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.