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 Chichón and Mount Pinatubo) are reported. Measurements of SO 2 , NO, NO 2 , HNO 3 , HCl, and O 3 clearly show the impact of volcanic injections to the stratosphere. Large amounts of SO 2 (up to 20 Mt) are observed to be injected by energetic volcanoes. Gaseous SO 2 is converted into sulfate aerosols within about 30 days. Reactive nitrogen (NO and NO 2 ) are reduced by up to 50% of their column amounts in midlatitudes. Some observations have shown HNO 3 amounts to be increased where NO 2 is decreased; other observations have not shown an HNO 3 increase. Heterogeneous reactions on the surfaces of sulfate aerosol particles are implicated in the conversion of NO and NO 2 into HNO 3 . The direct injection of HCl 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 Chichón and Mount Pinatubo eruptions.