Remote Sensing of Tropospheric Aerosols from Space: Past, Present, and Future

Tropospheric aerosols are important components of the earth–atmosphere–ocean system, affecting climate through three primary mechanisms. First, direct radiative forcing results when radiation is scattered or absorbed by the aerosol itself. Scattering of shortwave radiation enhances the radiation reflected back to space, therefore increasing the reflectance (albedo) of the earth and cooling the climate system. Absorption of solar and longwave radiation alters the atmospheric heating rate, which in turn may result in changes to the atmospheric circulation. Second, indirect radiative forcing results when enhanced concentrations of aerosol particles modify cloud properties, resulting in more cloud drops, albeit smaller in size, that generally increase the albedo of clouds in the earth’s atmosphere. Additional cloud properties beyond cloud droplet concentration and particle size, such as enhanced liquid water content and increased cloud lifetime, have also been observed under certain circumstances. Finally, aerosol particles can have an indirect effect on heterogeneous chemistry, which in turn can influence climate by modifying the concentration of climate-influencing constituents (such as greenhouse gases). These three mechanisms are discussed in further detail by Schwartz et al. (1995). Aerosol forcing of climate reemerged in the last few decades of the twentieth century as a key climate change component causing significant uncertainty in the radiative forcing of climate (Twomey et al. 1984; Remote Sensing of Tropospheric Aerosols from Space: Past, Present, and Future