Mount Pinatubo is painting sunsets of a different color

September 23, 1991|By Douglas Birch

Move over Monet. Here comes Mount Pinatubo.

Using a highly corrosive acid as pigment, a dreamy impressionist style and a canvas as sweeping as the horizon, the Philippine volcano has begun painting North American sunrises and sunsets in shell pinks, brilliant reds and luminous blues.

And the one-volcano show is expected to remain open in these latitudes for at least the next several years.

Chris Seymour, a park ranger and nature interpreter at Assateague Island National Seashore, said he has basked in some striking late-afternoon colors in recent weeks while standing near the shore of the Sinepuxent Bay.

"We get some very pretty sunsets here anyway. But there's definitely beena change," he said. "It's sort of a rosy pink color. There's less orange and more of a reddish pink glow to them."

Peter Martin, a naturalist with the Irvine Natural Science Center in Baltimore County, said he noticed the vivid colors at dusk and didn't realize they were related to the Philippine volcano. "I did notice them the last couple of nights, where the sun itself was a rosy, pinkish-type color," he said. "It was quite interesting to look at."

"The colors are spread out more," said Todd Ullery, education specialist for the Davis Planetarium at the Maryland Science Center, who has been explaining the colorful sunsets and their relationship to Mount Pinatubo to curious callers. "It is definitely redder. But as the sun gets lower and darker in the sky you'll also see yellows and greens and a deeper blue."

Arlin Krueger, an atmospheric scientist with NASA's Goddard Space Flight Center in Greenbelt, said Pinatubo's June eruption could have some less benign effects. It could accelerate the destruction of atmospheric ozone, and lower temperatures around the globe slightly over the next several years.

The eruption, which produced an ash cloud that forced the decision to close Clark Air Base, pumped millions of tons of gases includng hydrogen sulfide and sulfur dioxide into the stratosphere, to a height of between 12 and 18 miles.

Within about a month, chemical reactions had changed most of the material into an airborne aerosol of tiny droplets of highly corrosive sulfuric acid.

Until the end of August, Pinatubo's acid droplets circled the Earth in a broad belt on either side of the equator that reached only as far north as southern Texas and Florida.

But in recent weeks "the northern edge of the aerosol cloud has been starting to break up into pieces, which move into the Northern Hemisphere," Dr. Krueger said. "Several of those pieces have come by, producing colorful sunsets or sunrises."

So far, Pinatubo's sunsets have come intermittently and unpredictably. But all that should change sometime after today, the autumnal equinox, when the atmospheric winds begin pumping air toward the arctic.

"Probably by late fall, the sunsets will be visible just about every night throughout the Northern Hemisphere," weather permitting, Dr. Krueger said.

Pinatubo's colorful palette is created by sunlight filtering through the sulfuric acid cloud, he said.

The thicker the layer of mist, the more light bends and the more it is separated into its distinct bands, the way light is bent and separated into a rainbow when it passes through a glass of water. From the viewer's perspective, at daybreak and sunset the sun's rays must pass through the thickest layer of the cloud -- so the bending and scattering is most pronounced.

"Think of Los Angeles on a smoggy day," said Mr. Ullery of the science center. "The sunsets are wonderful, and that's because all the material

that makes up the smog . . . changes the sky color."

The cloud's staggering height in the atmosphere means it reflects the sun's rays long after the sun has sunk below the horizon. That gives the sky an eerie glow until late in the evening.

The June eruption, which created the biggest volcanic cloud since Mount Katmai in Alaska in 1912, blasted enough sulfuric acid in the stratosphere to etch sunsets over most of the United States for the next two or three years.

Other volcanoes have been spewing ash and gases around the Pacific in recent months, including Cerro Hudson in Chile, Mount Unzen in Japan and Mount Malindang, also in the Philippines. Only one has created a high-altitude cloud similar to Pinatubo's: Cerro Hudson, which erupted Aug. 12. That cloud, which is expected to circle the Earth in the Antarctic region for the next several months, is only about one-tenth the size of Pinatubo's.

Some scientists think that large volcanic clouds like Pinatubo's may influence global climate -- accelerating the destruction of atmospheric ozone, for example, or moderating global warming, where rising levels of gases produced by automobiles, industry and agriculture are suspected of trapping heat from the sun and causing temperatures to rise.

The cloud, Dr. Krueger said, is expected to act like an awning, reflecting some sunlight and cooling the Earth's surface over the next year, reducing the average temperature, perhaps by up to 2 degrees Fahrenheit.

At the same time, Pinatubo's droplet cloud may help destroy ozone, which protects Earth from the sun's harmful ultraviolet rays, by providing millions of tiny platforms where certain nitrogen molecules can react.

Those reactions are part of the cycle by which man-made chemicals called chlorofluorocarbons release chlorine atoms, each of which can dissolve tens of thousands of ozone molecules.

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