A Large Volcanic Eruption Could Damage Ozone

by Philip Shabecoff

A major volcanic eruption could significantly accelerate the destruction of the earth's protective ozone layer by industrial chemicals, two atmospheric scientists have reported.

Such large eruptions are rare, usually occurring not more than once or twice a century, but scientists who reviewed the new findings said they underscored the urgency of efforts to protect the ozone layer.

In a report last month in the Journal of Geophysical Research, David J. Hoffman and Susan Solomon said that the same sort of chemical reactions that have taken place on ice clouds over Antarctica can occur anywhere in the world on sulfuric acid droplets from a volcanic eruption. Over Antarctica, the reactions depleted the ozone layer by up to 50 percent in the spring of 1987.

Dangers from Depletion

The depletion of atmospheric ozone, now widely attributed to emissions of chlorofluorocarbons and other industrial gases into the upper atmosphere, permits substantially higher levels of ultraviolet radiation from the sun to penetrate to the surface of the earth. Such radiation can increase skin cancer and cause eye cataracts and damage to the immune systems in humans, and it can harm plant, animal and marine life.

Dr. Hoffman, a physicist at the University of Wyoming, and Dr. Solomon, an atmospheric chemist with the Aeronomy Laboratory of the National Oceanic and Atmospheric Administration, note that several scientific studies have suggested that the extinction of dinosaurs may have been linked to volcanic activity.

Evidence in recent years indicates that some volcanoes spew out chlorine compounds in addition to the large volumes of sulfuric acid produced in all eruptions. The chlorine compounds react with the sulfuric acid, thinning the ozone. Scientists theorize that the resulting increase in radiation killed the dinosaurs.

Although volcanic activity was much greater in that period than it is now, Dr. Solomon said in a telephone interview, the level of chlorine being sent into the atmosphere by human activity is now far higher than the amount of chlorine that could come from natural sources.

A large volcanic eruption, such as one that destroyed the island of Krakatoa in 1883, could establish conditions for catastrophic additional depletion of stratospheric ozone, the paper suggests.

Such a large eruption, the paper says, "would provide an important test of this dinosaur extinction theory and perhaps determine whether contemporary biological systems may also go the way of the dinosaurs."

Dr. Solomon emphasized that the eruption would have to be powerful enough to propel the sulfuric acid into the stratosphere. Smaller eruptions, she said, tend to send gases no higher than the lower atmosphere.

A scientist who reviewed the paper, Dr. Ralph Cicerone, director of the atmospheric chemistry division of the National Center for Atmospheric Research, said the possibility that destruction of the ozone layer could be accelerated by volcanic activity is "a potential bombshell.

"What is startling about the paper is that while there have been volcanoes throughout history, now we have put chlorine in the air with chlorofluorocarbons and that means volcanic activity is at least partially capable of destroying the ozone layer," Dr. Cicerone said.

Dr. Hoffman and Dr. Solomon said that after the eruption of El Chichon in Mexico in 1982 there was a sharp reduction in stratospheric ozone in the mid-latitudes of both hemispheres, for which there appeared to be no explanation.

Since then, studies have found that ice formations on clouds in Antarctica provided a surface for chemical reactions in which freed chlorine radicals from chlorofluorocarbons react with and destroy ozone, a gas comprised of three oxygen molecules.

The two scientists developed computer models that suggested that sulfuric acid droplets from El Chichon could also provide a surface for chemical reactions with the industrial gases, although the reaction is somewhat slower and less intense than it is on the ice crystals.

The findings suggest that chemical reactions that can destroy the ozone layer can happen anywhere in the world if there is a large volcanic eruption, Dr. Solomon said. She said there are other sources of sulfuric acid in the upper atmosphere that provide a surface for the reaction of industrial chlorine, which is spread around the world, to destroy ozone.

Margaret A. Tolbert, a chemist at SRI International, a research organization in California, said the Hoffman-Solomon study shows that the kind of chemical reactions studied intensively over Antarctica can be found anywhere in the world.

Under an international agreement in 1987, production of chlorofluorocarbons is being limited to 1986 levels by most producing countries. The countries have also committed themselves to reduce production by 50 percent by the end of the century. But since the agreement was reached, evidence has shown that ozone depletion is more rapid than previously realized, and many of the countries have called for a complete elimination of the chemicals.