Atmospheric aerosol particles play a critical role in the Earth’s radiation budget, yet the global radiative forcing by aerosols is widely recognized as a major uncertainty in our understanding of the climate [IPCC, 2007]. The radiative characteristics of aerosol particles are determined by their shape, size, total amount and chemical composition [Kaufman et al., 1997a]. Overall though, aerosols have a cooling effect at the Earth’s surface by reducing the amount of solar radiation arriving at the surface below the layer of aerosols in the atmosphere. This cooling effect by aerosols is achieved by increasing the planetary albedo at the top-of-the-atmosphere (TOA) through directly scattering some of incoming sunlight back into space. However, some of the radiation can also be absorbed in the atmosphere by aerosols and reemitted. The volcanic eruption of Mount Pinatubo in 1991 provides an excellent natural experiment to demonstrate the surface cooling effect of aerosols. In the following two years after the eruption the average global surface temperature was reduced by about half a degree Celsius, principally owing to the scattering of sunlight by volcanically enhanced stratospheric sulfate aerosol [Hansen et al., 1992]. In addition, to the direct influence that aerosols have on the climate system, aerosols have an indirect effect on the radiative forcing through their interaction with cloud droplets and influence on cloud albedo.