Measuring Volcanic Gases: The Answer is Blowin’ in the Wind — USGS Volcano Watch

Measurement of volcanic gases is critical for both public safety and understanding volcanic activity—and everything we measure relies on the wind.

The USGS Hawaiian Volcano Observatory (HVO) currently operates 19 permanent gas monitoring stations, and 7 portable instruments for eruption response. These can be divided into two categories based on what they measure: (1) gas concentrations; and (2) emission rates. 

Gas concentration instruments include multi-GAS stations that measure a combination of gases (CO2, H2O, SO2, and H2S) and high-resolution stations that can measure a single gas (SO2) down to very low concentrations. These instruments draw in or 'sip' samples of volcanic plume to tell us which gases are present and the ratios of these gases to each other, which is important for understanding the volcanic system.

Cartoon schematic of a volcanic plume from Halemaʻumaʻu blowing over permanent gas monitoring stations (diamonds) southwest of Kīlauea summit during normal trade wind conditions. Red is a Multi-GAS station; orange are high-resolution stations; yellow are Flyspec Array stations. Satellite imagery from Google Earth.

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Emission rate instrumentation, which includes the innovative Flyspec Array, UV-camera, and the vehicle-mounted DOAS, measures the plume’s absorption of ultraviolet light from the sun via remote sensing. This allows us to determine how much SO2 is coming out of the volcano, though only during daylight hours.

All these instruments require cooperation from the gases themselves: the plume must pass by or over the instrument for a measurement to be made.

The volcanic plume, however, doesn’t move on its own. It relies on the wind to carry it in any given direction. The job of volcano gas scientists is to chase around and measure this shifting, transient blob of gas—this is not an easy task!

Imagine a scenario where seismometers didn’t work every time it rained. Perhaps because the ground becomes too squishy and dampens the seismic signals. That’s not really what happens, but let’s pretend. In this scenario, we’d be able to feel an earthquake and know when it happened, but we’d have no way of measuring its magnitude just because the ground is wet. Thankfully, in the real-world, seismometers work no matter the weather condition—but gas instruments do not. They need the wind to be in right direction and the right speed to make a useful measurement.

At Kīlauea volcano, the dominant trade winds mean that near-surface winds blow from the northeast most of the year. For this reason, HVO’s permanent gas monitoring stations are positioned to the southwest (downwind) of Halemaʻumaʻu, Kīlauea’s summit crater.

If the wind direction is reversed relative to normal trade winds (a condition we call “Kona winds”) then although we can see the plume, smell it, and even taste it, we have no easy way of measuring it because the wind is blowing the gas away from our permanent sensors. 

Similarly, if the wind is too slow (below about 4 m/s, which is 9 mph or 8 knots), then the plume can loft straight up and once again miss our sensors. Alternatively, if the wind is too strong then it effectively dilutes the plume, spreading it thin and making it difficult for our sensors to measure. How strong is too strong depends on how much gas is in the plume, but during inter-eruptive periods with relatively little gas the cut-off is around 12 m/s, which is 27 mph or 23 knots.

Another complication is that volcanoes don’t always erupt from the same location. In the most recent eruption at Kīlauea, fissures opened in the upper Southwest Rift Zone—downwind of nearly the entire gas monitoring network. Only one instrument, a high-resolution station called HRPKE, was located near the eruptive vents, a few hundred meters to the west/northwest of the fissures. However, the winds were northerly that day, and were blowing the thick eruptive plume to the south, away from HRPKE. It didn’t record a wisp of gas until several hours into the eruption when the wind turned more easterly, finally blowing the plume to the station.

Effective gas measurements require an alignment of four things: wind direction, wind speed, sometimes daylight, and always luck. Volcano gas researchers at the USGS continue to develop new technologies to help us win this sophisticated game of chase, so that we can inform the public about this ever-shifting volcanic hazard. So, if you check the public webpage and it looks like there isn’t any gas reported, it may just mean the wind is not cooperating and we’re still chasing down the plume.

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Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week's article is by Research Corporation of the University of Hawaiʻi gas technician Christine Sealing.