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The sunrise is seen from Brainard Lake Recreation Area near Ward on Tuesday.
The sunrise is seen from Brainard Lake Recreation Area near Ward on Tuesday.

Editor’s note: This story has been updated to correct the height to which sulfur dioxide was sent by the volcano’s eruption.

Photographers across Boulder County, and even those not ready with their cameras, have been struck by a deep purple they are seeing at sunrise and sunset in recent weeks, and it’s not just their vivid imaginations.

There’s a very real cause for the unusual band of color visible, conditions permitting at dawn and dusk, and it lies more than 5,000 miles away in the remote Kuril Islands off the eastern coastline of Russia. Scientists are linking the purple to sulfur dioxide gas particles blasted into the stratosphere, 11 miles above the Earth’s surface, by the June 22 eruption of a volcano called Raikoke, northeast of Japan and south of the Bering Sea.

“Volcanic eruptions have been creating purple sunsets for a long time,” said Brian Toon, a professor in the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder. “The last time I saw one was in ‘91 or ‘92, following Mount Pinatubo, which was the largest injection of sulfur dioxide into the stratosphere in the 20th Century.”

It was June 1991 that Mount Pinatubo erupted in the Philippines, pouring thousands of tons of sulfur dioxide gas into the atmosphere in the second-largest volcanic eruption of the 20th century. The largest of the 20th Century occurred in 1912 at Novarupta in Katmai National Park and Preserve on the the Alaska Peninsula. But because it was a high latitude eruption, Toon said it did not spread as much atmospheric debris as the subsequent Pinatubo event.

Toon said, “I started seeing this one in sunsets around Labor Day, and I’ve seen them from California and Boulder and from the airport. It’s a little easier to see them if you are further away from the mountains — and if it’s not cloudy. If you have a nice clear day, it’s something to watch for.”

Local scientists have been doing more than watching it. Researchers at CU Boulder have used a high-altitude balloon to detect particles in the stratosphere linked to the eruption. These aerosols scatter sunlight as it passes through the air. In combination with the absorption of light by the ozone layer, that creates the purple shade that has been enhancing sunrises and sunsets.

Lars Kalnajs, also in the CU Boulder Atmospheric and Oceanic Sciences department, with his team launched a balloon in Aug. 28 near Laramie, Wyo. to a height of about 90,000 feet, as part of their ongoing research in support of NASA’s observational satellite Stratospheric Aerosol and Gas Experiment (SAGE) III.

In doing so, they found obvious evidence of the Raikoke event.Their preliminary data showed some aerosol layers in the stratosphere 20 times thicker than normal, in the wake of that eruption.

“We knew we might see it, and we did see it,” said Doug Goetz, a post-doctoral researcher at CU Boulder’s Laboratory of Atmospheric and Space Physics, one of the handful of scientists participating in the launch.

“It makes you realize that you don’t have to put a whole lot of aerosols into the stratosphere to change its composition,” Kalnajs said in a statement. “This was a relatively small volcanic eruption, but it was enough to impact most of the northern hemisphere.”

Goetz has been making less scientific observations as well.

“I was up in the Snowy Range in Wyoming a couple of weekends ago, and I snapped a couple good pictures of it,” he said.

Toon said Raikoke was probably responsible for only about 10% as much sulfur dioxide in the atmosphere as Pinatubo.

“Pinatubo made measurable changes in the climate and ocean. This is a much smaller volcanic cloud, and it probably will  make a few changes that science can measure, but the average person is not going to feel any effects, except for the purple twilight,” Toon said,

“It’s a rare thing and quite pretty. I don’t know how long it’s going to last. It could last for a little while, or it could fade quickly. Go out and look for it while you can. It hasn’t happened for the last 30 years, so it’s something you won’t see every day.”

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