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Because of this, “measuring ozone alone is not sufficient,” said Melo. Instead, itis necessary to measure a variety of different chemicals and to understand thedynamic processes that lead to ozone formation and destruction.CFCs affect the balance by destroying stratospheric ozone. They not only reducethe ozone shield around the globe, but also create large ozone holes over theAntarctic each year and significant losses over the Arctic. Many of thesechemicals were banned nearly two decades ago and there is evidence that theozone layer is slowly starting to recover.To produce its maps of the vertical profile of ozone in the atmosphere, OSIRISdepends on two phenomenon: the scattering of sunlight in the atmosphere andthe airglow, faint emissions caused by chemical processes in the upperatmosphere. When sunlight falls on the atmosphere, different atoms andmolecules both scatter and absorb it at specific wavelengths and OSIRIS canidentify these characteristic signatures.“Both the absorption of sunlight and the airglow are fingerprints of the atoms andmolecules in the atmosphere,” said Llewellyn. “If you can measure theirbrightness very accurately, you can measure their abundance very accurately.”This is a challenge, however, because the instrument records all the signatures itfinds in the atmosphere. “You’re not able to pick out just one little piece,” saidLlewellyn. Heavy-duty computing and modelling are needed to tease out thespecific fingerprints that are of greatest interest.Different techniques are used to detect the fingerprints of different atmosphericcomponents at different altitudes. In the stratosphere, the region of the upperatmosphere where ozone is most abundant, both it and nitrogen dioxide aredetected by measuring the absorption of sunlight shining through theatmosphere. Sulphate particles in the stratosphere are detected using a newtechnique that measures how the particles scatter sunlight.Some ozone also exists in the mesosphere, the region above the stratosphere;although the volume is not as great, it still plays an important role. This ozone ismeasured using airglow emissions, as is mesospheric water vapour.OSIRIS’s unique process of atmospheric tomography enables scientists to lookfor structures in the airglow. Like the CAT scan that Llewellyn compares it to, theprocess requires a series of stacked two-dimensional images which are thenanalyzed by computer to produce a three-dimensional representation of theinternal structure of the object being studied—in this case, components of theatmosphere such as mesospheric ozone and water vapour.60
In addition, the scientists use what Llewellyn describes as a tomography-liketechnique to process images obtained by detecting the absorption of sunlightpassing through the atmosphere. These types of images are used to measurestratospheric ozone and nitrogen dioxide.The amount of data processing required is staggering. At first, Llewellyn, whocame up with the idea along with a colleague, Ian McDade, thought there wassimply no practical way to do it. “We realized that no matter how fast computerswere, it was going to be a very slow process; we would never keep up with therate at which the data came in.”However, never one to say never, Llewellyn set Degenstein, then a graduatestudent, to the task. With the approval of the Canadian Space Agency, whichwas funding the research, Degenstein spent two years working with the Ottawacompany, Routes AstroEngineering, that was building the instrument. During thattime, he developed a system that can process 100 minutes of data in fiveminutes—20 times faster than “real time.”“He did make it sit up and beg,” said Llewellyn proudly. “We were overjoyed, butwe still hadn’t flown at that stage.” Several people told him that it was “awonderful idea but it can never work.” Even when they got their first flight resultsand presented them at an international conference, the skepticism persisted.Llewellyn said: “People looked at it and said, ‘Yeah, fine; we don’t believe you.’ ”It took a few years for them to convince the skeptics but “now it’s the acceptedway of looking for structures in the atmosphere. Everybody knows what theyhave to do next.”In the meantime, the science team has been expanding the range of atmosphericcomponents that OSIRIS is looking at. Melo noted that the instrument unique inits ability to measure water vapour at altitudes of about 80 to 90 kilometres. Thisis in the upper mesosphere, one of the highest regions of the atmosphere.The water content of the atmosphere is a key component in cloud formation andOSIRIS is currently studying an intriguing phenomenon called polar mesosphericclouds, the highest clouds in the atmosphere. Some scientists believe they’reoccurring more frequently and could be “a true canary in the coal mine for globalchange,” Llewellyn said.However, Melo noted that “the processes that lead to the formation of theseclouds are not fully understood and that’s where OSIRIS is contributing.” Shesaid that measuring polar mesospheric clouds started out as a secondary61
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