Susan Solomon National Oceanic and Atmosphere Administration, USA

One Hundred Reasons to be a Scientist SCIENCE OFFERS AN IMPORTANT INPUT Susan Solomon National Oceanic and Atmosphere Administration, USA I began life in Chicago, and first got hooked on science watching the undersea adventures of Jacques Cousteau on TV. In high school, my confidence that science was the right choice for me was boosted when I was lucky enough to take third place in a nationwide science fair with a project that measured the amount of oxygen in gas mixtures. While in undergraduate school studying chemistry at the Illinois Institute of Technology in Chicago, I was fascinated to learn of work being done regarding the chemistry of the atmosphere of the planet Jupiter. That’s what started me on the path to doing chemistry on a planet instead of in a test tube. After graduating from IIT I went to © Courtesy of Carlye Calvin graduate school in chemistry at the University of California at Berkeley. My doctoral dissertation was about chemistry on a planet—not Jupiter but on Earth. I earned my doctorate in chemistry in 1981. Prior to the discovery of the ozone hole, my work focused on what you might call some esoteric aspects of understanding the atmosphere. I was looking at things like the impact of natural factors including the aurora on the chemistry of the mesosphere, thermosphere, and stratosphere. Then the ozone hole was discovered, and that changed everything. I was intrigued by the observation, and one of the first things I thought about, coming from this mesosphere/thermosphere kind of work, was whether reactive nitrogen from phenomena like solar protons could be responsible. I convinced myself, however, that that hypothesis didn’t fit the data. So then I thought about what could be producing this kind of effect, and I began to wonder whether surface reactions on polar stratospheric cloud surfaces could be the cause. There had already been satellite measurements of polar stratospheric clouds, but they were considered a curiosity rather than a topic of any importance. But the ozone changes of the Antarctic weren’t being seen in the Arctic, and Antarctica really is the coldest place on Earth, which leads to many more clouds there than in the warmer Arctic. No one had imagined that surface chemistry involving chlorine could seriously impact the stratosphere, but I starting thinking about ways in which ozone might be depleted through reactions involving chlorine that might happen on those clouds. I ended up proposing in my Nature paper21 that a reaction between hydrochloric acid and chlorine nitrate could be occurring on the surfaces of polar stratospheric clouds. That reaction doesn't happen in the gas phase, but it happens quite readily on surfaces due to the incorporation of the acid onto the surface. Then the chlorine 21 Solomon, S., R.R. Garcia, F.S. Rowland, and D.J. Wuebbles, On the depletion of Antarctic ozone, Nature, 321, 755-758, 1986. the abdus salam international centre for theoretical physics 225 One Hundred Reasons to be a Scientist nitrate comes along and you get a reaction occurring in a completely different phase that is quite amazingly fast. So that was my proposal and it turned out to be the right answer. I also got involved in the observational side of the issue because I strongly argued that we ought to go to the Antarctic and make some measurements to figure out what's going on. It’s one thing to see the ozone drop. It’s another thing to measure the chemicals that actually influence ozone and to be able to make a science-based statement about why it’s changing, whatever that would be. I argued that we ought to have a ground-based expedition to Antarctica, and I was fortunate enough to lead one in 1986 and again in 1987. We worked from McMurdo Station, Antarctica, where among other things we measured how the air above the frozen south absorbed the moonlight of the long polar night. I was incredibly fortunate in that two of my colleagues in the Aeronomy Laboratory had designed and built a very high- quality, extremely sensitive instrument for measuring the intensity of incoming light in the visible region of the spectrum. We could not only measure ozone with it—but also nitrogen dioxide and chlorine dioxide, two very important chemicals. And chlorine dioxide turned out to be there in huge amounts—without reactions on those clouds it couldn’t have been present. So we made some of the first measurements that showed what caused the ozone hole. Being in the Antarctic for three months was one of the most exciting, challenging, fantastic experiences of my life. When the door of the airplane opened and that incredibly cold (-40° C) air rushed in and hit my face, it felt like being on another planet. I have never seen such unspoiled, natural beauty. The intense purple and blue colors of Antarctic twilight are incredible. The polar stratospheric clouds—those very clouds that facilitate the depletion of the ozone—are wonderful to see. They resemble tiny suspended rainbows. I was really honored in 1994, when the Solomon Glacier (78°23'S, 162°30'E) and Solomon Saddle (78°23'S, 162°39'E) were named in honor of my leadership in Antarctic research. I also was stunned when I received the United States National Medal of Science, which is the highest honor an American scientist can receive, for insights in explaining the Antarctic ozone hole. Other topics I have enjoyed studying include how volcanoes, though not damaging to the ozone layer by themselves, can speed up CFC-induced ozone destruction. After explosive volcanic eruptions, liquid sulfate aerosols can form, and those surfaces can cause major ozone loss at mid-latitudes via processes analogous to that in the Antarctic. I have also done some work on the issue of gases other than carbon dioxide that could contribute to global warming. Among other things, one question my colleagues and I have probed is the role of perfluorinated chemicals like CF4, SF6, and others. There’s not a lot of that stuff in the atmosphere today, so I’m not saying they are significant contributors to today’s global warming. But we’ve shown that these molecules live for literally thousands of years—they may as well be immortal—and they are potent absorbers of infrared light, hence greenhouse gases. A molecule that can outlast the pyramids of Egypt might be one to think about venting to the atmosphere especially carefully. In fact, maybe one of the most interesting overlaps between ozone depletion and climate change is that the ozone-depletion issue shows the need for science to help understand not just impacts but also time scales in environmental problems. It’s all part of using science to help inform society about the questions that have to be asked— not just what is or may be happening now, but if something does happen tomorrow, how long will we have to live with it? 226 the abdus salam international centre for theoretical physics One Hundred Reasons to be a Scientist Recently, I’ve taken a change of direction doing work more related to understanding how various molecules in the lower atmosphere actually absorb incoming sunlight. It’s relevant to the climate problem, but it really addresses the fundamentals of how the system works. So I’m moving into an area that is related to the climate problem, but a little more esoteric. I’m coming back to something like my early work on auroras and things like that, helping to lay the basis for understanding how radiation propagates—how sunlight really gets in through the atmosphere. I think of myself as a very privileged person. I have the satisfaction of doing things that are relevant. Having a goal and the idea of public service are absolutely consistent with what I do as a scientist. This planet supports many people and will have many more who will be putting all kinds of chemicals and substances in the atmosphere, so the 21st century will have enormous opportunities for people in many areas of environmental chemistry, including climate change. Science has a very important role to play in serving society, helping to understand what is happening and why. But in my opinion, that is where my job as a scientist stops and those of others—the economists and the politicians—begins. Science is an important input to many societal choices, but it is only one input. And when we keep our science focussed on the historic standards of scientific values of objectivity, leaving politics and personal opinion to others, I think we contribute the most we can to the world. the abdus salam international centre for theoretical physics 227 .

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