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January-February 2018

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The Ozone Hole: A Story of Healing and Hope

This fall, as the days are growing shorter in the Northern Hemisphere and longer over the southern half of the globe, scientists who focus on the upper atmosphere are turning their attention to the sky over Antarctica, as they have been doing since the late 1970s. They hope to find that the Antarctic's “ozone hole” is continuing to heal.

The stratospheric ozone layer protects life on Earth by absorbing ultraviolet light (UV), especially the range of wavelengths known as UV B, which can damage deoxyribonucleic acid (DNA), the basic building block of all of earth's plant and animal life.

Before 1979, scientists had not observed any signs that upper atmospheric ozone was decreasing. Thus, they were stunned when the British Antarctic Survey announced that year that the amount of ozone over its Rotha Station had thinned dramatically.

If the reports from Antarctica were accurate and a sign of a global threat to the ozone layer, all life on earth could be affected: scientists had to find out what's going on. Researchers had been observing stratospheric ozone since early in the 20th century, and they knew that ozone blocks most of the most dangerous types of ultraviolet energy from reaching earth's surface where it could damage all living things.

Scientists had started paying more attention to the ozone layer in the 1970s when the United States and other nations were talking about building fleets of supersonic airplanes that would enable thousands of people to zoom around the world faster than the speed of sound, high in the stratosphere (where ozone does its protective work). Chemists who specialize in the upper atmosphere worried that emissions from these jets would damage the ozone level.

Fortunately, that problem never materialized. Neither the 14 British-French Concorde supersonic jets that flew regularly at altitudes up to 60,000 feet between London or Paris and New York City, nor the few flights of the Soviet Union's Tupolev Tu-144 between 1978 and 2003, created enough emissions to harm stratospheric ozone, before airlines retired the planes.

Nevertheless, the possibility that humans could harm it led to more research into the ozone layer and the establishment of more observations from the ground, by balloons that carried measuring devices into the stratosphere, and from satellites.

Scientists who focus on the upper atmosphere wondered about the report from Antarctica because satellite measurements showed no ozone loss. However, when NASA researchers looked closely at the satellite data, they discovered that a computer program designed to toss out flawed data had been deleting the Antarctic readings. When NASA corrected this flaw, measurements from Antarctica showed that earth's natural sunscreen was thinning dramatically over Antarctica each Southern Hemisphere spring and then recovering in the summer. Some started calling the large, thin spot in the ozone layer “the ozone hole.”

These observations left scientists with three questions:

  • 1.

    What was destroying great amounts of stratospheric ozone over Antarctica?

  • 2.

    Was this happening only over Antarctica?

  • 3.

    Why did ozone disappear in the spring as the sun was rising and then recover during the summer?

Scientists came up with three general kinds of hypotheses about what was happening:

  • a.

    dynamic, involving air movements

  • b.

    solar, involving changes in solar output

  • c.

    chemical, involving then-unknown reactions, possibly involving substances called chlorofluorocarbons, or CFCs

As examples, one kind of dynamic theory proposed that as the sun begins rising over Antarctica, it warms lower-level air that contains little ozone, causing this air to rise and dilute stratospheric ozone. One solar activity theory suggested that the sun's increased activity, such as in the early 1980s, had increased stratospheric nitrogen oxides and other ozone destroying substances. The chemical hypothesis was that previously unknown atmospheric chemical reactions involving chlorine from CFCs were to blame.

Each Type of UV Is a Different Wavelength Band

  • UV A: Longer wavelengths that cause sunburn and premature skin aging

  • UV B: Wavelengths that damage plant and animal DNA if they reach the surface; a healthy ozone layer blocks most of this radiation

  • UV C: The strongest UV radiation. Fortunately for life on earth, upper atmospheric oxygen destroys all UV C radiation to start the series of chemical reactions that create stratospheric ozone, as shown in the “ozone and the stratosphere” illustration.

Of the solar UV energy reaching the equator, 95% is UVA and 5% is UVB.

This chemical hypothesis made sense because in 1974, Sherwood Rowland and Mario Molina of the University of California at Irvine showed that ultraviolet energy in the stratosphere could break apart CFCs in the stratosphere, thus freeing them to attack the ozone layer. In 1995, Rowland and Molina, along with Paul Jozef Crutzen of The Netherlands, who had done other pioneering research on threats to ozone by other atmospheric gases, won the Nobel Prize in Chemistry.

An apparent weak point of the chemical hypotheses was that ozone destruction requires tiny amounts of ozone and chlorine in the stratosphere's extremely thin air to come together in a way that allows the chlorine to break apart ozone molecules. This seemed highly unlikely: the sky is huge and the molecules are tiny.

Scientists Flock to Antarctica Looking for Answers

The National Science Foundation, NOAA, NASA, the University of Wyoming, and the State University of New York at Stony Brook quickly organized the National Ozone Expedition, which sent teams of scientists to Antarctica in 1986 and again in 1987. Their first job was to confirm the reports of ozone depletion, and if the reports were true, try to figure out what was going on. They quickly confirmed the ozone hole was real.

The team, led by Susan Solomon, a NOAA atmospheric scientist, and David Hoffman, then at the University of Wyoming, traveled to Antarctica and discovered that a previously unknown kind of high-altitude chemical reaction was destroying ozone each Antarctic spring and early summer. (Other scientists did not succeed in confirming the solar nor the dynamic hypothesis.)

Susan Solomon, a chemist who led the team that confirmed the creation of the ozone hole in 1986 and 1987.

The 1986 trip to Antarctica and another the following year found that the amount of chlorine dioxide in the stratosphere above Antarctica was 100 times greater than predicted. This was the solid evidence that pointed to chlorine chemistry as the cause of the Antarctic ozone hole.

So, how do the tiny molecules of ozone and chlorine get together? These molecules condense onto the ice that creates polar stratospheric clouds that form over Antarctica each winter. When the upper atmosphere over Antarctica warms in September with the return of sunlight, these clouds evaporate, releasing the chlorine to begin destroying ozone.

In 1987 most of the world's nations, including the United States, signed the “Montreal Protocol on Substances that Deplete the Ozone Layer,” which is an international treaty designed to protect the ozone layer by phasing out the production of substances responsible for ozone depletion. It's been revised several times since then to keep up with the development of new substances that could harm ozone and to take into account increased understanding of the upper atmosphere.

CFCs Seemed Harmless for a Long Time

CFCs go back to 1928 when Thomas Midgley (1889–1944) and his colleagues at General Motors, which owned the Frigidaire Corp., invented chemicals called chlorofluorocarbons as a working fluid for refrigerators. These molecules were a real lifesaver; unlike working fluids then being used, they were harmless even though they were made of chlorine, fluorine, and carbon. While fluorine and chlorine are dangerous, they are firmly locked up in CFCs, as far as Midgely, his colleagues, and other scientists could tell at the time. (They had no particular reason to think about what would happen to them in the stratosphere.)

However, researchers discovered in 1974 that ultraviolet energy could break apart the millions of CFC molecules in the atmosphere that had been leaking from discarded refrigerators and air conditioners and other uses. By this time, the many uses of CFCs included as solvents to clean electronic parts and as spray-can propellants. In fact, a major source of CFCs in the atmosphere was solvents that escaped while being used to clean some parts of the increasing numbers of electronic devices, such as television sets, being produced.

Do Spray Cans Harm the Ozone Layer?

When the danger to the ozone layer first came to public attention in the 1970s CFCs were commonly used as propellants in spray cans. That was then, not now. In the 1970s, manufacturers in the United States and many other nations stopped using CFCs in aerosol cans, and the United States banned their use. Spray cans sold in the U.S. are ozone safe, but you can't be sure of those from other nations.

All Indications Are That It's Working

In November 2015, scientists at the Massachusetts Institute of Technology (MIT) and other institutions published a study in the journal Science saying that they had identified the “first fingerprints of healing” of the Antarctic ozone layer. They found that the 2015 ozone hole had “shrunk by more than 4 million square kilometers—about half the area of the contiguous United States—since 2000, when ozone depletion was at its peak.”

The team found that gaseous volcanic eruptions had from time to time slowed, but not stopped, the recovery. An MIT press release quoted Susan Solomon—one of the leaders of the 1986–1987 ozone hole expeditions to Antarctica, who is now a professor at MIT—as saying “the ozone hole appears to be on a healing path.”

Kilauea Crater, Hawaii Volcanoes National Park.

“We can now be confident that the things we've done have put the planet on a path to heal,” Solomon said, “Which is pretty good for us, isn't it? Aren't we amazing humans, that we did something that created a situation that we decided collectively, as a world, ‘Let's get rid of these molecules’? We got rid of them, and now we're seeing the planet respond.”

 

 JACK WILLIAMS was the founding weather editor of USA TODAY and is now a freelance writer. He is the author or co-author of seven books, five of which are about weather.

 

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