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Dependence on temperature and pressure

The values of some spectral line parameters depend on environmental temperature and pressure. The database contains the values of these parameters at Tref and Pref of corresponding data source.

Temperature dependence of the line intensity

To calculate the intensity Sj of jth spectral line at temperatures different from the reference temperature of the data source, one uses the following expression

Sj(T) = Sj(Tref) · rQ · rB · rE , (1)

where S(Tref) is the intensity of the line j at Tref , rQ is the ratio of total internal partition functions at  Tref and T

rQ = Q(Tref)/Q(T) .
(2)

The values of the partition functions Q(T) for isotopologues presented in HITRAN are calculated by Fortran program, TIPS.for in the temperature interval from 70 to 3000 K. The values of the partition functions Q(T) for isotopologues not presented in HITRAN are provided by authors of corresponding linelists.

rB accounts for the ratio of Boltzmann populations

rB = exp⁡(−c2·Ejl/T)/exp⁡(−c2·Ejl/Tref) . (3)

rE accounts the effect of stimulated emission

rE = (1 - exp(-c2·WNj/T))/(1- exp(-c2·WNj/Tref)) . (4)

In expessins (3) and (4) Ejl is the lower-state energy of jth line, WNj is the wavennumber of jth line, c2 is the second radiation constant.


Temperature and pressure dependence of the line width

The half-width at half maximum (HWHM) of spectral lineis used on simulation of spectrum functions for building of profile of spectral line (see Line profiles).


The Doppler half-width of the line j is independent of the pressure and calculated by the expression following from expression (2) in previous paragraph

Dj(T) = Dj(Tref)·sqrt(T/Tref) . (5)

The Lorentz half-width of the line j at temperature T, pressure P and partial pressure Pself  is calculated as

Lj(T,P) = (Tref/T)Njt · (Ljenv(Tref,Pref) · (P - Pself) + Ljself · Pself) , (6)

where Njt -temperature-dependence exponent defined above.


Temperature and pressure dependence of the line position

The air pressure leads to a shift of the line position. This shift is given by expression

WNj(P) = WNj + Pjshift · P/Pref , (7)

where WNj is the wavenumber of the line j, Pjshift is the pressure shift of the line j at Pref .
The pressure shift should also include a temperature dependence, but that effect is not considered now.