Aerosol retrieval from combined spectral extinction and aureole measurements

Abstract

One of the main problems in retrieval of microstructure of aerosol in atmospheric column is connected with derivation of the single scattered radiance. In the present work, the ways to solve (or avoid) this problem for aureole measurements using spectral aerosol extinction data are considered. Aerosol optical thickness (AOT) in visible and near-IR is determined mostly by submicron aerosol fraction (except for the situations with extremely high coarse aerosol loading, like a dust storm). This fraction is also predominant in side scattering. So, it may be supposed that the single scattering phase function out of aureole region can be derived from the spectral AOT by means of inversion and following Mie calculations for retrieved submicron aerosol. For rough estimations, it is sufficient to calculate the Angstrom exponent and take the phase function for the corresponding power law distribution. Analysis of the model and derived from the experimental data size spectra showed that inversion of the spectral extinction allows to recover the phase function in the angular range 20 - 150 deg. with accuracy, comparable with the measurement accuracy. The phase function obtained gives the information needed for the account of multiple scattering in the aureole region. The asymmetry of clear sky radiance due to multiple scattering is much less than for single scattering. Numerical simulations showed that aerosol size spectra, retrieved from measured and single scattered aureole radiance, differ only in the region of small particles (less than wavelength). Thus, the aerosol size distribution can be obtained as a combination of the results of the inversion of AOT (submicron fraction) and aureole brightness (coarse fraction). The limits of applicability of the approaches proposed are discussed. The examples of the results obtained for different model and experimental size distributions are given.