Tuesday, 16 January 2007: 8:45 AM
An Airborne High Spectral Resolution Lidar based on an Iodine Absorption Filter
207B (Henry B. Gonzalez Convention Center)
Aerosols directly influence the fluxes of solar and terrestrial radiation within the atmosphere by absorption and scattering of light. The quantification of this effect accounts for accurate determination of the aerosol's optical properties. Conventional backscatter lidars are widely used for aerosol ranging due to their simple design and low instrumental requirements. However, by this technique, aerosol extinction profiles can only be derived by inverting the lidar signal and assuming the lidar ratio, which generally is a highly uncertain quantity. Using a HSRL, the lidar signal is split in two parts and spectrally filtered in order to separate the Mie- from the Rayleigh spectral components. The extreme narrow-band optical filter in the molecular channel suppresses the aerosol backscatter whereas the combined channel detects the intensity of the total backscatter spectrum. Thus a well calibrated HSRL overcomes the limitations of a conventional backscatter lidar and can directly measure the aerosol backscatter, optical depth and extinction. Profiles and cross sections of the lidar ratio can be deduced from the measured quantities so that no doubtful assumptions have to be made. We present an airborne high spectral resolution lidar based on an iodine vapour absorption filter and a high power, frequency doubled Nd:YAG laser. The instrument is capable to measure atmospheric backscatter and linear depolarization at 1064nm and 532nm, extinction-corrected backscatter within the HSRL channel at 532nm as well as water vapour absorption at 935nm with the DIAL-technique. With these measurements the atmospheric aerosol can be characterized with regard to its lidar ratio, depolarization ratio and infrared-to-green backscatter ratio. These quantities only depend on the microphysical and chemical properties and the size distributions of the aerosol. These data products can be of great use for validating spaceborne passive instruments which measure aerosol optical depth but have a lowered accuracy over the continents. Active spaceborne instruments like backscatter lidars can use the measured lidar ratio in order to enhance their measurement accuracy.