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Professor Paul Crutzen Professor Mario Molina Professor F. Sherwood Rowland
By Sean Henahan, Access Excellence

STOCKHOLM, Sweden- Three noted chemistry researchers have been awarded the Nobel Prize in Chemistry for atmospheric studies which led to an understanding of how the ozone layer forms and decomposes. The Royal Swedish Academy of Sciences praised the researchers' contribution "to our salvation from a global environmental problem that could have catastrophic consequences."

The one million dollar prize will be shared by F. Sherwood Rowland, Bren Professor of Chemistry, UC Irvine; Mario Molina, who currently is a member of the Department of Earth, Atmospheric and Planetary Sciences at Massachusetts Institute of Technology; and Paul Crutzen, a professor at Max-Planck Institute for Chemistry in Germany and adjunct professor at Scripps Institution of Oceanography at UC San Diego.

The award culminates 20 years of study on the ozone layer by the researchers. The three winners of the Nobel Prize in Chemistry are the first ever to receive the award for atmospheric chemistry or environmental science.

Although the layer of ozone in the (8 to 12 miles)--is relatively thin, it is crucial to life on earth because it absorbs the majority of the sun's ultraviolet radiation before it reaches the earth. The earth's stratosphere--the region 12 to 20 kilometers highearth contains small quantities of ozone a gas with molecules consisting of three oxygen atoms. Ozone (O3) is formed when ordinary oxygen molecules (O2) are split by ultraviolet radiation and then recombine with other oxygen molecules. If all the ozone in the atmosphere were compressed to a pressure corresponding to that at the earth's surface, the layer would be only 3 mm thick. But even though ozone occurs in such small quantities, it plays an exceptionally fundamental part in life on earth.

This is because ozone, together with ordinary molecular oxygen, is able to absorb the major part of the sun's ultra-violet radiation and therefore prevent this dangerous radiation from reaching the surface. Without a protective ozone layer in the atmosphere, animals and plants could not exist, at least upon land. The new Nobel recipients research revealed much about the processes that regulate the atmosphere's ozone content.

In 1970, Dr. Crutzen showed that the nitrogen oxides NO and NO2 react with ozone to hasten its destruction without being destroyed themselves. These nitrogen oxides occur when microorganisms in soil produce nitrous oxide (N2O).

Five years later, Drs. Molina and Rowland published a landmark article in Nature showing the threat to the ozone layer from chlorofluorocarbon (CFC) gases, which were being used widely in spray cans, refrigerators, and plastic foams. The two scientists realized that CFCs, which are chemically stable, could gradually be transported through normal air circulation to the stratosphere. There, intense ultraviolet light could break up the molecules, releasing chlorine, which catalyzes ozone destruction (just as nitrogen oxides do).

Molina and Rowlands' hypotheses was highly controversial when first proposed and brought a storm of protest from industrial producers of ozone. However, the researchers gained considerable support in 1985 when a drastic seasonal depletion of stratospheric ozone over Antarctica--the "ozone hole"--was discovered.

Satellite view of ozone hole (NASA)

Why is the depletion so rapid and severe over the Antarctic? Crutzen and colleagues found that the chemical reactions were taking place on the surfaces of polar stratospheric clouds. These rare clouds are formed by water and nitric acid at extremely cold temperatures, primarily over Antarctica. Through United Nations involvement, the Montreal Protocol on prohibition of CFC emissions was signed in 1987 and strengthened thereafter. With a few exceptions, the most dangerous gases will be completely banned by 1996. Still, the long lifetime of these gases means that damage to the ozone layer is likely to worsen for some years to come. Even if protocol guidelines are followed, it will take at least 100 years for the ozone layer to recover. However, in a recent development, the Republicans in the US Congress have introduced legislation to overturn U.S. participation in the international ban on the production of chlorofluorocarbons, or CFCs.


In 1930 the English physicist Sidney Chapman formulated the first photochemical theory for the formation and decomposition of ozone in the atmosphere. This theory, which describes how sunlight converts the various forms of oxygen from one to another, explains why the highest contents of ozone occur in the layer between 15 and 50 km, termed the ozone layer. Later measurements, however, showed appreciable deviations from Chapman's theory. The calculated ozone contents were considerably higher than the observed ones. This led scientists to look for other chemical reactions contributing to the reduction of the ozone content. Some years later the Belgian Marcel Nicolet contributed important knowledge of how the decomposition of ozone was enhanced by the presence of the hydrogen radicals OH and HO

The power of nitrogen oxides to decompose ozone was noted by the American researcher Harold Johnston, who carried out extensive laboratory studies of the chemistry of nitrogen compounds. In 1971 he pointed out the possible threat to the ozone layer that the planned fleet of supersonic aircraft and supersonic travel (SST) might represent. These aircraft would be capable of releasing nitrogen oxides right in the middle of the ozone layer at altitudes of 20 km. Crutzen's and Johnston's work gave rise to a very intensive debate among researchers as well as among technologists and decision-makers.

The next important development in ozone chemistry came in 1974, when Molina and Rowland published their widely noted Nature article on the threat to the ozone layer from chlorofluorocarbon (CFC) gases - "freons" - used in spray bottles, as the cooling medium in refrigerators and elsewhere and plastic foams.

Molina and Rowland based their conclusions on two important contributions by other researchers. James Lovelock had recently developed a highly sensitive device of measuring extremely low organic gas contents in the atmosphere, the electron capture detector. Using this he could now demonstrate that the exclusively man-made, chemically inert, CFC gases had already spread globally throughout the atmosphere. Richard Stolarski and Ralph Cicerone had shown that free chlorine atoms in the atmosphere can decompose ozone catalytically in similar ways as nitrogen oxides do.

The ozone controversy raises the issue of how mankind is affecting the climate. Ozone, like carbon dioxide and methane, is a greenhouse gas that contributes to high temperatures at the surface of the earth. (CFC gases have a similar effect). Model calculations have shown that the climate is specially sensitive to changes in the ozone content in the lower layers, the troposphere. Here the ozone content has increased markedly during the past century, chiefly because of the release of nitric oxide, carbon monoxide and gaseous hydrocarbons from vehicles and industrial processes and from the combustion of biomass in the tropics. The elevated ozone content in lower atmospheric layers is itself an environmental problem for the damage it can cause to crops and human health.

Thanks to the Royal Swedish Academy of Sciences for providing resources used in the preparation of this article.


Graedel, T. E. and Crutzen, P.J., Atmosphere, climate and change. Scientific American Library, 1995.

Rowland, F. S. and Molina, M. J., Ozone depletion: 20 years after the alarm, Chemical and Engineering News 72, 8-13, 1994

Scientific assessment of ozone depletion 1994, WMO Report 37, World Meteorological Organization and United Nations Environment Programme, Geneva, 1995.

Toon, O. B. and Turco, R. P., Polar stratospheric clouds and ozone depletion, Scientific American 264, 68-74, 1991.

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