Broadband lidar measurements of tropospheric water vapor profiles

A new technique has been developed for the measurement of vertical profiles of water vapor in the troposphere. The method, which is also appropriate for the measurement of gases such as ozone and nitrogen dioxide, is based on the application of differential optical absorption spectroscopy (DOAS) to backscattered photons from a broadband lidar (10-20 nm at full width half maximum) operating in the UV-visible region of the electromagnetic spectrum. A Nd:YAG pumped broadband dye laser is used as the light source, and backscattered photons from the atmosphere are both time and spectrally resolved using an imaging spectrometer with a charge-coupled device (CCD) detector. The new broadband lidar circumvents many of the difficulties and inaccuracies involved with traditional narrowband differential absorption lidar (DIAL) water vapor observations, for example, the laser bandwidth and wavelength-positioning requirements demanded by the narrowness of the water vapor transitions. Water vapor mixing ratio profiles measured over Cambridge, England, in 1996 with the new instrument demonstrate good levels of agreement with quasi-simultaneous radiosonde profiles from nearby weather stations.