Atmospheric stability in conjunction with a temperature inversion above 10 km pressure altitude, caused by the absorption of solar radiation by ozone, results in an accumulation of aerosol. The major precursor gas of the stratospheric aerosol is sulfur dioxide from volcanic eruptions. Major volcanic eruptions increase stratospheric aerosol surface area and volume by several orders of magnitude. Direct climatic forcing and changes in heterogeneous chemical reaction rates are the consequence. Anthropogenic influence on the stratospheric aerosol loading appears negligible. Soot aerosol from aircraft amounts to only a fraction of 1% of the sulfuric acid aerosol. Current stratospheric soot aerosol loadings are commensurate with emissions from commercial aircraft. Projected fuel consumption of a future supersonic fleet will increase the stratospheric soot aerosol burden by a factor of 2-3. Any effects from this increase will be confined to the northern hemisphere, because inter-hemispheric mixing of stratospheric aerosol is inhibited.