A NONLOCAL THERMODYNAMIC-EQUILIBRIUM RADIATIVE-TRANSFER MODEL FOR INFRARED EMISSIONS IN THE ATMOSPHERE OF MARS .2. DAYTIME POPULATIONS OF VIBRATIONAL LEVELS
Ma. Lopezvalverde et M. Lopezpuertas, A NONLOCAL THERMODYNAMIC-EQUILIBRIUM RADIATIVE-TRANSFER MODEL FOR INFRARED EMISSIONS IN THE ATMOSPHERE OF MARS .2. DAYTIME POPULATIONS OF VIBRATIONAL LEVELS, J GEO R-PLA, 99(E6), 1994, pp. 13117-13132
A non-local thermodynamic equilibrium (LTE) radiative transfer model h
as been applied to study the CO2 and CO emissions in the infrared (1-2
0 mum) in the atmosphere of Mars during daytime conditions. An extensi
ve set of vibrational-translational (V-T) and vibrational-vibrational
(V-V) collisional exchanges among the vibrational levels responsible f
or these emissions has been considered. Radiative transfer has been in
cluded 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 nu2 and nu3 modes of CO2 axe presented. The CO2(0,nu
2,0) levels follow LTE up to about 80 km at daytime, some 5 km lower t
han at nighttime conditions. The absorption of solar radiation at 1.6,
2.0, and 2.7 mum, and subsequent relaxation by V-V and radiative proc
esses, significantly populates these levels in the lower thermosphere,
increasing all their vibrational temperatures with respect to nightti
me conditions. Solar excitation and radiative transfer in 4.3 mum cons
titute the main sources of excitation of the (0,0(0),1) level in the t
hermosphere, where this level shows a very large vibrational temperatu
re. The V-V transfer from highly excited CO2 levels is even larger tha
n the direct radiative excitation of the (0,0(0),1) level in the mesos
phere. The model predicts that the known inversion population between
this vibrational level and the lower (0,2(0),0) and (1,0(0),0) levels
will occur in the high mesosphere and above. The CO(1) level also show
s much larger populations than during nighttime conditions, due to dir
ect solar absorption at 4.7 mum and the role played by radiative trans
fer. A sensitivity study of the effect of current uncertainties in rat
e constants on the level populations is also presented. The uncertaint
ies in the rate for nu3 quanta exchange among CO2 levels have signific
ant effects on the deactivation of high energy states, leading to chan
ges of importance in the daytime populations of the 2.7-mum states in
the mesosphere and in the (0,0(0),1) level in the lower thermosphere.