We're giving thanks for clean air, but what's that new smell?
Sulfur dioxide and hydrogen sulfide gases themselves are not visible, but dramatic plumes are sometimes visible at Kīlauea Volcano's summit (shown here) and Pu‘u ‘Ō‘ō. These plumes are a result of atmospheric conditions rather than increased volcanic activity, and frequently occur when warm volcanic gases condense as they are released into cooler air temperatures of early mornings or evenings. USGS photo.
In this season of giving thanks, Island of Hawai‘i residents and visitors can be thankful for the return of good air quality, generally free of volcanic air pollution.
Since the sharp drop in Kīlauea's volcanic activity and gas emissions in early August 2018, there has been dramatic improvement in the island's air quality. The rest of the state can also take comfort in the low levels of volcanic gases, because wintertime southerly winds will have much less volcanic pollution to blow along the island chain.
The main culprit in the formation of volcanic air pollution, or vog, at Kīlauea Volcano is sulfur dioxide (SO2
). Since SO2
is released when magma
is at a shallow depth (less than 1 kilometer or 0.6 miles beneath the surface), the current lack of activity means that Kīlauea is releasing only a small amount of this familiar gas.
Currently, less than 200 tons of SO2
are emitted from the volcano each day. This is more than 20 times less than average emissions during the 10 years of lava lake
activity at Halema‘uma‘u, and at least 200 times less than peak emissions during the 2018 lower East Rift Zone eruption.
With these much lower emissions, many people have expressed surprise that a strong and slightly unfamiliar smell can be detected from Kīlauea during certain wind conditions. A change in the chemistry of emitted sulfur gases is responsible for this new aroma.
A small amount of hydrogen sulfide gas (H2
S), the smelly cousin of SO2
, is being produced. With the current volcanic conditions, deeper magma has led to cooler vent
temperatures. Without shallow magma to boil off ground water, the sub-surface environment is also much wetter.
These cooler and wetter conditions cause a small amount of H2
S to form, in addition to the SO2
S is most commonly detected during interruptions in trade wind conditions and in locations downwind of Kīlauea's summit, Pu‘u ‘Ō‘ō, and the 2018 lower East Rift Zone fissure
Sulfur dioxide gas, which produces a sharp pungent aroma like that emitted when setting off fireworks or striking a kitchen match, is noticeable to most people at 0.3 to 1 parts per million (ppm)—0.3 to 1 parts gas in 1 million parts of air. In some sensitive asthmatics, lung function changes have been observed with SO2
concentrations as low as 0.1 ppm, well below the level detected by most human noses.
On the other hand, people can usually smell the rotten egg odor of H2
S at lower concentrations ranging from 0.0005 to 0.3 ppm. H2
S is present at Kīlauea in tiny amounts, but that little bit can be quite noticeable.
The smell of H2
S is a familiar odor to people from hot spring or geothermal areas. It is also produced by decaying organic material (anaerobic digestion) and is released by sewers and swamps. Even the human body produces a small amount of H2
The State of Hawaii has set a "nuisance level" for H2
S at 0.025 ppm, based on the odor threshold. Negative symptoms of H2
S exposure do not occur until concentrations are well above the odor threshold.
According to the Occupational Safety and Health Administration (OSHA), prolonged exposure to 2‒5 ppm may cause headaches, eye irritation, nausea or breathing problems in some asthmatics (https://www.osha.gov/SLTC/hydrogensulfide/hazards.html). Measured concentrations in populated areas around Kīlauea are less than 1 ppm.
S can be detected by humans at very low concentrations, a person's sense of smell to the gas is lost at high concentrations. For instance, two to five minutes of exposure at 100 ppm can cause a sensory adaptation known as "olfactory fatigue." But concentrations of H2
S measured at Kīlauea, even directly at volcanic vents
, are well below this level.
For those of us who have spent decades living with the familiar aroma of "classic" vog, the introduction of smelly H2
S can be curious or even disconcerting. As Kīlauea Volcano's next chapter of activity unfolds, when magma eventually rises toward the surface, we can expect a decrease in H2
S emissions and a return to the more familiar smell of the SO2
and particle-dominated vog.
Volcano Activity Update
Kīlauea is not erupting. Low rates of seismicity
, and gas release have not changed significantly over the past week.
continue to occur primarily at Kīlauea's summit area and south flank, with continued small aftershocks of the May 4, 2018, magnitude-6.9 quake. Seismicity
remains low in the lower East Rift Zone (ERZ).
signals are consistent with magma
refilling the middle ERZ. At the summit, tiltmeters showed little change this week, except for a small DI (deflation-inflation) sequence.
Last week, sulfur dioxide gas emissions averaged 25 tonnes/day at the summit and 75 tonnes/day at Pu‘u ‘Ō‘ō. No sulfur dioxide was detected by our instruments in the lower ERZ.
Hazardous conditions still exist at both the lower ERZ and summit. Residents in the lower Puna District and Kīlauea summit areas on the Island of Hawai‘i should stay informed and heed Hawai‘i County Civil Defense closures, warnings, and messages (http://www.hawaiicounty.gov/active-alerts).
The USGS Volcano Alert level for Mauna Loa remains at NORMAL.
No Hawaii earthquakes
received three or more felt reports (minimum to be recounted here) this past week.