Thus far we have discussed the macrophysical requirements for cloud formation. Since each cloud droplet forms on an aerosol particle, these particles will be discussed in this chapter. An aerosol is defined as a dispersed system of solid or liquid particles suspended in a carrier gas, in our case air. Although technically the term “aerosol” includes both the particles and the carrier gas, in atmospheric science it is common to use the term “aerosols” just for the solid or liquid particles and to neglect the carrier gas.

Aerosol particles can be classified in terms of their chemical composition and according to their physical characteristics such as size, shape, density and mass, as will be discussed in Section 5.1. Because aerosol particles vary in size from a few nanometers to several micrometers, it has been found useful to describe their number, surface and mass concentrations in terms of size ranges, called modes, using log-normal size distributions (Section 5.2). Aerosol particles can be directly emitted into the atmosphere or form by gasto- particle conversion. These mechanisms and the emissions of the most important aerosol types are the subject of Section 5.3. Aerosol particles are removed from the atmosphere by dry and wet scavenging (Section 5.4). The difference between their formation and removal rates determines their abundance (burden) in the atmosphere as well as their atmospheric lifetime. Their lifetime is strongly dependent on their size but also on the altitude in the atmosphere (Section 5.5). The chapter concludes with a summary of the most important aerosol processes in Section 5.6.

Chemical and physical characteristics of aerosol particles

Depending on the type of particle, aerosols have differing properties that determine their role in atmospheric processes. A number of fundamental properties can be used to characterize aerosol particles; these include chemical and physical properties.

Chemical characteristics

The chemical characterization of an aerosol particle at the level of chemical compounds would usually be too complicated. Whereas a sulfuric acid particle nucleated from its gaseous precursors can be described by a single chemical formula, a mineral dust particle or a combustion-generated carbonaceous particle consists of a vast multitude of different chemical compounds.