Remote sensing of atmospheric aerosol properties

Aerosol characteristics and distributions are still one of least understood features of the lower atmosphere. A technique has been developed that promises to help fill the voids in this knowledge during the next several years. Based on ideas developed on the mid-1980s, several doctoral students have developed and applied the capabilities of using the polarization ratio of the scattering phase function to determine atmospheric aerosol properties. Bistatic and multistatic lidar measurements use the polarization ratio of the scattering phase function to calculate profiles of the aerosol number density, size distribution, and type. These parameters can be determined for spherical particles in the size range between about 50 nm and 20 microns. Analysis of the aerosol concentration and size distribution requires adopting a mathematical shape function, usually a log-normal distribution of spherical particles. Information on aerosol type can be roughly determined based on the refractive index of the scatterers and depolarization of the scattered radiation as a function of wavelength. Measurement programs have investigated vertical profiles of layered haze to determine the particle characteristics, and horizontal measurements have been used to characterize the aerosols in different types of fog. One project has studied the case of relatively dense fog, where multiple scattering dominates. Laser remote sensing techniques provide important tools for determining most of the characteristics of aerosols, including their physical and chemical properties. A better understanding of the spatial and temporal distribution of aerosols is needed to investigate the sources, processes of formation and growth, and the roles aerosols play in establishing the planetary albedo and radiative transfer into space.