Observations of the impact of volcanic activity on stratospheric chemistry

The basic stratospheric chemical and radiative processes which could be modified by volcanic injections to the stratosphere are reviewed. Observed effects after two major volcanic eruptions (El Chichon and Mount Pinatubo) are reported. Measurements of SO2, NO, NO2, HNO3, HCl, and O-3 clearly show the impact of volcanic injections to the stratosphere. Large amounts of SO2, (up to 20 Mt) are observed to be injected by energetic volcanoes. Gaseous SO2 is converted into sulfate aerosols within about 30 days. Reactive nitrogen (NO and NO2) are reduced by up to 50% of their column amounts in midlatitudes. Some observations have shown HNO3 amounts to be increased where NO2 is decreased; other observations have not shown an HNO3 increase. Heterogeneous reactions on the surfaces of sulfate aerosol particles are implicated in the conversion of NO and NO2 into HNO3. The direct injection of KCI by volcanic eruptions may increase the local column by up to 40%. Satellite observations have revealed local ozone decreases in the range of 5 to 10% of the column following El Chichon and Mount Pinatubo eruptions.