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UNSW : UNSW Atmosphere Study Guide
Atmosphere Study Guide 33 Explain (article four) Twenty years after the signing of a landmark treaty to phase out CFCs, Susan Solomon reflects on her role in healing the hole in the ozone layer. IT WAS THE PERFECT NIGHT. In September of 1986, I stood on a roof at the McMurdo Antarctic research station, in temperatures of -40°C. I took turns with members of my team to constantly adjust mirrors, which had been set up to direct light from the Moon into our lab below. The fates seemed to align in our favour on September 17, 18, and 19, as the recently detected hole in the ozone layer sat right above us, while the Moon loomed its largest and dipped close to the horizon. Our goal was to tease out the vital information about the atmosphere that was imprinted in the moonlight. As incoming light travels through the atmosphere, its various colours or wavelengths are absorbed by many different molecules. This is what allows ozone to absorb ultraviolet light from the Sun, thereby protecting life below from its dangerous DNA-damaging effects; that absorption is an ozone ‘fingerprint’. Careful measurement of the wavelengths of light that hit the Earth’s surface can provide fingerprints of many other chemicals in the atmosphere, and provide a basis to test ideas about chemistry and even some aspects of atmospheric winds. We chose moonlight because the concentrations of the chemicals we were looking for were expected to be higher at night – and the closer the Moon is to the horizon, the longer the path its light takes through the atmosphere and the clearer the absorption fingerprints. So these very special, fully moonlit nights presented our best chance to obtain data that would help us piece together the puzzle of what was causing the then- mysterious ozone hole over the Antarctic. We took the measurements with a sense of excitement and impending discovery. Nine months earlier, I had been the primary author of a paper proposing that the ozone hole might be caused by an unanticipated chemical reaction involving chlorine on the surfaces of the polar stratospheric clouds that form in the uniquely frigid Antarctic stratosphere. These ethereally beautiful clouds form higher in the atmosphere than clouds can usually form, beginning when temperatures drop below about -70°C. Antarctica really is the coldest place on Earth, and these clouds are observed more often there than anywhere else, even the Arctic. Despite their height, they can sometimes be viewed by the naked eye, and we began to see them within a few days of our arrival in Antarctica. Our best photographs did not do justice to the true colours of these iridescent marvels, which gleam in the twilight sky at altitudes from about 10 to 25 km, where they can continue to reflect light long after the Sun has dipped below the horizon. The source of nearly all atmospheric chlorine is the chlorofluorocarbons (CFCs) that were ubiquitous to modern life until about the past decade: they were found in refrigerators, air conditioning, solvents and many other products. My speculation was that polar clouds were converting chlorine to forms that could rapidly devour ozone. If proven true, this theory would lay the blame for the disappearance of the Antarctic ozone on humankind. Several other different theories had also been proposed, and only data – such as those we were collecting – could resolve the matter. If chlorine were the cause of the ozone hole, we expected to measure much larger signatures of chlorine dioxide in those moonbeam fingerprints than seen elsewhere. However, if for example nitrogen chemistry were the culprit, we expected to find enhanced nitrogen dioxide signatures. Miraculously, our data bore fruit within hours of being collected. The chemical signature of the light revealed elevated levels of chlorine dioxide in Antarctica unlike levels anywhere else on Earth. While our measurements were the first, they were by no means the only piece of evidence that was needed to establish the cause of the ozone hole beyond reasonable doubt. Independent observations of many other chemicals quickly followed. By the following year, governments across the planet had agreed to the Montréal Protocol, which set out to limit and eventually phase out the production and use of CFCs. Unfortunately, CFCs live in the atmosphere for many decades, so those already emitted are decaying slowly and will continue to maintain annual ozone holes (which grow in the Antarctic spring to cover an area bigger than the U.S. and shrink in the summer) for decades to come. Nevertheless if we meet our global targets for phasing out the chemicals responsible, we hope to begin to see the ozone hole start to close sometime in the 2020s. If the ozone hole had been due to something other than chlorine, the scientific community would have been pleased to bring that news to the world. However, the evidence for chlorine’s role quickly became overwhelming. By its nature and culture, science seeks the truth. This guarantees the right answers for society in the end, and that’s the beauty and value of research. The challenge is to communicate these aspects of the nature of science, along with what we know and don’t know. In the tale of the ozone hole we have been fortunate in being able to tell a story that has resonated around the world. – Susan Solomon Hole in the heavens These moonlit nights were our best chance to piece together the puzzle of what was causing the ozone hole. NASA A computer-generated NASA image showing the stratospheric ozone hole (in blue) over the Antarctic on 6 October 1986, just after Solomon’s team collected their data.