Volcano Watch — Gases from Kīlauea

With the return of kona winds during the past few weeks, east Hawaii has been experiencing poor air quality, caused by the volcanic fume from the ongoing eruption on Kīlauea Volcano's East Rift Zone.

We receive many questions from people who assume that the dramatic change in air quality is caused by changes in eruptive activity or gas emissions from the volcano. The change, however, is entirely meteorologic in nature.

During periods of tradewinds, the volcanic fume, which consists dominately of water vapor, carbon dioxide, and sulfur dioxide, is blown to the southwest along the coast until it reaches South Point. It then wraps around the south flank of Mauna Loa Volcano and ends up along the Kona coast. There, the daytime onshore breeze carries it inland, and the nightime downslope breeze carries it back offshore.

With continuous trade wind conditions, such as occur during the summer months, the volcanic smog, or vog, accumulates in this back-and- forth wind pattern along the Kona coast.

During periods of kona winds or light variable winds, which occur predominately during the winter months, the volcanic fume is blown towards the northwest, north, or northeast. These wind directions generally result in accumulation of vog in east Hawaii. Because most of east Hawaii is located closer to the sources of the gas at Pu'u 'O'o and Kīlauea's summit, the vog has less chance to be diluted by mixing with clean air.

Recently, sulfur dioxide gas (the same sulfur gas emitted by the volcano) leaked at the BHP Hawaii Petroleum Refinery on O'ahu. The total gas emitted during a 34-hour period was about 8,900 pounds. Recalculating the emission rate to a daily rate yields about 6,250 pounds per day (slightly more than three tons per day).

In contrast, Kīlauea Volcano emits about 150 to 200 tons of sulfur dioxide from the summit caldera (mainly from Halema'uma'u) and an additional 700 to 1,250 tons per day from Pu'u 'O'o when in eruption.

However, because the leak at the refinery was from a point source and because there was limited dilution with clean air, the concentration of sulfur dioxide in the air exceeded 500 parts per million, a level which sent 29 workers to the hospital. We can see the importance of the extent to which the emitted gas mixes and is diluted by clean air when trying to assess the possible health consequences of exposure to sulfur dioxide gas.

The highest concentrations of sulfur dioxide we generally encounter during our work around the active eruption is about 100 parts per million. In the past week, the maximum concentration, averaged over an hour (recorded at an air quality station near the National Park headquarters) was 0.65 parts per million.

We also measured concentrations at the Observatory of 0.7 to 0.8 parts per million on December 7, when the light winds blew the plume from Halema'uma'u towards the Observatory. The highest 24-hour average value, recorded on December 1, was 0.1 part per million. For comparison, the Environmental Protection Agency's primary health standard for sulfur dioxide is 0.14 parts per million, averaged over a 24-hour period.

The sulfur dioxide released by the volcano reacts chemically with sunlight, oxygen, dust particles, and water in the air to form a mixture of sulfate aerosols (tiny particles and droplets), sulfuric acid, and other oxidized sulfur compounds. Together, the gas and aerosol mixture produces the hazy atmospheric condition known as vog.

Some areas of east Hawaii can expect the vog as frequently as we have kona winds. Of course, those in West Hawaii get a "breath of fresh air" only during these rather infrequent days of Kona wind, although it usually takes more than a day for the air to clear.