Nobel Chemistry Prize Winner Spurred By Wide-Ranging Interests
When the 1995 Nobel Prizes were announced last October, Paul J. Crutzen was on vacation in Spain with his wife, Terttu. So when journalists called at the Max Planck Institute for Chemistry in Mainz, Germany, where Crutzen is director of the Air Chemistry Division, they interviewed one of his senior colleagues instead. When informed that Crutzen had won the Nobel Prize in Chemistry, the colleague replied: "What for?"
The question seemed to imply surprise that Crutzen had won the prize, according to Thomas E. Graedel, distinguished member of the technical staff at AT&T Bell Laboratories, Murray Hill, N.J., who tells the anecdote. "But what the colleague meant was: There are so many things that Crutzen could have been cited for - for example, his work on biomass burning, his work on defining the methane cycle in the atmosphere, or his work on defining the sulfur cycle in the stratosphere," says Graedel. "He got into and defined many areas that nobody else up to that time had paid much attention to."
Crutzen shared the 1995 Nobel Prize in Chemistry with F. Sherwood Rowland, Bren Professor of Chemistry at the University of California, Irvine, and Mario J. Molina, Martin Professor of Environmental Science in the departments of chemistry and earth, atmospheric, and planetary sciences at Massachusetts Institute of Technology. "Their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone," was cited by the Royal Swedish Academy of Sciences.
The academy is the body that decides on the chemistry awards. In last year's announcement, the academy referred to Crutzen's work, published in 1970, that showed that the nitrogen oxides NO and NO2 react catalytically with ozone, thus accelerating the rate of ozone breakdown to O2 in the stratosphere. These nitrogen oxides are formed principally by decay of nitrous oxide (N2O) which originates from microbiological transformations in the soil. According to the academy, "The connection demonstrated by Crutzen between microorganisms in the soil and the thickness of the ozone layer is one of the motives for the recent rapid development of research on global biogeochemical cycles."
"His extraordinary curiosity and breadth of interest is what distinguishes him," says Graedel. "He, more than any other atmospheric scientist I know, has been willing to embrace almost every science that in any way touches on the atmosphere."
And his talents extend beyond atmospheric science. He is fluent in four languages - Dutch (his native language), English, German, and Swedish - and also has a knowledge of French.
The story of his journey to the Nobel Prize is as remarkable and unusual as are his achievements.
It started humbly. His father, a waiter, "was often unemployed," noted Crutzen in his Nobel lecture. His six years at elementary school in Amsterdam were substantially disrupted as a result of the German occupation of the Netherlands during World War II. During the final months of the war, "there was a severe lack of food and heating fuels," says Crutzen. "Also water for drinking, cooking, and washing was available only for a few hours per day."
It was not until 1945, when he was almost 12 years old, that he could start normal school education. After qualifying as a waterways construction engineer in 1954, he worked for four years at Amsterdam's Bridge Construction Bureau. In 1958, he married Terttu Soininen, a Finnish student, and moved to Gävle, a small Swedish town about 120 miles north of Stockholm, where he found a job in a house construction office.
The move north was motivated by one of Crutzen's passions. A picture on the cover of his recent book, "Atmosphere, Climate, and Change"(Scientific American Library, 1995), coauthored with Graedel, provides the clue. Pieter Brueghel the Younger's painting "Winter Landscape" (1601, Kunsthistorisches Museum, Vienna) shows people skating. A print of the work also hangs in Crutzen's office.
"I had a desire to go north, especially in winter, which was my favorite season at that time, because of skating and skiing," Crutzen tells C&EN. "Skating is a very important sport for Dutch people. I used to enjoy long-distance skating on the canals."
The same year that he moved to Sweden, Crutzen applied for a job as a computer programmer at the department of meteorology at Stockholm Högskola (which became Stockholm University in 1961). "I had not the slightest experience in computer programming but was fortunate to be chosen," he says. He moved to Stockholm in 1959 and for the next few years worked on various meteorological projects, including some of the first numerical weather models.
"In the early sixties, he worked as a programmer and at the same time attended lectures in mathematics, statistics, and meteorology," explains Henning Rodhe, professor of chemical meteorology at Stockholm University and a fellow student of Crutzen's at the university for several years during the 1960s.
Crutzen obtained the equivalent of a master of science degree in 1963 and then registered for further graduate work in meteorology. For this, he started working on a model of the distribution of the different forms of oxygen in the stratosphere, mesosphere, and lower thermosphere.
"I was not trained as a chemist at all, although it would have been wonderful if that had been the case. So, in principle, I had to learn it on my own," says Crutzen.
"He was a very hardworking student and very well focused," comments Rodhe. "He became fascinated with the stratospheric ozone problem and at a very early stage worked on it independently."
"It was thought that the chemistry of ozone in the stratosphere was well known," says Crutzen. "But I saw that much of it was based on guesswork. For instance, the rate constants of the reactions of hydrogen oxides with ozone were guessed. I showed that those rate constants gave roughly the right amount of total ozone, but the vertical distribution of ozone came out wrong. So I started thinking about solutions to that - for example, the catalytic depletion of ozone with nitrogen oxides."
He obtained a "Filosofie Licentiat" (equivalent to a Ph.D. degree) in 1968. "But I waited until 1970 before I published the paper containing the catalytic reactions," explains Crutzen. The paper, titled "The Influence of Nitrogen Oxides on the Atmosphere Ozone Content" [Q. J. R. Meteorol. Soc., 96, 320 (1970)], was published during a two-year postdoctoral fellowship at the department of atmospheric physics at the Clarendon Laboratory of Oxford University. "It was such a hit, and I was really working on it with very little chemical background," says Crutzen. "It became quite clear to me that I had touched a hot topic. I therefore decided to study the chemistry of nitrogen oxides and hydrogen oxides in more detail."
In 1971, he returned to the University of Stockholm and studied the impact of high-flying aircraft exhausts on the photochemistry of tropospheric and stratospheric ozone. He was awarded a "Filosofie Doctor" (doctor of science degree) for a dissertation on the topic in 1973. Large fleets of supersonic aircraft did not materialize, although, as Crutzen notes, this was "largely for economic reasons."
From 1974 until 1980, Crutzen continued his research at the National Center for Atmospheric Research in Boulder, Colo. During this period, he showed that the burning of savannah grasses and agricultural waste in the tropics and subtropics had an impact on the oxidizing capacity of the lower atmosphere and pointed out that the burden of this anthropogenic pollution caused increased formation of ozone in the lower troposphere in these regions.
"Biomass burning is not clean burning," says Crutzen. "Billions of tons of carbon are burned producing pollutants such as carbon monoxide, hydrocarbons such as methane, and nitrogen oxides in similar amounts to those produced by industrial processes. This leads to a dirty atmosphere and ozone formation over the rural areas of the tropics and subtropics during the dry season."
Crutzen emphasized the tropics in his Nobel lecture on the role of ozone on tropospheric chemistry presented at the Royal Swedish Academy of Sciences last December. He thinks the tropics represent one of the main challenges to atmospheric chemistry at the moment. "We have so few data and so little information about the basic processes taking place in the tropics. We need to know much more about the chemistry there."
In the early 1980s, Crutzen applied his biomass burning expertise to study the potential atmospheric effects of the widespread use of nuclear weapons. He pointed out that during a large nuclear war, the atmosphere would be loaded with huge quantities of pollutants produced by fires. This pollution would lead to a so-called nuclear winter.
"The issue is very simple," says Crutzen. "You bring lots of black particulate matter into the atmosphere which absorbs the solar energy that would normally reach the Earth's surface. So it gets warm higher up in the atmosphere and colder at the surface."
John W. Birks, chemistry professor at the University of Colorado, Boulder, spent a sabbatical with Crutzen in Mainz on this study. Their conclusion, in 1982, that a large-scale nuclear war could "jeopardize agricultural production for a large part of the human population" was subsequently confirmed by a group of scientists in a major international study conducted under the auspices of the Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (ICSU). The study concluded that far more people could die as a result of the climatic and other environmental consequences of a nuclear war than directly by the explosions.
Crutzen has strong views on nuclear weapons: Although he considers that the nuclear winter idea is not one of his greatest scientific contributions, he believes it is his most important politically. He thinks that the use of these weapons with all their "horrific consequences" is, in the long-term, inevitable. He therefore agrees "wholeheartedly" with Joseph Rotblat, who in his 1995 Nobel Peace Prize address in Oslo, Norway, last December stressed that, "We must get rid of all nuclear weapons."
In the mid-1980s, Crutzen began to tackle another global atmospheric problem, the "ozone hole," which was first observed in 1985. He was one of the first scientists to show that aerosols played a role in annual depletion of stratospheric ozone above Antarctica. His work was based on a mechanism he had proposed in the 1970s for the formation of sulfate aerosols in the stratosphere.
"I proposed that carbonyl sulfide produced at low altitudes, in the soils maybe, or in the oceans, would make it up into the stratosphere and be oxidized there to sulfur dioxide and then further to sulfuric acid," says Crutzen.
"Polar stratospheric clouds grow on these sulfuric acid particles," explains Thomas Peters, leader of the aerosol group in the Mainz institute. "These clouds are formed at very low temperatures and contain nitric acid trihydrate particles."
"Heterogeneous processes occur in the stratosphere," explains Geert K. Moortgat, leader of the kinetics and photochemistry group at the institute. "Sulfuric acid is an important surface catalytic agent" that can transform inactive chlorine species into radicals - such as chlorine and chlorine monoxide - that destroy ozone. Chlorofluorocarbons are the source of most of the chlorine in the stratosphere.
The idea that chlorine is activated on the surfaces of stratospheric ice particles was originally put forward in 1986 by Susan Solomon, a senior scientist at the National Oceanic & Atmospheric Administration, Boulder, Colo., and a former research student of Crutzen's.
"One of the big questions right now is under what conditions do these droplets freeze," says Peters.
Crutzen is also working with Christoph Brühl, a colleague at the institute, to compare data on ozone and chlorine and fluorine species in the stratosphere obtained from the National Aeronautics & Space Administration's Upper Atmosphere Research Satellite with data calculated from a two-dimensional model.
But the main focus of Crutzen's work at the moment is on the chemistry of the marine boundary layer. "This is the first few kilometers above the ocean," explains Crutzen. "Sea salt puts halogens into the atmosphere, and we have proposed that photochemical reactions liberate bromine in the aerosol, and that this then has an effect on ozone chemistry and even on sulfur dioxide oxidation in the troposphere."
Crutzen loves to discover new topics to work on. His research is curiosity driven. "From time to time I sit in the library for a few hours and just go through the literature," says Crutzen. "Most of my ideas I get by reading not only work in my own discipline but also very much in other disciplines such as biology, ecology, and agronomy."
He says he spends almost all his time thinking either consciously or unconsciously about atmospheric chemistry. He has been propelled forward by the various successes in his career stretching from his 1970 paper on nitrogen oxides to his Nobel Prize in Chemistry. "Coming basically from nowhere, without much background in chemistry, and then being able to contribute to our knowledge of atmospheric chemistry over about 25 years has been an enormous motivation," he concludes.
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