Retrieval of aerosol optical depth over water and over land from multi-spectral electro-optical sensors on satellites

Aerosols have been identified as an important factor in the regulation of the Earth climate. The incoming solar radiation is scattered by aerosols, which induces a negative (cooling) effect on the atmospheric radiation balance. The present contribution is focused on the detection of aerosols by using their scattering properties. The concentrations and occurrence of aerosols are highly variable in both space and time, due to the occurrence of many direct sources with strongly different source strengths, atmospheric residence times of several days, and a variety of transport mechanisms that disperse the aerosol throughout the troposphere. To obtain information on the regional and global distributions of aerosols, and thus on their effects on climate and other important issues such as health and environmental effects, satellite remote sensing is a very promising tool. Different satellite instruments are becoming available that have been designed to derive specific information that can be used for aerosol retrieval. The problem to derive aerosol properties from radiation received by satellite-based sensors is to separate the contribution due to reflection at the surface from the contribution due to scattering by aerosols. It was thought that aerosol retrieval was only possible over water, i.e. dark surfaces with very low reflection. However, at the TNO Physics and Electronics Laboratory (TNO-FEL), algorithms have been developed that can also be applied over land. An overview of algorithms presently available at TNO-FEL is presented.