Gas escapes from magma as it rises toward the surface, erupts, and as it cools and crystallizes below ground. Scientists have long recognized that dissolved gases are the driving force of volcanic eruptions, but only recently have new techniques permitted routine, precise measurement of different types of volcanic gases released into the atmosphere. Hawai'i and Kīlauea in particular are excellent places to develop and test new techniques.
A primary goal of HVO's gas monitoring effort is to determine changes in emission rates of certain gases, chiefly sulfur dioxide (SO2) and carbon dioxide (CO2). Changes are compared with other monitoring information to assess magma supply and eruption rates, issue eruption warnings, improve gas-hazard assessments and vog forecasts, and better understand how Hawaiian volcanoes work.
HVO uses several methods to measure SO2 gas emissions at Kīlauea, and a part of HVO's gas monitoring and research program is to investigate which technique is most useful under certain conditions. CO2 emission measurements are challenging at Kīlauea and Mauna Loa because gas sources are likely spread out over a large area. Lesser species such as CO, HCL, and HF are also measured, but not as frequently.
Sulfur dioxide gas absorbs ultraviolet light from the sun. Conceptually, the more SO2 there is in a volcano's gas plume, the more ultraviolet light energy is absorbed before it reaches the ground. Scientists measure the absorption of ultraviolet (UV) sunlight passing through the gas plume by using one or more upward-looking UV spectrometer beneath and just outside the plume during daylight hours.
This is done in three ways at Kīlauea. The first technique involves a spectrometer strapped on the roof of a vehicle (or in the back of a helicopter). The vehicles is driven (or flown) several times underneath the summit and East Rift Zone gas plumes during normal trade-wind conditions—the plumes are blown across Crater Rim Drive at the summit and Chain of Craters road downwind of Pu‘u ‘Ō‘ō. The second techniques uses a network of 10 stationary UV-spectrometers, called FLYSPECs, is positioned downwind of the summit plume make measurements every 10 seconds during daylight hours, and the data are sent by radio to HVO in real time.
A third technique for measuring SO2 emissions from the lava lake in Halema‘uma‘u Crater was installed in 2013—the world's first continuously-operating ultraviolet (UV) camera system for monitoring SO2 emissions in a volcanic plume. The custom-designed system utilizes two UV cameras tuned to specific frequency bands, a UV spectrometer and telescope for automated calibration of the camera images, solar-powered batteries, and high-speed telemetry for transmitting data to HVO. The system is powered down at night and starts up again in the morning. The camera image refresh rate is set at approximately every 5 seconds, a suitable time resolution for determining plume speed and identification of short-duration degassing events. All data acquisition, evaluation, and processing is performed by software running on servers at HVO.
The ever-growing archive of continuous SO2 imagery and high-resolution emission rates from the summit vent is a new, invaluable resource for understanding gas release and eruptive processes at Kīlauea Volcano. Ultimately, a long term record of SO2 emissions (figure) will add to our knowledge of volcanic degassing, its relationship to eruptive behavior, and impacts on the environment.
In addition to these UV absorption measurements, scientists at HVO also use an instrument called a Fourier transform infrared (FTIR) spectrometer to quantify gases in Kīlauea'S plumes. For these measurements, an infrared telescope is pointed at a heat source such as the lava lake, a lava fountain, an infrared lamp or the sun. Infrared radiation is gathered and coupled into the spectrometer unit. Here, the spectrum is recorded and analyzed for the absorption by different gases in the plume. This method is sensitive to H2O, SO2, CO2, CO, HCl and HF, so the data contain detailed information on the broader chemical composition of the gas plume.
Information about gas compositions can also provide insights into the nature of volcanic processes. When FTIR was first used to measure gases at Pu‘u ‘Ō‘ō in 2004, scientists discovered that different types of volcanic activity were driven by different modes of degassing. For example, persistent, continuous release of H20-rich, CO2-poor gas occurred when magma ascended with bubbles that burst right at the surface. The release of CO2-rich gas accompanied loud jetting and increased glow within small eruptive vents at Pu‘u ‘Ō‘ō, interpreted to be driven by gas slugs rising from depths of several hundred meters.
Presently, HVO maintains a single gas and temperature monitoring station in the summit caldera of Mauna Loa. The instrument measures SO2, CO2, and fumarole temperature and radios these data back to HVO. Thus far in the current episode of unrest at Mauna Loa, no significant changes have been detected in either gas output or temperature. More gas monitoring instrumentation is planned for Mauna Loa as funding and other resources allow. HVO is also working with colleagues at NO‘A‘ā's Mauna Loa Observatory on the north flank to examine the volcanic contribution to atmospheric CO2 measured by their extremely sensitive instrumentation.