Aerosols are liquid or solid particles of between 10^-3 and 10 microns. They have various origins and are of different types. The major classes of aerosol are as follows: stratospheric aerosols of sulfuric acid, mainly from volcanic eruptions, tropospheric marine aerosols from the oceans, desertic aerosols of dust from desert or semi-desert areas, anthropogenic aerosols from urban pollution or fires, and aerosols from chemical transformations. An important topic in the Global Change program is the study of the biogeochemical cycle of tropospheric aerosols, and specifically the generation of aerosols from the surface, their uplift and transport as well as their interaction with other cycles.

Although minor constituents of the atmosphere, aerosols have both a direct and indirect impact on climate. Their impact is direct through the diffusion and absorption of solar radiation, leading to a cooling effect; and indirect, through their interaction with clouds. Aerosols act as condensation nuclei and affect the microphysical properties of clouds, which in turn drive the Earth's radiation budget (clouds formed in this way would reflect more solar radiation). The effect of aerosols might therefore counterbalance global warming due to greenhouse gases.

The extremely variable nature of their physical and chemical properties, together with their distribution over time and space, make the study of aerosols quite complex.

Furthermore, aerosols also affect satellite-based remote sensing of the Earth in the visible domain. An improved characterization of their optical properties is a key issue in improving the quality of products based on data from future spaceborne imaging instruments.

POLDER contribution to the retrieval of aerosols is illustrated in Nature, September 2002 : "A satellite view of aerosols in the climate system", Yoram J. Kaufman, Didier Tanré & Olivier Boucher.

POLDER's original capabilities are such that new approaches may be proposed for the global mapping of aerosols (at a resolution of 20x20 km) above oceans and land.

The ocean is almost "black" in the near infrared and its contribution is quite constant at 565 nm wavelength (away from the glitter). The aerosol over ocean inversion scheme is based on the spectral dependence in the 565-865 nm range and on the directional information of the radiance and polarized radiance. The outputs are the Aerosols Optical Thickness and the bimodal aerosol size distribution. The coarse mode of aerosols is a mixture of spherical and non spherical particles.

It is much more complicated to obtain the optical properties of aerosols over land, as their contribution to the top of the atmosphere signal is at most of the same order as the surface contribution. However, the contribution of aerosols outweighs any others in polarized light. The operational algorithm over land, therefore, only uses polarized channels at 443, 670 and 865 nm. It allows to detect the smallest particles (anthropogenic aerosols).

Aerosol index from POLDER 1 - December 1996 (Source: LOA)