The stratospheric aerosol layer, a few kilometers thick and centered a few kilometers above the tropopause, is currently believed to be composed of Aitken nuclei, composed principally of ammonium sulfate, and large particles greater than 1 µ; particles larger than 5 µ contain little sulfate and are believed to be of meteoritic origin.With acceptance of the direct or indirect terrestrial origin of the stratospheric ammonium sulfate particles, three processes can be suggested for their occurrence in the stratosphere: (1) coagulation of stratospheric Aitken nuclei previously formed in the troposphere; (2) oxidation of SO2 of terrestrial origin in the stratosphere to H2SO4 followed by nucleation and internal growth; and (3) reactions involving hydrous stratospheric ions such as (H3O)+.n(H2O) which act as primary nuclei and facilitate reaction of gaseous constituents of terrestrial origin such as NH3, O2, and HCl.