Microlensing and absorption of light in atmospheric aerosol/soot mixtures

Scavenging of soot by sulfate aerosols in the atmosphere is related to visibility studies, air-pollution monitoring and determination of the Earth´s radiation budget. On the one hand, carbonaceous soot, the chief particulate absorber of sunlight (Horvath 1993), is mainly generated by burning vegetation, forest fires, and in the process of incomplete combustion of hydrocarbons in air. On the other hand, sulfate aerosols usually consist of aqueous sulfuric acid droplets with 75% H₂SO₄ (Kent et al 1983). Sulfurous gases, which ultimately give rise to sulfuric acid droplets, are constantly injected in the lower atmosphere by anthropogenic enterprises. Soot grains may remain interstitial to the sulfate droplets, thus forming an external mixture. They may also be swallowed by sulfate droplets; that case is usually referred to as internal mixture (Stephens and Tsay 1990). Available indirect mode-matching (IMM) solutions to electromagnetic (EM)-wave scattering from a sphere with an eccentric spherical inclusion (Skaropoulos et al. 1994), as well as from two non-overlapping spheres (Ioannidou et al. 1995), are applied to calculate absorption by aerosol-soot mixtures in the visible and near-infrared bands. Moreover, maps of the EM field throughout a composite sulfate/soot particle are presented, highlighting the phenomenon of microlensing by the sulfate droplet