O2-O2 and N2-N2 CIA incompatible with CTM
flag enabled for these two molecules
Notes
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.
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 [cm^{5}/molec^{2}] for different temperatures.
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.
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).
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