Derivation of aerosol optical properties from four-wavelength lidar observations

A multi-wavelength Mie lidar is a powerful tool to investigate the optical properties of aerosol particles along with their vertical profile information. In the usual analysis with the Fernald method, however, it is required to assume both the lidar ratio S1 and the extinction coefficients at the far end boundary. For a multi-wavelength lidar, appropriate choices of these parameters are indispensable to derive consistent profiles from the actual data. In this work, we propose two algorithms for the analysis of four-wavelength lidar data. The first algorithm is more comprehensive in that it adopts the direct fitting of the lidar A-scopes to the theoretical curves that are based on a look-up table: the table is pre-calculated for various combinations of the extinction coefficient, S1 parameter, complex refractive index, and aerosol size distribution. As a result, the vertical profiles of these parameters are determined along with the extinction profile. The second approach is a pragmatic one, and it relies on the sun photometer data simultaneously measured with the lidar data. By assuming a constant S1 value for each wavelength in the lower troposphere, a consistent set of S1 is determined by fitting the observed profiles to reference profiles that are relevant to the aerosol optical thicknesses from the sun photometer measurement.