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COSPEC setup on Sugar Bowl (northwest flank of Mount St. Helens) to measure SO2 emissions from top of dome.
-- USGS Photo by Lyn Topinka, October 21, 1983

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Geologists collect gas samples around the dome. Samples are gathered from vents on the dome and crater floor, and are used to monitor changes in chemical composition. Additionally, sulfur dioxide gas is measured from specially-equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano. During eruptions, emission rates typically increase to 5 to 10 times their pre-eruptive value.
-- USGS Photo by Thomas J. Casadevall, September 24, 1981

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Fumarole sampling at Mount Baker's summit.
-- USGS Photo by W. Chadwick, 1981

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Fumarole sampling, Mount Mageik, Alaska
-- USGS Photo by Bea Ritchie, July 1995

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