https://doi.org/10.3389/feart.2018.00214
“Retrieving accurate volcanic sulfur dioxide (SO2) gas emission rates is important for a variety of purposes. It is an indicator of shallow subsurface magma, and thus may signal impending eruption or unrest. SO2 emission rates are significant for accurately assessing climate impact, and providing context for assessing environmental, agricultural, and human health effects during volcanic eruptions. The U.S. Geological Survey Hawaiian Volcano Observatory uses an array of ten fixed, upward-looking ultraviolet spectrometer systems to measure SO2 emission rates at 10-s sample intervals from the Kīlauea summit. We present Kīlauea SO2 emission rates from the volcano’s summit and middle East Rift Zone during 2014–2017 and discuss the major sources of error for these measurements. Due to the wide range of SO2 emissions encountered at the summit vent, we used a variable wavelength spectral analysis range to accurately quantify both high and low SO2 column densities. We compare measured emission rates from the fixed spectrometer array to independent road and helicopter-based traverse measurements and evaluate the magnitudes and sources of uncertainties for each method. To address the challenge of obtaining accurate plume speed measurements, we examine ground-based wind-speed, plume speed tracking via spectrometer, and SO2 camera derived plume speeds. Our analysis shows that: (1) the summit array column densities calculated using a dual fit window, are within -6 to +22% of results obtained with a variety of other conventional and experimental retrieval methods; (2) emission rates calculated from the summit array located ∼3 km downwind provide the best, practical estimate of summit SO2 release under normal trade wind conditions; (3) ground-based anemometer wind speeds are 22% less than plume speeds determined by cross-correlation of plume features; (4) our best estimate of average Kīlauea SO2 release for 2014–2017 is 5100 t/d, which is comparable to the space-based OMI emissions of 5518 t/d; and (5) short-term variability of SO2 emissions reflects Kīlauea lava lake dynamics.”
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Mobile UV Spectrometer Instruments
In this study, two types of mobile instruments were used to collect moderate resolution spectra of incident scattered solar UV radiation. For one, we used a mobile DOAS instrument consisting of an Ocean Optics spectrometer (either USB2000+ or SD2000) connected to an upward-looking telescope with a fused silica fiber optic cable. The spectrometers had an optical resolution of about 0.7 nm and recorded radiance in the 280–420 nm wavelength interval. For traverse measurements, the telescope was mounted on the outside of the vehicle (either car or helicopter), while the rest of the instrumentation was placed inside. A laptop computer acquired spectra with the Mobile DOAS software (e.g., Galle et al., 2003). The other instrument was the standard mobile version of the FLYSPEC described in section “Spectrometer Array” and by Horton et al. (2006).
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