Interview with Prof. Paul Josef Crutzen


On 10 December 1995, in Stockholm's Concert Hall, Profs Paul Josef Crutzen, Mario Jose Molina and Sherwood Rowland received from King Carl XVI Gustav of Sweden the 1995 Nobel Prize for Chemistry for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. At a banquet the same day, Prof. Rowland made the following statement:

Your Majesties, your Royal Highnesses, ladies and gentlemen. My friends, Paul J. Crutzen and Mario Molina, and I are most grateful for the honours bestowed upon us today, for the Nobel Prize is the ultimate in recognition in the scientific world. It is immensely satisfying to us that our efforts to understand the chemistry of ozone in the atmosphere have been judged worthy of this honour.

The atmosphere and its manifold changes have held fascination for men and women ever since human beings have trod this Earth. Its study played an integral role in the evolution of natural philosophy from which all of our present sciences have sprung. The scientific and technological developments of the past several decades, not available to our predecessors in past millennia, have provided the theories and tools which have now permitted us to develop a significant understanding of several atmospheric processes which affect the concentration of ozone in our stratosphere.


Mr Crutzen

Paul J. Crutzen

This ozone is vital to us and to all other species living on the sunlit Earth because it both establishes the temperature structure of the atmosphere and simultaneously protects us against damage from the most energetic solar ultraviolet radiation. We now know that ozone is subject to transformation by long-lived chemicals, both natural and man-made, released at the Earth's surface, and substantial reductions in its concentration could have a strongly deleterious effect upon mankind and upon the rest of the biosphere.

Our gratitude extends well beyond our own personal satisfaction because these honours also confer high scientific approval upon the field of atmospheric chemistry and upon environmental science in general. The current understanding of the atmosphere has progressed over the past two decades through the skillful and dedicated work of many hundreds of our colleagues in the field of atmospheric chemistry. Their work has collectively evolved to the point that the nations of the world have accepted through the Montreal Protocol of the United Nations the need for careful monitoring and, in some instances, control of gaseous emissions to the atmosphere.

Once again, speaking on behalf of Paul Crutzen, Mario Molina and myself, we thank you all for the wonderful honours given to us today.

Dr Taba met Paul Crutzen in 1959 at the Meteorological Institute of Stockholm University. He had no academic degree and no previous training in the field of meteorology or related disciplines. His first impression was that Paul was going to have a difficult time in an institution which was in the forefront of meteorological research and where visitors were left on their own to conduct their research. He was small (for a Netherlander), unpretentious and quiet but intelligent and kind.

It would have been difficult to imagine that, one day, Paul would bring to the meteorological community the highest scientific recognition, unprecedented in the field of the atmospheric sciences. Yet it happened.

Paul's maternal grandparents were of mixed German and Polish origin. His father came from vaals, a little town in the southern corner of the country. His parents met in Amsterdam, where Paul was born on 3 December 1933. From his parents, Paul inherited a cosmopolitan view of the world. Despite having lived and worked in several countries since 1958, he remains a citizen of the Netherlands. His six years of elementary school coincided with World War II. The last months of the War were terrible, particularly during the bitter "winter of famine". Some relief came at the beginning of 1945, when the Swedish Red Cross dropped food supplies by parachute. Paul did not have the slightest idea how important Sweden would become in his life. In 1946, he entered the Higher Citizen School and finished in 1951. As part of the school curriculum, all students had to become proficient in three foreign languages: English, French and German. Paul learned these languages mainly from his parents: German from his mother and French from his father. During his school years, he spent a considerable time practising sport; his greatest passion being for long-distance skating on the canals and lakes. He played chess and read widely about travel and astronomy. Because of ill health, Paul could not pass exams qualifying him for a university stipend and, in order not to be a financial burden on his parents, chose to attend the Middle Technical School to train as a civil engineer.

From the summer of 1954 until February 1958 (with a 21-month interruption for compulsory military service), he worked at the Bridge Construction Bureau of the City of Amsterdam. During a trip to Switzerland, he met Terttu Soininen, a student of Finnish history and literature at the University of Helsinki; after intensive efforts, Paul persuaded her to marry him. Apparently, that was an excellent choice. In 1958, they settled in Gavle, a little town about 200 km north of Stockholm, where he found a job in a construction bureau. Paul and Terttu have two daughters. They and a grandson were all together during the Nobel Prize award week; a second grandson has been born since.

Paul's most ardent desire was to make an academic career. One day, at the beginning of 1958, he saw an advertisement in a Swedish newspaper by the Department of Meteorology of Stockholm University announcing an opening for a computer programmer. In spite of his complete lack of knowledge and experience in that field, he applied, was interviewed and chosen from among many candidates. In 1959, the family moved to Stockholm. About a year earlier, the founder of the Institute, Prof. C. G. Rossby, had died and had been succeeded by Dr Bert Bolin1. Until 1966, Paul was mainly involved in computer programming and helping to build and run some of the first numerical barotropic weather prediction models. The University of Stockholm housed the fastest BESK computer and its successor, FACIT. Of course, the great advantage of being in the Institute was that he could follow some lectures. In 1963, he obtained the degree of Filosofie Kandi-dat (the equivalent to an M.Sc.) in mathematics, statistics and meteorology. Oddly enough, he could not take any courses in physics and chemistry since those topics required extensive laboratory experiments for which Paul had no time.

Around 1965, Paul was given the task of helping a scientist from the USA develop a numerical model for the oxygen allotrope distribution in the stratosphere, mesosphere and lower thermo-sphere; through this work, he became interested in the photochemistry of atmospheric ozone. He embarked on an extensive study of the scientific literature, thus setting up the "initial conditions" for his scientific career. He obtained the degree of Filosofie Licentiat in 1968 (corresponding to a Ph.D.) and a D.Sc, in 1973, with the highest distinction. His dissertations focused on the photochemistry of stratospheric and tropospheric ozone.

From 1969 to 1971, Crutzen worked at the European Space Research Organization at Oxford University's Clarendon Laboratory (United Kingdom) with a postdoctoral fellowship. He became, successively, research scientist, senior scientist and director at the National Center for Atmospheric Research (NCAR), Boulder, Colorado, USA; consultant to the US National Oceanic and Atmospheric Administration (NOAA); adjunct professor at Colorado State University; director at the Max-Planck Institute for Chemistry, Germany; and part-time professor of geophysical sciences at the University of Chicago, USA (1987-1991). Since 1992 he has been Director of the Max-Planck Institute and part-time professor at the Scripps Institute of Oceanography and the University of Utrecht, the Netherlands.

Nobel ceremony


Stockhom, 10 December 1995—Paul Crutzen receives the Nobel Prize for Chemistry from the King of Sweden. Photo: Reportagebild

Paul is the recipient of numerous awards, including the Tyler Prize for Environment. He holds several honorary degrees, is a member of many committees and the author of about 200 published works. His research interests include the global modelling of atmospheric chemical processes at all levels; interactions of atmospheric chemistry with climate, the potential role of halogen photochemistry with ozone; and the role of biomass burning.

Reminiscing with Dr Taba about his time in Stockholm, Paul Crutzen spoke with great respect and affection of his supervisors and colleagues, in particular Bert Bolin, Bo Döös2 and George Witt. They gave him freedom to choose his research topics and encouragement but never claimed any credit.

Dr Taba was happy to meet Paul in Mainz, Germany, in December 1997 for the purposes of this interview. The last time he saw him, he had been a young student struggling for a university degree; now he was a Nobel laureate but the sense of humour and engaging smile had not changed.

H.T. — Could you describe, in simple terms, what is ozone and why the study of atmospheric ozone has assumed such importance?

P.J.C. — Ozone (O3) is a form of oxygen containing three atoms per molecule instead of the normal two atoms. It is a strong absorber of ultraviolet (UV) radiation. About a century ago, Cornu found that the spectra of all astronomical sources, including the Sun, showed cut-off at 300 nm and concluded that some absorber in the atmosphere was responsible. A few years later, Hartley observed the absorption spectrum of ozone in the laboratory and suggested that that gas was responsible for the UV cut-off. Ever since, measurements of the strength of this absorption have been used to measure the ozone amount from the ground, as well as from rockets, balloons and satellites. This absorption, which is highly beneficial to people and to many organisms, is disturbing to astronomers in their professional capacity (solar radiation around 300 nm is dangerous).

Most ozone is located in the stratosphere between about 15 and 25 km and as such we do not need to breathe it. It is a radiatively active trace gas and is a product of natural and human-induced chemical and photochemical reactions in the lower and upper atmosphere. Increases in ozone in the lower parts of the atmosphere are caused by the emission of common air pollutants, while the observed decreases in the upper regions occur as a consequence of the presence of chlorofluorocarbons (CFCs) and can be intensified by the release of additional nitrogen, chlorine and bromine compounds. Potential problems related to the increase or decrease of atmospheric ozone have led to a huge acceleration in research into the chemistry and meteorology of ozone since 1970.

H.T. —Could you say something about the formation of ozone; to begin with in the stratosphere?

P.J.C. — In 1930, the British scientist, Sydney Chapman, proposed that the formation of "odd oxygen"—O and O3—was due to photolysis of molecular oxygen O2 by solar radiation at wavelengths shorter than 240 nm.

O2 + hv → 2O           (ג ≤240nm)      (1)

This atomic oxygen reacts quickly with molecular oxygen O2 to form ozone O3. However, unless some other energy-absorbing molecule which we can call M (N2 and O2) is present, the ozone will rapidly decompose again. Therefore:

O + O2 + M → O3 + M                       (2)

Photolysis of ozone by absorption of solar radiation cleaves the O3 molecule. Therefore:

O3 + hv → O + O2                            (3)

The two relations (2) and (3) lead to the rapid establishment of a steady state for the concentration of O and O3 without affecting the concentration of odd oxygen. The destruction of odd oxygen counteracting its production by (1) above occurs by reaction (4).

O + O3 → 2O2                                  (4)

Until about the mid-1960s, the above theory was considered adequate to explain the ozone concentration distribution in the stratosphere. Later on, it appeared clear that reaction (4) was too slow to balance the production of odd oxygen by (1). In 1950, David Bates and Marcel Nicolet3 with Sydney Chapman—all great pioneers of upper atmospheric photochemistry research—proposed that catalytic reactions involving hydroxyl (OH) and HO2 radicals could counterbalance the production of odd oxygen in the meso- and thermosphere. Building on their work and laboratory studies conducted by others (e.g. the 1967 Nobel Prize laureate in chemistry, Prof. N. Norrish), the ozone destruction reactions involving OH and HO2 radicals as catalysts were postulated by J. Hampson and G. Hunt. Therefore:

OH + O3 → HO2 + O2       (5)

HO2 + O3 → OH + 2O2      (6)

2O3 → 3O2

The primary source for the OH radicals was photolysis of O3 by solar UV radiation of wavelengths shorter than about 320 nm, which leads to electronically excited O(1D) atoms, of which only a small fraction react with water vapour. Most O(1D) reacts with O2 and N2 to reproduce O3, rapidly producing a null cycle with no effect on concentrations of ozone or odd oxygen. Therefore:

O3 + hv  →  O(1D) + O2                    (7)

O(1D) + H2O  →  2OH                       (8)

O(1D) + M  →  O + M                        (9)

In the absence of laboratory measurements for the rate constants (5) and (6) and in order for these reactions to counterbalance the production of odd oxygen by reaction (1), Hunt and Hampson simply assumed the rate constants for these reactions.

H.T. — In your Filosofie Licentiat thesis (1968) at the University of Stockholm, you described the vertical distribution of ozone above 25 km differently from the previously proposed theories. Why?

P.J.C. — I thought that the rate constants chosen could not explain the vertical distribution of ozone in the photochemically dominated stratosphere above 25 km. Furthermore, I pointed out that the above choice of rate of constants would also lead to an unrealistically rapid loss of ozone in the troposphere. Anticipating a possible role of OH radicals in tropospheric chemistry, I also briefly mentioned the potential importance of reaction between OH with methane (CH4). We now know that reactions (5) and (6) proceed more than 10 times slower than those postulated by Hunt and Hampson and that oxidation and OH chemistry play a large role in tropospheric chemistry. As regards stratospheric ozone chemistry, I discarded the theory of Hampson and Hunt and concluded that at least part of the solution of the problem of the ozone distribution might be the introduction of photochemical processes other than those treated so far.

H.T. — In 1970, you said that the influence of nitrogen oxide compounds on the photochemistry of the ozone layer should be investigated. Could you expand on that?

P.J.C. — For about two years, I had been thinking about a potential role of NO and NO2 in catalysing O3 destruction, but no measurements of stratospheric nitrogen oxides NOX were available to confirm my thoughts. By the summer of 1969, I had joined the Department of Physics at the Clarendon Laboratory of Oxford University as a postdoctoral fellow. The head of the research group, Dr (now Sir) John Houghton, hearing of my idea on the potential role of NO and NO2, handed me a solar spectrum taken on board a balloon by Dr David Murray and coworkers of the University of Denver and suggested that it might reveal the presence of nitric acid (HNO3). After some analysis, I could derive the approximate amounts of stratospheric HNO3, including a rough idea of its vertical distribution. I did not have the opportunity to write up the result because, at about the same time, Rhine et al. published a paper showing a substantial vertical HNO3 column above 18.8 km. With this information, I knew that active nitrogen oxides (NOX = NO + NO2) should also be present in the atmosphere as a result of the following reactions:

OH + NO2 (+M) → HNO3 (+M)       (10)

HNO3 + hv        → OH + NO2        (11)

H.T. —  What happened then?

P.J.C. — By that time, I had enough confidence to submit my paper on catalytic ozone destruction by NO and NO2 to the Quarterly Journal of the Royal Meteorological Society. It was based on the simple catalytic set of the following reactions:

NO + O3  → NO2 + O2               (12)

O + NO2  → NO + O2                (13)

O + O3    → 2O               (12)+ (13)

The net result of reactions (12) and (13) is equivalent to the direct reaction (4). The rate of net reaction, however, can be greatly enhanced by relatively small quantities of NOx of the order of a few nanomole per mole. I also included a calculation of the vertical distribution of stratospheric HNO3. As the source of stratospheric NOX, I initially accepted the proposal by Bates and Hays that about 20 per cent of photolysis of N2O would yield N and NO. Subsequent work showed that this reaction does not take place. It was soon shown, however, that NO could also be formed to a lesser extent—but still in significant quantities—by the oxidation of nitrous oxide N2O by O(1D). It was further shown by Davis et al. that reaction (13) proceeds about 3.5 times faster than I had originally used.

N2O + O(1D) → 2NO                  (14)

A few years later, it was also shown that earlier estimates of O3 production by reaction (1) and (2) had been too large because of overestimations of both the absorption cross-sections of molecular oxygen and solar intensities in the ozone-producing 200-240 nm wavelength region. As a result of these developments, it became clear that enough NO is produced in reaction (14) to make reactions (12) and (13) the most important ozone loss reactions in the stratosphere in the altitudes between about 25 and 45 km. N2O is a natural product of microbiological processes in soils and waters. Therefore, a number of anthropogenic activities such as applications of nitrogen fertilizers in agriculture also lead to significant N2O emissions. The rate of increase in the atmospheric N2O concentrations for the past decades has been 0.2-0.3 per cent per year. This, however, was not known in 1971. The discovery of an indirect role of a primary biospheric product on the chemistry of the ozone layer has greatly stimulated interest in bringing biologists and atmospheric scientists together.

H. T. —  What are the impacts of supersonic stratospheric transport on ozone?

P.J.C. — While in Oxford, I obtained a copy of a report sponsored by the Massachusetts Institute of Technology entitled the Study of Critical Environmental Problems, which was carried out in July of 1970. The report also considered the potential impact of the introduction of large stratospheric fleets of supersonic aircraft such as Boeing (USA), Concorde (France and the United Kingdom) and Tupolev (USSR). The report provided me with the first quantitative information on the stratospheric inputs of NOx that would result from these operations. By comparing these with the production of NOx by reaction (14), I realized that we could be faced with a severe global environmental problem.

Paul Crutzen


Paul Crutzen with the catalytic ozone destruction cycle by NO and NO2 he proposed in 1970

Although my paper on the important catalytic role of NOx on ozone destruction had already been published in April 1970, the participants in the study had clearly not taken any note of it, since their conclusion was the following: "The direct role of CO, CO2, NO, NO2, SO2 and hydrocarbons in altering the heat budget is small. It is also unlikely that their involvement in ozone photochemistry is as insignificant as water vapour.". Of course, I was most upset by this statement and wrote a rude word in the margin of the text, which I cannot repeat here. At this stage, I decided to extend my 1970 study by treating in much more detail the chemistry of oxides of nitrogen NO, NO2, NO3, N2O4, N2O5, HNO3 and the hydrogen oxides, building on a literature review by Nico-let. I soon got into difficulties. Adopting Nico-let's reaction scheme, I calculated high concentrations of N2O4, a problem I soon overcame when I realized that this compound was thermally unstable, a fact not considered by Nico-let. A greater headache was caused by the following reactions

N2O5 + H2O  →  2HNO3                    (15)

O + HNO3    →  OH + NO3                 (16)

for which the only laboratory studies available at that time had yielded high rate coefficients. A combination of reactions (15) and (16) with those rates of constants would give a large source of OH radicals, which would lead to prohibitively rapid catalytic ozone loss. Knowing the great societal implications of my hypothesis, but having no formal training in chemistry, I discussed the problems with my colleagues and produced extensive model calculations on vertical distributions of these trace gases. My paper on this subject was published in 1971. Because of the major problems I had encountered, I did not make any calculations of ozone depletion. I drew attention only to the potential seriousness of the problem, based on a comparison of natural NOx inputs in the stratosphere by reaction (14) and by the planned supersonic aircraft fleet.

H. T. —  What about the supersonic controversy in the USA?

P.J.C. — Unknown to me, a debate on the potential environmental impact of supersonic stratospheric transport (SST) had already erupted in the USA, which first concentrated on the enhanced catalytic ozone destruction by OH and HO2 radicals resulting from the release of water vapour (H2O) in the engine exhausts. In mid-1971, a workshop was organized in Boulder, Colorado, by an advisory board of the Department of Commerce, to which Prof. Harold Johnston of the University of California, Berkely, was invited. He did not know my publication of 1970 on the role of NOX in catalysing ozone destruction. He pointed out that the role of NOx in reducing stratospheric ozone had been grossly underestimated. In his paper published the same year, Johnston stated that the oxides of nitrogen from SST exhaust posed a much greater threat to the ozone layer than the increase in water and that the projected increase in the stratospheric oxides of nitrogen could reduce the ozone shield by a factor of about 2. I did not know Johnston but I soon developed a great respect for him. Needless to say, I fully agreed with him on the potential severe consequences for stratospheric ozone and was happy to have support from such an eminent scientist. For a thorough resume of the controversies between scientist and industry (and between meteorologists and chemists), I refer to Johnston's article "Atmospheric ozone", which was published in 1992 in the Annual Reviews of Physical Chemistry. Johnston's publications also removed several of the major reaction kinetic problems I had encountered in my 1971 study, in particular reactions described by relation (15) and (16) mentioned before. As to the supersonic flights, in large part a result of the proposal by Johnston that NOx emissions could severely harm the ozone layer, major research programmes started, such as the Climate Impact Assessment Programme (CIAP) organized by the US Department of Transportation and a similar programme in France and the United Kingdom. The outcome of CIAP was: "We recommend that national and international regulatory authorities be alerted to the existence of potentially serious problems arising from the growth of future fleets of stratospheric airlines, both subsonic and supersonic". In any event, the proposed large fleets of SSTs never materialized, mainly for economic reasons.

H. T. —  You returned to Stockholm University in 1971 and two years later you submitted your inaugural dissertation. What was the theme of your thesis and what can you tell us about this important event?

P.J.C. — Back at Stockholm University, I devoted myself mainly to studies concerning the input of NOX releases from SSTs. In May 1973, I submitted my inaugural dissertation "On the photochemistry of ozone in the stratosphere and troposphere and pollution of the stratosphere by high-flying aircraft" to the Faculty of Natural Sciences. I was awarded the degree of Doctor of Science with the highest distinction, only the third time this had ever happened in the history of Stockholm University (earlier the Hogskola). This was one of the last occasions in which the classical and rather solemn Filosofie Doktor, similar to the Habituation in Germany and France, was awarded. I had to dress up as for the Nobel Prize ceremonies. First and second "opponents" were Drs John Houghton and Richard Wayne of the University of Oxford, who wore their college gowns for the occasion. Dr Wayne also served as a most capable, non-obligatory third opponent, whose task was to make fun of the candidate. Unfortunately, the classical doctoral degree has since been abolished, so I was one of the last to go through the procedure.

H.T. —  Let us talk about tropospheric ozone.

P.J.C. —In 1971, a paper by Hiram Levy proposed that OH radicals could also be produced in the troposphere by the action of solar UV radiation on ozone and that they could be responsible for the oxidation of CH4 and CO. This was a major step forward. In spite of low atmospheric concentrations, this ultra-minor constituent, and not abundant O2, is responsible for the oxidation of almost all compounds emitted into the atmosphere by natural processes and anthropogenic activities. We may call OH the atmosphere's detergent. As a follow-up to this paper, I proposed that in situ chemical processes could produce and destroy ozone in quantities larger than the estimated downward flux of ozone from the stratosphere to the troposphere. This thought was initially not well received in the meteorology community. A couple of years later, together with two of my students in Boulder, Jack Fishman and Susan Solomon, we presented the first observational evidence for a strong in situ tropospheric ozone chemistry. A recent ozone budget calculated with a three-dimensional chemical transport model of the troposphere shows clearly the dominance of in situ tropospheric ozone production and destruction. The calculations also indicate a clear increase in tropospheric ozone concentrations over the past few centuries. From the same model, we also calculated the OH concentration distributions for pre-industrial and present conditions, which are strongly affected by changing atmospheric inputs of NOx, CH4 and CO. The dominance of OH concentrations due to the high photochemical activity in the tropics clearly points to the great importance of the tropics in atmospheric chemistry. Despite this fact, research on low-latitude chemistry is greatly neglected, with the consequence that we do not even have satisfactory statistics on the ozone distribution in this part of the world. The chemical composition of the tropical and subtropical atmosphere is already substantially affected by human activity, particularly by biomass burning. In the future, major changes are going to occur here.

H.T. — What about pollution of the stratosphere by chlorine compounds?

P.J.C.A number of studies, in particular those by Stolarski and Cicerone, drew attention to the potentially large efficiency of CI and CIO in destroying stratospheric ozone. Stolarski and Cicerone mainly considered volcanic and space shuttle injections as a potential source of chlorine compounds, but concluded that they were rather minor. In the autumn of 1973 and early 1974, I spent some time looking for potential anthropogenic sources of chlorine in the stratosphere, such as DDT and other pesticides. Then I read a paper by James Lovelock and co-workers, who reported atmospheric measurements of chloro-fluorocarbon compounds over the Atlantic, stating that they were unusually stable chemically and might persist and accumulate in the atmosphere. At about the same time, a preprint of a paper by M. J. Molina and F.S. Rowland with the title "Stratospheric sink for chlorofluoromethanes— chlorine atom catalysed destruction of ozone", was sent to me by the authors. I realized its importance immediately and decided to mention it briefly during a presentation on stratospheric ozone to the Royal Swedish Academy of Sciences in Stockholm. What I did not know was that the press was also invited. A few days later, an article appeared in Svenska Dagbladet, drawing public attention to the topic. I was then visited by representatives of the German chemical company Hoechst and also by Prof. Rowland, whom I met for the first time. In September 1974, I published a model analysis of the potential ozone depletion resulting from the continued use of CFCs, which indicated the possibility of as much as 40 per cent ozone depletion near 40 km altitude as a result of continued use of these compounds at 1974 rates. Research on stratospheric chemistry intensified even more than before, with major emphasis on chlorine chemistry.

H.T. — In 1974 you went to Boulder, Colorado. What were your assignments?

P.J.C. — I had two assignments on a half-time basis: one was as consultant at the NOAA Aeron-omy Laboratory and the other at the NCAR Upper Atmosphere Project. The NOAA group, experts on ion chemistry, had just decided to direct their considerable experimental skills to studies of stratospheric chemistry. My task was to guide them in that direction. Under the leadership of its director, Dr Eldon Ferguson, and other outstanding scientists, this group made major contributions to stratospheric research, including such activities as air sampling with balloon-borne evacuated cans, so-called "salad bowls" for later gas chromatographic analysis', optical measurements of vertical abundances and distributions of NO2 and NO3; the design and operation of an instrument to measure extremely low water-vapour mixing ratios; and laboratory simulations of important, but hitherto little-known, rate coefficients of important reactions. At NCAR, the emphasis was placed more on infrared spectrographic measurements by John Gille and Bill Mankin—work that also developed into satellite-borne experiments. The other important activity was the analysis of the vertical distributions of less reactive gases such as CH4, H2O and CFCs, employing the cryogenic sampling technique pioneered by Drs Ed Martell and Dieter Ehhalt.

H.T. — In 1977, you took up the directorship of the Air Quality Division of NCAR. Was this a scientific or administrative appointment?

P.J.C.The assignment was partially administrative but I did continue with my scientific work, however. Fortunately, in Nelder Medrud, whom I knew from Stockholm, I had a highly competent administrative officer. I promoted work on both stratospheric and tropospheric chemistry; my own research was mostly devoted to the development of photochemical models, conducted mostly with my students Jack Fishman, Susan Solomon and Bob Chatfield. Together with Pat Zimmerman, we started studies on atmosphere-biosphere interactions, especially the release of hydrocarbons from vegetation and pollutant emissions from biomass burning in the tropics. I also tried to strengthen interactions between atmospheric chemists and meteorologists to improve the interpretation of the chemical measurements obtained during various field campaigns. Such interdisciplinary research was a challenge. During this period, as part of various activities in the USA and elsewhere, much of my research was also devoted to the issue of anthropogenic, chlorine-catalysed ozone destruction.

H.T. — Were you surprised by the discovery of the ozone hole in 1985 by Joe Farman and his colleagues?

P.J.C.Yes, of course, but the discovery of the ozone hole came during a period in which I was heavily involved in various international studies on the potential environmental impacts of a major nuclear war between North Atlantic Treaty Organization and Warsaw Pact nations. Many scientists were researching the ozone hole and I stayed out of it, initially. Then, in early 1986, I attended a scientific workshop, in Boulder, Colorado, which brought me up to date with the various theories that had been proposed to explain the ozone hole phenomenon. Although it turned out that some of the hypotheses had elements of truth (particularly the idea put forward by Solomon, Rowland and colleagues of reactive chlorine release from reactions on the surface of stratospheric ice particles), I felt dissatisfied with the treatment of the chemistry in the heterogeneous phase. On my flight back to Germany, I gave some thought to the problem and realized that if HNO3 and NOx were removed from the gas phase into the particulate phase, then an important defence against the attack of CIOx on O3 would be removed (like two mafia families, CIOx and NOx fight each other, to the advantage of ozone). I contacted Dr Frank Arnold of the Max-Planck Institute in Heidelberg to explain my idea about NOx removal from the gas phase. After about a week he had shown that this could indeed be the case and that under stratospheric conditions, solid nitric and trihy-drate particles could be formed at temperatures below about 200K, i.e. a temperature more than 10K higher than that needed for water ice-particle formation. We published our paper in Nature in 1986. On the surface of these particles, CIONO2 and HCI can react with each other (as shown or postulated by Solomon, Rowland, Molina and David Golden and co-workers), producing the CIOx which rapidly destroys O3. Such reactions cannot take place in the gas phase. Thus, the Antarctic ozone depletion appears to be connected with the extremely low prevailing temperatures which lead to condensation of water and nitric acid to form "polar stratospheric clouds". The ozone-decomposing chemical reactions are greatly reinforced by the presence of cloud particles. This understanding has led to an exciting new branch of atmospheric chemistry: heterogeneous chemical reactions on particle surfaces. They are also very important in the troposphere.

H.T. — Do you think there will be a concerted reaction to this discovery by the nations of the world?

P.J.C.The ozone hole is a drastic example of man-made chemical instability, which developed at a location most remote from the industrial releases of the chemicals responsible. The general validity of the chain of events leading to chlorine activation has been confirmed by ground-based and airborne in situ radical observations, showing large enhancements in CIO concentrations in the cold region of the lower stratosphere, coincident with a rapid decline in ozone concentrations. Together with other observations, this confirms the correctness of the postulated ozone depletion theory. In the meantime, the seriousness of this global problem has been recognized by all nations of the world; international agreements have been signed and enacted to halt the production of CFCs and halons since the beginning of 1996.

Nobel prize award


Stockhom, Sweden, December 1995—After dinner at the royal palace during the Nobel Prize award week (from left to right): HRH Princess Victoria, HM Queen Sylvia, HM King Carl XVI Gustav, Paul Crutzen and Terttu Crutzen Photo: Reportagetjänst

Had Joe Farman and his colleagues not persevered in making their measurements in the harsh Antarctic environment since the late 1950s, the discovery of the ozone hole would have been substantially delayed and there may have been far less urgency to reach international agreement on the phasing-out of CFC production. There might also have been a substantial risk of an ozone hole developing in the higher latitudes of the northern hemisphere.

H.T. — What about the release into the environment of other products, such as bromine?

P.J.C. — While the establishment of an instability in the O2/CIO2 system requires chlorine activation by heterogeneous reactions on solid or supercooled liquid particles, this is not required for inorganic bromine, which is normally largely present in its activated forms, owing to gas-phase photochemical reactions. This makes bromine almost 100 times more dangerous for ozone than chlorine on an atom-to-atom basis. This provokes the nightmarish thought that, if the chemical industry had developed organobromine compounds instead of CFCs—or, alternatively, if chlorine chemistry had behaved more like that of bromine—then, without any preparation, we would have been faced with a catastrophic ozone hole everywhere and at all seasons during the 1970s, probably before the atmospheric chemists had developed the necessary knowledge to identify the problem and the appropriate techniques for the required critical measurements. Noting that nobody had worried about the atmospheric consequences of the release of chlorine or bromine before 1974, I can only conclude that mankind has been extremely lucky. This shows that we should always be on our guard for the potential consequences of new products released into the environment. Continued surveillance of the composition of the stratosphere remains a matter of high priority for many years ahead.

H.T. — What about biomass burning?

PJ.C. By the end of the 1970s, considerable attention was being given to the possibility of a large net source of atmospheric CO2 arising from tropical deforestation. I recognized, however, that biomass burning was not only a source of CO2, but also of a great number of photochemically and radiatively active trace gases. Furthermore, biomass burning in the tropics is not only restricted to forest conversion but is also a common activity related to agricultural practices, involving the burning of savanna grasses, wood and agricultural wastes. Because biomass burning releases substantial quantities of reactive trace gases, such as hydrocarbons, CO and NOx in photochemically active environments, large quantities of ozone were expected to be formed in the tropics and sub-tropics during the dry season. Several measurement campaigns in South America and in Africa, starting in 1979 and 1980 with NCAR's Quemadas expedition in Brazil, have confirmed this expectation. The effects of biomass burning are especially noticeable in the southern hemisphere, as satellite observations clearly show.

H. T. — You were involved in various international studies on the potential environmental impacts of a major nuclear war. Could you tell us about it?

P.J.C.My research interests both in the effects of NOX on stratospheric ozone and in biomass burning explain my involvement in "nuclear winter" studies. In 1981, the editor of Ambio had to exert a great deal of pressure to persuade me to contribute to a special issue on the environmental consequences of a major nuclear war. My initial thought was that a nuclear war would be so terrible that the atmospheric consequences would be minor. I was supposed to make an update on predictions of the destruction of ozone by the NOx that would be produced and carried up by the fireballs into the stratosphere. Prof. John Birks of the University of Colorado, Boulder, joined me in this study. Although the ozone depletion effects were significant, it was also apparent to us that these effects could not compete with the direct impacts of the nuclear explosions. However, we then considered the potential climatic effects of the large amounts of sooty smoke from fires in forests and in urban and industrial centres and oil storage facilities, which would reach the middle and higher troposphere. Our conclusion was that the absorption of sunlight by the black smoke could lead to darkness and strong cooling at the Earth's surface and a heating of the atmosphere at higher elevations, thus creating atypical, large-scale meteorological and climatic conditions that would jeopardize agricultural production for a large part of the human population. Our idea was picked up by several research groups, in particular in the USA and the USSR, who showed with their models that there might even be sub-freezing temperatures over much of the Earth. A major international study of the issue, which was conducted by a group of scientists working under the auspices of the Scientific Committee on Problems of the Environment (International Council of Scientific Unions) supported the initial hypothesis and concluded that far more people would die because of the climatic and environmental consequences of a nuclear war than as a direct result of the explosion.

I do not count the nuclear winter idea among my greatest scientific achievements (in fact, the hypothesis cannot be tested without performing the "experiment"!). I am convinced, from a political point of view, however, that, it is by far the most important, because it magnifies and highlights the dangers of nuclear war and convinces me that, in the long run, mankind can escape such horrific consequences only if nuclear weapons are totally abolished by international agreement.

H.T. — Could you say something about the relation between ozone and climate?

P.J.C. — Ozone is a significant greenhouse gas with an infra-red absorption band in the atmospheric window region, centred at 9.6 urn. Although the amount of ozone in the troposphere is only about 10 per cent of that of the stratosphere, its effective long-wave contribution to the optical depth of tropospheric ozone is larger. Of greatest importance would be any changes that might take place in ozone concentrations in the tropopause regions as a result of human activity, such as those caused by H2O, NO, SO2 and particulate emissions from expanding fleets of civil aircraft flying in the stratosphere and upper troposphere. This may lead to increasing temperatures and ozone concentrations in the lower stratosphere and dynamical and chemical effects which we do not understand. However, increasing HNO3 and H2O concentrations in the lower stratosphere in the aircraft exhaust may increase the likelihood of polar stratospheric particle formation and ozone destruction. Such a course of events is also promoted by cooling of the stratosphere through increasing concentrations of CO2. This cooling effect also increases with height in the stratosphere and mesosphere. The implications of this for the future dynamics of the stratosphere, mesosphere and lower thermo-sphere are likewise a topic deserving of considerable attention. Changes in chemical and radiative conditions in the lower stratosphere may create feedback that we need to understand well, involving potential changes of tropopause height and temperatures, stratospheric water vapour concentration, lower stratospheric cloud characteristics and the tropospheric hydrological cycle. Recent observations of increasing trends of water vapour concentrations in the lower stratosphere over Boulder emphasize this point. All these factors should be scientifically explored before decisions are taken on vast expansions of aircraft operations in the stratosphere.

H.T. —  What are your current interests?

P.J. C.— Realizing the great importance of heterogeneous reactions in stratospheric chemistry, I have been involved, together with my former Netherlands students Jos Lelieveld (now Professor at the University of Utrecht) and Frank Den-tener, in particular, in studies on the effects of reactions taking place in cloud droplets and tropo-spheric aerosol particles. In general, such reactions result in the removal of NOx and lower concentrations of O3 and OH. The role of rapid transport of reactive compounds from the planetary boundary layer into the upper troposphere is another topic which I have been involved in with some of my students over the past decade. This may have important effects on the chemistry of the upper troposphere and even the lower stratosphere. My great interest in the role of clouds in atmospheric chemistry has brought me in close contact with a scientifically powerful research group at the University of California, San Diego, headed by my good friend, Prof. V. Ramanathan. A new project which greatly interests me and my collaborators is the possibility of halogen (chlorine, bromine, iodine) chemistry in the marine boundary layer. It is already known that bromine activation can explain the near-zero O3 concentrations at the surface which are often found in the polar marine boundary layer during springtime. In our most recent papers, we discuss the possibility that bromine activation and even iodine chemistry may also occur in other marine regions and seasons. These ideas will be tested by field programmes and, if confirmed, introduced in advanced photochemical-transport models. The project is funded by the European Union and the modelling work is being conducted by a consortium of researchers from France, Germany, Italy, the Netherlands and Sweden.

H.T. — Which are the areas in which future research should be directed?

P.J.C. — Despite the fundamental progress that has been made over the past decades, much research will be needed to fill major gaps in our knowledge of atmospheric chemistry. I mention some of the research areas here:

Observation of tropospheric ozone

There are still major uncertainties concerning budget and global concentration distribution. Everywhere, especially in the tropics and subtropics, there is a severe lack of data on tropospheric ozone concentrations. This knowledge gap is serious, considering the enormous role of tropical ozone in the formation of OH and thus the oxidation efficiency of the atmosphere, the already recognized large anthropogenic impact on ozone through biomass burning and the expected major agricultural and industrial expansion of human activities in this part of the world.


Long-term observations of properties of the atmosphere


Mr Crutzen with Mr Houghton and Mr Wayne


Paul Crutzen (centre) defended his D.Sc. at the University of Stockholm in 1973. He is pictured here with his "opponents", John Houghton (right) and Richard Wayne

Major findings over the past decades have demonstrated the value of long-term observations of important chemical properties of the atmosphere. One example was the discovery of the rapid depletion of stratospheric ozone over the Antarctic during the spring months. Another was the recent unexpected major temporary break in the trends of CH4 and CO, observed by scientists in Boulder. Since the beginning of this decade, there has been a clear decline in CO concentrations, possibly leading to increases in OH concentrations.

Cloud transport

The role of clouds as transporters of chemical constituents such as reactive hydrocarbons, CO and NO and their oxidation products from the boundary layer to the middle and upper troposphere (and possibly into the lower stratosphere) should be better understood and quantified, so that they can be parameterized for inclusion in large-scale photochemical models of the atmosphere. Similarly, the production of NO by lightning and its vertical redistribution by convective storms should also be much better quantified, both for marine and continental conditions. Current uncertainties of NO production by lightning are at least a factor of 4.

General interaction with hydrometeors

The interactions of chemical constituents emanating from the boundary layer with liquid and solid hydrometeors in the clouds will be of special importance. There is, for instance, the question why strong ozone formation has not been noticed around the most convective regions in the continental tropics, in which large amounts of forest-derived reactive hydrocarbons such as isoprene (C5H8) and their oxidation products are rapidly lifted to the middle and upper troposphere and mixed with lightning-produced NO to provide favourable conditions for photochemical ozone formation.

Photolysis rates in cloudy atmospheres

Regarding the photochemistry taking place in cloudy atmospheric conditions, recent observations of unexpectedly high absorption of solar radiation in cloudy atmospheres point to the possibility that multiple scattering in broken cloud systems may lead to strongly enhanced photolysis rates and photochemical activity, leading, for example, especially above or in the upper parts of the clouds, to much higher O3 destruction and OH production than thought so far. The influence of clouds on the photochemi-cally active UV radiation field is a potentially important research topic, which should be pursued by measurements and the development of appropriate radiative transfer models.

Biogenic sources of hydrocarbons, CO and NO

The continental biosphere is a large source of hydrocarbons. Quantification of these sources in terms of physical (e.g. temperature, humidity, light levels) and biogeochemical (soil physical and chemical properties, land use) parameters are urgently needed for inclusion in atmospheric models. The hydrocarbon oxidation mechanisms in the atmosphere should also be better understood, so that formation of ozone, carbon monoxide, partially oxidized gaseous hydrocarbons and organic aerosol must be better quantified and parameterized for inclusion in chemical transport models.

Heterogeneous reactions on particles

The issue of interactions between gases and atmospheric aerosol is largely unexplored and hardly considered in tropospheric chemistry models. Examples are the interactions of SO2 and H2SO4 with, and uptake in, seasalt, and soil dust in the marine boundary layer. The lack of consideration of such processes may well have led to an overestimation of the cooling effects by sulphate aerosol in climate models. In general, we need much better information on the physico-chemical properties and the global distribution of various kinds of aerosols: soil dust, organic aerosol resulting from anthropogenic and vegetation-derived hydrocarbons, aerosol containing sulphate, aerosol produced by biomass burning (smoke and soot), and seasalt particles. Of special importance are those aerosols which can act as cloud condensation nuclei. Much emphasis has been placed in the recent decade on the climate-cooling effects of anthropogenic sulphate aerosols. The role of the other aerosols has been greatly underestimated.

H. T. — You have enjoyed a successful scientific life. Is there anything else you wish you had done?

P.J.C. —After a slow start due to unavoidable circumstances, my life has been highly exciting and I am happy to say that my interest and enthusiasm for atmospheric chemistry research is not dwindling. In this field so much is to be discovered that even I, as an amateur, without a formal chemistry background, could make discoveries. How much more is there left for others to do!

The only thing I have missed in my scientific career is the direct, hands-on involvement in experiments. Maybe that is something to think about after my official retirement in four years from now.

H. T. — Paul, you have taken a major step towards a deeper understanding of the chemistry of the ozone layer by demonstrating the important role played by nitrogen oxides. Thanks to your ideas, the connection between micro-organisms in the soil and the thickness of the ozone layer became one of the motors in the rapid development of research in biochemical cycles. Thank you for according me this interview.


  • 1 Interviewed WMO Bulletin 37 (4) [back]
  • 2 Interviewed WMO Bulletin 46 (3) [back]
  • 3 Interviewed WMO Bulletin 39 (4) [back]


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