RFM Flags

CIA Flag: Include Collision-Induced Absorption


Avaiable from RFM v4.34 onwards
Include Collision-Induced absorption (currently for CO2, CH4, O2 and N2)
Driver Table requires *CIA section to specify .cia filenames

Neglect any CIA effects
Ignore *CIA section

O2-O2 and N2-N2 CIA incompatible with CTM flag enabled for these two molecules

  1. Collision-Induced Absorption (CIA) is an additional absorption that arises from collisions between pairs of molecules, and consequently depends on the product of the concentration of both molecules (or the square of the concentration if just a single species is involved) rather than just the concentration of a single molecule.
  2. The HITRAN web-site lists files of CIA coefficients in a standardised format for different pairs of molecules (eg O2-O2, O2-N2), which are read directly by the RFM. These represent tabulations of the CIA, in units of [cm5/molec2] for different temperatures.
  3. Interpolation: to look up the CIA for a particular molecule pair at wavenumber ν and temperature T the procedure is first to establish a pair of tabulations which include ν and bracket T, interpolate both to ν and then linearly interpolate for T. If ν is outside the wavenumber axis the CIA value is assumed to be zero. If T is outside the range of tabulation temperatures, the CIA at the closest temperature is used.
  4. The current RFM O2 (1360-1805 cm-1) and N2 (2120-2605 cm-1) continua (enabled with the CTM flag) are also CIA features, although these have been 'normalised' for standard air mixtures, ie 80/20 mix of N2/O2. Consequently the absorption generated using the N2-N2 CIA files for these spectral ranges is about 80% of the absorption from the CTM flag, and for O2-O2 it is about 20% (the collisional effect of O2-N2 being approximately the same as O2-O2 and N2-N2).
  5. Ideally, there would be an additional O2-N2 CIA file representing the CIA effect at these spectral ranges so that CIA in air could be accurately modelled. But there isn't (the current HITRAN O2-N2 CIA file only covers >7500 cm-1), so,
    For midinfrared O2 and N2 continuum effects in air, the recommendation is to use the RFM CTM flag

None reported.