Absolutely calibrated Raman lidar as a reference tool in atmospheric humidity profiling

Raman lidars exploit the proportionality between the intensity of scattered by a Raman process laser radiation and the number density of the scattering molecules to derive water vapor mixing ratio profile. Water vapor/air mixing ratio is directly proportional to the ratio of the measured intensities of Raman scattering from water vapor and nitrogen molecules. The coefficient of proportionality, commonly denoted as calibration constant, depends on the instrument parameters and the spectroscopic parameters of the scattering molecule and is the primary factor defining lidar measurement accuracy. Derivation of the calibration constant using the above mentioned parameters is possible but leads to high uncertainty. Therefore, Raman lidars are calibrated against reference instruments- a radio sonde or a microwave radiometer. The accuracy of such calibration is thus defined by the accuracy of the reference instrument. Since these reference instruments have inferior, compared to the potential Raman lidar accuracy, this type of calibration impairs the lidar accuracy. A new first principle calibration method, based on the use of gravimetrically produced water vapor/air mixture will be presented. The method allows deriving the calibration constant with uncertainty lower than 0.1%, traceable to the primary standards of mass and length. Apart from the use for operational profiling, the calibrated in such a way lidar has the potential to become a reference in radio-sonde, microwave radiometer and GPS water vapor validation and/or calibration.