RFM Spectroscopic Data
Contents
05APR25

Introduction

Spectroscopic data for each molecule can be represented in one of three forms
Line parameters
Set of parameters required to model each individual vibrational/rotational transition
Absorption cross-section
Tabulated values of absorption coefficient as a function of wavenumber, pressure and temperature
Collision-Induced Absorption (CIA)
Transient absorption features generated during collisions between two (same or different) molecules, tabulated as a function of wavenumber and temperature
In addition, the RFM can use Look Up Tables (LUTs) which are an (RFM-specific) alternative representation of spectroscopic data, replacing either line or absorption cross-section (but not CIA) data.

Line Parameters - HITRAN .par files

List of HITRAN .par file parameters used by the RFM
SymbolParameter Units
M Molecule number [n/a]
I Isotope number [n/a]
ν Vacuum Wavenumber cm-1
S Intensity cm-1/(molec cm-2)
γair Air-broadened half-width cm-1 atm-1
γself Self-broadened half-width cm-1 atm-1
E" Lower-state energy cm-1
nair Temperature dependence exponent for γair [n/a]
δair Air pressure-induced line shift cm-1 atm-1
V',V" Vibrational quanta (via HITBIN, for non-LTE) [n/a]
Q',Q" Rotational quanta (for CO2 line-mixing) [n/a]
HITRAN [Web-site] is the most widely-used compilation of spectroscopic line parameters.

The basic form consists of text records of 160-characters (.par file), one record for each transition, ordered by increasing wavenumber.

The 2020 edition contained over 7 million records spanning 0–7×106 cm-1 (i.e. wavelengths down to 1.4 nm), for 55 different molecules.

The parameters used by the RFM are listed in the table.

While the RFM can use this .par file directly, it can be quite slow, and the recommendation is to use the HITBIN program to convert it to a binary format. The binary format is also required for non-LTE calculations, selection of vibrational bands and use of the internal CO2 line-coupling model.

There are also some local Fortran/Python programs for manipulating the HITRAN parameter files:

SUBHIT
Subset HITRAN .par file. Can be used to reduce original .par file to containing only molecules/spectral range of interest.
MRGHIT
Merge HITRAN .par files. Can be used to merge different .par files, eg to replace molecule within the original file with a different set.

Additional Line Parameters - HITRAN database

A limitation of the standard HITRAN .par format is that it does not allow for any additional line parameters beyond the basic set required for the standard Voigt lineshape, and even those are restricted to measurements with 'air' as the broadening gas (with the exception of self-broadened half-width).

Nowadays HITRAN is actually a database, and the 160-character .par line list is just one way of accessing the information. In fact the user has complete flexibility in extracting any subset of parameters in any order in different formats [Example].

Unfortunately this flexibility makes reading the extracted data more complicated and, unlike the standard .par structure, there is no guarantee that valid data exists for all the selected fields (in fact most of the 'non-standard' fields are filled with '#' characters, i.e., blank), so any code reading the data has to make decisions on whether to ignore the line completely or use default values.

See [HITRAN Database Files] for RFM implementation.

Absorption Cross-sections

Certain molecules, usually the more complex type, have lines which are too closely spaced to be resolved. For these, the absorption coefficient k(ν) [cm2/molec] is tabulated directly from lab measurements under particular pressure and temperature conditions. These are the HITRAN Absorption Cross-Section data files.

The data for each tabulation of k(ν) at a particular (p,T), (p in this case is measured in Torr, 760 Torr = 1 atmosphere) is provided as a separate file. Each tabulation is on a regular spectral grid, although not necessarily the same for all (p,T) — at lower pressures the spectral features start to resolve and higher spectral sampling is necessary.

The RFM expects cross-section data as a single file for each molecule, which can be constructed by simply concatenating the HITRAN files in any order of (p,T). Some caution is required here: the HITRAN data are often a collection from different sources and simply combining all the available data can lead to systematic errors when interpolating in the (p,T) domain (particularly noticeable when evaluating Jacobians). It is generally recommended that only data from a single source is combined.

Eg using the linux cat command to create a combined file sf6.xsc for data in the spectral range 925–955 cm-1

cat SF6*925*xsc > sf6.xsc
However, where the cross-section data covers different bands (eg 810–880 cm-1 and 1050–1120 cm-1 for CFC-11) all the data for each band should be concatenated first, and then data for the individual bands concatenated. [Note: for RFM v5.10 and later versions, the files can be concatenated in any order] For example, using the linux cat command with the CFC-11 (CCl3F) data to create a combined file f11.xsc
cat CCl3F*810*xsc CCl3F*1050*xsc > f11.xsc

HITRAN Collision-Induced Absorption

Collision Induced Absorption data are a relatively new (since 2012) third type of HITRAN spectroscopic data. As with absorption cross-sections these are tabulations of absorption coefficient as a function of wavenumber c(ν) but representing the absorption due to collisions between a specific pair of molecules rather than generic pressure broadening. As a consequence the absorption coefficient c is in units of cm5/molec2 (so that, mulitplied by the product of the densities of the two molecules, the result cρ1ρ2 is cm-1, as with conventional absorption kρ). The tabulations are only functions of temperature.

The data are available as single files for each pair of molecules, with all temperatures and spectral ranges contained in the same file. The ordering is band-by-band, and within each band, tabulations for different temperatures.

The RFM can use these data files directly.

HITEMP Data

HITEMP is a HITRAN-style line parameter compilation including many more transitions for the major molecules which are only apparent at higher temperatures.

Since it is in the same 160-character .par file format as the standard HITRAN line list the RFM can use these files either directly or converted to binary. However since HITEMP contains only a limited set of molecules you might first want to merge this with the standard HITRAN list using MRGHIT.

The RFM uses the 2020 Gamache TIPS functions which are valid up to 9000K.

UV Data

Radiative transfer in the ultra-violet is dominated by molecular scattering so the RFM cannot be used to calculate meaningful radiances. However it is still useful for calculating the molecular absorption component.

The HITRAN 2016 line database extends into the UV (>25 000 cm-1, <0.4 µm) but only for a limited set of molecules with well-resolved lines: H2O (<25 711 cm-1), OH (<35 875 cm-1), HF (<32 352 cm-1) and H2 (<36 406 cm-1)

For other gases, absorptions are available as cross-section tabulations, in the same format as for the infrared heavy molecules, and so can be handled by the RFM in the same way. Note that for molecules which are normally represented in the infrared by HITRAN line data (eg O3) to specify cross-section data you have to append the letter 'x' (eg 'O3x') in the RFM driver table *GAS section — see also Recognised Molecules below, molecule numbers 171–184.

GEISA Data

GEISA [Web-site] is an alternative source of spectroscopic line parameters and cross-section parameters.

GEISA line data [format] contains the same set of essential parameters required by the RFM, but in a different format. Also the list of molecules and isotopologues is slightly different to HITRAN. The RFM cannot read GEISA data records directly, but a Fortran program is provided to convert GEISA to HITRAN .par format, which can then be read directly by the RFM, or converted to binary.

geihit
Convert GEISA 2015 data to HITRAN 2016 .par format

GEISA absorption cross-section data [format] are also in a different format to HITRAN (although often from the same orginal source). As with the line parameter data, there is a Fortran convversion program:

geixsc
Convert GEISA 2015 absorption cross-section data to HITRAN .xsc format

Recognised Molecules

The following is a list of all the molecules (Names in Red) recognised by RFM. If the last letter is 'q' this is a line molecule which is normally expected to be represented by tabulated cross-section data, and vice-versa for 'x'.

Internally the RFM assigns each molecule a number (set in subroutine molidx_sub.f90), although for tabulated cross-section data (.xsc files) it is possible to add arbitrary molecules which are, internally, assigned temporary numbers. This is possible for cross-section molecules since it is only necessary to associated the molecule's name with a matching cross-section file and atmospheric profile. However, for molecules described by line parameters, additional information such as molecular weight (for Doppler widths), permitted isotopologues and TIPS data is also required, requiring amendments to the code.
Molecules 1–55 follow the assigments for HITRAN 2020, 56 –57 are extra molecules assigned these indices in the associated TIPS data, and 61–63 are assigned to additional molecules defined in the GEISA 2015 database.
ID Molecule ID Molecule ID Molecule ID Molecule ID Molecule
1 H2O 2 CO2 3 O3 4 N2O 5 CO
6 CH4 7 O2 8 NO 9 SO2 10 NO2
11 NH3 12 HNO3 13 OH 14 HF 15 HCl
16 HBr 17 HI 18 ClO 19 OCS 20 H2CO
21 HOCl 22 N2 23 HCN 24 CH3Cl 25 H2O2
26 C2H2 27 C2H6 28 PH3 29 COF2 30 SF6q
31 H2S 32 HCOOH 33 HO2 34 O 35 ClONO2q
36 NO+ 37 HOBr 38 C2H4 39 CH3OHq 40 CH3Br
41 CH3CNq 42 CF4q 43 C4H2 44 HC3N 45 H2
46 CS 47 SO3 48 C2N2 49 COCl2 50 SO
51 CH3F* 52 GeH4* 53 CS2 54 CH3I 55 NF3
56 C3H4 57 CH3
60 * 61 C3H8q 62 HNC 63 C2H6q
Key Standard HITRAN line molecules
*Changed with RFM v5.2. Was (51) C3H4, (52) CH3, (60) GeH4, while 53–57 were undefined
HITRAN line data normally/better represented as cross-sections
Additional line molecule indices defined in 2020 TIPS data
Additional line molecule indices defined in the GEISA database
Molecules 100+ are RFM-specific indices for species represented by infrared absorption cross-section data.
ID Molecule ID Molecule ID Molecule ID Molecule ID Molecule
100 Aerosol
General Heavy Molecules
101 ClONO2 102 N2O5 103 SF6 104 CCl4 105 HNO4
106 SF5CF3 107 BrONO2 108 ClOOCl 109 X109 110 X110
CFCs (ChloroFluoroCarbons)
111 F11 (CCl3F) 112 F12 (CCl2F2) 113 F113 (C2Cl3F3) 114 F114 (C2Cl2F4) 115 F115 (C2ClF5)
116 F116 (C2F6) 117 F13 (CClF3) 118 F14 (CF4)
HCFCs (HydroChloroFluoroCarbons)
121 F21 (CHCl2F) 122 F22 (CHClF2) 123 F123 (CHCl2CF3) 124 F124 (CHClFCF3) 125 F141b (CH3CCl2F)
126 F142b (CH3CClF2) 127 F225ca (CHCl2CF2CF3) 128 F225cb (CClF2CF2CHClF)
HFCs (HydroFluoroCarbons)
131 F125 (CHF2CF3) 132 F32 (CH2F2) 133 F134 (CHF2CHF2) 134 F134a (CFH2CF3) 135 F143 (CF3CH3)
136 F143a (CF3CH3) 137 F152a (CH3CHF2) 138 F365mfc
MHCs (Methylated Hydrocarbons)
141 CH3OH (Methanol) 142 CH3CN (AcetoNitrile) 143 CH3CHO (Acetaldehyde) 144 Acetone (CH3COCH3) 145 PAN (CH3C(O)OONO2)
NMHCs (Non-Methylated Hydrocarbons)
151 C2H6x (Ethane) 152 C3H8 (Propane) 153 C6H6 (Benzene) 154 C2H2x (Acetylene) 155 C2H4x (Ethylene)
Halocarbons
161 C4F8 (Octafluorocyclobutane)
UV Cross-sections of simple molecules
171 O3x 172 N2Ox 173 SO2x 174 NO2x 175 H2COx
176 BrO 177 NO3 178 OClO
UV Cross-sections of heavy molecules
181 C7H8 (Toluene) 182 oxylene (o-C8H10) 183 mxylene (m-C8H10) 184 pxylene (p-C8H10)
Key Extinction cross-section [km-1]
Molecular cross-section [cm2/molec]
Dummy names for user-supplied cross-section molecules
Cross-section normally represented as HITRAN line data
Additional GEISA Cross-section molecules
UV Cross-sections
Some molecules are represented by both HITRAN line parameters and as cross-section data.