<figure><img src="image_21.png" /><figcaption>**Fig. 19.** Overview of the <sup>14</sup>N<sup>16</sup>O (top), <sup>15</sup>N<sup>16</sup>O (middle) and <sup>14</sup>N<sup>18</sup>O (bottom) line lists in HITRAN2020, when compared to HITRAN2016 (indicated by black points).</figcaption></figure>

(where $\gamma_{air} = 0.79\gamma_{N_2} + 0.21\gamma_{O_2}$) is calculated from rotationally-dependant $N_2$ and $O_2$ half-widths provided by separate Padé functions (Eq. (1)) for the $\Omega = 1/2$ and $3/2$ components. The HITEMP update includes transitions with maximum $J = 184.5$, so care was taken to ensure good performance at high-J. The self-broadening half-widths were also revised as part of the HITRAN and HITEMP updates and were fit to the same functional form. The coefficients of Eq. (1) for $N_2$, $O_2$- and self-broadening of the $\Omega = 1/2$ and $3/2$ components of NO are given in Table 3 of Ref. [55].

In addition, the pressure-induced line shifts for air ($\delta_{air}$) that are measured for the fundamental vibrational band [345] have been extended to additional vibrational bands (up to $\Delta\nu = 6$) using the method outlined by Hartmann and Boulet [346]. This same method was previously applied to CO [257] for HITRAN2016.

The HITRAN2020 update for NO also includes the NOname line lists of <sup>15</sup>N<sup>16</sup>O and <sup>14</sup>N<sup>18</sup>O, with the same intensity thresholds as <sup>14</sup>N<sup>16</sup>O applied. The extended coverage when compared to HITRAN2016 is also shown in Fig. 19. Isotopologue analysis of human breath [347] highlighted issues for the intensities of <sup>15</sup>N<sup>16</sup>O in HITRAN2016. The present update contains intensities from NOname for each isotopologue and is therefore expected to address this issue. The NOname line lists for the minor isotopologues <sup>14</sup>N<sup>17</sup>O, <sup>15</sup>N<sup>18</sup>O and <sup>15</sup>N<sup>17</sup>O (with abundances of $3.7 \times 10^{-4}$, $7.3 \times 10^{-6}$ and $1.4 \times 10^{-6}$, respectively) have not been added to HITRAN at this time as they are not expected to contribute significantly to terrestrial atmospheric spectra.

NO also absorbs strongly in the near ultraviolet. In particular, in the 200–230 nm (44,000–50,000 cm<sup>-1</sup>) region its spectrum overlaps those of ammonia, $O_2$ and $O_3$. Qu et al. have recently developed a spectroscopic model which covers the $\beta$, $\gamma$ and $\delta$ bands of NO [348] and associated line list [349] which will be considered for future inclusion in the database.

## 2.9. SO₂: sulfur dioxide (molecule 9)

Sulfur dioxide is an abundant pollutant in the terrestrial atmosphere, being produced by natural (such as volcanoes [350]) and anthropogenic (including coal burning [351]) sources. Characterization of SO₂ is necessary for atmospheric chemistry and climate models, due to its large effect on sulfate aerosols [350]. It is present on Venus, contributing to the Venusian sulfur cycle [352], and SO₂ is also being actively searched for in the atmosphere of Mars [353,354] as it is believed to play an important role in the atmospheres of rocky planets, including exoplanets

[355]. The recently selected NASA DaVinci+ mission to Venus will be equipped with the Tunable Laser Spectrometer (TLS), which will be specifically targeting transitions of different isotopologues of SO₂ to obtain information on sulfur isotope ratios [356]. It is therefore important to expand the amount of the isotopologues of this molecule in HITRAN.

### 2.9.1. Description of new line lists

Recent developments in SO₂ spectroscopy have laid the groundwork for a major update and expansion of the corresponding line list in HITRAN. In particular, semi-empirical line lists from Huang et al. [357], and Naumenko et al. [358], have been identified as major sources for the potential improvement. Both line lists actively employed ab initio intensities (sometimes adjusted with empirical values) and systematically verified energy levels available from MARVEL analyses. In the MARVEL work on the SO₂ molecule [58,59], three sulfur isotopologues, <sup>32</sup>S<sup>16</sup>O₂, <sup>33</sup>S<sup>16</sup>O₂, and <sup>34</sup>S<sup>16</sup>O₂, were analyzed, resulting in 15,130, 5852, and 10,893 validated empirical ro-vibrational energy levels, respectively [359]. The <sup>32</sup>S<sup>16</sup>O₂, <sup>33</sup>S<sup>16</sup>O₂, and <sup>34</sup>S<sup>16</sup>O₂ MARVEL datasets, collated from the literature, contain 40,269, 15,628, 31,080 ro-vibrational lines, respectively [359]. Both Huang et al. [357], (hereafter referred to as Ames) and Naumenko et al. [358] (hereafter referred to as Naumenko) line lists and their evaluations against available laboratory spectra are briefly described below.

From 2014 to 2016, NASA Ames-296K ab initio IR line lists were published for <sup>32</sup>S<sup>16</sup>O₂, <sup>33</sup>S<sup>16</sup>O₂, <sup>34</sup>S<sup>16</sup>O₂ [360,361], and <sup>16</sup>O<sup>32</sup>S<sup>18</sup>O [362]. They were computed on a PES empirically refined with selected lines taken from HITRAN2012 with line position uncertainty codes ≥4 (see Table 2) [15] (unchanged in HITRAN2016 [16]), and an original ab initio dipole moment surface determined at the CCSD(T)/aug-cc-pV(Q+d)Z level of theory. Compared to the bands in HITRAN2012 [15], the agreement for intensities was usually better than 90-95% [360,361]. The consistency of the Ames-296K line lists has been systematically investigated to explore the possibility of further improvements [363,364]. Using the same potential and dipole surfaces, Underwood et al. [365] reported a hot <sup>32</sup>S<sup>16</sup>O₂ line list containing 1.3 billion lines. With more complete calculations at higher $J$ values and extending to a higher energy range, the list was expected to provide the most complete coverage for 0-8000 cm<sup>-1</sup> and temperatures up to 2000 K.

Although line position predictions from the ab initio works described above have matched recent experiments with $\sigma_{RMS} = 0.01–0.02$ cm<sup>-1</sup>, this accuracy is not sufficient for atmospheric applications. In order to improve the line positions, experimental line positions and Effective Hamiltonian (EH) models based upon ro-vibrational energy levels published between 2009 and 2017 were collected and analyzed for <sup>32</sup>S<sup>16</sup>O₂ [366–373], <sup>33</sup>S<sup>16</sup>O₂ [374,375], <sup>34</sup>S<sup>16</sup>O₂ [376–379], and <sup>16</sup>O<sup>32</sup>S<sup>18</sup>O [380–384]. With $J \le 75$ (the limit of the Ames-296K line lists) and $S_{296K} \ge 10^{-26}$ cm/molecule, 26,464, 25,089, 20,820 lines can be matched for <sup>32</sup>S<sup>16</sup>O₂, <sup>34</sup>S<sup>16</sup>O₂, and <sup>16</sup>O<sup>32</sup>S<sup>18</sup>O, respectively. These “New Line Sets” and other <sup>32</sup>S<sup>16</sup>O₂ and <sup>34</sup>S<sup>16</sup>O₂ data from HITRAN2016 [16] were updated with reliable ground state EH models [371,384,385]. A complete list of observed and calculated energy levels of <sup>33</sup>S<sup>16</sup>O₂ [386] was incorporated at this stage. The resulting energy levels and other published EH model based levels were taken as the “corrected” reference energy set. Transitions were extracted from the Ames-296K line lists if their lower and upper state energy levels could be matched and replaced by the reference set values. Those extracted lines formed the “Extended Line Sets”. These contain 195,425/162,403/242,889/123,441 lines for <sup>32</sup>S<sup>16</sup>O₂/<sup>34</sup>S<sup>16</sup>O₂/<sup>16</sup>O<sup>32</sup>S<sup>18</sup>O/<sup>33</sup>S<sup>16</sup>O₂, in the range of 0–4151/3465/2974/2625 cm<sup>-1</sup>, with maximum $K''_a = 42/39/32/35$ for 16/13/11/6 vibrational states and 79/59/39/20 bands. Similarly, an “Ames+MARVEL” line set was extracted from the Ames-296K line lists.
line lists by matching to the published MARVEL level set. These include 195 882/159 729/79 927 lines for $^{32}$S$^{16}$O$_2/^{34}$S$^{16}$O$_2/^{33}$S$^{16}$O$_2$, with maximum $K''_a = 35/29/22$. Uniform criteria were adopted for both line sets: 1) line position difference $\le 0.10 \text{ cm}^{-1}$ (for $E'$, $E''$, and transition wavenumber), 2) ro-vibrational quantum numbers match, 3) $S_{296K} \ge 10^{-26} \text{ cm/molecule}$. Compared to HITRAN2016 [16], many more $^{32}$S$^{16}$O$_2$ bands have been added up to 4200 cm$^{-1}$. However, the $3\nu_1$, $2\nu_1 + \nu_3$, and $\nu_1 + 2\nu_3$ bands of $^{32}$S$^{16}$O$_2$ are still missing from the Ames line sets. See more details in Huang et al. [357].

The Ames line sets did not actively utilize the published experimental EH models (or the EH models published along with the MARVEL analysis) to derive a complete energy level set and use it in the match analysis, except for those of the ground states and $^{33}$S$^{16}$O$_2$. For example, the $^{34}$S$^{16}$O$_2$ EH models of the $3\nu_2$ and $2\nu_1 + \nu_3$ states were published by Lafferty and Flaud [387] but they are not available in the Ames line set (nor the MARVEL level set). Therefore, in addition to the $S_{296K}$ and $J/K_a$ cutoffs, the effective coverage and quality are further restricted by the number and accuracy of reported measured lines and EH-based levels. Levels extrapolated from outdated EH models may coexist with those derived from the latest experiments. This potentially would result in a $0.001-0.01 \text{ cm}^{-1}$ discrepancy within the "Expanded Line Set". Future updates to the Ames lists are planned that will fix such minor inconsistencies, enable provision of reliable uncertainty indices to line positions, and re-evaluate the intensities of the major isotopologues with an even more accurate dipole moment surface. Additional Ames-296K line list information for other isotopologues with combinations of isotopes $^{17}$O, $^{18}$O, $^{33}$S, $^{34}$S, $^{35}$S, and $^{36}$S [363,364] is available at huang.seti.org/SO2/so2.html, including an "Expanded Line Set" reported for $^{32}$S$^{18}$O$_2$ [357].

The Naumenko $^{32}$S$^{16}$O$_2$ line list is based primarily on the experimental energy levels derived in the aforementioned MARVEL work by Tóbiás et al. [359]. Moreover, additional experimental energy levels obtained from the identification of the FTS spectra of SO$_2$ from Vasilenko et al. [388] and Naumenko et al. [389] were also utilized. The uncertainty of the majority of the transition wavenumbers determined this way correspond to HITRAN uncertainty code 4 (see Table 2). For completeness, effective Hamiltonian calculations performed in Tóbiás et al. [359] were used for the rest of the lines which in turn have uncertainties in the range $0.001-0.01 \text{ cm}^{-1}$ (code 3).

The Naumenko et al. [358] line list employs *ab initio* intensity values from Underwood et al. [365]. Exceptions are for the $\nu_1 + \nu_2$, $\nu_2 + \nu_3$, $\nu_1 + \nu_3$ and $\nu_1 + \nu_2 + \nu_3$ bands, where intensity values are from the recent experimental data from Ulenikov et al. [383] and Borkov et al. [390]. The estimated uncertainties of the intensities are around 5–10% on average. Overall, the Naumenko et al. [358] line list ranges from 0.025 and 4159 cm$^{-1}$ and consists of 549,200 transitions with a $10^{-30} \text{ cm/molecule}$ intensity cutoff.

### 2.9.2. Validation

Both $^{32}$S$^{16}$O$_2$ line lists (Naumenko and Ames) were converted into the HITRAN format. This includes addition of air- and self-broadening data (as well as temperature dependence of the former) using the procedure derived by Tan et al. [391]. The Ames intensities were scaled by the HITRAN isotopic abundance 0.9457 (note that only the two most abundant isotopologues were available in HITRAN prior to the present edition). The Naumenko line list contains only the principal $^{32}$S$^{16}$O$_2$ isotopologue.

In addition, there were further requirements when generating the Ames $^{32}$S$^{16}$O$_2$ line list following recommendations in that article. It was suggested using the $^{32}$S$^{16}$O$_2$ "Ames+MARVEL" line list which uses MARVEL levels matched to Ames lines. Then supplement the $^{32}$S$^{16}$O$_2$ "Ames+Marvel" line list with other transitions from the $^{32}$S$^{16}$O$_2$ "Expanded" line list which uses matched Experimental and HITRAN Effective Hamiltonian (EH) model levels to Ames lines. The article also recommended applying cutoffs for $J \le 50$ and $K_a \le 25$ for the most reliable Ames-296K intensity. All recommendations were applied for the $^{32}$S$^{16}$O$_2$ Ames line list [357] prior to comparisons with laboratory data. Line-to-line comparisons of all the $^{32}$S$^{16}$O$_2$ spectral bands were generated to evaluate the position, intensity and lower-state energy differences between the Naumenko line list [358], the $^{32}$S$^{16}$O$_2$ Ames line list [357] and the $^{32}$S$^{16}$O$_2$ HITRAN2016 [16] line list.

The Naumenko and Ames line lists were compared against PNNL [244] laboratory data at 5°C, 25°C and 50°C with nitrogen as the buffer gas for each data set at a pressure of 1 atm. There is a degree of uncertainty for these comparisons in that the PNNL data are N$_2$-broadened and HITRAN does not contain N$_2$-broadening for SO$_2$, therefore air-broadening has been used. Also, the PNNL [244] measurements are limited to frequencies $\ge 600 \text{ cm}^{-1}$, making far-IR comparisons unavailable. In addition to the PNNL comparisons, the line lists were validated at high resolution against an FTS experimental spectrum of SO$_2$ recorded at Old Dominion University (ODU) in 2015 (using the experimental setup previously described for CH$_4$ [392]). This spectrum covered the $\nu_1 + \nu_3$ and $2\nu_3$ bands of SO$_2$ with a sample pressure of 10 Torr (0.0132 atm), temperature of 297 K, and resolution of 0.015 cm$^{-1}$.

The result of the $^{32}$S$^{16}$O$_2$ line list comparisons showed that in the majority of the bands where HITRAN had data, both line lists exhibited an improvement in residuals and contained multiple bands not previously available in HITRAN. The only exception is the $\nu_1+3\nu_2$ band which was only available in HITRAN2016 and missing from the Naumenko and Ames $^{32}$S$^{16}$O$_2$ line lists: therefore this band is being retained for HITRAN2020. The Ames line list [357] lacked some observable transitions. This is due to the $J \le 50$ and $K_a \le 25$ limits and a cutoff for intensities beyond $10^{-26} \text{ cm/molecule}$. Sample comparisons to PNNL [244] laboratory data is available in Figs. 20 and 21. These figures include data from $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ in HITRAN2016 [16], $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ from Huang et al. [357], $^{32}$S$^{16}$O$_2$ from Naumenko et al. [358] supplemented by the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list. Overall, the final HITRAN2020 $^{32}$S$^{16}$O$_2$ line list contains data from Naumenko et al. [358] supplemented by a single band from HITRAN2016 ($\nu_1+3\nu_2$) along with some position changes in the 1337-1376 cm$^{-1}$ region where the Ref. [358] values were changed to HITRAN2016 values after detailed comparisons to laboratory spectra.

The $^{34}$S$^{16}$O$_2$ isotopologue line list generated by Huang et al. [357] is available in several line list sets. One is labeled the "Expanded" set and another is labeled the "Ames+Marvel" set. As recommended by the article in Huang et al. [357], the following cutoffs were applied for $J \le 50$ and $K_a \le 25$. In following the recommendations by the authors, the "Ames+Marvel" set was supplemented with other transitions from the "Expanded" set to generate the final $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list.

The resulting $^{34}$S$^{16}$O$_2$ line list was evaluated by first converting it into the HITRAN format and then scaling the intensities by the HITRAN isotopic abundance $4.195 \times 10^{-2}$. Next, line-by-line comparisons of the available ground state to ground state, $\nu_1$, $\nu_1 + \nu_3$ and $\nu_3$ bands in HITRAN were used to evaluate the position, intensity and lower-state energy differences between the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list and the HITRAN2016 [16] $^{34}$S$^{16}$O$_2$ line list. From these comparisons and from comparisons against PNNL data, it was determined that the HITRAN2016 main bands had more reliable intensity and position data. HITRAN2016 also contained some transitions that were missing from the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list. Therefore, for the ground state to ground state, $\nu_1$, $\nu_1 + \nu_3$ and $\nu_3$ bands, HITRAN2016 data are retained while all other $^{34}$S$^{16}$O$_2$ Huang et al. [357] bands are added.

As was the case for the principal isotopologue, the procedure derived by Tan et al. [391] was used to populate line-shape parameters.
line lists by matching to the published MARVEL level set. These include 195 882/159 729/79 927 lines for $^{32}$S$^{16}$O$_2/^{34}$S$^{16}$O$_2/^{33}$S$^{16}$O$_2$, with maximum $K''_a = 35/29/22$. Uniform criteria were adopted for both line sets: 1) line position difference $\le 0.10 \text{ cm}^{-1}$ (for $E'$, $E''$, and transition wavenumber), 2) ro-vibrational quantum numbers match, 3) $S_{296K} \ge 10^{-26} \text{ cm/molecule}$. Compared to HITRAN2016 [16], many more $^{32}$S$^{16}$O$_2$ bands have been added up to 4200 cm$^{-1}$. However, the $3\nu_1$, $2\nu_1 + \nu_3$, and $\nu_1 + 2\nu_3$ bands of $^{32}$S$^{16}$O$_2$ are still missing from the Ames line sets. See more details in Huang et al. [357].

The Ames line sets did not actively utilize the published experimental EH models (or the EH models published along with the MARVEL analysis) to derive a complete energy level set and use it in the match analysis, except for those of the ground states and $^{33}$S$^{16}$O$_2$. For example, the $^{34}$S$^{16}$O$_2$ EH models of the $3\nu_2$ and $2\nu_1 + \nu_3$ states were published by Lafferty and Flaud [387] but they are not available in the Ames line set (nor the MARVEL level set). Therefore, in addition to the $S_{296K}$ and $J/K_a$ cutoffs, the effective coverage and quality are further restricted by the number and accuracy of reported measured lines and EH-based levels. Levels extrapolated from outdated EH models may coexist with those derived from the latest experiments. This potentially would result in a $0.001-0.01 \text{ cm}^{-1}$ discrepancy within the "Expanded Line Set". Future updates to the Ames lists are planned that will fix such minor inconsistencies, enable provision of reliable uncertainty indices to line positions, and re-evaluate the intensities of the major isotopologues with an even more accurate dipole moment surface. Additional Ames-296K line list information for other isotopologues with combinations of isotopes $^{17}$O, $^{18}$O, $^{33}$S, $^{34}$S, $^{35}$S, and $^{36}$S [363,364] is available at huang.seti.org/SO2/so2.html, including an "Expanded Line Set" reported for $^{32}$S$^{18}$O$_2$ [357].

The Naumenko $^{32}$S$^{16}$O$_2$ line list is based primarily on the experimental energy levels derived in the aforementioned MARVEL work by Tóbiás et al. [359]. Moreover, additional experimental energy levels obtained from the identification of the FTS spectra of SO$_2$ from Vasilenko et al. [388] and Naumenko et al. [389] were also utilized. The uncertainty of the majority of the transition wavenumbers determined this way correspond to HITRAN uncertainty code 4 (see Table 2). For completeness, effective Hamiltonian calculations performed in Tóbiás et al. [359] were used for the rest of the lines which in turn have uncertainties in the range $0.001-0.01 \text{ cm}^{-1}$ (code 3).

The Naumenko et al. [358] line list employs *ab initio* intensity values from Underwood et al. [365]. Exceptions are for the $\nu_1 + \nu_2$, $\nu_2 + \nu_3$, $\nu_1 + \nu_3$ and $\nu_1 + \nu_2 + \nu_3$ bands, where intensity values are from the recent experimental data from Ulenikov et al. [383] and Borkov et al. [390]. The estimated uncertainties of the intensities are around 5–10% on average. Overall, the Naumenko et al. [358] line list ranges from 0.025 and 4159 cm$^{-1}$ and consists of 549,200 transitions with a $10^{-30} \text{ cm/molecule}$ intensity cutoff.

### 2.9.2. Validation

Both $^{32}$S$^{16}$O$_2$ line lists (Naumenko and Ames) were converted into the HITRAN format. This includes addition of air- and self-broadening data (as well as temperature dependence of the former) using the procedure derived by Tan et al. [391]. The Ames intensities were scaled by the HITRAN isotopic abundance 0.9457 (note that only the two most abundant isotopologues were available in HITRAN prior to the present edition). The Naumenko line list contains only the principal $^{32}$S$^{16}$O$_2$ isotopologue.

In addition, there were further requirements when generating the Ames $^{32}$S$^{16}$O$_2$ line list following recommendations in that article. It was suggested using the $^{32}$S$^{16}$O$_2$ "Ames+MARVEL" line list which uses MARVEL levels matched to Ames lines. Then supplement the $^{32}$S$^{16}$O$_2$ "Ames+Marvel" line list with other transitions from the $^{32}$S$^{16}$O$_2$ "Expanded" line list which uses matched Experimental and HITRAN Effective Hamiltonian (EH) model levels to Ames lines. The article also recommended applying cutoffs for $J \le 50$ and $K_a \le 25$ for the most reliable Ames-296K intensity. All recommendations were applied for the $^{32}$S$^{16}$O$_2$ Ames line list [357] prior to comparisons with laboratory data. Line-to-line comparisons of all the $^{32}$S$^{16}$O$_2$ spectral bands were generated to evaluate the position, intensity and lower-state energy differences between the Naumenko line list [358], the $^{32}$S$^{16}$O$_2$ Ames line list [357] and the $^{32}$S$^{16}$O$_2$ HITRAN2016 [16] line list.

The Naumenko and Ames line lists were compared against PNNL [244] laboratory data at 5°C, 25°C and 50°C with nitrogen as the buffer gas for each data set at a pressure of 1 atm. There is a degree of uncertainty for these comparisons in that the PNNL data are N$_2$-broadened and HITRAN does not contain N$_2$-broadening for SO$_2$, therefore air-broadening has been used. Also, the PNNL [244] measurements are limited to frequencies $\ge 600 \text{ cm}^{-1}$, making far-IR comparisons unavailable. In addition to the PNNL comparisons, the line lists were validated at high resolution against an FTS experimental spectrum of SO$_2$ recorded at Old Dominion University (ODU) in 2015 (using the experimental setup previously described for CH$_4$ [392]). This spectrum covered the $\nu_1 + \nu_3$ and $2\nu_3$ bands of SO$_2$ with a sample pressure of 10 Torr (0.0132 atm), temperature of 297 K, and resolution of 0.015 cm$^{-1}$.

The result of the $^{32}$S$^{16}$O$_2$ line list comparisons showed that in the majority of the bands where HITRAN had data, both line lists exhibited an improvement in residuals and contained multiple bands not previously available in HITRAN. The only exception is the $\nu_1+3\nu_2$ band which was only available in HITRAN2016 and missing from the Naumenko and Ames $^{32}$S$^{16}$O$_2$ line lists: therefore this band is being retained for HITRAN2020. The Ames line list [357] lacked some observable transitions. This is due to the $J \le 50$ and $K_a \le 25$ limits and a cutoff for intensities beyond $10^{-26} \text{ cm/molecule}$. Sample comparisons to PNNL [244] laboratory data is available in Figs. 20 and 21. These figures include data from $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ in HITRAN2016 [16], $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ from Huang et al. [357], $^{32}$S$^{16}$O$_2$ from Naumenko et al. [358] supplemented by the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list. Overall, the final HITRAN2020 $^{32}$S$^{16}$O$_2$ line list contains data from Naumenko et al. [358] supplemented by a single band from HITRAN2016 ($\nu_1+3\nu_2$) along with some position changes in the 1337-1376 cm$^{-1}$ region where the Ref. [358] values were changed to HITRAN2016 values after detailed comparisons to laboratory spectra.

The $^{34}$S$^{16}$O$_2$ isotopologue line list generated by Huang et al. [357] is available in several line list sets. One is labeled the "Expanded" set and another is labeled the "Ames+Marvel" set. As recommended by the article in Huang et al. [357], the following cutoffs were applied for $J \le 50$ and $K_a \le 25$. In following the recommendations by the authors, the "Ames+Marvel" set was supplemented with other transitions from the "Expanded" set to generate the final $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list.

The resulting $^{34}$S$^{16}$O$_2$ line list was evaluated by first converting it into the HITRAN format and then scaling the intensities by the HITRAN isotopic abundance $4.195 \times 10^{-2}$. Next, line-by-line comparisons of the available ground state to ground state, $\nu_1$, $\nu_1 + \nu_3$ and $\nu_3$ bands in HITRAN were used to evaluate the position, intensity and lower-state energy differences between the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list and the HITRAN2016 [16] $^{34}$S$^{16}$O$_2$ line list. From these comparisons and from comparisons against PNNL data, it was determined that the HITRAN2016 main bands had more reliable intensity and position data. HITRAN2016 also contained some transitions that were missing from the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list. Therefore, for the ground state to ground state, $\nu_1$, $\nu_1 + \nu_3$ and $\nu_3$ bands, HITRAN2016 data are retained while all other $^{34}$S$^{16}$O$_2$ Huang et al. [357] bands are added.

As was the case for the principal isotopologue, the procedure derived by Tan et al. [391] was used to populate line-shape parameters
<figure><img src="image_24.png" /><figcaption>**Fig. 20.** Validation of calculated absorption cross sections ($\sigma$) covering the $v_1$, $v_3$ and $v_1 + v_3$ vibrational bands against PNNL laboratory data [244]. In each panel: $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ HITRAN2016 data [16], $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ Ames data [357], and $^{32}$S$^{16}$O$_2$ Naumenko data [358] (supplemented by the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list) have been compared to the PNNL SO$_2$ spectrum at 25°C and 1 atm (N$_2$ buffer gas). Note that the final HITRAN2020 SO$_2$ data consists of the following; the $^{32}$S$^{16}$O$_2$ isotopologue provided by Naumenko et al. [358] with $v_1 + 3v_2$ band from HITRAN2016 being retained, the $^{34}$S$^{16}$O$_2$ isotopologue provided by Huang et al. [357] in addition to several strong bands from HITRAN2016, the $^{33}$S$^{16}$O$_2$ and $^{16}$O$^{32}$S$^{18}$O isotopologues provided by Huang et al. [357].</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. 20.** Validation of calculated absorption cross sections ($\sigma$) covering the $v_1$, $v_3$ and $v_1 + v_3$ vibrational bands against PNNL laboratory data [244]. In each panel: $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ HITRAN2016 data [16], $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ Ames data [357], and $^{32}$S$^{16}$O$_2$ Naumenko data [358] (supplemented by the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list) have been compared to the PNNL SO$_2$ spectrum at 25°C and 1 atm (N$_2$ buffer gas). Note that the final HITRAN2020 SO$_2$ data consists of the following; the $^{32}$S$^{16}$O$_2$ isotopologue provided by Naumenko et al. [358] with $v_1 + 3v_2$ band from HITRAN2016 being retained, the $^{34}$S$^{16}$O$_2$ isotopologue provided by Huang et al. [357] in addition to several strong bands from HITRAN2016, the $^{33}$S$^{16}$O$_2$ and $^{16}$O$^{32}$S$^{18}$O isotopologues provided by Huang et al. [357].</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Fig. 21.** Validation of calculated absorption cross sections ($\sigma$) covering the $v_1 + v_2$, $v_2 + v_3$ and $2v_1$ vibrational bands against PNNL laboratory data [244]. These bands were not previously included in HITRAN2016 [16]. In each panel, spectra calculated from $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ Ames data [357], and $^{32}$S$^{16}$O$_2$ Naumenko data [358] (supplemented by the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list) have been compared to the PNNL laboratory SO$_2$ spectra at 25°C and 1 atm (N$_2$ buffer gas). Note that the final HITRAN2020 SO$_2$ data consists of the following; the $^{32}$S$^{16}$O$_2$ isotopologue provided by Naumenko et al. [358] with $v_1 + 3v_2$ band from HITRAN2016 being retained, the $^{34}$S$^{16}$O$_2$ isotopologue provided by Huang et al. [357] in addition to several strong bands from HITRAN2016, the $^{33}$S$^{16}$O$_2$ and $^{16}$O$^{32}$S$^{18}$O isotopologues provided by Huang et al. [357].</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. 21.** Validation of calculated absorption cross sections ($\sigma$) covering the $v_1 + v_2$, $v_2 + v_3$ and $2v_1$ vibrational bands against PNNL laboratory data [244]. These bands were not previously included in HITRAN2016 [16]. In each panel, spectra calculated from $^{32}$S$^{16}$O$_2$, $^{34}$S$^{16}$O$_2$ Ames data [357], and $^{32}$S$^{16}$O$_2$ Naumenko data [358] (supplemented by the $^{34}$S$^{16}$O$_2$ Huang et al. [357] line list) have been compared to the PNNL laboratory SO$_2$ spectra at 25°C and 1 atm (N$_2$ buffer gas). Note that the final HITRAN2020 SO$_2$ data consists of the following; the $^{32}$S$^{16}$O$_2$ isotopologue provided by Naumenko et al. [358] with $v_1 + 3v_2$ band from HITRAN2016 being retained, the $^{34}$S$^{16}$O$_2$ isotopologue provided by Huang et al. [357] in addition to several strong bands from HITRAN2016, the $^{33}$S$^{16}$O$_2$ and $^{16}$O$^{32}$S$^{18}$O isotopologues provided by Huang et al. [357].</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Fig. 22.** The HITRAN2016 SO₂ line list [16] is shown in the upper panel, which can be compared to the spectral coverage of the new HITRAN2020 SO₂ line list (using data from Refs. [357,358] shown in the lower panel. These intensities have been scaled to their "natural" abundance (see Table 1) per HITRAN convention.</figcaption></figure>

rameters. Additionally, the ³⁴S¹⁶O₂ Huang et al. [357] line list was supplemented into the ³²S¹⁶O₂ Huang et al. [357] line list prior to validations against the PNNL [244] and the ODU laboratory data. HITRAN2016 [16] contains only a few bands for this isotopologue, which is evident from these comparisons. However, the ³⁴S¹⁶O₂ Huang et al. [357] line list contains many more transitions/bands and therefore provide a more complete ³⁴S¹⁶O₂ line list.

The ³³S¹⁶O₂ isotopologue is available in several line list sets generated by Huang et al. [357]. One labeled the "Expanded" set and another labeled the "Ames+Marvel" set. As recommended by Huang et al. [357] the line list was limited to $J \le 50$ and $K_a \le 25$. The "Ames+Marvel" set was used as a basis and then supplemented with additional transitions from the "Expanded" set to generate the final ³³S¹⁶O₂ line list for HITRAN.

The resulting intensities of the ³³S¹⁶O₂ line list were scaled by the HITRAN isotopic abundance $7.464 \times 10^{-3}$. The ³³S¹⁶O₂ isotopologue was not included in HITRAN2016 [16], therefore the ³³S¹⁶O₂ line list from Huang et al. [357] was added to HITRAN2020.

The ¹⁶O³²S¹⁸O isotopologue was available in the "Expanded" line list generated by Huang et al. [357]. As recommended, the line list was limited to $J \le 50$ and $K_a \le 25$ and the resulting ¹⁶O³²S¹⁸O intensities were scaled by the HITRAN isotopic abundance $3.793 \times 10^{-3}$. The ¹⁶O³²S¹⁸O isotopologue was not included in HITRAN2016 [16], therefore the ¹⁶O³²S¹⁸O line list from Huang et al. [357] was added to HITRAN2020.

As was the case for the two most abundant isotopologues, the air-broadening coefficients (and their temperature dependence), as well as self-broadening coefficients, were applied to the ³³S¹⁶O₂ and ¹⁶O³²S¹⁸O Huang et al. [357] line list using the procedure described by Tan et al. [391].

The half-widths of the SO₂ lines by the ambient pressure of CO₂ and their temperature dependence, introduced to HITRAN2016 by Wilzewski et al. [393] have been revised. The new values are based on the semi-empirical algorithm originally developed in Ref. [394]; however for the inclusion into HITRAN2020 this algorithm was updated to account for recent experimental measurements by Borkov et al. [390].

Finally, Fig. 22 compares the new HITRAN2020 line list for SO₂ (for all isotopologues) against the previous SO₂ line list in HITRAN2016 [16] and demonstrates the significant increase in spectral coverage.

## 2.10. NO₂: nitrogen dioxide (molecule 10)

Nitrogen dioxide (NO₂) plays an important role in the photochemistry of Earth's atmosphere. Combustion of fossil fuels at high temperatures (particularly from diesel engines), along with biomass burning and soil emissions are primary contributors to tropospheric NO₂ [395]. NO₂ also impacts air quality and is detrimental to human health [396]; therefore concentrations are routinely monitored, such as during COVID-19 restrictions [397].

For HITRAN2020, the NO₂ spectral coverage has been significantly extended. Fig. 23 (left panel) displays an overview comparison of the ¹⁴NO₂ line lists of HITRAN2016 and HITRAN2020. While HITRAN2016 contained only lines below 3074.153 cm⁻¹, the present version has been extended into the NIR. Selected data from NDSD-1000 [398,399] up to 4775 cm⁻¹ (with an intensity threshold of $S = 1.0 \times 10^{-25}$ cm/molecule) have been added for HITRAN2020. Comparisons to PNNL absorption cross sections [244] over the 4060–4775 cm⁻¹ spectral region necessitated a scaling of the NDSD-1000 intensities by a factor of 3.5 in this region (see Ref. [55] for further details).

Data obtained from extensive CRDS measurements [400–408] in the 5800–8000 cm⁻¹ region has also been added for HITRAN2020. The CRDS measured line positions and intensities were used to fit effective Hamiltonian and effective dipole moment parameters for the separate groups of interacting bands [400–408]. The obtained sets of effective parameters were presently used for the generation of the HITRAN2020 lists. As illustrated in Fig. 23 (right panel), different intensity cutoffs were applied depending on the study (in some cases, the addition to HITRAN has been extrapolated beyond the observed data). The smallest intensity for this region is $S = 1.0 \times 10^{-29}$ cm/molecule and the total number vibrational bands that are included in HITRAN for ¹⁴NO₂ has increased to 48.

For the added bands, semi-empirical approaches [409,410] were used for the calculation of the self- and air-broadening coefficients as well as their temperature exponents [398]. The empirical parameters used in these approaches were fitted to the measured self-broadening coefficients [411] and to the measured N₂- and O₂-broadening coefficients [412]. The vibrational dependence of the line-broadening coefficients was found to be small (less than 4%) [398] and thus neglected in the production of the NO₂ line list.
<figure><img src="image_24.png" /><figcaption>**Fig. 22.** The HITRAN2016 SO₂ line list [16] is shown in the upper panel, which can be compared to the spectral coverage of the new HITRAN2020 SO₂ line list (using data from Refs. [357,358] shown in the lower panel. These intensities have been scaled to their "natural" abundance (see Table 1) per HITRAN convention.</figcaption></figure>

rameters. Additionally, the ³⁴S¹⁶O₂ Huang et al. [357] line list was supplemented into the ³²S¹⁶O₂ Huang et al. [357] line list prior to validations against the PNNL [244] and the ODU laboratory data. HITRAN2016 [16] contains only a few bands for this isotopologue, which is evident from these comparisons. However, the ³⁴S¹⁶O₂ Huang et al. [357] line list contains many more transitions/bands and therefore provide a more complete ³⁴S¹⁶O₂ line list.

The ³³S¹⁶O₂ isotopologue is available in several line list sets generated by Huang et al. [357]. One labeled the “Expanded” set and another labeled the “Ames+Marvel” set. As recommended by Huang et al. [357] the line list was limited to $J \le 50$ and $K_a \le 25$. The “Ames+Marvel” set was used as a basis and then supplemented with additional transitions from the “Expanded” set to generate the final ³³S¹⁶O₂ line list for HITRAN.

The resulting intensities of the ³³S¹⁶O₂ line list were scaled by the HITRAN isotopic abundance $7.464 \times 10^{-3}$. The ³³S¹⁶O₂ isotopologue was not included in HITRAN2016 [16], therefore the ³³S¹⁶O₂ line list from Huang et al. [357] was added to HITRAN2020.

The ¹⁶O³²S¹⁸O isotopologue was available in the “Expanded” line list generated by Huang et al. [357]. As recommended, the line list was limited to $J \le 50$ and $K_a \le 25$ and the resulting ¹⁶O³²S¹⁸O intensities were scaled by the HITRAN isotopic abundance $3.793 \times 10^{-3}$. The ¹⁶O³²S¹⁸O isotopologue was not included in HITRAN2016 [16], therefore the ¹⁶O³²S¹⁸O line list from Huang et al. [357] was added to HITRAN2020.

As was the case for the two most abundant isotopologues, the air-broadening coefficients (and their temperature dependence), as well as self-broadening coefficients, were applied to the ³³S¹⁶O₂ and ¹⁶O³²S¹⁸O Huang et al. [357] line list using the procedure described by Tan et al. [391].

The half-widths of the SO₂ lines by the ambient pressure of CO₂ and their temperature dependence, introduced to HITRAN2016 by Wilzewski et al. [393] have been revised. The new values are based on the semi-empirical algorithm originally developed in Ref. [394]; however for the inclusion into HITRAN2020 this algorithm was updated to account for recent experimental measurements by Borkov et al. [390].

Finally, Fig. 22 compares the new HITRAN2020 line list for SO₂ (for all isotopologues) against the previous SO₂ line list in HITRAN2016 [16] and demonstrates the significant increase in spectral coverage.

## 2.10. NO₂: nitrogen dioxide (molecule 10)

Nitrogen dioxide (NO₂) plays an important role in the photochemistry of Earth's atmosphere. Combustion of fossil fuels at high temperatures (particularly from diesel engines), along with biomass burning and soil emissions are primary contributors to tropospheric NO₂ [395]. NO₂ also impacts air quality and is detrimental to human health [396]; therefore concentrations are routinely monitored, such as during COVID-19 restrictions [397].

For HITRAN2020, the NO₂ spectral coverage has been significantly extended. Fig. 23 (left panel) displays an overview comparison of the ¹⁴NO₂ line lists of HITRAN2016 and HITRAN2020. While HITRAN2016 contained only lines below 3074.153 cm⁻¹, the present version has been extended into the NIR. Selected data from NDSD-1000 [398,399] up to 4775 cm⁻¹ (with an intensity threshold of $S = 1.0 \times 10^{-25}$ cm/molecule) have been added for HITRAN2020. Comparisons to PNNL absorption cross sections [244] over the 4060–4775 cm⁻¹ spectral region necessitated a scaling of the NDSD-1000 intensities by a factor of 3.5 in this region (see Ref. [55] for further details).

Data obtained from extensive CRDS measurements [400–408] in the 5800–8000 cm⁻¹ region has also been added for HITRAN2020. The CRDS measured line positions and intensities were used to fit effective Hamiltonian and effective dipole moment parameters for the separate groups of interacting bands [400–408]. The obtained sets of effective parameters were presently used for the generation of the HITRAN2020 lists. As illustrated in Fig. 23 (right panel), different intensity cutoffs were applied depending on the study (in some cases, the addition to HITRAN has been extrapolated beyond the observed data). The smallest intensity for this region is $S = 1.0 \times 10^{-29}$ cm/molecule and the total number vibrational bands that are included in HITRAN for ¹⁴NO₂ has increased to 48.

For the added bands, semi-empirical approaches [409,410] were used for the calculation of the self- and air-broadening coefficients as well as their temperature exponents [398]. The empirical parameters used in these approaches were fitted to the measured self-broadening coefficients [411] and to the measured N₂- and O₂-broadening coefficients [412]. The vibrational dependence of the line-broadening coefficients was found to be small (less than 4%) [398] and thus neglected in the production of the NO₂ line list.
<figure><img src="image_27.png" /><figcaption>**Fig. 23.** Overview of the NO₂ line list for the main isotopologue, ¹⁴N¹⁶O₂. Left panel: Comparison of the HITRAN2016 and HITRAN2020 versions (red and black circles, respectively). Right panel: Enlargement of the 5800-8000 cm⁻¹ region that has been updated using CRDS measurements. The HITRAN2020 update (black circles) has been overlaid by the original CRDS data for each band system [400–408], which are identified by color.</figcaption></figure>

It is worth noting that the minimum intensity of the ¹⁴NNO₂ line list is different below and above 5000 cm⁻¹. However, the HITRAN list is not complete in both regions as some relatively strong bands are not provided due to the lack of measurements. New observations, such as of the v₁ + v₂ + v₃ band near 3600 cm⁻¹ [413], will help to improve the completeness for future editions of HITRAN.

Finally, it should be mentioned that the statistical weights of the lower and upper states and the Einstein-A coefficients for the NO₂ HITRAN2016 bands with unresolved hyperfine structure were corrected.

### 2.10.1. Addition of ¹⁵N¹⁶O₂

¹⁵N¹⁶O₂ is the second most abundant isotopologue of nitrogen dioxide, with a 0.003646 concentration of total NO₂ [57]. The v₃ band of ¹⁵N¹⁶O₂ is centered at 1582.1029 cm⁻¹ and becomes the first band of this isotopologue to be included into HITRAN.

The ¹⁵N¹⁶O₂ data included for HITRAN2020 contain 5860 transitions between 1500.73 and 1660.45 cm⁻¹ for the v₃ fundamental band. These transitions have a maximum intensity of 2.0 × 10⁻²⁶ cm/molecule, with Nmax = 77 and Ka = 0–15. The line positions and intensities were generated during two analyses: Ref. [414] used laboratory FTS spectra recorded at high-resolution (0.006 cm⁻¹ unapodized) to generate a preliminary line list, which was later refined by Perrin et al. [415]. The ¹⁵N¹⁶O₂ line list was restricted to the strong v₃ band, although the resonances that couple the strong v₃ band with the very dark 2v₂ and v₁ bands were explicitly accounted for. Experimental data concerning absolute line intensities are presently unavailable for ¹⁵N¹⁶O₂; therefore the line intensity calculation was performed using the transition moment constants which are quoted for the v₃ band of ¹⁴N¹⁶O₂ in Ref. [416]. The total internal partition sums for ¹⁵N¹⁶O₂ have been calculated for HITRAN2020 [417] and are described in Section 6.4.

Perrin et al. [415] have used this line list to detect atmospheric ¹⁵N¹⁶O₂ for the first time from balloon-borne solar occultation spectra measured by the JPL MklV FTS. Their retrieved ¹⁵NO₂/¹⁴NO₂ ratio is within 5% of the expected 0.00364 isotopic value (at an altitude of 15–35 km) and validate the calculated intensities. Over the same altitude range the RMS spectral fitting residuals reduce significantly as a result of including the new ¹⁵N¹⁶O₂ line list. Thus improving the accuracy of retrievals for all gases that absorb in the 1550–1650 cm⁻¹ region (e.g., ¹⁴NO₂, H₂O, HDO, O₂).

Future improvements for the NO₂ line list can be achieved by using the line list for the 1153–4775 cm⁻¹ spectral region [413] generated from recently recorded high-resolution FTS measurements (and added to the GEISA database [418]). For the 1500–1750 cm⁻¹ and 2780–2920 cm⁻¹ spectral regions, Jacquinet-Husson et al. [418] fully updated the line lists for the first hot

bands (v₂ + v₃ – v₂ and v₁ + v₂ + v₃ – v₂, respectively) and also included the v₃ and v₁ + v₃ bands of ¹⁵N¹⁶O₂. Several weak cold bands in the 2000–4500 cm⁻¹ region together with several higher order hot bands in the 1500–1750 cm⁻¹ and 2780–2920 cm⁻¹ spectral regions are also included for ¹⁴N¹⁶O₂, whenever possible. Finally, Perrin et al. [413] performed a validation of this new line list and inter-comparisons with the NO₂ lists present in the HITRAN and HITEMP [55] databases. The most significant differences concern vibration-rotation bands that have a weak IR signature for typical terrestrial atmospheric conditions, but for hot bands may have an impact for NO₂ in high-temperature gas conditions.

### 2.10.2. NO₂ added to HITEMP

The HITEMP database [53] has been expanded to include NO₂ [55] and is based on the NDSD-1000 line list [398,399]. NO₂ was added to HITEMP prior to the 5800–8000 cm⁻¹ (¹⁴NO₂) and ¹⁵NO₂ HITRAN additions described above. To maintain consistency, these updates will be incorporated into HITEMP in due course. Readers should refer to Ref. [55] for a full description of the HITEMP line list for NO₂.

### 2.11. NH₃: ammonia (molecule 11)

Ammonia (NH₃) is integral to global agriculture through its use as an industrial fertilizer [419]. Consequently, anthropogenic emissions are a large contributor to the atmospheric abundance [420], but enhancements can also occur through natural events such as wildfires [421]. The abundance of NH₃ impinges on air quality [422] and its abundance can now be monitored over urban [423], industrial [424], and rural [425] regions using satellite and ground-based observations. Furthermore, NH₃ is a well known constituent of gas giants atmospheres and recent measurements from the Juno mission demonstrate complex weather processes with large-scale NH₃ variability [426,427]. The absorption of NH₃ is also expected to contribute to the opacities of cool brown dwarfs [428] and exoplanets [429].

The “CoYuTe” *ab initio* line list has been calculated as part of the ExoMol project based on a spectroscopically determined PES and an *ab initio* DMS [430]. CoYuTe spans transitions with wavenumbers up to 20,000 cm⁻¹ and is designed for use at temperatures up to 1500 K [431]; even before improvement of the energy levels using MARVEL, it is significantly more accurate than the BYTe line list [432] used to populate earlier versions of HITRAN. In parallel there was an extensive update of the ¹⁴NH₃ MARVEL database [433] which considered 46,115 transitions up to 7500 cm⁻¹ yielding 4936 unique, labeled empirical ro-vibrational energy levels. These MARVEL energy levels were used in the CoYuTe line list to ensure experimental accuracy in the wavenumbers of the vast majority of transitions lying below 7000 cm⁻¹ and above the
<figure><img src="image_25.png" /><figcaption>**Fig. 23.** Overview of the NO₂ line list for the main isotopologue, ¹⁴N¹⁶O₂. Left panel: Comparison of the HITRAN2016 and HITRAN2020 versions (red and black circles, respectively). Right panel: Enlargement of the 5800-8000 cm⁻¹ region that has been updated using CRDS measurements. The HITRAN2020 update (black circles) has been overlaid by the original CRDS data for each band system [400–408], which are identified by color.</figcaption></figure>

It is worth noting that the minimum intensity of the ¹⁴NNO₂ line list is different below and above 5000 cm⁻¹. However, the HITRAN list is not complete in both regions as some relatively strong bands are not provided due to the lack of measurements. New observations, such as of the v₁ + v₂ + v₃ band near 3600 cm⁻¹ [413], will help to improve the completeness for future editions of HITRAN.

Finally, it should be mentioned that the statistical weights of the lower and upper states and the Einstein-A coefficients for the NO₂ HITRAN2016 bands with unresolved hyperfine structure were corrected.

### 2.10.1. Addition of ¹⁵N¹⁶O₂

¹⁵N¹⁶O₂ is the second most abundant isotopologue of nitrogen dioxide, with a 0.003646 concentration of total NO₂ [57]. The v₃ band of ¹⁵N¹⁶O₂ is centered at 1582.1029 cm⁻¹ and becomes the first band of this isotopologue to be included into HITRAN.

The ¹⁵N¹⁶O₂ data included for HITRAN2020 contain 5860 transitions between 1500.73 and 1660.45 cm⁻¹ for the v₃ fundamental band. These transitions have a maximum intensity of 2.0 × 10⁻²⁶ cm/molecule, with Nmax = 77 and Ka = 0–15. The line positions and intensities were generated during two analyses: Ref. [414] used laboratory FTS spectra recorded at high-resolution (0.006 cm⁻¹ unapodized) to generate a preliminary line list, which was later refined by Perrin et al. [415]. The ¹⁵N¹⁶O₂ line list was restricted to the strong v₃ band, although the resonances that couple the strong v₃ band with the very dark 2v₂ and v₁ bands were explicitly accounted for. Experimental data concerning absolute line intensities are presently unavailable for ¹⁵N¹⁶O₂; therefore the line intensity calculation was performed using the transition moment constants which are quoted for the v₃ band of ¹⁴N¹⁶O₂ in Ref. [416]. The total internal partition sums for ¹⁵N¹⁶O₂ have been calculated for HITRAN2020 [417] and are described in Section 6.4.

Perrin et al. [415] have used this line list to detect atmospheric ¹⁵N¹⁶O₂ for the first time from balloon-borne solar occultation spectra measured by the JPL MklV FTS. Their retrieved ¹⁵NO₂/¹⁴NO₂ ratio is within 5% of the expected 0.00364 isotopic value (at an altitude of 15–35 km) and validate the calculated intensities. Over the same altitude range the RMS spectral fitting residuals reduce significantly as a result of including the new ¹⁵N¹⁶O₂ line list. Thus improving the accuracy of retrievals for all gases that absorb in the 1550–1650 cm⁻¹ region (e.g., ¹⁴NO₂, H₂O, HDO, O₂).

Future improvements for the NO₂ line list can be achieved by using the line list for the 1153–4775 cm⁻¹ spectral region [413] generated from recently recorded high-resolution FTS measurements (and added to the GEISA database [418]). For the 1500–1750 cm⁻¹ and 2780–2920 cm⁻¹ spectral regions, Jacquinet-Husson et al. [418] fully updated the line lists for the first hot

bands (v₂ + v₃ - v₂ and v₁ + v₂ + v₃ - v₂, respectively) and also included the v₃ and v₁ + v₃ bands of ¹⁵N¹⁶O₂. Several weak cold bands in the 2000–4500 cm⁻¹ region together with several higher order hot bands in the 1500–1750 cm⁻¹ and 2780–2920 cm⁻¹ spectral regions are also included for ¹⁴N¹⁶O₂, whenever possible. Finally, Perrin et al. [413] performed a validation of this new line list and inter-comparisons with the NO₂ lists present in the HITRAN and HITEMP [55] databases. The most significant differences concern vibration-rotation bands that have a weak IR signature for typical terrestrial atmospheric conditions, but for hot bands may have an impact for NO₂ in high-temperature gas conditions.

### 2.10.2. NO₂ added to HITEMP

The HITEMP database [53] has been expanded to include NO₂ [55] and is based on the NDSD-1000 line list [398,399]. NO₂ was added to HITEMP prior to the 5800–8000 cm⁻¹ (¹⁴NO₂) and ¹⁵NO₂ HITRAN additions described above. To maintain consistency, these updates will be incorporated into HITEMP in due course. Readers should refer to Ref. [55] for a full description of the HITEMP line list for NO₂.

### 2.11. NH₃: ammonia (molecule 11)

Ammonia (NH₃) is integral to global agriculture through its use as an industrial fertilizer [419]. Consequently, anthropogenic emissions are a large contributor to the atmospheric abundance [420], but enhancements can also occur through natural events such as wildfires [421]. The abundance of NH₃ impinges on air quality [422] and its abundance can now be monitored over urban [423], industrial [424], and rural [425] regions using satellite and ground-based observations. Furthermore, NH₃ is a well known constituent of gas giants atmospheres and recent measurements from the Juno mission demonstrate complex weather processes with large-scale NH₃ variability [426,427]. The absorption of NH₃ is also expected to contribute to the opacities of cool brown dwarfs [428] and exoplanets [429].

The “CoYuTe” *ab initio* line list has been calculated as part of the ExoMol project based on a spectroscopically determined PES and an *ab initio* DMS [430]. CoYuTe spans transitions with wavenumbers up to 20,000 cm⁻¹ and is designed for use at temperatures up to 1500 K [431]; even before improvement of the energy levels using MARVEL, it is significantly more accurate than the BYTe line list [432] used to populate earlier versions of HITRAN. In parallel there was an extensive update of the ¹⁴NH₃ MARVEL database [433] which considered 46,115 transitions up to 7500 cm⁻¹ yielding 4936 unique, labeled empirical ro-vibrational energy levels. These MARVEL energy levels were used in the CoYuTe line list to ensure experimental accuracy in the wavenumbers of the vast majority of transitions lying below 7000 cm⁻¹ and above the
<figure><img src="image_28.png" /><figcaption>**Fig. 24.** Overview of the <sup>14</sup>NH<sub>3</sub> data in HITRAN2020. The location of the retained HITRAN2016 data and new additions from CoYuTe/MARVEL, Cacciani et al. [437] and Vander Auwera and Vanfleteren [438] have been indicated.</figcaption></figure>

HITRAN2016 intensity cutoff. These new MARVEL energy levels and the CoYuTe line list were used to complement the HITRAN2016 dataset in which a significant number of lines were missed in the 4000–7000 cm<sup>-1</sup> region.

### 2.11.1. Addition of selected bands from the CoYuTe/MARVEL line list

The performance of the CoYuTe/MARVEL line list at modeling room-temperature experimental FTS spectra (0.01 cm<sup>-1</sup> resolution) between 2400 and 5500 cm<sup>-1</sup> [434,435] and 4800–7500 cm<sup>-1</sup> [436] (as well as lower resolution cross-sections from PNNL [244]), has been compared to HITRAN2016 [16]. Throughout these spectral regions there were instances where CoYuTe/MARVEL provided position and/or intensity improvements when compared to HITRAN; however, there were also a number of occasions when the HITRAN2016 data were favorable, with many instances where the performance were comparable. A more intensive and thorough assessment of the line-by-line improvements provided by the CoYuTe/MARVEL line list is planned for the future. At this time, only transitions from vibrational bands not previously contained in HITRAN are considered for inclusion. For NH<sub>3</sub>, the lower-state energies in HITRAN refer to the lowest allowed lower-state energy and values in the CoYuTe/MARVEL line list have been reduced by 0.7934 cm<sup>-1</sup>: the energy difference between the symmetric and anti-symmetric J'' = K'' = 0 levels of the ground vibrational state.

A previous lack of assignments for NH<sub>3</sub> in the 5500–6350 cm<sup>-1</sup> spectral region means that the CoYuTe/MARVEL line list is missing the majority of transitions recently observed by Cacciani et al. [437]. Of the weak transitions in this range that were included in the CoYuTe/MARVEL list, the predicted intensities appeared to be overestimated when compared to measured values. Therefore, only the CoYuTe/MARVEL transitions of selected bands < 5500 cm<sup>-1</sup> (with intensities > 1.0 × 10<sup>-25</sup> cm/molecule) were included as part of HITRAN2020. Fig. 24 summarizes the new CoYuTe/MARVEL additions to HITRAN below 5500 cm<sup>-1</sup>.

The CoYuTe/MARVEL lines that were not included as part of the present update will be thoroughly evaluated as a subsequent update to HITRAN2020. In addition, the intensities for the 50–660 cm<sup>-1</sup> spectral region measured by Sung et al. [439] will also be considered.

### 2.11.2. The 5500–6350 cm⁻¹ region

In previous editions of HITRAN, this spectral region contained very limited transitions of NH<sub>3</sub> due to a lack of measurements, but this region is part of the 1.6 µm atmospheric transparency window and allows for additional opportunities for NH<sub>3</sub> retrievals. Recently, Cacciani et al. [437] have analyzed an archived FTS spectrum of NH<sub>3</sub> (1.8 Torr at 21.5°C) between 5500 and 6350 cm<sup>-1</sup> recorded at Kitt Peak NSO in 1991 with a resolution of 0.01 cm<sup>-1</sup>. A multi-line fitting procedure was used to obtain the positions and intensities

of 2779 lines, of which 1762 lines were assigned to 29 vibrational bands.

A small number of transitions (between 6300 and 6350 cm<sup>-1</sup>) partially overlap the data provided by Sung et al. [440], which was included as part of HITRAN2012 [15]. Comparison of these intensities showed a systematic offset of ~10–15% (within the uncertainty of the experiment), but a good agreement was found when compared to ab initio line intensities [431] for many more lines (see Fig. 9 of Ref. [437]). Therefore, the positions and intensities from Cacciani et al. [437] have been added to HITRAN, and replace the previous values [440] for the 6300–6350 cm<sup>-1</sup> overlap region. For inclusion to HITRAN, the lower-state energies in Ref. [437] have been reduced by 0.7934 cm<sup>-1</sup> to account for the lowest allowed lower-state energy.

### 2.11.3. The 7400–8600 cm⁻¹ region

Relying on two high-resolution Fourier-transform spectra recorded at room temperature, the positions and intensities of 1936 lines of ammonia were measured in the range 7400–8600 cm<sup>-1</sup> [438]. These lines are not assigned and lower-state energies are therefore not available. The line positions were calibrated using 145 water-vapor lines observed in the ranges 5255–5536 cm<sup>-1</sup> and 7054–7398 cm<sup>-1</sup> and reference line positions reported by Toth [61] and available in HITRAN2016 [16]. The accuracy of the calibrated line positions was estimated to range from 0.001 to 0.002 cm<sup>-1</sup> from the lower to the upper limits of the spectral range considered, while the accuracy of the line intensities was estimated to be around 10% or better. Line positions and intensities measured in the range 6800–7000 cm<sup>-1</sup> of the same two spectra with measurements reported by Sung et al. [440] agreed within these estimated accuracies (see Figs. 2 and 3 of Ref. [438]). Partially-assigned lines measured in the 7400–8640 cm<sup>-1</sup> range of a FTS spectrum recorded at NSO by C. De Bergh in 1980 [441] were incorporated in HITRAN2016 [16]. Comparisons of these line positions and intensities with the new measurements [438] put forward discrepancies within the uncertainties stated in HITRAN2016 [16] for the line positions, but (significantly) larger for the intensities of strong lines (S > 10<sup>-22</sup> cm/molecule; see Figs. 6 and 7 of Ref. [438]). Note that less-accurately measured positions and intensities of 1985 lines were also reported in the range 7400–8600 cm<sup>-1</sup> [438], leading to a combined total of 3921 lines.

The Vander Auwera T. Vanfleteren [438] positions and intensities were compared to the empirical line list of Ref. [436]. Empirical lower-state energies have been matched to 578 lines of Ref. [438] to improve the temperature extrapolation of this spectral range. For all other lines, a default value is used (E'' = 333 cm<sup>-1</sup>). The performance of the Vander Auwera and Vanfleteren [438] line lists (all 3921 lines) was validated against HITRAN2016 by modeling a FTS spectrum of NH<sub>3</sub> (5.0 Torr at 21.5°C and recorded at the NSO in 1980). This same spectrum was previously analysed
<figure><img src="image_26.png" /><figcaption>**Fig. 24.** Overview of the <sup>14</sup>NH<sub>3</sub> data in HITRAN2020. The location of the retained HITRAN2016 data and new additions from CoYuTe/MARVEL, Cacciani et al. [437] and Vander Auwera and Vanfleteren [438] have been indicated.</figcaption></figure>

HITRAN2016 intensity cutoff. These new MARVEL energy levels and the CoYuTe line list were used to complement the HITRAN2016 dataset in which a significant number of lines were missed in the 4000–7000 cm<sup>-1</sup> region.

### 2.11.1. Addition of selected bands from the CoYuTe/MARVEL line list

The performance of the CoYuTe/MARVEL line list at modeling room-temperature experimental FTS spectra (0.01 cm<sup>-1</sup> resolution) between 2400 and 5500 cm<sup>-1</sup> [434,435] and 4800–7500 cm<sup>-1</sup> [436] (as well as lower resolution cross-sections from PNNL [244]), has been compared to HITRAN2016 [16]. Throughout these spectral regions there were instances where CoYuTe/MARVEL provided position and/or intensity improvements when compared to HITRAN; however, there were also a number of occasions when the HITRAN2016 data were favorable, with many instances where the performance were comparable. A more intensive and thorough assessment of the line-by-line improvements provided by the CoYuTe/MARVEL line list is planned for the future. At this time, only transitions from vibrational bands not previously contained in HITRAN are considered for inclusion. For NH<sub>3</sub>, the lower-state energies in HITRAN refer to the lowest allowed lower-state energy and values in the CoYuTe/MARVEL line list have been reduced by 0.7934 cm<sup>-1</sup>: the energy difference between the symmetric and anti-symmetric J'' = K'' = 0 levels of the ground vibrational state.

A previous lack of assignments for NH<sub>3</sub> in the 5500–6350 cm<sup>-1</sup> spectral region means that the CoYuTe/MARVEL line list is missing the majority of transitions recently observed by Cacciani et al. [437]. Of the weak transitions in this range that were included in the CoYuTe/MARVEL list, the predicted intensities appeared to be overestimated when compared to measured values. Therefore, only the CoYuTe/MARVEL transitions of selected bands < 5500 cm<sup>-1</sup> (with intensities > 1.0 × 10<sup>-25</sup> cm/molecule) were included as part of HITRAN2020. Fig. 24 summarizes the new CoYuTe/MARVEL additions to HITRAN below 5500 cm<sup>-1</sup>.

The CoYuTe/MARVEL lines that were not included as part of the present update will be thoroughly evaluated as a subsequent update to HITRAN2020. In addition, the intensities for the 50–660 cm<sup>-1</sup> spectral region measured by Sung et al. [439] will also be considered.

### 2.11.2. The 5500–6350 cm⁻¹ region

In previous editions of HITRAN, this spectral region contained very limited transitions of NH<sub>3</sub> due to a lack of measurements, but this region is part of the 1.6 µm atmospheric transparency window and allows for additional opportunities for NH<sub>3</sub> retrievals. Recently, Cacciani et al. [437] have analyzed an archived FTS spectrum of NH<sub>3</sub> (1.8 Torr at 21.5°C) between 5500 and 6350 cm<sup>-1</sup> recorded at Kitt Peak NSO in 1991 with a resolution of 0.01 cm<sup>-1</sup>. A multi-line fitting procedure was used to obtain the positions and intensities

of 2779 lines, of which 1762 lines were assigned to 29 vibrational bands.

A small number of transitions (between 6300 and 6350 cm<sup>-1</sup>) partially overlap the data provided by Sung et al. [440], which was included as part of HITRAN2012 [15]. Comparison of these intensities showed a systematic offset of ~10–15% (within the uncertainty of the experiment), but a good agreement was found when compared to ab initio line intensities [431] for many more lines (see Fig. 9 of Ref. [437]). Therefore, the positions and intensities from Cacciani et al. [437] have been added to HITRAN, and replace the previous values [440] for the 6300–6350 cm<sup>-1</sup> overlap region. For inclusion to HITRAN, the lower-state energies in Ref. [437] have been reduced by 0.7934 cm<sup>-1</sup> to account for the lowest allowed lower-state energy.

### 2.11.3. The 7400–8600 cm⁻¹ region

Relying on two high-resolution Fourier-transform spectra recorded at room temperature, the positions and intensities of 1936 lines of ammonia were measured in the range 7400–8600 cm<sup>-1</sup> [438]. These lines are not assigned and lower-state energies are therefore not available. The line positions were calibrated using 145 water-vapor lines observed in the ranges 5255–5536 cm<sup>-1</sup> and 7054–7398 cm<sup>-1</sup> and reference line positions reported by Toth [61] and available in HITRAN2016 [16]. The accuracy of the calibrated line positions was estimated to range from 0.001 to 0.002 cm<sup>-1</sup> from the lower to the upper limits of the spectral range considered, while the accuracy of the line intensities was estimated to be around 10% or better. Line positions and intensities measured in the range 6800–7000 cm<sup>-1</sup> of the same two spectra with measurements reported by Sung et al. [440] agreed within these estimated accuracies (see Figs. 2 and 3 of Ref. [438]). Partially-assigned lines measured in the 7400–8640 cm<sup>-1</sup> range of a FTS spectrum recorded at NSO by C. De Bergh in 1980 [441] were incorporated in HITRAN2016 [16]. Comparisons of these line positions and intensities with the new measurements [438] put forward discrepancies within the uncertainties stated in HITRAN2016 [16] for the line positions, but (significantly) larger for the intensities of strong lines (S > 10<sup>-22</sup> cm/molecule; see Figs. 6 and 7 of Ref. [438]). Note that less-accurately measured positions and intensities of 1985 lines were also reported in the range 7400–8600 cm<sup>-1</sup> [438], leading to a combined total of 3921 lines.

The Vander Auwera T. Vanfleteren [438] positions and intensities were compared to the empirical line list of Ref. [436]. Empirical lower-state energies have been matched to 578 lines of Ref. [438] to improve the temperature extrapolation of this spectral range. For all other lines, a default value is used (E'' = 333 cm<sup>-1</sup>). The performance of the Vander Auwera and Vanfleteren [438] line lists (all 3921 lines) was validated against HITRAN2016 by modeling a FTS spectrum of NH<sub>3</sub> (5.0 Torr at 21.5°C and recorded at the NSO in 1980). This same spectrum was previously analysed
<figure><img src="image_29.png" /><figcaption>**Fig. 25.** A demonstration of the improvements for the NH₃ line list in HITRAN2020 over the 7400–8600 cm⁻¹ spectral region (a), with zoomed in examples of when HITRAN2016 (b) or Vander Auwera and Vanfleteren [438] (c) contribute to the HITRAN2020 data. In each upper panel, a FTS NSO spectrum with 5.0 Torr of NH₃ at 21.5°C is compared to calculated spectra using HITRAN2016 (using the data of Ref. [441]), Ref. [438] and the combined HITRAN2020 line lists. The lower panels display the obs-calc residuals.</figcaption></figure>

by Barton et al. [441] and the corresponding data added to HITRAN2016 [16]. This reanalysis for this work demonstrated that Ref. [438] provided many improvements, such as for strong lines (as shown in Fig. 25c), but these comparisons also demonstrated that some blended features were better modeled by HITRAN2016 (Fig. 25b). The vast majority of transitions in this region are unasigned; therefore the calculated spectra were assessed in their ability to reproduce the experimental observations. The residuals were analysed on a line-by-line basis, and those parameters from each list (either HITRAN2016 or Ref. [438]) that provided the smallest residual were used for HITRAN2020. The NH₃ line list for HITRAN2020 is also included in the comparison plots of Fig. 25 and can be seen to have the smallest residual in each case.

Additional FTS spectra (10.0 Torr of NH₃ combined with a 0.5 m path length) measured by Beale et al. [436] also cover the 7400–8600 cm⁻¹ region. However, the SNR for this region was not sufficient to provide detailed comparisons. Nevertheless, the overestimation of strong line intensities identified by Vander Auwera and Vanfleteren [438] for HITRAN2016 (and shown in Fig. 25c) were clearly observed when comparing calculated spectra, with the HITRAN2020 calculated spectrum providing the smallest residual.

### 2.11.4. Additional updates and broadening parameters for ¹⁴NH₃

The lower-state energies of 12 transitions were noted to give an incorrect temperature extrapolation when used at higher temperatures [434]. To correct this issue, the lower-state energies of these transitions have been reverted to values from HITRAN2008 [14] and the assignment removed. Provisional comparisons to atmospheric and laboratory spectra highlighted 8 lines near 4415 cm⁻¹ (with intensities greater than 10⁻²¹ cm/molecule), which were absent in previous editions of HITRAN. These transitions were assigned using Ref. [433] and added to HITRAN (with appropriate lower-state energy).

In addition, it was noted by Yurchenko [442] that 3 lines at 5014.4776, 5084.8734, 5104.2963 cm⁻¹ identified as belonging to ¹⁵NH₃ had intensities overestimated by (at least) an order of magnitude. These anomalous intensities are due to stronger coincident ¹⁴NH₃ transitions and they have therefore been attributed to ¹⁴NH₃ and the assignment has been removed.

The air- and self-broadening coefficients and temperature dependence from Nemtchinov [443] have been applied to all new transitions. For the current work, the functions used to calculate air-broadening have been extended to improve comparisons with atmospheric and laboratory spectra. For transitions with $J \ge 9$ (and $K \le 9$), the coefficients are fixed at the values used for and corresponding value of $KJ = 9$. For transitions with $J \ge 9$ and $K > 9$, a constant value of 0.0906 cm⁻¹/atm is used.

To support the application of HITRAN data to modeling of planetary atmospheres, H₂-, He- and CO₂-broadening parameters were introduced for NH₃ as part of HITRAN2016 [393]. In addition, H₂O-broadening parameters have since been added to HITRAN by Tan et al. [49]. These parameters are necessary for accurate modelling of the atmosphere of Jupiter and have been validated against experimental measurements under Jovian conditions [444]. As part of this work, the self-broadening parameter for the R(0) ground-state transition at 4.67463 cm⁻¹ was empirically adjusted to 0.250 cm⁻¹/atm, for better agreement with observation.

### 2.11.5. ¹⁵NH₃

Notwithstanding the recognized important role of ¹⁵NH₃ in the determination of the ¹⁵N/¹⁴N ratio in space [445] where ammonia is abundant in various environments and objects, the extent and quality of spectroscopic information contained in HITRAN2016 for this isotopologue was not on par with that of ¹⁴NH₃ (with the exception of the MW region).

Recently, the spectroscopic analysis of *a* and *s* inversion levels of the bending states up to about 3000 cm⁻¹ has been completed for ¹⁵NH₃ at experimental accuracy [446,447], as a result of new high-resolution spectra recorded using the FTS at the Canadian Light Source (CLS) synchrotron and the Bomem DA8 interferometer in Bologna. Overall, 13,288 inversion-rotation-vibration transitions in the $v_2$, $2v_2$, $v_4$, $3v_2$, and $v_2 + v_4$ cold bands, in the $2v_2 \leftarrow v_2$, $2v_2 \leftarrow v_4$, $v_4 \leftarrow v_2$, $v_4 \leftarrow 2v_2$, $3v_2 \leftarrow v_2$, $3v_2 \leftarrow 2v_2$, $3v_2 \leftarrow v_4$, $3v_2 \leftarrow v_2 + v_4$, $v_2 + v_4 \leftarrow v_2$, $v_2 + v_4 \leftarrow 2v_2$, $v_2 + v_4 \leftarrow 3v_2$ and $v_2 + v_4 \leftarrow v_4$ hot bands, and the inversion-rotation transitions in the studied excited states have been assigned and analyzed [446,447].

This analyses allowed creating a model that can reproduce the line positions of the assigned transitions well within experimen-
<figure><img src="image_27.png" /><figcaption>**Fig. 25.** A demonstration of the improvements for the NH₃ line list in HITRAN2020 over the 7400–8600 cm⁻¹ spectral region (a), with zoomed in examples of when HITRAN2016 (b) or Vander Auwera and Vanfleteren [438] (c) contribute to the HITRAN2020 data. In each upper panel, a FTS NSO spectrum with 5.0 Torr of NH₃ at 21.5°C is compared to calculated spectra using HITRAN2016 (using the data of Ref. [441]), Ref. [438] and the combined HITRAN2020 line lists. The lower panels display the obs-calc residuals.</figcaption></figure>

by Barton et al. [441] and the corresponding data added to HITRAN2016 [16]. This reanalysis for this work demonstrated that Ref. [438] provided many improvements, such as for strong lines (as shown in Fig. 25c), but these comparisons also demonstrated that some blended features were better modeled by HITRAN2016 (Fig. 25b). The vast majority of transitions in this region are unasigned; therefore the calculated spectra were assessed in their ability to reproduce the experimental observations. The residuals were analysed on a line-by-line basis, and those parameters from each list (either HITRAN2016 or Ref. [438]) that provided the smallest residual were used for HITRAN2020. The NH₃ line list for HITRAN2020 is also included in the comparison plots of Fig. 25 and can be seen to have the smallest residual in each case.

Additional FTS spectra (10.0 Torr of NH₃ combined with a 0.5 m path length) measured by Beale et al. [436] also cover the 7400–8600 cm⁻¹ region. However, the SNR for this region was not sufficient to provide detailed comparisons. Nevertheless, the overestimation of strong line intensities identified by Vander Auwera and Vanfleteren [438] for HITRAN2016 (and shown in Fig. 25c) were clearly observed when comparing calculated spectra, with the HITRAN2020 calculated spectrum providing the smallest residual.

### 2.11.4. Additional updates and broadening parameters for ¹⁴NH₃

The lower-state energies of 12 transitions were noted to give an incorrect temperature extrapolation when used at higher temperatures [434]. To correct this issue, the lower-state energies of these transitions have been reverted to values from HITRAN2008 [14] and the assignment removed. Provisional comparisons to atmospheric and laboratory spectra highlighted 8 lines near 4415 cm⁻¹ (with intensities greater than 10⁻²¹ cm/molecule), which were absent in previous editions of HITRAN. These transitions were assigned using Ref. [433] and added to HITRAN (with appropriate lower-state energy).

In addition, it was noted by Yurchenko [442] that 3 lines at 5014.4776, 5084.8734, 5104.2963 cm⁻¹ identified as belonging to ¹⁵NH₃ had intensities overestimated by (at least) an order of magnitude. These anomalous intensities are due to stronger coincident ¹⁴NH₃ transitions and they have therefore been attributed to ¹⁴NH₃ and the assignment has been removed.

The air- and self-broadening coefficients and temperature dependence from Nemtchinov [443] have been applied to all new transitions. For the current work, the functions used to calculate air-broadening have been extended to improve comparisons with atmospheric and laboratory spectra. For transitions with $J \ge 9$ (and $K \le 9$), the coefficients are fixed at the values used for and corresponding value of $KJ = 9$. For transitions with $J \ge 9$ and $K > 9$, a constant value of 0.0906 cm⁻¹/atm is used.

To support the application of HITRAN data to modeling of planetary atmospheres, H₂-, He- and CO₂-broadening parameters were introduced for NH₃ as part of HITRAN2016 [393]. In addition, H₂O-broadening parameters have since been added to HITRAN by Tan et al. [49]. These parameters are necessary for accurate modelling of the atmosphere of Jupiter and have been validated against experimental measurements under Jovian conditions [444]. As part of this work, the self-broadening parameter for the R(0) ground-state transition at 4.67463 cm⁻¹ was empirically adjusted to 0.250 cm⁻¹/atm, for better agreement with observation.

### 2.11.5. ¹⁵NH₃

Notwithstanding the recognized important role of ¹⁵NH₃ in the determination of the ¹⁵N/¹⁴N ratio in space [445] where ammonia is abundant in various environments and objects, the extent and quality of spectroscopic information contained in HITRAN2016 for this isotopologue was not on par with that of ¹⁴NH₃ (with the exception of the MW region).

Recently, the spectroscopic analysis of *a* and *s* inversion levels of the bending states up to about 3000 cm⁻¹ has been completed for ¹⁵NH₃ at experimental accuracy [446,447], as a result of new high-resolution spectra recorded using the FTS at the Canadian Light Source (CLS) synchrotron and the Bomem DA8 interferometer in Bologna. Overall, 13,288 inversion-rotation-vibration transitions in the $v_2$, $2v_2$, $v_4$, $3v_2$, and $v_2 + v_4$ cold bands, in the $2v_2 \leftarrow v_2$, $2v_2 \leftarrow v_4$, $v_4 \leftarrow v_2$, $v_4 \leftarrow 2v_2$, $3v_2 \leftarrow v_2$, $3v_2 \leftarrow 2v_2$, $3v_2 \leftarrow v_4$, $3v_2 \leftarrow v_2 + v_4$, $v_2 + v_4 \leftarrow v_2$, $v_2 + v_4 \leftarrow 2v_2$, $v_2 + v_4 \leftarrow 3v_2$ and $v_2 + v_4 \leftarrow v_4$ hot bands, and the inversion-rotation transitions in the studied excited states have been assigned and analyzed [446,447].

This analyses allowed creating a model that can reproduce the line positions of the assigned transitions well within experimen-
tal uncertainties. This was achieved through a fit with an effective Hamiltonian that included all symmetry-allowed interactions between (and within) the studied excited states. This Hamiltonian is derived according to the most recent results of the principal isotopologue of ammonia [448,449]. In both studies [446,447] the experimental values have also been compared with their corresponding *ab initio* values calculated in Ref. [442].

The line positions calculated using the effective Hamiltonian are now included as part of HITRAN2020. The line intensity of each transition is calculated from the *ab initio* Einstein-A coefficients computed in Ref. [442] for $J \le 18$. The lower-state energies of Refs. [446,447] have been reduced by $0.7577 \text{ cm}^{-1}$ for HITRAN to account for the energy difference between the symmetric and anti-symmetric $J'' = K'' = 0$ levels of the ground vibrational state. Isotopologue dependence of the line shape parameters was ignored, and the same algorithms that were used for the principal isotopologue for self-, air- [443], H$_2$, He, CO$_2$ [393], and H$_2$O [49] broadening (and associated temperature dependencies) have also been applied for $^{15}$NH$_3$.

It should be noted, that the quantum number notation used for $^{15}$NH$_3$ is currently dissimilar to the principal isotopologue in HITRAN. The global quantum number notation used for $^{14}$NH$_3$ was updated in HITRAN2012 [15] to: $v_1, v_2, v_3, v_4, L_3, L_4, L, \Gamma_{vib}$ (refer to Table 7 of Ref. [15] for parameter descriptions, but note that the order of the parameters in the “.par” file is as specified here). However, the global quantum number notation for $^{15}$NH$_3$ remains as: $v_1, v_2, v_3, v_4, S$ (parameter descriptions are given in Table 7 of Ref. [13]). For future editions of HITRAN, the consistency of the quantum number notation between ammonia isotopologues will be addressed.

### 2.11.6. Future expansion above 12,000 cm$^{-1}$

The HITRAN2020 NH$_3$ data extend up to $10,349 \text{ cm}^{-1}$, but recent spectroscopic analyses have provided line lists into the NIR and visible regions. These works [450,451] will be considered for future updates of the database and are briefly summarized below.

Near $12,500 \text{ cm}^{-1}$, the positions and intensities of 1114 ammonia lines observed in the $12,491-12,810 \text{ cm}^{-1}$ region have been measured [450] using FTS absorption spectra.

In addition, Zobov et al. [451] have reported analysis of the green and red bands of NH$_3$ for the $15,200-15,700 \text{ cm}^{-1}$ and $17,950-18,250 \text{ cm}^{-1}$ spectral regions, respectively, based on reanalysed Kitt Peak archive absorption spectra from 1980. Assignments were made up to $J = 7$ and were used to refine the PES [430] used for the CoYuTe line list [431]. Irwin et al. [452] used the CoYuTe line list to model low- to medium-resolution spectra of Jupiter at visible wavelengths with promising results, although application to high-resolution studies is not recommended

### 2.12. HNO$_3$: nitric acid (molecule 12)

Unchanged.

### 2.13. OH: hydroxyl radical (molecule 13)

The OH radical is a key species in atmospheric science. In the terrestrial troposphere, OH is the most important oxidizing agent for organic molecules, and in the upper atmosphere produces prominent airglow emission. Atmospheric OH abundances are measured by pure rotational transitions [453,454], vibration-rotation transitions (Meinel bands) [455], and by the $A^2\Sigma^+ - X^2\Pi$ electronic transition [456]. OH is also prominent in many astronomical objects [457,458] and is a strong emitter in high-temperature sources such as flames [459]. The line positions of the Meinel bands ($X^2\Pi$ vibration-rotation bands) of $^{16}$OH were updated using mainly calculated values from the PGOPHER [460] fit

of Brooke et al. [461] for $v = 0-13$ based on the earlier [462] analysis. Many data sources were included from the literature including some $B^2\Sigma^+ - X^2\Pi$ bands for the high vibrational levels ($v > 10$). The maximum rotational levels reported were extrapolated to $5-10$ $J$ values beyond the last observed level and range from 60.5 for $v = 0$ to 13.5 for $v = 13$.

Noll et al. [455] analyzed Meinel band airglow spectra recorded with the astronomical echelle spectrograph UVES at the European Southern Observatory. The line positions agreed very well with the line list provided by Brooke et al. [461] except for some transitions involving higher rotational levels of $v = 5$ and 6. The observed lines of Noll et al. were then included in the Brooke et al. fit using the same number of line parameters. The spectroscopic constants changed slightly, and a new line list was created with PGOPHER for HITRAN2020.

The line positions for the $A^2\Sigma^+ - X^2\Pi$ electronic transition of $^{16}$OH in HITRAN2020 are based on the analysis of Yousefi et al. [463]. Data from the literature was used for $v' = 0-4$ in the $A^2\Sigma^+$ state, and the $X^2\Pi$ state constants for $v'' = 0-9$ were held constant at the values determined by Brooke et al. [461]. The primary source of these data was the $A^2\Sigma^+ - X^2\Pi$ 0-0, 1-1, and 2-2 bands analyzed by Stark et al. [464] from FT emission spectra and recalibrated using the frequency comb measurements of Fast et al. [465]. PGOPHER was used to make a line list for all A-X bands with $\Delta v \le 5$ and extrapolated to 5-7 $J$'s higher than the observed values which range from $J = 12.5$ ($v' = 4$) to 30.5 ($v' = 0$).

The line strengths for the Meinel bands are those calculated by Brooke et al. [461] from two *ab initio* dipole moment functions and the RKR potential curve. The Herman-Wallis effect was taken into account by calculating N-dependent dipole matrix elements using LeRoy's LEVEL program [466]. LEVEL does not include electron spin (Hund's case (b) matrix elements) while PGOPHER uses J-dependent matrix elements (Hund's case (a)) as input. As described by Brooke et al., the LEVEL output was transformed into the Hund's case (a) dipole matrix elements for PGOPHER. The line list intensities were validated by comparison with the observed Herman-Wallis effect in the 2-0 band emission spectrum.

Noll et al. [455] compared the observed airglow intensities for many bands with the Brooke et al. Einstein-A values and found generally good agreement for the P- and R-branches, but the Q-branches showed discrepancies. These discrepancies vary considerably depending on the band; a recent check using the 2-0 band in the laboratory spectrum shows satisfactory agreement with the Brooke et al. values for the P-, Q-, and R-branches. The discrepancies reported in Ref. [455] are attributed to interactions with the $A^2\Sigma^+$ state that are not included in the intensity calculation. Work is continuing in order to improve the line strength values.

The line strengths for $A^2\Sigma^+ - X^2\Pi$ bands are from the calculations of Ref. [463]. This work uses the same methodology [467] as described above for the Meinel bands. A new *ab initio* transition dipole function was calculated and RKR potentials were used in LEVEL to compute N-dependent transition dipole matrix elements. The Herman-Wallis effect was included in the PGOPHER intensity calculations but, in contrast to the Meinel bands, was relatively small. The intensities in the line list were compared with a laboratory emission spectrum and with astronomical spectra: satisfactory agreement was obtained.

All line positions and intensities (of $^{16}$OH) for the Meinel bands and $A^2\Sigma^+ - X^2\Pi$ band have been updated for HITRAN2020, except for hyperfine transitions of the pure rational transitions. The $^{18}$OH and $^{16}$OD isotopologues remain unchanged.

The air-broadening coefficients applied to all additional OH Meinel band transitions follow the linearly dependent values (from $N = 1$ to 4) described in HITRAN1996 [11], with a constant value of $\gamma_{air} = 0.040 \text{ cm}^{-1}/\text{atm}$ for $N \ge 5$. For A-X transitions, the N-dependant values from Gillis et al. [468] are used (introduced
tal uncertainties. This was achieved through a fit with an effective Hamiltonian that included all symmetry-allowed interactions between (and within) the studied excited states. This Hamiltonian is derived according to the most recent results of the principal isotopologue of ammonia [448,449]. In both studies [446,447] the experimental values have also been compared with their corresponding *ab initio* values calculated in Ref. [442].

The line positions calculated using the effective Hamiltonian are now included as part of HITRAN2020. The line intensity of each transition is calculated from the *ab initio* Einstein-A coefficients computed in Ref. [442] for $J \le 18$. The lower-state energies of Refs. [446,447] have been reduced by $0.7577 \text{ cm}^{-1}$ for HITRAN to account for the energy difference between the symmetric and antisymmetric $J'' = K'' = 0$ levels of the ground vibrational state. Isotopologue dependence of the line shape parameters was ignored, and the same algorithms that were used for the principal isotopologue for self-, air- [443], H₂, He, CO₂ [393], and H₂O [49] broadening (and associated temperature dependencies) have also been applied for $^{15}$NH₃.

It should be noted, that the quantum number notation used for $^{15}$NH₃ is currently dissimilar to the principal isotopologue in HITRAN. The global quantum number notation used for $^{14}$NH₃ was updated in HITRAN2012 [15] to: $v_1, v_2, v_3, v_4, L_3, L_4, L, \Gamma_{vib}$ (refer to Table 7 of Ref. [15] for parameter descriptions, but note that the order of the parameters in the “.par” file is as specified here). However, the global quantum number notation for $^{15}$NH₃ remains as: $v_1, v_2, v_3, v_4, S$ (parameter descriptions are given in Table 7 of Ref. [13]). For future editions of HITRAN, the consistency of the quantum number notation between ammonia isotopologues will be addressed.

### 2.11.6. Future expansion above 12,000 cm⁻¹

The HITRAN2020 NH₃ data extend up to $10,349 \text{ cm}^{-1}$, but recent spectroscopic analyses have provided line lists into the NIR and visible regions. These works [450,451] will be considered for future updates of the database and are briefly summarized below.

Near $12,500 \text{ cm}^{-1}$, the positions and intensities of 1114 ammonia lines observed in the $12,491–12,810 \text{ cm}^{-1}$ region have been measured [450] using FTS absorption spectra.

In addition, Zobov et al. [451] have reported analysis of the green and red bands of NH₃ for the $15,200–15,700 \text{ cm}^{-1}$ and $17,950–18,250 \text{ cm}^{-1}$ spectral regions, respectively, based on reanalysed Kitt Peak archive absorption spectra from 1980. Assignments were made up to $J = 7$ and were used to refine the PES [430] used for the CoYuTe line list [431]. Irwin et al. [452] used the CoYuTe line list to model low- to medium-resolution spectra of Jupiter at visible wavelengths with promising results, although application to high-resolution studies is not recommended

### 2.12. HNO₃: nitric acid (molecule 12)

Unchanged.

### 2.13. OH: hydroxyl radical (molecule 13)

The OH radical is a key species in atmospheric science. In the terrestrial troposphere, OH is the most important oxidizing agent for organic molecules, and in the upper atmosphere produces prominent airglow emission. Atmospheric OH abundances are measured by pure rotational transitions [453,454], vibration-rotation transitions (Meinel bands) [455], and by the $A^2\Sigma^+-X^2\Pi$ electronic transition [456]. OH is also prominent in many astronomical objects [457,458] and is a strong emitter in high-temperature sources such as flames [459]. The line positions of the Meinel bands ($X^2\Pi$ vibration-rotation bands) of $^{16}$OH were updated using mainly calculated values from the PGOPHER [460] fit

of Brooke et al. [461] for $v = 0–13$ based on the earlier [462] analysis. Many data sources were included from the literature including some $B^2\Sigma^+-X^2\Pi$ bands for the high vibrational levels ($v > 10$). The maximum rotational levels reported were extrapolated to $5–10 \text{ J}$ values beyond the last observed level and range from 60.5 for $v = 0$ to 13.5 for $v = 13$.

Noll et al. [455] analyzed Meinel band airglow spectra recorded with the astronomical echelle spectrograph UVES at the European Southern Observatory. The line positions agreed very well with the line list provided by Brooke et al. [461] except for some transitions involving higher rotational levels of $v = 5$ and 6. The observed lines of Noll et al. were then included in the Brooke et al. fit using the same number of line parameters. The spectroscopic constants changed slightly, and a new line list was created with PGOPHER for HITRAN2020.

The line positions for the $A^2\Sigma^+-X^2\Pi$ electronic transition of $^{16}$OH in HITRAN2020 are based on the analysis of Yousefi et al. [463]. Data from the literature was used for $v' = 0–4$ in the $A^2\Sigma^+$ state, and the $X^2\Pi$ state constants for $v'' = 0–9$ were held constant at the values determined by Brooke et al. [461]. The primary source of these data was the $A^2\Sigma^+-X^2\Pi$ 0–0, 1–1, and 2–2 bands analyzed by Stark et al. [464] from FT emission spectra and recalibrated using the frequency comb measurements of Fast et al. [465]. PGOPHER was used to make a line list for all A-X bands with $\Delta v \le 5$ and extrapolated to 5-7 J's higher than the observed values which range from $J = 12.5$ ($v' = 4$) to 30.5 ($v' = 0$).

The line strengths for the Meinel bands are those calculated by Brooke et al. [461] from two *ab initio* dipole moment functions and the RKR potential curve. The Herman-Wallis effect was taken into account by calculating N-dependent dipole matrix elements using LeRoy's LEVEL program [466]. LEVEL does not include electron spin (Hund's case (b) matrix elements) while PGOPHER uses J-dependent matrix elements (Hund's case (a)) as input. As described by Brooke et al., the LEVEL output was transformed into the Hund's case (a) dipole matrix elements for PGOPHER. The line list intensities were validated by comparison with the observed Herman-Wallis effect in the 2-0 band emission spectrum.

Noll et al. [455] compared the observed airglow intensities for many bands with the Brooke et al. Einstein-A values and found generally good agreement for the P- and R-branches, but the Q-branches showed discrepancies. These discrepancies vary considerably depending on the band; a recent check using the 2-0 band in the laboratory spectrum shows satisfactory agreement with the Brooke et al. values for the P-, Q-, and R-branches. The discrepancies reported in Ref. [455] are attributed to interactions with the $A^2\Sigma^+$ state that are not included in the intensity calculation. Work is continuing in order to improve the line strength values.

The line strengths for $A^2\Sigma^+-X^2\Pi$ bands are from the calculations of Ref. [463]. This work uses the same methodology [467] as described above for the Meinel bands. A new *ab initio* transition dipole function was calculated and RKR potentials were used in LEVEL to compute N-dependent transition dipole matrix elements. The Herman-Wallis effect was included in the PGOPHER intensity calculations but, in contrast to the Meinel bands, was relatively small. The intensities in the line list were compared with a laboratory emission spectrum and with astronomical spectra: satisfactory agreement was obtained.

All line positions and intensities (of $^{16}$OH) for the Meinel bands and $A^2\Sigma^+-X^2\Pi$ band have been updated for HITRAN2020, except for hyperfine transitions of the pure rational transitions. The $^{18}$OH and $^{16}$OD isotopologues remain unchanged.

The air-broadening coefficients applied to all additional OH Meinel band transitions follow the linearly dependent values (from $N = 1$ to 4) described in HITRAN1996 [11], with a constant value of $\gamma_{air} = 0.040 \text{ cm}^{-1}/\text{atm}$ for $N \ge 5$. For A-X transitions, the N-dependant values from Gillis et al. [468] are used (introduced
as part of HITRAN2000 [12]), with a constant value of $γ_{air} = 0.0526 \text{ cm}^{-1}\text{/atm}$ for $N \ge 4$. For typical atmospheric modeling applications, the self-broadening contribution of OH is expected to be negligible; however a default estimated value of $γ_{self} = 0.30 \text{ cm}^{-1}\text{/atm}$ has been applied for all transitions to avoid null values. A value of $n_{air} = 0.66$ is applied for the temperature dependence of the air-broadened half-widths of all additional lines [11].

$H_2$- and He-broadening coefficients (and their temperature dependencies) for OH have also been estimated and introduced to HITRAN and are described in detail by Tan et al. [266].

The line list of OH described above was also used to update the HITEMP database [53]. An intensity threshold of $S = 1.0 \times 10^{-99}$ cm/molecule has been applied to the HITRAN2020 update, but all lines have been added to HITEMP.

## 2.14. HF: hydrogen fluoride (molecule 14)

Unchanged.

## 2.15. HCl: hydrogen chloride (molecule 15)

Unchanged.

## 2.16. HBr: hydrogen bromide (molecule 16)

Unchanged.

## 2.17. HI: hydrogen iodide (molecule 17)

Unchanged.

## 2.18. ClO: chlorine monoxide (molecule 18)

Unchanged.

## 2.19. OCS: carbonyl sulfide (molecule 19)

Carbonyl sulfide (OCS) is the most abundant sulfur-containing gas in the terrestrial atmosphere, with a tropospheric mixing ratio of about 500 ppt [469]. It makes a significant contribution to the formation of stratospheric sulfate aerosols and influences the radiative properties of the Earth's atmosphere, climate change, and stratospheric ozone concentration [470–474]. Carbonyl sulfide is released to the atmosphere by biomass burning, oceans, the oxidation of dimethyl sulfide and carbon disulfide ($CS_2$), and several anthropogenic sources (coal combustion, aluminum production, and sulfur recovery) [475,476]. OCS is also present in the atmosphere of Venus [477].

When retrieving OCS abundances from infrared atmospheric spectra measured by the JPL MKIV Fourier transform infrared (FTIR) spectrometer, Toon et al. [478] have identified that several bands of observable intensity were missing from the HITRAN database in the region of the strong $ν_3$ fundamental. To mitigate this deficiency, Toon et al. [478] used 709 unassigned lines, spectral parameters of which were inferred empirically from available laboratory data.

In the course of preparing the HITRAN2020 edition, it was recognized (based on the line positions) that many of these “missing” lines were present in the original unpublished line list from Brown and Fayt [479] (briefly explained in Ref. [12]) which formed a substantial bulk of the HITRAN2000 [12] data for the OCS molecule. These particular lines from Brown and Fayt [479] were, however, omitted from HITRAN because of some concerns regarding the accuracy of their intensities. They were included in the GEISA database [418] and given vibrational assignments. For HITRAN2020

these new bands for the $^{16}\text{O}^{12}\text{C}^{32}\text{S}$, $^{16}\text{O}^{12}\text{C}^{34}\text{S}$, $^{16}\text{O}^{13}\text{C}^{32}\text{S}$ isotopologues were therefore based on simultaneous analyses of the lines from Toon et al. [478] and previously unused data from Brown and Fayt [479]. Effectively, the line intensities from Ref. [12] were scaled to the experimental data [478] by factors of 1.1579 and 2.2592 for the 1111–1110 and 0331–0330 bands of the $^{16}\text{O}^{12}\text{C}^{32}\text{S}$ isotopologue, respectively. Moreover, in the course of analyzing lines from Toon et al. [478], 91 lines of the $ν_3$ band of the $^{16}\text{O}^{13}\text{C}^{34}\text{S}$ isotopologue (band center 2008.46 cm$^{-1}$) were identified, which were not present in Ref. [12], HITRAN, nor GEISA. Therefore, line positions and intensities of these lines have been fit to obtain spectroscopic parameters for this isotopologue. These parameters were used to compute the line positions and intensities of 221 lines up to $J = 110$. As a result of this analysis, the lines of the $^{16}\text{O}^{13}\text{C}^{34}\text{S}$ isotopologue now make their debut in HITRAN. The abundance of $4.675 \times 10^{-4}$ is calculated for this isotopologue with $Q(296K) = 2546.53$ [417]. Uncertainty codes 3 for the line positions and 3 for the line intensities (see Table 2) were used in the case of the new added bands. Fig. 26 gives an overview comparison between HITRAN2016 and the new added bands in HITRAN2020 in the 0–8000 cm$^{-1}$ spectral range.

### 2.19.1. He-, $H_2$-, and $CO_2$-broadening parameters

The He-broadened and $H_2$-broadened half-widths of OCS were added to the HITRAN2016 database using the semi-empirical models by Wilzewski et al. [393]. The recent experimental data by Hays et al. [480] (using an E-band chirped pulse spectrometer) showed that the previous values of $γ_{He}$ in HITRAN were approximately half the values obtained in these experiments. A new semi-empirical model, based on a Padé approximant (Eq. (1)), has been used to update the He-line broadening parameters for $J'' \le 64$. The new fit (which ignored the vibrational dependence) incorporated the early experimental results from Refs. [481,484] as well as the recent measurements by Hays et al. [480]. The $H_2$-broadened half-widths were also updated using a Padé approximant fitted to the data from Broquier et al. [483]. These $H_2$- and He-broadening for OCS will be described in detail in Ref. [266].

The $CO_2$-broadened half-widths of the OCS lines were also updated based on semi-empirical calculations from Ref. [485]. Their temperature dependencies were also updated based on the same work, introducing rotational dependence, while previously only a constant value was used.

## 2.20. $H_2CO$: formaldehyde (molecule 20)

### 2.20.1. He-, $H_2$-, and $CO_2$-broadening parameters

The He-broadening of formaldehyde ($H_2CO$) lines has been measured in the millimeter-wave spectral region in the early work of Nerf [486], and more recently, in the $2ν_5$ band by cavity-enhanced absorption spectroscopy [487], as well as two strongest transitions in the $ν_6$ band by direct absorption spectroscopy [488]. A Padé approximant (Eq. (1)) has been applied to fit the ratio of all collected experimental data to HITRAN air-broadening values as a function of the index $J + 0.2K_a$. It was then used to generate $γ_{He}$ for all $H_2CO$ lines in the database from the available air-broadened values.

As for $H_2$-broadening of $H_2CO$ lines, experiments were carried out in both the millimeter-wave region [486] and the 9–11 µm region [489]. A good agreement was claimed in comparison with these two experiments indicating no vibrational dependence in these data. A similar approach has been carried out to obtain $γ_{H_2}$ using the Padé approximant with additional fitting to an extra constrained point at a higher $J + 0.2K_a$ value.

The $CO_2$-broadening of two strongest transitions of $H_2CO$ in the $ν_6$ band was measured by Wang et al. [488]. These measurements were used to scale the air-broadening parameters in the line list to
as part of HITRAN2000 [12]), with a constant value of $γ_{air} = 0.0526 \text{ cm}^{-1}\text{/atm}$ for $N \ge 4$. For typical atmospheric modeling applications, the self-broadening contribution of OH is expected to be negligible; however a default estimated value of $γ_{self} = 0.30 \text{ cm}^{-1}\text{/atm}$ has been applied for all transitions to avoid null values. A value of $n_{air} = 0.66$ is applied for the temperature dependence of the air-broadened half-widths of all additional lines [11].

$H_2$- and He-broadening coefficients (and their temperature dependencies) for OH have also been estimated and introduced to HITRAN and are described in detail by Tan et al. [266].

The line list of OH described above was also used to update the HITEMP database [53]. An intensity threshold of $S = 1.0 \times 10^{-99}$ cm/molecule has been applied to the HITRAN2020 update, but all lines have been added to HITEMP.

2.14. HF: hydrogen fluoride (molecule 14)

Unchanged.

2.15. HCl: hydrogen chloride (molecule 15)

Unchanged.

2.16. HBr: hydrogen bromide (molecule 16)

Unchanged.

2.17. HI: hydrogen iodide (molecule 17)

Unchanged.

2.18. ClO: chlorine monoxide (molecule 18)

Unchanged.

2.19. OCS: carbonyl sulfide (molecule 19)

Carbonyl sulfide (OCS) is the most abundant sulfur-containing gas in the terrestrial atmosphere, with a tropospheric mixing ratio of about 500 ppt [469]. It makes a significant contribution to the formation of stratospheric sulfate aerosols and influences the radiative properties of the Earth's atmosphere, climate change, and stratospheric ozone concentration [470–474]. Carbonyl sulfide is released to the atmosphere by biomass burning, oceans, the oxidation of dimethyl sulfide and carbon disulfide ($CS_2$), and several anthropogenic sources (coal combustion, aluminum production, and sulfur recovery) [475,476]. OCS is also present in the atmosphere of Venus [477].

When retrieving OCS abundances from infrared atmospheric spectra measured by the JPL MKIV Fourier transform infrared (FTIR) spectrometer, Toon et al. [478] have identified that several bands of observable intensity were missing from the HITRAN database in the region of the strong $ν_3$ fundamental. To mitigate this deficiency, Toon et al. [478] used 709 unassigned lines, spectral parameters of which were inferred empirically from available laboratory data.

In the course of preparing the HITRAN2020 edition, it was recognized (based on the line positions) that many of these “missing” lines were present in the original unpublished line list from Brown and Fayt [479] (briefly explained in Ref. [12]) which formed a substantial bulk of the HITRAN2000 [12] data for the OCS molecule. These particular lines from Brown and Fayt [479] were, however, omitted from HITRAN because of some concerns regarding the accuracy of their intensities. They were included in the GEISA database [418] and given vibrational assignments. For HITRAN2020

these new bands for the $^{16}\text{O}^{12}\text{C}^{32}\text{S}$, $^{16}\text{O}^{12}\text{C}^{34}\text{S}$, $^{16}\text{O}^{13}\text{C}^{32}\text{S}$ isotopologues were therefore based on simultaneous analyses of the lines from Toon et al. [478] and previously unused data from Brown and Fayt [479]. Effectively, the line intensities from Ref. [12] were scaled to the experimental data [478] by factors of 1.1579 and 2.2592 for the 1111–1110 and 0331–0330 bands of the $^{16}\text{O}^{12}\text{C}^{32}\text{S}$ isotopologue, respectively. Moreover, in the course of analyzing lines from Toon et al. [478], 91 lines of the $ν_3$ band of the $^{16}\text{O}^{13}\text{C}^{34}\text{S}$ isotopologue (band center 2008.46 cm$^{-1}$) were identified, which were not present in Ref. [12], HITRAN, nor GEISA. Therefore, line positions and intensities of these lines have been fit to obtain spectroscopic parameters for this isotopologue. These parameters were used to compute the line positions and intensities of 221 lines up to $J = 110$. As a result of this analysis, the lines of the $^{16}\text{O}^{13}\text{C}^{34}\text{S}$ isotopologue now make their debut in HITRAN. The abundance of $4.675 \times 10^{-4}$ is calculated for this isotopologue with Q(296K) = 2546.53 [417]. Uncertainty codes 3 for the line positions and 3 for the line intensities (see Table 2) were used in the case of the new added bands. Fig. 26 gives an overview comparison between HITRAN2016 and the new added bands in HITRAN2020 in the 0–8000 cm$^{-1}$ spectral range.

2.19.1. He-, $H_2$-, and $CO_2$-broadening parameters

The He-broadened and $H_2$-broadened half-widths of OCS were added to the HITRAN2016 database using the semi-empirical models by Wilzewski et al. [393]. The recent experimental data by Hays et al. [480] (using an E-band chirped pulse spectrometer) showed that the previous values of $γ_{He}$ in HITRAN were approximately half the values obtained in these experiments. A new semi-empirical model, based on a Padé approximant (Eq. (1)), has been used to update the He-line broadening parameters for $J'' \le 64$. The new fit (which ignored the vibrational dependence) incorporated the early experimental results from Refs. [481,484] as well as the recent measurements by Hays et al. [480]. The $H_2$-broadened half-widths were also updated using a Padé approximant fitted to the data from Broquier et al. [483]. These $H_2$- and He-broadening for OCS will be described in detail in Ref. [266].

The $CO_2$-broadened half-widths of the OCS lines were also updated based on semi-empirical calculations from Ref. [485]. Their temperature dependencies were also updated based on the same work, introducing rotational dependence, while previously only a constant value was used.

2.20. $H_2CO$: formaldehyde (molecule 20)

2.20.1. He-, $H_2$-, and $CO_2$-broadening parameters

The He-broadening of formaldehyde ($H_2CO$) lines has been measured in the millimeter-wave spectral region in the early work of Nerf [486], and more recently, in the $2ν_5$ band by cavity-enhanced absorption spectroscopy [487], as well as two strongest transitions in the $ν_6$ band by direct absorption spectroscopy [488]. A Padé approximant (Eq. (1)) has been applied to fit the ratio of all collected experimental data to HITRAN air-broadening values as a function of the index $J + 0.2K_a$. It was then used to generate $γ_{He}$ for all $H_2CO$ lines in the database from the available air-broadened values.

As for $H_2$-broadening of $H_2CO$ lines, experiments were carried out in both the millimeter-wave region [486] and the 9–11 µm region [489]. A good agreement was claimed in comparison with these two experiments indicating no vibrational dependence in these data. A similar approach has been carried out to obtain $γ_{H_2}$ using the Padé approximant with additional fitting to an extra constrained point at a higher $J + 0.2K_a$ value.

The $CO_2$-broadening of two strongest transitions of $H_2CO$ in the $ν_6$ band was measured by Wang et al. [488]. These measurements were used to scale the air-broadening parameters in the line list to
<figure><img src="image_30.png" /><figcaption>**Fig. 26.** Overview of the line lists of $^{16}	ext{O}^{12}	ext{C}^{32}	ext{S}$, $^{16}	ext{O}^{12}	ext{C}^{34}	ext{S}$, $^{16}	ext{O}^{13}	ext{C}^{32}	ext{S}$, $^{16}	ext{O}^{12}	ext{C}^{33}	ext{S}$, $^{18}	ext{O}^{12}	ext{C}^{32}	ext{S}$, and $^{16}	ext{O}^{13}	ext{C}^{34}	ext{S}$ isotopologues in HITRAN2016 and the new added bands in HITRAN2020 in the 0–8000 cm$^{-1}$ range.</figcaption></figure>

obtain the estimates for CO$_2$-broadened half-widths. For the lack of measurements, the same temperature dependence as for air-broadening was used.

### 2.20.2. Future work

Formaldehyde has been the subject of a recent MARVEL study (see paper published as part of this special issue [490]). This study assembled and validated 16,596 non-redundant transitions from the literature, with a few newly measured as part of the study, giving 5029 empirical energy levels determined to high-resolution accuracy. These empirical levels were then used to replace the computed energy levels in the ExoMol AYTY line list [491] giving 367 779 transitions with empirically-determined wavenumbers of which 183,673 are more intense than the HITRAN cutoff at 296 K. This updated line list will be considered for a future HITRAN upgrade for formaldehyde.

### 2.21. HOCl: hypochlorous acid (molecule 21)

For typical atmospheric modeling applications, the self-broadening contribution of HOCl is expected to be negligible; however a default estimated value of $\gamma_{self} = 0.15$ cm$^{-1}$/atm has been applied for all transitions to avoid null values.

### 2.22. N$_2$: molecular nitrogen (molecule 22)

Unchanged.

### 2.23. HCN: hydrogen cyanide (molecule 23)

#### 2.23.1. H$^{12}$CN

HCN is a product of biomass burning in the Earth's atmosphere [492] and it is studied as a possible indicator for bacterial lung infection [493]. HCN, and its isomer HNC which is not included in HITRAN, are important astrophysical species. HCN has recently been detected in Pluto's atmosphere [494] and the atmosphere of exoplanet 55 Cancri e [495]. Isotopologue ratios such as H$^{12}$CN/H$^{13}$CN encode information on the thermal and chemical histories of a variety of solar system materials and provide insights into their origins [496].

For the main isotopologue H$^{12}$C$^{14}$N, the HITRAN entry has been expanded using data from the ExoMol [342] line list due to Barber et al. [497]. This line list is based on the *ab initio* transition intensities of Harris et al. [498] with empirical energy levels from the experiments of Mellau [499]. All lines stronger than 10$^{-30}$ cm/molecule at 296 K and wavenumbers up to 4001 cm$^{-1}$ not in HITRAN2016 were added. HITRAN2016 contained 58,109 lines; the new release more than doubles this number to 131,031 lines.

A new, significantly improved line list for H$^{12}$C$^{14}$N called "MOMENT-90" is published as part of this special issue [500]. A unique feature of this polyatomic line list comes from the fact that all the line positions, even for the very weak lines that are hardly observable experimentally, were derived from experimental energy levels obtained from the corresponding high-temperature studies [499]. At the same time, this large set of empirical vibrational-rotational energy levels made it possible to improve the fitting procedure used to determine the PES and the nonadiabatic correction used in the variational calculations. The new intensities show many differences from the intensities given in HITRAN2016. The accuracy of the calculated line intensities were demonstrated using a variety of absorption and emission spectra. This line list will form the basis for a future update and is highly recommended for practical usage

#### 2.23.2. H$^{13}$CN

The spectrum of H$^{13}$C$^{14}$N at wavenumbers below 3405 cm$^{-1}$ was included in HITRAN2004 based on the experiments of Maki et al. [501] and Maiwald et al. [502]. Similar to the subsequent updates in HITRAN for the parent isotopologue, we add many new hot and combinationa bands for H$^{13}$C$^{14}$N effectively extending the data to higher wavenumbers based on the use of empirically corrected variational line lists. A new line list was computed using the semi-empirical potential energy surface (PES) and *ab initio* dipole moment surface (DMS) by Makhnev et al. [503]. The update considered wavenumbers up to 8000 cm$^{-1}$ and, limited to transitions which have intensity greater than 10$^{-29}$ cm/molecule at 296 K (at natural abundance). Wavenumbers for the majority of these lines were generated using the empirical energy levels of Ref. [504] but the transition intensities are the *ab initio* ones. These intensities can be considered fairly reliable based on comparisons with the
<figure><img src="image_28.png" /><figcaption>**Fig. 26.** Overview of the line lists of ¹⁶O¹²C³²S, ¹⁶O¹²C³⁴S, ¹⁶O¹³C³²S, ¹⁶O¹²C³³S, ¹⁸O¹²C³²S, and ¹⁶O¹³C³⁴S isotopologues in HITRAN2016 and the new added bands in HITRAN2020 in the 0–8000 cm⁻¹ range.</figcaption></figure>

obtain the estimates for CO₂-broadened half-widths. For the lack of measurements, the same temperature dependence as for air-broadening was used.

### 2.20.2. Future work

Formaldehyde has been the subject of a recent MARVEL study (see paper published as part of this special issue [490]). This study assembled and validated 16,596 non-redundant transitions from the literature, with a few newly measured as part of the study, giving 5029 empirical energy levels determined to high-resolution accuracy. These empirical levels were then used to replace the computed energy levels in the ExoMol AYTY line list [491] giving 367 779 transitions with empirically-determined wavenumbers of which 183,673 are more intense than the HITRAN cutoff at 296 K. This updated line list will be considered for a future HITRAN upgrade for formaldehyde.

### 2.21. HOCl: hypochlorous acid (molecule 21)

For typical atmospheric modeling applications, the self-broadening contribution of HOCl is expected to be negligible; however a default estimated value of $γ_{self} = 0.15$ cm⁻¹/atm has been applied for all transitions to avoid null values.

### 2.22. N₂: molecular nitrogen (molecule 22)

Unchanged.

### 2.23. HCN: hydrogen cyanide (molecule 23)

#### 2.23.1. H¹²CN

HCN is a product of biomass burning in the Earth's atmosphere [492] and it is studied as a possible indicator for bacterial lung infection [493]. HCN, and its isomer HNC which is not included in HITRAN, are important astrophysical species. HCN has recently been detected in Pluto's atmosphere [494] and the atmosphere of exoplanet 55 Cancri e [495]. Isotopologue ratios such as H¹²CN/H¹³CN encode information on the thermal and chemical histories of a variety of solar system materials and provide insights into their origins [496].

For the main isotopologue H¹²C¹⁴N, the HITRAN entry has been expanded using data from the ExoMol [342] line list due to Barber et al. [497]. This line list is based on the *ab initio* transition intensities of Harris et al. [498] with empirical energy levels from the experiments of Mellau [499]. All lines stronger than 10⁻³⁰ cm/molecule at 296 K and wavenumbers up to 4001 cm⁻¹ not in HITRAN2016 were added. HITRAN2016 contained 58,109 lines; the new release more than doubles this number to 131,031 lines.

A new, significantly improved line list for H¹²C¹⁴N called "MOMENT-90" is published as part of this special issue [500]. A unique feature of this polyatomic line list comes from the fact that all the line positions, even for the very weak lines that are hardly observable experimentally, were derived from experimental energy levels obtained from the corresponding high-temperature studies [499]. At the same time, this large set of empirical vibrational-rotational energy levels made it possible to improve the fitting procedure used to determine the PES and the nonadiabatic correction used in the variational calculations. The new intensities show many differences from the intensities given in HITRAN2016. The accuracy of the calculated line intensities were demonstrated using a variety of absorption and emission spectra. This line list will form the basis for a future update and is highly recommended for practical usage

#### 2.23.2. H¹³CN

The spectrum of H¹³C¹⁴N at wavenumbers below 3405 cm⁻¹ was included in HITRAN2004 based on the experiments of Maki et al. [501] and Maiwald et al. [502]. Similar to the subsequent updates in HITRAN for the parent isotopologue, we add many new hot and combinationa bands for H¹³C¹⁴N effectively extending the data to higher wavenumbers based on the use of empirically corrected variational line lists. A new line list was computed using the semi-empirical potential energy surface (PES) and *ab initio* dipole moment surface (DMS) by Makhnev et al. [503]. The update considered wavenumbers up to 8000 cm⁻¹ and, limited to transitions which have intensity greater than 10⁻²⁹ cm/molecule at 296 K (at natural abundance). Wavenumbers for the majority of these lines were generated using the empirical energy levels of Ref. [504] but the transition intensities are the *ab initio* ones. These intensities can be considered fairly reliable based on comparisons with the
<figure><img src="image_31.png" /><figcaption>Fig. 27. Comparison of the new line list for H$^{13}$CN and that from the HITRAN2016 edition$^{-1}$.</figcaption></figure>

recent experimental study by Guay et al. [505] which used a free-
running, all-fiber dual electro-optic frequency comb system. An
overview of the H$^{13}$C$^{14}$N line list in HITRAN2020 is given in Fig. 27.

2.23.3. H₂- and He-broadening parameters

There are a number of experimental measurements regarding
He-broadening in early works [506–510]; however there is gen-
erally poor agreement between studies. A critical evaluation has
been made to filter selected data, and the rotational distribution
of the He-broadening line width (γ<sub>He</sub>) is described using a Padé
approximant (Eq. (1)) with J'' ≤ 16.

The microwave transient emission technique has been used
to study the *l*-doublet transitions of HCN with H₂-broadening
[507,510]. Later, frequency-stabilized tunable diode laser spectrom-
eters have been used to study the H₂-broadening line width in
the *v*₂ band [508,511]. More recently, new experimental measure-
ments of H₂-broadening in the millimeter-wave region have been
reported [512]. Comprehensive comparisons with all experimental
results exhibit a strong rotational dependence, while they appear
to be vibrationally independent. A similar procedure has been used
to derive the semi-empirical H₂-broadening line width (γH₂) based
on the Padé approximant for transitions with *J*'' ≤ 31.

2.24. CH<sub>3</sub>Cl: methyl chloride (molecule 24)

Unchanged.

2.25. H₂O₂: hydrogen peroxide (molecule 25)

For the self-broadening half-width (which previously had values
of zero), a default estimate value of 0.3 cm⁻¹/atm has now been
assigned to all transitions for this molecule.

2.26. C₂H₂: acetylene (molecule 26)

Acetylene is a minor trace gas in the terrestrial atmosphere,
primarily originating from combustion sources, and therefore its
concentration is highly correlated with carbon monoxide [513]. It
is also present in the atmospheres of solar system gas giants and
their satellites [514–516]. The acetylene (C₂H₂) molecule is a pro-
totype system for molecular dynamics with a very rich IR spectra
as a consequence of strong couplings between vibrational modes.
However, the NIR spectral region in HITRAN had been missing
many spectral details.

The global modeling of the C₂H₂ spectrum in the frame of the polyad model has been developed at ULB-Brussels [517], and IAO-Tomsk [518,519]. The approximate relations of the vibrational modes give rise to polyads such that P = 5V₁ + 3V₂ + 5V₃ + V₄ + V₅ (where Vᵢ are the vibrational normal mode quantum numbers, with i = 1 – 5). The vibrational assignments used for C₂H₂ in previous editions of HITRAN only indicated the total bending angular momentum, |l₄ + l₅|. However, as many more bands have been added to HITRAN in recent additions, it has become necessary to separate the bending angular momentum quantum numbers to avoid degeneracy and allow unique identification. For HITRAN2020, the vibrational assignment has been updated for all acetylene transitions of the three isotopologues so that V₁, V₂, V₃, V₄, V₅, l₄, l₅,+/-, u/g quantum numbers are now used to identify each vibrational state (see the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the ".par" format).

For HITRAN2016, numerous bands were added in the 13–248 cm⁻¹ and 390–634 cm⁻¹ spectral regions. The last region was supplemented and extended (in the 390–893 cm⁻¹ region) based on the recent ΔP = 1 work of Jacquemart et al. [520]. This work used a multi-spectrum analysis of FTS measurements, and line intensities were measured for 18 bands (only four of them previously reported). The very good predictability of the theoretical model developed in IAO-Tomsk has been used to include new hot bands in this region as well as to improve line positions and/or intensities of the existing HITRAN2016 bands.

Comparisons to N₂-broadened (1 atm) PNNL spectra [244] of C₂H₂ for the beginning of the ΔP = 6 region indicated absorption features of some bands not present in HITRAN. Based on the global model developed in IAO-Tomsk [519], 8 hot bands have been added to HITRAN between 3738 and 3996 cm⁻¹ (see Table 9). Note that under atmospheric pressures, Q-branches of acetylene are affected by line-mixing. This line-mixing is especially the case for the intense Q-branches of ¹²C₂H₂ located at 730, 3881, 3896 and 4090 cm⁻¹. Using a Voigt line profile for these Q-branches in atmospheric retrievals will lead to systematic residuals due to line-mixing. Studies are in progress to model line-mixing effects for Q-branches of acetylene under atmospheric conditions and will be considered for updates to HITRAN.

Lyulin and Campargue [521] collected together the recent ex-
perimental studies covering the 5850–6341 cm⁻¹ and 7000–9415
cm⁻¹ spectral regions that used FTS measurements for the stronger
<figure><img src="image_29.png" /><figcaption>Fig. 27. Comparison of the new line list for H$^{13}$CN and that from the HITRAN2016 edition$^{-1}$.</figcaption></figure>

recent experimental study by Guay et al. [505] which used a free-
running, all-fiber dual electro-optic frequency comb system. An
overview of the H$^{13}$C$^{14}$N line list in HITRAN2020 is given in Fig. 27.

2.23.3. H₂- and He-broadening parameters

There are a number of experimental measurements regarding
He-broadening in early works [506–510]; however there is gen-
erally poor agreement between studies. A critical evaluation has
been made to filter selected data, and the rotational distribution
of the He-broadening line width (γ<sub>He</sub>) is described using a Padé
approximant (Eq. (1)) with J'' ≤ 16.

The microwave transient emission technique has been used
to study the *l*-doublet transitions of HCN with H₂-broadening
[507,510]. Later, frequency-stabilized tunable diode laser spectrom-
eters have been used to study the H₂-broadening line width in
the *v*₂ band [508,511]. More recently, new experimental measure-
ments of H₂-broadening in the millimeter-wave region have been
reported [512]. Comprehensive comparisons with all experimental
results exhibit a strong rotational dependence, while they appear
to be vibrationally independent. A similar procedure has been used
to derive the semi-empirical H₂-broadening line width (γH₂) based
on the Padé approximant for transitions with *J*'' ≤ 31.

2.24. CH<sub>3</sub>Cl: methyl chloride (molecule 24)

Unchanged.

2.25. H₂O₂: hydrogen peroxide (molecule 25)

For the self-broadening half-width (which previously had values
of zero), a default estimate value of 0.3 cm⁻¹/atm has now been
assigned to all transitions for this molecule.

2.26. C₂H₂: acetylene (molecule 26)

Acetylene is a minor trace gas in the terrestrial atmosphere,
primarily originating from combustion sources, and therefore its
concentration is highly correlated with carbon monoxide [513]. It
is also present in the atmospheres of solar system gas giants and
their satellites [514–516]. The acetylene (C₂H₂) molecule is a pro-
totype system for molecular dynamics with a very rich IR spectra
as a consequence of strong couplings between vibrational modes.
However, the NIR spectral region in HITRAN had been missing
many spectral details.

The global modeling of the C₂H₂ spectrum in the frame of the polyad model has been developed at ULB-Brussels [517], and IAO-Tomsk [518,519]. The approximate relations of the vibrational modes give rise to polyads such that P = 5V₁ + 3V₂ + 5V₃ + V₄ + V₅ (where Vᵢ are the vibrational normal mode quantum numbers, with i = 1 – 5). The vibrational assignments used for C₂H₂ in previous editions of HITRAN only indicated the total bending angular momentum, |l₄ + l₅|. However, as many more bands have been added to HITRAN in recent additions, it has become necessary to separate the bending angular momentum quantum numbers to avoid degeneracy and allow unique identification. For HITRAN2020, the vibrational assignment has been updated for all acetylene transitions of the three isotopologues so that V₁, V₂, V₃, V₄, V₅, l₄, l₅,+/-, u/g quantum numbers are now used to identify each vibrational state (see the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the ".par" format).

For HITRAN2016, numerous bands were added in the 13–248 cm⁻¹ and 390–634 cm⁻¹ spectral regions. The last region was supplemented and extended (in the 390–893 cm⁻¹ region) based on the recent ΔP = 1 work of Jacquemart et al. [520]. This work used a multi-spectrum analysis of FTS measurements, and line intensities were measured for 18 bands (only four of them previously reported). The very good predictability of the theoretical model developed in IAO-Tomsk has been used to include new hot bands in this region as well as to improve line positions and/or intensities of the existing HITRAN2016 bands.

Comparisons to N₂-broadened (1 atm) PNNL spectra [244] of C₂H₂ for the beginning of the ΔP = 6 region indicated absorption features of some bands not present in HITRAN. Based on the global model developed in IAO-Tomsk [519], 8 hot bands have been added to HITRAN between 3738 and 3996 cm⁻¹ (see Table 9). Note that under atmospheric pressures, Q-branches of acetylene are affected by line-mixing. This line-mixing is especially the case for the intense Q-branches of ¹²C₂H₂ located at 730, 3881, 3896 and 4090 cm⁻¹. Using a Voigt line profile for these Q-branches in atmospheric retrievals will lead to systematic residuals due to line-mixing. Studies are in progress to model line-mixing effects for Q-branches of acetylene under atmospheric conditions and will be considered for updates to HITRAN.

Lyulin and Campargue [521] collected together the recent ex-
perimental studies covering the 5850–6341 cm⁻¹ and 7000–9415
cm⁻¹ spectral regions that used FTS measurements for the stronger
<caption>**Table 9**</caption>

Hot bands of acetylene included in HITRAN between 3738 and 3996 cm⁻¹. N is the number of transitions per band, v<sub>min</sub> and v<sub>max</sub> are the minimum and maximum wavenumbers (in cm⁻¹), and S<sub>sum</sub> is the sum of line intensities (in 10⁻²⁰ cm/molecule).

<table><thead><tr><th>Upper state<sup>a</sup></th><th>Lower state<sup>a</sup></th><th>N</th><th>v<sub>min</sub></th><th>v<sub>max</sub></th><th>S<sub>sum</sub></th></tr></thead><tbody><tr><td>01031 3 -1 u</td><td>00010 1 0 g</td><td>263</td><td>3744</td><td>3969</td><td>0.58</td></tr><tr><td>00120 2 0 u</td><td>00010 1 0 g</td><td>250</td><td>3770</td><td>3984</td><td>0.58</td></tr><tr><td>01031 1 -1 ±u</td><td>00010 1 0 g</td><td>205</td><td>3794</td><td>3977</td><td>0.36</td></tr><tr><td>00120 0 0 +u</td><td>00010 1 0 g</td><td>101</td><td>3817</td><td>3975</td><td>0.24</td></tr><tr><td>00111 1 1 g</td><td>00001 0 1 u</td><td>240</td><td>3772</td><td>3980</td><td>0.20</td></tr><tr><td>00111 1 -1 ±g</td><td>00001 0 1 u</td><td>233</td><td>3780</td><td>3975</td><td>0.16</td></tr><tr><td>01022 2 0 g</td><td>00001 0 1 u</td><td>220</td><td>3756</td><td>3963</td><td>0.13</td></tr><tr><td>01022 2 -2 ±g</td><td>00001 0 1 u</td><td>251</td><td>3749</td><td>3959</td><td>0.12</td></tr></tbody></table>

bands [522–524] and Cavity Ring Down Spectroscopy (CRDS) for the weaker absorption windows between bands [525–527]. Additional CRDS [528] and FTS [529,530] studies covered the 5693–5882 cm⁻¹ and 9280–10,740 cm⁻¹ spectral ranges, respectively. Following Ref. [521], these studies have been compiled into an empirical line list for HITRAN. The line list includes numerous ¹²C₂H₂ and ¹²C¹³CH₂ bands that have not previously been included in HITRAN. Fig. 28 displays the significant number of additional bands that have been included in the NIR for HITRAN2020.

As part of the ExoMol project [531], the “aCeTY” line list has been built for ¹²C₂H₂ [532] using the MARVEL acetylene database wherever possible [533]. Comparisons of the aCeTY line list and global model developed in IAO-Tomsk with measurements in the ΔP = 1 region are presented in Jacquemart et al. [520]. The line lists from aCeTY and the ASD-1000 database [518] are intended for use at high temperature, therefore further comparisons between models and measurements will be investigated for updates to HITEMP [53].

In addition, H₂⁻, He- and CO₂-broadening coefficients that were introduced to HITRAN by Wilzewski et al. [393] have been extended to the new transitions for C₂H₂.

A small number of Einstein-A coefficients and statistical weights have also been corrected for some of the bands of C₂H₂.

## 2.27. C₂H₆: ethane (molecule 27)

Ethane (C₂H₆) is the most abundant non-methane hydrocarbon (NMHC) in the atmosphere of the outer planets [534] and Titan [535], playing an important role as a tracer of atmospheric chemistry and dynamics. Ethane is also an important constituent of comets and their gaseous envelopes [536]. The relative abundance of isotopic species of ethane, such as D/H ratio from C₂H₅D/C₂H₆, can carry valuable information about the atmospheric formation and chemical evolution. In this work, we have expanded the ethane line list in HITRAN to include the v₅, v₇ and underlying combination bands of ¹²C₂H₆ and the v₄, v₁₂, and 2v₆ bands of ¹²C₂H₅D from recent model predictions validated through a laboratory study.

### 2.27.1. Region of v₅ and v₇ fundamentals (2800–3071 cm⁻¹)

Until this present edition, HITRAN contained only strong Q-branch lines of the v₇ band in the spectral region around 3.3 µm. Nevertheless, these lines alone are insufficient to correctly interpret atmospheric and planetary spectra and a better high-resolution spectroscopic model was needed. This spectral range is dominated by the C-H stretching fundamental of v₅ (parallel band) and v₇ (degenerate perpendicular band), and the v₈ + v₁₁ combination band (v₈ and v₁₁ are the degenerate antisymmetric and symmetric deformations of the two methyl groups, respectively). The characterization of rotational structure in this complex molecule is non-trivial because the v₇ band is severely perturbed by overtones and combination states (with a low-frequency torsional mode, v₄ at 289 cm⁻¹) that are in Fermi or Coriolis resonance with v₇ [537].

For the HITRAN2020 edition, we expand and advance the v₇ band at 3.3 µm based on Refs. [536,538], add a linelist for the v₅ band of ethane at 3.4 µm based on Radeva et al. [539], and add combination bands that include the strong v₈ + v₁₁ band based on Lattanzi et al. [538]. These references, and a summary of how their data were adapted to HITRAN, are described below.

### 2.27.2. Line list from Ref. [538]

Relying on a high-resolution FTS spectrum recorded at 229 K in Brussels and line positions measured in a Doppler-limited spectrum recorded at 119 K using a tunable difference-frequency laser spectrometer [538,540], the 2860–3060 cm⁻¹ region of ethane was re-investigated.

This work led to some progress in the understanding of the complex network of interacting vibrational levels occurring in this

<figure><img src="image_32.png" /><figcaption>**Fig. 28.** An overview of the NIR bands of C₂H₂ that have been added to HITRAN based on FTS and CRDS measurements [521–530].</figcaption></figure>

<footnote><sup>a</sup>The upper and lower vibrational states have the format V₁, V₂, V₃, V₄, V₅, I₄, I₅, ±, u/g. The full FORTRAN descriptors for the global and local quanta are provided in the Supplementary Material.</footnote>
<caption>**Table 9**</caption>

Hot bands of acetylene included in HITRAN between 3738 and 3996 cm⁻¹. N is the number of transitions per band, v<sub>min</sub> and v<sub>max</sub> are the minimum and maximum wavenumbers (in cm⁻¹), and S<sub>sum</sub> is the sum of line intensities (in 10⁻²⁰ cm/molecule).

<table><thead><tr><th>Upper state<sup>a</sup></th><th>Lower state<sup>a</sup></th><th>N</th><th>v<sub>min</sub></th><th>v<sub>max</sub></th><th>S<sub>sum</sub></th></tr></thead><tbody><tr><td>01031 3 -1 u</td><td>00010 1 0 g</td><td>263</td><td>3744</td><td>3969</td><td>0.58</td></tr><tr><td>00120 2 0 u</td><td>00010 1 0 g</td><td>250</td><td>3770</td><td>3984</td><td>0.58</td></tr><tr><td>01031 1 -1 ±u</td><td>00010 1 0 g</td><td>205</td><td>3794</td><td>3977</td><td>0.36</td></tr><tr><td>00120 0 0 +u</td><td>00010 1 0 g</td><td>101</td><td>3817</td><td>3975</td><td>0.24</td></tr><tr><td>00111 1 1 g</td><td>00001 0 1 u</td><td>240</td><td>3772</td><td>3980</td><td>0.20</td></tr><tr><td>00111 1 -1 ±g</td><td>00001 0 1 u</td><td>233</td><td>3780</td><td>3975</td><td>0.16</td></tr><tr><td>01022 2 0 g</td><td>00001 0 1 u</td><td>220</td><td>3756</td><td>3963</td><td>0.13</td></tr><tr><td>01022 2 -2 ±g</td><td>00001 0 1 u</td><td>251</td><td>3749</td><td>3959</td><td>0.12</td></tr></tbody></table>

bands [522–524] and Cavity Ring Down Spectroscopy (CRDS) for the weaker absorption windows between bands [525–527]. Additional CRDS [528] and FTS [529,530] studies covered the 5693–5882 cm⁻¹ and 9280–10,740 cm⁻¹ spectral ranges, respectively. Following Ref. [521], these studies have been compiled into an empirical line list for HITRAN. The line list includes numerous ¹²C₂H₂ and ¹²C¹³CH₂ bands that have not previously been included in HITRAN. Fig. 28 displays the significant number of additional bands that have been included in the NIR for HITRAN2020.

As part of the ExoMol project [531], the “aCeTY” line list has been built for ¹²C₂H₂ [532] using the MARVEL acetylene database wherever possible [533]. Comparisons of the aCeTY line list and global model developed in IAO-Tomsk with measurements in the ΔP = 1 region are presented in Jacquemart et al. [520]. The line lists from aCeTY and the ASD-1000 database [518] are intended for use at high temperature, therefore further comparisons between models and measurements will be investigated for updates to HITEMP [53].

In addition, H₂⁻, He- and CO₂-broadening coefficients that were introduced to HITRAN by Wilzewski et al. [393] have been extended to the new transitions for C₂H₂.

A small number of Einstein-A coefficients and statistical weights have also been corrected for some of the bands of C₂H₂.

## 2.27. C₂H₆: ethane (molecule 27)

Ethane (C₂H₆) is the most abundant non-methane hydrocarbon (NMHC) in the atmosphere of the outer planets [534] and Titan [535], playing an important role as a tracer of atmospheric chemistry and dynamics. Ethane is also an important constituent of comets and their gaseous envelopes [536]. The relative abundance of isotopic species of ethane, such as D/H ratio from C₂H₅D/C₂H₆, can carry valuable information about the atmospheric formation and chemical evolution. In this work, we have expanded the ethane line list in HITRAN to include the v₅, v₇ and underlying combination bands of ¹²C₂H₆ and the v₄, v₁₂, and 2v₆ bands of ¹²C₂H₅D from recent model predictions validated through a laboratory study.

### 2.27.1. Region of v₅ and v₇ fundamentals (2800–3071 cm⁻¹)

Until this present edition, HITRAN contained only strong Q-branch lines of the v₇ band in the spectral region around 3.3 µm. Nevertheless, these lines alone are insufficient to correctly interpret atmospheric and planetary spectra and a better high-resolution spectroscopic model was needed. This spectral range is dominated by the C-H stretching fundamental of v₅ (parallel band) and v₇ (degenerate perpendicular band), and the v₈ + v₁₁ combination band (v₈ and v₁₁ are the degenerate antisymmetric and symmetric deformations of the two methyl groups, respectively). The characterization of rotational structure in this complex molecule is non-trivial because the v₇ band is severely perturbed by overtones and combination states (with a low-frequency torsional mode, v₄ at 289 cm⁻¹) that are in Fermi or Coriolis resonance with v₇ [537].

For the HITRAN2020 edition, we expand and advance the v₇ band at 3.3 µm based on Refs. [536,538], add a linelist for the v₅ band of ethane at 3.4 µm based on Radeva et al. [539], and add combination bands that include the strong v₈ + v₁₁ band based on Lattanzi et al. [538]. These references, and a summary of how their data were adapted to HITRAN, are described below.

### 2.27.2. Line list from Ref. [538]

Relying on a high-resolution FTS spectrum recorded at 229 K in Brussels and line positions measured in a Doppler-limited spectrum recorded at 119 K using a tunable difference-frequency laser spectrometer [538,540], the 2860–3060 cm⁻¹ region of ethane was re-investigated.

This work led to some progress in the understanding of the complex network of interacting vibrational levels occurring in this

<figure><img src="image_30.png" /><figcaption>**Fig. 28.** An overview of the NIR bands of C₂H₂ that have been added to HITRAN based on FTS and CRDS measurements [521–530].</figcaption></figure>

<footnote><sup>a</sup>The upper and lower vibrational states have the format V₁, V₂, V₃, V₄, V₅, I₄, I₅, ±, u/g. The full FORTRAN descriptors for the global and local quanta are provided in the Supplementary Material.</footnote>
energy range (see Fig. 2 of Lattanzi et al. [538]). In particular, 572 line positions belonging to $^pP$ and $^rR$ transitions in the $v_7$ band (maximum $J = 30$), $^rQ_0$, $^pP_1$ and $^rR_1$ transitions in the $v_8 + v_{11}$ band, and $^pP_6$ transitions in the $v_3 + 2v_4 + v_8$ band were least-squares fit to a Hamiltonian. The model involved the $v_7$ degenerate vibrational level and four degenerate perturbers, i.e., the $v_8 + v_{11}$, $v_3 + 2v_4 + v_8$, $v_4 + v_{11} + v_{12}$ and $v_3 + 3v_4 + v_{12}$ vibrational levels. Although RMS deviations as large as 0.018 cm⁻¹ were obtained, indicating that the analysis is far from complete, a line list was generated because it still provided a much improved description of the 3.3 µm region of the ethane spectrum. Positions, relative intensities, and lower-state energies of 4969 lines associated with transitions belonging to five perpendicular bands ($v_8 + v_{11}$, $v_4 + v_{11} + v_{12}$, $v_3 + 3v_4 + v_{12}$, $v_8 + v_{11}$ and $v_3 + 2v_4 + v_8$) were calculated between 2900 and 3071 cm⁻¹, relying on the model and parameters involved therein and resulting from the least squares analysis. The content of the line list is summarized in Table 8 of Ref. [538]. As detailed in Ref. [538], incorrectly predicted line positions were recomputed using empirical upper-state energies. These altered positions are indicated by the HITRAN error code of 4 (see Table 2), while a conservative error code of 2 was assigned to the remaining predicted positions. The predicted relative line intensities were normalized by inspection of observed and calculated spectra (HITRAN error code = 2). The Lattanzi et al. [538] line list covers the 2900–3071 cm⁻¹ region.

### 2.27.3. Line lists from Refs. [536,539]

These models of $v_5$ and $v_7$ were generated by characterizing the upper ro-vibrational states using linear progressions of $J$ and $K$. For the ground vibrational state, spectroscopic constants from Pine and Lafferty [537] were used, with specific corrections for some $J/K$ ladders (see details in Villanueva et al. [536]). For the $v_5$ model, as explained in Radeva et al. [539], the upper-state rotational constants were not present in the literature. Therefore they were obtained by fitting experimental data given in Ref. [541] for each $K$ ladder. For the band intensity of the $v_5$ band, parameters reported in Ref. [542] were employed.

The $v_7$ upper-state ro-vibrational structure was derived by fitting to experimental data as presented in Ref. [543], in which cross-sections for ethane in the 3-µm region at temperatures between 194 and 297 K and total pressures from 0.0689 Torr to 763.48 Torr were reported. Using this dataset, we identified 466 lines, which were consolidated with 122 lines reported in Ref. [542] and 66 reported in Ref. [541], ultimately deriving rotational constants for 30 $K$-ladders of the $v_7$ band of ethane. Our model does provide good results for the selected lines (standard deviation of 0.005 cm⁻¹ for the 654 lines), but because of the numerous perturbations, their validity is relatively uncertain.

Determining accurate band intensities from experimental data in this highly active spectral region can be complex, in particular for ethane at 3.3 µm, since multiple fundamental (e.g., $v_7$ and $v_5$), combination (e.g., $v_8 + v_{11}$), and hot-bands (e.g., $v_7 + v_4 - v_4$) overlap at these wavelengths. As reported in Ref. [543], accurate absorption cross sections for ethane at these wavelengths were determined, with an overall uncertainty of 4%. Their cross-sections were calibrated against PNNL spectra [244]. Considering these new absorption cross-sections and taking into account the first torsional hot-band, we derived a band intensity of 301 cm⁻²atm⁻¹ for the $v_7$ band [536].

### 2.27.4. Combining the line lists based on validations against laboratory data

The three line lists described above were cross-evaluated against each other, HITRAN2016 data, and the experimental cross-sections from Refs. [543,544]. To that end, HAPI [52] was used

to generate cross-sections under the same thermodynamic conditions and resolution as experimental data and the synthetic cross-sections were compared with the experimental ones. It was found that data from Refs. [536,538] both agree quite well with the experimental data near the $v_7$ band center, with both line lists being superior to the HITRAN2016 data except for the region around the $^pQ_7$ manifold near 2976 cm⁻¹ where HITRAN was based on purely empirical data from Pine and Lafferty [537]. As rotational quanta increase, Ref. [538] produced much better agreement with the experimental data. At around 3070 cm⁻¹, the deviations of the Ref. [536] line list from experimental data becomes so significant (up to 0.5 cm⁻¹) that it was decided to not use this list in the 3071–3100 cm⁻¹ interval, which is not available in Ref. [538]. In summary, the $v_7$ band and combination bands were taken from Ref. [538]. However in selected spectral windows where the residuals based on the Ref. [536] data were better, the latter line list was employed. In the small spectral window around 2976 cm⁻¹ HITRAN2016 data were retained (although several lines had to be reassigned to the $v_8 + v_{11}$ band).

Only the Ref. [539] line list is available for the $v_5$ band. Validations have shown substantially larger disagreements than those observed with either of the line lists in the $v_7$ band. Some notable modifications were therefore applied to the line list from Ref. [539]. First, intensities for all the lines have been reduced by 20% to better agree with both sets of experimental cross-sections. The line positions for many lines with $K > 1$ appeared to strongly deviate from their observed values. For instance, the deviations from experimental values for lines with $K=2$ ranged from 0.004 cm⁻¹ (for $J=2$) up to 0.17 cm⁻¹ (for $J=21$). We therefore applied a third order polynomial correction in $J$ to adjust the line positions of transitions with $K = 2$ and 3, but further refinements are needed in the future. Considering the rapidly growing deviations (with rotational quanta), the Ref. [539] line list was also truncated by applying an intensity cutoff of 10⁻²⁴ cm/molecule (as opposed to 10⁻³³ cm/molecule used in the original line list). After these modifications the resulting line list produced satisfactory agreement with laboratory cross-sections. However, further improvements in this region, including addition of the hot bands, would clearly be beneficial.

Fig. 29 provides an overview of the ethane spectra in the 3.3-µm spectral region, showing experimental cross-sections from Ref. [544] in the lower panel, and those generated with HAPI using HITRAN2016 and HITRAN2020.

### 2.27.5. $^{12}$CH$_3$ $^{12}$CH$_2$D

Mono-deuterated ethane is the third most abundant isotope of ethane, with a HITRAN abundance of 9.131×10⁻⁴ [57]. The deuterium substitution reduces the symmetry, which results in significantly more transitions being visible in the mid-infrared. It also slightly offsets the bright series of Q-branches around 2980 cm⁻¹, which are characteristic for $C_2H_6$, allowing the possibility of remote observations of the D/H ratio in this spectral range. Doney et al. [545] determined line positions and relative intensities of transitions in the C-D (centered around 2170 cm⁻¹) and C-H (2850–3030 cm⁻¹) stretches, capturing the $v_4$ band (2170 cm⁻¹), the $2v_7$ band (2770 cm⁻¹), as well as a series of bands between 2850 and 3030 cm⁻¹ (2$v_{14}$, 2$v_6$, 2$v_5$, $v_1$, $v_2$ and $v_{12}$). The assignments were made by employing *ab initio* CCSD(T)/ANO1 calculations. The study was based on spectra recorded at high resolution using a Bruker IFS-125HR spectrometer equipped with a cryogenic Herriot cell at JPL [546,547]. For the assignments in Ref. [545], spectra were recorded at 85 K, at very low pressures below 0.0022 Torr with a pathlength of 20.941 m. The model includes transitions up to $J'' \le 22$, $K''_a \le 10$ and $K''_c \le 18$, with uncertainties of the order of ~0.05 cm⁻¹. Although the model captures most of the strong transitions, further work is needed to refine the description
energy range (see Fig. 2 of Lattanzi et al. [538]). In particular, 572 line positions belonging to $^pP$ and $^rR$ transitions in the $v_7$ band (maximum $J=30$), $^rQ_0$, $^pP_1$ and $^rR_1$ transitions in the $v_8 + v_{11}$ band, and $^pP_6$ transitions in the $v_3 + 2v_4 + v_8$ band were least-squares fit to a Hamiltonian. The model involved the $v_7$ degenerate vibrational level and four degenerate perturbers, i.e., the $v_8 + v_{11}$, $v_3 + 2v_4 + v_8$, $v_4 + v_{11} + v_{12}$ and $v_3 + 3v_4 + v_{12}$ vibrational levels. Although RMS deviations as large as 0.018 cm⁻¹ were obtained, indicating that the analysis is far from complete, a line list was generated because it still provided a much improved description of the 3.3 µm region of the ethane spectrum. Positions, relative intensities, and lower-state energies of 4969 lines associated with transitions belonging to five perpendicular bands ($v_8 + v_{11}$, $v_4 + v_{11} + v_{12}$, $v_3 + 3v_4 + v_{12}$, $v_8 + v_{11}$ and $v_3 + 2v_4 + v_8$) were calculated between 2900 and 3071 cm⁻¹, relying on the model and parameters involved therein and resulting from the least squares analysis. The content of the line list is summarized in Table 8 of Ref. [538]. As detailed in Ref. [538], incorrectly predicted line positions were recomputed using empirical upper-state energies. These altered positions are indicated by the HITRAN error code of 4 (see Table 2), while a conservative error code of 2 was assigned to the remaining predicted positions. The predicted relative line intensities were normalized by inspection of observed and calculated spectra (HITRAN error code = 2). The Lattanzi et al. [538] line list covers the 2900–3071 cm⁻¹ region.

### 2.27.3. Line lists from Refs. [536,539]

These models of $v_5$ and $v_7$ were generated by characterizing the upper ro-vibrational states using linear progressions of $J$ and $K$. For the ground vibrational state, spectroscopic constants from Pine and Lafferty [537] were used, with specific corrections for some $J/K$ ladders (see details in Villanueva et al. [536]). For the $v_5$ model, as explained in Radeva et al. [539], the upper-state rotational constants were not present in the literature. Therefore they were obtained by fitting experimental data given in Ref. [541] for each $K$ ladder. For the band intensity of the $v_5$ band, parameters reported in Ref. [542] were employed.

The $v_7$ upper-state ro-vibrational structure was derived by fitting to experimental data as presented in Ref. [543], in which cross-sections for ethane in the 3-µm region at temperatures between 194 and 297 K and total pressures from 0.0689 Torr to 763.48 Torr were reported. Using this dataset, we identified 466 lines, which were consolidated with 122 lines reported in Ref. [542] and 66 reported in Ref. [541], ultimately deriving rotational constants for 30 $K$-ladders of the $v_7$ band of ethane. Our model does provide good results for the selected lines (standard deviation of 0.005 cm⁻¹ for the 654 lines), but because of the numerous perturbations, their validity is relatively uncertain.

Determining accurate band intensities from experimental data in this highly active spectral region can be complex, in particular for ethane at 3.3 µm, since multiple fundamental (e.g., $v_7$ and $v_5$), combination (e.g., $v_8 + v_{11}$), and hot-bands (e.g., $v_7 + v_4 - v_4$) overlap at these wavelengths. As reported in Ref. [543], accurate absorption cross sections for ethane at these wavelengths were determined, with an overall uncertainty of 4%. Their cross-sections were calibrated against PNNL spectra [244]. Considering these new absorption cross-sections and taking into account the first torsional hot-band, we derived a band intensity of 301 cm⁻²atm⁻¹ for the $v_7$ band [536].

### 2.27.4. Combining the line lists based on validations against laboratory data

The three line lists described above were cross-evaluated against each other, HITRAN2016 data, and the experimental cross-sections from Refs. [543,544]. To that end, HAPI [52] was used

to generate cross-sections under the same thermodynamic conditions and resolution as experimental data and the synthetic cross-sections were compared with the experimental ones. It was found that data from Refs. [536,538] both agree quite well with the experimental data near the $v_7$ band center, with both line lists being superior to the HITRAN2016 data except for the region around the $^pQ_7$ manifold near 2976 cm⁻¹ where HITRAN was based on purely empirical data from Pine and Lafferty [537]. As rotational quanta increase, Ref. [538] produced much better agreement with the experimental data. At around 3070 cm⁻¹, the deviations of the Ref. [536] line list from experimental data becomes so significant (up to 0.5 cm⁻¹) that it was decided to not use this list in the 3071–3100 cm⁻¹ interval, which is not available in Ref. [538]. In summary, the $v_7$ band and combination bands were taken from Ref. [538]. However in selected spectral windows where the residuals based on the Ref. [536] data were better, the latter line list was employed. In the small spectral window around 2976 cm⁻¹ HITRAN2016 data were retained (although several lines had to be reassigned to the $v_8 + v_{11}$ band).

Only the Ref. [539] line list is available for the $v_5$ band. Validations have shown substantially larger disagreements than those observed with either of the line lists in the $v_7$ band. Some notable modifications were therefore applied to the line list from Ref. [539]. First, intensities for all the lines have been reduced by 20% to better agree with both sets of experimental cross-sections. The line positions for many lines with $K>1$ appeared to strongly deviate from their observed values. For instance, the deviations from experimental values for lines with $K=2$ ranged from 0.004 cm⁻¹ (for $J=2$) up to 0.17 cm⁻¹ (for $J=21$). We therefore applied a third order polynomial correction in $J$ to adjust the line positions of transitions with $K=2$ and 3, but further refinements are needed in the future. Considering the rapidly growing deviations (with rotational quanta), the Ref. [539] line list was also truncated by applying an intensity cutoff of 10⁻²⁴ cm/molecule (as opposed to 10⁻³³ cm/molecule used in the original line list). After these modifications the resulting line list produced satisfactory agreement with laboratory cross-sections. However, further improvements in this region, including addition of the hot bands, would clearly be beneficial.

Fig. 29 provides an overview of the ethane spectra in the 3.3-µm spectral region, showing experimental cross-sections from Ref. [544] in the lower panel, and those generated with HAPI using HITRAN2016 and HITRAN2020.

### 2.27.5. $^{12}$CH$_3$ $^{12}$CH$_2$D

Mono-deuterated ethane is the third most abundant isotope of ethane, with a HITRAN abundance of 9.131×10⁻⁴ [57]. The deuterium substitution reduces the symmetry, which results in significantly more transitions being visible in the mid-infrared. It also slightly offsets the bright series of Q-branches around 2980 cm⁻¹, which are characteristic for $C_2H_6$, allowing the possibility of remote observations of the D/H ratio in this spectral range. Doney et al. [545] determined line positions and relative intensities of transitions in the C-D (centered around 2170 cm⁻¹) and C-H (2850–3030 cm⁻¹) stretches, capturing the $v_4$ band (2170 cm⁻¹), the $2v_7$ band (2770 cm⁻¹), as well as a series of bands between 2850 and 3030 cm⁻¹ (2$v_{14}$, 2$v_6$, 2$v_5$, $v_1$, $v_2$ and $v_{12}$). The assignments were made by employing *ab initio* CCSD(T)/ANO1 calculations. The study was based on spectra recorded at high resolution using a Bruker IFS-125HR spectrometer equipped with a cryogenic Herriot cell at JPL [546,547]. For the assignments in Ref. [545], spectra were recorded at 85 K, at very low pressures below 0.0022 Torr with a pathlength of 20.941 m. The model includes transitions up to $J'' \le 22$, $K''_a \le 10$ and $K''_c \le 18$, with uncertainties of the order of ~0.05 cm⁻¹. Although the model captures most of the strong transitions, further work is needed to refine the description
<figure><img src="image_33.png" /><figcaption>**Fig. 29.** The ethane spectra in the 3.3-µm spectral region, showing experimental cross-sections from Ref. [544] in the lower panel, and those generated with HAPI (under the same thermodynamic conditions) using HITRAN2016 and HITRAN2020 in the top and middle panels, respectively.</figcaption></figure>

of weak transitions in the 2850–3030 cm⁻¹ region. No hot bands are included in this line list.

For inclusion into HITRAN, the intensities of the C₂H₅D transitions have been calibrated against additional experimental spectra recorded using the same setup, but at higher pressures (2.023 and 0.1367 Torr), shorter path lengths (0.2038 and 0.1526 m), but at intermediate cold and room temperatures (130 and 298 K). This line list will be provided as one of the immediate updates to the official release of HITRAN2020.

### 2.27.6. Line-shape parameters

For all of the new bands of ethane (including the deuterated isotopologue) self- and air-broadening half-widths, and their temperature dependences, were estimated using the expressions reported by Devi et al. [548,549] from measurements in the Q-branch of the ν₉ band near 822 cm⁻¹. The parameters involved in these expressions were applied from $K'' = 0$ to $K_{max} = 3$ for the

broadening coefficients and $K'' = 0$ to $K_{max} = 7$ for their temperature dependence, while those provided for $K_{max}$ were used for transitions with $K'' > K_{max}$. The uncertainties for these pressure-induced coefficients are conservatively set (error code = 2, see Table 2) with the warning that the uncertainty is unknown for $J'' > 31$. Finally, a constant value of -0.004 cm⁻¹atm⁻¹ (error code = 1) was estimated for air pressure induced shifts, from the average of two air-broadening measurements at 296 K for rQ₀ and pQ₃ of the ν₇ band [541]. For pressure-shifts, we considered the N₂-broadened pressure-induced shifts of -0.004 cm⁻¹atm⁻¹ reported in Ref. [541] from rQ₀ and pQ₃.

For the ν₄ torsional band at 35 µm region [550], the temperature dependence exponent of the air-broadened line half-widths, $n_{N_2}$, listed in HITRAN2016 [16] had a truncation error which removed the integer part when the exponent is greater than 1. This issue has been fixed for HITRAN2020. In addition, the self-broadening values in that band were previously given as a con-
<figure><img src="image_31.png" /><figcaption>**Fig. 29.** The ethane spectra in the 3.3-µm spectral region, showing experimental cross-sections from Ref. [544] in the lower panel, and those generated with HAPI (under the same thermodynamic conditions) using HITRAN2016 and HITRAN2020 in the top and middle panels, respectively.</figcaption></figure>

of weak transitions in the 2850–3030 cm⁻¹ region. No hot bands are included in this line list.

For inclusion into HITRAN, the intensities of the C₂H₅D transitions have been calibrated against additional experimental spectra recorded using the same setup, but at higher pressures (2.023 and 0.1367 Torr), shorter path lengths (0.2038 and 0.1526 m), but at intermediate cold and room temperatures (130 and 298 K). This line list will be provided as one of the immediate updates to the official release of HITRAN2020.

### 2.27.6. Line-shape parameters

For all of the new bands of ethane (including the deuterated isotopologue) self- and air-broadening half-widths, and their temperature dependences, were estimated using the expressions reported by Devi et al. [548,549] from measurements in the Q-branch of the ν₉ band near 822 cm⁻¹. The parameters involved in these expressions were applied from $K'' = 0$ to $K_{max} = 3$ for the

broadening coefficients and $K'' = 0$ to $K_{max} = 7$ for their temperature dependence, while those provided for $K_{max}$ were used for transitions with $K'' > K_{max}$. The uncertainties for these pressure-induced coefficients are conservatively set (error code = 2, see Table 2) with the warning that the uncertainty is unknown for $J'' > 31$. Finally, a constant value of -0.004 cm⁻¹atm⁻¹ (error code = 1) was estimated for air pressure induced shifts, from the average of two air-broadening measurements at 296 K for rQ₀ and pQ₃ of the ν₇ band [541]. For pressure-shifts, we considered the N₂-broadened pressure-induced shifts of -0.004 cm⁻¹atm⁻¹ reported in Ref. [541] from rQ₀ and pQ₃.

For the ν₄ torsional band at 35 µm region [550], the temperature dependence exponent of the air-broadened line half-widths, $n_{N_2}$, listed in HITRAN2016 [16] had a truncation error which removed the integer part when the exponent is greater than 1. This issue has been fixed for HITRAN2020. In addition, the self-broadening values in that band were previously given as a con-
stant, while in HITRAN2020 they correspond to those in Ref. [548] as was originally intended.

## 2.28. PH3: phosphine (molecule 28)

On Earth, phosphine (PH₃) is a trace constituent of the lower troposphere with very low, but highly variable, atmospheric concentrations [551]. PH₃ has also long been observed in the atmospheres of Jupiter and Saturn due to prominent spectral IR features [552] and is used as a tracer for tropospheric dynamics in gas giant planets [553]. While PH₃ is associated with anaerobic ecosystems of Earth, and notoriously toxic for humans [554], it has also been proposed as a potential biosignature gas in anoxic exoplanets [555].

Recently, a tentative detection of the R(0) rotational transition in the atmosphere of Venus using mm-wave observations [556] has prompted significant discussion relating to the chemical (and suggested biological) production pathways that can account for the observed concentration. However, a number of followup studies [557–560] have cast substantial doubt on the original detection and conclusions. Nevertheless, these recent works have contributed to a surge of interest in accurate PH₃ spectroscopic parameters.

The IR spectrum of PH₃ forms distinct polyad bands due to the approximate relationship of the vibrational modes: $v_1 \approx 2v_2 \approx v_3 \approx 2v_4 \approx v_2 + v_4$. For HITRAN2020, the line positions and intensities in the dyad (750–1500 cm⁻¹), pentad (1750–2600 cm⁻¹) and octad (2750–3650 cm⁻¹) spectral regions have been extended or updated, while pure rotational transitions remain unchanged from HITRAN2016 [16].

Kleiner and Devi [561] produced an extensive line list covering the pentad region, based upon the experimental measurements of Refs. [562,563] that were described in HITRAN2016. The Ref. [561] line list consists of 9894 transitions of the $2v_2$, $2v_4$, $v_2 + v_4$, $v_1$, $v_3$ bands. Line positions were obtained by fitting 3403 experimental measurements with a Hamiltonian including the interactions within the $V_2 = 2$, $V_4 = 2$, $V_2 = V_4 = 1$, $V_1 = 1$ and $V_3 = 1$ upper states (up to $J = 14$) [563], which were combined with empirically-determined transition frequencies calculated from accurate ground-state energy levels [564]. Line intensities were based on a fit to 1579 selected transitions [563] with RMS deviations of 7.7%. The methods used for both energy level and intensity fittings are described in Ref. [565].

The octad region has been analysed by Nikitin et al. [566] between 2733–3660 cm⁻¹ using a global approach. This analysis builds on a previous model for the lower three polyads [567], which was noted to have poor simulation of spectra above 3100 cm⁻¹. The new model extended the assignments in Ref. [568] for the octad region and has been validated against FTS spectra [244,568,569] with a quoted RMS deviation of 0.0018 cm⁻¹ and 11% for the positions and intensities, respectively.

Recently, Rey et al. [570] have produced an updated vibration-rotation line list of PH₃ in several steps. As PH₃ is a semi-rigid molecule without large amplitude vibrations, the normal mode representation provides an adequate description of the nuclear motions. At the first step, the full nuclear motion Hamiltonian was built in the Eckart-Watson form [571] from the PES reported by Nikitin et al. [572] using the reduction techniques described in Ref. [573]. For a full account of symmetry, the energy levels and transitions were computed by the variational method using the irreducible tensor operators following the technique of Refs. [574,575]. At the second step, the ab initio PES was empirically optimized to match precisely the four observed fundamental band origins. The line intensities were computed from the ab initio DMS of Nikitin et al. [576]. The RMS deviations for energy levels up to $J = 20$ between the variational calculations and the empirically-fitted effective Hamiltonian were 0.04, 0.05 and 0.07 cm⁻¹ for the dyad, pentad and octad, respectively.

Additionally, to improve the accuracy of the line positions, we have followed the strategy successfully applied for methane line lists [309], which consists of making empirical corrections using a set of experimental vibration-rotation energy levels. Finally, a data set composed of 14,400 energy levels up to the octad ($J_{max} = 25$) and obtained from previous analyses [566–568] was used to provide empirical corrections for more than 100,000 line positions. The corresponding line list is available via the TheoReTS web site [310]. To determine the preferred line intensities and positions for each spectral region, comparisons have been made to high-resolution FTS spectra recorded at PNNL covering the dyad [577] and pentad [562] regions (resolutions of 0.0020 cm⁻¹ and 0.0115 cm⁻¹, respectively), along with those obtained from the McMath-Pierce FTS at Kitt Peak [568] for the octad region (0.0115 cm⁻¹ resolution). Further FTS measurements from PNNL [244], which include all three polyads at lower resolution (0.112 cm⁻¹), have also been used.

For the dyad region, the $v_2$, $v_4$ and $2v_2-v_2$ bands from HITRAN2016 have been supplemented with the $v_2 + v_4 - v_4$, $v_2 + v_4 - v_2$ and $2v_4 - v_4$ hot bands from Rey et al. [570].

The line list of Kleiner and Devi [561] was intended to be used for updating the pentad region in HITRAN2016. However, the recent theoretical work of Ref. [570] was shown to provide a significant improvement toward the edges of the pentad region as higher rotational levels (i.e., $J'' > 14$, $K'' > 12$) were not included in the analysis of Ref. [561]. Fig. 30 demonstrates the significant improvement when compared to PNNL spectra at 25°C [244]. The positions and intensities of Ref. [570] have therefore been adopted for all pentad transitions in HITRAN2020.

In HITRAN2016, the octad region primarily constituted unassigned empirical lines, which restricts the applicable temperature range. The line lists provided by Nikitin et al. [566,570] are fully assigned and compared to HITRAN2016 in their ability to reproduce observations. It should be noted that comparisons for the $3v_2$ band are hindered by low signal to noise in the PNNL spectra. High-resolution comparisons demonstrated that the Ref. [566] list yields the best performance with smallest RMS residuals across the octad region and has been used to update HITRAN. The $4v_2-v_2$ hot band from HITRAN2016 has been retained as these lines were observed in the spectra of Ref. [568]. The octad region of the PH₃ now includes quantum assignments for all transitions.

Sousa-Silva et al. [578] refined an earlier PES [579] to produce the “SAITY” line list covering the spectral range up 10 000 cm⁻¹ and temperatures up to 1500 K. A calculated spectrum for the pentad region is included in the comparisons of Fig. 30. While SAITY is primarily intended for high-temperature simulations, these line predictions can be used to advance the assignment of experimental spectra. Furthermore, a MARVEL project is in progress which will allow many of the lines in SAITY to reach experimental accuracy.

There is potential for further empirical improvements to line positions in the dyad, pentad and octad regions. This analysis will be considered for future updates for PH₃.

### 2.28.1. Line-shape parameters of PH3

The air-broadening half-widths introduced in HITRAN2008 [14] have been extended to all new transitions and are based on scaled N₂-broadening measurements [568,580–582]. The linear relationship for the temperature dependence exponents of the air-broadening coefficients $n_{air} = 0.702 - 0.01J''$ [582,583] has also been extended to all new transitions. For HITRAN2020, all self-broadening half-widths have been updated with the method described in Ref. [566] for the octad region, using an empirical function developed for the $v_3$ band [562]. The self-broadening coefficients are given by

$$ \gamma_{self} = 0.1172 - 9.257 \times 10^{-5} [J_m(J_m + 1) + K_m^2] \quad (3) $$
stant, while in HITRAN2020 they correspond to those in Ref. [548] as was originally intended.

## 2.28. PH₃: phosphine (molecule 28)

On Earth, phosphine (PH₃) is a trace constituent of the lower troposphere with very low, but highly variable, atmospheric concentrations [551]. PH₃ has also long been observed in the atmospheres of Jupiter and Saturn due to prominent spectral IR features [552] and is used as a tracer for tropospheric dynamics in gas giant planets [553]. While PH₃ is associated with anaerobic ecosystems of Earth, and notoriously toxic for humans [554], it has also been proposed as a potential biosignature gas in anoxic exoplanets [555].

Recently, a tentative detection of the R(0) rotational transition in the atmosphere of Venus using mm-wave observations [556] has prompted significant discussion relating to the chemical (and suggested biological) production pathways that can account for the observed concentration. However, a number of followup studies [557–560] have cast substantial doubt on the original detection and conclusions. Nevertheless, these recent works have contributed to a surge of interest in accurate PH₃ spectroscopic parameters.

The IR spectrum of PH₃ forms distinct polyad bands due to the approximate relationship of the vibrational modes: $v_1 \approx 2v_2 \approx v_3 \approx 2v_4 \approx v_2 + v_4$. For HITRAN2020, the line positions and intensities in the dyad (750–1500 cm⁻¹), pentad (1750–2600 cm⁻¹) and octad (2750–3650 cm⁻¹) spectral regions have been extended or updated, while pure rotational transitions remain unchanged from HITRAN2016 [16].

Kleiner and Devi [561] produced an extensive line list covering the pentad region, based upon the experimental measurements of Refs. [562,563] that were described in HITRAN2016. The Ref. [561] line list consists of 9894 transitions of the $2v_2$, $2v_4$, $v_2 + v_4$, $v_1$, $v_3$ bands. Line positions were obtained by fitting 3403 experimental measurements with a Hamiltonian including the interactions within the $V_2 = 2$, $V_4 = 2$, $V_2 = V_4 = 1$, $V_1 = 1$ and $V_3 = 1$ upper states (up to $J = 14$) [563], which were combined with empirically-determined transition frequencies calculated from accurate ground-state energy levels [564]. Line intensities were based on a fit to 1579 selected transitions [563] with RMS deviations of 7.7%. The methods used for both energy level and intensity fittings are described in Ref. [565].

The octad region has been analysed by Nikitin et al. [566] between 2733–3660 cm⁻¹ using a global approach. This analysis builds on a previous model for the lower three polyads [567], which was noted to have poor simulation of spectra above 3100 cm⁻¹. The new model extended the assignments in Ref. [568] for the octad region and has been validated against FTS spectra [244,568,569] with a quoted RMS deviation of 0.0018 cm⁻¹ and 11% for the positions and intensities, respectively.

Recently, Rey et al. [570] have produced an updated vibration-rotation line list of PH₃ in several steps. As PH₃ is a semi-rigid molecule without large amplitude vibrations, the normal mode representation provides an adequate description of the nuclear motions. At the first step, the full nuclear motion Hamiltonian was built in the Eckart-Watson form [571] from the PES reported by Nikitin et al. [572] using the reduction techniques described in Ref. [573]. For a full account of symmetry, the energy levels and transitions were computed by the variational method using the irreducible tensor operators following the technique of Refs. [574,575]. At the second step, the ab initio PES was empirically optimized to match precisely the four observed fundamental band origins. The line intensities were computed from the ab initio DMS of Nikitin et al. [576]. The RMS deviations for energy levels up to $J = 20$ between the variational calculations and the empirically-fitted effective Hamiltonian were 0.04, 0.05 and 0.07 cm⁻¹ for the dyad, pentad and octad, respectively.

Additionally, to improve the accuracy of the line positions, we have followed the strategy successfully applied for methane line lists [309], which consists of making empirical corrections using a set of experimental vibration-rotation energy levels. Finally, a data set composed of 14,400 energy levels up to the octad ($J_{max} = 25$) and obtained from previous analyses [566–568] was used to provide empirical corrections for more than 100,000 line positions. The corresponding line list is available via the TheoReTS web site [310]. To determine the preferred line intensities and positions for each spectral region, comparisons have been made to high-resolution FTS spectra recorded at PNNL covering the dyad [577] and pentad [562] regions (resolutions of 0.0020 cm⁻¹ and 0.0115 cm⁻¹, respectively), along with those obtained from the McMath-Pierce FTS at Kitt Peak [568] for the octad region (0.0115 cm⁻¹ resolution). Further FTS measurements from PNNL [244], which include all three polyads at lower resolution (0.112 cm⁻¹), have also been used.

For the dyad region, the $v_2$, $v_4$ and $2v_2-v_2$ bands from HITRAN2016 have been supplemented with the $v_2 + v_4 - v_4$, $v_2 + v_4 - v_2$ and $2v_4 - v_4$ hot bands from Rey et al. [570].

The line list of Kleiner and Devi [561] was intended to be used for updating the pentad region in HITRAN2016. However, the recent theoretical work of Ref. [570] was shown to provide a significant improvement toward the edges of the pentad region as higher rotational levels (i.e., $J'' > 14$, $K'' > 12$) were not included in the analysis of Ref. [561]. Fig. 30 demonstrates the significant improvement when compared to PNNL spectra at 25°C [244]. The positions and intensities of Ref. [570] have therefore been adopted for all pentad transitions in HITRAN2020.

In HITRAN2016, the octad region primarily constituted unassigned empirical lines, which restricts the applicable temperature range. The line lists provided by Nikitin et al. [566,570] are fully assigned and compared to HITRAN2016 in their ability to reproduce observations. It should be noted that comparisons for the $3v_2$ band are hindered by low signal to noise in the PNNL spectra. High-resolution comparisons demonstrated that the Ref. [566] list yields the best performance with smallest RMS residuals across the octad region and has been used to update HITRAN. The $4v_2-v_2$ hot band from HITRAN2016 has been retained as these lines were observed in the spectra of Ref. [568]. The octad region of the PH₃ now includes quantum assignments for all transitions.

Sousa-Silva et al. [578] refined an earlier PES [579] to produce the “SAITY” line list covering the spectral range up 10 000 cm⁻¹ and temperatures up to 1500 K. A calculated spectrum for the pentad region is included in the comparisons of Fig. 30. While SAITY is primarily intended for high-temperature simulations, these line predictions can be used to advance the assignment of experimental spectra. Furthermore, a MARVEL project is in progress which will allow many of the lines in SAITY to reach experimental accuracy.

There is potential for further empirical improvements to line positions in the dyad, pentad and octad regions. This analysis will be considered for future updates for PH₃.

### 2.28.1. Line-shape parameters of PH₃

The air-broadening half-widths introduced in HITRAN2008 [14] have been extended to all new transitions and are based on scaled N₂-broadening measurements [568,580–582]. The linear relationship for the temperature dependence exponents of the air-broadening coefficients $n_{air} = 0.702 - 0.01J''$ [582,583] has also been extended to all new transitions. For HITRAN2020, all self-broadening half-widths have been updated with the method described in Ref. [566] for the octad region, using an empirical function developed for the $v_3$ band [562]. The self-broadening coefficients are given by

$$ \gamma_{self} = 0.1172 - 9.257 \times 10^{-5} [J_m(J_m + 1) + K_m^2] \quad (3) $$
<figure><img src="image_34.png" /><figcaption>**Fig. 30.** Spectra for the pentad region of PH₃ compared to the PNNL absorption cross sections at 25°C [244]. The upper panel displays calculated spectra using the HITRAN2020 (see text for details), HITRAN2016 [16], Kleiner and Devi [561] and Sousa-Silva et al. [578] line lists. The lower panel displays the obs-calc residuals, where the observation corresponds to the PNNL spectrum and calculated spectra are identified by the legend (all are shown in the upper panel).</figcaption></figure>

where $J_m$ and $K_m$ refer to the maximum values of $J$ and $K$, respectively, for each transition. A minimum value of $\gamma_{self} = 0.05 \text{ cm}^{-1}/\text{atm}$ is applied when the predicted values become too small.

Due to the presence of PH₃ in the atmospheres of Jupiter and Saturn, there have been numerous studies aimed at measuring the H₂- and He-broadening half-widths as well as their temperature dependencies [562,568,580,582–591]. For HITRAN, H₂- and He-broadening coefficients and temperature dependencies have been introduced for PH₃, which will be described in detail in Ref. [266].

While updating HITRAN, the Einstein-A coefficients for all E symmetry transitions have been corrected.

Line-mixing coefficients for phosphine have not been introduced to HITRAN yet, although values are available from experimental [562,589] and theoretical [591] studies. This will be considered for the future.

## 2.29. COF₂: carbonyl fluoride (molecule 29)

Unchanged.

## 2.30. SF₆: sulfur hexafluoride (molecule 30)

Sulfur hexafluoride (SF₆) line lists in the $\nu_3$ (stretching) and $\nu_4$ (bending) regions have been largely updated recently thanks to new global analyses of high-resolution infrared spectra [592,593]. For the main isotopologue, ³²SF₆, these lists now contain some hot bands. This amounts to more than 350,000 calculated lines in the SHeCaSDa database (Sulfur Hexafluoride Calculated Spectroscopic Database) [594] which are included in the present HITRAN2020 edition.

Interestingly, the amount of lines for this molecule reduce by about a factor of eight. The reason is a substantially smaller cutoff in rotational quanta. Indeed, as it is demonstrated in the *ab initio* work [595] from the TheoReTs group [310], one needs to include a large amount of hot-bands in order to model even room temperature absorption accurately. Ref. [595] demonstrated that their *ab initio* calculations are able to model the PNNL spectra [244] accurately. The corresponding line list by Rey et al. [596] will be considered for future updates, although it contains billions of lines. Even with the use of “effective” lines proposed in Ref. [56], it will

likely contain tens of millions of transitions and therefore SF₆ line list will continue to reside in the supplementary folder of static files. It is worth reminding the HITRAN users that there is a comprehensive set of experimental cross-sections, which are provided in HITRAN for this molecule (see Section 3.1 for details).

For the ³³SF₆, ³⁴SF₆ and ³⁶SF₆ minor isotopologues, only the $\nu_3$ fundamental band are present [597] in the SHeCaSDa database, but it will be considered to include those data [596] in the future.

## 2.31. H₂S: hydrogen sulfide (molecule 31)

There are no changes to the line positions or intensities of this molecule. However, some recent works offer promising potential for updates in the near future. Recent semi-empirical NIR line lists from Ulenikov et al. [598,599] could be a potential source for improvements to the intensities of relevant bands in future updates.

The line positions throughout the database can be further revised with some recent data including that from the MARVEL analyses [600]. A total of 44,325 measured and assigned transitions were collected in the MARVEL database [600] and a careful analysis of these transitions resulted in 7436 empirical ro-vibrational energy levels up to 16,890 cm⁻¹; these empirical energy levels have already been used to improve the ExoMol line list for H₂S [601]. Self-broadening parameters from Ref. [602] could also be used as a source for future updates.

### 2.31.1. He-, H₂-, and CO₂-broadening parameters

The rotational dependence of He-broadening for H₂S lines was studied in the $\nu_2$ band [603,604], and in the $\nu_1$ and $\nu_3$ band [605]. An optimal set of parameters for a model inter-molecular potential that provides the best reproduction of noble gas broadening coefficients for H₂S lines was reported by Starikov et al. [606]. However, significant discrepancies take place for almost all transitions in the $\nu_2$ band for H₂S-He, and in some cases can reach 100%. We excluded the early work of Ref. [603] because of large discrepancies, and utilized experimental results from Kissel et al. [604,605]. The broadening coefficients for $\gamma_{He}$ decrease rapidly with increasing rotational quantum numbers. A Padé approximant (Eq. (1)) has been applied to fit all collected experimental data, and a semi-empirical model has been used to generate $\gamma_{He}$ for H₂S lines in the database versus the index $J + 0.2K_a$.
<figure><img src="image_32.png" /><figcaption>**Fig. 30.** Spectra for the pentad region of PH₃ compared to the PNNL absorption cross sections at 25°C [244]. The upper panel displays calculated spectra using the HITRAN2020 (see text for details), HITRAN2016 [16], Kleiner and Devi [561] and Sousa-Silva et al. [578] line lists. The lower panel displays the obs-calc residuals, where the observation corresponds to the PNNL spectrum and calculated spectra are identified by the legend (all are shown in the upper panel).</figcaption></figure>

where *J*<sub>m</sub> and *K*<sub>m</sub> refer to the maximum values of *J* and *K*, respectively, for each transition. A minimum value of γ<sub>self</sub> = 0.05 cm<sup>-1</sup>/atm is applied when the predicted values become too small.

Due to the presence of PH₃ in the atmospheres of Jupiter and
Saturn, there have been numerous studies aimed at measuring the
H₂- and He-broadening half-widths as well as their temperature
dependencies [562,568,580,582–591]. For HITRAN, H₂- and He-
broadening coefficients and temperature dependencies have been
introduced for PH₃, which will be described in detail in Ref. [266].

While updating HITRAN, the Einstein-A coefficients for all *E*
symmetry transitions have been corrected.

Line-mixing coefficients for phosphine have not been intro-
duced to HITRAN yet, although values are available from experi-
mental [562,589] and theoretical [591] studies. This will be con-
sidered for the future.

2.29. COF<sub>2</sub>: carbonyl fluoride (molecule 29)

Unchanged.

2.30. SF<sub>6</sub>: sulfur hexafluoride (molecule 30)

Sulfur hexafluoride (SF₆) line lists in the ν₃ (stretching) and
ν₄ (bending) regions have been largely updated recently thanks to
new global analyses of high-resolution infrared spectra [592,593].
For the main isotopologue, ³²SF₆, these lists now contain some hot
bands. This amounts to more than 350,000 calculated lines in the
SHeCaSDa database (Sulfur Hexafluoride Calculated Spectroscopic
Database) [594] which are included in the present HITRAN2020
edition.

Interestingly, the amount of lines for this molecule reduce by
about a factor of eight. The reason is a substantially smaller cutoff
in rotational quanta. Indeed, as it is demonstrated in the *ab initio*
work [595] from the TheoReTs group [310], one needs to include a
large amount of hot-bands in order to model even room temper-
ature absorption accurately. Ref. [595] demonstrated that their *ab
initio* calculations are able to model the PNNL spectra [244] accu-
rately. The corresponding line list by Rey et al. [596] will be con-
sidered for future updates, although it contains billions of lines.
Even with the use of "effective" lines proposed in Ref. [56], it will

likely contain tens of millions of transitions and therefore SF₆ line
list will continue to reside in the supplementary folder of static
files. It is worth reminding the HITRAN users that there is a com-
prehensive set of experimental cross-sections, which are provided
in HITRAN for this molecule (see Section 3.1 for details).

For the ³³SF₆, ³⁴SF₆ and ³⁶SF₆ minor isotopologues, only the ν₃
fundamental band are present [597] in the SHeCaSDa database, but
it will be considered to include those data [596] in the future.

2.31. H₂S: *hydrogen sulfide* (molecule 31)

There are no changes to the line positions or intensities of this
molecule. However, some recent works offer promising potential
for updates in the near future. Recent semi-empirical NIR line lists
from Ulenikov et al. [598,599] could be a potential source for im-
provements to the intensities of relevant bands in future updates.

The line positions throughout the database can be further re-
vised with some recent data including that from the MARVEL anal-
yses [600]. A total of 44,325 measured and assigned transitions
were collected in the MARVEL database [600] and a careful anal-
ysis of these transitions resulted in 7436 empirical ro-vibrational
energy levels up to 16,890 cm⁻¹; these empirical energy levels
have already been used to improve the ExoMol line list for H₂S
[601]. Self-broadening parameters from Ref. [602] could also be
used as a source for future updates.

2.31.1. He-, H₂-, and CO₂-broadening parameters

The rotational dependence of He-broadening for H₂S lines was
studied in the ν₂ band [603,604], and in the ν₁ and ν₃ band [605].
An optimal set of parameters for a model inter-molecular potential
that provides the best reproduction of noble gas broadening coeffi-
cients for H₂S lines was reported by Starikov et al. [606]. However,
significant discrepancies take place for almost all transitions in the
ν₂ band for H₂S-He, and in some cases can reach 100%. We ex-
cluded the early work of Ref. [603] because of large discrepancies,
and utilized experimental results from Kissel et al. [604,605]. The
broadening coefficients for γ<sub>He</sub> decrease rapidly with increasing ro-
tational quantum numbers. A Padé approximant (Eq. (1)) has been
applied to fit all collected experimental data, and a semi-empirical
model has been used to generate γ<sub>He</sub> for H₂S lines in the database
versus the index *J* + 0.2*K<sub>a</sub>.
The H₂-broadening for H₂S lines has been measured from a pulse-driven diode laser spectrometer [607] in the ν₂ band, and also calculated based on experimental results [608]. For HITRAN, the H₂-broadening is fit using a similar procedure to that of H-broadening, with a Padé approximant (Eq. (1)) being applied to the experimental results covering J'' up to 12.

Experimentally determined CO₂-broadening parameters for 39 transitions of H₂S in the ν₂ band were presented in Ref. [607] with uncertainties about 2%. Just as in the case of H₂- and H-broadening, a semi-empirical method was used to model these parameters.

These H₂-, He-, and CO₂-broadening for H₂S will be described in detail in Ref. [266].

## 2.32. HCOOH: formic acid (molecule 32)

Formic acid is one of the most abundant organic acids in the terrestrial atmosphere and is being monitored by different remote sensing instruments, including IASI [609] and ACE [610]. In this edition, a line list for the ν₇ and ν₉ fundamentals of HCOOH at 16 µm has been included. All simulations and fits described in this section were performed using PGOPHER [460]. Line positions were computed using the spectroscopic constants previously reported for the ground vibrational state and the strongly interacting 7¹ and 9¹ vibrational states of the normal isotopologue of formic acid [611]. This corresponds to the inclusion of 53 diagonal parameters and 12 off-diagonal parameters. The 7¹–0 and 9¹–0 bands are hybrid a/b-type and c-type, respectively. Their relative transition moments were determined by performing a fit to the intensities of 2239 isolated peaks (using 3125 transitions) identified in a high resolution (0.00096 cm⁻¹) experimental spectrum of HCOOH (similar to that reported in Ref. [612]) with absorbances (base-10) ranging from 0.1 to 1; they are µₐ/µ₏/µₑ = 1/-1.62/3.45. The experimental spectrum, which covered 400-1250 cm⁻¹ (8-25 µm) allowed scaling the calculated relative line intensities within the 16 µm bands to the known spectral line intensities, within the 9 µm bands (already in HITRAN) [613]. The intensities included in the 16-µm line list cover seven orders of magnitude (2.24 × 10⁻²⁷ to 2.24 × 10⁻²⁰ cm/molecule). Fig. 31 provides a comparison of the experimental and simulated spectra in arbitrarily chosen regions.

One should take note, as pointed out by Kochanov et al. [614], that infrared spectra calculated using HITRAN HCOOH data underestimates cross-sections when compared to the PNNL spectra [244] by about 40%. Not surprisingly, the new FIR bands introduced here exhibit the same trend because they were scaled based on the HITRAN data for the ν₆ band. The source of this discrepancy remains unclear. One of the possible explanations could be the lack of hot bands in HITRAN, but it would unlikely to be responsible for a uniform 40% difference across all spectral regions. Further inves-

<figure><img src="image_35.png" /><figcaption>**Fig. 31.** Top panel: Experimental (upward) and simulated (downward) spectra of HCOOH within the ν₉ fundamental using a Gaussian FWHM of 0.00171 cm⁻¹. Bottom panel: Experimental (upward) and simulated (downward) spectra spectra within the ν₆ fundamental using a Gaussian FWHM of 0.00245 cm⁻¹.</figcaption></figure>

tigations are needed to understand the source of the discrepancy of the HITRAN and PNNL intensities.

## 2.33. HO₂: hydroperoxyl radical (molecule 33)

Unchanged.

## 2.34. O: atomic oxygen (“molecule” 34)

Unchanged.

## 2.35. ClONO₂: chlorine nitrate (molecule 35)

Unchanged.

## 2.36. NO⁺: nitric oxide cation (molecule 36)

For typical atmospheric modeling applications, the self-broadening contribution of NO⁺ is expected to be negligible; however the previous default value of γself = 0.05 cm⁻¹/atm (as used for pure rotational lines) has been extended for all transitions to avoid null values.

## 2.37. HOBr: hypobromous acid (molecule 37)

For typical atmospheric modeling applications the self-broadening of HOBr is expected to be negligible, however a default estimated value of γself = 0.15 cm⁻¹/atm has been applied for all transitions to avoid null values.

## 2.38. C₂H₄: ethylene (molecule 38)

Unchanged.

The 3 µm region (C-H stretch) is presently under reinvestigation for both line positions and line intensities thanks to new experimental spectra and to the tensorial formalism developed in Division [615]. This line-by-line analysis will be considered for a future HITRAN update for this molecule. A complete analysis of the 10-µm region for the ¹³C₂H₄ isotopologue is also planned. Finally, the *ab initio* line lists for different isotopologues [616–618] from the TheoReTs database [310] will also be evaluated for future updates.

## 2.39. CH₃OH: methanol (molecule 39)

Unchanged.

## 2.40. CH₃Br: methyl bromide (molecule 40)

Unchanged.

## 2.41. CH₃CN: methyl cyanide (molecule 41)

Unchanged.

The ν₄ band of methyl cyanide was introduced into HITRAN2008 [14]. The data were based on a multispectrum analysis of this band and a preliminary model of the positions and intensities [619]. A complex model of low-lying vibrational states was recently expanded to include extensive Δν₄ = 1 data [620]. These are ν₄ transition frequencies up to *J* = 61 and *K* = 13 along with rotational data up to *J* = 79 and *K* = 16. These new data will allow for a substantial improvement to the database in this spectral region.

For bands involving ν₈ at longer wavelengths, a case study describing the Δν₈ = 0, 1, and 2 states that employed IR and very extensive rotational data was presented by Müller et al. [621]. We expect to include these line lists as forthcoming updates to HITRAN2020.
The H₂-broadening for H₂S lines has been measured from a pulse-driven diode laser spectrometer [607] in the ν₂ band, and also calculated based on experimental results [608]. For HITRAN, the H₂-broadening is fit using a similar procedure to that of H-broadening, with a Padé approximant (Eq. (1)) being applied to the experimental results covering J'' up to 12.

Experimentally determined CO₂-broadening parameters for 39 transitions of H₂S in the ν₂ band were presented in Ref. [607] with uncertainties about 2%. Just as in the case of H₂- and H-broadening, a semi-empirical method was used to model these parameters.

These H₂-, He-, and CO₂-broadening for H₂S will be described in detail in Ref. [266].

## 2.32. HCOOH: formic acid (molecule 32)

Formic acid is one of the most abundant organic acids in the terrestrial atmosphere and is being monitored by different remote sensing instruments, including IASI [609] and ACE [610]. In this edition, a line list for the ν₇ and ν₉ fundamentals of HCOOH at 16 µm has been included. All simulations and fits described in this section were performed using PGOPHER [460]. Line positions were computed using the spectroscopic constants previously reported for the ground vibrational state and the strongly interacting 7¹ and 9¹ vibrational states of the normal isotopologue of formic acid [611]. This corresponds to the inclusion of 53 diagonal parameters and 12 off-diagonal parameters. The 7¹–0 and 9¹–0 bands are hybrid a/b-type and c-type, respectively. Their relative transition moments were determined by performing a fit to the intensities of 2239 isolated peaks (using 3125 transitions) identified in a high resolution (0.00096 cm⁻¹) experimental spectrum of HCOOH (similar to that reported in Ref. [612]) with absorbances (base-10) ranging from 0.1 to 1; they are µₐ/µ₏/µₑ = 1/-1.62/3.45. The experimental spectrum, which covered 400-1250 cm⁻¹ (8-25 µm) allowed scaling the calculated relative line intensities within the 16 µm bands to the known spectral line intensities, within the 9 µm bands (already in HITRAN) [613]. The intensities included in the 16-µm line list cover seven orders of magnitude (2.24 × 10⁻²⁷ to 2.24 × 10⁻²⁰ cm/molecule). Fig. 31 provides a comparison of the experimental and simulated spectra in arbitrarily chosen regions.

One should take note, as pointed out by Kochanov et al. [614], that infrared spectra calculated using HITRAN HCOOH data underestimates cross-sections when compared to the PNNL spectra [244] by about 40%. Not surprisingly, the new FIR bands introduced here exhibit the same trend because they were scaled based on the HITRAN data for the ν₆ band. The source of this discrepancy remains unclear. One of the possible explanations could be the lack of hot bands in HITRAN, but it would unlikely to be responsible for a uniform 40% difference across all spectral regions. Further investigations are needed to understand the source of the discrepancy of the HITRAN and PNNL intensities.

## 2.33. HO₂: hydroperoxyl radical (molecule 33)

Unchanged.

## 2.34. O: atomic oxygen (“molecule” 34)

Unchanged.

## 2.35. CLONO₂: chlorine nitrate (molecule 35)

Unchanged.

## 2.36. NO⁺: nitric oxide cation (molecule 36)

For typical atmospheric modeling applications, the self-broadening contribution of NO⁺ is expected to be negligible; however the previous default value of γself = 0.05 cm⁻¹/atm (as used for pure rotational lines) has been extended for all transitions to avoid null values.

## 2.37. HOBr: hypobromous acid (molecule 37)

For typical atmospheric modeling applications the self-broadening of HOBr is expected to be negligible, however a default estimated value of γself = 0.15 cm⁻¹/atm has been applied for all transitions to avoid null values.

## 2.38. C₂H₄: ethylene (molecule 38)

Unchanged.

The 3 µm region (C-H stretch) is presently under reinvestigation for both line positions and line intensities thanks to new experimental spectra and to the tensorial formalism developed in Division [615]. This line-by-line analysis will be considered for a future HITRAN update for this molecule. A complete analysis of the 10-µm region for the ¹³C₂H₄ isotopologue is also planned. Finally, the *ab initio* line lists for different isotopologues [616–618] from the TheoReTs database [310] will also be evaluated for future updates.

## 2.39. CH₃OH: methanol (molecule 39)

Unchanged.

## 2.40. CH₃Br: methyl bromide (molecule 40)

Unchanged.

## 2.41. CH₃CN: methyl cyanide (molecule 41)

Unchanged.

The ν₄ band of methyl cyanide was introduced into HITRAN2008 [14]. The data were based on a multispectrum analysis of this band and a preliminary model of the positions and intensities [619]. A complex model of low-lying vibrational states was recently expanded to include extensive Δν₄ = 1 data [620]. These are ν₄ transition frequencies up to *J* = 61 and *K* = 13 along with rotational data up to *J* = 79 and *K* = 16. These new data will allow for a substantial improvement to the database in this spectral region.

For bands involving ν₈ at longer wavelengths, a case study describing the Δν₈ = 0, 1, and 2 states that employed IR and very extensive rotational data was presented by Müller et al. [621]. We expect to include these line lists as forthcoming updates to HITRAN2020.

<figure><img src="image_33.png" /><figcaption>**Fig. 31.** Top panel: Experimental (upward) and simulated (downward) spectra of HCOOH within the ν₉ fundamental using a Gaussian FWHM of 0.00171 cm⁻¹. Bottom panel: Experimental (upward) and simulated (downward) spectra within the ν₆ fundamental using a Gaussian FWHM of 0.00245 cm⁻¹.</figcaption></figure>