Kenneth A. McGee Michael P. Doukas David R. Sherrod William E. Scott Peter H. Stauffer Terrence M. Gerlach 2008 Airborne surveillance of gas emissions began at Mount St. Helens on September 27, 2004. Reconnaissance measurements--SO₂ column abundances and CO₂ , SO₂ , and H₂ S concentrations--showed neither a gas plume downwind of the volcano nor gas sources within the crater. Subsequent measurements taken during the period of unrest before the eruption began on October 1 and for several days after October 1 showed only small point sources of gas within the crater. These sources defined a pattern of scrubbed degassing that evolved from near-zero emissions, to scattered CO₂ -only sources, to growing sources of CO₂ with minor H₂ S and SO₂ , and finally to myriad sources of CO₂ with increasingly SO₂ - dominant sulfur gases. Scrubbing strongly hydrolyzed SO₂ but also affected CO₂ and H₂ S. application/pdf 10.3133/pp175026 en U.S. Geological Survey Emission rates of CO2, SO2, and H2S, scrubbing, and preeruption excess volatiles at Mount St. Helens, 2004-2005 reports