NON LOCAL THERMODYNAMIC-EQUILIBRIUM (LTE) ATMOSPHERIC LIMB EMISSION AT 4.6 MU-M - 2 - AN ANALYSIS OF THE DAYTIME WIDE-BAND RADIANCES AS MEASURED BY UARS IMPROVED STRATOSPHERIC AND MESOSPHERIC SOUNDER

An analysis of the measurements taken by the improved stratospheric an d mesospheric sounder (ISAMS) in its carbon monoxide wideband channel around 4.6 mu m at daytime is presented. The radiances show a good sig nal to noise ratio up to the lower thermosphere (about 120 km) and hav e been shown to be mainly due to emission from the weak CO2 4.3 mu m i sotopic and hot bands. They exhibit a very clear dependence with the s olar illumination at tangent heights above about 60 km, where they hav e been found to be almost exclusively determined by the solar elevatio n. Below about 50 km they are dominated by the variations of the kinet ic temperature. The measurements have been analyzed in the 50-100 km r ange by using a detailed non local thermodynamic equilibrium (LTE) mod el of the CO2 states emitting in the 4.3 mu m spectral region and the GENLN2 line-by-line radiance code. A large number (up to 32) of CO2 is otopic and hot bands emit significantly in this spectral region. The N 2O(v(3)=1) and two O-3(v(1)+v(3)) bands also give contributions in the stratosphere and lower mesosphere. The CO(1-->0) band is of relative importance only in the lower thermosphere. The absolute radiances as w ell as the solar zenith angle dependence are well reproduced by the mo del. The dependence on the solar zenith angle is due to the absorption of solar radiation in the CO2 near-infrared bands. A sensitivity stud y of the radiances was also conducted. The major conclusions are (1) t he inclusion of the excitation of N-2(1) from the electronic energy of O(D-1) was required to explain the radiances in the lower mesosphere; (2) the value for the rate of the vibrational exchange between CO2(v( 1),v(2),1) and N-2(1) is very similar to the laboratory measurements E nd to that used in the analysis of the Spectral Infrared Rocket Experi ment (SPIRE) 4.3 mu m CO2 atmospheric limb radiances; and (3) the CO2 volume mixing ratio (vmr) in the 70-100 km region is significantly sma ller than that measured in rocket experiments and similar to that dedu ced from the atmospheric trace molecule spectroscopy (ATMOS) absorptio n measurements. A reanalysis pf the SAMS 4.3 mu m limb radiance measur ements has also been conducted with the same non-LTE model and the GEN LN2 line-by-line radiance code. The values for the parameters derived from the ISAMS data also explain the SAMS measurements very satisfacto rily.