TITLE:
    A non-local thermodynamica equilibrium radiative transfer model
    for infrared emissions in the atmosphere of Mars. 
    2. Daytime populations of vibrational levels 

AUTHORS:
   Lopez-Valverde, M. A. and M. Lopez-Puertas 

REFERENCE:
   J. Geophys. Res. - Planets, vol.99 , pp.13117-13132, 1994 

ABSTRACT:

A non-local thermodynamic equilibrium (LTE)  radiative transfer model has
been applied to study the CO2 and CO emissions in the infrared
(1-20 mu) in the atmosphere of Mars during daytime conditions. An
extensive set of vibrational--translational (V--T) and
vibrational--vibrational (V--V) collisional exchanges among the vibrational
levels responsible for these emissions has been considered. Radiative
transfer has been included for most of the transitions and its importance
illustrated for some of them. The populations of the most important 
vibrational levels of CO and of the v2 and v3 modes of CO2 are
presented. The CO2(0,v2,0) levels follow LTE up to about 80 km at
daytime, some 5 km lower than at nighttime conditions. The absorption of
solar radiation at 1.6, 2.0, and 2.7-mu, and subsequent relaxation by
V--V and radiative processes, significantly populates these levels in the
lower thermosphere, increasing all their vibrational temperatures with
respect to nighttime conditions. Solar excitation and radiative transfer in
4.3-mu constitute the main sources of excitation of the (0,0,1)
level in the thermosphere, where this level shows a very large vibrational
temperature. The V--V transfer from highly excited CO2 levels is even
larger than the direct radiative excitation of the (0,0,1) level in the
mesosphere. The model predicts that the known inversion population between
this vibrational level and the lower (0,2,0) and (1,0,0) levels will
occur in the high mesosphere and above. The CO(1) level also shows much
larger populations than during nighttime conditions, due to direct solar
absorption at 4.7-mu and the role played by radiative transfer. A
sensitivity study of the effect of current uncertainties in rate constants
on the level populations is also presented. The uncertainties in the rate
for v3 quanta exchange among CO_2 levels have significant effects on
the deactivation of high energy states, leading to changes of importance in
the daytime populations of the 2.7-mu states in the mesosphere and in the
(0,0,1) level in the lower thermosphere.