Carbonaceous aerosols are increasingly recognized as an important atmospheric constituent. These small atmospheric particles are predominately soot produced by incomplete combustion of fossil fuels, biofuels, and outdoor biomass that generally form through condensation of vaporized organic matter. However, biogenic emissions from trees, other vegetation, and animals are also sources of carbonaceous aerosols. Elemental carbon, in the form of graphite, is the main cause of the blackness of soot; it absorbs sunlight strongly and almost uniformly across the solar spectrum. However, the graphite seldom is pure carbon, instead involving varying proportions of other atoms. Furthermore, the carbonaceous aerosols include an enormous variety of organic compounds of carbon.