CO2 DIAL Sensitivity Studies for Measurements of Atmospheric Trace Gases
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There is great need for atmospheric trace constituent measurements with higher resolution than attainable with passive radiometers. Infrared (IR) differential absorption lidar (DIAL), which depends on Mie scattering from aerosols, has special advantages for tropospheric and lower stratospheric applications and has great potential importance for measurements from Shuttle and aircraft (refs. 1, 2, 3, 4). DIAL data reduction involves comparing large amplitude signals which have small differences. The accuracy of the trace constituent concentration inferred from DIAL measurements depends strongly on the errors in determining the amplitude of the signals. Thus, the commonly used SNR expression (signal divided by noise in the absence of signal) is not adequate to describe DIAL-measurement accuracy and must be replaced by an expression which includes the random coherent (speckle) noise within the signal (refs. 5, 6). A comprehensive DIAL computer algorithm (ref. 7) is modified to include heterodyne and direct detection with speckle noise. Results of studies for monitoring vertical distribution of O3, H2O, and NH3 from ground aircraft and Shuttle platforms are extended to include measurements of ground level plumes with larger than ambient densities, from ground and aircraft platforms. Results are shown for plumes of NH3 but these results can be generalized in terms of the plume optical depth.
KeywordsDirect Detection Speckle Noise Pulse Pair Heterodyne Detection Ground Reflection
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