Retrieval of aerosol type and optical thickness over the Mediterranean from SeaWiFS images using an automatic neural classification method

Abstract

We present an automatic classification method based on topological neural network algorithms to retrieve aerosol optical properties from multi-spectral ocean-color satellite imagery. The first step of the method consisted in an unsupervised classification of a large set of clear-sky top of the atmosphere reflectance spectra measured by the sensor. We used the so-called Kohonen map which aggregates similar spectra into a reduced set of pertinent groups. The second step consisted in labeling these groups by clustering them with synthetic TOA reflectance spectra whose optical properties (i.e., aerosol type or optical thickness) are known. These synthetic spectra have been computed using a radiative transfer model. In the present study, we dealt with five aerosol types (maritime, coastal, tropospheric, oceanic and mineral) and several aerosol optical thickness values ranging from 0.05 to 0.8. These simulated spectra were then projected onto the Kohonen map to label each group of the map. The last step consisted in applying this method to the SeaWiFS imagery of the Mediterranean region for the years 1999 and 2000. The Kohonen map was "educated" from pixels randomly extracted during the year 1999 in this region. We accounted for the viewing geometry of the sensor by clustering the simulated spectra into ten groups of similar geometries, as defined by both scattering and sun zenith angles. The analysis of SeaWiFS images was performed pixel-by-pixel by selecting the suitable labeling (in terms of viewing geometry), then by identifying the closest spectrum in the Kohonen map, which finally gives the aerosol optical properties. This method led to accurate and coherent results, as shown by the comparison with in situ aerosol measurements provided by the AERONET station at Lampedusa and by the study of two aerosol events over the Mediterranean. One of the major advantages of this method is that it enables us to automatically identify the aerosol type and to retrieve the aerosol optical properties with a better accuracy than classical methods such as those used by SeaWifs. It gives accurate results for optical thickness values larger than 0.35 and is able to retrieve dust aerosols such as African dust aerosol (absorbing aerosol). These should ensure a more precise inversion of ocean-color imagery where the knowledge of atmospheric optical parameters is essential. Moreover the method is able to give probabilities for the estimate values of aerosol properties. © 2005 Elsevier Inc. All rights reserved.