The increasing availability of multi-band satellite sensors in the VIS-SWIR region (e.g., MODIS and MTI) supports the use of more effective multispectral algorithms for retrieving aerosol optical properties. The most prevalent multispectral algorithm applied over land [1] makes use of SWIR measured reflectance for 'dark pixels' to ascertain VIS surface reflectance values needed for aerosol property retrievals. Over oceans VIS-NIR data in ratio form [2] or in difference form [3] are used to specify aerosol properties.

However, aerosol optical properties are not all mutually independent, meaning single-solutions are not necessarily obtained, even with increased spectral measurements. Such ambiguity can be reduced by configuring the retrieval algorithm to emphasize one aerosol property. In this presentation two optical properties - aerosol optical thickness t and single-scattering albedo w - are retrieved using signature differences: spectral, spatial, or temporal, along with spectral models of the two properties t(l) and w(l). Measured spectral differences are basic to assessing the wavelength variations in aerosol properties, in particular, to evaluate the parameters in the aerosol models. Temporal difference data emphasize the changing magnitude of t, with aerosol type being similar for the two measurements. The spatial differences considered here are within the same area but at locations having considerably different surface reflectance values, such as vegetated land and water. Spatial differences eliminate the need to specify aerosol reflectance, which is dominated by t, - emphasizing instead aerosol transmittance, which is strongly dependent on the magnitude of w. The assumption is made of uniform aerosol within the area of spatial-difference data. Using these techniques, multiple values are retrieved from a number of VNIR-SWIR measurements. As a minimum four measurements are needed to retrieve two values of t, giving aerosol type (Angstrom exponent), and two values of w, providing aerosol spectral absorption.

VNIR-SWIR data from the Department of Energy MTI satellite are used to demonstrate the retrieval of aerosol properties. The MTI sensor has four VIS-NIR bands with spatial resolution 5 m, and six NIR-SWIR bands of resolution 20 m. Recent MTI data in the vicinity of the Plymouth Power Plant, located on the coast of Massachusetts, are used to demonstrate aerosol property retrieval using difference approaches. The retrieved aerosol optical thickness values are inter-compared with results from more commonly used algorithms and with AERONET measurements in the region.

References: [1] Kaufman, Y.J., A.E. Wald, L.A. Remer, B-C. Gao, R-R. Li, and L. Flynn, 1997: The MODIS 2.1 mm Channel - Correlation with Visible Reflectance for Use in Remote Sensing of Aerosol. IEEE Trans. Geosci. Remote Sens., 35, 1286-1298. [2] Durkee, P.A., F. Pfeil, E. Frost, and R. Shema, 1991: Global Analysis of Aerosol Particle Characteristics. Atmos. Environ., 25A, 2457-2471. [3] Chou, M-D., P-K. Chan, and M. Wang, 2001: Aerosol Radiative Forcing Derived from SeaWiFS-Retrieved Aerosol Optical Properties. J. Atmos. Sci., 58, (Special GACP Issue).

This work was sponsored by the National Oceanographic and Atmospheric Administration under Air Force Contract F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Air Force.