Three Chemistry Laureates Have Made Pioneering Contributions To Knowledge

Contributions To Knowledge Date: March 4, 1996 (The Scientist, Vol:10, #5, pg.9 & 12, March 4, 1996) (Copyright ©, The Scientist, Inc.) In December, Paul Crutzen, Mario Molina, and F. Sherwood Rowland received the 1995 Nobel Prize in chemistry for their work toward understanding how stratospheric ozone is naturally produced and destroyed and wherein human activities are capable of reducing worldwide ozone. The many other scientists who contributed atmospheric observations, laboratory measurements, and theoretical developments toward the same goal surely join me in celebrating the receipt of the Nobel Prize by these three leaders. Although ozone represents less than one part per million of the global atmosphere, it is the primary filter that determines how much biologically damaging solar ultraviolet-B radiation reaches the ground. As a general rule, at any latitude the annual average ultraviolet-B radiation would increase if the annual average overhead ozone should decrease. About 90 percent of the Earth's ozone is in the stratosphere, a region about 9 to 30 miles above the planet's surface. By 1958, there were about 100 ground-based instruments that measured the amount and vertical distribution of overhead ozone. Between 1978 and 1991, instruments aboard the Nimbus-7 satellite daily measured the amount and vertical distribution of the atmospheric ozone worldwide. Many other stratospheric measurements have been made from the ground, aircraft, rockets, balloons, satellites, and the space shuttle (Scientific Assessment of Ozone Depletion: 1994, World Meteorological Organization Global Ozone Research and Monitoring Project -- Report No. 37). Between 1930 and 1970, several scientists unsuccessfully tried to identify the sources of natural ozone destruction. The turning point in our understanding of the Earth's natural ozone balance occurred in 1970, when Paul Crutzen proposed a homogeneous catalytic chain reaction involving the nitrogen oxides, NO and NO2, as a strong natural source of ozone destruction (P.J. Crutzen, Quarterly Journal of the Royal Meteorological Society, 96:320-5, 1970). Crutzen's great article published in October 1971 (P.J. Crutzen, Journal of Geophysical Research, 76:7311-27) succeeded in organizing the major chemical mechanisms that naturally form and destroy atmospheric ozone. He identified and quantified the source of stratospheric nitrogen oxides as an indirect byproduct of the biological nitrogen cycle: Legumes and soil bacteria, for example, fix atmospheric nitrogen (N2) to provide soil nutrients for plants and then to animals, both of which return to the soil. Other soil bacteria eventually return N2 and some nitrous oxide (N2O) to the atmosphere. Nitrous oxide -- not to be confused with the highly reactive oxides of nitrogen (NO and NO2) -- is stable in the troposphere. Here it is slowly transported to the midstratosphere, where it is broken down by far ultraviolet radiation, but some reacts to form chemically active nitric oxide (NO). Nitric oxide reacts with ozone to form active nitrogen dioxide. These two chemical species carry out Crutzen's catalytic cycle, the most important process that naturally destroys ozone in the stratosphere. Crutzen's 1970 article had immediate practical application concerning the planned United States fleet of 500 supersonic transports. I showed in August 1971 some plausible long-term stratospheric distributions of nitrogen oxides from aircraft engine exhaust that gave large calculated ozone reductions (H.S. Johnston, Science, 173:517-22, 1971). This concern directly led the U.S. Congress in September 1971 to establish the first major program of atmospheric research, the U.S. Climatic Impact Assessment Program (CIAP). Extensive stratospheric measurements were made, and Crutzen's 1970 and 1971 postulates were confirmed. In 1974, Molina and Rowland deduced the atmospheric fate of chlorofluorocarbons (CFCs) (M.J. Molina, F.S. Rowland, Nature, 249:810-14, 1974; F.S. Rowland, M.J. Molina, Reviews of Geophysics and Space Physics, 13:1-36, 1975), with heavy use of Crutzen's nitrous oxide model as a hand-in-glove analogy for atmospheric CFCs. Citing recent measurements of CFCs in the troposphere and industry records of CFCs production, Molina and Rowland concluded that CFCs were inert in the troposphere, as is nitrous oxide. Nitrous oxide was known from CIAP observations to be slowly transported up into the stratosphere and to be broken apart by ultraviolet radiation there. CFCs surely must be similarly transported and broken down by ultraviolet radiation to feed atomic chlorine (Cl) and chlorine monoxide (ClO) into their ozone-destroying catalytic cycle. Using these facts and logic, they found that long-continued production and release of CFCs at the 1973 rate would approximately double the number of ozone-destroying catalysts in the worldwide stratosphere, with probable reduction of global ozone. After CIAP terminated in 1975, new concerns raised by Molina and Rowland led Congress to assign long-term stratospheric ozone research to the National Aeronautics and Space Administration (NASA), which collaborated with other nations and the World Meteorological Organization. Many new discoveries were made in stratospheric science during the next 20 years. The postulates of Molina and Rowland were confirmed by atmospheric measurements. A surprise to Crutzen, Molina, Rowland, and everyone else in the atmospheric chemistry community came in 1985 when the "Antarctic ozone hole" was reported. A massive expedition to Antarctica provided direct atmospheric measurements that proved beyond reasonable doubt that Cl from CFCs was largely responsible for the great ozone reductions seen in the austral spring (S. Solomon, Nature, 347:347-54, 1990). The explanation of the Antarctic ozone hole involves new heterogeneous chemical reactions on ice particles and on nitric acid ice particles. By 1994, NASA scientists had interpreted 15 years of satellite ozone data, reporting a global average ozone reduction of about 5 percent. This worldwide ozone reduction is caused by sulfuric acid aerosols in the lower stratosphere that catalytically convert inactive forms of chlorine into ozone-destroying chlorine oxide, and bromine reactions are similarly involved. Through the leverage of recently understood homogeneous and heterogeneous catalysis in the stratosphere, rather minor human activities that inject only about one megaton per year of ozone-destroying CFCs into the stratosphere have reduced the much larger mass of global ozone by about 5 percent. The discovery, understanding, and significance of these global changes would not have been made in this generation except for the three 1995 Nobel laureates' work, for which we should all be grateful. Harold S. Johnston is a professor of chemistry at the University of California, Berkeley.

Three Chemistry Laureates Have Made Pioneering Contributions To Knowledge

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