Aerosol Assumptions

The requirement for a-priori assumptions is caused by the lack of free parameters in satellite retrievals. Aerosol is defined by at least five different parameters (concentration, composition [defined by the real and the imaginary part of the refractive index], size [represented by size-distribution parameters] and shape). Characterizing aerosol is usually more complicated, because aerosol constitutes always a (frequently internally) mixture of many components. And each component has its own set of parameters. Added complexity comes from spectral dependencies (refractive index) and from dependencies on ambient relative humidity (refractive index, size) for some components. In addition, also nonsphericity must be considered for mineral aerosol (Mishchenko et al., 1997).

In contrast, a retrieval based on a single measurement contains only one piece of information, and this information is usually only valid for the spectral region of the measurement. Thus, assumptions are required for all remaining parameters that define the aerosol. Remote sensing from space at multiple spectral bands and/or multiple viewing angles and/or polarization may not provide unique solutions, but at least it can provide additional bounds for these a-priori assumptions (Mishchenko and Travis, 1997). Such advanced aerosol retrievals will possible with future satellites.

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