ABSTRACT
One cubic centimeter of atmospheric air contains approximately 2.5×1019 molecules. About 1000 of these molecules may be charged (ions). The molecules of N2, O2, and the various trace gases have sizes (diameters) of about 3×10−8 cm. The average distance between the molecules is about ten times the molecular size. In addition to the molecules and the ions, one cubic centimeter of air also contains a substantial number of particles varying in size from a few times the molecular size to several microns (µm, 1µm=10−4 cm). In relatively clean air there are about 1000 particles with diameters 0.001 µm to 50.0 µm while in polluted air there can be 100,000 or more, including pollen, bacteria, dust, and industrial emissions. These particles, which can be both beneficial and detrimental, arise from a number of natural sources as well as from the activities of the Earth’s inhabitants. The particles can have complex chemical compositions and morphologies, and may even be radioactive or toxic. A suspension of particles in a gas is known as an aerosol. Atmospheric aerosol is of global interest and has an important impact on our lives.
