Differential absorption Lidar measurements of H_2O and O_2 using a coherent white light continuum

摘要

We applied a broadband and coherent white light continuum to differential absorption lidar (DIAL) detection of H_2O and O_2 profiles in the troposphere. The white light continuum can be generated by focusing high intensity femtosecond laser pulses at 800 run into a Kr gas cell covering a broad spectral range from UV to mid-IR. Thus, the use of white light continuum potentially enables the DIAL measurement of several greenhouse and/or pollutant gases simultaneously while minimizing the lead time for developing a tunable light source. In order to demonstrate such capability, here we report the lidar measurements of H_2O and O_2. These molecular species exhibit absorption lines in the near IR region where relatively high intensity of the white light continuum is available. The white light continuum was transmitted through the atmosphere collinearly to the axis of a receiver telescope. Backscattered light was passed through bandpass filters (H_2O On: 725 and 730 nm, H_2O Off: 750 ran, O_2 On: 760 nm, O_2 Off: 780 nm), and was detected by a photomultiplier tube. The detection wavelengths were selected consecutively by rotating the filter wheels that contain five bandpass filters with an interval of 1 minute. In addition, we propose a method for retrieving vertical profiles of H_O by considering wavelength dependence of the aerosol extinction coefficient a and backscatter coefficient ss. These results show that for achieving precise retrieval of H_2O distribution, one needs to reduce the effect of aerosol temporal variations by means of long-time accumulation or simultaneous detection of the On- and Off-wavelength signals.