Satellite observations (reviewed by King et al., 1999) are naturally suited to the global coverage demanded by regional variations in aerosols. Aerosol optical depth can be retrieved over the ocean in clear-sky conditions from satellite measurements of irradiance and a model of aerosol properties (Mishchenko et al., 1999). These have been retrieved from satellite instruments such as AVHRR (Husar et al., 1997; Higurashi and Nakajima, 1999), METEOSAT (e.g., Jankowiak and Tanré, 1992; Moulin et al., 1997), ATSR (Veefkind et al., 1999), and OCTS (Nakajima et al., 1999). More recently-dedicated instruments such as MODIS and POLDER have been designed to monitor aerosol properties (Figure 5.3) (Deuzé et al., 1999; Tanré et al., 1999; Boucher and Tanré, 2000). Aerosol retrievals over land, especially over low albedo regions, are developing rapidly but are complicated by the spectral and angular dependence of the surface reflectivity (e.g., Leroy et al., 1997; Wanner et al., 1997; Soufflet et al., 1997). The TOMS instrument has the capability to detect partially absorbing aerosols over land and ocean but the retrievals are only semi-quantitative (Hsu et al., 1999). Comparisons of ERBE and SCARAB data with radiative transfer models show that aerosols must be included to accurately model the radiation budget (Cusack et al., 1998; Haywood et al., 1999).
There is not enough information content in a single observed quantity (scattered light) to retrieve the aerosol size distribution and the vertical profile in addition to the optical depth. Light scattering can be measured at more than one wavelength, but in most cases no more than two or three independent parameters can be derived even from observations at a large number of wavelengths (Tanré et al., 1996; Kaufman et al., 1997). Observations of the polarisation of backscattered light have the potential to add more information content (Herman et al., 1997), as do observations at multiple angles of the same point in the atmosphere as a satellite moves overhead (Flowerdew and Haigh, 1996; Kahn et al., 1997; Veefkind et al., 1998).
In addition to aerosol optical depth, the vertically averaged Ångström exponent (which is related to aerosol size), can also be retrieved with reasonable agreement when compared to ground-based sunphotometer data (Goloub et al., 1999; Higurashi and Nakajima, 1999). Vertical profiles of aerosols are available in the upper troposphere and stratosphere from the SAGE instrument but its limb scanning technique cannot be extended downward because of interference from clouds. Active sensing from space shows promise in retrieving vertical profiles of aerosols (Osborn et al., 1998).
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