Particle size distributions and extinction determined by a unique bistatic lidar technique

A study has been conducted to investigate the application of a unique bistatic lidar receiver to remotely determine properties of the lower tropospheric aerosols, particularly optical extinction, mean radius and size distribution width. The motivation for this study is to advance understanding of aerosols near the ground with a possible long term goal of calculating extinction at any wavelength. Single ended remote sensing instruments, whether using lasers, radars, or microwaves, have difficulties determining absolute extinction along a propagation path. This is due to the large variations in the ratio of the forward scattering to the backward scattering between different particles (scattering phase function). The bistatic linear array receiver was developed to provide necessary information on the scattering phase function of the aerosols. The bistatic remote receiver utilizes a linear photodiode array to image the radiation scattered from any high power CW or pulsed laser system. By observing the angular scattering variation along a horizontal path, additional information contained in the scattering angle phase function can be obtained. A technique has been developed to estimate particle size and distribution widths (of spherical scatters) by comparing two transmitted E-field components, one parallel and one perpendicular to the scattering plane. Polarizers are used on the receiver to measure the cross polarization to determine the amount of multiple scattering and nonsphericity of the particles in the scattering volume. The first studies of this measurement technique have been conducted in a marine/coastal environment during the CASE I (Coastal Aerosol Scattering Experiment) program at the NASA flight center on Wallops Island, VA during September 1995. The atmospheric conditions during this measurement program correspond to cases of high relative humidity where the use of a spherical model to describe the scatterers would be reasonable. Results, which can be represented by trimodal lognormal size distributions, were obtained from both clear and hazy/misty nights. Extinction calculated from the size distributions is compared with extinction from the Raman lidar at two different wavelengths. On calm evenings with high humidity and decreasing temperature, the inversions also show increasing particle sizes consistent with radiation fog formation. The results show remarkable agreement with a tri-modal aerosol distribution used in a spherical model (Mie theory).