Measuring volcanic gases: continuous on-site gas monitoring

Scientist working on continuous gas monitoring equipment, Kilauea Volcano, Hawai`i

Continuous gas-monitoring site, Kilauea Volcano
Photograph by J.D. Griggs on June 11, 1991

Continuous gas-monitoring stations have the advantage of being able to detect both short-lived degassing events that occur in time periods lasting minutes to hours as well as long-term changes that occur over days to years. A station can be tailored to monitor fumarole gases, the air, or soil gases depending on the goals of the specific volcano monitoring strategy. Data are typically taken from the sensor(s) every few minutes and sent by radio, satellite, or telephone line to a receiving site.

The descriptions below highlight the continuous gas monitoring stations currently used by USGS scientists to monitor activity at Kilauea Volcano, Hawai`i and Long Valley Caldera, California.

Continuous gas monitoring at Kilauea Volcano, Hawai`i

Gas studies at the USGS Hawaiian Volcano Observatory (HVO) are conducted in order to assess current eruptive activity and hazards of Kilauea and Mauna Loa volcanoes and to improve our understanding of how these and other volcanoes work. Hawaiian volcanoes, because of their accessibility and effusive nature, serve as an ideal test sites for developing new techniques for volcanic gas studies. Among these techniques is continuous gas monitoring.

Continuous gas monitoring near vents

HVO scientists working on gas-monitoring stations, Kilauea Volcano, Hawai`i

Station near Pu`u `O`o

A small computer at this gas monitoring site, located on Kilauea's east rift zone, controls instrumentation which periodically samples the air near the Pu`u `O`o vent (cone in background). Chemically-selective sensors for SO2 and CO2 measure gas concentrations and a wind sensor measures wind speed and direction. Data from this solar-powered station are transmitted to HVO every 10 minutes, providing near real time data on degassing from Pu`u `O`o.

Air quality monitoring in Hawai`i Volcanoes National Park

HVO scientists working on air-quality sensor, Kilauea Volcano, Hawai`i

monitoring station

HVO scientists working on air-quality sensor, Kilauea Volcano, Hawai`i

inside trailer

An ambient air quality monitoring station located at the summit of Kilauea is operated cooperatively by the National Park Service and the USGS. The station measures concentrations of SO2 gas down to the part per billion (ppb) level in order to provide information about the impact of volcanic emissions on air quality. Data from this site help confirm the importance of wind speed and direction on the geographic distribution and concentration of volcanic air pollution. This station is part of a nationwide network of air monitoring sites that operate within National Parks.

During specific wind regimes, emissions from the main degassing sources of Kilauea impact the populated summit area. Data from this site show that federal health standards for SO2 (145 ppb) have been exceeded on more than 80 occasions during the past 13 years. An alert system using the data from this station informs staff of the Hawai`i Volcanoes National Park of the presence of potentially unhealthy concentrations of SO2 gas during episodes of very poor air quality.

Continuous gas monitoring at Long Valley caldera, California

A network of gas-monitoring stations is currently tracking CO2 concentrations in the shallow soils at several locations around Mammoth Mountain volcano and nearby areas. Each station collects data from its sensors at least every hour and transmits the stored data every three hours to a GOES geostationary satellite. The data are evaluated by scientists at the USGS Cascades Volcano Observatory.

Scientists from Pensylvannia State University, in cooperation with the USGS, operate 1-2 monitoring stations that measure carbon dioxide soil efflux for part of the year.

CO2 degassing event captured by continuous monitoring

Graph showing carbon dioxide gas in soil (percent) and earthquakes between September 1997 and January 1998

CO2 (percent) in soil and earthquakes in Long Valley caldera,
Sept. 1997 - Jan. 1998

The CO2 monitoring stations around Mammoth Mountain recorded a large degassing event in the fall of 1997. The event lasted about two months. The elevated CO2 concentration correlated well to a period of intense seismic activity in the south-central part of Long Valley caldera. The earthquakes occurred in response to the intrusion of magma beneath the caldera and right-lateral faulting in the south moat seismic zone.

Programming GOES satellite transmitter

Scientist programs a GOES satellite transmitter, Long Valley caldera, California

Long Valley caldera

USGS scientist programs a GOES satellite transmitter at a carbon dioxide monitoring site near Mammoth Lakes, California. Satellite telemetry offers the advantage of being able to locate monitoring stations without regard to access to telephone lines or short-distance radio links.


McGee, K.A., Sutton, A.J., and Sato, Motoaki, 1987, Use of satellite telemetry for monitoring active volcanoes, with a case study of a gas-emission event at Kilauea Volcano, December 1982, in Decker, R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, p. 821-825.

McGee, K.A., and Sutton, A.J., 1994, Eruptive activity at Mount St. Helens, Washington, USA, 1984-1988: a gas geochemistry perspective: Bulletin of Volcanology, v. 56, n. 6-7, p. 435-446.

McGee, K.A. , Gerlach, T.M., Kessler, R., and Doukas, M.P., 2000, Geochemical evidence for a magmatic CO2 degassing event at Mammoth Mountain, California, September-December 1997: Journal of Geophysical Research, v. 105, n. B4, p. 8447-8456.

McGee, K.A., and Gerlach, T.M., 1998, Annual cycle of magmatic CO2 in a tree kill soil at Mammoth Mountain: Implications for soil acidification: Geology, v. 26, n. 5, p. 463-466.

Other methods of monitoring volcanic gases