In this lecture fundamental properties and basic mechanisms connected with atmospheric ice particles are presented.
Special emphasis is placed on the tropopause region and the lower stratosphere.
The mechanisms of homogeneous and heterogeneous nucleation leading to the formation of these clouds are briefly discussed. Then the importance of the presence of the various cloud types for the atmospheric gas phase is assessed. This includes a description of the heterogeneous reactions and the underlying microphysics occurring on ice particles as well as sulfuric acid droplets and ternary solution droplets. Also the heterogeneous chemistry of polar stratospheric clouds and its influence on the stratospheric ozone are described. The experimental results obtained from in-situ measurements with high flying research aircraft are placed into the framework of this theoretical background.