A new technique to find both real and imaginary index of refraction of atmospheric aerosols from clear sky radiance measurements.

Abstract

A new iterative technique to find both the real and imaginary index of refraction of atmospheric aerosols has been developed which utilizes polarized sky radiance measurements in the solar principal plane. The technique requires that the aerosol size distribution, Rayleigh optical depth, aerosol optical depth, surface albedo, and preferably the surface anisotropic characteristics be known from correlative measurements. The calculated optimal radiance ratio, which is the ratio of maximum sensitivity to the index being retrieved with minimum sensitivity to other parameters, is compared with the measured optimal radiance ratio. The retrieved index is found through iterative steps to minimize the difference between the measurement ratio and calculated ratio. The degree to which the uncertainties of the ensemble of parameters that affect retrieved index is assessed. Mie single scattering sensitivity simulations and radiative transfer simulations using a Gauss-Sidel routine are made to determine the sensitivity of polarized sky radiance to index, size distribution, surface albedo, and surface reflectance anisotropy. The results indicate that this inversion technique is viable, especially to retrieve the effective real index of refraction. While the retrieval of imaginary index did not work as consistently well as the retrieval of real index, the imaginary index retrieval yielded promising results with larger values of imaginary index (∼0.020). This technique also offers new insight into the effect surface anisotropy has on sky radiance.