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.