Bistatic LIDAR Measurements of Lower Tropospheric Aerosols.

A study was conducted to investigate the application of a bistatic lidar receiver to remotely determine properties of lower tropospheric aerosols, particularly optical extinction, median radius and size distribution width. The motivation for this study is to advance our understanding of optical scattering by 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 distribution width. A bistatic linear array receiver was developed to collect information on the scattering phase function of tropospheric aerosols. The first studies using the bistatic lidar were conducted in a marine coastal environment during the Wallops CASE I 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 Mie theory and a lognormal particle distribution can describe the measurements from this bistatic receiver. 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^circ to 22 ^circC. A radiation fog mode of nearly monodispersed particles grew from 6.46 mu m 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 mum. An error analysis determines that the mean particle radius of the radiation fog, measured with the bistatic lidar, is known to an accuracy of +/-0.41 mum. 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.