Measurement of range-resolved water vapor concentration by coherent CO2 differential absorption lidar

The feasibility of DIAL (Differential Absorption Lidar) species concentration measurements using a coherent CO2 lidar was examined. A computer simulation estimated errors in coherent and incoherent CO2 DIAL measurements due to speckle, noise, turbulence, and atmospheric inhomogeneities. Results indicated that direct-detection is the preferable mode at shorter ranges while heterodyne detection provides better sensitivity beyond a few km. The NOAA pulsed lidar was used to investigate statistical properties of the aerosol backscattered returns which are processed to obtain DIAL measurements. Fluctuations due to speckle and noise necessitate averaging of returns from multiple pulses for accurate concentration estimates. Other atmospheric effects can be minimized by judicious signal processing. The first reported range-resolved coherent DIAL water vapor measurements were made with the lidar to ranges beyond 10 km.