Emission rates of CO2, SO2, and H2S, scrubbing, and preeruption excess volatiles at Mount St. Helens, 2004-2005

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.