VIBRATIONAL TEMPERATURES AND RADIATIVE COOLING OF THE CO2 15 MU-M BANDS IN THE MIDDLE ATMOSPHERE

Altitude and latitude distributions of the populations of the bending and symmetric stretching levels of CO2 and of the cooling rates induced by the bands originating from them in the middle atmosphere (40-120 km), have been calculated for the CIRA 1986 reference atmosphere using a non-LTE radiative transfer model. A sensitivity study of the more uncertain parameters that strongly affect these quantities has been undertaken. These factors are the carbon dioxide and atomic oxygen volume mixing ratios, and the rate coefficient for the deactivation of the bending mode of CO2 by atomic oxygen. The model makes use of the most recent values of these parameters: the abundances of CO2 in the upper mesosphere and lower thermosphere as measured by, and derived from, rocket and satellite instruments; the global distribution of atomic oxygen density calculated by Rees and Fuller-Rowell (1988) and the measurements of Thomas (1990); and the fast rate for the deactivation Of CO2(0, nu-2, 0) by O(3P), recently derived by Sharma and Wintersteiner (1990). Among the most important results are the following: (i) the CO2 15-mu-m fundamental band is close to LTE (T(v) - T(k) less-than-or-equal-to 6 K) up to about 100 km altitude; (ii) the first and second hot bands exhibit high vibrational temperatures, especially in the daytime summer mesopause region; (iii) the cooling rates by the CO2 15-mu-m bands in the lower thermosphere can be very large, depending on the CO2 abundance. Finally, our results have been compared with other recent non-LTE models in order to quantify the effects of different radiative transfer schemes on the calculations of vibrational temperatures and cooling rates.