nm

The line list for the A-band in HITRAN2012 originates from the data from the Long et al. [250,251] papers. Here an update of the principal isotopologue magnetic dipole allowed transitions is given that incorporates a self-consistent model of collisional line-shapes, collisional line mixing and collision induced absorption in a multispectrum fit to a wide range of experimental data sets described in Drouin et al. [252] (hereafter referred to as Drouin 2017). The lineshape chosen was the speed-dependent Voigt profile and effects of line mixing were introduced as a fixed W-matrix derived from the theoretical line mixing formalism of Tran, Boulet and Hartmann in 2006 [253]. Strong correlation was observed between the scaled line-mixing parameterization and the collision-induced absorption derived as remaining non-resonant absorption. Correlation was also observed between the line-by-line parameters correlations in the R-branch band-head region and the scaled line-mixing parameterization. The self and air-broadening parameters as well as air-pressure shift parameters do not deviate significantly from HITRAN 2012, with smooth trends that closely match the polynomial expressions of Long et al. that were used in the prior compilation. Unlike HITRAN 2012, minor erratic deviations of these parameters (which were minimized in the chosen scaling of the line mixing) are retained in the compilation for self-consistency. The self-pressure shift parameters are provided for the first time in this HITRAN edition, and these values differ systematically by $0.006 \text{ cm}^{-1} \text{ atm}^{-1}$ from those given by Long et al. [251]. The air broadening temperature dependence is also systematically shifted to higher values in comparison to HITRAN 2012. The self-broadening temperature dependence is also provided for this band for the first time in

HITRAN and it is even more systematically shifted in comparison to published values from Brown & Plymate [254]. Temperature dependences of the pressure-shift are also included in the new line-by-line parameterization; there is no prior parameterization for comparisons of these values. The final fitted line-by-line parameter is the 'speed-dependence' of the pressure broadening, which replaces the Dicke parameter determined in Long's work and included in the supplemental listing of HITRAN 2012. Qualitative comparison of these two 'narrowing' parameters shows that there is a similar $m$ dependence in each parameter set, each with a more pronounced slope at $m < 0$ (the P-branch). Use of the speed-dependent Voigt allows this lineshape profile to be cast within the more encompassing Hartmann-Tran profile. Line-by-line line-mixing parameters (Rosenkranz parameters) were not fitted in Drouin 2017 [252], but rather a full W-matrix formalism based on theoretical work, empirically modified to improve the multi-spectrum fit quality, was implemented. The empirical corrections that attenuated line mixing at high rotational excitation (see Tran and Hartmann, [255]) were abandoned in favor of the removal of mixing between differing spin-states. The effects of this correction are clearly evident in the derived Rosenkranz parameters, which oscillate between even and odd $J$ (or $m$) values when the spin-changing collisional mixing is allowed, but follow a smooth trend with $J$ (or $m$) when they are removed. Predoi-Cross et al. [256,257] provide fitted room-temperature Rosenkranz parameters (and other lineshape parameters) that agree with the parameters of Drouin 2017. The modified line mixing parameterization significantly alters the derived collision-induced absorption, with this absorption appearing narrower and stronger, with a shape and extent that conforms more closely to the resonant band. The collision-induced absorption derived for air is provided separately in the corresponding section of the HITRAN database, see Section 4 for details.

For the convenience of HITRAN users, two transformations have been applied to the line-shape/line-mixing parameterization given by Drouin 2017. First, given the primary usage of the oxygen database to model the Earth's atmosphere, the foreign-broadening line-shape parameters presented in Drouin 2017 have been converted to air-broadening parameters using the binary mixing ratio $[N_2]_i: [O_2] = 0.79:0.21$. Second, the scaled, full-W-matrices for foreign and self line mixing and the associated temperature dependence were evaluated for 1 atm of air at four 'standard' temperatures and then composed into first order line-by-line Rosenkranz parameters. This representation of the line mixing is based on similar representations provided for the fine-structure line mixing in the microwave spectrum and is both easier to tabulate as well as more efficient to compute. We note that the multidimensional oxygen A-band cross-section table (ABSCO 5.0), now in use for the reduction of OCO-2 [64] atmospheric data, utilizes the parameterization of Drouin 2017 in its native form, and was calculated with the same source code as the multi-spectrum fitting program. The differences between the ABSCO 5.0 table and the HITRAN database are due to the two cosmetic modifications indicated above, as well as the replacement of line centers with the more precise values derived from the work of Yu et al. [240] described above.

### 2.7.3. New addition of $b^1Σ_g^+(ν = 3) - X^3Σ_g^-(ν = 0)$ and $b^1Σ_g^+(ν = 2) - X^3Σ_g^-(ν = 1)$ bands and merging in the UV lines for $^{16}O_2$

Atmospheric spectra can show where important bands are missing. Fig. 18 shows a high-air mass ground-based Kitt Peak spectrum fitted using the HITRAN 2012 line list. It is clear that the $b^1Σ_g^+(ν = 3) - X^3Σ_g^-(ν = 0)$ band of $O_2$ at $0.58 \text{ µm}$ is missing in the compilation. The line list for this band was calculated and while there were data to calculate line positions [258], no information
was available for intensities, and this atmospheric spectrum was used to scale the intensities to fit the observations. We also note that line positions used in HITRAN2016 for this band differ from those in Yu et al. [240]. This is due to the fact that the measurements in Ref. [258] were carried out using a relatively high pressure of oxygen. It is accounted for in the fit here but not completely in Ref. [240]. Broadening and shift parameters for this band were estimated using the approach described in Gordon et al. [259]. Fig. 18b shows that the inclusion of this band makes the difference and it was included into HITRAN2016.

In a similar fashion it was discovered that it is important to include the $b^l \sum_g^+(v=2) - X^3 \sum_g^-(v=1)$ band (basically a hot band underlying the B-band). Intensities for this band were also determined based on atmospheric spectra.

Finally, the UV bands of oxygen are not stored separately anymore and one can retrieve them from www.hitran.org in the same manner as the other bands.

### 2.7.4. B-band

Recently Domysławska et al. [260] developed a dataset of the line-shape parameters for the B-band of self-broadened $O_2$ (the main isotopologue), based on accurate CRDS measurements [261,262]. The dataset includes 49 lines from the P and R branches. It was observed that, at the considered level of signal-to-noise ratio, the spectra are well reproduced by the speed-dependent Voigt profile with quadratic speed dependence. In the near future we plan to incorporate these data into HITRAN within the formalism of the Hartmann-Tran profile by setting the frequency of the velocity-changing collisions, $v_{VC}$, and the correlation parameter, $\eta$, to zero.

### 2.8. NO (molecule 8)

Nitric oxide is an important constituent of the terrestrial atmosphere, where it is commonly detected and quantified by its fundamental IR band. The rotational spectrum plays an important role for the detection in diverse interstellar sources, but may also be used for detection in the Earth's upper atmosphere; Müller et al. [263] give a brief overview with appropriate references.

Positions and intensities for all stable NO isotopologues and for the first excited state in the pure rotation transition (1-1) of the principal isotopologue were based on Müller et al. [263] who performed a combined fit of all available rotational data together with heterodyne IR data of the fundamental band of the main species. All experimental data were carefully evaluated, a few poorly-fitting transition frequencies were omitted, and the uncertainties of some additional data were slightly adjusted as detailed in Ref. [263].

The $^{14}$N$^{16}$O line position and intensity data consist of millimeter and lower submillimeter data from Pickett et al. [264] and tunable far-IR data from Varberg et al. [265] in $v=0$. The $\Lambda$-doubling transitions in $v=0$ were mainly taken from Meerts and Dymanus [266]; additional data were taken from Refs. [267,268]. Further $\Lambda$-doubling transitions in $v=0$ and 1 were taken from Dale et al. [269] and Lowe et al. [270]. Also used in the fit were the $v=1-0$ heterodyne IR data from Hinz et al. [271] and the very accurate data from Saupe et al. [272].

Millimeter and lower submillimeter data of $^{15}$N$^{16}$O were contributed by Saleck et al. [273], and tunable far-IR data by Varberg et al. [265]. The $\Lambda$-doubling transitions were taken from Meerts and Dymanus [266]. Millimeter and lower submillimeter data of the isotopologue $^{14}$N$^{18}$O were also published by Saleck et al. [273], and tunable far-IR data by Müller et al. [263]. Finally, millimeter data of $^{14}$N$^{17}$O and $^{15}$N$^{18}$O were taken from Saleck et al. [274].

A very accurate value of the ground-state electric dipole

moment of $^{14}$N$^{16}$O was published in Ref. [268]; a value for $v=1$ was determined in Ref. [275]. The strength of the magnetic dipole transition was derived from the electron spin-rotation parameter $\gamma$ employing Curl's relationship [276] to evaluate the g-factor of the respective isotopologue and in the appropriate vibrational state.

The data set has been extended considerably by FTIR data of $^{14}$N$^{16}$O reaching high $v$ and by FTIR data of minor isotopic species from Ref. [277]. This will be used for future editions of HITRAN and HITEMP [278]. The spectroscopic model developed in this study has also been used to compute g-factors for the system [279].

### 2.9. SO₂ (molecule 9)

Since the 2008 edition of HITRAN, the air-broadening coefficients of sulfur dioxide have been a constant, $\gamma_{air} = 0.1025 \text{ cm}^{-1}$, for line positions lower than $3000 \text{ cm}^{-1}$; with $\gamma_{air} = 0.1 \text{ cm}^{-1}$ for line positions larger than $3000 \text{ cm}^{-1}$ (see HITRAN2008 paper [198] for details). Recently, new measurements have been made of the air-broadening coefficients of $SO_2$ from infrared and submillimeter-wave spectra [280,281]. When analyzing these data together with previously available experiments [282–285], a slow decrease of the air-broadening coefficients with increasing $(J'' + 0.2Ka'')$ lower-state quantum number combination was observed (see the report by Tan et al. [286] for details). The vibrational dependence of the broadening parameters is demonstrated to be very small or negligible.

A linear extrapolation (see Eq. (1)) up to $(J'' + 0.2Ka'') \le 62$ (with error code 4 (10–20%)) was used, while for the transitions with $(J'' + 0.2Ka'') > 62$, the value corresponding to the experimental value with the largest $(J'' + 0.2Ka'')$ was used.

$$ \gamma_{air} = 0.10731 - 2.88311 \times 10^{-4}(J'' + 0.2K_a'') \quad (1) $$

A complete line-by-line database of room temperature self-broadening coefficients for about 1650 $SO_2$ transitions has been compiled by Tasinato et al. [287]. These data are used to update the self-broadening coefficients for the corresponding transitions. The error code has been set to 5, since we estimate the accuracy to be 5–6%. We also use the average values from Ref. [287] (with error code 4) of the same quantum numbers $(J'', K_a'')$ for all the other transitions with corresponding rotational quantum numbers. More details about this update of air and self-broadening parameters can be found in the report by Tan et al. [286].

Another update to the $SO_2$ data is the addition of pressure shifts, line widths and their temperature-dependence exponents in $H_2$, He and $CO_2$ environments for every HITRAN transition. This update is described in detail by Wilzewski et al. [4]. While shifts of the $SO_2$ lines due to $H_2$, He or $CO_2$ were set to the default value of zero for every line because no reports of these parameters exist in available peer-reviewed sources, broadening parameters and their temperature exponents have been added based on Refs. [280,281,284,288,289]. Especially in the case of the $SO_2–CO_2$ system, the amount of available data used to construct the line list of broadening parameters is minimal, and future measurements of the $SO_2–H_2$, $-He$ and $-CO_2$ systems would help to improve the present semi-empirical parameter sets.

There has been significant recent progress in the computation of extensive, accurate theoretical line lists for both room temperature [290,291] and hot [292] $SO_2$. The intensities in these line lists are accurate enough to provide alternatives for missing bands of both the main and the minor isotopologues; this should be considered in future updates. They can be supplemented with line positions calculated using constants from recent works from Ulenikov et al. [293–297] and references therein.
<figure><img src="image_23.png" /><figcaption>**Fig. 19.** Air-broadening line widths of NH₃ with different temperatures for aR(0,0), aR(4,3), aR(7,6) and aR(8,7) transitions. The temperature-dependence exponent *n*<sub>air</sub> is determined from the slope of the least-squares fits of -ln*n*<sub>air</sub><sup>0</sup>(*T*) vs. ln*T*.</figcaption></figure>

2.10. NO₂ (molecule 10)

Unchanged.

2.11. NH₃ (molecule 11)

NH₃ is a very important constituent of the atmospheres of giant planets and small, cold stars. It is also abundant in various environments of the interstellar medium and occurs in the terrestrial atmosphere. The isotopologue with ¹⁵N can be very important for studying isotopic fractionation or for studying the opacity of lines pertaining to ¹⁴NH₃.

Positions and intensities of the ¹⁵NH₃ pure-tunneling and ro-
tation-tunneling transitions were taken from the CDMS. It is based
on rotation-tunneling transition frequencies from Belov et al. [298]
and Winnewisser et al. [299], on pure tunneling data from Kuko-
lich [300], in part corrected by Hougen [301], and from Sasada
[302]. Also included are FIR data from Urban et al. [303] and
Carlotti et al. [304]. Initial spectroscopic parameters were taken
from a modified fit of ¹⁴NH₃ data by Chen et al. [305]. Quantum
chemical calculations and trial fits suggested that the C rotational
constant cannot be determined reliably from these data. Therefore,
its value was kept fixed to the ¹⁴NH₃ value derived from Ref. [306]
modified by the ¹⁴NH₃/¹⁵NH₃ difference calculated from energies
calculated by Huang et al. [306]. The dipole moment and its first
order J and K distortion corrections were taken from Tanaka et al.
[307]. The rotation-tunneling transitions extend to J = 14 [304],
and the pure tunneling transitions [302] even higher in J. The
predictions should show FIR accuracy at least up to J = 14, but

predictions higher than *J* = 15 should be viewed with increasing
caution. Moreover, frequencies and possibly also intensities of the
weak Δ*K* = 3 transitions should be viewed with some caution.

HITRAN2012 only contained ammonia transitions up to
7000 cm⁻¹. Recently, Barton et al. [308] analyzed Fourier Trans-
form (FT) absorption spectra recorded at the National Solar Ob-
servatory by C. De Bergh in 1980, providing partially-assigned lists
of measured line positions and intensities. The present update
includes 8468 lines in the region 7400–8640 cm⁻¹ [308]; 2474 of
these lines are fully assigned using the quantum numbers re-
commended by Down et al. [309]. Barton et al. [310] have also
analyzed and partially assigned a further spectrum covering the
8800 to 10,400 cm⁻¹ region. These data were also included in
HITRAN2016.

At lower wavenumbers, comparisons between the HITRAN data
and comprehensive variational line lists [311,312] give good agree-
ment below 4000 cm⁻¹. However, comparisons with these line lists
suggest that the current HITRAN compilation is missing a significant
number of lines in the 4000–7000 cm⁻¹ region. Attempts to resolve
this problem using a combination of available variational line lists
referred to as BYTe [311,312] and empirical energy levels provided
using the MARVEL procedure [313] to give a hybrid line list referred
to as "BARVEL" proved to be inferior to HITRAN2012 when compared
with cross sections from PNNL [314]. It would appear that further
work is needed on both the computed line list and the MARVEL
dataset in this region. Work in both these directions is currently
being undertaken and, in particular, a new high-accuracy *ab initio*
potential energy surface [315] should provide a much better starting
point for improved nuclear motion calculations.
<table><caption>**Table 8**
Comparison of the HNO₃ line parameters in the 7.6-µm region in the 2012 and 2016 editions of HITRAN.</caption><thead><tr><th colspan="8">(a) HITRAN2012</th></tr><tr><th>Band</th><th>N</th><th>S<sub>tot</sub> (10<sup>-18</sup>)</th><th>ν<sub>min</sub></th><th>ν<sub>max</sub></th><th>S<sub>min</sub> (10<sup>-23</sup>)</th><th>S<sub>max</sub> (10<sup>-21</sup>)</th></tr></thead><tbody><tr><td>ν<sub>3</sub></td><td>21,308</td><td>25.37</td><td>1098.376</td><td>1387.849</td><td>1.037</td><td>31.33</td></tr><tr><td>ν<sub>4</sub></td><td>19,584</td><td>12.78</td><td>1229.867</td><td>1387.561</td><td>1.037</td><td>18.67</td></tr><tr><td>Sum</td><td></td><td>38.15</td><td></td><td></td><td></td><td></td></tr></tbody><thead><tr><th colspan="8">(b) HITRAN2016</th></tr><tr><th>Band</th><th>N</th><th>S<sub>tot</sub> (10<sup>-18</sup>)</th><th>ν<sub>min</sub></th><th>ν<sub>max</sub></th><th>S<sub>min</sub> (10<sup>-25</sup>)</th><th>S<sub>max</sub> (10<sup>-21</sup>)</th></tr></thead><tbody><tr><td>ν<sub>3</sub></td><td>16,408</td><td>24.94</td><td>1252.010</td><td>1394.177</td><td>4.910</td><td>32.0</td></tr><tr><td>ν<sub>4</sub></td><td>18,105</td><td>9.834</td><td>1238.929</td><td>1387.081</td><td>4.020</td><td>10.7</td></tr><tr><td>2ν<sub>6</sub></td><td>2451</td><td>0.1194</td><td>1243.465</td><td>1348.275</td><td>4.624</td><td>3.660</td></tr><tr><td>ν<sub>5</sub> + ν<sub>9</sub></td><td>13,817</td><td>0.7163</td><td>1246.929</td><td>1390.071</td><td>2.081</td><td>3.543</td></tr><tr><td>ν<sub>7</sub> + ν<sub>8</sub></td><td>11,125</td><td>0.7615</td><td>1246.422</td><td>1395.679</td><td>2.314</td><td>5.017</td></tr><tr><td>3ν<sub>9</sub></td><td>13,894</td><td>1.177</td><td>1233.107</td><td>1388.497</td><td>4.582</td><td>2.378</td></tr><tr><td>Sum</td><td></td><td>37.55</td><td></td><td></td><td></td><td></td></tr><tr><td>ν<sub>3</sub> + ν<sub>9</sub> - ν<sub>4</sub></td><td>12,106</td><td>1.408</td><td>1271.050</td><td>1394.899</td><td>5.285</td><td>1.798</td></tr></tbody></table>

Note: N is the number of lines, $ν_{min}$ and $ν_{max}$ (cm$^{-1}$) are the lower and upper ranges of the band, $S_{min}$ and $S_{max}$ are the smallest and largest line intensity (cm$^{-1}$/molecule cm$^{-2}$) at 296 K) and $S_{tot}$ is the sum of the line intensities.

Pressure-broadening coefficients and their temperature dependence exponent were calculated from the polynomial coefficients of Nemtchinov et al. [316] derived from the $ν_2$ band measurements. Nemtchinov et al. suggested that the rotational dependence of the air- and self-broadening half width, $γ_{air}$ and $γ_{self}$, can be represented using the following polynomial:

$$γ^0(m, K) = β_0 + β_1 m + β_2 K + β_3 m^2 + β_4 K^2 + β_5 mK \quad (2)$$

where $m = |J - J, J + 1|$ for the P, Q and R branches, and $β_i$ are the polynomial coefficients. This polynomial was used only for $J \le 8$ with error code 6 for both $γ_{air}$ and $γ_{self}$, and for the other transitions with, $J > 8$ the extrapolated value corresponding to the largest $J$ and $K$ was used. For instance, when $J > 8$ the default values $γ_{air} = 0.0906 \text{ cm}^{-1}\text{/atm}$ and $γ_{self} = 0.5 \text{ cm}^{-1}\text{/atm}$ have been used for all these transitions, and the error code has been set to 5 for both $γ_{air}$ and $γ_{self}$.

The temperature dependence exponents have also been measured in the $ν_2$ band by Nemtchinov et al. [316] but for broadening by O₂ and N₂ separately. We note that in the literature many researchers determine the temperature exponent to be

$$n_{\text{air}} = 0.79n_{\text{N2}} + 0.21n_{\text{O2}} \quad (3)$$

by analogy with the standard approximation

$$γ_{\text{air}} = 0.79γ_{\text{N2}} + 0.21γ_{\text{O2}} \quad (4)$$

We note however that Eq. (3) and Eq. (4) contradict each other because the variation of the air-broadening coefficients with temperature T is given by:

$$\frac{γ_{\text{air}}(T)}{γ_{\text{air}}(T_{\text{ref}})} = \left(\frac{T_{\text{ref}}}{T}\right)^{n_{\text{air}}} \quad (5)$$

Here $T_{\text{ref}}$ is the reference temperature. Then we have:

$$n_{\text{air}} = - \frac{\ln γ_{\text{air}}(T_{\text{ref}}) - \ln γ_{\text{air}}(T)}{\ln T_{\text{ref}} - \ln T} \quad (6)$$

where $γ_{air}$ can be defined using Eq. (4). Therefore the measured half widths induced by O₂ and N₂ at 200, 255 and 296 K can be determined from the values of $n_{air}$ as the slope of the least-squares fits of $-\ln γ_{air}^0(T)$ vs $\ln T$, as shown in Fig. 19.

The broadening algorithm described above as well as the

<figure><img src="image_24.png" /><figcaption>**Fig. 20.** Overview of the Fourier transform spectrum recorded at high resolution in Giessen University in 2004 [335] and comparisons with the models performed using the HITRAN2012 and HITRAN2016 editions.</figcaption></figure>

temperature dependence procedure is also used throughout the entire database for both isotopologues ¹⁴NH₃ and ¹⁵NH₃.

Semi-empirical line widths, pressure shifts and temperature-dependence exponents of ammonia perturbed by H₂, He and CO₂ have been added to the database for the first time based on the algorithm described in Wilzewski et al. [4]. This algorithm makes use of the results from Refs. [316–320] (for H₂), Refs. [317,319–325] (for He) and Refs. [321,326,327] (for CO₂). Even though the new parameters were constructed from a relatively large number of data-reporting articles, the database would profit from future studies of NH₃ absorption lines perturbed by H₂, He and CO₂ for validation purposes as described in Ref. [4].

## 2.12. HNO₃ (molecule 12)

For nitric acid (HNO₃), the three strongest absorption band systems, namely the {ν₅, 2ν₉}, {ν₃, ν₄} interacting bands, and the $ν_2$ band, are located at 11, 7.6 and 5.8 µm respectively. Although two times weaker than the bands observed in the two other ranges, the 11-µm band system is of particular atmospheric interest since it coincides with a rather clear atmospheric window. As the HITRAN2012 database [1] provides a reliable description of the nitric
acid spectrum at 11 µm (the line list includes the $v_5$ and $2v_9$ cold bands for the H$^{14}$N$^{16}$O$_3$ and H$^{15}$N$^{16}$O$_3$ isotopologues, as well as the $v_5 + v_9 - v_9$, $3v_9 - v_9$, $v_5 + v_7 - v_7$ and $v_5 + v_6 - v_6$ hot bands of H$^{14}$N$^{16}$O$_3$), this region is used for HNO$_3$ retrievals by numerous satellite or balloon-borne instruments [328–333].

However, retrievals of nitric acid at altitudes higher than ~35 km, where it is less abundant, require use of the stronger infrared signatures at 5.8 or 7.6 µm. Because the first spectral region overlaps rather strongly with water vapor absorption in atmospheric spectra, the 7.6-µm band system is preferable. However, most satellite or balloon-borne instruments avoided using this latter region up to now for HNO$_3$ retrievals because of the poor quality of the available HNO$_3$ line positions and line intensities.

For HNO$_3$, the 7.6-µm spectral region in HITRAN2012 is limited to the $v_3$ and $v_4$ cold bands of the main isotopologue, H$^{14}$N$^{16}$O$_3$. In addition, the quality of the line positions and intensities available therein is rather poor, mainly because the theoretical model used at that time [334] accounted only for the interactions coupling energy levels belonging to the $v_3$ and $v_4$ bright states but was neglecting interactions with several dark states present in the same energy range.

Therefore it was decided to update the HNO$_3$ line list at 7.6 µm. First, a complete reinvestigation of the line positions for nitric acid at 7.6 µm was performed [335]. In contrast to the previous analysis [334], the new Hamiltonian model accounted properly for the various vibration-rotation resonances and torsional effects affecting the $v_3$ and $v_4$ bright states and the four dark states $2v_6$, $3v_9$, $v_5 + v_9$ and $v_7 + v_8$. Additionally, the $v_3 + v_9 - v_9$ hot band was identified for the first time [335].

At the end of this first study [335], a new list of line positions and of preliminary line intensities was generated. This list included contribution from the $v_3$ and $v_4$ cold bands, the $2v_6$, $3v_9$, $v_5 + v_9$, $v_7 + v_8$ dark bands, and the $v_3 + v_9 - v_9$ hot band of the main isotopologue. The final line intensities at 7.6 µm were generated through a calibration process during which the HNO$_3$ vol mixing ratio profiles retrieved from the "Michelson Interferometer for Passive Atmospheric Sounding" (MIPAS) limb emission radiances in the 11- and 7.6-µm domains were compared. This line list at 7.6 µm, which was also validated using the available laboratory information (individual line intensities and absorption cross sections), proved to provide an improved description of the 7.6-µm region absorption of nitric acid [336].

Table 8 compares the HNO$_3$ line lists at 7.6 µm given in the 2012 and 2016 releases of HITRAN showing significant changes. Finally, Fig. 20 compares the observed spectrum in the 7.6-µm spectral region (laboratory spectrum recorded in Giessen) to synthetic spectra generated using HITRAN2012 and HITRAN2016 showing the improvements brought by the new data as shown by the residuals. In conclusion, it is expected that this study should help to improve HNO$_3$ satellite retrievals by allowing measurements to be performed simultaneously in the 11- and 7.6-µm spectral regions.

In the HITRAN2008 paper, the several-item roadmap for the improvements to the nitric acid spectroscopy was presented. A majority of these problems have now been solved. However, the biggest challenge still remains, the lack of a line-by-line list for the $v_1$ band of HNO$_3$ centered at 3551 cm$^{-1}$. It is a pronounced feature that interferes with other gases absorbing at these wavenumbers. Fig. 5 in the paper by Toon et al. [147] shows how the lack of spectroscopic parameters negatively affects retrievals from the Mk IV balloon observations. Low-resolution spectra of this band exist in the PNNL database [314] but at a limited set of thermodynamic conditions, while ab initio calculations for this molecule [337,338] do not yet allow accurate atmospheric retrievals.

### 2.13. OH (molecule 13)

The line list for the hydroxyl radical remained unchanged. The recent study by Brooke et al. [339] could be considered for updating intensities of the weak lines (this is more relevant for HITEMP [278]) in the near future.

### 2.14. HF (molecule 14)

Transitions of HF with changes in vibrational quantum number, $\Delta v$, larger than 10 were removed from the database along with transitions with $\Delta v > 8$ for DF. This removal was carried out because there were numerical issues in calculating the intensities of these transitions. Medvedev et al. [340] explain how the use of double precision in calculating overtone intensities may lead to numerical errors and that quadruple precision is needed. Additional numerical errors occur depending on the interpolation techniques used for the dipole moment functions as explained in Medvedev et al. [341].

Broadening, shifting and temperature-dependence parameters of HF due to the pressure of H$_2$, He and CO$_2$ have been added to HITRAN for the first time. The semi-empirical procedure described in Wilzewski et al. [4] was developed using results from Ref. [342] (for H$_2$), Refs. [343–345] (for CO$_2$) and Refs. [346–348] (for He). To expand and validate the present parameters, more measurements are needed, while it would also be valuable to have more dedicated data for DF.

### 2.15. HCl (molecule 15)

The intensity issue mentioned above for HF concerning the removal of high overtones of all isotopologues was likewise applied to HCl (i.e. transitions with $\Delta v > 8$ were removed for all four isotopologues of hydrogen chloride).

Ref. [4] describes in detail the addition of H$_2$-, He- and CO$_2$-broadening, -shifting and temperature-dependence parameters of HCl to the database. Semi-empirical HCl-CO$_2$ broadening and shifting parameters for each HITRAN transition are based on Refs. [345,349] and sophisticated extrapolations of these results, while Refs. [350,351] were used to construct HCl-He linewidths and shifts. For the HCl-H$_2$ system, the new parameters are based on Refs. [352–355]. In the future, measurements of the temperature-dependence of foreign-broadened HCl spectral lines will be very helpful additions to the database.

### 2.16. HBr (molecule 16)

Unchanged.

### 2.17. HI (molecule 17)

Unchanged.

### 2.18. CIO (molecule 18)

Unchanged.

### 2.19. OCS (molecule 19)

Carbonyl sulfide is the most abundant sulfur-containing species in the free troposphere, often monitored using optical remote sensing in the 5-µm region [356]. OCS is present in the atmosphere of Venus [357]. The increasing interest for the application of very sensitive cavity ring down spectroscopy to the in situ detection of trace species [358] makes the near infrared range more and more attractive. However, the HITRAN2012 database provided spectroscopic
information for OCS only up to 4200 cm⁻¹ [1]. Recently, Fourier transform spectra of carbonyl sulfide were recorded between 6170 and 6680 cm⁻¹ and between 7700 and 8160 cm⁻¹ using a femto/OPO laser absorption source and cavity enhanced spectroscopy [359]. The strongest bands observed in these two regions are 3ν₁+2ν₃ (near 6640 cm⁻¹) and 2ν₁+3ν₃ (near 7812 cm⁻¹), respectively. The line positions thus measured (with an accuracy estimated to be 0.001 cm⁻¹ for not-too-weak unblended lines) were used to update a global polyad model of carbonyl sulfide (see Ref. [360] and references therein).

This global polyad model was then used to generate a list of line positions, assignments, lower-state energies, and relative intensities for the main isotopologue. Because quantitative intensity information could not be retrieved from the measured spectra, the predicted relative intensities had to be scaled relying on additional information. A scaling factor was determined for each of the two spectral regions by matching calculated spectra of the 3ν₁+2ν₃ and 2ν₁+3ν₃ bands to their Fourier transform spectra, recorded previously [360]. Because predicted relative intensities only make sense for transitions involving levels belonging to the same polyads, these two scaling factors could only be applied to bands corresponding to transitions involving upper vibrational levels belonging to the same polyads as the upper levels of the 3ν₁+2ν₃ and 2ν₁+3ν₃ bands. Line intensities could therefore be obtained for cold bands matching this constraint, and hot bands involving variations of the vibrational quantum numbers identical to these cold bands (thus assuming that the vibrational dependence of the transition moments is negligible). Because isotopic substitution is not expected to significantly change the transition moments, line intensities were also obtained for the same bands belonging to ¹⁶O¹²C³⁴S, ¹⁶O¹³C³²S and ¹⁶O¹²C³³S. All these predicted bands cover the ranges 6484–6650 cm⁻¹ (region A) and 7728–7821 cm⁻¹ (region B). These two regions, involving a total of 3660 predicted lines, are presented in Figs. 13 and 14 of Ref. [359]. Note that only lines having intensities larger than 3.0 × 10⁻²⁸ (region A) and 6.7 × 10⁻²⁸ (region B) cm⁻¹/(molecule cm⁻²) at 296 K were considered. These intensity thresholds are slightly lower than the experimental limits. The supplementary material of Ref. [359] also includes a list of line positions, assignments, and lower-state energies for 3417 additional lines of ¹⁶O¹²C³²S, ¹⁶O¹²C³⁴S, ¹⁶O¹³C³²S and ¹⁶O¹²C³³S, covering the 6200–6449 cm⁻¹ and 7901–8120 cm⁻¹ ranges. Because of the constraint mentioned above, the relative intensities of these lines could not be scaled and were set to zero, and they were not included in HITRAN2016.

The spectroscopic information included in the new edition of HITRAN is the data of regions A and B obtained as described above, to which 10 lines belonging to the 2ν₁+2ν₂°+2ν₃ band of the ¹⁶O¹³C³²S isotopologue (between 6650 and 6660 cm⁻¹) were added. The origin of this band is located near 6650.773 cm⁻¹. Relying on predictions of the global polyad model, which turned out to be as close as a few 10⁻³ cm⁻¹ to the observed positions, 17 P and R branch lines of the 2ν₁+2ν₂°+2ν₃ band of ¹⁶O¹³C³²S could be observed, their positions measured in the spectra of Ref. [359] and included in the global modeling to yield improved predictions. These improved predictions are included in this edition of HITRAN. Note that the upper level of this band is the highest ever observed for ¹⁶O¹³C³²S. In the supplementary material of Ref. [359], 152 pairs of upper vibrational levels have identical assignments as a result of the combined effects of heavy mixing near avoided crossings and the assignments of the levels being based on the largest coefficient of their eigenvectors. To remove these duplicate assignments, one vibrational level in each pair was manually re-assigned through careful examination of the corresponding energy level structures. Additionally, it was found that the line intensities provided in the supplementary material of Ref. [359] could be affected by the number of significant digits used when storing

intermediate results. The line intensities provided in this edition of HITRAN correct this problem and are listed with the appropriate number of significant digits. The HITRAN line list for OCS contains self- and air-broadening coefficients for all the lines calculated using the Padé approximant proposed of Koshelev and Tretyakov [361].

H₂-, He- and CO₂-broadened linewidths of OCS were added to HITRAN for the first time based on semi-empirical models derived in Wilzewski et al. [4]. The following experimental data were used to derive these models: Ref. [362] (for H₂), Refs. [362–365] (for He) and Ref. [366] (for CO₂). There are no lineshifts and temperature-dependence parameters due to H₂, He or CO₂ available in the literature.

## 2.20. H₂CO (molecule 20)

Unchanged.

## 2.21. HOCl (molecule 21)

The ν₂ band region of hypochlorous acid, HOCl, was first introduced in HITRAN2004 [3] based on the line list from Vander Auwera et al. [367]. It was found that 49 lines of the principal isotopologue of HOCl in HITRAN had an incorrect vibrational assignment being attributed to the ν₂ band, while in reality they are 2ν₃ lines. Also, the quantum rotational assignment of the line at 1237.62928 cm⁻¹ was incorrect. All of the assignments are now fixed, based on the correct information provided in the original publication [367].

## 2.22. N₂ (molecule 22)

It was found that the abundance of the ¹⁴N¹⁵N isotopologue was incorrectly calculated in the original release of HITRAN2012; this issue with the intensities has been fixed in the latest edition.

## 2.23. HCN (molecule 23)

Hydrogen cyanide is a trace atmospheric species that can result from biomass burning [368–370]; its presence in the troposphere is being routinely monitored [371,372]. The use of HCN spectra for diagnostic applications in medicine are also being investigated [373]. HCN, and its isomer HNC (which is not yet in HITRAN), are important astronomical species with, for example, the recent tentative detection of HCN in the super-Earth exoplanet 55 Cancri e [374].

The spectrum of ¹H¹²C¹⁴N at wavenumbers below 3500 cm⁻¹ has been included in HITRAN for some time. Here we extend the data available to higher wavenumbers. These data were based on the semi-empirical line list generated by Barber et al. [375] as part of the ExoMol project [376]. For full line lists appropriate for higher temperatures and covering both HCN and HNC the reader should consult these sources directly.

HITRAN has been updated with ¹H¹²C¹⁴N lines for wave-numbers above 3500 cm⁻¹ and limited to transitions which have intensity greater than 10⁻²⁹ cm⁻¹/(molecule·cm⁻²); these criteria give a total of 55,153 lines in that spectral region. Wave-numbers for the majority of these lines were taken from the empirical energy levels of Mellau [377,378]. However, transition intensities are ones computed by Harris et al. [379] using the *ab initio* dipole moment and potential energy surface of Van Mourik et al. [380]. The Harris et al. calculations are not sufficiently accurate for transition frequencies but the evidence is that the transition intensities can be considered fairly reliable [381]. An overview of the HCN line list in HITRAN2016 is given in Fig. 21.

The air-broadening half widths were calculated using semi-
<figure><img src="image_25.png" /><figcaption>Fig. 21. Overview of the transitions of HCN in HITRAN2016 with new bands revealed.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Fig. 22. Comparison of the SAITY line list [424] with the phosphine data from HITRAN2012 at room temperature; also shown are the recent results of Malathy Devi et al. [426].</figcaption></figure>

empirical expressions derived in Yang et al. [382]. Self-broadening parameters were calculated using the procedure explained in the HITRAN2004 paper [3], see Eq. (20) there and discussion around it.

## 2.24. CH<sub>3</sub>Cl (molecule 24)

The parameters of methyl chloride for 43,147 lines (7079 of which are new), were calculated based on a recent analysis [383] of Fourier transform spectra in the range of 1900–2600 cm<sup>-1</sup>. The data from the supplementary file of Ref. [383] were used in the update. The error code 4 was given for line positions and the error code 5 (5–10%)– for line intensities, see Ref. [383] for details on uncertainties. We also recalculated the Einstein A coefficients for

the lines which had changed intensities. In addition, a couple of issues that have been identified in the 3-µm region of CH<sub>3</sub>Cl in HITRAN2012 were fixed. In particular, the air-broadening temperature dependencies, n<sub>air</sub>, of the lines around 3 µm used to be negative while they are supposed to be positive. And all of the lines in the 3-µm band have a shift of –0.02 cm<sup>-1</sup>/atm. We compared the calculated spectra from the HITRAN database with the PNNL database [314] and the results show that such a strong shift of the lines appears to be incorrect. Therefore the shifts were set to be zero and n<sub>air</sub> to be positive around 3 µm. In a similar fashion we removed the shift of –0.02 cm<sup>-1</sup>/atm in the ν<sub>2</sub> + ν<sub>6</sub> band around 5 µm.

Both experimental measurements and calculations have been
made of the self- and air- broadening coefficients of CH₃Cl recently, allowing for a relatively sophisticated update of these parameters in HITRAN. The available results have been collected and evaluated to update the database for CH₃Cl. The details are as follows:

For self-broadening coefficients of CH₃Cl:

1. The CH₃³⁵Cl self-broadening coefficients at reference temperature 296 K for the R-branch (ΔK=0) were computed using a semi-empirical (SE) method described by Dudaryonok et al. [384]. We used these semi-empirical calculations for R-branch and P-branch (for all ΔK values) lines in all of the bands of the two isotopologues CH₃³⁵Cl and CH₃³⁷Cl considered in the database. The average value of RMS-deviations of the SE CH₃Cl self-broadening coefficients from the experimental data of Ref. [385] is 0.0408 cm⁻¹/atm. We used the ratio of the individual half widths to this value to estimate the uncertainties and determine a corresponding error code.

Only R-branch transitions for quantum number ranges of 0 ≤ *J* ≤ 70, 0 ≤ *K* ≤ 20 are available from Ref. [1]. The empirical relation enables evaluation of the P-branch and Q-branch broadening coefficients from those of the R-branch

$$γ_P(J, K) \approx γ_R(J - 1, K) \approx γ_Q(J, K) \qquad (7)$$

2. Experimental results collected from the literature were applied to corresponding transitions with the same *J* and *K*. These experimental values supersede the calculated ones:

(a) Experimental results from Ref [386]. are used for the same isotopologue, (J',K'-J'',K'') transitions in all of the bands, with the error code set to 5 (5–10%).

(b) Experimental results from Ref [385]. are for both pure rotational (J = 6, 31, 37, 40, 45, 50) and ν₁ bands of CH₃Cl. The measured self-broadening coefficients for pure rotational transitions are used for the corresponding transitions in the database. Experimental results for the ν₁ band are used for transitions sharing the same quantum numbers throughout other bands.

For air-broadening coefficients of CH₃Cl:

1. The air-broadening coefficients and associated temperature exponents for both CH₃³⁵Cl and CH₃³⁷Cl ro-vibrational transitions of R-branches (ΔK = 0) are calculated by a semi-classical (SC) approach [387]. The SC calculation are used for R-branch and P-branch (for all ΔK values) lines in all of the bands assuming γP(J,K) ≈ γR(J - 1,K).

2. Experimental results from Ref. [388] and Ref. [389] are used for the transitions sharing the same rotational quantum numbers.

For air-broadening temperature dependencies of CH₃Cl:

The air-broadening temperature exponents have been updated using the SC calculations of Buldyreva [387]. Only the ro-vibrational transitions of R-branches and P-branches were calculated. The values for Q-branches were kept at the constant value of 0.70 for nP(J,K) ≈ nR(J - 1,K).

## 2.25. H₂O₂ (molecule 25)

For hydrogen peroxide, H₂O₂, the measurements of the air-broadening half widths from Refs. [390–392] were included for the corresponding transitions. The majority of the air-broadened coefficients in the database still have the same value of 0.1 cm⁻¹/atm estimated from the measurements of Malathy Devi et al. [392].

Currently the H₂O₂ line list contained in HITRAN only covers

wavelengths longer than 5 µm. PNNL [314] and recent ab initio calculations [393,394] show strong features near 2.75 µm associated with the OH stretch. More laboratory work is needed to obtain reliable line lists in that region.

## 2.26. C₂H₂ (molecule 26)

New bands of acetylene from the wavenumber regions 13 – 248 cm⁻¹ and 390 – 634 cm⁻¹ were added using the results of Refs. [395–397]. The new data incorporated into HITRAN2016 are based mainly on the global modeling of both line positions and intensities performed in Tomsk within the framework of the method of effective operators [395]. The new measurements performed by Jacquemart et al. [397] considerably extend the available experimental data in the 390–634 cm⁻¹ region and allow an increase in the number of the fitted effective dipole moment (EDM) parameters responsible for the line intensities in the 13–634 cm⁻¹ region. This extended set of EDM parameters has better extrapolation abilities compared to that published in Ref. [395]. Using the eigenfunctions of the effective Hamiltonian from Ref. [395] and the new set of EDM parameters, a new list of line positions and intensities was generated. This list was used to update the C₂H₂ line list in the 13 – 248 cm⁻¹ and 390 – 634 cm⁻¹ regions.

Global modeling of the rovibrational energies of the bending vibrational states has also been performed in the Jet Propulsion Laboratory (JPL) [396] using an EH for the bending modes. The comparisons to the observed line positions showed that the JPL calculated line positions are slightly closer to the observations for the high values of the angular momentum quantum numbers than those derived from Ref. [395]. Thus it was decided to use JPL line positions (when available) for these bands.

The line list has been completed by other parameters (self- and air-broadening coefficients, temperature exponents of air-broadening coefficients, air-shift coefficients) as described in Section 4 of Jacquemart et al. [398].

Uncertainty codes 4 (0.0001 to 0.001 cm⁻¹) for the line positions and 6 (2–5%) for the line intensities were used in the case of the observed bands. For predicted bands, an uncertainty code 3 (0.001 to 0.01 cm⁻¹) for the line positions and 4 (10–20%) for the line intensities were adopted. For the JPL line positions, the uncertainty code 5 (0.00001 to 0.0001 cm⁻¹) was used.

The labeling V₁ V₂ V₃ V₄ ℓ₄ ℓ₅ J ε of the ro-vibrational states has been used in the list; see Jacquemart et al. [397] for details.

In addition, it was discovered that the Q-branch in the ν₂ + 2ν₄ + ν₅ band near 2.58 µm was missing from the database. It is now provided based on the model of Lyulin and Perevalov [395].

In our effort to expand the database to spectral line parameters relevant to the study of planetary atmospheres, we have compiled semi-empirical parameter sets of C₂H₂ broadened and shifted by the pressure of H₂, He and CO₂ together with the corresponding temperature dependence of the linewidth. This update was described in detail in Wilzewski et al. [4]. These new parameters were constructed from Refs. [257,399–402] for H₂, Refs. [403,404] for He and Refs. [405,406] for CO₂.

Further studies of these line-shape parameters are important to improve our current update. This is especially true for the C₂H₂–CO₂ system, where almost no shift parameters and no temperature-dependence values have been reported in the literature to this point.

We note a recent study by Lyulin and Campargue [407] where empirical line lists for some of the NIR bands of acetylene are provided. These will be considered for future updates of HITRAN.
<figure><img src="image_27.png" /><figcaption>**Fig. 23.** Comparison of experimental PNNL absorbance cross-sections [314] of PH₃ in the dyad and pentad ranges (in black) with variational calculations Rey et al. [205] (upside down in red).</figcaption></figure>

## 2.27. C₂H₆ (molecule 27)

Ethane (C₂H₆) is the second most abundant hydrocarbon after CH₄ in the stratospheres of the outer planets (see for instance Ref. [408]) and Titan [409]. It therefore plays an important role as a tracer of atmospheric chemistry and dynamics. C₂H₆ is a direct product of CH₄ photochemistry and is a key constituent in photochemical modeling. In addition to the v₉ fundamental of ethane, which is the strongest band seen in Titan observed in the 10-µm terrestrial window, the weak v₄ torsional band at ~35 µm (289 cm⁻¹) has been detected on Titan by Cassini/CIRS [410]. Although the torsional band is not infrared active in the lowest order and hence very weak in a laboratory setting, the features near 35 µm are seen relatively strong in the Cassini/CIRS spectrum of Titan. This new development in the far-infrared enables astronomers to probe the lower altitudes of Titan's atmosphere.

### 2.27.1. $^{12}$C₂H₆

Since the last update in HITRAN2012 [1] for $^{12}$C₂H₆ at 12 µm reported by Devi et al. [411,412] and at 7 µm reported by di Lauro et al. [413], significant progress has been made for ethane in the far- and mid-infrared regions. Moazzen-Ahmadi et al. [414] studied the features in the 35-µm region both in laboratory measurements and theoretical modeling, which included the torsional fundamental v₄, and the first torsional hot band, 2v₄-v₄, of $^{12}$C₂H₆ and the Q-branch of the $^{13}$CH₃$^{12}$CH₃ v₆ band. Multiple sets of pure ethane spectra were obtained at cold temperatures down to 166 K at the Jet Propulsion Laboratory using a multipass absorption cell with 52-m path length. Starting with the earlier global analyses of ethane considering inter-vibrational interactions among the torsional levels of v₃, v₉, 3v₄ bands [415], the observed spectra were fit by adjusting independent dipole constants and the same value for self-broadening parameter for every transition. The observed spectra were reproduced to the residuals of 1% (the reader is referred to Fig. 1 in Ref. [415]).

It was found that the modeling of the transition intensities required an expansion of the dipole moment operator to higher order; this introduced Herman-Wallis like terms. The line

positions and line intensities of the torsional band (v₄) of $^{12}$C₂H₆ at 35 µm reported by Moazzen-Ahmadi et al. [414] have been adopted in this update. The uncertainties for line positions are set to no better than 0.0003 cm⁻¹ (HITRAN code = 4), while those of the line intensities are conservatively set to be 5 to 10% (HITRAN code = 5). For the pressure-broadened widths of the torsional band (v₄) at 35 µm, it should be noted that self- and N₂-broadened widths are listed in this version, for which we adopted the values calculated by using the linear expressions for v₉ transitions reported by Devi et al. [411,412], similar to what was done for C₂H₆ in the 7-µm region in HITRAN2008 [198]. The uncertainties for widths are conservatively set to 10-20% (HITRAN code = 4). The uncertainties for the temperature dependence exponents of the widths are thought to be 10-25% (HITRAN code = 3).

In the near future two minor isotopologues of ethane could be updated in the 12 µm region; one is for the $^{13}$C singly-enriched v₁₂ band of ethane ($^{13}$CH₃$^{12}$CH₃) at 12.2 µm measured by Devi et al. [416] and the other for the singly-deuterated ethane (C₂H₅D) at 12.5 µm studied by Daly et al. [417]. In particular, new studies were reported for $^{13}$CH₃$^{12}$CH₃ in the v₁₂ band in the 12 µm region by Devi et al. [416] and in the 7 µm region by di Lauro et al. [418] for the v₄, v₉, v₁₀ and v₆ + v₁₁ bands between 1345 and 1557 cm⁻¹. Their intensities were reported at 130 K with partially completed theoretical modeling.

## 2.28. PH₃ (molecule 28)

Phosphine (PH₃) is a molecule of interest in astronomy, and its features have long been observed and identified in the spectra of Jupiter and Saturn (e.g. Refs. [419–422]). Because of its great importance in the Earth's and other planetary atmospheres including the interstellar medium, PH₃ has been the subject of a large number of experimental and theoretical investigations. Far infrared (FIR) line parameters of PH₃ were included for the first time in the HITRAN2012 database [1]. No updates have been made for any of the spectral line parameters for the infrared (IR) bands of phosphine since the HITRAN2008 database [198]. After the release of the HITRAN2012 database, there have been a few experimental