Frequently Asked Questions about volcanic gas at Kīlauea

Color photograph showing a volcanic crater emitting volcanic gases — Puʻu ʻŌʻō crater is obscured by heavy volcanic gas fumes, Kīlauea. Photo taken in 2008. (Public domain.)

1. What gases are emitted by Kīlauea and other active volcanoes?

Ninety-nine percent of the gas molecules emitted during a volcanic eruption are water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). The remaining one percent is comprised of small amounts of hydrogen sulfide, carbon monoxide, hydrogen chloride, hydrogen fluoride, and other minor gas species.

Color photograph of person next to instrument, measuring gas — USGS HVO geochemist measuring gases released from Kīlauea with a Fourier transform infrared (FTIR) spectrometer, an instrument that detects gas compositions on the basis of absorbed infrared radiation. (Public domain.)

2. Who monitors volcanic gases emitted by Kīlauea?

The U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) determines the amount and composition of gases emitted by Kīlauea Volcano (read Monitoring Volcanic Gas at Hawaiian Volcanoes to learn more). Changes in gas emissions can reveal clues about the inner workings of a volcano, so they are measured on a regular basis.

HVO scientists share scientific data and work closely with other government agencies responsible for making decisions regarding public safety, including Hawai‘i County Civil Defense, Hawai‘i State Department of Health, and Hawai‘i Volcanoes National Park.

Color photograph showing person and instrument measuring volcanic gas — Camera captures images of SO2 released from Kīlauea's summit vent. HVO scientist tests the network link between the instrument and the Hawaiian Volcano Observatory. (Credit: Christoph Kern, Cascades Volcano Observatory. Public domain.)

3. How do HVO scientists measure volcanic gas emissions at Kīlauea?

USGS HVO uses both remote and direct sampling techniques to measure the composition and emission rate of gas from Kīlauea Volcano. Gas emissions are measured "campaign style," that is, scientists go to the field to collect the data, as well as continuously by stationary instruments.

To determine how much sulfur dioxide (SO₂) is emitted, HVO scientists measure the amount of ultraviolet (UV) radiant energy absorbed by the volcanic gas plume as sunlight passes through it. One way they do this is by attaching a miniature-UV spectrometer to a vehicle and driving the upward-looking instrument beneath the plume. Using these data, scientists can compute the SO₂ emission rate, typically reported in units of metric tons per day (one metric ton equals 2,200 pounds) or kilograms per second.

Also at the summit of Kīlauea, a stationary array of 10 upward-looking UV spectrometers known as FLYSPECs automatically measures SO₂ emission rates as the plume travels overhead.

During steady trade winds, the FLYSPEC systems measure SO₂ every 10 seconds and send data to HVO via radio. In the absence of trade winds, SO₂ emission rate measurements are more challenging to acquire. Generally, steady trade winds and little or no rain result in the highest quality data.

HVO also uses an ultraviolet camera to image the plume and provide emission rate information.

Both the FLYSPEC and ultraviolet camera techniques require moderate emission rates of SO₂ to provide useful information; for low emission rates, HVO relies on the vehicle-based emission rate measurements.

In addition to emission rates of SO₂, HVO also measures ambient concentrations of volcanic gases at Kīlauea. Three stations near the summit report SO₂ concentrations every ten minutes. Another station, called a MultiGAS, at the summit reports concentrations of SO₂, CO₂, H₂S, and water vapor eight times per day. We also have portable MultiGAS sensors that can be carried or mounted on a UAS (unoccupied aircraft system) to determine concentrations of those same four gases in various places.

Another tool used to measure the relative abundance of different gases, including SO₂, CO₂, hydrogen chloride (HCl), hydrogen fluoride (HF), carbon monoxide (CO) and water vapor (H_2O), is the Fourier Transform infrared spectrometer (FTIR). The FTIR measures the amount of energy absorbed by gases along the path between the spectrometer and an infrared source, such as incandescent lava or a lamp.

We can also collect gas directly into a specialized glass bottle or metal tube. By collecting direct samples, we get a fuller picture of all gas species that Kīlauea is emitting by obtaining chemical analysis of the gas samples. HVO collects direct samples at least four times per year at Sulphur Banks, near Kīlauea’s summit in Hawai‘i Volcanoes National Park.

4. How much sulfur dioxide (SO₂) gas comes from Kīlauea?

Kīlauea has previously emitted between 500 and 10,000 metric tons of sulfur dioxide gas (SO₂) each day (t/d) during periods of continuous eruption, like that between 1983 and early 2018. Regular emission rate measurements began at Kīlauea in 1979, when a few hundred tons of SO₂ were released each day from the summit area. The onset of the East Rift Zone (ERZ) eruption at Pu‘u‘ō‘ō in 1983 was accompanied by a large increase in SO₂ release: emissions averaged about 1500-2000 t/d during the first decades of the ERZ activity. Another large increase in emissions occurred in 2008 when the summit lava lake eruption began. While Pu‘u‘ō‘ō was the main source of gas release from 1979–2007, from 2008 to early 2018, Kīlauea Volcano's summit vent in Halema‘uma‘u Crater was the dominant gas source. Prior to 2018, Kīlauea emitted more than five times as much SO₂ as the top emitting power plant in the United States, but less than 1 percent of global SO₂ generated by humans. During Kīlauea’s eruption in 2018, emission rates at the Kīlauea summit increased briefly during the explosive activity in May, but then decreased steadily. In the lower East Rift Zone, measured emission rates were the highest measured at Kīlauea, over 100,000 t/d for much of the three months of the 2018 fissure eruption. Following the end of the 2018 eruption, emission rates in the lower East Rift Zone, at Pu‘u‘ō‘ō, and at the summit decreased drastically. By the end of 2018, Kīlauea was emitting only about 50 t/d of SO₂. SO₂ emissions remained below 50 t/d until a summit eruption began on December 20, 2020. At the onset of the new eruption in December 2020, Kīlauea summit emission rates were 30,000–40,000 t/d. SO₂ emissions progressively dropped to around 2,500 t/d on January 11, 2021, telling us that the eruption rate decreased. SO₂ emissions have recently varied between 300-1200 t/d.

Color photograph showing emissions from volcanic vent — Volcanic-gas plume rises from Halema‘uma‘u Crater, Kīlauea Volcano, Hawai‘i. Photo taken in 2008. (Public domain.)

5. Why do I still smell sulfur when SO₂ emissions are low?

SO₂ is emitted primarily when lava is on the surface or if magma is very shallow and close to the surface (within 100s of meters or yards). With no lava presently erupting from anywhere on Kīlauea, SO₂ emissions are very low. Since the end of the 2018 eruption, car-based measurements using an ultraviolet spectrometer on nearby roads show that SO₂emissions are less than 50 t/d at the summit, less than 100 t/d at Pu‘u ‘Ō‘ō, and below detection limits (<~20 t/d) in the lower East Rift Zone. Thus, total Kīlauea SO₂ emissions are less than a few hundred t/d of SO₂. The significant decrease in SO₂ emissions has resulted in drastically reduced vog downwind of Kīlauea. SO₂ emissions are decreased, but some SO₂ is still present. Additionally, the cooler and wetter volcanic conditions mean H₂S is also present. Both gases smell ‘like sulfur’, but while SO₂ is a ‘sharper’ smell (like a struck match), H₂S smells like rotten eggs. Human noses are very sensitive to H₂S, so even with the current low emission rates, minor amounts are noteable.

6. Will Kīlauea emit high levels of SO₂again?

Kīlauea is one of the best-studied volcanoes on Earth, but much about the volcano's behavior remains unknown. To forecast future behavior of active volcanoes, scientists often look to the past. The first written accounts of Halema‘uma‘u were recorded by William Ellis when he trekked to Kīlauea's summit in 1823. Since then, Halema‘uma‘u eruptions have ranged in duration from over a century to less than day. Kīlauea could erupt again, but we don't yet know when or where.