Intercomparing the Twomey method with a multimodal lognormal approach to retrieve the aerosol size distribution

This paper concentrates on intercomparing an objective algorithm to estimate the columnar aerosol size distribution from aerosol spectral extinction and a widespread robust nonparametric inversion approach. The nonparametric approach which has been considered is the Phillips-Twomey second-derivative regularization method. The parametric approach assumes that the size distribution is the linear combination of a set of lognormal functions (typically, from 10 to 30 distributions are considered) which have suitable mode radii and standard deviations. Intercomparisons have been performed on the basis of aerosol optical thicknesses derived by high-resolution solar radiation absorption spectra in the spectral region 0.40-1.1 mu m. Absorption spectra were recorded by a grating spectrometer with a resolution of 0.5 nm. Each inversion, either parametric or nonparametric, was obtained by using more than 450 data points at a time which, from a statistical point of view, allowed us to obtain high-quality results. For the cases presented here, we found that the two approaches agree fairly well in indicating the most significant characteristics of the aerosol size distribution.