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Sulfur dioxide scrubbing by liquid water masked SO2 emissions from shallow magma during the 1992 eruptions of Crater Peak and effectively prevented observation of SO2 emissions from shallow magma both before and after explosive eruptions and seismic crises. Airborne ultraviolet correlation spectrometer (COSPEC) measurements from July 22, 1991, to September 24, 1992, indicate only background to minor (<100 t/d) noneruptive SO2 emissions from Crater Peak, even though this period included the onset of precursory seismicity beneath Crater Peak (August 1991), the peaking in frequency of volcano-tectonic earthquakes and the initiation of volcanic tremor bursts (June 5, 1992), and three explosive eruptions that produced SO2 emissions of 200 to 400 kilotons (kt). The low, nonexplosive SO2 emissions are explained by hydrolysis of SO2 to aqueous H2S and sulfate from interactions with liquid water: 4H2O(l) + 4SO2(aq) ===> H2S(aq) + 3H+(aq) + 3HSO4-(aq). Sulfur dioxide hydrolysis also explains the increase in the sulfate content of Crater Peak lake water prior to the first eruption, the strong H2S odor during periods of background to low SO2 emission, the TOMS evidence for significant H2S emissions during the explosive eruptions, and the observed decline of SO2 during periods of volcanic tremor. Abundant, local sources of melt water and a high permeability for the Mount Spurr volcanic edifice are probably the chief factors responsible for masking SO2 emissions by scrubbing, and possibly for quenching shallow intrusions that were ascending. Large SO2 emissions unencumbered by scrubbing were only possible during the three explosive eruptions when magma penetrated through liquid water zones under Crater Peak and reached the surface. Nonexplosive SO2 emissions of as much as 750 t/d were possible, however, for a brief period when dry pathways to the surface existed from September 25 until about October 10, 1992. Airborne infrared spectrometer (MIRAN) measurements of CO2 emissions indicate that in addition to the degassing of magma through dry pathways, degassing through boiling water with the loss of SO2 by scrubbing was also important during that time. The CO2 emission data indicate that magma degassing was taking place, and CO2/SO2 values calculated from MIRAN and COSPEC data are in the range 10 to 100, which supports the hypothesis of SO2 loss by scrubbing. Because of its strong preference for the vapor phase during boiling, CO2 emissions from degassing magma are less likely to be masked by the presence of water, whereas SO2 emissions may be lost totally from interactions with water; thus misleading COSPEC results are obtained. We recommended prompt and early monitoring of CO2 when Cook Inlet volcanoes become restless.
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The 1992 eruptions of Crater Peak vent, Mount Spurr Volcano, Alaska
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