Iterative method for the retrieval of the aerosol backscatter coefficient and the Angstrom exponent from bi-wavelength lidar data in low- transmittance atmospheres

摘要

A method based on the advantage of a bi-wavelength lidar system has been developed and tested. The method departs from the fact that in nearly transparent atmospheres (weak extinctions), the ratio of the lidar signals at both wavelengths gives the Angstroem exponent in backscatter with very low uncertainty. The method consists in (ⅰ) assuming a linear relationship between the Angstroem exponent in backscatter and the Angstroem exponent in extinction (usually called Angstroem exponent), (ⅱ) approximating by a first order development the ratio of the transmittances, (ⅲ) and forcing the resulting Angstroem exponent. Profiles of the Angstroem exponent, and the aerosol backscatter coefficient at 1064 and 532 nm were retrieved. The aerosol backscatter coefficient profiles were compared to Klett inversion results in the lowest part of the atmosphere containing aerosols, [0-3 km], in a rural environment in summertime: the agreement is better than 12 % at 1064 nm, and better than 35 % at 532 nm, indicating a greater dependency to the initial assumptions in the visible range.