Study Of Laser Radar For Fine-Detection Of Aerosol Profiles

Atmospheric aerosol is one of the important parameters for investigating the atmospheric physics and climate change,because its absorption,scattering and distribution of density influence the balance of Earth's radiation,atmospheric climate conditions and air pollution index directly.Fine-detection of the production,the transportation,optics and the physical property of aerosols as well as investigation of its space-time change rule have the significant research meaning and the social benefit to study the atmospheric environment,enhance early warning forecast ability of the natural disaster,especially to investigate the problem of global climate warming and the sand storm early warning forecast and the urban aerosol physical optics properties.Mie scattering lidar is one of the powerful tools for detection of the optical characteristics of aerosol with real-time because of its compactness,relatively low cost and easy of operation. However,because the retrieval of the optical properties of aerosol from the Mie lidar retum signal needs some assumption of weather conditions,which limits its measurement accuracy, therefore,the measurement uncertainty is still an intrinsic problem of the Mie lidar.In order to overcome the shortage of the Mie scattering lidar,in this paper,two kinds of lidar techniques which are based on Raman scattering and Rayleigh scattering,respectively,are proposed for fine-detection of aerosol profiles,and the feasibility of those lidar systems are confirmed by use of the experimental observation and the numerical simulation.Raman scattering lidar technique utilizes the dependence relationship between Raman scattering signal of atmospheric molecular and extinction coefficient of aerosol for fine detection of aerosol extinction profiles:A tripled Nd:YAG pulsed laser and a 250-mm-diameter Cassegrainian telescope are employed as transmitter and receiver respectively,a high-resolution plane reflection grating separates the vibrational Raman signal of N₂ at a central wavelength of 387nm.The first order of the grating(355nm)is used as Mie lidar to verify the feasibility of Raman method and to retrieve the aerosol extinction coefficient.The two scattering signals are detected by two photomultiplier tubes with a pre-amplifier in analog detection mode and recorded by analog-digital conversion.Preliminary experiments show that the Raman lidar system has the capability of fine-detection of aerosol profiles up to a height of 3km with 250mJ of laser energy and 8 minute integration time.High-spectral-resolution lidar(HSRL)technique separates the Mie-scattering signal caused by aerosol and Rayleigh-scattering signal caused by molecular by using the high-spectral-resolution spectroscopic filter,and then achieves accurate measurement of aerosol optical properties.An injection seeded single frequency pulsed Nd:YAG laser at 355nm is employed as the transmitter,a high-resolution blazed grating and a Fabry-Perot etalon filter are used to block the solar background and to separate the Rayleigh- and Mie-scattering components respectively.As a result,the aerosol extinction and lidar ratio can be obtained accurately without needs of assumption condition.The numerical simulation based on a standard atmospheric model shows that the lidar system has the capability of measuring the aerosols with a signal to noise ratio of more than 10 up to a height of 10km at daytime with 0.3×10⁹Wm^-2sr^-1nm^-1solar irradiance.