Volcano Watch — Increases in vog may not mean increases in volcanic activity

Sometimes people call the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) and ask us if more gas is coming out of the volcano because air quality where they are calling from seems worse, or maybe because the amount of visible fume at Pu`u `O`o or Halema`uma`u seems greater. We at HVO usually say that the "emission rate " of sulfur dioxide (SO₂) from the volcano, which we measure regularly, is fairly constant. SO₂ is the major contributor to poor air quality near Kīlauea. But the caller rejoins, "I know that the fume is more concentrated, I can see it and smell it! If the concentration is higher, there must be more gas coming out, right? What 's going on here?! "

The key words in the foregoing paragraph are "emission rate " and "concentration. " Emission rate is a measure of how much SO₂ gas flows out of the volcano per unit of time. For Kīlauea, we measure and report the SO₂ emission rates in units of metric tonnes per day, with a metric tonne equal to 2,200 pounds. During periods of steady eruptive activity, the volcano emits about 2,000 tonnes of SO₂—enough pure SO₂ gas each day to fill 100 Goodyear blimps!

Concentration, by contrast, is a measure of how much gas or vog (volcanic smog, which is a mixture of fine particles and gas) is suspended in a volume of air at a particular place and time. Usually concentration is reported as parts per million for a gas or liquid, or as milligrams per cubic meter for particles.

To better understand a part per million (ppm), imagine filling up a 7,000 gallon water catchment tank with Margarita mix. Next, measure out about two thirds of a shot of Tequila and poor it into the tank and stir well. You have just made a Margarita with a Tequila concentration of 1 ppm. The Margarita seems pretty weak, but curiously, the human nose can easily detect SO₂ at a concentration of about half of this, or 0.5 ppm. On Kīlauea, if you 're immediately downwind of Pu`u `O`o or Halema`uma`u where the gas comes out, the concentration can be tens or even hundreds of parts per million. If, on the other hand, you are 20 km directly downwind, the concentration may be only 1 or 2 parts per million.

Thus, if the gas emission rate from Kīlauea is a constant 2000 tonnes per day, is the concentration of SO₂constant also? The answer to this question is literally blowing in the wind. If the wind direction is constant, then the gas concentration that an individual experiences at a given distance downwind from the emissionsource should be directly related to wind speed, a situation analogous to sitting in a restaurant "downwind " from a table full of chain smokers.

Intuitively, if the wind speed increases, perhaps by opening the window, the smoke from the adjacent table (or fume from the volcano) becomes more diluted, and the concentration drops. Conversely, as the wind blows more slowly, second-hand smoke (or vog) accumulates, resulting in higher concentrations (and, in the case of the smokers, possible lawsuits).

Wind direction is also critical for determining gas or vog concentrations at a given location. The prevailing northeasterly trade winds typically transport the constant airborne stream of vog around the southern end of the island and up the Kona coast. In contrast, when trade winds are absent or kona winds (light winds from the south and southwest) are present, much of the vog stays on the eastern side of the island.

Finally, steam vents and fuming areas like Halema`uma`u or Pu`u `O`o appear to release more gas during increases in atmospheric humidity. This is purely a meteorological phenomenon that causes warm, moist fume and steam from volcanic vents to cool, condense, and become more visible in air that is already completely saturated with water vapor. Our ability to see vog is also affected by humidity. The small particles which comprise vog are sub-micron in diameter and actually absorb excess humidity, causing them to become more visible. Consequently, a voggy day can appear to become more voggy simply by an increase in atmospheric humidity.

Having discussed the possibilities for false alarms in identifying changes in gas emissions, it turns out that there really have been substantial changes in gas release for Kīlauea during the first six months of this year. East Rift SO₂ emissions from the Pu`u `O`o area decreased as much as 90 % for a brief period. Emissions dropped nearly 70 percent during the short-lived eruptive episode at Napau crater at the end of January, and an additional 20 percent following episode 54. Emissions stayed low until well after molten lava reappeared in Pu`u `O`o in late February. Residents, especially on the Kona side, recognized and applauded the dramatic decrease in vog from early February through early-May, when SO₂ emissions really began picking back up. As of early June, the volcano was back to a "typical " SO₂ emission rate of 2,000 tonnes per day. Because of the substantial increase in East Rift emissions since May, we are once again receiving the numerous questions and complaints about vog from residents and visitors to west Hawai`i. Yes, Big Island residents, there really is more gas out there this time!

Volcano Activity Update

Kīlauea 's East Rift zone eruptive activity continued during the past week with intermittent action from three areas. The spatter cone within Pu`u `O`o Crater has continued to produce flows which episodically resurface the crater floor. Fountains from the west flank vent have intermittently continued sending flows to the west and north for distances of less than 1 km, providing visitors to the Pu`u Huluhulu viewing area with a fine show during clear weather. Other small channeled lava flows originating from a perched lava pond on the south side of Pu`u `O`o extend for less than 1 km to the south.

There were no felt earthquakes reported this week.