Lidar detection of small aerosol size distribution

The major unknown in the global climate radiation balance calculations is the effect of aerosols. The extinction of aerosols depends upon the wavelength, size, concentration, composition, and to a lesser extent, shape of the aerosols. Thus, methods are needed to determine and model these quantities. The size distribution of larger aerosols can be monitored with multistatic lidar, at least in the spherical approximation. We can use this approximation in humid environments, and for old desert dusts in which the aspect ratio is typically below two. Aerosols that are small compared to the incident wavelength present a Rayleigh-like scattering dependence, and the size cannot be determined using multistatic lidar techniques. We discuss the analysis of true extinction from Raman lidar measurements at several wavelengths for determining the size distribution of aerosols. The Angstrom ratio, which is the natural log of the extinction ratio divided by the natural log of the wavelength ratio, has been used in column-integrated measurements to classify aerosols. Lidar backscatter Angstrom ratio measurements have also been used to classify aerosols as a function of range. However, the use for aerosol size distribution has not been investigated in detail before this work. We find, from Raman lidar measurements, Mie models of extinction and backscatter Angstrom ratios, that small aerosols make a significant contribution to optical scattering, and find that size information can be extracted from the lidar data.