Bistatic Lidar Measurements of Lower Tropospheric Aerosols
Document ID: 114
Doctoral Dissertation
The Pennsylvania State University
The Graduate School
Department of Electrical Engineering
Abstract
A study has been conducted to investigate the application of a bistatic lidar receiver to remotely determine properties of lower tropospheric aerosols, particularly the optical extinction, median radius and size distribution width. The motivation for this study is to advance our understanding of the optical scattering of aerosols with a possible long term goal of calculating extinction at many wavelengths. Single-ended remote sensing instruments, whether laser, radar, or microwave based, have difficulties determining absolute extinction along a propagation path. This is due to the large variations in the ratio of forward scatter to backward scatter for different sizes and types of particles. In fact, large variations in particle scattering as a function of angle can be used to identify particle shape, size, and width of the distribution. A bistatic linear array receiver was developed to collect information on the scattering phase function of tropospheric aerosols. Additional wavelength and angle measurements are not always useful, however, in order to estimate extinction from a single-ended instrument this additional information is necessary.
The first studies using the bistatic lidar were conducted in a marine coastal environment during the Wallops CASE I (Coastal Aerosol Scattering Experiment) program. The measurements were made under sufficiently high relative humidities that it is reasonable to use a spherical model to describe the scatterers. One of the objectives of this research effort is to determine how well a spherical scattering model (Mie theory) and a lognormal particle distribution can describe the measurements from this bistatic receiver. The results show remarkable agreement with the model on certain evenings, while at times the measurements and model show significant differences.
A trimodal lognormal distribution of aerosols was observed on September 14, 1995 to increase in size during a period of several hours when the relative humidity remained constant at 92 % and the temperature decreased from 23° to 22° C. A radiation fog mode of nearly monodispersed particles grew from 6.46 µm to 8.91 µm over a period of 2.5 hours. During this same period a smaller and broader mode narrowed its distribution width as it grew from 0.166 µm to 0.237 µm. Many of the measured profiles are very sensitive to small changes in the size distribution and therefore add confidence to these extinction calculations. An error analysis determines that the mean particle radius of the radiation fog, measured from the bistatic lidar, is known to an accuracy of ±0.41 µm. Extinction coefficients calculated from the bistatic receiver data are compared with the monostatic lidar extinction determined using the Raman molecular profiles, which have been shifted to wavelengths at 607 and 530 nm. The error analysis is used to determine the dependence of extinction on each of the nine parameters. As the particle sizes grow, the bistatic lidar prediction of the extinction agrees with the extinction measured from the Raman lidar. On nights when Mie theory describes the results, an analysis is provided which examines the range of size distributions that will provide a consistent fit to the data. It has been determined that this bistatic measurement technique along with Mie theory provides a useful method to characterize aerosol scattering in a humid coastal marine environment.
| Citation: | T. D. Stevens, "Bistatic Lidar Measurements of Lower Tropospheric Aerosols", The Pennsylvania State University, Doctoral Dissertation, May 1996, 247 pages |