Characteristics of atmospheric aerosols based on optical remote sensing

Laser remote sensing techniques now provide an important tool for determining most of the characteristics of aerosols, including their physical and chemical properties. Examples are selected from measurements to show the types of information contained in the optical scattering signatures. Improvements in understanding the distribution of aerosols, their sources, their processes of formation and growth, and their role in establishing the planetary albedo and the radiative transfer into space are critically important for improving the predictions of our changing climate.

Multi-wavelength backscatter measurements from Rayleigh and Raman lidar techniques provide signals that are used to profile the properties for describing the transmission of radiation through an atmospheric column. The Rayleigh lidar signals provide backscatter coefficients and the Raman lidar signals backscattered from N2 and O2 provide true extinction profiles. The combination of these two sets of data gathered simultaneously makes a most important contribution to understanding the radiation transmission through the atmosphere. In addition, the same lidar beams can be used for making multistatic measurements of the polarization ratio of the scattering phase function. The multistatic measurements at several wavelengths are analyzed to determine profiles of the aerosol number density, size, size distribution, and type. These parameters can be determined for spherical particles in the size range between about 50 nm and 20 µm. Analysis of the size distribution requires adopting a mathematical shape function, usually a log-normal distribution. Information on aerosol type can be roughly determined based on chemistry and shape estimates from refractive index of the scatterers and depolarization of the scattered radiation as a function of wavelength.

Raman and DIAL lidar measurements provide the opportunities for measuring the profiles of chemical species which absorb radiation; thereby reducing the atmospheric transmission, as in the case of the infrared region greenhouse gases. Multi-wavelength DIAL, hyper-spectral measurements, and the recent developments of supercontinuum lidar techniques (SAL, Supercontinuum Absorption Lidar or SAS, Supercontinuum Absorption Spectroscopy) promise to provide the fundamental data to describe the optical absorption that limits the radiation balance between Earth and space.