[en] Some of the important chemical and physical properties of gases and aerosols are described. These properties are required to determine the fate of airborne pollutants. In order to understand the reaction mechanisms and rates in the troposphere, the following chemical elements are discussed in the order of their increasing complexity in atmospheric reactions: hydrogen, oxygen, nitrogen, sulfur, and carbon. The tropospheric aerosol is considered from the point-of-view of an evolving population of particles. Particle and population properties are described, and examples of atmospheric aerosol distributions are presented. Finally, the sources and properties of airborne radionuclides from light-water reactors are discussed