Volcano Watch: How We Know How Much Sulfur Dioxide Volcanoes Emit

Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

The left diagram shows how incoming UV would get absorbed by the SO2 plume (faded red arrows) along a spectrometer traverse under the plume (yellow arrow). Under clear sky, no UV is absorbed (blue arrows). Inset is a cartoon diagram of a plume ‘slice’ that the spectrometer measures. At right is the telescope of the UV spectrometer mounted to an HVO vehicle during the 2018 lower East Rift Zone eruption. The telescope is aimed up at the sky, and a fiber optic cable (taped to car window) connects the bottom of the telescope to the spectrometer inside the car. Also visible is a wire attached to a GPS antenna on top of the car. USGS images.

Volcanic gases are an important part of eruptions—they help magma to rise within the earth and erupt, they can tell us how much lava is being erupted, and the volcanic air pollution (vog) they cause can be a hazard. So it is important for the USGS Hawaiian Volcano Observatory (HVO) to measure how much of what kind of gas is being emitted by our volcanoes.

When people think about gas measurements, gas concentration—or how much gas there is in one spot at one time—is often what comes to mind. Knowing concentrations of gases like sulfur dioxide (SO2), for example, is important for understanding implications for human health during volcanic eruptions.

Another number that volcanologists report is the emission rate of SO2—how much SO2 is released over time by a volcano. But perhaps counterintuitively to those who are not volcanic gas specialists, the emission rate is not actually determined by directly measuring concentrations of SO2.

To measure SO2 emission rates, we begin by mounting an ultraviolet (UV) spectrometer to a car, aircraft, or backpack frame. Since SO2 is invisible and may not perfectly coincide with visible parts of the plume, we assess where the SO2 should be based on wind direction. Then, starting under clear sky on one side of the plume, we traverse underneath the entire width of the plume, and end up back under clear sky on the other side.

The spectrometer looks up at the sky and, because SO2 absorbs UV radiation, it detects less incoming UV when it is under the gas plume where there is SO2. It measures how much SO2 is above it in the vertical ‘path’ where the spectrometer is looking—the ‘concentration-path-length’.

Concentration-path-length combines concentration and path into a single unit, ppm∙m (parts per million meters). A 1-meter (about one yard) thick plume with a concentration of 10 ppm (parts per million) of SO2 is equivalent to 10 ppm∙m. So is a 10-meter (about 11 yards) thick plume with a concentration of only 1 ppm of SO2. The amount of SO2 is the same, it’s just distributed differently.

All those concentration-path-length measurements put together across the plume’s width make a ‘slice’, or cross-section, through the plume, showing how much SO2 was above the spectrometer at each point. That slice, since it incorporates the plume width in meters, is the area of the gas in a cross-section of plume, with units of ppm∙m2 (parts per million square meters).

Once we have that cross-section, we use plume speed (in meters/second) to determine how many of those cross-sections—and how much gas—are passing overhead in a certain amount of time. That brings us to units of ppm∙m3/s (parts per million cubic meters per second)—which is a volume of gas with a certain concentration of SO2 each second.

Because we know how much a molecule of SO2 weighs, we can convert that volume into a mass (in kilograms or metric tonnes), and we can convert seconds to days. That’s how we derive our emission rates of SO2, which are usually presented in units of tonnes/day (t/d).

So how do SO2 emission rates from the current eruption compare to previous eruptions at Kīlauea?

When HVO began to routinely use these UV measurements in 1979, the summit averaged about 500 t/d of SO2 or less. Between 1983 and 2008, Kīlauea’s Pu‘u ‘Ō‘ō eruption averaged around 2,000 t/d. After higher emission rates early in the 2008–2018 summit eruption, the lava lake emissions stabilized near 5,000 t/d while Pu‘u ‘Ō‘ō’s emissions fell to a few hundred t/d.

The 2018 eruption had incredibly high emission rates of nearly 200,000 t/d, the highest recorded emissions from Kīlauea. Following the 2018 activity, total Kīlauea emissions dropped to only about 30 t/d.

At the onset of the new eruption in December 2020, Kīlauea summit emission rates were 30,000–40,000 t/d. Since the north fissure activity ceased on December 26, 2020, SO2 emissions have progressively dropped to around 2,500 t/d on January 11, 2021, telling us that the eruption rate has decreased.

Even with decreased levels of SO2 being emitted, measurements are still important because of hazards associated with vog, and because of what we can learn about eruption dynamics. So whatever happens next, HVO will continue using our ‘gas math’ to keep an eye on Kīlauea’s SO2 emissions.