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.
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.
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. HVO also uses an ultraviolet camera to image the plume and provide emission rate information.
During steady trade winds, the stationary 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.
When plume geometry allows, the amount of carbon dioxide (CO2) emitted by Kīlauea is measured using a small infrared analyzer (LI-COR). Scientists drive this instrument right through the gas cloud where it continuously and directly samples the ground-level cross-section of the plume.
Another tool used to measure the relative abundance of gases, including SO2, CO2, hydrogen chloride (HCl), hydrogen fluoride (HF), carbon monoxide (CO) and water vapor (H20), 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.
Kīlauea emits between 500 and 10,000 metric tons of sulfur dioxide gas (SO2) each day during periods of continuous eruption. 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 eruption began. While Pu‘u ‘Ō‘ō was the main source of gas release from 1979–2007, since 2008, Kīlauea Volcano's summit vent in Halema?uma?u Crater has been the dominant gas source. Currently, Kīlauea emits 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.
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. Thus, the current summit activity is not out of the ordinary. It could end tomorrow or go on for decades.