Volcano Watch — Gas numbers are up but emission rates are not

Sulfur Dioxide (SO₂) gas emission rates from Kīlauea volcano are measured by HVO staff using an ultraviolet (UV) spectrometer that can measure the amount of UV light it receives. If we make measurements under clear sky and compare them with measurements taken beneath the Kīlauea gas plume, we find that less UV light reaches the spectrometer when it is under the gas plume. We can use the difference in these measurements to calculate how much UV light is absorbed by the SO₂ gas and, after a few more calculations, how much gas is in the plume.

Beginning in 1979, an upward-looking spectrometer was mounted atop our field vehicle and regularly driven beneath the gas plume coming from Kīlauea summit. The data were summed over the length of the vehicle traverse and multiplied by the average wind speed to obtain an emission rate.

This method, pioneered at Kīlauea and used at volcanoes worldwide, works best for gas plumes that appear visually thin and wispy. This was the case at Kīlauea’s summit through 2007 and along Chain of Craters Road, where SO₂ in the plume emanating from the long-running east rift eruption was measured. Through 2007, the East Rift Zone (ERZ) emissions were about 1700 tonnes per day (t/d) — much higher than the 150-200 t/d produced at the summit.

As chronicled in an earlier Volcano Watch, this all changed in early 2008 (http://hvo.wr.usgs.gov/volcanowatch/view.php?id=177), when summit emissions leading to the beginning of the eruption at Halema‘uma‘u in March ramped up to levels we’d not seen before. During the first half of 2008, the summit plume also became milky in appearance. We knew this meant that our tried-and-true vehicle-based technique would significantly underestimate SO₂ emission rates. But while these measurements were less accurate, the relative changes continued to be useful for tracking changes in volcanic activity.

The easiest way to improve our measurements was to locate them farther from the vent where the plume is spread out and less dense. This works for ERZ measurements and we will continue to report the vehicle-based numbers. Unfortunately, it doesn’t work at the summit of Kīlauea because there are no roads far enough out into the Ka‘ū Desert to make a constructive difference.

In 2009, the American Recovery and Reinvestment Act made funds available to HVO for improved monitoring instrumentation. Using those funds, we enlisted the help of colleagues at the University of Hawai‘i at Mānoa and collaborated on an experiment involving a fixed array of 10 upward-looking spectrometers located more than twice the distance downwind of where the vehicle-based measurements were made. That fixed array system became operational last year, and since then, we’ve been optimizing its performance while comparing it with vehicle-based values and other monitoring data.

The time has come for HVO to start reporting these new values, even as we continue to improve the system. We are confident that the fixed-array values more accurately estimate the summit emission rates than the vehicle-based values used between 2008 and 2013. The comparisons between vehicle-based and fixed-array techniques over the past few months have shown that the actual emission rate values were several times higher than our pre-2014 estimates.

As an example, the vehicle-based SO₂ emission rate measurements for December 11, 2013, were about 600 t/d while the fixed array measurements were between 1,300 and 3,100 t/d during that week. The difference is not a simple factor by which we can multiply the vehicle-based numbers to get improved accuracy. The best measurements must be made in a different way or at a different location or both - we are doing both.

As of January 1, 2014, HVO will report the range of SO₂ emission rates from the summit of Kīlauea volcano based on measurements made with our fixed-array technique, so expect to see a dramatic jump in these values. But keep in mind that the actual SO₂ coming out of Kīlauea has not changed – just our way of measuring and reporting it.

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Volcano Activity Update

A lava lake within the Halema‘uma‘u Overlook vent produced nighttime glow that was visible via HVO's Webcam during the past week. The lava-lake level fluctuated in response to 1–2-day-long deflation-inflation cycles (DI events) at the summit.

On Kīlauea's East Rift Zone, the Kahauale‘a 2 flow continues to advance slowly into the forest northeast of Pu‘u ‘Ō‘ō. The active front of the flow was about 5.8 km (3.6 miles) northeast of Pu‘u ‘Ō‘ō on December 11, and there is no indication that it has advanced significantly since then.

There were no earthquakes reported felt on the Island of Hawai‘i in the past week.