What is "vog"? How is it related to sulfur dioxide (SO2) emissions?

Vog (volcanic smog) is a visible haze comprised of gas and an aerosol of tiny particles and acidic droplets created when sulfur dioxide (SO2) and other gases emitted from a volcano chemically interact with sunlight and atmospheric oxygen, moisture, and dust. Volcanic gas emissions can pose environmental and health risks to nearby communities.  

Vog is a hazard that's associated with Hawaiian volcanoes in particular. See the Hawaii Interagency Vog Information Dashboard for detailed information and current conditions.

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Image: Vog from Kilauea
January 31, 2008

Vog from Kilauea

The rim of Kīlauea Volcano’s summit caldera, normally clear on trade-wind days (left), became nearly obscured by vog (right) on some non-trade wind days beginning in 2008, when sulfur dioxide emissions from the volcano’s summit increased to unusually high levels. (This photo has been edited.)

video thumbnail: Gas-Pistoning at Drainhole Vent in Pu'u 'O'o Crater (June 28, 2006)
June 27, 2006

Gas-Pistoning at Drainhole Vent in Pu'u 'O'o Crater (June 28, 2006)

Gas-pistoning is an interesting phenomenon seen at Kilauea and other volcanoes. It is caused by the accumulation of gas within, or the rise of a gas slug through, a column of lava. In either case, the gas pushes up the overlying lava (the "piston"). Eventually, the gas breaches the surface and escapes, sometimes as a forceful jet of fume and spatter. The lava then drains

Image shows two scientists on the slopes of Mount St. Helens with steam rising around them
September 24, 1981

Gas Sampling around the Mount St. Helens Dome

USGS geologists gathered samples by hand from vents on the dome and crater floor. Additionally, sulfur dioxide gas was measured from a specially equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano.

Image: Halema'uma'u Vent Gas Plume

Halema'uma'u Vent Gas Plume

Over the past several days, the lava surface within the vent in Halema'uma'u has occasionally, and temporarily, reached to within about 115 m (375 ft) below the floor of Halema'uma'u Crater, as seen in this photo. During these high-lava stands, the gas plume is generally fairly wispy, providing the rare naked-eye view of the lava surface. The far (north) side of the vent