## 2.42. CF₄: carbon tetrafluoride (molecule 42)

The line list is unchanged, but the quantum number format has been made consistent with similar species. See the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the `.par` format.

Recent and ongoing global analyses of carbon tetrafluoride (CF₄) [622] already partly included in the TFMeCaSDa database (TetraFluoro-Methane Calculated Spectroscopic Database) [594] should lead, in the near future, to further improvements for this molecule (especially concerning hot bands) that will be considered for the next HITRAN update.

Another alternative source of data is the TheoReTs [310] line list calculated with help of *ab initio* methods. Completeness of line lists is essential for appropriate atmospheric retrievals. CF₄ is heavier than methane and has two low-lying bending frequencies at 440 and 640 cm⁻¹ leading to a huge number of IR-active transitions belonging to the hot bands. A major challenge concerns the modeling of these hot bands that strongly contribute to the absorption, even at room temperature. It has recently been shown [573] that converged opacity calculations for CF₄ in the IR using global variational methods requires the same amount of computational effort at room temperature as lighter systems (such as methane) for very elevated temperatures of about 1000 K. The corresponding CF₄ line list in the 0–4000 cm⁻¹ region, generated from potential energy and *ab initio* dipole moment surfaces by Rey et al. [573], contained about 2 billion transitions at room temperature. These data are currently too big for the standard HITRAN format but are accessible via the TheoReTs [310] information system in a hybrid compressed form. The initially computed full line-by-line lists were partitioned into two sets to accelerate modeling of spectral functions as described in Refs. [310,573] which also demonstrate very good agreement with the PNNL database [244]. In the future, this list could be accommodated into HITRAN with the use of “effective” lines, as proposed in Ref. [56] for methane.

## 2.43. C₄H₂: diacetylene (molecule 43)

Unchanged.

## 2.44. HC₃N: cyanoacetylene (molecule 44)

Cyanoacetylene is a molecule of notable astrochemical importance. It has been detected in a large number of astronomical environments (see Ref. and references therein), including planetary atmospheres [624], comets [625], and nearby galaxies [626]. Several laboratory works have been carried out that have mainly explored the rotational and ro-vibrational spectra of HC₃N. In 2017, a detailed global analysis of this molecule was published [623], which included pure rotational transitions in the ground and some excited vibrational states and ro-vibrational transitions in the 450–1350 cm⁻¹, involving all the energy levels lower than 1000 cm⁻¹. Such analysis has been extended to the far-infrared region below 450 cm⁻¹ and to the stretching region between 2034 and 3360 cm⁻¹. The newly recorded spectra and the related new global fit carried out in Bologna will be a subject for an upcoming publication [627]. All the experimental and theoretical details and the treatment of the data will be reported in Ref. [627]. From this analysis, a new line list of rotational and ro-vibrational transitions has been compiled and introduced in the HITRAN2020 database. The line list reports transition wavenumbers calculated with the best spectroscopic parameters obtained from a global fit. The main advantage of the new line list is the completeness of the data and their consistency, confirmed by the quality of the global fit. For the first time, the line list includes ro-vibrational transitions relative

to all seven vibrational modes of HC₃N up to 3400 cm⁻¹ (fundamentals, combinations, overtones, and their associated hot-bands) and rotational data in the ground and many vibrational states of all normal modes. The fundamental band $ν_7$, the lowest bending mode at 221.8 cm⁻¹, has been detected directly in the far infrared region for the first time. This is important for an accurate derivation of its vibrational energy and therefore for the analysis of many hot bands which originate from it. The global fit also accounts for two anharmonic resonance networks, and their upscale by one quantum of $ν_7$.

It is important to point out that the accuracy of the line positions for the $ν_5$ and $ν_6$ band systems is at least one order of magnitude better than that reported in the previous line list (although the differences do not exceed 0.001 cm⁻¹ and in fact is much better than that for most of the lines). Very limited extrapolations to J values higher than the observed ones have been made. With that being said, the intensities of lines in these fundamentals in the new list exceed the intensities in HITRAN2016 (which originate from Ref. [628]). The origins of this discrepancy remain to be determined. Considering that the line list from Ref. [628] is used in Titan studies and no issues were reported in the literature, we retain HITRAN2016 parameters in the corresponding spectral regions (460–560 cm⁻¹ and 620–750 cm⁻¹). This will be a subject of further investigation.

The intensities of the $ν_1$ fundamental reported in Ref. [625] are a factor of 3 stronger than those in the new line list adapted for HITRAN2020. This also will be a subject of further investigation.

## 2.45. H₂: molecular hydrogen (molecule 45)

The ro-vibrational spectra of molecular hydrogen are relevant for the atmospheres of the giant planets in the solar system and some types of super-Earth exoplanets [629]. Molecular hydrogen was first introduced in HITRAN2012 for the electric quadrupole and dipole lines in H₂ and HD isotopologues, respectively [15]. Default broadening values were used for all the lines. In HITRAN2016 [16], the electric quadrupole lines for HD were added and the line-shape parameters for the HT profile (based on experimental spectra) were added for self-perturbed H₂ [165].

In HITRAN2020, a comprehensive dataset of beyond-Voigt line-shape parameters for He-perturbed H₂ lines was added [630]. The dataset is based on *ab initio* quantum-scattering calculations and was validated on highly-accurate CRDS spectra to sub-percent levels [630]. It covers a wide temperature range from 20 to 1000 K: the temperature dependencies of all the six line-shape parameters (broadening and shift, $γ_0$ and $δ_0$, speed dependence of broadening and shift, $γ_2$ and $δ_2$, real and imaginary parts of the Dicke parameter, $\tilde{\nu}_{opt}^r$ and $\tilde{\nu}_{opt}^i$) are represented with the double-power-law (DPL) approximation [119,120] that recently was adopted in HITRAN [120] but will be made available after the official release of HITRAN2020. To make the self-perturbed H₂ line-shape parameter datasets [165] consistent with the He-perturbed H₂ dataset [630] and with the DPL format [120], the self-perturbed H₂ dataset, introduced into HITRAN in 2016 [165], was transformed into the DPL format [120].

Recently, it was demonstrated that for high-J levels of H₂ the intensities of the Q-branch lines are considerably influenced by the magnetic dipole contribution [631]. The H₂ line list will be updated in the near future to account for this effect.

The hydrogen-rich atmospheres are dominated by H₂ and He, but also contain the HD isotopologue whose low abundance is compensated by much larger intensities of dipole lines (compared to quadrupole lines in H₂). Therefore a complete dataset for planetary applications should include four systems: He-perturbed H₂, He-perturbed HD, H₂-perturbed HD and self-perturbed H₂. In the
## 2.42. CF₄: carbon tetrafluoride (molecule 42)

The line list is unchanged, but the quantum number format has been made consistent with similar species. See the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the `.par` format.

Recent and ongoing global analyses of carbon tetrafluoride (CF₄) [622] already partly included in the TFMeCaSDa database (TetraFluoro-Methane Calculated Spectroscopic Database) [594] should lead, in the near future, to further improvements for this molecule (especially concerning hot bands) that will be considered for the next HITRAN update.

Another alternative source of data is the TheoReTs [310] line list calculated with help of *ab initio* methods. Completeness of line lists is essential for appropriate atmospheric retrievals. CF₄ is heavier than methane and has two low-lying bending frequencies at 440 and 640 cm⁻¹ leading to a huge number of IR-active transitions belonging to the hot bands. A major challenge concerns the modeling of these hot bands that strongly contribute to the absorption, even at room temperature. It has recently been shown [573] that converged opacity calculations for CF₄ in the IR using global variational methods requires the same amount of computational effort at room temperature as lighter systems (such as methane) for very elevated temperatures of about 1000 K. The corresponding CF₄ line list in the 0–4000 cm⁻¹ region, generated from potential energy and *ab initio* dipole moment surfaces by Rey et al. [573], contained about 2 billion transitions at room temperature. These data are currently too big for the standard HITRAN format but are accessible via the TheoReTs [310] information system in a hybrid compressed form. The initially computed full line-by-line lists were partitioned into two sets to accelerate modeling of spectral functions as described in Refs. [310,573] which also demonstrate very good agreement with the PNNL database [244]. In the future, this list could be accommodated into HITRAN with the use of “effective” lines, as proposed in Ref. [56] for methane.

## 2.43. C₄H₂: diacetylene (molecule 43)

Unchanged.

## 2.44. HC₃N: cyanoacetylene (molecule 44)

Cyanoacetylene is a molecule of notable astrochemical importance. It has been detected in a large number of astronomical environments (see Ref. and references therein), including planetary atmospheres [624], comets [625], and nearby galaxies [626]. Several laboratory works have been carried out that have mainly explored the rotational and ro-vibrational spectra of HC₃N. In 2017, a detailed global analysis of this molecule was published [623], which included pure rotational transitions in the ground and some excited vibrational states and ro-vibrational transitions in the 450–1350 cm⁻¹, involving all the energy levels lower than 1000 cm⁻¹. Such analysis has been extended to the far-infrared region below 450 cm⁻¹ and to the stretching region between 2034 and 3360 cm⁻¹. The newly recorded spectra and the related new global fit carried out in Bologna will be a subject for an upcoming publication [627]. All the experimental and theoretical details and the treatment of the data will be reported in Ref. [627]. From this analysis, a new line list of rotational and ro-vibrational transitions has been compiled and introduced in the HITRAN2020 database. The line list reports transition wavenumbers calculated with the best spectroscopic parameters obtained from a global fit. The main advantage of the new line list is the completeness of the data and their consistency, confirmed by the quality of the global fit. For the first time, the line list includes ro-vibrational transitions relative

to all seven vibrational modes of HC₃N up to 3400 cm⁻¹ (fundamentals, combinations, overtones, and their associated hot-bands) and rotational data in the ground and many vibrational states of all normal modes. The fundamental band $ν_7$, the lowest bending mode at 221.8 cm⁻¹, has been detected directly in the far infrared region for the first time. This is important for an accurate derivation of its vibrational energy and therefore for the analysis of many hot bands which originate from it. The global fit also accounts for two anharmonic resonance networks, and their upscale by one quantum of $ν_7$.

It is important to point out that the accuracy of the line positions for the $ν_5$ and $ν_6$ band systems is at least one order of magnitude better than that reported in the previous line list (although the differences do not exceed 0.001 cm⁻¹ and in fact is much better than that for most of the lines). Very limited extrapolations to J values higher than the observed ones have been made. With that being said, the intensities of lines in these fundamentals in the new list exceed the intensities in HITRAN2016 (which originate from Ref. [628]). The origins of this discrepancy remain to be determined. Considering that the line list from Ref. [628] is used in Titan studies and no issues were reported in the literature, we retain HITRAN2016 parameters in the corresponding spectral regions (460–560 cm⁻¹ and 620–750 cm⁻¹). This will be a subject of further investigation.

The intensities of the $ν_1$ fundamental reported in Ref. [625] are a factor of 3 stronger than those in the new line list adapted for HITRAN2020. This also will be a subject of further investigation.

## 2.45. H₂: molecular hydrogen (molecule 45)

The ro-vibrational spectra of molecular hydrogen are relevant for the atmospheres of the giant planets in the solar system and some types of super-Earth exoplanets [629]. Molecular hydrogen was first introduced in HITRAN2012 for the electric quadrupole and dipole lines in H₂ and HD isotopologues, respectively [15]. Default broadening values were used for all the lines. In HITRAN2016 [16], the electric quadrupole lines for HD were added and the line-shape parameters for the HT profile (based on experimental spectra) were added for self-perturbed H₂ [165].

In HITRAN2020, a comprehensive dataset of beyond-Voigt line-shape parameters for He-perturbed H₂ lines was added [630]. The dataset is based on *ab initio* quantum-scattering calculations and was validated on highly-accurate CRDS spectra to sub-percent levels [630]. It covers a wide temperature range from 20 to 1000 K: the temperature dependencies of all the six line-shape parameters (broadening and shift, $γ_0$ and $δ_0$, speed dependence of broadening and shift, $γ_2$ and $δ_2$, real and imaginary parts of the Dicke parameter, $\tilde{\nu}_{opt}^r$ and $\tilde{\nu}_{opt}^i$) are represented with the double-power-law (DPL) approximation [119,120] that recently was adopted in HITRAN [120] but will be made available after the official release of HITRAN2020. To make the self-perturbed H₂ line-shape parameter datasets [165] consistent with the He-perturbed H₂ dataset [630] and with the DPL format [120], the self-perturbed H₂ dataset, introduced into HITRAN in 2016 [165], was transformed into the DPL format [120].

Recently, it was demonstrated that for high-J levels of H₂ the intensities of the Q-branch lines are considerably influenced by the magnetic dipole contribution [631]. The H₂ line list will be updated in the near future to account for this effect.

The hydrogen-rich atmospheres are dominated by H₂ and He, but also contain the HD isotopologue whose low abundance is compensated by much larger intensities of dipole lines (compared to quadrupole lines in H₂). Therefore a complete dataset for planetary applications should include four systems: He-perturbed H₂, He-perturbed HD, H₂-perturbed HD and self-perturbed H₂. In the
near future, we plan to add a comprehensive dataset for the H-perturbed HD lines (both dipole and quadrupole).

Unlike the principal isotopologue, HD transitions in HITRAN have only Voigt values which are known to be not efficient for this molecule and were rather arbitrarily assigned 0.05 cm⁻¹/atm for self- and air-width by default. However, this value seems to be overestimated approximately by a factor of five when they are compared to laboratory measurements [632]. Recently, a new laboratory study of pure rotational transitions has reported the line-shape parameters of HD [633], confirming that the HITRAN default values are indeed overestimated. Thus, an update is planned for the near future based on the new experimental values, which will include the broadening and frequency shifts of HD by self and H₂ and their temperature dependences for R(0)-R(3) transitions [634].

## 2.46. CS: carbon monosulfide (molecule 46)

Unchanged.

### 2.46.1. Future work

The CS line list in HITRAN includes the four most abundant isotopologues ($^{12}\text{C}^{32}\text{S}$, $^{12}\text{C}^{34}\text{S}$, $^{13}\text{C}^{32}\text{S}$, $^{12}\text{C}^{33}\text{S}$) with line positions based on lower-state energies provided by CDMS [635] and intensities calculated from Einstein-A coefficients of Ref. [636]. In the time since these data were added to HITRAN, empirically-corrected *ab initio* line lists for the $X^1\Sigma^+$ electronic ground state of CS has been calculated by Paulose et al. [637] as part of the ExoMol project [342]. Eight isotopologues of CS ($^{12}\text{C}^{32}\text{S}$, $^{12}\text{C}^{33}\text{S}$, $^{12}\text{C}^{34}\text{S}$, $^{12}\text{C}^{36}\text{S}$, $^{13}\text{C}^{32}\text{S}$, $^{13}\text{C}^{33}\text{S}$, $^{13}\text{C}^{34}\text{S}$, and $^{13}\text{C}^{36}\text{S}$) were included and cover frequencies up to 11 000 cm⁻¹. More recently, Hou and Wei [638] have calculated comprehensive empirically-based line positions with *ab initio* intensities for the same eight isotopologues as Ref. [637] with frequencies extending up to 15 000 cm⁻¹.

The $^{12}\text{C}^{32}\text{S}$ intensities from CDMS [635], Exomol [637], and Ref. [638] have been compared to bands available in HITRAN [15]. Generally, there is good agreement between all sources for the $\Delta\nu = 0$ bands. For the 1–0 band, the intensities of Hou and Wei [638] appear ~5% weaker than the other studies, whereas for the 2–0 band the intensities of Ref. [638] are ~50% stronger than HITRAN (the weakest). The differences exhibited for the 2–0 transitions indicate that further validation of the line lists from Refs. [637,638] are required. Therefore these works will be considered for inclusion into future editions of HITRAN.

## 2.47. SO₃: sulfur trioxide (molecule 47)

Unchanged.

## 2.48. C₂N₂: cyanogen (molecule 48)

Unchanged.

## 2.49. COCl₂: phosgene (molecule 49)

Unchanged.

## 2.50. SO: sulfur monoxide (molecule 50)

Sulfur monoxide is among the sulfur-containing compounds detected on Venus (see for instance Ref. [639]). This molecule has also been detected in the atmospheres of Jupiter's moon Io [640] and comets [641]. To aid interpretation of the spectra of planetary atmospheres a line list for this molecule was added to HITRAN.

The line positions, lower-state energies, and intensities for the three most abundant isotopologues of sulfur monoxide ($^{32}\text{S}^{16}\text{O}$,

$^{34}\text{S}^{16}\text{O}$, and $^{32}\text{S}^{18}\text{O}$) were calculated using the SPCAT program [642]. For the principal isotopologue, 0-0, 1-1 and 2-2 bands were calculated, using constants provided by M.-A. Martin-Drumel (Paris) based on the fit of measurements from Ref. [643], and other available data. Note that these constants differ slightly from the ones reported in Ref. [643]. The value of the dipole moment is adapted from the CDMS database [635] which is in turn based on the values reported in Refs. [644,645]. There is a notable difference in the line positions for transitions with higher rotational quanta when compared with the CDMS catalogue (containing 0-0 and 1-1 bands) and especially the JPL catalogue [227], which is based on less recent results than CDMS. The intensities agree well with the CDMS catalogue for the 0-0 band but differ noticeably (about 14%) for the 1-1 band. This is due to a systematic difference of almost 30 cm⁻¹ in the lowerstate energies. The lower-state energies calculated for HITRAN agree very well with literature values. For the $^{34}\text{S}^{16}\text{O}$ and $^{32}\text{S}^{18}\text{O}$ isotopologues, the 0-0 band was calculated based on constants from Martin-Drumel et al. [643] and the same dipole moment that was used for the principal isotopologue. There is a good agreement for intensities with the CDMS catalogue, but line positions deviate noticeably, especially with the increase of rotational quanta. It should be noted that the wavenumber format for SO in the traditional "par" output is set to F12.9 for transitions below 1.0 cm⁻¹, F12.8 for transitions 1.0 to 10.0 cm⁻¹, and F12.7 for transitions 10.0 to 100.0 cm⁻¹.

The $a^1\Delta-X^3\Sigma^-$ and $b^1\Sigma^+-X^3\Sigma^-$ electronic transitions of $^{32}\text{S}^{16}\text{O}$ have been added to HITRAN based on the work of Ref. [646]. Fits to spectroscopic data in the literature (including Ref. [643] and references therein) were performed using PGOPHER [460]. The SO line list includes the transitions involving vibrational levels $v=0-6$ for the $X^3\Sigma^-$ state, $v=0-5$ for the $a^1\Delta$ state, and $v=0-2$ for the $b^1\Sigma^+$ state. For electric dipole transitions, the transition dipole moment matrix elements were obtained from *ab initio* calculations, but for magnetic dipole transitions, the transition dipole moment matrix elements were scaled to experimental values [647]. These matrix elements were used in PGOPHER to provide Einstein-A coefficients that were then converted to line intensities for inclusion to HITRAN. All lower-state energies have been adjusted by 5.5913 cm⁻¹ to shift the zero energy to the lowest-lying energy level (to be consistent with the database formalism and the MW line list described above). The magnetic dipole transitions have been indicated by "d" in the lower-state quanta (see the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the "par" format).

No broadening parameters for SO are available in the literature. For that reason they have been estimated from those of the isoelectronic oxygen molecule. For air- and self-broadening, the functions used for the oxygen A-band have been applied from Robichaud et al. [325], with a default value for temperature dependence. It is worth pointing out that because SO has been observed on planets with a history of volcanic activity, measurements and broadening by pressure of CO₂ would be very welcomed.

## 2.51. CH₃F: methyl fluoride (molecule 51)

A line list for methyl fluoride (CH₃F) has been introduced to HITRAN for the first time. This tetrahedral molecule is present in traces in the terrestrial atmosphere and participates in global warming [648]. A line list for the intense $v_6$ band around 1200 cm⁻¹ has now been included in HITRAN, based on the work of Ref. [649] for line positions, on Ref. [650] for line intensities and self-broadening coefficients, and on Ref. [651] for air-broadening coefficients. For some of the lines, the quantum assignment is incomplete in the sense that A1 and A2 symmetry components are not distinguished. In the future a more detailed quantum assignment will be provided.
near future, we plan to add a comprehensive dataset for the H-perturbed HD lines (both dipole and quadrupole).

Unlike the principal isotopologue, HD transitions in HITRAN have only Voigt values which are known to be not efficient for this molecule and were rather arbitrarily assigned 0.05 cm⁻¹/atm for self- and air-width by default. However, this value seems to be overestimated approximately by a factor of five when they are compared to laboratory measurements [632]. Recently, a new laboratory study of pure rotational transitions has reported the line-shape parameters of HD [633], confirming that the HITRAN default values are indeed overestimated. Thus, an update is planned for the near future based on the new experimental values, which will include the broadening and frequency shifts of HD by self and H₂ and their temperature dependences for R(0)-R(3) transitions [634].

## 2.46. CS: carbon monosulfide (molecule 46)

Unchanged.

### 2.46.1. Future work

The CS line list in HITRAN includes the four most abundant isotopologues ($^{12}$C$^{32}$S, $^{12}$C$^{34}$S, $^{13}$C$^{32}$S, $^{12}$C$^{33}$S) with line positions based on lower-state energies provided by CDMS [635] and intensities calculated from Einstein-A coefficients of Ref. [636]. In the time since these data were added to HITRAN, empirically-corrected *ab initio* line lists for the X$^1\Sigma^+$ electronic ground state of CS has been calculated by Paulose et al. [637] as part of the ExoMol project [342]. Eight isotopologues of CS ($^{12}$C$^{32}$S, $^{12}$C$^{33}$S, $^{12}$C$^{34}$S, $^{12}$C$^{36}$S, $^{13}$C$^{32}$S, $^{13}$C$^{33}$S, $^{13}$C$^{34}$S, and $^{13}$C$^{36}$S) were included and cover frequencies up to 11 000 cm⁻¹. More recently, Hou and Wei [638] have calculated comprehensive empirically-based line positions with *ab initio* intensities for the same eight isotopologues as Ref. [637] with frequencies extending up to 15 000 cm⁻¹.

The $^{12}$C$^{32}$S intensities from CDMS [635], Exomol [637], and Ref. [638] have been compared to bands available in HITRAN [15]. Generally, there is good agreement between all sources for the $\Delta\nu = 0$ bands. For the 1–0 band, the intensities of Hou and Wei [638] appear ~5% weaker than the other studies, whereas for the 2–0 band the intensities of Ref. [638] are ~50% stronger than HITRAN (the weakest). The differences exhibited for the 2–0 transitions indicate that further validation of the line lists from Refs. [637,638] are required. Therefore these works will be considered for inclusion into future editions of HITRAN.

## 2.47. SO$_3$: sulfur trioxide (molecule 47)

Unchanged.

## 2.48. C$_2$N$_2$: cyanogen (molecule 48)

Unchanged.

## 2.49. COCl$_2$: phosgene (molecule 49)

Unchanged.

## 2.50. SO: sulfur monoxide (molecule 50)

Sulfur monoxide is among the sulfur-containing compounds detected on Venus (see for instance Ref. [639]). This molecule has also been detected in the atmospheres of Jupiter's moon Io [640] and comets [641]. To aid interpretation of the spectra of planetary atmospheres a line list for this molecule was added to HITRAN.

The line positions, lower-state energies, and intensities for the three most abundant isotopologues of sulfur monoxide ($^{32}$S$^{16}$O,

$^{34}$S$^{16}$O, and $^{32}$S$^{18}$O) were calculated using the SPCAT program [642]. For the principal isotopologue, 0-0, 1-1 and 2-2 bands were calculated, using constants provided by M.-A. Martin-Drumel (Paris) based on the fit of measurements from Ref. [643], and other available data. Note that these constants differ slightly from the ones reported in Ref. [643]. The value of the dipole moment is adapted from the CDMS database [635] which is in turn based on the values reported in Refs. [644,645]. There is a notable difference in the line positions for transitions with higher rotational quanta when compared with the CDMS catalogue (containing 0-0 and 1-1 bands) and especially the JPL catalogue [227], which is based on less recent results than CDMS. The intensities agree well with the CDMS catalogue for the 0-0 band but differ noticeably (about 14%) for the 1-1 band. This is due to a systematic difference of almost 30 cm⁻¹ in the lowerstate energies. The lower-state energies calculated for HITRAN agree very well with literature values. For the $^{34}$S$^{16}$O and $^{32}$S$^{18}$O isotopologues, the 0-0 band was calculated based on constants from Martin-Drumel et al. [643] and the same dipole moment that was used for the principal isotopologue. There is a good agreement for intensities with the CDMS catalogue, but line positions deviate noticeably, especially with the increase of rotational quanta. It should be noted that the wavenumber format for SO in the traditional “.par” output is set to F12.9 for transitions below 1.0 cm⁻¹, F12.8 for transitions 1.0 to 10.0 cm⁻¹, and F12.7 for transitions 10.0 to 100.0 cm⁻¹.

The $a^1\Delta-X^3\Sigma^-$ and $b^1\Sigma^+-X^3\Sigma^-$ electronic transitions of $^{32}$S$^{16}$O have been added to HITRAN based on the work of Ref. [646]. Fits to spectroscopic data in the literature (including Ref. [643] and references therein) were performed using PGOPHER [460]. The SO line list includes the transitions involving vibrational levels $v=0-6$ for the $X^3\Sigma^-$ state, $v=0-5$ for the $a^1\Delta$ state, and $v=0-2$ for the $b^1\Sigma^+$ state. For electric dipole transitions, the transition dipole moment matrix elements were obtained from *ab initio* calculations, but for magnetic dipole transitions, the transition dipole moment matrix elements were scaled to experimental values [647]. These matrix elements were used in PGOPHER to provide Einstein-A coefficients that were then converted to line intensities for inclusion to HITRAN. All lower-state energies have been adjusted by 5.5913 cm⁻¹ to shift the zero energy to the lowest-lying energy level (to be consistent with the database formalism and the MW line list described above). The magnetic dipole transitions have been indicated by “d” in the lower-state quanta (see the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the “.par” format).

No broadening parameters for SO are available in the literature. For that reason they have been estimated from those of the isoelectronic oxygen molecule. For air- and self-broadening, the functions used for the oxygen A-band have been applied from Robichaud et al. [325], with a default value for temperature dependence. It is worth pointing out that because SO has been observed on planets with a history of volcanic activity, measurements and broadening by pressure of CO₂ would be very welcomed.

## 2.51. CH$_3$F: methyl fluoride (molecule 51)

A line list for methyl fluoride (CH$_3$F) has been introduced to HITRAN for the first time. This tetrahedral molecule is present in traces in the terrestrial atmosphere and participates in global warming [648]. A line list for the intense $\nu_6$ band around 1200 cm⁻¹ has now been included in HITRAN, based on the work of Ref. [649] for line positions, on Ref. [650] for line intensities and self-broadening coefficients, and on Ref. [651] for air-broadening coefficients. For some of the lines, the quantum assignment is incomplete in the sense that A1 and A2 symmetry components are not distinguished. In the future a more detailed quantum assignment will be provided.
## 2.52. GeH₄: Germane (molecule 52)

Germane (GeH₄) is a tetrahedral molecule of interest for the study of the atmospheres of giant planets Jupiter and Saturn. The need for accurate line lists for this molecule has been especially renewed recently by the availability of spectroscopic measurements from the JIRAM (Jovian InfraRed Auroral Mapper) infrared spectrometer aboard NASA's Juno spacecraft [652]. Germane abundance retrieval in the Jovian atmosphere allows one to probe the planet's troposphere below the ammonia cloud level.

During the past few years, a new experimental and modeling study on this molecule has been undertaken by French (LISA in Créteil, ICB in Dijon) and Belgian (at ULB in Brussels) groups to measure high-resolution infrared spectra of the fundamental bands of germane in order to obtain accurate line positions and line intensities. The molecule was studied in natural abundance and effective Hamiltonian and dipole moment parameters were retrieved using the Dijon tensorial formalism and programs [653] for the five main isotopologues: ⁷⁴GeH₄ (36.52 %), ⁷²GeH₄ (27.41 %), ⁷⁰GeH₄ (20.51 %), ⁷³GeH₄ (7.76 %), and ⁷⁶GeH₄ (7.46 %). Deuterated species have not been considered in these studies. A first paper was dedicated to the stretching dyad $v_1/v_3$ around 2100 cm⁻¹ [654] and a second one to the bending dyad $v_2/v_4$ around 900 cm⁻¹ [655].

The retrieved molecular parameters allowed the production of calculated germane line lists for both regions. These data were first used to setup GeCaSDa (Germane Calculated Spectroscopic Database) [594], which can be accessed either directly (http://vamdc.icb.cnrs.fr) or through the VAMDC (Virtual Atomic and Molecular Data Centre) portal [656].

The germane line list for HITRAN contains line positions and absolute line intensities for approximately 12,200 lines for each of the five isotopologues (with 60,878 lines in total). These lines pertain to the $v_1, v_2, v_3$ and $v_4$ fundamental bands only, in the 648–2271 cm⁻¹ wavenumber range [654,655]. The lower intensity threshold for calculations is set to $10^{-23}$ cm⁻¹/(molecule cm⁻²), which is sufficiently strong to avoid including exaggerated extrapolations from assigned experimental lines. The Einstein-A coefficients in HITRAN have been recalculated to enable consistency with the total internal partition sums of Ref. [417] described in Section 6.4.

For completeness, we also mention the work on germane by O. Ulenikov's group in Tomsk, Russia (see for instance Ref. [657] and references therein), where similar results have been obtained. More recently, a comprehensive study of the 1400–1950 cm⁻¹ spectral region includes analysis of the $2v_2, 2v_4$, and $v_2 + v_4$ bands [658]. This line list has not been included into HITRAN, but the broadening measurements have been used to estimate self-broadening coefficients for HITRAN. A linear fit to the self-broadening coefficients of Ref. [658] yields $\gamma_{self} = 0.07615 – 0.00040''$. This has been used to calculate the self-broadening for all bands (of all germane isotopologues) in the HITRAN line list. There are no known measurements of air-broadening therefore an average value of $\gamma_{air} = 0.06$ cm⁻¹/atm and $n_{air} = 0.75$ have been used based on comparisons to the methane parameters in HITRAN.

## 2.53. CS₂: carbon disulfide (molecule 53)

Carbon disulfide (CS₂) has been introduced in the line-by-line part of HITRAN for the first time. CS₂ is a molecule of interest in atmospheric environmental chemistry, medical diagnostics, and studies of planetary atmospheres. It has a significant effect on the global atmospheric sulfur budget and the oxidation of carbon disulfide in the atmosphere is a major source of OCS [470–473]. The CS₂ molecule has been detected in comets [659,660] and in Jupiter's atmosphere after the collision of the Shoemaker-Levy 9 comet [661]. The principal sources of CS₂ are industrial and nat-

ural processes (volcanic eruptions, evaporation from the oceans, soils, biomass burning, and petroleum refining) [662,663]. In addition, exposure to CS₂ can cause accelerated atherosclerosis and coronary artery disease [664,665], and therefore it is essential to monitor its concentrations in relevant production sites.

The HITRAN2020 carbon disulfide line list contains 83,420 transitions of the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues in the 1.2–6466.4 cm⁻¹ spectral range. The line list is described in detail in Ref. [666]; therefore here we only briefly summarize the main characteristics. In the HITRAN2020 edition, the CS₂ molecule was given number “53” to be consistent with the Total Internal Partition Sums (TIPS2017) program [178] (isotopologue ID: ¹²C³²S₂: 1, ³²S¹²C³⁴S: 2, ³²S¹²C³³S: 3, and ¹³C³²S₂: 4). Although CS₂ has the same symmetry as CO₂, the quantum notation that was chosen for this molecule is more in line with that used for other linear molecules, including OCS. In particular, the labeling $v_1v_2l_2v_3$ of the vibrational states was used (see the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the “.par” format). The calculations of the line positions and intensities were performed by applying the PGO-PHER program [460] using a large set of measured line positions available in the literature, including the most recent high-precision dual-comb laser spectroscopy measurements [667] and transition dipole moments for each measured band. A global least-squares fit of measured line positions to the corresponding spectroscopic parameters for the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues was carried out. A unique set of parameters for each lower and upper state was obtained. The maximum rotational angular momentum in the line list was set to $J = 150$. In addition, the perturbed line positions of the $3v_3, v_1+3v_3$, and $3v_1+3v_3$ bands of the ¹²C³²S₂ isotopologue and the $3v_3$ band of the ³²S¹²C³⁴S isotopologue were replaced by their experimental values from Refs. [668–670]. The calculations of line intensities for 423 bands of the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues have been carried out. For vibrational CS₂ bands known experimentally from the literature, the corresponding transitions of the dipole moments were fit to the measured line intensities. Different scaling factors were used to correct the line intensities for the carbon disulfide bands not having dedicated intensity measurements. In this case, validation and correction of the calculated line intensities have been performed using the PNNL [244] spectrum which covers the 600–6500 cm⁻¹ spectral range. The comparison between the CS₂ line list (i.e., HITRAN line list) and experimental PNNL spectrum can be found in Figs. 9 and 10 of Ref. [666]. The overall agreement of the line positions and intensities is fairly good except for the spectral region around of 1535 cm⁻¹, where there is not enough data to calculate the line parameters for all the hot bands. More dedicated experimental and theoretical studies of the intensities in multiple bands are needed. Also, in the course of recalculation of the partition sums (TIPS-2021) for HITRAN2020 [417], it was found that the values at 296 K for ¹²CS₂ and ¹³CS₂ differed to their previous values (TIPS-2017) [178] by around 30%. It is important to note that the intensity cutoff $10^{-30}$ cm/molecule at 296 K, was applied when adapting the line list from Karlovets et al. [666] to HITRAN.

The HITRAN line list allowed one to extend the knowledge about the CS₂ line parameters in the 1–600 cm⁻¹ spectral region dominated by the $v_2$ band of ¹²C³²S₂. The line intensities of this band were calculated using data from Ref. [671] which may be considered to be imprecise; therefore, new experiments for this band are welcomed. Uncertainty codes for the line positions and the line intensities used in the CS₂ line list are described in Ref. [666].

Fig. 32 shows an overview of the line lists for the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues in the 0–7000 cm⁻¹ region.
## 2.52. GeH₄: Germane (molecule 52)

Germane (GeH₄) is a tetrahedral molecule of interest for the study of the atmospheres of giant planets Jupiter and Saturn. The need for accurate line lists for this molecule has been especially renewed recently by the availability of spectroscopic measurements from the JIRAM (Jovian InfraRed Auroral Mapper) infrared spectrometer aboard NASA's Juno spacecraft [652]. Germane abundance retrieval in the Jovian atmosphere allows one to probe the planet's troposphere below the ammonia cloud level.

During the past few years, a new experimental and modeling study on this molecule has been undertaken by French (LISA in Créteil, ICB in Dijon) and Belgian (at ULB in Brussels) groups to measure high-resolution infrared spectra of the fundamental bands of germane in order to obtain accurate line positions and line intensities. The molecule was studied in natural abundance and effective Hamiltonian and dipole moment parameters were retrieved using the Dijon tensorial formalism and programs [653] for the five main isotopologues: ⁷⁴GeH₄ (36.52 %), ⁷²GeH₄ (27.41 %), ⁷⁰GeH₄ (20.51 %), ⁷³GeH₄ (7.76 %), and ⁷⁶GeH₄ (7.46 %). Deuterated species have not been considered in these studies. A first paper was dedicated to the stretching dyad $v_1/v_3$ around 2100 cm⁻¹ [654] and a second one to the bending dyad $v_2/v_4$ around 900 cm⁻¹ [655].

The retrieved molecular parameters allowed the production of calculated germane line lists for both regions. These data were first used to setup GeCaSDa (Germane Calculated Spectroscopic Database) [594], which can be accessed either directly (http://vamdc.icb.cnrs.fr) or through the VAMDC (Virtual Atomic and Molecular Data Centre) portal [656].

The germane line list for HITRAN contains line positions and absolute line intensities for approximately 12,200 lines for each of the five isotopologues (with 60,878 lines in total). These lines pertain to the $v_1, v_2, v_3$ and $v_4$ fundamental bands only, in the 648–2271 cm⁻¹ wavenumber range [654,655]. The lower intensity threshold for calculations is set to $10^{-23}$ cm⁻¹/(molecule cm⁻²), which is sufficiently strong to avoid including exaggerated extrapolations from assigned experimental lines. The Einstein-A coefficients in HITRAN have been recalculated to enable consistency with the total internal partition sums of Ref. [417] described in Section 6.4.

For completeness, we also mention the work on germane by O. Ulenikov's group in Tomsk, Russia (see for instance Ref. [657] and references therein), where similar results have been obtained. More recently, a comprehensive study of the 1400–1950 cm⁻¹ spectral region includes analysis of the $2v_2, 2v_4$, and $v_2 + v_4$ bands [658]. This line list has not been included into HITRAN, but the broadening measurements have been used to estimate self-broadening coefficients for HITRAN. A linear fit to the self-broadening coefficients of Ref. [658] yields $\gamma_{self} = 0.07615 – 0.00040''$. This has been used to calculate the self-broadening for all bands (of all germane isotopologues) in the HITRAN line list. There are no known measurements of air-broadening therefore an average value of $\gamma_{air} = 0.06$ cm⁻¹/atm and $n_{air} = 0.75$ have been used based on comparisons to the methane parameters in HITRAN.

## 2.53. CS₂: carbon disulfide (molecule 53)

Carbon disulfide (CS₂) has been introduced in the line-by-line part of HITRAN for the first time. CS₂ is a molecule of interest in atmospheric environmental chemistry, medical diagnostics, and studies of planetary atmospheres. It has a significant effect on the global atmospheric sulfur budget and the oxidation of carbon disulfide in the atmosphere is a major source of OCS [470–473]. The CS₂ molecule has been detected in comets [659,660] and in Jupiter's atmosphere after the collision of the Shoemaker-Levy 9 comet [661]. The principal sources of CS₂ are industrial and nat-

ural processes (volcanic eruptions, evaporation from the oceans, soils, biomass burning, and petroleum refining) [662,663]. In addition, exposure to CS₂ can cause accelerated atherosclerosis and coronary artery disease [664,665], and therefore it is essential to monitor its concentrations in relevant production sites.

The HITRAN2020 carbon disulfide line list contains 83,420 transitions of the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues in the 1.2–6466.4 cm⁻¹ spectral range. The line list is described in detail in Ref. [666]; therefore here we only briefly summarize the main characteristics. In the HITRAN2020 edition, the CS₂ molecule was given number “53” to be consistent with the Total Internal Partition Sums (TIPS2017) program [178] (isotopologue ID: ¹²C³²S₂: 1, ³²S¹²C³⁴S: 2, ³²S¹²C³³S: 3, and ¹³C³²S₂: 4). Although CS₂ has the same symmetry as CO₂, the quantum notation that was chosen for this molecule is more in line with that used for other linear molecules, including OCS. In particular, the labeling $v_1v_2l_2v_3$ of the vibrational states was used (see the Supplementary Material of this paper for a description of the upper- and lower-state quanta in the “.par” format). The calculations of the line positions and intensities were performed by applying the PGO-PHER program [460] using a large set of measured line positions available in the literature, including the most recent high-precision dual-comb laser spectroscopy measurements [667] and transition dipole moments for each measured band. A global least-squares fit of measured line positions to the corresponding spectroscopic parameters for the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues was carried out. A unique set of parameters for each lower and upper state was obtained. The maximum rotational angular momentum in the line list was set to $J = 150$. In addition, the perturbed line positions of the $3v_3, v_1+3v_3$, and $3v_1+3v_3$ bands of the ¹²C³²S₂ isotopologue and the $3v_3$ band of the ³²S¹²C³⁴S isotopologue were replaced by their experimental values from Refs. [668–670]. The calculations of line intensities for 423 bands of the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues have been carried out. For vibrational CS₂ bands known experimentally from the literature, the corresponding transitions of the dipole moments were fit to the measured line intensities. Different scaling factors were used to correct the line intensities for the carbon disulfide bands not having dedicated intensity measurements. In this case, validation and correction of the calculated line intensities have been performed using the PNNL [244] spectrum which covers the 600–6500 cm⁻¹ spectral range. The comparison between the CS₂ line list (i.e., HITRAN line list) and experimental PNNL spectrum can be found in Figs. 9 and 10 of Ref. [666]. The overall agreement of the line positions and intensities is fairly good except for the spectral region around of 1535 cm⁻¹, where there is not enough data to calculate the line parameters for all the hot bands. More dedicated experimental and theoretical studies of the intensities in multiple bands are needed. Also, in the course of recalculation of the partition sums (TIPS-2021) for HITRAN2020 [417], it was found that the values at 296 K for ¹²CS₂ and ¹³CS₂ differed to their previous values (TIPS-2017) [178] by around 30%. It is important to note that the intensity cutoff $10^{-30}$ cm/molecule at 296 K, was applied when adapting the line list from Karlovets et al. [666] to HITRAN.

The HITRAN line list allowed one to extend the knowledge about the CS₂ line parameters in the 1–600 cm⁻¹ spectral region dominated by the $v_2$ band of ¹²C³²S₂. The line intensities of this band were calculated using data from Ref. [671] which may be considered to be imprecise; therefore, new experiments for this band are welcomed. Uncertainty codes for the line positions and the line intensities used in the CS₂ line list are described in Ref. [666].

Fig. 32 shows an overview of the line lists for the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues in the 0–7000 cm⁻¹ region.
<figure><img src="image_36.png" /><figcaption>Fig. 32. Overview of the line lists of the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues in HITRAN.</figcaption></figure>

We obtained the air- and self-broadening parameters of CS₂ and the temperature dependence exponents of the half-widths based on the measured or theoretically calculated data in the literature. To populate these parameters for all the lines of CS₂, the available results were fit using the Padé approximants (Eq. (1)) and these parameters ($\gamma_{air}$, $\gamma_{self}$, and $n_{air}$) were predicted for all the lines including the transitions with higher quantum numbers. The pressure shifts of CS₂ were not yet added to the database because of the lack of measured data for this parameter.

## 2.54. CH₃I: methyl iodide (molecule 54)

Methyl iodide (CH₃I), a naturally occurring halogenated volatile organic compound, is an important carrier of iodine from the ocean to the atmosphere and plays a crucial role in the chemistry of the atmosphere [672,673]. In addition, it is used in several industrial and agricultural applications. Examples include use as a methylation reagent in organic synthesis, a fumigant in buildings and soils, and as a pesticide. In nuclear power plants [674], methyl iodide is mainly produced in the containment by the reaction of iodine with organic coatings of the enclosure under ionizing radiation. In the case of a severe nuclear accident, iodine fission products represent a major part of the released radioactivity and are of deep concern due to the affinity of iodine with the thyroid. Therefore, it is crucial to monitor the release of iodine compounds into the atmosphere as part of nuclear safety and radio-protection. Relevant to all these applications is the capability to install leak detectors as well as to monitor personal exposure limits. Implementation of optical detection schemes is hindered by the lack of accurate spectroscopic models based on high-precision laboratory measurements.

### 2.54.1. $v_6$ band at 11.2 µm

A relatively strong $v_6$ band of CH₃I is located around 893 cm⁻¹, coinciding with the 11-µm transparency window in the atmosphere [21,186], could be a good candidate for detection of this molecule in atmospheric spectra. Detailed studies concerning the line positions and intensities of the $v_6$ fundamental and interacting $2v_3$ bands were recently carried out [675,676]. For the computation of the line positions and intensities, the hyperfine structure due to the iodine nuclear quadrupole moment was accounted for explicitly (Fig. 33), together with the vibration-rotation resonances

<figure><img src="image_37.png" /><figcaption>Fig. 33. Example of a calculation of methyl iodide transitions with and without accounting for the hyperfine structure.</figcaption></figure>

which perturb them. Transitions from both the $v_6$ and $2v_3$ bands have been included for HITRAN2020.

### 2.54.2. $v_4$ band at 3 µm

The reported line positions and intensities of the $v_4$ band and nearby $v_3 + v_4 - v_3$ hot band are based on a high-precision measurement using optical frequency comb Fourier transform spectroscopy [677]. The details of the mid-IR frequency comb source [678], the home-built fast-scanning FTS [679], as well as the auto-balancing detection scheme in the FTS [680] and a Herriot cell are presented elsewhere. The high-resolution spectra were recorded in the region from 2800 to 3160 cm⁻¹ with sampling point spacing of 11 MHz, utilizing the sub-nominal resolution interleaving scheme [681,682]. The measured spectrum, shown in Fig. 34(a), contains three main ro-vibrational features: the parallel vibrational overtone and combination bands centered around 2850 cm⁻¹, the strong symmetric stretch $v_1$ band centered at 2971 cm⁻¹, and the asymmetric stretch $v_4$ band centered at 3060 cm⁻¹. Based on the analysis of these spectra, the $v_4$ band and the nearby $v_3 + v_4 - v_3$ hot band are included in the HITRAN2020 database.

Fig. 34 (b) shows the measured (black) spectrum of 0.11 mbar of pure CH₃I at 296 K together with the simulations of the $v_4$ band (red) and the $v_3 + v_4 - v_3$ hot band (blue). Fig. 34(c) shows a further enlarged section of Fig. 34(b) around a $Q_K(J)$ sub-branch.
<figure><img src="image_34.png" /><figcaption>Fig. 32. Overview of the line lists of the ¹²C³²S₂, ³²S¹²C³⁴S, ³²S¹²C³³S, and ¹³C³²S₂ isotopologues in HITRAN.</figcaption></figure>

We obtained the air- and self-broadening parameters of CS₂ and the temperature dependence exponents of the half-widths based on the measured or theoretically calculated data in the literature. To populate these parameters for all the lines of CS₂, the available results were fit using the Padé approximants (Eq. (1)) and these parameters ($\gamma_{air}$, $\gamma_{self}$, and $n_{air}$) were predicted for all the lines including the transitions with higher quantum numbers. The pressure shifts of CS₂ were not yet added to the database because of the lack of measured data for this parameter.

## 2.54. CH₃I: methyl iodide (molecule 54)

Methyl iodide (CH₃I), a naturally occurring halogenated volatile organic compound, is an important carrier of iodine from the ocean to the atmosphere and plays a crucial role in the chemistry of the atmosphere [672,673]. In addition, it is used in several industrial and agricultural applications. Examples include use as a methylation reagent in organic synthesis, a fumigant in buildings and soils, and as a pesticide. In nuclear power plants [674], methyl iodide is mainly produced in the containment by the reaction of iodine with organic coatings of the enclosure under ionizing radiation. In the case of a severe nuclear accident, iodine fission products represent a major part of the released radioactivity and are of deep concern due to the affinity of iodine with the thyroid. Therefore, it is crucial to monitor the release of iodine compounds into the atmosphere as part of nuclear safety and radio-protection. Relevant to all these applications is the capability to install leak detectors as well as to monitor personal exposure limits. Implementation of optical detection schemes is hindered by the lack of accurate spectroscopic models based on high-precision laboratory measurements.

### 2.54.1. $v_6$ band at 11.2 µm

A relatively strong $v_6$ band of CH₃I is located around 893 cm⁻¹, coinciding with the 11-µm transparency window in the atmosphere [21,186], could be a good candidate for detection of this molecule in atmospheric spectra. Detailed studies concerning the line positions and intensities of the $v_6$ fundamental and interacting $2v_3$ bands were recently carried out [675,676]. For the computation of the line positions and intensities, the hyperfine structure due to the iodine nuclear quadrupole moment was accounted for explicitly (Fig. 33), together with the vibration-rotation resonances

<figure><img src="image_35.png" /><figcaption>Fig. 33. Example of a calculation of methyl iodide transitions with and without accounting for the hyperfine structure.</figcaption></figure>

which perturb them. Transitions from both the $v_6$ and $2v_3$ bands have been included for HITRAN2020.

### 2.54.2. $v_4$ band at 3 µm

The reported line positions and intensities of the $v_4$ band and nearby $v_3 + v_4 - v_3$ hot band are based on a high-precision measurement using optical frequency comb Fourier transform spectroscopy [677]. The details of the mid-IR frequency comb source [678], the home-built fast-scanning FTS [679], as well as the auto-balancing detection scheme in the FTS [680] and a Herriot cell are presented elsewhere. The high-resolution spectra were recorded in the region from 2800 to 3160 cm⁻¹ with sampling point spacing of 11 MHz, utilizing the sub-nominal resolution interleaving scheme [681,682]. The measured spectrum, shown in Fig. 34(a), contains three main ro-vibrational features: the parallel vibrational overtone and combination bands centered around 2850 cm⁻¹, the strong symmetric stretch $v_1$ band centered at 2971 cm⁻¹, and the asymmetric stretch $v_4$ band centered at 3060 cm⁻¹. Based on the analysis of these spectra, the $v_4$ band and the nearby $v_3 + v_4 - v_3$ hot band are included in the HITRAN2020 database.

Fig. 34 (b) shows the measured (black) spectrum of 0.11 mbar of pure CH₃I at 296 K together with the simulations of the $v_4$ band (red) and the $v_3 + v_4 - v_3$ hot band (blue). Fig. 34(c) shows a further enlarged section of Fig. 34(b) around a $Q_K(J)$ sub-branch.
<figure><img src="image_38.png" /><figcaption>**Fig. 34.** (a) The broadband high-resolution spectrum of pure CH₃I measured at 0.03 mbar in the range from 2800–3160 cm⁻¹ using comb-based FTS [677]. (b) The absorption coefficient, α of the ν₄ band measured at 0.11 mbar of pure CH₃I (black) together with the simulations of the ν₄ band (red) and the ν₃ + ν₄ – ν₃ hot band (blue) obtained using PGOPHER. (c) Zoom in region around one of the Qₓ(J) sub-branches of the ν₄ band.</figcaption></figure>

The spectra of these two bands were simulated and assigned using PGOPHER [460]. The overall band structure, as well as the relative intensities of the individual lines in the simulations, agree very well with the experimental data. A least-square fit of the assigned transitions to the measured spectrum provided accurate upper-state rotational constants of both bands. The hyperfine splittings due to the ¹²⁷I iodine nuclear quadrupole moment are observed for transitions with *J* ≤ 2K. Future work will involve further analysis of the hyperfine splittings and the analysis of the measured ν₁ band and the parallel vibration bands.

For inclusion to HITRAN, transitions with *J* < 25 contain hyperfine splitting. All CH₃I lower-state energies (including transitions for the ν₆ region) have been adjusted by 1.0 × 10⁻⁴ cm⁻¹ to account for the lowest allowed energy level. Sadiek et al. [677] provided line intensities for 207 transitions obtained using a multi-spectrum fitting procedure and these intensities were adapted for HITRAN. Line intensities for the remaining transitions of the ν₄ band and ν₃ + ν₄ – ν₃ hot band have been estimated by scaling the relative PGOPHER intensities. A single scale factor was determined by taking an average ratio of the empirical intensities from Sadiek et al. [677] to their corresponding PGOPHER relative intensities. A dependence on the rotational quanta was observed in this comparison. It has been shown [683] that Herman-Wallis coefficients are necessary for determining accurate line intensities for CH₃I and will therefore be considered for future intensity analyses.

The self-and air-broadening parameters for all CH₃I transitions are based on recent studies by Raddaoui et al. [648,684]. The values are based on FTS analysis of the ν₆ band, which proposed a set of smoothed empirical parameters to calculate widths up to *J* = 80. Empirical parameters are fitted through the *K*-rotational dependencies observed for transitions having the same lower *J* value. The rotational *J*- and *K*-dependencies calculated by this model reproduced the measurements (around 1000) with a sub-percent av-

erage discrepancy and one standard deviation of around 7% both for self- and air-broadening coefficients. The smoothed parameters of Table 4 from Ref. [648] and Table 5 from Ref. [684] for self- and air-broadening coefficients, respectively, have been used to generate broadening coefficients at 296 K for all CH₃I transitions. To avoid extrapolating the self-broadening coefficients to *J''* and *K''* pairs far beyond observation, a minimum value of 0.10 cm⁻¹/ atm has been used. An error code of 5 (see Table 2) has been used for both self- and air-broadening coefficients.

There are no line shifts and temperature dependence parameters for this molecule available in the literature. Default values for temperature dependence (fixed to 0.75) of air-broadening coefficients have been used for all CH₃I transitions. For atmospheric applications, a study of the temperature dependence of the line-shape parameters of CH₃I (190–300 K) will be required.

### 2.55. NF₃: nitrogen trifluoride (molecule 55)

The nitrogen trifluoride (NF₃) line list is presented in the HITRAN database for the first time. This line list includes more than 40 cold and 680 hot sub-bands and covers the spectral range up to 2200 cm⁻¹. NF₃ is known as an anthropogenic greenhouse gas with a high global warming potential of about 17 000 [685–687]. The concentration of NF₃ has been increasing in the Earth's atmosphere during the past decade [688,689] because this gas is widely used in the semiconductor industry. The NF₃ molecule is semirigid, belonging to the C₃v point group; the symmetry properties and selection rules are quite similar to those of other symmetric tops included in the previous HITRAN2016 release like PH₃ or CH₃D, for which the reader can find detailed discussions in Ref. [690] (and references therein). However, nitrogen trifluoride has lower vibrational modes and smaller rotational constants resulting in a much more congested infrared spectra, which leads to complicated line-by-line analyses using a purely empirical approach. This is particularly true for line intensity determination because of numerous overlapping hot bands, even at room temperature. The HITRAN2020 version includes the global combined line list of Egorov et al. [691], which was based on large-scale variational calculations with ab initio PES and DMS constructed at the CCSD(T)/CVQZ and CCSD(T)/AVQZ levels of theory. The effective Hamiltonian of NF₃ was then obtained by the six-order contact transformation approach using the MOL_C_T computational code [214,692] from the ab initio PES. The effective polyad model included six groups of vibrational states: from ground up to icosad. The ab initio parameters of the effective model were refined using the MIRS computational code [693,694] and experimental transitions of NF₃ existing in the literature. In particular, the experimental line positions from the following works were used for the fine tuning of the energy levels: ν₄(E) [695]; ν₂(A₁), ν₂ + ν₄(E), and 2ν₂(A₁) [696]; 2ν₄(A₁, E) [697]; ν₁(E) [698]; ν₃(E) [699]; ν₁ + ν₄(E) [700,701]; 2ν₃(A₁, E) and ν₁ + ν₃(E) [702]; ν₂ + ν₃(E), 2ν₁(A₁) and ν₁ + ν₂ + ν₄(E) [703]. The empirical parameters of the ground state were taken from Ref. [697] where the “loop-method” was applied to determine the *K*-dependent parameters (C₀, Dₖ, Hₖ etc.) by combining the experimental pure rotational transitions with those from the ground state combination differences. All NF₃ line intensities were calculated variationally from the pure ab initio DMS and are available in the TheoReTS web site (http://theorets.univ-reims.fr; http://theorets.tsu.ru). The first experimental studies of NF₃ integrated cross sections with medium spectral resolution were conducted in Ref. [704] and then revisited in Ref. [685]. The present NF₃ line list has been validated in detail in the work of Egorov et al. [691] by comparison with the PNNL absorption coefficients [244] above 600 cm⁻¹. The line positions and intensities in the region of the 2ν₃(A₁, E) and ν₁ + ν₃(E) bands were additionally validated using low-temperature (196 K) FTS spectra [705]. The
<figure><img src="image_36.png" /><figcaption>**Fig. 34.** (a) The broadband high-resolution spectrum of pure CH₃I measured at 0.03 mbar in the range from 2800–3160 cm⁻¹ using comb-based FTS [677]. (b) The absorption coefficient, α of the ν₄ band measured at 0.11 mbar of pure CH₃I (black) together with the simulations of the ν₄ band (red) and the ν₃ + ν₄ – ν₃ hot band (blue) obtained using PGOPHER. (c) Zoom in region around one of the Qₓ(J) sub-branches of the ν₄ band.</figcaption></figure>

The spectra of these two bands were simulated and assigned using PGOPHER [460]. The overall band structure, as well as the relative intensities of the individual lines in the simulations, agree very well with the experimental data. A least-square fit of the assigned transitions to the measured spectrum provided accurate upper-state rotational constants of both bands. The hyperfine splittings due to the ¹²⁷I iodine nuclear quadrupole moment are observed for transitions with *J* ≤ 2K. Future work will involve further analysis of the hyperfine splittings and the analysis of the measured ν₁ band and the parallel vibration bands.

For inclusion to HITRAN, transitions with *J* < 25 contain hyperfine splitting. All CH₃I lower-state energies (including transitions for the ν₆ region) have been adjusted by 1.0 × 10⁻⁴ cm⁻¹ to account for the lowest allowed energy level. Sadiek et al. [677] provided line intensities for 207 transitions obtained using a multi-spectrum fitting procedure and these intensities were adapted for HITRAN. Line intensities for the remaining transitions of the ν₄ band and ν₃ + ν₄ – ν₃ hot band have been estimated by scaling the relative PGOPHER intensities. A single scale factor was determined by taking an average ratio of the empirical intensities from Sadiek et al. [677] to their corresponding PGOPHER relative intensities. A dependence on the rotational quanta was observed in this comparison. It has been shown [683] that Herman-Wallis coefficients are necessary for determining accurate line intensities for CH₃I and will therefore be considered for future intensity analyses.

The self-and air-broadening parameters for all CH₃I transitions are based on recent studies by Raddaoui et al. [648,684]. The values are based on FTS analysis of the ν₆ band, which proposed a set of smoothed empirical parameters to calculate widths up to *J* = 80. Empirical parameters are fitted through the *K*-rotational dependencies observed for transitions having the same lower *J* value. The rotational *J*- and *K*-dependencies calculated by this model reproduced the measurements (around 1000) with a sub-percent av-

erage discrepancy and one standard deviation of around 7% both for self- and air-broadening coefficients. The smoothed parameters of Table 4 from Ref. [648] and Table 5 from Ref. [684] for self- and air-broadening coefficients, respectively, have been used to generate broadening coefficients at 296 K for all CH₃I transitions. To avoid extrapolating the self-broadening coefficients to *J''* and *K''* pairs far beyond observation, a minimum value of 0.10 cm⁻¹/ atm has been used. An error code of 5 (see Table 2) has been used for both self- and air-broadening coefficients.

There are no line shifts and temperature dependence parameters for this molecule available in the literature. Default values for temperature dependence (fixed to 0.75) of air-broadening coefficients have been used for all CH₃I transitions. For atmospheric applications, a study of the temperature dependence of the line-shape parameters of CH₃I (190–300 K) will be required.

### 2.55. NF₃: nitrogen trifluoride (molecule 55)

The nitrogen trifluoride (NF₃) line list is presented in the HITRAN database for the first time. This line list includes more than 40 cold and 680 hot sub-bands and covers the spectral range up to 2200 cm⁻¹. NF₃ is known as an anthropogenic greenhouse gas with a high global warming potential of about 17 000 [685–687]. The concentration of NF₃ has been increasing in the Earth's atmosphere during the past decade [688,689] because this gas is widely used in the semiconductor industry. The NF₃ molecule is semirigid, belonging to the C₃v point group; the symmetry properties and selection rules are quite similar to those of other symmetric tops included in the previous HITRAN2016 release like PH₃ or CH₃D, for which the reader can find detailed discussions in Ref. [690] (and references therein). However, nitrogen trifluoride has lower vibrational modes and smaller rotational constants resulting in a much more congested infrared spectra, which leads to complicated line-by-line analyses using a purely empirical approach. This is particularly true for line intensity determination because of numerous overlapping hot bands, even at room temperature. The HITRAN2020 version includes the global combined line list of Egorov et al. [691], which was based on large-scale variational calculations with ab initio PES and DMS constructed at the CCSD(T)/CVQZ and CCSD(T)/AVQZ levels of theory. The effective Hamiltonian of NF₃ was then obtained by the six-order contact transformation approach using the MOL_C_T computational code [214,692] from the ab initio PES. The effective polyad model included six groups of vibrational states: from ground up to icosad. The ab initio parameters of the effective model were refined using the MIRS computational code [693,694] and experimental transitions of NF₃ existing in the literature. In particular, the experimental line positions from the following works were used for the fine tuning of the energy levels: ν₄(E) [695]; ν₂(A₁), ν₂ + ν₄(E), and 2ν₂(A₁) [696]; 2ν₄(A₁, E) [697]; ν₁(E) [698]; ν₃(E) [699]; ν₁ + ν₄(E) [700,701]; 2ν₃(A₁, E) and ν₁ + ν₃(E) [702]; ν₂ + ν₃(E), 2ν₁(A₁) and ν₁ + ν₂ + ν₄(E) [703]. The empirical parameters of the ground state were taken from Ref. [697] where the “loop-method” was applied to determine the *K*-dependent parameters (C₀, Dₖ, Hₖ etc.) by combining the experimental pure rotational transitions with those from the ground state combination differences. All NF₃ line intensities were calculated variationally from the pure ab initio DMS and are available in the TheoReTS web site (http://theorets.univ-reims.fr; http://theorets.tsu.ru). The first experimental studies of NF₃ integrated cross sections with medium spectral resolution were conducted in Ref. [704] and then revisited in Ref. [685]. The present NF₃ line list has been validated in detail in the work of Egorov et al. [691] by comparison with the PNNL absorption coefficients [244] above 600 cm⁻¹. The line positions and intensities in the region of the 2ν₃(A₁, E) and ν₁ + ν₃(E) bands were additionally validated using low-temperature (196 K) FTS spectra [705]. The
<table><caption>**Table 10**
Absorption cross sections added to HITRAN2020 for remote sensing of the terrestrial atmosphere.</caption><thead><tr><th>Molecule</th><th>Temperature range (K)</th><th>Pressure range (Torr)</th><th>Number of P,T sets</th><th>Spectral range (cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td>CCl<sub>3</sub>F (CFC-11)</td><td>192 – 293</td><td>7.5 – 760</td><td>30</td><td>710 – 1290</td></tr><tr><td>CH<sub>3</sub>CCl<sub>2</sub>F (HCFC-141b)</td><td>188 – 295</td><td>7.5 – 761</td><td>30</td><td>705 – 1280</td></tr><tr><td>SF<sub>6</sub></td><td>189 – 294</td><td>7.5 – 751</td><td>37</td><td>780 – 1100</td></tr><tr><td>CF<sub>4</sub> (CFC-14)</td><td>190 – 296</td><td>7.5 – 760</td><td>34</td><td>1190 – 1336</td></tr></tbody></table>

NF₃ line list is now provided in the HITRAN database with averaged values for air- and self-broadened half-widths ($γ_{air} = 0.1$ cm⁻¹atm⁻¹, $γ_{self} = 0.5$ cm⁻¹atm⁻¹) as well as the temperature dependence exponent ($n_{air} = 0.55$). It is worth mentioning that only the principal isotopologue ($^{14}$NF₃) is included at this time with the abundance of 0.9964. The error codes (explained in Table 2) for all *ab initio* line intensities were set to 4, and set to 3 for the line positions of the ones corrected to the experiment bands, and to 2 for the *ab initio* line positions of all other bands. Due to a very large size of the file and the fact that *ab initio* line positions in some of the bands may deviate from the observed values, the line list for NF₃ have been placed in the folder with the other static line lists, e.g. SF₆, ClONO₂ and CF₄.

# 3. Absorption cross-sections

## 3.1. IR cross-sections

The comprehensive update of the infrared absorption cross-sections carried out for HITRAN2016 [706] dramatically extended the number of compounds represented in this section to almost 300. For HITRAN2020, some additional updates were introduced and are described below.

### 3.1.1. Halogenated species of atmospheric interest

Four high-spectral-resolution absorption cross-section datasets of the halogenated species trichlorofluoromethane (CCl₃F a.k.a. CFC-11), 1,1-dichloro-1-fluoroethane (CH₃CCl₂F a.k.a. HCFC-141b), sulfur hexafluoride (SF₆), and carbon tetrafluoride (CF₄ a.k.a. CFC-14) have been added to HITRAN2020 (see Table 10). Covering a wide range of atmospherically relevant pressures and temperatures, these datasets are intended for use by the atmospheric remote-sensing community, particularly for the interpretation of measurements by atmospheric infrared limb sounders such as the ACE-FTS [707]. CFC-11 was one of the first chlorofluorocarbons (CFCs) developed in the 1930s as inexpensive, reliable, safe and non-toxic refrigerants for domestic use. Its applications ranged from refrigerators and air conditioners to propellants in spray cans and blowing agents in foam production. As the use of CFCs became widespread, their atmospheric concentrations steadily rose. However, the discovery that they were destroying stratospheric ozone led to the signing of the 1987 Montreal Protocol. Designed to protect the Earth's ozone layer, the Protocol mandated the phasing out of CFC production. Although phased out, CFC-11 is still emitted into the atmosphere from existing “banks” (e.g., old refrigerators and air conditioners containing CFCs), but overall its atmospheric abundance is now decreasing. Despite the general success of the Montreal protocol, it was recently discovered that some countries have been emitting CFC-11 from around 2013 [708,709], although these emissions had largely been curbed by 2019 [710,711]. Hydrochlorofluorocarbons (HCFCs) were initially adopted as ‘transitional’ CFC replacements because of their shorter atmospheric lifetimes on account of their more efficient reaction with OH in the troposphere, and their reduced stratospheric ozone depletion potentials. With the worldwide CFC phase out achieved under the

terms of the Montreal Protocol, the focus has now shifted to HCFCs themselves, with a final phase out currently scheduled for 2030 for developed countries and 2040 in the developing world. HCFC-141b is primarily used as a foam blowing agent, a solvent in electronics, and for precision cleaning applications. Like other HCFCs, its atmospheric abundance continues to increase. Both CF₄ and SF₆ belong to the class of source gases known as F-gases, with fluorine as the only halogen attached to either carbon, sulfur, or nitrogen. These gases are not ozone-depleting and are not regulated by the Montreal Protocol. They both have small natural sources: degassing of the Earth's crust sustains an atmospheric background of 34.7 ppt for CF₄ [712] and up to 0.01 ppt for SF₆ [713]. However, anthropogenic emissions of these species now dominate. As these species are potent greenhouse gases with very long atmospheric lifetimes, they fall within the remit of the Kyoto Protocol. CF₄ and SF₆ have leaked into the atmosphere from a number of industrial applications: CF₄ from the production of aluminium and the manufacture of microchips in the semiconductor industry, and SF₆ as an insulating medium in high-voltage electrical equipment, in particular in electricity distribution systems, magnesium production, and semiconductor manufacturing. Due to their very long atmospheric lifetimes, SF₆ and CF₄ in the stratosphere are useful tracers for age of air. Remote-sensing measurements of both species have the potential to investigate changes in the Brewer-Dobson circulation due to climate change.

3.1.1.1. Trichlorofluoromethane (CCl₃F, CFC-11). The CFC-11 cross section dataset in previous HITRAN compilations was provided by Varanasi et al. [12,714] and has been used extensively for remote-sensing applications. For HITRAN2020, this has been replaced by a new dataset from Harrison [715], determined from spectra recorded using a high-resolution FTS (Bruker IFS 125HR) and a 26-cm-pathlength cell at spectral resolutions between 0.01 and 0.03 cm⁻¹ (see Table 10). This new dataset resolves a number of issues with the Varanasi et al. data, namely a more accurately calibrated wavenumber scale, more consistent integrated band intensities, improved signal-to-noise, no channel fringing, and a wider range of pressures and temperatures. The Varanasi et al. dataset is now available in the HITRAN alternate folder.

3.1.1.2. 1,1-Dichloro-1-fluoroethane (CH₃CCl₂F, HCFC-141b). The HITRAN2016 compilation included three 760-Torr-N₂-broadened HCFC-141b cross sections (278, 298, and 323 K) at 0.112 cm⁻¹ spectral resolution, and seven cross sections (223, 233, 243, 253, 263, 273, and 283 K) for pure HCFC-141b at 0.02 cm⁻¹ resolution [716]. However, these do not account for air-broadening below 760 Torr, so are not the most appropriate for use in remote sensing of the Earth's atmosphere. HITRAN2020 now includes air-broadened HCFC-141b cross sections [717] over a range of pressures and temperatures appropriate for atmospheric conditions (Table 10); these are derived from spectra recorded at spectral resolutions between 0.01 and 0.03 cm⁻¹ using a high-resolution FTS (Bruker IFS 125HR) and a 26-cm-pathlength cell.

3.1.1.3. Sulfur hexafluoride (SF₆). As was discussed in Section 2.30, the extent of the line list for sulfur hexafluoride does not allow
<table><caption>**Table 10**
Absorption cross sections added to HITRAN2020 for remote sensing of the terrestrial atmosphere.</caption><thead><tr><th>Molecule</th><th>Temperature range (K)</th><th>Pressure range (Torr)</th><th>Number of P,T sets</th><th>Spectral range (cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td>CCl<sub>3</sub>F (CFC-11)</td><td>192 – 293</td><td>7.5 – 760</td><td>30</td><td>710 – 1290</td></tr><tr><td>CH<sub>3</sub>CCl<sub>2</sub>F (HCFC-141b)</td><td>188 – 295</td><td>7.5 – 761</td><td>30</td><td>705 – 1280</td></tr><tr><td>SF<sub>6</sub></td><td>189 – 294</td><td>7.5 – 751</td><td>37</td><td>780 – 1100</td></tr><tr><td>CF<sub>4</sub> (CFC-14)</td><td>190 – 296</td><td>7.5 – 760</td><td>34</td><td>1190 – 1336</td></tr></tbody></table>

NF₃ line list is now provided in the HITRAN database with averaged values for air- and self-broadened half-widths ($γ_{air} = 0.1$ cm⁻¹atm⁻¹, $γ_{self} = 0.5$ cm⁻¹atm⁻¹) as well as the temperature dependence exponent ($n_{air} = 0.55$). It is worth mentioning that only the principal isotopologue ($^{14}$NF₃) is included at this time with the abundance of 0.9964. The error codes (explained in Table 2) for all *ab initio* line intensities were set to 4, and set to 3 for the line positions of the ones corrected to the experiment bands, and to 2 for the *ab initio* line positions of all other bands. Due to a very large size of the file and the fact that *ab initio* line positions in some of the bands may deviate from the observed values, the line list for NF₃ have been placed in the folder with the other static line lists, e.g. SF₆, ClONO₂ and CF₄.

# 3. Absorption cross-sections

## 3.1. IR cross-sections

The comprehensive update of the infrared absorption cross-sections carried out for HITRAN2016 [706] dramatically extended the number of compounds represented in this section to almost 300. For HITRAN2020, some additional updates were introduced and are described below.

### 3.1.1. Halogenated species of atmospheric interest

Four high-spectral-resolution absorption cross-section datasets of the halogenated species trichlorofluoromethane (CCl₃F a.k.a. CFC-11), 1,1-dichloro-1-fluoroethane (CH₃CCl₂F a.k.a. HCFC-141b), sulfur hexafluoride (SF₆), and carbon tetrafluoride (CF₄ a.k.a. CFC-14) have been added to HITRAN2020 (see Table 10). Covering a wide range of atmospherically relevant pressures and temperatures, these datasets are intended for use by the atmospheric remote-sensing community, particularly for the interpretation of measurements by atmospheric infrared limb sounders such as the ACE-FTS [707]. CFC-11 was one of the first chlorofluorocarbons (CFCs) developed in the 1930s as inexpensive, reliable, safe and non-toxic refrigerants for domestic use. Its applications ranged from refrigerators and air conditioners to propellants in spray cans and blowing agents in foam production. As the use of CFCs became widespread, their atmospheric concentrations steadily rose. However, the discovery that they were destroying stratospheric ozone led to the signing of the 1987 Montreal Protocol. Designed to protect the Earth's ozone layer, the Protocol mandated the phasing out of CFC production. Although phased out, CFC-11 is still emitted into the atmosphere from existing "banks" (e.g., old refrigerators and air conditioners containing CFCs), but overall its atmospheric abundance is now decreasing. Despite the general success of the Montreal protocol, it was recently discovered that some countries have been emitting CFC-11 from around 2013 [708,709], although these emissions had largely been curbed by 2019 [710,711]. Hydrochlorofluorocarbons (HCFCs) were initially adopted as 'transitional' CFC replacements because of their shorter atmospheric lifetimes on account of their more efficient reaction with OH in the troposphere, and their reduced stratospheric ozone depletion potentials. With the worldwide CFC phase out achieved under the

terms of the Montreal Protocol, the focus has now shifted to HCFCs themselves, with a final phase out currently scheduled for 2030 for developed countries and 2040 in the developing world. HCFC-141b is primarily used as a foam blowing agent, a solvent in electronics, and for precision cleaning applications. Like other HCFCs, its atmospheric abundance continues to increase. Both CF₄ and SF₆ belong to the class of source gases known as F-gases, with fluorine as the only halogen attached to either carbon, sulfur, or nitrogen. These gases are not ozone-depleting and are not regulated by the Montreal Protocol. They both have small natural sources: degassing of the Earth's crust sustains an atmospheric background of 34.7 ppt for CF₄ [712] and up to 0.01 ppt for SF₆ [713]. However, anthropogenic emissions of these species now dominate. As these species are potent greenhouse gases with very long atmospheric lifetimes, they fall within the remit of the Kyoto Protocol. CF₄ and SF₆ have leaked into the atmosphere from a number of industrial applications: CF₄ from the production of aluminium and the manufacture of microchips in the semiconductor industry, and SF₆ as an insulating medium in high-voltage electrical equipment, in particular in electricity distribution systems, magnesium production, and semiconductor manufacturing. Due to their very long atmospheric lifetimes, SF₆ and CF₄ in the stratosphere are useful tracers for age of air. Remote-sensing measurements of both species have the potential to investigate changes in the Brewer-Dobson circulation due to climate change.

3.1.1.1. Trichlorofluoromethane (CCl₃F, CFC-11). The CFC-11 cross section dataset in previous HITRAN compilations was provided by Varanasi et al. [12,714] and has been used extensively for remote-sensing applications. For HITRAN2020, this has been replaced by a new dataset from Harrison [715], determined from spectra recorded using a high-resolution FTS (Bruker IFS 125HR) and a 26-cm-pathlength cell at spectral resolutions between 0.01 and 0.03 cm⁻¹ (see Table 10). This new dataset resolves a number of issues with the Varanasi et al. data, namely a more accurately calibrated wavenumber scale, more consistent integrated band intensities, improved signal-to-noise, no channel fringing, and a wider range of pressures and temperatures. The Varanasi et al. dataset is now available in the HITRAN alternate folder.

3.1.1.2. 1,1-Dichloro-1-fluoroethane (CH₃CCl₂F, HCFC-141b). The HITRAN2016 compilation included three 760-Torr-N₂-broadened HCFC-141b cross sections (278, 298, and 323 K) at 0.112 cm⁻¹ spectral resolution, and seven cross sections (223, 233, 243, 253, 263, 273, and 283 K) for pure HCFC-141b at 0.02 cm⁻¹ resolution [716]. However, these do not account for air-broadening below 760 Torr, so are not the most appropriate for use in remote sensing of the Earth's atmosphere. HITRAN2020 now includes air-broadened HCFC-141b cross sections [717] over a range of pressures and temperatures appropriate for atmospheric conditions (Table 10); these are derived from spectra recorded at spectral resolutions between 0.01 and 0.03 cm⁻¹ using a high-resolution FTS (Bruker IFS 125HR) and a 26-cm-pathlength cell.

3.1.1.3. Sulfur hexafluoride (SF₆). As was discussed in Section 2.30, the extent of the line list for sulfur hexafluoride does not allow
modeling of complete spectral regions under atmospheric conditions, because of lack of hot bands. Therefore, it has always been recommended that HITRAN users make use of absorption cross sections of SF₆ for remote sensing purposes; in previous compilations this has meant using the dataset derived from measurements by Varanasi et al. [12,718]. For HITRAN2020, this dataset has been replaced by a new one from Harrison [719], which covers a wider range of pressures and temperatures, notably at the low-pressure end, has a more accurately calibrated wavenumber scale, with improved signal-to-noise, more consistent integrated band intensities, and no channel fringes. This new dataset has been determined from spectra of pure and air-broadened SF₆ recorded at spectral resolutions between 0.002 and 0.03 cm⁻¹ using a high-resolution FTS (Bruker IFS 125HR) and a 26-cm-pathlength cell. The previous dataset is now available in the HITRAN alternate folder.

### 3.1.1.4. Carbon tetrafluoride (CF₄, CFC-14).
In previous compilations, the IR absorption cross sections of CF₄ available for remote sensing were derived from measurements in the Varanasi group [12,720]. For HITRAN2020, this cross-section dataset has been replaced by a new one from Harrison [721], determined from spectra of pure and air-broadened CF₄ recorded at spectral resolutions between 0.0018 and 0.03 cm⁻¹ using a high-resolution FTS (Bruker IFS 125HR) and 5 cm⁻¹ and 26 cm-pathlength sample cells. The new dataset covers a wider range of pressures and temperatures, notably at the low-pressure end, and corrects problems with the under-resolved low-pressure measurements. Furthermore, it has a more accurately calibrated wavenumber scale, more consistent integrated band intensities, and improved signal-to-noise. The previous Varanasi et al. dataset is now available in the HITRAN alternate folder.

### 3.1.1.5. Dinitrogen pentoxide (N₂O₅).
Cross-sections of dinitrogen pentoxide covering the spectral range of 555–1765 cm⁻¹ were originally added to HITRAN in the 1992 edition [10] based on Ref. [722]. For the 2004 edition [13], these cross-sections were replaced by those from Ref. [723]. However, only 540–1380 cm⁻¹ was covered by the 2004 update, while there is still a clear need for having the data in the higher wavenumber region. Therefore, cross-sections from Ref. [722] in the 1680–1765 cm⁻¹ window previously available in HITRAN, but omitted in the 2004 edition, have made their way back to HITRAN. However, they have been modified in the following way. The overlapping regions of Refs. [722,723] were compared, and it became apparent that in order to make them consistent, a factor of 1.13 had to be applied to the Ref. [722] cross-sections. Hence, in the 1680–1765 cm⁻¹ window, cross-sections from Cantrell et al. [722] have been multiplied by 1.13 and added to the database for HITRAN2020.

## 3.1.2. Planetary-relevant broadening and high temperatures

The vast majority of absorption cross-sections in HITRAN2016 were measurements of pure gases (or samples broadened by air or N₂). To extend the applicability of the HITRAN database to planetary environments, many line-by-line molecules in HITRAN now contain collisional broadening parameters for H₂, He, CO₂ and H₂O (see Section 2). This allows the HITRAN line lists to be applied in radiative-transfer calculations of planetary atmospheres. Similarly, absorption cross-sections broadened by planetary-relevant species (H₂, He, CO₂, N₂) and covering appropriate temperatures (including higher temperatures applicable to exoplanets) are also being added to HITRAN. In HITRAN2020 some of the first “planetary” cross-sections have been included, but a larger update of relevant cross-sections is planned in the near future.

The lower atmosphere of Titan predominantly consists of N₂ (~94.2%) and CH₄ (~5.6%) with a small H₂ contribution (0.1%) and other carbon-rich compounds [724]. Therefore, the N₂-broadened

absorption cross-sections already contained in HITRAN can be applied to the study of Titan. These will be supplemented with N₂-broadened absorption cross-sections of many hydrocarbon compounds recorded with experimental conditions (pressure, composition and temperature) chosen to represent those found in Titan's atmosphere. N₂-broadened absorption cross-sections for the following molecules have been added to the database for HITRAN2020: propylene [725], propane [726], n-butane [727], isobutane [728], and benzene [729,730]. Further N₂-broadened absorption cross-sections will be included as an update to HITRAN2020 and include Refs. [731–734] and additional works.

For the atmospheres of outer planets and many exoplanets, the dominant constituents are typically H₂ and He. Again, many measurements have been recorded with experimental conditions (pressure, composition and temperature) with broadening applicable to the outer planets. For HITRAN2020, H₂-broadened absorption cross-sections for propane [735] and isobutane [728] have been added to the database, with He-broadened absorption cross-sections also included for propane [735]. Further absorption cross-sections broadened by H₂ and He will be included as an update to HITRAN2020 and include Refs. [728–733,735–739] and additional works.

The absorption cross-sections in HITRAN are typically recorded at room temperatures (and below) as appropriate for the terrestrial atmosphere. Absorption cross-sections of spectroscopically-challenging compounds are predicted to be abundant (with respect to the terrestrial atmosphere) in planetary atmospheres at elevated temperatures. A series of absorption cross-sections based on FTIR measurements at high-temperatures (up to 700 K) have been added to HITRAN2020 for ethane [544], propane [740], and propylene [741]. Further high-temperature (up to 1600 K) absorption cross sections have been measured for hydrocarbons [742–747], alcohols [748] and aldehydes [749] using either an FTS and a tube furnace setup or a rapid-tuning broad scan external cavity quantum cascade laser in conjunction with shock tube facilities. Therefore, to increase the applicability of the HITRAN data to planetary environments, these high-temperature absorption cross-sections (among other works) will be considered for future updates to HITRAN.

## 3.2. UV cross-sections

### 3.2.1. O₃

A new UV cross-section database for ozone (referred to as DLR-O3-2020), which straddles the peak of the Hartley band and covers the spectral range 28,000–42,000 cm⁻¹ (357–238 nm) and temperature range 193–293 K was generated within the framework of the ESA project SEOM-IAS, ESA/AO/1-7566/13/I-BG [750]. The database was tested for analysis of OMI observations and found to be satisfactory [751].

FTS transmittance measurements were performed using a Bruker IFS 125 HR spectrometer in combination with a coolable 22.15-cm-long single-pass cell. Measurements of absorption spectra were performed under “sealed-off” conditions. Ozone was prepared from O₂ in a silent discharge and purified and handled applying procedures similar to those given in Ref. [219]. Because decomposition of ozone was negligible at low temperatures, number densities could be derived from absolute pressure measurements. Absorption cross-sections span a large dynamic range from 3 × 10⁻²² to 1.0 × 10⁻¹⁷ cm²/molecule, requiring pressure measurements over the range 0.1 to 30 mbar. In total, 191 spectra were measured. The transmittance spectra are provided with an uncalibrated wavenumber scale, which is justified by the low-resolution nature of the O₃ spectra. It was confirmed that, due to this smooth spectral dependence, the cross-sections do not depend markedly
modeling of complete spectral regions under atmospheric conditions, because of lack of hot bands. Therefore, it has always been recommended that HITRAN users make use of absorption cross sections of SF₆ for remote sensing purposes; in previous compilations this has meant using the dataset derived from measurements by Varanasi et al. [12,718]. For HITRAN2020, this dataset has been replaced by a new one from Harrison [719], which covers a wider range of pressures and temperatures, notably at the low-pressure end, has a more accurately calibrated wavenumber scale, with improved signal-to-noise, more consistent integrated band intensities, and no channel fringes. This new dataset has been determined from spectra of pure and air-broadened SF₆ recorded at spectral resolutions between 0.002 and 0.03 cm⁻¹ using a high-resolution FTS (Bruker IFS 125HR) and a 26-cm-pathlength cell. The previous dataset is now available in the HITRAN alternate folder.

### 3.1.1.4. Carbon tetrafluoride (CF₄, CFC-14).
In previous compilations, the IR absorption cross sections of CF₄ available for remote sensing were derived from measurements in the Varanasi group [12,720]. For HITRAN2020, this cross-section dataset has been replaced by a new one from Harrison [721], determined from spectra of pure and air-broadened CF₄ recorded at spectral resolutions between 0.0018 and 0.03 cm⁻¹ using a high-resolution FTS (Bruker IFS 125HR) and 5 cm⁻¹ and 26 cm-pathlength sample cells. The new dataset covers a wider range of pressures and temperatures, notably at the low-pressure end, and corrects problems with the under-resolved low-pressure measurements. Furthermore, it has a more accurately calibrated wavenumber scale, more consistent integrated band intensities, and improved signal-to-noise. The previous Varanasi et al. dataset is now available in the HITRAN alternate folder.

### 3.1.1.5. Dinitrogen pentoxide (N₂O₅).
Cross-sections of dinitrogen pentoxide covering the spectral range of 555–1765 cm⁻¹ were originally added to HITRAN in the 1992 edition [10] based on Ref. [722]. For the 2004 edition [13], these cross-sections were replaced by those from Ref. [723]. However, only 540–1380 cm⁻¹ was covered by the 2004 update, while there is still a clear need for having the data in the higher wavenumber region. Therefore, cross-sections from Ref. [722] in the 1680–1765 cm⁻¹ window previously available in HITRAN, but omitted in the 2004 edition, have made their way back to HITRAN. However, they have been modified in the following way. The overlapping regions of Refs. [722,723] were compared, and it became apparent that in order to make them consistent, a factor of 1.13 had to be applied to the Ref. [722] cross-sections. Hence, in the 1680–1765 cm⁻¹ window, cross-sections from Cantrell et al. [722] have been multiplied by 1.13 and added to the database for HITRAN2020.

## 3.1.2. Planetary-relevant broadening and high temperatures

The vast majority of absorption cross-sections in HITRAN2016 were measurements of pure gases (or samples broadened by air or N₂). To extend the applicability of the HITRAN database to planetary environments, many line-by-line molecules in HITRAN now contain collisional broadening parameters for H₂, He, CO₂ and H₂O (see Section 2). This allows the HITRAN line lists to be applied in radiative-transfer calculations of planetary atmospheres. Similarly, absorption cross-sections broadened by planetary-relevant species (H₂, He, CO₂, N₂) and covering appropriate temperatures (including higher temperatures applicable to exoplanets) are also being added to HITRAN. In HITRAN2020 some of the first “planetary” cross-sections have been included, but a larger update of relevant cross-sections is planned in the near future.

The lower atmosphere of Titan predominantly consists of N₂ (~94.2%) and CH₄ (~5.6%) with a small H₂ contribution (0.1%) and other carbon-rich compounds [724]. Therefore, the N₂-broadened

absorption cross-sections already contained in HITRAN can be applied to the study of Titan. These will be supplemented with N₂-broadened absorption cross-sections of many hydrocarbon compounds recorded with experimental conditions (pressure, composition and temperature) chosen to represent those found in Titan's atmosphere. N₂-broadened absorption cross-sections for the following molecules have been added to the database for HITRAN2020: propylene [725], propane [726], n-butane [727], isobutane [728], and benzene [729,730]. Further N₂-broadened absorption cross-sections will be included as an update to HITRAN2020 and include Refs. [731–734] and additional works.

For the atmospheres of outer planets and many exoplanets, the dominant constituents are typically H₂ and He. Again, many measurements have been recorded with experimental conditions (pressure, composition and temperature) with broadening applicable to the outer planets. For HITRAN2020, H₂-broadened absorption cross-sections for propane [735] and isobutane [728] have been added to the database, with He-broadened absorption cross-sections also included for propane [735]. Further absorption cross-sections broadened by H₂ and He will be included as an update to HITRAN2020 and include Refs. [728–733,735–739] and additional works.

The absorption cross-sections in HITRAN are typically recorded at room temperatures (and below) as appropriate for the terrestrial atmosphere. Absorption cross-sections of spectroscopically-challenging compounds are predicted to be abundant (with respect to the terrestrial atmosphere) in planetary atmospheres at elevated temperatures. A series of absorption cross-sections based on FTIR measurements at high-temperatures (up to 700 K) have been added to HITRAN2020 for ethane [544], propane [740], and propylene [741]. Further high-temperature (up to 1600 K) absorption cross sections have been measured for hydrocarbons [742–747], alcohols [748] and aldehydes [749] using either an FTS and a tube furnace setup or a rapid-tuning broad scan external cavity quantum cascade laser in conjunction with shock tube facilities. Therefore, to increase the applicability of the HITRAN data to planetary environments, these high-temperature absorption cross-sections (among other works) will be considered for future updates to HITRAN.

## 3.2. UV cross-sections

### 3.2.1. O₃

A new UV cross-section database for ozone (referred to as DLR-O3-2020), which straddles the peak of the Hartley band and covers the spectral range 28,000–42,000 cm⁻¹ (357–238 nm) and temperature range 193–293 K was generated within the framework of the ESA project SEOM-IAS, ESA/AO/1-7566/13/I-BG [750]. The database was tested for analysis of OMI observations and found to be satisfactory [751].

FTS transmittance measurements were performed using a Bruker IFS 125 HR spectrometer in combination with a coolable 22.15-cm-long single-pass cell. Measurements of absorption spectra were performed under “sealed-off” conditions. Ozone was prepared from O₂ in a silent discharge and purified and handled applying procedures similar to those given in Ref. [219]. Because decomposition of ozone was negligible at low temperatures, number densities could be derived from absolute pressure measurements. Absorption cross-sections span a large dynamic range from 3 × 10⁻²² to 1.0 × 10⁻¹⁷ cm²/molecule, requiring pressure measurements over the range 0.1 to 30 mbar. In total, 191 spectra were measured. The transmittance spectra are provided with an uncalibrated wavenumber scale, which is justified by the low-resolution nature of the O₃ spectra. It was confirmed that, due to this smooth spectral dependence, the cross-sections do not depend markedly
on the air pressure. Consequently, the total pressure dependence of the cross-sections was neglected.

A multi-spectrum fitting approach as described in Ref. [218] was applied for each of the 6 temperatures in the range 193–293 K. Above 35000 cm⁻¹ the original spectral resolution of 3.3 cm⁻¹ was decreased to ~7.7 cm⁻¹ by Gaussian smoothing. Uncertainties for each spectral point were propagated from the noise in the transmittance spectra.

Polynomials of first- (>37000 cm⁻¹) and second-order (<37000 cm⁻¹) in temperature (in K) were then fitted to the absorption cross sections for each spectral point. This fit reveals systematic errors and reduces the statistical uncertainty. Absorption cross-sections were calculated from the polynomials at temperatures (193, 213, 233, 253, 273, 293 K) near the measurement conditions and are given in the database. The relative systematic uncertainties in the DLR-O3-2020 absorption cross-sections are 0.3%.

This is the first work where a large wavelength range relevant for ozone remote sensing was measured entirely with the FTS technique and a single spectrometer. We find differences of relative absorption cross-sections between the DLR-O3-2020 data with those from older references and inconsistencies among previous data on the order of few percent, which can be attributed to radiometric problems in the grating instruments. This result is supported by the good agreement of relative absorption cross sections with UV FTS measurements in the range 310–350 nm by the Bremen spectroscopy group [752]. The high quality of the DLR-O3-2020 data can be seen by the good agreement with recent high-accuracy, room temperature ozone cross-sections, which include the 254 nm value recommended by Hodges et al. [753] $u_r = 0.31\%$ (DLR-O3-2020 data 0.64(35)% below) and the laser-based measurement at 325.126 nm of Janssen et al. [754] $u_r = 0.09\%$ (DLR-O3-2020 data 0.94(31)% below). We note that as described in Ref. [753], the choice of the cross-section at 254 nm constitutes the spectroscopic anchor point for SI-traceable measurements of ozone mole fraction in the lower atmosphere. These observations are ubiquitous and are based on an international network of ground-level standard reference photometers (SRPs) operating at 254 nm that are dedicated to environmental monitoring. Currently, these instruments use the outdated cross-section value reported by Hearn et al. [755], which has a value that is about 1.2% greater and sixfold more uncertain than that recommended by Hodges et al. [753]. We note that there will be a globally coordinated change to the new cross-section value lasting 3 to 5 years, with a target change date of January 1, 2024. For physically consistent comparisons with ozone mixing ratios provided by ground-based SRPs to be based on the updated cross-section at 254 nm, future satellite retrievals of ozone abundance that use the DLR-O3-2020 data may be multiplied by 1.0064. Nevertheless, there remains some debate whether this scaling factor can be uniformly applied to the DLR-O3-2020 cross-section data at all wavelengths. At the moment, a broadband correction of this form will be left to the discretion of the user.

The measurement database, calculated absorption cross sections with statistical uncertainties, and polynomials can be downloaded from Ref. [750].

### 3.2.2. SO₂

Starting with the HITRAN2008 edition [14], UV cross-sections of SO₂ from Refs. [756,757] were used in the HITRAN database. While being of excellent quality, they only cover a relatively high (by terrestrial atmosphere standards) temperature range of 298–358 K. A new UV database in the spectral range 23,000–36,000 cm⁻¹ (435–278 nm) and temperature range 193–293 K was generated within the framework of the ESA project SEOM-IAS, ESA/AO/1-7566/13/IBG. A detailed publication is in preparation [758].

FTS transmittance measurements were performed using a Bruker IFS 125 HR spectrometer in combination with a 22.15 cm coolable single pass cell with a maximum optical path difference of 0.3 cm. Measurements of absorption spectra were performed under sealed-off conditions. SO₂ was supplied by Linde with a specified purity of 99.98%.

Since SO₂ is stable, number densities could be derived from absolute pressure measurements. The absorption cross sections have a large dynamic range from 10⁻²⁴ to 10⁻¹⁸ cm²/molecule, requiring a pressure range from 1.0 to 1000 mbar. In total, 87 spectra were measured. A calibration factor of 1.000003905 was applied, deduced from the calibration factor of a CH₄ measurement around 6000 cm⁻¹ with a correction accounting for the different input aperture diameter.

A multi-spectrum fitting approach as described in Ref. [218] was applied for each of the 6 temperatures in the range 193 to 293 K. Uncertainties in the baseline were reduced by fixing the absorption cross sections in the ranges <23,500 cm⁻¹ and 24,00024,400 cm⁻¹ to zero. Uncertainties for each spectral point were propagated from the noise in the transmittance spectra.

In contrast to ozone, SO₂ showed high resolution features with widths down to the Doppler limit. An error-free absorption cross sections database would require air-broadened measurements with sub-Doppler instrumental resolution. To avoid self-broadening, SO₂ pressures must be below 10 mbar, which in turn requires multi-reflection cell measurements. In principle this could have been carried out, but is impractical due to resource limitations. Thus, it was decided to measure pure SO₂ up to 1000 mbar with a moderate resolution of 1.7 cm⁻¹. In order to assess the systematic errors associated with this approach, Doppler-limited measurements have been carried out as well as self-broadened measurements with sufficient instrumental resolution to have monochromatic spectra (not provided here). The impact under conditions relevant to the TROPOMI instrument was found to be small within the requirements. The total column error was <2% for all atmospheric scenarios with SO₂ columns up to 2000 DU.

Polynomials of second-order in temperature (in K) were then fitted to the absorption cross sections for each spectral point in the range 25,000–36,000 cm⁻¹ (400–278 nm). This fit reveals systematic errors and improves the statistical uncertainty. Indeed, a fourth-order offset polynomial had to be fitted for the 213 K absorption cross sections in the range 25,000–30,900 cm⁻¹ to reduce the residuals of the temperature dependence fit. The statistical errors of the absorption cross sections were propagated into the polynomial coefficients errors and are used together with the $\chi^2$ of the polynomial fit to calculate absorption cross sections at about measurement temperatures.

The new data overcome some problems with the data sets used so far. The data from Hermans and Vandaele [756,757] are applied for high atmospheric SO₂ load in the spectral region 360–390 nm (27800–25640 cm⁻¹) and are extrapolated to lower temperature since measurements covered only 298–358 K. In the regions for lower volcanic (325–335 nm, 30,770–29,850 cm⁻¹) and background (312–326 nm, 32,050–30,675 cm⁻¹) SO₂ the data from Bogumil et al. [759] are used for atmospheric retrieval. These data have an inconsistency between 203 and 223 K in the temperature dependence of the absorption cross sections. Applying the new absorption cross section data would yield maximum SO₂ column changes of 5% with respect to the Hermans and Vandaele data and 16% with respect to the Bogumil et al. data.

The measurement database, calculated (semi-empirical) absorption cross sections with statistical uncertainties and polynomials can be downloaded from Ref. [760]. The cross-sections from this work have been added to the HITRAN2020 database.
on the air pressure. Consequently, the total pressure dependence of the cross-sections was neglected.

A multi-spectrum fitting approach as described in Ref. [218] was applied for each of the 6 temperatures in the range 193–293 K. Above 35 000 cm⁻¹ the original spectral resolution of 3.3 cm⁻¹ was decreased to ~7.7 cm⁻¹ by Gaussian smoothing. Uncertainties for each spectral point were propagated from the noise in the transmittance spectra.

Polynomials of first- (>37 000 cm⁻¹) and second-order (<37 000 cm⁻¹) in temperature (in K) were then fitted to the absorption cross sections for each spectral point. This fit reveals systematic errors and reduces the statistical uncertainty. Absorption cross-sections were calculated from the polynomials at temperatures (193, 213, 233, 253, 273, 293 K) near the measurement conditions and are given in the database. The relative systematic uncertainties in the DLR-O3-2020 absorption cross-sections are 0.3%.

This is the first work where a large wavelength range relevant for ozone remote sensing was measured entirely with the FTS technique and a single spectrometer. We find differences of relative absorption cross-sections between the DLR-O3-2020 data with those from older references and inconsistencies among previous data on the order of few percent, which can be attributed to radiometric problems in the grating instruments. This result is supported by the good agreement of relative absorption cross sections with UV FTS measurements in the range 310–350 nm by the Bremen spectroscopy group [752]. The high quality of the DLR-O3-2020 data can be seen by the good agreement with recent high-accuracy, room temperature ozone cross-sections, which include the 254 nm value recommended by Hodges et al. [753] $u_r = 0.31\%$ (DLR-O3-2020 data 0.64(35)% below) and the laser-based measurement at 325.126 nm of Janssen et al. [754] $u_r = 0.09\%$ (DLR-O3-2020 data 0.94(31)% below). We note that as described in Ref. [753], the choice of the cross-section at 254 nm constitutes the spectroscopic anchor point for SI-traceable measurements of ozone mole fraction in the lower atmosphere. These observations are ubiquitous and are based on an international network of ground-level standard reference photometers (SRPs) operating at 254 nm that are dedicated to environmental monitoring. Currently, these instruments use the outdated cross-section value reported by Hearn et al. [755], which has a value that is about 1.2% greater and sixfold more uncertain than that recommended by Hodges et al. [753]. We note that there will be a globally coordinated change to the new cross-section value lasting 3 to 5 years, with a target change date of January 1, 2024. For physically consistent comparisons with ozone mixing ratios provided by ground-based SRPs to be based on the updated cross-section at 254 nm, future satellite retrievals of ozone abundance that use the DLR-O3-2020 data may be multiplied by 1.0064. Nevertheless, there remains some debate whether this scaling factor can be uniformly applied to the DLR-O3-2020 cross-section data at all wavelengths. At the moment, a broadband correction of this form will be left to the discretion of the user.

The measurement database, calculated absorption cross sections with statistical uncertainties, and polynomials can be downloaded from Ref. [750].

### 3.2.2. SO₂

Starting with the HITRAN2008 edition [14], UV cross-sections of SO₂ from Refs. [756,757] were used in the HITRAN database. While being of excellent quality, they only cover a relatively high (by terrestrial atmosphere standards) temperature range of 298–358 K. A new UV database in the spectral range 23,000–36,000 cm⁻¹ (435–278 nm) and temperature range 193–293 K was generated within the framework of the ESA project SEOM-IAS, ESA/AO/1-7566/13/I-BG. A detailed publication is in preparation [758].

FTS transmittance measurements were performed using a Bruker IFS 125 HR spectrometer in combination with a 22.15 cm coolable single pass cell with a maximum optical path difference of 0.3 cm. Measurements of absorption spectra were performed under sealed-off conditions. SO₂ was supplied by Linde with a specified purity of 99.98%.

Since SO₂ is stable, number densities could be derived from absolute pressure measurements. The absorption cross sections have a large dynamic range from 10⁻²⁴ to 10⁻¹⁸ cm²/molecule, requiring a pressure range from 1.0 to 1000 mbar. In total, 87 spectra were measured. A calibration factor of 1.000003905 was applied, deduced from the calibration factor of a CH₄ measurement around 6000 cm⁻¹ with a correction accounting for the different input aperture diameter.

A multi-spectrum fitting approach as described in Ref. [218] was applied for each of the 6 temperatures in the range 193 to 293 K. Uncertainties in the baseline were reduced by fixing the absorption cross sections in the ranges <23,500 cm⁻¹ and 24,00024,400 cm⁻¹ to zero. Uncertainties for each spectral point were propagated from the noise in the transmittance spectra.

In contrast to ozone, SO₂ showed high resolution features with widths down to the Doppler limit. An error-free absorption cross sections database would require air-broadened measurements with sub-Doppler instrumental resolution. To avoid self-broadening, SO₂ pressures must be below 10 mbar, which in turn requires multi-reflection cell measurements. In principle this could have been carried out, but is impractical due to resource limitations. Thus, it was decided to measure pure SO₂ up to 1000 mbar with a moderate resolution of 1.7 cm⁻¹. In order to assess the systematic errors associated with this approach, Doppler-limited measurements have been carried out as well as self-broadened measurements with sufficient instrumental resolution to have monochromatic spectra (not provided here). The impact under conditions relevant to the TROPOMI instrument was found to be small within the requirements. The total column error was <2% for all atmospheric scenarios with SO₂ columns up to 2000 DU.

Polynomials of second-order in temperature (in K) were then fitted to the absorption cross sections for each spectral point in the range 25,000–36,000 cm⁻¹ (400–278 nm). This fit reveals systematic errors and improves the statistical uncertainty. Indeed, a fourth-order offset polynomial had to be fitted for the 213 K absorption cross sections in the range 25,000–30,900 cm⁻¹ to reduce the residuals of the temperature dependence fit. The statistical errors of the absorption cross sections were propagated into the polynomial coefficients errors and are used together with the $\chi^2$ of the polynomial fit to calculate absorption cross sections at about measurement temperatures.

The new data overcome some problems with the data sets used so far. The data from Hermans and Vandaele [756,757] are applied for high atmospheric SO₂ load in the spectral region 360–390 nm (27800–25640 cm⁻¹) and are extrapolated to lower temperature since measurements covered only 298–358 K. In the regions for lower volcanic (325–335 nm, 30,770–29,850 cm⁻¹) and background (312–326 nm, 32,050–30,675 cm⁻¹) SO₂ the data from Bogumil et al. [759] are used for atmospheric retrieval. These data have an inconsistency between 203 and 223 K in the temperature dependence of the absorption cross sections. Applying the new absorption cross section data would yield maximum SO₂ column changes of 5% with respect to the Hermans and Vandaele data and 16% with respect to the Bogumil et al. data.

The measurement database, calculated (semi-empirical) absorption cross sections with statistical uncertainties and polynomials can be downloaded from Ref. [760]. The cross-sections from this work have been added to the HITRAN2020 database.
# 4. Collision-induced absorption

## 4.1. Description of 2019 update

Collision-induced absorption (CIA) is the process of absorption of light by pairs of colliding molecules, due to the dipole moment induced by interactions between the colliding molecules. That is, CIA is an additional source of absorption, not just broadening of the existing monomer absorption lines. However, often CIA will lead to absorption in the same spectral regions as monomer absorption, leading to a broad feature underneath sharp absorption lines. The contribution of this continuum absorption is especially important for forbidden transitions, where monomer transitions are weak, or conversely if the absorption lines are saturated, such that it becomes difficult to extract information from their intensity. In the far infrared, CIA contributes appreciably to the heat balance of planetary atmospheres due to absorption of outgoing blackbody radiation. In the terrestrial atmosphere, CIA by $N_2$ and $O_2$ molecules is important for the remote sensing applications [311,761–766], and in exoplanetary atmospheres $O_2–O_2$ collision absorption is an important target as a possible biomarker [767]. Collision-induced absorption involving $N_2$ and $CO_2$ molecules is important in the atmospheres of Titan and Venus, respectively, whereas collision partners such as He, $H_2$ and $CH_4$ are important in gas giants and brown dwarfs [768]. Collision-induced absorption by $CH_4$, $CO_2$, and $H_2$ was suggested to affect the modeling of Venusian and Martian atmospheres in the early stages of geological history [769,770].

HITRAN has a section devoted to collision-induced absorption, which was introduced in 2012 [771]. This section has recently been updated [772]. The temperature and spectral ranges for the bands included for each collisional pair can be found in Table 11. The main updates involved the rototranslational (RT) and vibrational bands of $N_2–N_2$, $CO_2–CO_2$, the RT band of $CO_2–H_2$ and $CO_2–CH_4$, collisions involving different molecules and helium atoms, $H_2–H_2$ at low temperatures, and electronic transitions in $O_2–O_2$ and $O_2–N_2$. The database now contains CIA for $N_2–N_2$, $N_2–H_2$, $N_2–CH_4$, $N_2–H_2O$, $N_2–O_2$, $O_2–O_2$, $O_2–CO_2$, $CO_2–CO_2$, $H_2–H_2$, $H_2–He$, $H_2–CH_4$, $H_2–H$, $H–He$, $CH_4–CH_4$, $CH_4–CO_2$, $CH_4–He$, and $CH_4–Ar$ collision pairs. Instructions for accessing the CIA section of the HITRAN database can be found on the HITRAN website (www.hitran.org/cia). A set of supplementary files is available in the “Alternate folder” which contain data that are not recommended in general but do have a clear advantage over the recommended data, such as extended temperature ranges, accounting for spin statistics, or are constructed to be consistent with a particular line list. Further updates of the CIA data are forthcoming, as described below.

## 4.2. Post-2019 updates and prospects for the future

### 4.2.1. $N_2–N_2$ rototranslational band

In the Karman et al. [772] effort, the Main folder RT spectra for $N_2–N_2$ were updated with the results of quantum mechanical line-shape calculations from Karman et al. [806]. Also, the results of experimental measurements from Sung et al. [792] were provided in the Alternate folder. In the current edition, we have updated the Main folder $N_2–N_2$ CIA spectra with the slightly refined results of the semi-classical trajectory-based simulation performed by Chistikov et al. [789] at 34 temperatures between 70 and 400 K.

The trajectory-based approach developed in Ref. [789] relies on the assumption that the collisional dynamics, as well as the interaction with the electromagnetic field, can be considered within the classical framework. On the one hand, the use of a classical approximation allows the extension of this approach to the molecular systems, for which quantum consideration is presently unfeasible.

On the other hand, immediately produced classical spectral profiles do not conform to the detailed balance principle [807], which is responsible for the striking asymmetry of the experimentally observed profiles. It is widely believed that this major defect of the classical approach can be approximately corrected through the use of the so-called desymmetrization procedure [807,808]. The latter, however, is not unambiguously defined (e.g., discussion in Ref. [808]). The use of a semi-empirically scaled variant of Egelstaff’s procedure described by Frommhold [807] instead of the Schofield’s procedure [809] adopted in Ref. [789] improved the agreement with the low-temperature measurements reported by Sung et al. [792].

Fig. 35 shows the results of the theoretical calculations for the $N_2–N_2$ RT band and experimental measurements [792] at two representative temperatures. It is seen that, at least in the vicinity of the absorption peak, the trajectory-based results [789] refined with the use of Egelstaff’s procedure demonstrate an improvement compared to the previous results of Karman et al. [806]. For the temperatures 78.3, 89.3, 109.6, and 129.0 K, the measured data at the peak exceed the Karman et al. [806] calculations by 18, 13, 12, and 10%, respectively. In contrast, the trajectory-based spectra underestimate absorption with respect to experimental data by 5, 3, 3, and 2%, respectively. Such a deviation, at least for temperatures in excess of 80 K, is not significant given a reported ±3% uncertainty in the measured absorption. The measurements at 78 K appear to be suffering from the systematic non-zero offset; therefore the discrepancy with theoretical results should be interpreted with caution. In the far wing, beyond 150 cm⁻¹, irrespective of the temperature, neither of the calculations demonstrate perfect overall agreement. We have to note, however, that at 109.6 and 129.0 K (not shown) the Karman et al. [806] calculations appear to agree better with the experimental measurements at the band wing.

For the millimeter wavelength range, Serov et al. [811] have recently shown that trajectory-based spectra issued from Chistikov et al. [789], as well as Borysow et al. model [810], are in good agreement with new resonator spectrometer measurements from 105 to 200 GHz and the results of previous experimental studies, e.g., Meshkov et al. [812]. The $N_2–N_2$ data provided in the previous effort [772] were found to significantly underestimate absorption in the 105-200 GHz range due to the choice of interpolating procedure, which is imperative to simulate continuous frequency dependence since the calculations in Karman et al. [806] were carried out on a rare frequency grid.

### 4.2.2. $O_2$ fundamental in $O_2–N_2$, $O_2–O_2$ and $O_2–air$

In the Richard et al. [771] effort, the $O_2$ fundamental band $O_2–O_2$ data were adopted from Baranov et al. [790]. These Main folder data remained unchanged in the 2019 update [772]. Here, we include the data for the $O_2$ fundamental in $O_2–N_2$ and $O_2–air$, which weren’t previously provided, and extend the data in the $O_2–O_2$ Alternate folder as described below.

Laboratory measurements of the absorption by pure $O_2$ and $O_2–N_2$ mixtures most recently have been reported in Thibault et al. [800], Orlando et al. [801], and Mate et al. [813]. The data from Mate et al. [813] will be retrieved from the authors and considered for future inclusion in the database. Thibault et al. [800] and Orlando et al. [801] performed comprehensive FTIR studies at 193–293 K with 0.5 cm⁻¹ resolution and at 225–356 K with 1.0 cm⁻¹ resolution, respectively. Both sets of measurements for $O_2–O_2$ were validated against the Baranov et al. [790] data. As seen in Fig. 36, Orlando et al. [801] data contain the most noise but extend to higher temperatures than those reported in Thibault et al. [800]. The absorption data from Thibault et al. [800] at 193–293 K and Orlando et al. [801] at 356 K converted to the HITRAN format are available in Main folder $O_2–N_2$ and Alternate folder $O_2–O_2$. The Orlando et al. [801] data at 356 K was cast to the same
# 4. Collision-induced absorption

## 4.1. Description of 2019 update

Collision-induced absorption (CIA) is the process of absorption of light by pairs of colliding molecules, due to the dipole moment induced by interactions between the colliding molecules. That is, CIA is an additional source of absorption, not just broadening of the existing monomer absorption lines. However, often CIA will lead to absorption in the same spectral regions as monomer absorption, leading to a broad feature underneath sharp absorption lines. The contribution of this continuum absorption is especially important for forbidden transitions, where monomer transitions are weak, or conversely if the absorption lines are saturated, such that it becomes difficult to extract information from their intensity. In the far infrared, CIA contributes appreciably to the heat balance of planetary atmospheres due to absorption of outgoing blackbody radiation. In the terrestrial atmosphere, CIA by $N_2$ and $O_2$ molecules is important for the remote sensing applications [311,761–766], and in exoplanetary atmospheres $O_2-O_2$ collision absorption is an important target as a possible biomarker [767]. Collision-induced absorption involving $N_2$ and $CO_2$ molecules is important in the atmospheres of Titan and Venus, respectively, whereas collision partners such as He, $H_2$ and $CH_4$ are important in gas giants and brown dwarfs [768]. Collision-induced absorption by $CH_4$, $CO_2$, and $H_2$ was suggested to affect the modeling of Venusian and Martian atmospheres in the early stages of geological history [769,770].

HITRAN has a section devoted to collision-induced absorption, which was introduced in 2012 [771]. This section has recently been updated [772]. The temperature and spectral ranges for the bands included for each collisional pair can be found in Table 11. The main updates involved the rototranslational (RT) and vibrational bands of $N_2-N_2$, $CO_2-CO_2$, the RT band of $CO_2-H_2$ and $CO_2-CH_4$, collisions involving different molecules and helium atoms, $H_2-H_2$ at low temperatures, and electronic transitions in $O_2-O_2$ and $O_2-N_2$. The database now contains CIA for $N_2-N_2$, $N_2-H_2$, $N_2-CH_4$, $N_2-H_2O$, $N_2-O_2$, $O_2-O_2$, $O_2-CO_2$, $CO_2-CO_2$, $H_2-H_2$, $H_2-He$, $H_2-CH_4$, $H_2-H$, $H-He$, $CH_4-CH_4$, $CH_4-CO_2$, $CH_4-He$, and $CH_4-Ar$ collision pairs. Instructions for accessing the CIA section of the HITRAN database can be found on the HITRAN website (www.hitran.org/cia). A set of supplementary files is available in the “Alternate folder” which contain data that are not recommended in general but do have a clear advantage over the recommended data, such as extended temperature ranges, accounting for spin statistics, or are constructed to be consistent with a particular line list. Further updates of the CIA data are forthcoming, as described below.

## 4.2. Post-2019 updates and prospects for the future

### 4.2.1. $N_2-N_2$ rototranslational band

In the Karman et al. [772] effort, the Main folder RT spectra for $N_2-N_2$ were updated with the results of quantum mechanical line-shape calculations from Karman et al. [806]. Also, the results of experimental measurements from Sung et al. [792] were provided in the Alternate folder. In the current edition, we have updated the Main folder $N_2-N_2$ CIA spectra with the slightly refined results of the semi-classical trajectory-based simulation performed by Chistikov et al. [789] at 34 temperatures between 70 and 400 K.

The trajectory-based approach developed in Ref. [789] relies on the assumption that the collisional dynamics, as well as the interaction with the electromagnetic field, can be considered within the classical framework. On the one hand, the use of a classical approximation allows the extension of this approach to the molecular systems, for which quantum consideration is presently unfeasible.

On the other hand, immediately produced classical spectral profiles do not conform to the detailed balance principle [807], which is responsible for the striking asymmetry of the experimentally observed profiles. It is widely believed that this major defect of the classical approach can be approximately corrected through the use of the so-called desymmetrization procedure [807,808]. The latter, however, is not unambiguously defined (e.g., discussion in Ref. [808]). The use of a semi-empirically scaled variant of Egelstaff’s procedure described by Frommhold [807] instead of the Schofield’s procedure [809] adopted in Ref. [789] improved the agreement with the low-temperature measurements reported by Sung et al. [792].

Fig. 35 shows the results of the theoretical calculations for the $N_2-N_2$ RT band and experimental measurements [792] at two representative temperatures. It is seen that, at least in the vicinity of the absorption peak, the trajectory-based results [789] refined with the use of Egelstaff’s procedure demonstrate an improvement compared to the previous results of Karman et al. [806]. For the temperatures 78.3, 89.3, 109.6, and 129.0 K, the measured data at the peak exceed the Karman et al. [806] calculations by 18, 13, 12, and 10%, respectively. In contrast, the trajectory-based spectra underestimate absorption with respect to experimental data by 5, 3, 3, and 2%, respectively. Such a deviation, at least for temperatures in excess of 80 K, is not significant given a reported ±3% uncertainty in the measured absorption. The measurements at 78 K appear to be suffering from the systematic non-zero offset; therefore the discrepancy with theoretical results should be interpreted with caution. In the far wing, beyond 150 cm⁻¹, irrespective of the temperature, neither of the calculations demonstrate perfect overall agreement. We have to note, however, that at 109.6 and 129.0 K (not shown) the Karman et al. [806] calculations appear to agree better with the experimental measurements at the band wing.

For the millimeter wavelength range, Serov et al. [811] have recently shown that trajectory-based spectra issued from Chistikov et al. [789], as well as Borysow et al. model [810], are in good agreement with new resonator spectrometer measurements from 105 to 200 GHz and the results of previous experimental studies, e.g., Meshkov et al. [812]. The $N_2-N_2$ data provided in the previous effort [772] were found to significantly underestimate absorption in the 105-200 GHz range due to the choice of interpolating procedure, which is imperative to simulate continuous frequency dependence since the calculations in Karman et al. [806] were carried out on a rare frequency grid.

### 4.2.2. $O_2$ fundamental in $O_2-N_2$, $O_2-O_2$ and $O_2$-air

In the Richard et al. [771] effort, the $O_2$ fundamental band $O_2-O_2$ data were adopted from Baranov et al. [790]. These Main folder data remained unchanged in the 2019 update [772]. Here, we include the data for the $O_2$ fundamental in $O_2-N_2$ and $O_2$-air, which weren’t previously provided, and extend the data in the $O_2-O_2$ Alternate folder as described below.

Laboratory measurements of the absorption by pure $O_2$ and $O_2-N_2$ mixtures most recently have been reported in Thibault et al. [800], Orlando et al. [801], and Mate et al. [813]. The data from Mate et al. [813] will be retrieved from the authors and considered for future inclusion in the database. Thibault et al. [800] and Orlando et al. [801] performed comprehensive FTIR studies at 193–293 K with 0.5 cm⁻¹ resolution and at 225–356 K with 1.0 cm⁻¹ resolution, respectively. Both sets of measurements for $O_2-O_2$ were validated against the Baranov et al. [790] data. As seen in Fig. 36, Orlando et al. [801] data contain the most noise but extend to higher temperatures than those reported in Thibault et al. [800]. The absorption data from Thibault et al. [800] at 193–293 K and Orlando et al. [801] at 356 K converted to the HITRAN format are available in Main folder $O_2-N_2$ and Alternate folder $O_2-O_2$. The Orlando et al. [801] data at 356 K was cast to the same
<table><caption>**Table 11**
Summary of the different bands available in the HITRAN CIA section, including Supplementary folders for all collisional systems.</caption><thead><tr><th>System</th><th>Folder</th><th>Spectral range (cm<sup>-1</sup>)</th><th>T range (K)</th><th># of sets</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="2">H<sub>2</sub>-H<sub>2</sub></td><td>Main</td><td>20-10 000</td><td>200-3000</td><td>113</td><td>[773]</td></tr><tr><td>Alternate</td><td>0-2400</td><td>40-400</td><td>120</td><td>[774]</td></tr><tr><td>H<sub>2</sub>-He</td><td>Main</td><td>20-20 000</td><td>200-9900</td><td>334</td><td>[775]</td></tr><tr><td>H<sub>2</sub>-H</td><td>Main</td><td>100-10 000</td><td>1000-2500</td><td>4</td><td>[776]</td></tr><tr><td>He-H</td><td>Main</td><td>50-11 000</td><td>1500-10 000</td><td>10</td><td>[777]</td></tr><tr><td>H<sub>2</sub>-CH<sub>4</sub></td><td>Main</td><td>0-1946</td><td>40-400</td><td>10</td><td>[778]</td></tr><tr><td>N<sub>2</sub>-He</td><td>Main</td><td>1-1000</td><td>300</td><td>1</td><td>[779]</td></tr><tr><td>CO<sub>2</sub>-He</td><td>Main</td><td>0-1000</td><td>300</td><td>1</td><td>[779]</td></tr><tr><td>CO<sub>2</sub>-Ar</td><td>Main</td><td>0-300</td><td>200-400</td><td>21</td><td>[780]</td></tr><tr><td>CH<sub>4</sub>-He</td><td>Main</td><td>1-1000</td><td>40-350</td><td>10</td><td>[781]</td></tr><tr><td>CH<sub>4</sub>-Ar</td><td>Alternate</td><td>1-697</td><td>70-296</td><td>5</td><td>[782]</td></tr><tr><td>CH<sub>4</sub>-CH<sub>4</sub></td><td>Alternate</td><td>0-990</td><td>200-800</td><td>7</td><td>[783]</td></tr><tr><td>CO<sub>2</sub>-H<sub>2</sub></td><td>Main</td><td>0-2000</td><td>200-350</td><td>4</td><td>[770]</td></tr><tr><td>CO<sub>2</sub>-CH<sub>4</sub></td><td>Main</td><td>1-2000</td><td>200-350</td><td>4</td><td>[770]</td></tr><tr><td rowspan="5">CO<sub>2</sub>-CO<sub>2</sub></td><td rowspan="5">Main</td><td>1-750</td><td>200-800</td><td>10</td><td>[784]</td></tr><tr><td>1000-1800</td><td>200-350</td><td>6</td><td>[785]</td></tr><tr><td>1000-1800</td><td>200-350</td><td>6</td><td>[786]</td></tr><tr><td>2510-2850</td><td>221-297</td><td>3</td><td>[787]</td></tr><tr><td>2850-3250</td><td>298</td><td>1</td><td>[787]</td></tr><tr><td>N<sub>2</sub>-H<sub>2</sub></td><td>Main</td><td>0-1886</td><td>40-400</td><td>10</td><td>[788]</td></tr><tr><td rowspan="7">N<sub>2</sub>-N<sub>2</sub></td><td rowspan="7">Main</td><td>0-450</td><td>70-200</td><td>14</td><td>[789]</td></tr><tr><td>0-550</td><td>210-300</td><td>10</td><td>[789]</td></tr><tr><td>0-650</td><td>310-400</td><td>10</td><td>[789]</td></tr><tr><td>1850-3000</td><td>301-363</td><td>5</td><td>[790]</td></tr><tr><td>2000-2698</td><td>228-272</td><td>5</td><td>[791]</td></tr><tr><td>4300-5000</td><td>200-330</td><td>14</td><td>[764]</td></tr><tr><td rowspan="10">O<sub>2</sub>-O<sub>2</sub></td><td rowspan="10">Main</td><td>30-300</td><td>78-129</td><td>4</td><td>[792]</td></tr><tr><td>1150-1950</td><td>193-353</td><td>15</td><td>[793]</td></tr><tr><td>7450-8491</td><td>296</td><td>1</td><td>[794]</td></tr><tr><td>9091-9596</td><td>293</td><td>1</td><td>[795]</td></tr><tr><td>10 512-11 228</td><td>293</td><td>1</td><td>[796]</td></tr><tr><td>12 600-13 839</td><td>296</td><td>1</td><td>[797]</td></tr><tr><td>14 206-14 898</td><td>293</td><td>1</td><td>[798]</td></tr><tr><td>15 290-16 664</td><td>203-287</td><td>4</td><td>[799]</td></tr><tr><td>16 700-29 800</td><td>203-293</td><td>5</td><td>[799]</td></tr><tr><td rowspan="8">Alternate</td><td>1300-1850</td><td>193-356</td><td>7</td><td>[800,801]</td></tr><tr><td>7583-8183</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>9060-9960</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>10 525-11 125</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>12 804-13 402</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>14 296-14 806</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>1300-1850</td><td>193-356</td><td>7</td><td>[800,801]</td></tr><tr><td>1850-3000</td><td>301-363</td><td>5</td><td>[790,802]</td></tr><tr><td rowspan="10">O<sub>2</sub>-N<sub>2</sub></td><td rowspan="10">Main</td><td>2000-2698</td><td>228-272</td><td>5</td><td>[791,802]</td></tr><tr><td>7450-8488</td><td>293</td><td>1</td><td>[794]</td></tr><tr><td>12 600-13 840</td><td>296</td><td>1</td><td>[797]</td></tr><tr><td>7583-8183</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>12 804-13 402</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>1850-3000</td><td>301-363</td><td>5</td><td>[790,802]</td></tr><tr><td>2000-2698</td><td>228-272</td><td>5</td><td>[791,802]</td></tr><tr><td>4300-5000</td><td>200-330</td><td>14</td><td>[764]</td></tr><tr><td>1300-1850</td><td>193-356</td><td>7</td><td>[800,801]</td></tr><tr><td>7450-8480</td><td>250-296</td><td>3</td><td>[794]</td></tr><tr><td rowspan="7">N<sub>2</sub>-Air</td><td rowspan="7">Main</td><td>9091-9596</td><td>293</td><td>1</td><td>[795]</td></tr><tr><td>10 512-11 228</td><td>293</td><td>1</td><td>[796]</td></tr><tr><td>12 600-13 839</td><td>300</td><td>1</td><td>[797]</td></tr><tr><td>12 990-13 220</td><td>298</td><td>1</td><td>[327]</td></tr><tr><td>7583-8183</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>9060-9960</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>10 525-11 125</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td rowspan="5">O<sub>2</sub>-Air</td><td rowspan="5">Main</td><td>12 802-13 402</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>14 206-14 806</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>1930-2830</td><td>250-350</td><td>11</td><td>[803]</td></tr><tr><td>0-1379</td><td>40-400</td><td>10</td><td>[804]</td></tr><tr><td>12 600-13 839</td><td>200-300</td><td>1</td><td>[805]</td></tr><tr><td>N<sub>2</sub>-H<sub>2</sub>O</td><td>Main</td><td></td><td></td><td></td><td></td></tr><tr><td>N<sub>2</sub>-CH<sub>4</sub></td><td>Alternate</td><td>0-1379</td><td></td><td></td><td></td></tr><tr><td>O<sub>2</sub>-CO<sub>2</sub></td><td>Main</td><td>12 600-13 839</td><td></td><td></td><td></td></tr></tbody></table>

frequency grid as was used in Thibault et al. [800] through cubic spline interpolation.

Following the concept introduced in the HITRAN2016 update [16], we provided O₂-air data useful for applications for the Earth's atmosphere. The data for O₂-air was represented as a sum of O₂-O₂ and O₂-N₂ continua taken with 79% and 21% weights

corresponding to the oxygen and nitrogen abundances in the atmosphere. For consistency, the O₂-air cross-sections for each temperature are calculated based on the O₂-O₂ and O₂-N₂ data from the same source. Thibault et al. [800] data at 193-293 K and Orlando et al. data at 356 K were taken to estimate the O₂-air cross-sections. In order to reduce the noise, the Orlando et al. [801] data
<table><caption>**Table 11**
Summary of the different bands available in the HITRAN CIA section, including Supplementary folders for all collisional systems.</caption><thead><tr><th>System</th><th>Folder</th><th>Spectral range (cm<sup>-1</sup>)</th><th>T range (K)</th><th># of sets</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="2">H<sub>2</sub>-H<sub>2</sub></td><td>Main</td><td>20-10 000</td><td>200-3000</td><td>113</td><td>[773]</td></tr><tr><td>Alternate</td><td>0-2400</td><td>40-400</td><td>120</td><td>[774]</td></tr><tr><td>H<sub>2</sub>-He</td><td>Main</td><td>20-20 000</td><td>200-9900</td><td>334</td><td>[775]</td></tr><tr><td>H<sub>2</sub>-H</td><td>Main</td><td>100-10 000</td><td>1000-2500</td><td>4</td><td>[776]</td></tr><tr><td>He-H</td><td>Main</td><td>50-11 000</td><td>1500-10 000</td><td>10</td><td>[777]</td></tr><tr><td>H<sub>2</sub>-CH<sub>4</sub></td><td>Main</td><td>0-1946</td><td>40-400</td><td>10</td><td>[778]</td></tr><tr><td>N<sub>2</sub>-He</td><td>Main</td><td>1-1000</td><td>300</td><td>1</td><td>[779]</td></tr><tr><td>CO<sub>2</sub>-He</td><td>Main</td><td>0-1000</td><td>300</td><td>1</td><td>[779]</td></tr><tr><td>CO<sub>2</sub>-Ar</td><td>Main</td><td>0-300</td><td>200-400</td><td>21</td><td>[780]</td></tr><tr><td>CH<sub>4</sub>-He</td><td>Main</td><td>1-1000</td><td>40-350</td><td>10</td><td>[781]</td></tr><tr><td>CH<sub>4</sub>-Ar</td><td>Alternate</td><td>1-697</td><td>70-296</td><td>5</td><td>[782]</td></tr><tr><td>CH<sub>4</sub>-CH<sub>4</sub></td><td>Alternate</td><td>0-990</td><td>200-800</td><td>7</td><td>[783]</td></tr><tr><td>CO<sub>2</sub>-H<sub>2</sub></td><td>Main</td><td>0-2000</td><td>200-350</td><td>4</td><td>[770]</td></tr><tr><td>CO<sub>2</sub>-CH<sub>4</sub></td><td>Main</td><td>1-2000</td><td>200-350</td><td>4</td><td>[770]</td></tr><tr><td>CO<sub>2</sub>-CO<sub>2</sub></td><td>Main</td><td>1-750</td><td>200-800</td><td>10</td><td>[784]</td></tr><tr><td></td><td></td><td>1000-1800</td><td>200-350</td><td>6</td><td>[785]</td></tr><tr><td></td><td></td><td>1000-1800</td><td>200-350</td><td>6</td><td>[786]</td></tr><tr><td></td><td></td><td>2510-2850</td><td>221-297</td><td>3</td><td>[787]</td></tr><tr><td></td><td></td><td>2850-3250</td><td>298</td><td>1</td><td>[787]</td></tr><tr><td>N<sub>2</sub>-H<sub>2</sub></td><td>Main</td><td>0-1886</td><td>40-400</td><td>10</td><td>[788]</td></tr><tr><td rowspan="7">N<sub>2</sub>-N<sub>2</sub></td><td rowspan="7">Main</td><td>0-450</td><td>70-200</td><td>14</td><td>[789]</td></tr><tr><td>0-550</td><td>210-300</td><td>10</td><td>[789]</td></tr><tr><td>0-650</td><td>310-400</td><td>10</td><td>[789]</td></tr><tr><td>1850-3000</td><td>301-363</td><td>5</td><td>[790]</td></tr><tr><td>2000-2698</td><td>228-272</td><td>5</td><td>[791]</td></tr><tr><td>4300-5000</td><td>200-330</td><td>14</td><td>[764]</td></tr><tr><td>Alternate</td><td>30-300</td><td>78-129</td><td>4</td><td>[792]</td></tr><tr><td rowspan="10">O<sub>2</sub>-O<sub>2</sub></td><td rowspan="10">Main</td><td>1150-1950</td><td>193-353</td><td>15</td><td>[793]</td></tr><tr><td>7450-8491</td><td>296</td><td>1</td><td>[794]</td></tr><tr><td>9091-9596</td><td>293</td><td>1</td><td>[795]</td></tr><tr><td>10 512-11 228</td><td>293</td><td>1</td><td>[796]</td></tr><tr><td>12 600-13 839</td><td>296</td><td>1</td><td>[797]</td></tr><tr><td>14 206-14 898</td><td>293</td><td>1</td><td>[798]</td></tr><tr><td>15 290-16 664</td><td>203-287</td><td>4</td><td>[799]</td></tr><tr><td>16 700-29 800</td><td>203-293</td><td>5</td><td>[799]</td></tr><tr><td>Alternate</td><td>1300-1850</td><td>193-356</td><td>7</td><td>[800,801]</td></tr><tr><td></td><td></td><td>7583-8183</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td></td><td></td><td>9060-9960</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td></td><td></td><td>10 525-11 125</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td></td><td></td><td>12 804-13 402</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td></td><td></td><td>14 296-14 806</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td rowspan="10">O<sub>2</sub>-N<sub>2</sub></td><td rowspan="10">Main</td><td>1300-1850</td><td>193-356</td><td>7</td><td>[800,801]</td></tr><tr><td>1850-3000</td><td>301-363</td><td>5</td><td>[790,802]</td></tr><tr><td>2000-2698</td><td>228-272</td><td>5</td><td>[791,802]</td></tr><tr><td>7450-8488</td><td>293</td><td>1</td><td>[794]</td></tr><tr><td>12 600-13 840</td><td>296</td><td>1</td><td>[797]</td></tr><tr><td>Alternate</td><td>7583-8183</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>12 804-13 402</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>1850-3000</td><td>301-363</td><td>5</td><td>[790,802]</td></tr><tr><td>2000-2698</td><td>228-272</td><td>5</td><td>[791,802]</td></tr><tr><td>4300-5000</td><td>200-330</td><td>14</td><td>[764]</td></tr><tr><td rowspan="10">N<sub>2</sub>-Air</td><td rowspan="10">Main</td><td>1300-1850</td><td>193-356</td><td>7</td><td>[800,801]</td></tr><tr><td>7450-8480</td><td>250-296</td><td>3</td><td>[794]</td></tr><tr><td>9091-9596</td><td>293</td><td>1</td><td>[795]</td></tr><tr><td>10 512-11 228</td><td>293</td><td>1</td><td>[796]</td></tr><tr><td>12 600-13 839</td><td>300</td><td>1</td><td>[797]</td></tr><tr><td>Alternate</td><td>12 990-13 220</td><td>298</td><td>1</td><td>[327]</td></tr><tr><td>7583-8183</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>9060-9960</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>10 525-11 125</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>12 802-13 402</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td>14 206-14 806</td><td>206-346</td><td>15</td><td>[795]</td></tr><tr><td rowspan="2">N<sub>2</sub>-H<sub>2</sub>O</td><td rowspan="2">Main</td><td>1930-2830</td><td>250-350</td><td>11</td><td>[803]</td></tr><tr><td>Alternate</td><td>0-1379</td><td>40-400</td><td>10</td><td>[804]</td></tr><tr><td>N<sub>2</sub>-CH<sub>4</sub></td><td>Main</td><td>12 600-13 839</td><td>200-300</td><td>1</td><td>[805]</td></tr><tr><td>O<sub>2</sub>-CO<sub>2</sub></td><td>Main</td><td></td><td></td><td></td><td></td></tr></tbody></table>

frequency grid as was used in Thibault et al. [800] through cubic spline interpolation.

Following the concept introduced in the HITRAN2016 update [16], we provided O₂-air data useful for applications for the Earth's atmosphere. The data for O₂-air was represented as a sum of O₂-O₂ and O₂-N₂ continua taken with 79% and 21% weights

corresponding to the oxygen and nitrogen abundances in the atmosphere. For consistency, the O₂-air cross-sections for each temperature are calculated based on the O₂-O₂ and O₂-N₂ data from the same source. Thibault et al. [800] data at 193-293 K and Orlando et al. data at 356 K were taken to estimate the O₂-air cross-sections. In order to reduce the noise, the Orlando et al. [801] data
<figure><img src="image_39.png" /><figcaption>**Fig. 35.** N₂-N₂ collision-induced absorption spectra at two temperatures. The black curves indicate results from the trajectory-based calculation [789], the blue curves denote the experimental measurements [792], the red diamonds denote the results from the quantum-mechanical calculation [806], and the grey curves denote Borysow model spectra [810]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_40.png" /><figcaption>**Fig. 36.** Overview of O₂-O₂ data in the O₂ fundamental. Solid lines refer to the Baranov et al. [790] data, dashed lines refer to the Thibault et al. [800] data in the left panel and the Orlando et al. [801] data in the right panel, respectively.</figcaption></figure>

<figure><img src="image_41.png" /><figcaption>**Fig. 37.** Overview of O₂-air data. The data for 193-293K and 356K are derived from the Thibault et al. [800] and Orlando et al. [801] measurements, respectively.</figcaption></figure>

were smoothed using a simple moving average routine with the window size of 15 cm⁻¹. Fig. 37 demonstrates that O₂-air absorption data, despite being compiled from two sources, exhibit consistent temperature dependence.

### 4.2.3. CO₂-Ar rototranslational band

Argon is known to be the third most abundant gas in the Earth's atmosphere, with a volume mixing ratio of 0.934%. It is more than twice as abundant as water vapor, and more than 20 times as abundant as carbon dioxide. Although largely inert in

absorption, argon is capable of modifying radiative properties of other molecular species by virtue of weak intermolecular perturbation.

We include the CO₂-Ar RT spectra issued from the trajectory-based simulation carried out in Ref. [780]. In these calculations, an array of up to 20 million classical trajectories was rendered through the solution of dynamical equations in Hamilton form in the laboratory frame of reference. The ensemble-averaged autocorrelation function of the induced dipole moment is obtained from these simulations, and its Fourier transform yields the absorption spectrum. The contributions to CIA profile from the free/quasibound and true bound states were calculated separately using the same computational approach. The trajectory-based calculations were performed using *ab initio* potential energy and induced dipole surfaces obtained with the coupled-cluster (CCSD(T)) method.

The absorption spectra at millimeter wavelengths in the CO₂-Ar mixtures have been recorded with a resonator spectrometer as is described in detail in Odintsova et al. [780]. These spectra were measured in the 105-240 GHz range at 297.3 K. An excellent agreement between reported experimental data and calculated profiles was achieved. In an earlier study, Oparin et al. [814] examined the CO₂-Ar RT band using classical trajectories method and simplified potential energy and induced dipole surfaces. A thorough comparison of the data from Ref. [814] with the results of the recent trajectory-based simulation in Odintsova et al. [780] showed good agreement of both sets of calculated CIA spectra over the entire range of CO₂-Ar RT band including the millimeter wavelength range. However, we consider the data from Ref. [780], which are based on *ab initio* potential energy and induced dipole surfaces, to be somewhat more precise. Moreover, the
<figure><img src="image_37.png" /><figcaption>**Fig. 35.** N₂-N₂ collision-induced absorption spectra at two temperatures. The black curves indicate results from the trajectory-based calculation [789], the blue curves denote the experimental measurements [792], the red diamonds denote the results from the quantum-mechanical calculation [806], and the grey curves denote Borysow model spectra [810]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>**Fig. 36.** Overview of O₂-O₂ data in the O₂ fundamental. Solid lines refer to the Baranov et al. [790] data, dashed lines refer to the Thibault et al. [800] data in the left panel and the Orlando et al. [801] data in the right panel, respectively.</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>**Fig. 37.** Overview of O₂-air data. The data for 193-293K and 356K are derived from the Thibault et al. [800] and Orlando et al. [801] measurements, respectively.</figcaption></figure>

were smoothed using a simple moving average routine with the window size of 15 cm⁻¹. Fig. 37 demonstrates that O₂-air absorption data, despite being compiled from two sources, exhibit consistent temperature dependence.

### 4.2.3. CO₂-Ar rototranslational band

Argon is known to be the third most abundant gas in the Earth's atmosphere, with a volume mixing ratio of 0.934%. It is more than twice as abundant as water vapor, and more than 20 times as abundant as carbon dioxide. Although largely inert in

absorption, argon is capable of modifying radiative properties of other molecular species by virtue of weak intermolecular perturbation.

We include the CO₂-Ar RT spectra issued from the trajectory-based simulation carried out in Ref. [780]. In these calculations, an array of up to 20 million classical trajectories was rendered through the solution of dynamical equations in Hamilton form in the laboratory frame of reference. The ensemble-averaged autocorrelation function of the induced dipole moment is obtained from these simulations, and its Fourier transform yields the absorption spectrum. The contributions to CIA profile from the free/quasibound and true bound states were calculated separately using the same computational approach. The trajectory-based calculations were performed using *ab initio* potential energy and induced dipole surfaces obtained with the coupled-cluster (CCSD(T)) method.

The absorption spectra at millimeter wavelengths in the CO₂-Ar mixtures have been recorded with a resonator spectrometer as is described in detail in Odintsova et al. [780]. These spectra were measured in the 105-240 GHz range at 297.3 K. An excellent agreement between reported experimental data and calculated profiles was achieved. In an earlier study, Oparin et al. [814] examined the CO₂-Ar RT band using classical trajectories method and simplified potential energy and induced dipole surfaces. A thorough comparison of the data from Ref. [814] with the results of the recent trajectory-based simulation in Odintsova et al. [780] showed good agreement of both sets of calculated CIA spectra over the entire range of CO₂-Ar RT band including the millimeter wavelength range. However, we consider the data from Ref. [780], which are based on *ab initio* potential energy and induced dipole surfaces, to be somewhat more precise. Moreover, the
<figure><img src="image_42.png" /><figcaption>Fig. 38. The temperature variation of the CO₂-Ar CIA RT band-shape in the 200-400 K range.</figcaption></figure>

Odintsova et al. [780] data cover a wider temperature range compared to the Oparin et al. data [814].

Overall, HITRAN CIA tabulates 21 spectra for the CO₂-Ar RT band from 200 K to 400 K with steps of 10 K. The temperature variation in the CO₂-Ar RT band-shape is shown in Fig. 38. The contribution from true bound states manifests itself as a secondary peak in the 3-7 cm⁻¹ interval, which becomes more pronounced at lower temperatures.

### 4.2.4. Outlook for the future

The CIA section of the HITRAN database underwent a substantial update in 2019 [772] featuring improvements to the existing data, extension of temperature and spectral ranges, and addition of new collisional pairs. Nevertheless, there is a growing demand to further improve and extend CIA data in HITRAN. This update represents a small revision of the CIA section. For the next update of the CIA section, data from a number of recent papers will be evaluated, such as measurements and semi-empirical models of CO₂-H₂ within RT band [769,815,816] and (1-0) band of H₂ [817] and CH₄-CO₂ within RT band [769,815,816], results of trajectory-based simulation of CH₄-N₂ RT band [818], measurements of a¹Δg(v = 1) ← X³Σ⁻g(v = 0) O₂-CO₂ [819] and a¹Δg ← X³Σ⁻g O₂-O₂ [820] CIA bands.

## 5. Aerosol refractive indices

HITRAN2020 contains refractive indices in the visible, infrared, and millimeter spectral ranges of many types of materials which make up cloud, aerosol particles, and planetary surfaces. The indices apply to materials found in the Earth's atmosphere and surface, and candidate exoplanet atmospheres. Knowing the real and imaginary indices of particles, as a function of wavelength, and the particle size distribution of cloud or aerosol particles, one can calculate the extinction, scattering, and absorptive properties of the atmospheric particles [821]. These particle optical depths add to gas optical depths, which in combination, determine the wavelength dependent total optical depths of an atmosphere. The interpretation of remote-sensing retrievals of gaseous species is limited frequently by how well one can separate gaseous opacity from that of clouds and aerosols. Clouds and aerosols also take part in chemical reactions in both the liquid and solid phases, with heterogeneous chemistry on Polar Stratospheric Clouds (PSCs) being a particularly important example [822]. Table 12 lists the HITRAN2020 indices. The listing is comprised of main and supplementary data sets. Several of the supplementary datasets are older, correspond to a limited number of wavelengths, or are not complete, e.g., just

the imaginary component is tabulated. Several of the supplementary datasets are for similar materials in the main set and can be used to compare indices from different laboratories. Table 12 begins with the venerable set of indices compiled at the Air Force Cambridge Research Laboratory in 1987 [823], which includes a wide variety of materials: Water, ice, sodium chloride, sea salt, water-soluble aerosol, ammonium sulfate, carbonaceous aerosol, volcanic dust, sulfuric acid, meteoric dust, quartz, hematite, and sand. Table 12 then lists indices of supercooled water, ice at various temperatures, materials which comprise PSC particles, Saharan dust, volcanic ash, secondary organic aerosol (SOA), brown carbon, biomass fire particles, flame particles, surface minerals, Titan tholins, and candidate exoplanet atmospheric particulates.

As discussed in the Bohren and Huffman [873] text on light scattering and Mie calculations, the complex refractive index m is a function of wavelength, with real m<sub>real</sub> and imaginary m<sub>imag</sub> components.

$$m = m_{\text{real}} + im_{\text{imag}} \quad (4)$$

A plane light wave of wavelength λ is attenuated along the propagation x-axis according to

$$E = E_0 \exp(-2\pi m_{\text{imag}} x/\lambda) \exp(i2\pi m_{\text{real}} x/\lambda - i2\pi ct/\lambda) \quad (5)$$

with time t and the speed of light c. The imaginary refractive index m<sub>imag</sub> therefore determines the amount of light absorption in a medium, attenuating the light intensity by exp(-4πm<sub>imag</sub>x/λ) along a path of distance x.

Exoplanet atmosphere particles are produced at a variety of temperatures from 700 K (e.g., ZnS) to 1725 K (e.g., SiO₂). HITRAN2020 includes most of the condensates tabulated by Wakeford and Sing [874], who studied hot-Jupiter exoplanet atmospheres. Dr. Harrald Mutschke of the Friedrich Schiller University Jena kindly provided the exoplanet indices to HITRAN. Additional exoplanet material indices not listed in Table 12 are accessible from the extensive Jena website (http://www.astro.uni-jena.de/Laboratory/OCDB/index.html). New indices in HITRAN2020 include the secondary organic aerosol (SOA) indices of Liu et al. [837,838] and Dingle et al. [864], volcanic ash indices of Deguine [836], and additional indices of Titan tholins (Imanaka et al. [849]). While primary organic aerosols are emitted into the troposphere directly by the biosphere, SOA is produced by a series of gas-phase organic chemistry reactions. The Liu et al. [837,838] indices (from 0.23 to 1.2 µm) and the Dingle et al. [864] indices (at 532 and 1064 nm) correspond to several sets of indices, each of which corresponds to a specific set of hydrocarbon, hydroxyl radical sources, and NO initial laboratory conditions. SOA comprises an important fraction of tropospheric aerosols. An example of the new volcanic ash indices is presented in Fig. 39, which displays the volcanic ash

<figure><img src="image_43.png" /><figcaption>Fig. 39. Variations in the real and imaginary indices in volcanic samples from six volcanoes [836].</figcaption></figure>
<figure><img src="image_40.png" /><figcaption>Fig. 38. The temperature variation of the CO₂-Ar CIA RT band-shape in the 200-400 K range.</figcaption></figure>

Odintsova et al. [780] data cover a wider temperature range compared to the Oparin et al. data [814].

Overall, HITRAN CIA tabulates 21 spectra for the CO₂-Ar RT band from 200 K to 400 K with steps of 10 K. The temperature variation in the CO₂-Ar RT band-shape is shown in Fig. 38. The contribution from true bound states manifests itself as a secondary peak in the 3-7 cm⁻¹ interval, which becomes more pronounced at lower temperatures.

### 4.2.4. Outlook for the future

The CIA section of the HITRAN database underwent a substantial update in 2019 [772] featuring improvements to the existing data, extension of temperature and spectral ranges, and addition of new collisional pairs. Nevertheless, there is a growing demand to further improve and extend CIA data in HITRAN. This update represents a small revision of the CIA section. For the next update of the CIA section, data from a number of recent papers will be evaluated, such as measurements and semi-empirical models of CO₂-H₂ within RT band [769,815,816] and (1-0) band of H₂ [817] and CH₄-CO₂ within RT band [769,815,816], results of trajectory-based simulation of CH₄-N₂ RT band [818], measurements of a¹Δg(v = 1) ← X³Σ⁻g(v = 0) O₂-CO₂ [819] and a¹Δg ← X³Σ⁻g O₂-O₂ [820] CIA bands.

## 5. Aerosol refractive indices

HITRAN2020 contains refractive indices in the visible, infrared, and millimeter spectral ranges of many types of materials which make up cloud, aerosol particles, and planetary surfaces. The indices apply to materials found in the Earth's atmosphere and surface, and candidate exoplanet atmospheres. Knowing the real and imaginary indices of particles, as a function of wavelength, and the particle size distribution of cloud or aerosol particles, one can calculate the extinction, scattering, and absorptive properties of the atmospheric particles [821]. These particle optical depths add to gas optical depths, which in combination, determine the wavelength dependent total optical depths of an atmosphere. The interpretation of remote-sensing retrievals of gaseous species is limited frequently by how well one can separate gaseous opacity from that of clouds and aerosols. Clouds and aerosols also take part in chemical reactions in both the liquid and solid phases, with heterogeneous chemistry on Polar Stratospheric Clouds (PSCs) being a particularly important example [822]. Table 12 lists the HITRAN2020 indices. The listing is comprised of main and supplementary data sets. Several of the supplementary datasets are older, correspond to a limited number of wavelengths, or are not complete, e.g., just

the imaginary component is tabulated. Several of the supplementary datasets are for similar materials in the main set and can be used to compare indices from different laboratories. Table 12 begins with the venerable set of indices compiled at the Air Force Cambridge Research Laboratory in 1987 [823], which includes a wide variety of materials: Water, ice, sodium chloride, sea salt, water-soluble aerosol, ammonium sulfate, carbonaceous aerosol, volcanic dust, sulfuric acid, meteoric dust, quartz, hematite, and sand. Table 12 then lists indices of supercooled water, ice at various temperatures, materials which comprise PSC particles, Saharan dust, volcanic ash, secondary organic aerosol (SOA), brown carbon, biomass fire particles, flame particles, surface minerals, Titan tholins, and candidate exoplanet atmospheric particulates.

As discussed in the Bohren and Huffman [873] text on light scattering and Mie calculations, the complex refractive index m is a function of wavelength, with real m<sub>real</sub> and imaginary m<sub>imag</sub> components.

$$m = m_{\text{real}} + im_{\text{imag}} \qquad (4)$$

A plane light wave of wavelength λ is attenuated along the propagation x-axis according to

$$E = E_0 \exp(-2\pi m_{\text{imag}}x/\lambda) \exp(i2\pi m_{\text{real}}x/\lambda - i2\pi ct/\lambda) \qquad (5)$$

with time t and the speed of light c. The imaginary refractive index m<sub>imag</sub> therefore determines the amount of light absorption in a medium, attenuating the light intensity by exp(-4πm<sub>imag</sub>x/λ) along a path of distance x.

Exoplanet atmosphere particles are produced at a variety of temperatures from 700 K (e.g., ZnS) to 1725 K (e.g., SiO₂). HITRAN2020 includes most of the condensates tabulated by Wakeford and Sing [874], who studied hot-Jupiter exoplanet atmospheres. Dr. Harrald Mutschke of the Friedrich Schiller University Jena kindly provided the exoplanet indices to HITRAN. Additional exoplanet material indices not listed in Table 12 are accessible from the extensive Jena website (http://www.astro.uni-jena.de/Laboratory/OCDB/index.html). New indices in HITRAN2020 include the secondary organic aerosol (SOA) indices of Liu et al. [837,838] and Dingle et al. [864], volcanic ash indices of Deguine [836], and additional indices of Titan tholins (Imanaka et al. [849]). While primary organic aerosols are emitted into the troposphere directly by the biosphere, SOA is produced by a series of gas-phase organic chemistry reactions. The Liu et al. [837,838] indices (from 0.23 to 1.2 µm) and the Dingle et al. [864] indices (at 532 and 1064 nm) correspond to several sets of indices, each of which corresponds to a specific set of hydrocarbon, hydroxyl radical sources, and NO initial laboratory conditions. SOA comprises an important fraction of tropospheric aerosols. An example of the new volcanic ash indices is presented in Fig. 39, which displays the volcanic ash

<figure><img src="image_41.png" /><figcaption>Fig. 39. Variations in the real and imaginary indices in volcanic samples from six volcanoes [836].</figcaption></figure>
<table><caption>**Table 12**
Refractive indices included in HITRAN2020.</caption><thead><tr><th>Compound</th><th>Measurement Specifics</th><th>Reference</th></tr></thead><tbody><tr><td>Water, ice, sodium chloride, sea salt, water soluble aerosol, ammonium sulfate, carbonaceous aerosol, volcanic dust, sulfuric acid, meteoric dust, quartz, hematite, sand</td><td>Room temperature, 0.2-40 µm</td><td>[823]</td></tr><tr><td>Water</td><td>27°C, 10-5000 cm<sup>-1</sup></td><td>[824]</td></tr><tr><td>Supercooled water</td><td>238 - 269 K, 1100 - 4500 cm<sup>-1</sup></td><td>[825]</td></tr><tr><td>Ice</td><td>266 K, 0.04µm - 2 m</td><td>[826]</td></tr><tr><td>Ice</td><td>130 - 210 K, 800 - 4000 cm<sup>-1</sup></td><td>[827]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>215 K, 499-6996 cm<sup>-1</sup></td><td>[828]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>273 - 298 K, 400 - 7500 cm<sup>-1</sup></td><td>[829]</td></tr><tr><td>Nitric acid (HNO<sub>3</sub>)</td><td>223 - 293 K, 450 - 6500 cm<sup>-1</sup></td><td>[830]</td></tr><tr><td>Ternary Solution (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O/HNO<sub>3</sub>)</td><td>203 - 293 K, 450 - 6500 cm<sup>-1</sup></td><td>[830]</td></tr><tr><td>NAD (nitric acid dihydrate)</td><td>160-190 K, 700-4700 cm<sup>-1</sup></td><td>[831]</td></tr><tr><td>NAT (nitric acid trihydrate)</td><td>160 K, 711-4004 cm<sup>-1</sup></td><td>[832]</td></tr><tr><td>Amorphous nitric acid (aNAM, aNAD, aNAT)</td><td>153 K, 482-7000 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>NAM (nitric acid monohydrate)</td><td>179 K, 482-6002 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>NAD</td><td>184 K, 482-6981 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>αNAT</td><td>181 K, 482-6989 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>βNAT</td><td>196 K, 482-6364 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>Saharan dust</td><td>0.30 - 0.95 µm</td><td>[834]</td></tr><tr><td>Volcanic ash</td><td>0.45 - 25 µm</td><td>[835]</td></tr><tr><td>Volcanic ash</td><td>690 - 32500 cm<sup>-1</sup></td><td>[836]</td></tr><tr><td>SOA</td><td>0.23 - 1.2 µm</td><td>[837]</td></tr><tr><td>SOA</td><td>0.23 - 1.2 µm</td><td>[838]</td></tr><tr><td>Organic acids (Oxalic, malonic, succinic, pinonic, pyruvic, phthalic)</td><td>0.25 - 1.1 µm</td><td>[839]</td></tr><tr><td>Brown carbon</td><td>0.2 - 1.2 µm</td><td>[840]</td></tr><tr><td>Burning vegetation</td><td>525-5000 cm<sup>-1</sup></td><td>[841]</td></tr><tr><td>Burning vegetation</td><td>0.35 - 1.5 µm</td><td>[842]</td></tr><tr><td>Carbon flame</td><td>0.4 - 0.7 µm, 25 - 600°C</td><td>[843]</td></tr><tr><td>Flame soot</td><td>0.2 - 38 µm</td><td>[844]</td></tr><tr><td>Minerals (clay, illite, kaolin, montmorillonite)</td><td>2.5 - 200 µm</td><td>[845]</td></tr><tr><td>Minerals (granite, montmorillonite)</td><td>5 - 40 µm</td><td>[846]</td></tr><tr><td>Titan tholins</td><td>0.02 - 920 µm</td><td>[847]</td></tr><tr><td>Titan tholins</td><td>0.2 - 1 µm</td><td>[848]</td></tr><tr><td>Titan tholins</td><td>2.5 - 25 µm</td><td>[849]</td></tr><tr><td>KCl</td><td>0.22 - 166 µm</td><td>[845]</td></tr><tr><td>ZnS</td><td>0.22 - 166 µm</td><td>[845]</td></tr><tr><td>SiO<sub>2</sub> (amorphous)</td><td>6.6 - 487 µm, 10 - 300 K</td><td>[850]</td></tr><tr><td>SiO<sub>2</sub> (crystalline)</td><td>6.25 µm - 10 µm, 300 - 928 K</td><td>[851]</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>7.8 - 200 µm</td><td>[852]</td></tr><tr><td>FeO</td><td>0.2 - 500 µm</td><td>[853]</td></tr><tr><td>CaTiO<sub>3</sub> (Perovskite)</td><td>2.0 - 500 µm</td><td>[854]</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>0.1 - 1000 µm</td><td>[855]</td></tr><tr><td>Fe<sub>2</sub>SiO<sub>4</sub> (Fayalite)</td><td>0.4 - 10 µm</td><td>[856]</td></tr><tr><td>Fe<sub>2</sub>SiO<sub>4</sub> (Fayalite)</td><td>2 µm - 10 µm</td><td>[857]</td></tr><tr><td>MgAl<sub>2</sub>O<sub>4</sub> (annealed)</td><td>1.6 - 6825 µm</td><td>[858]</td></tr><tr><td>MgAl<sub>2</sub>O<sub>4</sub> (natural)</td><td>2.0 µm - 10 µm</td><td>[858]</td></tr><tr><td>Mg<sub>2</sub>SiO<sub>4</sub></td><td>0.19 - 948 µm</td><td>[859]</td></tr><tr><td>MgSiO<sub>3</sub></td><td>0.2 - 500 µm</td><td>[859]</td></tr><tr><td>TiO<sub>2</sub> (Rutile)</td><td>0.47 - 36.2 µm</td><td>[860]</td></tr><tr><td>TiO<sub>2</sub> (Anatase)</td><td>2.0 - 5843 µm</td><td>[860]</td></tr><tr><td>TiO<sub>2</sub> (Brookite)</td><td>2.0 - 5843 µm</td><td>[861]</td></tr><tr><td>Supplementary</td><td></td><td></td></tr><tr><td>Water and Ice</td><td>0.67 - 2.5 µm, imaginary</td><td>[862]</td></tr><tr><td>Saharan Dust</td><td>0.35 - 0.65 µm</td><td>[863]</td></tr><tr><td>SOA</td><td>0.375 and 0.632 µm, various radical sources</td><td>[864]</td></tr><tr><td>SOA</td><td>0.532 µm, various cases</td><td>[865]</td></tr><tr><td>Diesel Soot</td><td>0.45 - 10 µm</td><td>[845]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>200-300 K, 825-4700 cm<sup>-1</sup></td><td>[866]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>183 - 293 K, 2 - 23 µm</td><td>[867]</td></tr><tr><td>Nitric acid (H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub>)</td><td>213-293 K, 2 - 23 µm</td><td>[867]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>Room temperature, 75 and 90% H<sub>2</sub>SO<sub>4</sub></td><td>[868]</td></tr><tr><td>Nitric acid (H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub>)</td><td>220 K, 754-4700 cm<sup>-1</sup></td><td>[869]</td></tr><tr><td>Nitric acid (H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub>)</td><td>Room temperature, 2 - 40 µm</td><td>[870]</td></tr><tr><td>Sulfuric and Nitric acids</td><td>Room temperature, 6 - 11 µm</td><td>[871]</td></tr><tr><td>Titan organic haze</td><td>0.532 µm (single wavelength)</td><td>[872]</td></tr></tbody></table>
<table><caption>**Table 12**
Refractive indices included in HITRAN2020.</caption><thead><tr><th>Compound</th><th>Measurement Specifics</th><th>Reference</th></tr></thead><tbody><tr><td>Water, ice, sodium chloride, sea salt, water soluble aerosol, ammonium sulfate, carbonaceous aerosol, volcanic dust, sulfuric acid, meteoric dust, quartz, hematite, sand</td><td>Room temperature, 0.2-40 µm</td><td>[823]</td></tr><tr><td>Water</td><td>27°C, 10-5000 cm<sup>-1</sup></td><td>[824]</td></tr><tr><td>Supercooled water</td><td>238 - 269 K, 1100 - 4500 cm<sup>-1</sup></td><td>[825]</td></tr><tr><td>Ice</td><td>266 K, 0.04µm - 2 m</td><td>[826]</td></tr><tr><td>Ice</td><td>130 - 210 K, 800 - 4000 cm<sup>-1</sup></td><td>[827]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>215 K, 499-6996 cm<sup>-1</sup></td><td>[828]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>273 - 298 K, 400 - 7500 cm<sup>-1</sup></td><td>[829]</td></tr><tr><td>Nitric acid (HNO<sub>3</sub>)</td><td>223 - 293 K, 450 - 6500 cm<sup>-1</sup></td><td>[830]</td></tr><tr><td>Ternary Solution (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O/HNO<sub>3</sub>)</td><td>203 - 293 K, 450 - 6500 cm<sup>-1</sup></td><td>[830]</td></tr><tr><td>NAD (nitric acid dihydrate)</td><td>160-190 K, 700-4700 cm<sup>-1</sup></td><td>[831]</td></tr><tr><td>NAT (nitric acid trihydrate)</td><td>160 K, 711-4004 cm<sup>-1</sup></td><td>[832]</td></tr><tr><td>Amorphous nitric acid (aNAM, aNAD, aNAT)</td><td>153 K, 482-7000 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>NAM (nitric acid monohydrate)</td><td>179 K, 482-6002 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>NAD</td><td>184 K, 482-6981 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>αNAT</td><td>181 K, 482-6989 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>βNAT</td><td>196 K, 482-6364 cm<sup>-1</sup></td><td>[833]</td></tr><tr><td>Saharan dust</td><td>0.30 - 0.95 µm</td><td>[834]</td></tr><tr><td>Volcanic ash</td><td>0.45 - 25 µm</td><td>[835]</td></tr><tr><td>Volcanic ash</td><td>690 - 32500 cm<sup>-1</sup></td><td>[836]</td></tr><tr><td>SOA</td><td>0.23 - 1.2 µm</td><td>[837]</td></tr><tr><td>SOA</td><td>0.23 - 1.2 µm</td><td>[838]</td></tr><tr><td>Organic acids (Oxalic, malonic, succinic, pinonic, pyruvic, phthalic)</td><td>0.25 - 1.1 µm</td><td>[839]</td></tr><tr><td>Brown carbon</td><td>0.2 - 1.2 µm</td><td>[840]</td></tr><tr><td>Burning vegetation</td><td>525-5000 cm<sup>-1</sup></td><td>[841]</td></tr><tr><td>Burning vegetation</td><td>0.35 - 1.5 µm</td><td>[842]</td></tr><tr><td>Carbon flame</td><td>0.4 - 0.7 µm, 25 - 600°C</td><td>[843]</td></tr><tr><td>Flame soot</td><td>0.2 - 38 µm</td><td>[844]</td></tr><tr><td>Minerals (clay, illite, kaolin, montmorillonite)</td><td>2.5 - 200 µm</td><td>[845]</td></tr><tr><td>Minerals (granite, montmorillonite)</td><td>5 - 40 µm</td><td>[846]</td></tr><tr><td>Titan tholins</td><td>0.02 - 920 µm</td><td>[847]</td></tr><tr><td>Titan tholins</td><td>0.2 - 1 µm</td><td>[848]</td></tr><tr><td>Titan tholins</td><td>2.5 - 25 µm</td><td>[849]</td></tr><tr><td>KCl</td><td>0.22 - 166 µm</td><td>[845]</td></tr><tr><td>ZnS</td><td>0.22 - 166 µm</td><td>[845]</td></tr><tr><td>SiO<sub>2</sub> (amorphous)</td><td>6.6 - 487 µm, 10 - 300 K</td><td>[850]</td></tr><tr><td>SiO<sub>2</sub> (crystalline)</td><td>6.25 µm - 10 µm, 300 - 928 K</td><td>[851]</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>7.8 - 200 µm</td><td>[852]</td></tr><tr><td>FeO</td><td>0.2 - 500 µm</td><td>[853]</td></tr><tr><td>CaTiO<sub>3</sub> (Perovskite)</td><td>2.0 - 500 µm</td><td>[854]</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>0.1 - 1000 µm</td><td>[855]</td></tr><tr><td>Fe<sub>2</sub>SiO<sub>4</sub> (Fayalite)</td><td>0.4 - 10 µm</td><td>[856]</td></tr><tr><td>Fe<sub>2</sub>SiO<sub>4</sub> (Fayalite)</td><td>2 µm - 10 µm</td><td>[857]</td></tr><tr><td>MgAl<sub>2</sub>O<sub>4</sub> (annealed)</td><td>1.6 - 6825 µm</td><td>[858]</td></tr><tr><td>MgAl<sub>2</sub>O<sub>4</sub> (natural)</td><td>2.0 µm - 10 µm</td><td>[858]</td></tr><tr><td>Mg<sub>2</sub>SiO<sub>4</sub></td><td>0.19 - 948 µm</td><td>[859]</td></tr><tr><td>MgSiO<sub>3</sub></td><td>0.2 - 500 µm</td><td>[859]</td></tr><tr><td>TiO<sub>2</sub> (Rutile)</td><td>0.47 - 36.2 µm</td><td>[860]</td></tr><tr><td>TiO<sub>2</sub> (Anatase)</td><td>2.0 - 5843 µm</td><td>[860]</td></tr><tr><td>TiO<sub>2</sub> (Brookite)</td><td>2.0 - 5843 µm</td><td>[861]</td></tr><tr><td>Supplementary</td><td></td><td></td></tr><tr><td>Water and Ice</td><td>0.67 - 2.5 µm, imaginary</td><td>[862]</td></tr><tr><td>Saharan Dust</td><td>0.35 - 0.65 µm</td><td>[863]</td></tr><tr><td>SOA</td><td>0.375 and 0.632 µm, various radical sources</td><td>[864]</td></tr><tr><td>SOA</td><td>0.532 µm, various cases</td><td>[865]</td></tr><tr><td>Diesel Soot</td><td>0.45 - 10 µm</td><td>[845]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>200-300 K, 825-4700 cm<sup>-1</sup></td><td>[866]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>183 - 293 K, 2 - 23 µm</td><td>[867]</td></tr><tr><td>Nitric acid (H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub>)</td><td>213-293 K, 2 - 23 µm</td><td>[867]</td></tr><tr><td>Sulfuric acid (H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O)</td><td>Room temperature, 75 and 90% H<sub>2</sub>SO<sub>4</sub></td><td>[868]</td></tr><tr><td>Nitric acid (H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub>)</td><td>220 K, 754-4700 cm<sup>-1</sup></td><td>[869]</td></tr><tr><td>Nitric acid (H<sub>2</sub>SO<sub>4</sub>/HNO<sub>3</sub>)</td><td>Room temperature, 2 - 40 µm</td><td>[870]</td></tr><tr><td>Sulfuric and Nitric acids</td><td>Room temperature, 6 - 11 µm</td><td>[871]</td></tr><tr><td>Titan organic haze</td><td>0.532 µm (single wavelength)</td><td>[872]</td></tr></tbody></table>
indices of Deguine et al. [836]. Due to the impact upon civil aviation by the Icelandic Eyjafjallajökull eruption in 2010, there is interest in the optical properties of dust emissions from active volcanoes. Fig. 39 displays the indices for six volcanoes, including Eyjafjallajökull. There is a sizable difference (by a factor of 1.5) in the imaginary indices near 10.6 µm for the Etna (Italy) and Grímsvötn (Iceland) volcanoes, and thus for the same particle size distribution, a sizable difference in the extinction spectra (a factor of 1.6 at 10.6 µm).

HITRAN2020 extends the HITRAN-RI program [875] that resides on the HITRAN website, by including a version of the program written in Python. HITRAN-RI is also written in the IDL (Interactive Design Language) and FORTRAN 90 programming languages, and all three versions apply the Bohren-Huffman [873] Mie code. The FORTRAN 90 version has been changed to read in ASCII files for compilation ease purposes. All programs and subdirectories are bundled together in a single tar file. In all three versions of the program, the user edits the directory path names in the `directory.dat` ASCII file, specifying the subdirectories that have the input data sets and the output subdirectory to which files are written to. One then examines the `indices.dat` ASCII file to determine which material to work with. The editing of the `work.dat` file just requires simply replacing integers and floating-point numbers with new values. The wavelength range, and the particle size distribution, are also specified in the `work.dat` file. The use of the `".dat"` files allows the user to specify the HITRAN-RI calculations without having to modify the source code. The HITRAN-RI program then calculates optical property spectra of extinction, scattering, absorption, single scattering albedo, and the asymmetry parameter. The IDL version of HITRAN-RI generates output postscript and NetCDF files of the input indices, particle size distribution, and wavelength dependent optical properties. As an instructional aid, test cases can be run. PDF versions of the original reference papers are contained in a subdirectory, while the refractive indices are stored in subdirectories in ASCII and NetCDF formats. The user can edit the `work.dat` file to instruct the program to read in user-specified refractive indices and/or the particle size distribution. Since all of the source code is fully accessible, HITRAN-RI can be modified as desired by the user.

# 6. Global data and software

## 6.1. Database structure and interface

HITRANonline, the software providing an online interface to the HITRAN database at https://hitran.org, has undergone a series of improvements and minor modifications since its release in 2015 [51]. In addition to bug-fixes and security patches, the entire code base was ported to Python 3 in 2019, using Django 2 as its web framework. At the same time the web server was configured to use the HTTPS protocol for secure communication with client computers. Errors in the HDF-5 output format were corrected and compatibility with the major web browsers on both Windows and Unix-like operating systems improved.

To access the database, HITRANonline requires users to register for an account by providing a name and using a valid email address. An ongoing inconvenience has been the large number of automated bots signing up fake accounts with the website that consequently required removal. This has been mitigated by the introduction of a question on the registration page which is found to be difficult to parse by a bot but intended to be easy for humans to answer correctly. The question is currently "1 + two"; for the avoidance of doubt, the correct answer to this question is "3" or "three".

A number of video tutorials (https://hitran.org/videos/) have been created to guide users through the main aspects of the

database and demonstrate how to navigate the website and make queries in different formats. These tutorials have proven to be very helpful to new users, and on aggregate they have been watched over 9500 times so far.

Recently, a new automated referencing system has been developed and implemented as part of the HITRAN project to provide consistent, accurate and detailed bibliographies for every source of data in the HITRAN database. Administrators using this system can obtain the complete bibliographic entry for the article they wish to cite by entering only its unique digital object identifier (DOI). The referencing program, which is available as open source software, is described by Skinner et al. [876]. It provides a convenient, customizable bibliographic system to allow database administrators to implement bibliographies in their database systems more efficiently and with fewer human errors.

## 6.2. HAPI2

The first generation of the HITRAN Application Programming Interface (HAPI) [52] has proven to be a convenient tool for acquiring and working with HITRAN data. The HAPI library provided a means of downloading and filtering the spectroscopic transitions for molecules provided by the HITRANonline [51] web server, using a range of partition sums and spectral line parameters. A significant feature of HAPI was the ability to calculate absorption coefficients based on the line-by-line spectroscopic parameters. For a more detailed description of this software library, we refer readers to the dedicated paper [52] and corresponding user manual available online (https://hitran.org/hapi).

Although the first generation of HAPI allows users to build new functions, it does not have the functionality to make use the whole range of spectroscopic data currently available in the HITRAN database. For instance, the first version of the REST-API used by HAPI only allowed line-by-line data to be downloaded. For this reason, an extended version of HAPI (with greater functionality) is provided as part of HITRAN2020. This extended version, named "HAPI2", includes all the functionality of HAPI but with a new Python library and has been designed to be backward-compatible. To take advantage of the more advanced features in the "second generation" extension library, users will be required to upgrade to HAPI2.

One main feature of HAPI2 will be the ability to consider more objects available for downloading. This essentially means users will now be able to access the vast library of absorption cross-sections, CIA, and more. This was achieved by revisiting the HITRAN server's REST API. A new version is able to access the information for a number of entities available in HITRAN. Among these entities are molecule information, reference sources, line-by-line transitions, monomer and collision-induced absorption cross-sections, and metadata on line parameters. An example of the REST API query for the water molecule and corresponding JSON response are shown in Fig. 40. Users should note that, in order to be able to query the new REST API, a valid API key is required in the request. An API key is a unique string identifier, which can be generated in the HITRANonline user profile by pressing the button "generate API key", as shown in Fig. 41.

Secondly, for applications that require numerous transitions to be considered in absorption coefficient calculations (such as at high-temperatures), the speed of calculation is of paramount importance. Although the first generation of HAPI [52] contained some Numpy-based optimizations [877], it lacked the means for fast cross-section computation. In HAPI2, efficient coding for HT and SDV profiles [74,878,879] that makes use of the "Just-in-time" compilation approach, has provided a significant speed increase for computations.
indices of Deguine et al. [836]. Due to the impact upon civil aviation by the Icelandic Eyjafjallajökull eruption in 2010, there is interest in the optical properties of dust emissions from active volcanoes. Fig. 39 displays the indices for six volcanoes, including Eyjafjallajökull. There is a sizable difference (by a factor of 1.5) in the imaginary indices near 10.6 µm for the Etna (Italy) and Grímsvötn (Iceland) volcanoes, and thus for the same particle size distribution, a sizable difference in the extinction spectra (a factor of 1.6 at 10.6 µm).

HITRAN2020 extends the HITRAN-RI program [875] that resides on the HITRAN website, by including a version of the program written in Python. HITRAN-RI is also written in the IDL (Interactive Design Language) and FORTRAN 90 programming languages, and all three versions apply the Bohren-Huffman [873] Mie code. The FORTRAN 90 version has been changed to read in ASCII files for compilation ease purposes. All programs and subdirectories are bundled together in a single tar file. In all three versions of the program, the user edits the directory path names in the `directory.dat` ASCII file, specifying the subdirectories that have the input data sets and the output subdirectory to which files are written to. One then examines the `indices.dat` ASCII file to determine which material to work with. The editing of the `work.dat` file just requires simply replacing integers and floating-point numbers with new values. The wavelength range, and the particle size distribution, are also specified in the `work.dat` file. The use of the `.dat` files allows the user to specify the HITRAN-RI calculations without having to modify the source code. The HITRAN-RI program then calculates optical property spectra of extinction, scattering, absorption, single scattering albedo, and the asymmetry parameter. The IDL version of HITRAN-RI generates output postscript and NetCDF files of the input indices, particle size distribution, and wavelength dependent optical properties. As an instructional aid, test cases can be run. PDF versions of the original reference papers are contained in a subdirectory, while the refractive indices are stored in subdirectories in ASCII and NetCDF formats. The user can edit the `work.dat` file to instruct the program to read in user-specified refractive indices and/or the particle size distribution. Since all of the source code is fully accessible, HITRAN-RI can be modified as desired by the user.

# 6. Global data and software

## 6.1. Database structure and interface

HITRANonline, the software providing an online interface to the HITRAN database at https://hitran.org, has undergone a series of improvements and minor modifications since its release in 2015 [51]. In addition to bug-fixes and security patches, the entire code base was ported to Python 3 in 2019, using Django 2 as its web framework. At the same time the web server was configured to use the HTTPS protocol for secure communication with client computers. Errors in the HDF-5 output format were corrected and compatibility with the major web browsers on both Windows and Unix-like operating systems improved.

To access the database, HITRANonline requires users to register for an account by providing a name and using a valid email address. An ongoing inconvenience has been the large number of automated bots signing up fake accounts with the website that consequently required removal. This has been mitigated by the introduction of a question on the registration page which is found to be difficult to parse by a bot but intended to be easy for humans to answer correctly. The question is currently "1 + two"; for the avoidance of doubt, the correct answer to this question is "3" or "three".

A number of video tutorials (https://hitran.org/videos/) have been created to guide users through the main aspects of the

database and demonstrate how to navigate the website and make queries in different formats. These tutorials have proven to be very helpful to new users, and on aggregate they have been watched over 9500 times so far.

Recently, a new automated referencing system has been developed and implemented as part of the HITRAN project to provide consistent, accurate and detailed bibliographies for every source of data in the HITRAN database. Administrators using this system can obtain the complete bibliographic entry for the article they wish to cite by entering only its unique digital object identifier (DOI). The referencing program, which is available as open source software, is described by Skinner et al. [876]. It provides a convenient, customizable bibliographic system to allow database administrators to implement bibliographies in their database systems more efficiently and with fewer human errors.

## 6.2. HAPI2

The first generation of the HITRAN Application Programming Interface (HAPI) [52] has proven to be a convenient tool for acquiring and working with HITRAN data. The HAPI library provided a means of downloading and filtering the spectroscopic transitions for molecules provided by the HITRANonline [51] web server, using a range of partition sums and spectral line parameters. A significant feature of HAPI was the ability to calculate absorption coefficients based on the line-by-line spectroscopic parameters. For a more detailed description of this software library, we refer readers to the dedicated paper [52] and corresponding user manual available online (https://hitran.org/hapi).

Although the first generation of HAPI allows users to build new functions, it does not have the functionality to make use the whole range of spectroscopic data currently available in the HITRAN database. For instance, the first version of the REST-API used by HAPI only allowed line-by-line data to be downloaded. For this reason, an extended version of HAPI (with greater functionality) is provided as part of HITRAN2020. This extended version, named "HAPI2", includes all the functionality of HAPI but with a new Python library and has been designed to be backward-compatible. To take advantage of the more advanced features in the "second generation" extension library, users will be required to upgrade to HAPI2.

One main feature of HAPI2 will be the ability to consider more objects available for downloading. This essentially means users will now be able to access the vast library of absorption cross-sections, CIA, and more. This was achieved by revisiting the HITRAN server's REST API. A new version is able to access the information for a number of entities available in HITRAN. Among these entities are molecule information, reference sources, line-by-line transitions, monomer and collision-induced absorption cross-sections, and metadata on line parameters. An example of the REST API query for the water molecule and corresponding JSON response are shown in Fig. 40. Users should note that, in order to be able to query the new REST API, a valid API key is required in the request. An API key is a unique string identifier, which can be generated in the HITRANonline user profile by pressing the button "generate API key", as shown in Fig. 41.

Secondly, for applications that require numerous transitions to be considered in absorption coefficient calculations (such as at high-temperatures), the speed of calculation is of paramount importance. Although the first generation of HAPI [52] contained some Numpy-based optimizations [877], it lacked the means for fast cross-section computation. In HAPI2, efficient coding for HT and SDV profiles [74,878,879] that makes use of the "Just-in-time" compilation approach, has provided a significant speed increase for computations.
Query: hitran.org/api/v2/<api_key>/molecules?id=1

<figure><img src="image_44.png" /><figcaption>**Fig. 40.** Sample JSON response for the REST API query requesting molecule information, with an additional restriction imposed on the HITRAN ID value. The JSON fields contain the information on the molecule entry with the `id=1` (water vapor).</figcaption></figure>

<figure><img src="image_45.png" /><figcaption>**Fig. 41.** An API key is required to make use of the full querying capabilities of HAPI2. Users can generate a personal API key from their HITRANonline user profile at www.hitran.org by clicking "Generate API key". The resulting API key will be displayed in the location indicated in the screenshot.</figcaption></figure>

In addition, among the prominent features of HAPI2 is the ability to account for the full line-mixing. Following the work of Hashemi et al. [161], the latest versions (starting from v1.1) of HAPI [52] can account for line-mixing effects through the implementation of the first-order Rosenkranz line-mixing parameters Y [232] into the Hartmann-Tran profile for the CO₂ molecule. Despite the advantages of the first-order approach (e.g., reduced number of collisional quantities and fast computations of the profiles), it fails to model the absorption for regions where lines are dense and strongly affected by line-mixing, such as Q-branches of CO₂. The HAPI2 extension includes the Python version of the line-mixing code by Lamouroux et al. [166] with account of the corrections made by Hashemi et al. [161]. Despite the structure of the Python version, it has essentially the same structure as the FORTRAN version [166] (and references therein), the major difference is that the database files are no longer provided. They will be constructed

from the code locally by the users, as a preliminary step of the first CO₂ line-mixing calculation. If the files were already built, this step will not be executed by the code, except if it is explicitly asked by the user.

As was the case for HAPI [52], the HAPI2 extension stores the downloaded files locally. Thus, the final prominent feature of HAPI2 is the ability to make use of the fast and flexible relational database on these local files. This gives users the ability to perform data mining on the stored sets of molecules, references, line parameters, and cross-sections, including user-supplied data sets. The data scheme used in this relational engine is an extension of the relational scheme constructed by Hill et al. [50]. In the HAPI2 implementation, this scheme was created using the SQLAlchemy (https://www.sqlalchemy.org/) library which provides back-ends for many existing database management systems such as SQLite, MySQL, PostgreSQL, and more. HAPI2 uses SQLite as the default database back-end. The back-ends in HAPI2 use the Object Relational Mapping (ORM) technique to connect with the low-level database. Some of the data filtering is available through the REST API on the server side (e.g., setting the wavenumber range for transitions and specifying the set of isotopologues). Nevertheless, the full capability of the data filtering can be done locally by the means of the standard SQLAlchemy ORM methods.

The HAPI2 extension is currently available on the Github repository (https://github.com/hitranonline/hapi2) as well as in the Python Package Index. A more complete description of the HAPI2 extension library will be described in a separate paper.

## 6.3. HAPIEST

The HITRAN Application Programming Interface and Efficient Spectroscopic Tools (HAPIEST) is a joint project which started in the Fall of 2017 as a collaboration between the HITRAN team and the State University of New York at Oswego. The purpose of HAPIEST is to simplify usage of HAPI to work efficiently with HITRANonline and to allow users who are not familiar with Python to access the spectroscopic data offered by HITRAN.

HAPIEST provides a cross-platform graphical user interactive that gives access to the basic features of HAPI (such as data fetching and filtering), as well as calculating and plotting spectral functions (absorption coefficients, and transmittance, absorption, and radiance spectra). Moreover, HAPIEST provides access to most of the controls that are involved in spectral filtering and simulation, and is distributed both as binary and source code. The most recent version of the source code can be found on Github (https://github.com/hitranonline/hapiest), as well as the most recent binary versions (https://github.com/hitranonline/hapiest/releases).

Using the HAPI and HAPI2 libraries, HAPIEST can retrieve the spectroscopic line-by-line data for any molecule or isotopologue in HITRAN. There are currently three functionalities provided in HAPIEST for line-by-line data: viewing, graphing, and data selection. The "view" feature allows line-by-line data to be viewed in a spreadsheet-like widget. This feature automatically paginates tables to save on system resources, but this page-length can be changed in the user-configurable settings. One major feature of HAPIEST is the graphing capability. HAPIEST is able to display the line-by-line data in graphical form by calculating absorption coefficients, as well as absorption, transmittance, and radiance spectra, with a variety of instrumental functions and line profiles. As part of the HAPIEST functionality, users are able to filter (e.g., by vibrational band) and plot results separately. When bands are plotted, HAPIEST creates a legend for band graphs which will display upper- and lower-quanta of each band. In addition, the absorption cross-section data can also be downloaded from the HITRANonline web server and plotted by HAPIEST. Multiple absorp-