Tropospheric aerosols significantly influence global climate, by changing the radiative energy balance as well as the hydrological cycle (e.g., IPCC [2007]). Also known as suspended airborne particles, or particulate matter (PM), aerosols are a component of smog and air pollution (e.g., USEPA [2003], [2002] and [1997]) and a regulated criteria air pollutant (e.g., http://www.epa.gov/oar/particlepollution/naaqs-rev2006.html). Aerosols are spatially and temporally inhomogeneous and, depending on aerosol type and meteorology, they may be found far from their sources. Spanning from nanometers (run) to tens of micrometers (urn) in radius, aerosols are efficient at scattering solar radiation back to space, thus affecting photochemistry (e.g., [1997]) and changing the effective albedo of the Earth. Passive satellite sensors, from their vantage point above the atmosphere, observe the scattered solar radiation to measure the global aerosol distribution [Kaufman et al., 1997a]. The fundamental unit of measure for aerosol remote sensing is known as the aerosol optical depth (AOD), which is the integral of the aerosol light extinction over vertical path through the atmosphere, and is a function of wavelength λ. Typically, AOD (measured at λ = 0.55μm) ranges from near zero in pristine conditions to 1.0, 2.0 or even 5.0, during episodes of heavy pollution, smoke or dust.