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Passive infrared (IR) standoff sensing and imaging of hazardous vapor plumes possess inherent unreliability due to its dependence on the volatile temperature difference. Active standoff chemical detection by the IR lidar is much less influenced by the field temperature variations as the passive IR sensing is. In this paper, the effect of the lidar pulse stability on the chemical sensing performance is studied. An analytical expression of the concentration limit of detection (LOD) of lidar measurement in the topographical scatterer mode with inclusion of transmitted pulse instability parameter (pulse energy dispersion D) was derived. Calculations done for the 8-14 μm range, 1 mJ/pulse, and 0.05 m2 telescope lidar show that in the short range, the pulse energy stability is a crucial lidar parameter. It has been found that in the subkilometer range the LOD value is proportional to D, and that it does not depend either on the laser pulse energy or the telescope mirror diameter. At the short R, deeper cooling of the photodetector does not reduce the LOD value either. In addition, research was done on the new kind of noise arising from the laser beam spot variability. The magnitude of this noise was found to be proportional to the covariance between the laser spot energy distribution on the scatterer surface and the reflection unevenness of that surface.