from the NASA website (link to .rtf file):
- How do volcanoes affect the atmosphere? As volcanoes erupt, they blast huge clouds into the atmosphere. These clouds are made up of particles and gases, including sulfur dioxide. Millions of tons of sulfur dioxide gas can reach the stratosphere from a major volcano. There, the sulfur dioxide converts to tiny persistent sulfuric acid (sulfate) particles, referred to as aerosols.
- How do volcanic emission influence climatic changes? Global cooling often has been linked with major volcanic eruptions. Sulfate particles reflect energy coming from the sun, preventing the sun's rays from heating Earth thus lowering temperatures in the troposphere, and changing atmospheric circulation patterns.