The Retrieval and Comparison of Aerosol Mean and Effective Radii Measured by SAGE II and SAGE III

The retrieval of aerosol mean and effective radii from wavelength-dependent aerosol extinction coefficients measured by SAGE II and SAGE III satellite experiments is discussed. The retrieval method used in this study is the linear minimizing error method. This approach assumes that the kth moment of the aerosol size distribution can be expressed as a linear combination of the aerosol extinction coefficients measured by SAGE II or SAGE III. By definition the aerosol mean and effective radii are ratios of different moments of the aerosol size distribution. The coefficients in the weighted linear combination are obtained by minimizing the average of retrieval errors over a set of testing size distributions. Since any subset of the measured extinction coefficients can be used for retrieval, the choice of aerosol channels for retrieval is determined by comparing the associated retrieval errors and using the subset that can minimize the retrieval error. The retrieval error as a function of the parameters in the size distribution is presented. Since both SAGE II and SAGE III conducted measurements between 2002 and 2005, there are occasions when their measurement locations on the same day are nearly coincident, thereby providing opportunities for comparing aerosol radii measured by these experiments. It is found that in general, the effective and mean radii measured by SAGE III are about 10 to 30% lower than the corresponding values measured by SAGE II in the main stratospheric aerosol layer between about 18 to 26 km. Larger differences are shown at altitudes close to 30 km and around the tropopause.