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Sulfur dioxide is emitted by volcanoes, produced by combustion of fossil fuels or smelting of ores, and is an intermediate product from organic sources in the ocean. It is rapidly oxidized to sulfuric acid, which causes acidic pollution of lakes and streams and forms an aerosol that is important in climate change. Volcanic sulfur dioxide is a useful marker for ash clouds that are a hazard to aircraft. Satellites offer the best platform to monitor SO2 sources and to track volcanic clouds. UV remote sensing instruments have measured eruption plume masses since 1978. Newer instruments are sensitive enough to also measure volcanic degassing, emissions from power plants, refineries, smelters, and heavy air pollution episodes. New retrieval algorithms have improved the data quality. The observations are used to constrain models of eruption processes and to monitor activity of all volcanoes in a consistent manner. The practical applications of the satellite data include aviation safety, air quality, environmental control, climate modeling, and atmospheric dynamics modeling.