It is particularly noteworthy that Paul Crutzen played significant roles in the formation of both IGAC and IGBP and is serving currently as the vice-chair of the IGAC Scientific Steering Committee. Sherry Rowland is also active in IGAC, measuring hydrocarbons and other species using aircraft over the globe, and Mario Molina is doing fundamental work in the laboratory on heterogeneous atmospheric chemical reactions very relevant to IGAC.
While we think of ozone depletion as a contemporary issue, the story really began more than two billion years ago when blue-green algae evolved on Earth. Their photosynthesis led to an oxygen-rich atmosphere. Oxygen, pumped with the sun's ultraviolet radiation, produced the first ozone layer. Perhaps around this same time, there also evolved bacteria with the capability to convert nitrogen compounds in soils and water into molecular nitrogen and nitrous oxide. The nitrogen would come to dominate our atmosphere, and with the nitrous oxide emissions came an important process limiting the thickness of the ozone layer.
The contemporary ozone depletion issues effectively began in the 1930s with the invention of an extremely useful class of nearly inert chemicals called chlorofluorocarbons (CFCs), and in the 1970s with proposals for a global fleet of supersonic commercial aircraft which would fly in and exhaust gases into the lower stratosphere.
Crutzen, Molina, and Rowland played leading roles in elucidating the ways in which these natural and artificial emissions affect the ozone layer which protects the global biosphere from harmful ultraviolet radiation. Their initial proposals instigated a large international research program on the ozone layer and also proved to be a catalyst for a much wider-ranging study of the complex chemical and biological connections which exist on Earth.
The first connections began to be made when Paul Crutzen published two papers in 1970 and 1971 proposing that catalytic reactions involving nitric oxide and nitrogen dioxide (let me call them the "Crutzen" reactions) are a major ozone destruction mechanism. In the natural stratosphere the major source of these nitrogen oxides is the reaction of electronically excited oxygen atoms (themselves produced from ozone) with nitrous oxide. As pointed out concurrently by Harold Johnston, supersonic aircraft deposit these catalytic nitrogen oxides directly into the stratosphere. The second connection was made when in two papers in 1974 and 1975, Mario Molina and F. Sherwood Rowland proposed that the nearly inert chlorofluorocarbons and chlorocarbons (CCs) were dissociated by ultraviolet light in the stratosphere to produce chlorine atoms and chlorine monoxide. Only a short time before that, it had been recognized that these chlorine species catalytically destroy ozone through the so-called "Stolarski-Cicerone" reactions which I name here after their discoverers. The CFCs were widely used in the 1970s for refrigeration, air conditioning, aerosol can propellants, solvents, plastic foam puffing agents, and a myriad of other applications. The major CC was trichloroethane (methyl chloroform) which was widely used as a cleaning agent in the electronics and automobile industries. Measurement of the CFCs and CCs in air became possible in the early 1970s with the invention of the electron capute detector by James Lovelock.
These early proposals of ozone depletion led to a rapid expansion of research in stratospheric chemistry. For a variety of reasons, including potential ozone depletion by the Crutzen reactions, plans for large supersonic aircraft fleets were shelved in the mid-1970s. There was also enough early confidence in the Molina-Rowland theory that several countries in the mid-1970s phased out the use of CFCs in certain trivial uses, particularly aerosol cans. Nevertheless, even as evidence for the Crutzen, Molina, and Rowland theories mounted, the observational evidence for actual depletion of ozone was equivocal. Due to changing wind patterns, the thickness of the stratospheric ozone layer is highly variable in space and time and therefore small long-term changes in its thickness are very difficult to detect. A series of international assessments was begun in order to periodically examine the validity of these ozone-depletion hypotheses. It was a watch-and-wait phase. The situation changed dramatically with the publication of the discovery of the Antarctic Ozone Hole by Joseph Farman and colleagues in 1985. A remarkable thinning of the ozone layer was occurring every spring over Antarctica and the thinning was increasing with time. However, this very evident ozone depletion was not explained by the then-current ozone-depletion theories. These theories did not include the chemistry instigated by reactions involving the stratospheric ice clouds prevalent over Antarctica in winter due to the extremely cold temperatures occurring there. The scientific assessments accelerated and the first significant CFC regulatory policy negotiations began with the 1985 Vienna Convention leading to the 1987 Montreal Protocol. Simultaneously, several researchers, including James Anderson, were gathering evidence for unexpected chlorine, bromine, and nitrogen chemistry in the Antarctic spring atmosphere. Theoretical and laboratory studies involving Susan Solomon, Molina, Crutzen, and others was establishing the fact that reactions on ice particles can lead to release of chlorine monoxide. A new catalytic cycle was demonstrated by Molina and colleagues involving the dimer of chlorine monoxide which operates efficiently in the Ozone Hole. The pieces of the scientific puzzle were beginning to come together and the chemical industry was at the same time gearing up to identify and manufacture suitable CFC and CC alternatives. I am glad to say that global observations of CFCs and CCs carried out in the Advanced Global Atmospheric Gases Experiment (AGAGE) and NOAA Climate Monitoring and Diagnostics Laboratory networks now show that the Montreal Protocol is indeed working. My AGAGE colleagues and I were able to report earlier this year that the major CC trichloroethane is the first ozone-depleting gas to actually show a dramatic decrease in the atmosphere. Carbon tetrachloride and CFC-11 are now also slowly decreasing.
The ozone depletion story is not ending however with the Nobel Awards and successful implementation of the Montreal Protocol. Removal of long-lived CFCs from the atmosphere will still take many decades so we will be living with a perturbed ozone layer well into the next century. Also, AGAGE and other measurements show nitrous oxide levels are continuing to rise slowly and we have still not established why. That is one of many problems that IGAC, with help from other IGBP projects, can hopefully soon solve.
Ozone is also a chemically and radiatively important species in the troposphere and the work by Paul Crutzen on tropospheric ozone over the past twenty years has been an important contribution to our current knowledge in this area. Paul, I, and many others are, however, frustrated at the lack of observations of tropospheric ozone necessary to define the global distribution and trends for this critically important gas. Hopefully, the International Tropospheric Ozone Years (ITOY) proposed as a major initiative under the IGAC Global Tropospheric Ozone Network (GLONET) Activity, will receive special impetus with the Nobel Committee's recognition of ozone research.
Thus there is still much work remaining. Nevertheless, the remarkable contributions by three members of our community are a great pleasure to acknowledge. Congratulations Paul, Mario, and Sherry for a job well done!
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