Multiple Scattering Measurements Using Multistatic Lidar
Document ID: 107
Doctoral Dissertation
The Pennsylvania State University
The Graduate School
College of Engineering
Abstract
This thesis addresses the problem of multiple scattering in atmosphere. Multiple scattering is an important factor in treating the penetration of radiation through an optically thick medium, such as clouds and fog. A multistatic lidar system, which was developed at Pennsylvania State University, has the ability to evaluate multiple scattering effects in a dense medium by measuring the polarization ratio of the scattering phase function at different scattering angles, and by measuring the radial distributions of a transmitted laser beam. Measurements of aerosol properties as a function of scattering angle are particularly important for extracting information on the characteristics of the optical scatterers, such as particles sizes and number densities. It was known that the width of the intensity peak is inversely dependent upon the particle size, and this factor also explains multiple scattering effects along the beam path. The effects of a multiple scattering during propagation of narrow light beams in aerosols have been investigated using the two features; the radial distribution of scattered light, and the changes of polarization ratio compared to single particle scattering.
Laboratory experiments were conducted in the Aerosol Research Chamber of the Defense Research and Development Canada (DRDC) in Quebec and in a small chamber that we constructed at PSU. These chamber environments have been used to conduct scattering experiments under conditions of a better controlled laboratory environment. An outdoor field study was performed in State College during the summer of 2007. The field experiments were made under sufficiently high relative humidity conditions of light and heavy fog conditions on these nights that it is reasonable to use a spherical model to describe scattering particles. These conditions provide excellent situations to investigate multiple scattering effects.
A theoretical investigation of multiple scattering using a Bistatic Monte Carlo (BMC) calculation method showed that multiple scattering effects increase as particle size, extinction and the penetration distance of a scattering medium increase. Multiple scattering calculations for atmospheric models having large extinction coefficients are comparable to or larger than those of single scattering. However, in case of a lower extinction coefficient, a single scattering component is always dominant in both forward and backward directions. Experimental results obtained from chamber tests revealed that even in the case when same particle distribution is considered, multiple scattering effects, associated with the density of scatterers in terms of optical depth, increase as the number density increases within the scattering volume. Specifically, multiple scattering increases depolarization of the scattered intensity as the scattering angle increases from 0° and 180°. Results of measurements of the radial distribution of a laser beam into optically dense media showed that multiple scattering effects increase with optical depth, lead to beam broadening along the beam path, and exhibit a slope in the gradient of the radial distribution of intensity that is much less than for the clear atmospheric condition. The results show good agreement with the previous calculations and measurements conducted by Bissonnette.
| Citation: | J. H. Park, "Multiple Scattering Measurements Using Multistatic Lidar", The Pennsylvania State University, Doctoral Dissertation, May 2008, 190 pages |