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HITRAN plugin: gas lines in the air

The hitran plugin adds line-by-line absorption and dispersion of air: H2O (required), CO2 and O2 (optional), from HITRAN line lists, as the air of your system. Water lines near 1 µm are only a few GHz wide, so they ring behind a femtosecond pulse for hundreds of picoseconds; with grid.tail that ring-down is carried without a nanosecond window on the nonlinear grid.

The line data are not part of the plugin or the engine: HITRAN has its own terms of use. You download them once.

Install and get the data

The plugin ships with the engine packages (--plugins hitran; the NativeAOT C library and the Python hosted engine have it compiled in). Download the lines with the HITRAN API (pip install hitran-api; the module is hapi, or hapi2 in the newer release):

python -m ultrafast_hitran download H2O                       # 6500-14000 cm-1 (714-1538 nm), main isotopologues
python -m ultrafast_hitran download H2O CO2 O2 --dir ./hitran --numin 6500 --numax 14000

(An isotopologue the server has no lines for in the range is skipped with a message; the download fails only when no line is found at all.)

import ultrafast_hitran as hit
hit.download(["H2O"], numin=6500, numax=14000)                 # writes ~/.ultrafast/hitran/H2O.csv

The files are looked up in the working directory, dataDir, $ULTRAFAST_HITRAN_DIR and ~/.ultrafast/hitran. Both the CSV written by the downloader (HAPI column names molec_id, local_iso_id, nu, sw, a, gamma_air, gamma_self, elower, n_air, delta_air) and HITRAN's own 160-character .par records are read. Main isotopologues are downloaded: H2O 161, 181, 171, 162; CO2 626, 636, 628, 627; O2 66, 68, 67. (Whether the HITRAN API's package name and its fetch signature are as assumed here could not be checked without a network: the downloader is tested with a fake API, so please report any mismatch.)

Use

// system file
"environment": { "air": { "kind": "hitran", "applyToGaps": true,
                          "temperatureC": 22, "pressurePa": 101325, "humidity": 0.45, "co2ppm": 420,
                          "species": { "H2O": true, "CO2": true } } }
// run spec
"grid": { "tail": { "windowS": 2e-9 } }          // the carrier is not snapped with a tail: it stays on the line you chose
from ultrafast import build as b
import ultrafast_hitran as hit
env = b.environment(air=hit.air(humidity=0.45, temperature_c=22, species=hit.species(h2o=True, co2=True)))
system = b.system("cell", [b.gap("g1", 10.0)], environment=env)
spec = b.run_spec(b.pulse(1.03e-6, 150e-15, energy=1e-6), grid=b.grid(tail=b.tail_grid(window_s=2e-9)))

C and C++ (ultrafast.h) take the same JSON; the plugin is registered when the library loads. A single gap can override the air ("air": {"humidity": 0.2} on the gap).

key of the hitran air default meaning
temperatureC, pressurePa, humidity (0..1), co2ppm 20, 101325, 0, 450 state of the air: Ciddor index and the number densities
species {"H2O": true} true, a file name, or {file, moleFraction, partitionExponent} per species (H2O, CO2, O2)
rangeCm1 [6500, 14000] lines kept (cm-1)
cutoffCm1 25 each line is evaluated within this distance of its centre
baseIndex ciddor the smooth index; vacuum leaves it out (n = 1 + lines)
dataDir, lineScale folder of the line files; a factor on all line strengths
applyToGaps false use this air as the "Air" of every gap (environment.air only)

environment.air is opt-in. Without applyToGaps: true the environment.air block does what it always did (it only supplies the air used for data quoted relative to air), so existing results do not change. When it is on, a note in the result says so, and gap elements may override it with their own air.

What the model is

For each line the complex Voigt line shape F = w(z) / (gamma_D sqrt(pi)), z = (nu - nu0 + i gamma_L) / gamma_D (w the Faddeeva function, Weideman's rational approximation) gives both parts, so absorption and dispersion are Kramers-Kronig consistent by construction (exp(-iωt) convention, n > 1 below a line):

alpha(nu) = n_d S Re F                 intensity absorption, 1/cm            kappa = n_d S Re F / (4 pi nu)
Dn(nu)    = - n_d S Im F / (4 pi nu)   (nu in cm-1)
  • Strength: S(T) = S(296 K) [Q(296)/Q(T)] exp(-c2 E'' (1/T - 1/296)) [1 - exp(-c2 nu/T)] / [1 - exp(-c2 nu/296)].
  • Pressure width (HWHM): gamma_L = (296/T)^n_air [gamma_air (p - p_self) + gamma_self p_self], pressure in atm. Shift: nu0 = nu + delta_air p.
  • Doppler HWHM: nu0 sqrt(2 k T ln2 / (m c^2)) (H2O at 1 µm and 296 K: 0.014 cm-1, FWHM 0.85 GHz).
  • Number density n_d = x p / (k T); the water-vapour mole fraction comes from the relative humidity with the CIPM-2007 saturation pressure (the same formula as the Ciddor index).
  • The lines are cut at +-cutoffCm1; there is no continuum absorption; the smooth Ciddor index stays.

Approximations to know about.

  • Partition sums. The downloader also writes <NAME>.partition.csv (iso,T,Q, 100-500 K in 1 K steps, the TIPS sums of hapi.partitionSum for each isotopologue it fetched). When that file lies next to the line file (or partitionFile names it) the strengths are scaled with the real Q(296)/Q(T) per isotopologue; the description of the model says "TIPS partition sums". Without it the model falls back to (T/296)^1.5 for H2O and (T/296) for CO2 and O2 (or your partitionExponent), which neglects the vibrational part and can be a percent or more off away from 296 K. The tables are not bundled (they are data you generate), and a temperature outside the table is an error.
  • delta_air is not temperature dependent; widths use a single n_air; no line mixing, no speed dependence.
  • Water lines are broader at 1 atm than the Doppler width: the Lorentz half width is about 0.1 cm-1 (3 GHz) for typical lines (check gamma_air, gamma_self of your lines), so the ring-down lasts a few 1/(2 pi gamma) = 50 ps e-folds.
  • The frame of a propagation step is referenced to the smooth index, not to the line (why).

Validation

HitranTests: Faddeeva values (w(0), w(iy), Dawson), continuity at the asymptotic switch; HITRAN .par and CSV round trips; Doppler width against the kinetic formula; a pressure-broadened (Lorentz) line against n_d S / (pi gamma_L) and the dispersion extremum n_d S / (8 pi^2 nu gamma_L); Kramers-Kronig by a numerical Hilbert transform of a Voigt line; the temperature and density scaling; energy lost in a gap against Beer-Lambert sum S (1 - exp(-alpha L)); and the ring-down of three lines in a 10 m gap on the tail grid (main N = 1024, tail N = 262 144) against the same system on one 2 ns window (N = 524 288) -- total energy to 1e-5, tail power 100-900 ps to 2 %, main peak power to 0.1 %. The same lines in a nonlinear gap, a radial gap and a vector radial gap reach the tail of the linear gap to 2 %. The always-on tests use no HITRAN data (the lines are synthetic, written in the HITRAN format).

Real lines against the reference implementation. plugins/hitran/tools/make_hapi_reference.py DIR downloads real lines (needs the network and pip install hitran-api) and lets HAPI -- the HITRAN team's own code, with TIPS partition sums -- compute its Voigt absorption coefficient at 250, 296 and 350 K; the opt-in test HitranTests.AgainstHapi_RealLines_AgreeWithTheReferenceImplementation (ULTRAFAST_HITRAN_VALIDATE_DIR=DIR) compares the plugin with it. With the same wing cutoff as HAPI's default (cutoffCm1: 5, 50 half widths), results of the run of 2026-10-29:

band lines (points on a line) integrated absorption, plugin / HAPI worst point on a line
H2O 9690-9730 cm-1 (1 µm) 1590 1.002 - 1.004 1.4 - 3.0 %
H2O 7180-7200 (1.39 µm) 1500 0.995 - 1.004 1.1 - 3.2 %
CO2 6970-7000 (1.43 µm) 5000-6000 1.002 - 1.004 1.2 - 2.8 %
O2 7870-7900 (1.27 µm) 3500-3900 1.005 - 1.007 2.6 - 2.8 %
O2 13110-13150 (A band, 0.76 µm) 1850-2500 1.005 - 1.007 2.6 - 3.9 %

The ratio does not change with temperature, so the TIPS scaling is right. With the plugin's default cutoff of 25 cm-1 the integrated absorption is 0.8 - 1.5 % higher: the far Lorentz wings of the 300 - 6000 lines add up (no line is accurate that far out; there is no continuum model to correct them). This compares the plugin with an independent implementation of the same physics, not with a measured spectrum: a comparison with a measured transmission of humid air is still open.