Remote sensing of tropospheric aerosol properties from space is difficult, as the aerosol associated backscatter signal is often at or below atmospheric (variability related) noise-levels. Considering the aerosol's (usually) sub-micrometer size, remote sensing is most promising at (atmospheric scattering dominated) regions of the solar spectrum. Multi-spectral radiometers and polarimeters (a few with multi-angular capabilities), as well as more recently CCD arrays, are used to probe changes in reflected sun-light in the ultraviolet, in the visible and/or in the near-infrared spectral region. However, before attributing changes in (solar) reflection to aerosol, impacts involving other contributors to solar reflection must be removed, most importantly reflections of clouds, molecular scattering and the earth's surface. Unfortunately, albedos from clouds and the earth's land surfaces are dominant modulators of solar radiation reflected to space. Thus, in aerosol retrievals from satellites these properties must be known at high accuracy. This is in sharp contrast to rather vague ideas on cloud cover and cloud microphy-sics or in contrast to a poor knowledge of surface conditions. To avoid these potential problems, aerosol retrievals from satellites are commonly limited to regions where albedo contributions below the aerosol can be neglected or can be expected to remain stable and small, such as over oceans.

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