Mario Molina and Susan Solomon

Mario Molina

Mario Molina. Courtesy Mario Molina.

In 1973 Mario Molina (b. 1943) was a postdoctoral researcher working in the laboratory of F. Sherwood Rowland at the University of California at Irvine, just south of Los Angeles, when he made an unsettling discovery. He had been investigating a class of compounds called chlorofluorocarbons, or CFCs. CFCs were used as refrigerants, aerosol sprays, and in making plastic foams. Molina wondered what happened to them once they were released into the atmosphere.

This was a hypothetical study based on computer modeling, but his results suggested that CFCs could, in theory, destroy an oxygen compound called ozone under the conditions that exist in the upper atmosphere. Far above the earth’s surface, a thin layer of ozone floats, protecting us from the sun’s ultraviolet radiation. Molina, just a young scientist at the time, was nervous about showing Rowland his theory of how CFCs might destroy ozone. But if CFCs really could reduce ozone at a predicted rate of 7 percent after 60 years, the world would be in trouble.

According to Molina’s theory, the same photons that break oxygen molecules apart to produce oxygen atoms can also break CFCs apart to release chlorine atoms, among other products. Since the chlorine molecule has an unpaired electron, it is a radical. Radicals are very reactive, so chlorine atoms react easily with molecules of ozone. When a chlorine atom encounters an ozone molecule, it takes one of the oxygen atoms away, leaving O2 and chlorine oxide (ClO). ClO is also a reactive radical and reacts quickly with another ozone molecule, converting it to two O2 molecules, and freeing the Cl to do still more damage. Since the radical Cl atom starts this whole reaction, but is not consumed in the process, it is a catalyst for the ozone destruction reaction.

Rowland took his protégé seriously. Over the next two decades he and Molina became voices alerting the world to the danger of CFCs and ozone depletion. They were not always heeded. Bans on CFCs in aerosol sprays went into effect first in the United States in 1978, and later in Canada, Norway, and Sweden. CFC use for other purposes only increased. Scientists, activists, politicians, and CFC-producing companies would argue for years over the merit of Molina’s theories.

Molina was born in Mexico City, where his father was a successful lawyer and a diplomat. As a child, he was fascinated with chemistry and converted one of the family bathrooms to a chemistry laboratory for himself. His aunt, Esther Molina, was a chemist, and she encouraged and mentored the boy by helping him carry out more advanced experiments than normally possible with a child’s chemistry set. Recognizing his passion for science, Molina’s parents sent him to a boarding school in Europe, where they thought his fascination with science would be nurtured.

Molina returned to Mexico City for college and earned an undergraduate degree in chemical engineering from the Universidad Nacional Autónoma de México in Mexico City. He then studied mathematics and science at universities in Germany and France to fill deficiencies in his academic background. Finally, he was accepted for graduate study at the University of California at Berkeley. After earning his Ph.D. he made his way to UC Irvine, where he joined Rowland’s group as a postdoctoral researcher. It was then that Molina and Rowland began wondering what might be happening to CFCs released into the atmosphere.

Later, Molina moved on from UC Irvine to work at NASA’s Jet Propulsion Laboratory near Pasadena, where he continued to investigate ozone depletion. Over the years evidence mounted in support of Molina’s theories, leading to increased international regulation of CFCs. But this did not happen easily, nor did it happen overnight. Before most of the world would listen, it took an alarming observation in 1983 by British scientist Joseph Farman and colleagues that the ozone levels above Antarctica had been dropping dramatically, by as much as 35 percent, during the Antarctic spring (September through December) compared with 1960s levels.

Susan Solomon

Susan Solomon in her office in Boulder, Colorado, next to a globe showing Antarctica. Courtesy the National Oceanic and Atmospheric Administration.

The race was on to discover what was causing the so-called “ozone hole.” Why was the ozone depletion happening so much faster than anyone had expected? And why over Antarctica? Why did the hole form so far from where CFCs were being used? To answer these questions, an expedition to Antarctica was organized in 1986, led by Susan Solomon (b. 1956) of the National Oceanic and Atmospheric Administration (NOAA). 

Solomon, a Chicago native, first got hooked on science as a child while watching television shows like The Undersea World of Jacques Cousteau. In high school she took third place in a national science fair with a project that measured the amount of oxygen in gas mixtures, foreshadowing her future work in atmospheric chemistry. After high school she studied chemistry at the Illinois Institute of Technology (IIT) in Chicago.

After getting her B.S. degree from IIT in 1977, she went to graduate school at the University of California at Berkeley, where she specialized in atmospheric chemistry. She earned her Ph.D. in 1981 and then joined NOAA. With regard to the ozone hole, she theorized that polar stratospheric clouds present at each of earth’s poles provided solid surfaces in the form of ice crystals on which reactions could take place. Like many other reactions, these were vastly speeded up by occurring on surfaces where one or more of the atoms involved are held immobilized for a time. Ozone breakdown could also occur at other latitudes, but at much slower rates.

Susan Solomon with her penguin pals. Courtesy the National Oceanic and Atmospheric Administration.

Susan Solomon with her penguin pals. Courtesy the National Oceanic and Atmospheric Administration.

Because the ozone hole opened up in the early Antarctic spring, Solomon and her team had to travel to McMurdo Base, Antarctica, in the late winter (August 1986) to study the hole as it formed, enduring brutally cold temperatures and nearly 24-hour-a-day darkness. The smoking gun that CFCs were the guilty party for ozone depletion would be the presence of ClO in the stratosphere at the same time and place as ozone loss was detected, which would make it hard to deny that CFCs were to blame for ozone loss. During this and a second expedition in 1987, Solomon and her team were able to gather enough data to show high levels of ClO being released by CFCs.  

Molina and Rowland were vindicated. In 1987 the Montreal Protocol for reducing named substances that deplete the ozone layer was opened for signature. By 2009 all nations in the United Nations had ratified the original protocol. In 1996 Molina and Rowland were awarded the Nobel Prize for chemistry, along with Swedish scientist Paul Crutzen, for the work they had done in helping unravel the mysteries and dangers of CFCs.

First at the Massachusetts Institute of Technology and then at the University of California, San Diego, and the Center for Atmospheric Sciences at the Scripps Institute of Oceanography, Molina continued his research on gas-phase chemistry, including the effects of pollutants in the atmosphere. Not forgetting his roots, he has also been studying strategies for making the air in cities cleaner. Mexico City has been the case study for this project.

Solomon has also investigated ozone depletion in the atmosphere elsewhere in the world. She has led missions to the Arctic to study a smaller ozone hole that has developed over the North Pole, and has helped show how volcanoes can speed up CFC-induced ozone destruction.

Today Solomon continues her work at NOAA and at the University of Colorado at Boulder. She has earned many honors for her work. In addition to having a glacier in Antarctica named in her honor in 1994, she was awarded the 1999 National Medal of Science, the highest scientific award bestowed by the U.S. government. In 2004 she received the international Blue Planet Prize, and in 2007 she shared in the Nobel Peace Prize awarded to the Intergovernmental Panel on Climate Change and former Vice President Al Gore, Jr. She is also the author of The Coldest March: Scott’s Fatal Antarctic Expedition (2002).

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