12 December, 2003
Remote Gas Sampling
While I have accompanied Shauna Mikelich up to the crater rim to pump gas directly from Mt. Erebus’s plume (see 12/08/03 The Pump Station), Dr. Clive Oppenheimer and Dawn Sweeney have been using remote sensing to measure the gas emissions from Mount Erebus. An older method, that we are still using this year, employs a COSPEC (Correlation Spectrometer). In this process, a large spectrometer is housed in the Lower Erebus Hut garage (Figures 1 and 2). On days when the plume is rising straight up in the sky, the COSPEC is aimed at the plume and can sample how much sulfur dioxide (SO2) is emitted from Erebus. Determination of gas concentration is based on the degree of absorption of UV radiation by SO2 parcticles in the plume (Figure 3). The new method employed this season is called DOAS (Differential Optical Absorption Spectrometry). This method is similar to COSPEC in that it also measures the amount of UV radiation transmitted though the plume in order to establish gas concentration. DOAS, however, is capable of measuring all of the gases emitted in a volcanic plume (sulfur dioxide, carbon dioxide, water vapor, etc) and weighs a fraction of the big, bulky COSPEC allowing for a greater range of remote sensing (Figures 4-8).
Gas emissions are important for understanding the internal plumbing of the volcano, the rate of magma recharge, and the potential volcanic activity and human health hazards. The major volcanic gasses (H2O, CO2, SO2, HCl and HF) are soluble (dissolved into the magma) at different pressures and temperatures. By determining the volume of each gas released in the plume, you can determine the maximum depth of the magma source (the closer to the surface of the Earth, the lower the pressure and therefore more of any given gas will be released). In the case of Mt. Pinatubo in the Philippines, the volume of SO2 released from the volcano increased dramatically leading up to the eruption as magma migrated up the volcanic conduit. Understanding what the baseline volumes are for volcanoes like Erebus, and how the volumes change as activity increases, may help in predicting eruptions. With Erebus, there is a continuously convecting lava lake, which means that gasses are released at the surface, but then recharged again at depth. Changes in the baseline levels of volcanic gasses may help to determine the convection rate. Finally, SO2, HCl, HF gasses pose a health hazard for humans and livestock and can affect agriculture.
Helping Dawn tote around her computer, DOAS, GPS, and battery backpack on foot gave me a great appreciation for the new, “lighter” system… and for Skidoos. I wouldn’t want to carry around the 85-pound COSPEC! As we scurried out of the way of an incoming helicopter, the backpack felt a lot heavier than the 20-pound battery that was in it. Dropping our load in the hut, we wandered out to watch one of the large 212 helicopters sling a broken Skidoo back to McMurdo. Frosted from head to toe with snow from the propeller wash, it was time to call it quits for the day and warm up in the hut.
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