Infrared radiative excitation in non-local thermodynamic equilibrium (non-LTE) regions of the Earth's atmospheric for the &ngr;3-mode vibrationally excited states of CO2 under sunlit conditions and the resulting 4.3-μm limb radiance are calculated using a line-by-line (LBL) radiative transfer model. Excited-state population densities and the corresponding vibrational temperature profiles are calculated for the important emitting states using a model which includes radiative absorption and emission as well as various collisional processes. The quenching of O(1D) by N2 has a greater impact on these population densities than has been previously reported in the literature. Integrated radiance in a limb view for the 4.3-μm bands is calculated from the model and compared with sunlit earthlimb measurements obtained by the Spectral Infrared Rocket Experiment (SPIRE). Solar pumping is the dominant excitation process for the 4.3-μm emitting states in the daytime. The major contribution to the total limb radiance for tangent heights of 55--95 km is made by the fluorescent states at approximately 3600 cm-1 which absorb sunlight at 2.7 μm and then emit preferentially at 4.3 μm. The predicted radiance is in good agreement with the SPIRE measurements for all tangent heights in the 50- to 130-km range. This is the first detailed comparison of results of a full line-by-line non-LTE radiative transfer calculation with 4.3-μm earthlimb radiance data. ¿ American Geophysical Union 1994