Retrieval of aerosol size distributions and integral properties from simulated extinction measurements at SAGE III wavelengths by the linear minimizing error method

Abstract

The retrieval of aerosol size distribution and integral properties from simulated aerosol extinction measurements of the new satellite instrument, the Stratospheric Aerosol and Gas Experiment (SAGE) III, is investigated. The retrieval method used in this study is the newly proposed linear minimizing error (LME) method. This approach assumes that the aerosol size distribution can be approximated by a histogram of number density as a function of particle size. The retrieved number density in each bin and the integral properties, including surface area and volume densities, are expressed as a linear combination of the SAGE III aerosol extinction coefficients at eight or fewer wavelengths. The coefficients in the weighted linear combination are obtained by minimizing the average of retrieval errors over a set of testing size distributions. The choice of aerosol channels for retrieval was first determined by comparing the average composite errors of the integral properties retrieved from different combinations of aerosol extinctions. The accuracy of the proposed method was studied by comparing the actual and retrieved properties for six aerosol size distributions. Results of this investigation indicate that unimodal, as well as bimodal, lognormal size distributions can be retrieved with reasonable accuracy. When all eight aerosol extinctions are available for retrieval, the average composite errors, which include systematic errors and errors due to measurement uncertainties for the retrieval of surface area and volume densities, are 9.4±4.3 and 4.7±2.0%, respectively. The retrieved aerosol integral properties are more accurate than those retrieved from SAGE II. However, it is not advantageous to use all eight SAGE III wavelengths for retrieval. The proper choice of aerosol extinctions for retrieval is elucidated. The derived formulas listed in this paper can be easily used by investigators to retrieve aerosol integral properties and estimate the retrieval errors from SAGE III aerosol extinctions at different altitudes for a variety of purposes. The application of the LME method to instrumental design is discussed.