Paul Josef Crutzen (1933–2021)

PERSONAL NEWS

Scientists across the globe deeply mourn pursuing university education with a sti- tergovernmental Panel on Climate the loss of Professor Paul J Crutzen. He pend. As he did not wish to burden his Change. At that time the University of was among the greatest scientific genius- parents financially anymore, he opted for Stockholm housed the fastest computers es and pioneers the world has seen in the a 3-year Middle Technical School degree in the world and for the next five years, past century. He passed away on 28 Jan- in civil engineering. The second year Paul helped build and run some of the uary 2021 in Mainz, Germany and is sur- consisted of practical work and enabled first numerical weather prediction mod- vived by his wife Mrs Terttu Crutzen, him to earn money. els, as well as a model for tropical cyc- daughters Ilona and Sylvia, and three From 1954 to 1959, he worked at the lones. Speaking of those times, Paul later grandchildren. Bridge Construction Bureau of the City reminisced that all code had to be written Be it his 1995 Nobel Prize winning of Amsterdam. In 1954 at the age of 21, in machine language and to ensure that work on atmospheric chemistry, which while vacationing on Mount Pilatus in computations did not yield wrong results, provided the foundation for understand- Switzerland, he met Terttu Soininen all operations had to yield numbers in the ing the natural ozone layer and human- from Finland who soon became the love range of –1 to +1, which furthermore induced damage to it, the habitability for of his life. They fell in love and got mar- meant that the equations had to be scaled humans on Earth in the aftermath of a ried in 1958. In his Nobel lecture2, Paul to stay within these limits! While work- nuclear war, perturbations to natural bio- made special mention of it as one of the ing he started attending theory courses in geochemical cycles through biomass best decisions of his life. He gratefully mathematics, statistics and meteorology burning and climate change, climate geo- acknowledged that without Terttu’s lov- and by 1963, he was able to fulfil the engineering, to name a few major ing care and extraordinary support, it requirements for the award of ‘Filosofie themes1, he exemplified through his liv- would have been impossible for him to Kandidat’ equivalent to an M.Sc. In ing example that the best scientific devote as much time as he did to science. 1965, he worked on a project to develop attributes (creativity, rigour, critical With his characteristic humour, Paul later a numerical model for the allotropic dis- thinking, sharing of ideas with peers also remarked that they chose to settle tribution of oxygen in the stratosphere, freely) when combined with the best down in Gavle, Sweden after marriage mesosphere and lower thermosphere. human attributes (compassion, humility, because Sweden happened to be mid-way This project kindled in him a keen interest and concern for nature and fellow human between Finland and the Netherlands! to know more about the photochemistry beings) could time and again manifest Still longing for an academic career, in of atmospheric ozone and he embarked spectacular achievements for the benefit 1958 when he saw a job advertisement on an intensive study of the literature on of the world. for a computer programmer he promptly this topic. Subsequently, for his Ph.D. Paul’s earthly journey commenced on applied. To his pleasant surprise, he was thesis research (‘Filosofie Licentiat’), 3 December 1933 in Amsterdam where selected from amongst many applicants therefore instead of choosing to further he was born to Anna Gurk and Jozef despite no prior experience. On 1 July develop the numerical model for tropical Crutzen. While his mother had German 1959, he started the new position in the cyclones, he chose to work on stratosphe- and Polish ancestry, his father was Department of Meteorology at the Uni- ric ozone chemistry. The proposal was Dutch. They belonged to a working-class versity of Stockholm. The Head of the generously accepted by his Ph.D. Advi- family and while growing up, conditions Institute was Bert Bolin, who later also sor Bert Bolin. In 1968 his thesis work were often tough. The brutal winter fa- served as the first Chairperson of the In- entitled ‘Determination of parameters mine of the 1944–45 and the Second World War were some major hardships that they endured. Paul was an avid sports lover. Playing football, cycling and long-distance skating on lakes were among his favourite activities. He also enjoyed chess, reading about adventur- ous expeditions and bridges. Due to the cosmopolitan atmosphere at home, he easily became fluent in French, German and English. He liked mathematics and physics too, but chemistry was not among his favourite subjects at school. As he neared completion of school education, he desired to pursue university studies in the natural sciences. However, in 1951 while appearing for the final school exams, he fell ill and had to take the written and oral exams under Left: Paul Crutzen in 2014 during the trip to revisit the place where he had first met high bouts of fever. This affected his Terttu 50 years ago in Switzerland. Right: Paul Crutzen with his wife Terttu when they final grades rendering him ineligible for invited the author’s family in Mainz 2017. Photo credit: Vinayak Sinha.

1102 CURRENT SCIENCE, VOL. 120, NO. 6, 25 MARCH 2021 PERSONAL NEWS appearing in the “dry” and the “wet” of ozone in the stratosphere and further- This work also suggested for the first photochemical theories for ozone in the more would result in unrealistically rapid time that the biosphere could affect even stratosphere’ was awarded the highest ozone loss in the troposphere. Discarding the remote stratosphere chemically. This distinction possible. This work also the theory proposed by Hunt and Hamp- connection brought together biologists sowed the seeds for his brilliant para- son to close the stratospheric ozone and atmospheric scientists and germi- digm shifting ground breaking subse- budget, he noted that the possibility of nated the seed for later unravelling also quent work carried out during his D.Sc. other photochemical processes should be other biosphere–stratospheric inter- (‘Filosofie Docktor’) research at Oxford examined and in particular the influence actions such as those due to carbonyl University. Ironically, Paul’s motivation of nitrogen compounds should be inves- sulphide and methane. Subsequently, by to work on stratospheric chemistry was tigated. However, no experimental data incorporating more accurate reaction rate also prompted by a desire to investigate existed to test Paul’s hypotheses. constants for reactions in the Chapman the pure science of ‘natural’ processes as After completing his Ph.D. from mechanism, and the more detailed HOx many of his colleagues were investigating Stockholm university in 1968 at the age and NOx chemistry, scientists could ac- the chemistry of acid rain and damage to of 35, in 1969 Paul joined as a post- curately account for the observed abun- ecosystems due to air pollution. What doctoral fellow of the European Space dance and distribution of stratospheric could be more remote and far away from Research Organization in the Clarendon ozone. Also in 1970, Paul heard about human influence than the stratosphere Laboratory of the Department of Physics the plans by many countries to deploy and what atmospheric region by virtue of at the University of Oxford, UK. Aware large stratospheric fleets of supersonic its chemical composition and radiation of Paul’s idea concerning the potential passenger aircraft through an MIT spon- regime could have fewer chemical reac- influence of nitrogen compounds on stra- sored study report. The report enabled tions to worry about – or so he thought! tospheric ozone chemistry, one day the him to make estimates of how much NOx The British scientist Sydney Chapman Head of the Research Group John might be released into the stratosphere had proposed a mechanism3 in 1930 to Houghton handed him a solar spectrum and the resultant ozone loss. The num- explain the photochemical production taken from a balloon flight by research- bers disturbed him greatly, as he could and loss of ozone by natural processes in ers at the University of Denver. This foresee a global environmental problem the stratosphere through a series of four spectrogram indicated the presence of ni- looming on the horizon because of ozone simple reactions involving only molecu- tric acid in the stratosphere. By analysing depletion in the stratosphere. Shockingly, lar oxygen, oxygen atoms and ozone. it, Paul was able to deduce the approx- the authors of the report had completely Due to the limitations in measuring rates imate amounts and potential distribution ignored Paul’s recent work and con- of these reactions accurately and paucity of stratospheric nitric acid. But before he cluded in their report that there would be of ozone measurements in the stratos- could write up the results, Rhine et al.5 little impact on the stratospheric heat and phere, it was thought until the early published their results on the distribution ozone budgets from the aircraft exhaust 1960s that this mechanism was adequate of nitric acid above 19 km. For nitric ac- emissions. Paul realized that his research to explain the observed stratospheric id to occur in the stratosphere, Paul sur- area had acquired new importance with ozone abundance. In fact, the shape of mised that nitrogen oxides (NO and NO2) major global implications. This moti- the vertical profile of ozone with a maxi- must also be present. Garnering suffi- vated him to extend his initial work and mum in the middle stratosphere was also cient confidence from these results, in publish an updated study7, incorporating well predicted by this mechanism. In the 1970 he proposed that significant ozone more detailed chemistry of nitrogen 1950s, Bates and Nicolet, together with loss could occur due to catalytic reac- oxides. The challenges before him in Chapman pointed out the presence of hy- tions involving ultra-trace amounts of doing so were immense. For one, unlike 6 droxyl and hydroperoxyl radicals (collec- NOx in the stratosphere . The source of today no critically evaluated recommen- tively called HOx) in the mesosphere and NOx was postulated to be photochemical dations for reaction rate constants were thermosphere. By the mid 1960s, it was reactions involving nitrous oxide (N2O), available. He pretty much had to compile becoming apparent through additional a chemically inert gas produced naturally and comprehend the chemistry on his observations and studies that the Chap- through microbe mediated nitrification own which was a major handicap, consi- man mechanism tended to over-estimate and denitrification processes in soil and dering his lack of formal training in the actual abundance of stratospheric water. As it applied later also for anthro- chemistry. He was assisted in these ozone significantly. Following up on the pogenically emitted chlorofluorocarbons efforts by Richard Wayne, a former stu- HOx study and laboratory experiments (CFCs), nitrous oxide does not undergo dent of Norrish and fellow co-worker at conducted by the 1967 Chemistry Nobel photochemical loss in the troposphere, the University of Oxford. Laureate Norrish, Hampson proposed resulting in lifetimes long enough to be In 1971 in the US, Harold Johnston of that HOx radicals could consume ozone transported to the stratosphere. These the University of California Berkeley, a in the upper atmosphere. These were ideas proved to be path breaking for the physical chemist who measured the rate subsequently incorporated into an at- time for more than one reason. The che- constants for several reactions relevant to 4 mospheric chemical model by Hunt . mistry of nitrogen oxides in the strato- NOx chemistry published his work in However, the values they chose for the sphere had not been considered Science8 which corroborated Paul’s con- reaction rate constants were too high. In previously. Also, the idea that a sub- cerns of potential damage to the natural his Ph.D. thesis work, Crutzen had stance present at more than 1000 times ozone layer in the stratosphere by a large shown that the choice of the rate con- lower abundance than stratospheric fleet of supersonic air craft. Harold stants employed by Hampson and Hunt ozone could be a significant sink for it, Johnston encountered fierce criticism could not explain the vertical distribution appeared somewhat fanciful at first sight. from many quarters for this work. The

CURRENT SCIENCE, VOL. 120, NO. 6, 25 MARCH 2021 1103 PERSONAL NEWS scientific evidence withstood the scrutiny pheric Research (NCAR) and National discoveries and serve as the basis for our and was irrefutable. Eventually, on ac- Oceanic and Atmospheric Administration present day understanding of atmospher- count of economic reasons, plans for a (NOAA) in Boulder, Colorado, USA. By ic chemistry processes, sources and sinks large supersonic civilian fleet thankfully 1980, he had become the Director of the of green-house gases and volatile organic got grounded! The work of Crutzen and Air Quality Division at NCAR. Also as compounds. In 1995, Paul Crutzen was Johnston led to funding of several new an Adjunct Professor at the Colorado jointly awarded the Nobel Prize in Che- research programmes such as the Climate State University, he actively participated mistry with Mario Molina and Sherwood Impact Assessment Program. In May in the teaching activities. During this pe- Rowland for their work on atmospheric 1973, Paul submitted his D.Sc. disserta- riod, motivated by Hiriam Levy’s work chemistry, with special mention of their tion entitled, ‘On the photochemistry of on tropospheric hydroxyl radicals11, to- work concerning the formation and de- ozone in the stratosphere and troposphere gether with his then Ph.D. students Susan composition of ozone. and pollution of the stratosphere by high- Solomon and Jack Fishman he started Perhaps also worth mentioning here is flying aircraft’ which was again awarded focussing on tropospheric chemistry. A the multi-nation Indian Ocean Experi- the highest distinction possible. number of new findings12–14 resulted ment conducted in 1999 in which Paul’s Then in 1974, Mario Molina and from these efforts which completely division partnered with Veerabhadran Sherwood Rowland published a paper in changed the then understanding of tro- Ramanathan of Scripps Institution of Nature9 pointing out that human emitted pospheric ozone and also pointed to its Oceanography17. This international effort chemicals called chlorofluorocarbons role as a potent green-house gas. Under- also involved leading Indian scientists (CFCs) would undergo photodissociation standing of tropospheric ozone and its like A. P. Mitra of National Physical at short wavelength UV radiation availa- chemical production through organic Laboratory, D. R. Sikka of Indian Insti- ble in the stratosphere and release reac- trace gases and nitrogen oxides, helped tute of Tropical Meteorology, Pune and tive chlorine atoms that could clear lingering notions about downward Shyam Lal of the Physical Research La- catalytically destroy stratospheric ozone. transport of ozone from the stratosphere boratory, Ahmedabad. A large number of In 1985, Farman et al.10 reported obser- being the major source of tropospheric young Indian scientists, who are leading vations which showed the formation of a ozone. While nitrogen oxides acted as a lights of Indian atmospheric science to- large ozone hole over the Antarctic in sink of ozone in the stratosphere, in the day such as S. K. Satheesh of the Indian springtime. This stunned everyone as the presence of reactive organic trace gases, Institute of Science, Bengaluru also be- remote and cold and dry Antarctic was they acted as a source of ozone in the nefitted from participation in INDOEX. literally the last place anyone expected troposphere. The findings triggered several new re- anthropogenic emissions to deplete the In 1980, Paul Crutzen returned to Eu- search programmes and national studies ozone layer. Subsequent research by a rope to succeed Christian Junge as the spearheaded by ISRO under K. Krishna- number of groups including those of new Director of the Atmospheric Chemi- moorthy and Shyam Lal. These have Crutzen, Molina and Rowland provided stry Department at the Max Planck Insti- contributed immensely to improve the further insights into the mechanism be- tute for Chemistry in Mainz, Germany. current understanding of the nexus hind the multiphase chemistry. It was In 1981 he was asked by the editor of between air pollutant emissions and cli- shown that the widespread ozone deple- Ambio to contribute to a special issue on mate over the Indian region. tion events in the Antarctic in springtime the environmental consequences of a ma- The term Anthropocene now used in required polar stratospheric clouds jor nuclear war. Together with John multiple disciplines is credited to Crut- (PSCs). PSCs are hydrated nitric acid Birks, he carried out research on the zen. The story18 goes that at a Scientific crystals that form at higher temperatures politically sensitive and surcharged is- Committee Meeting of the International (~197 K), than normal clouds in the cold sue. They pointed out that an atomic war Geosphere–Biosphere Program (IGBP) and dry wintertime conditions of the iso- would cause huge fires to run through fo- in Mexico in 2000, while listening to the lated Antarctic vortex. Once formed, rests and cities, and the sooty smoke par- review of the unparalleled global envi- their surfaces catalyse conversion of hy- ticles from the fires would absorb ronmental change that had occurred in drogen chloride and chlorine nitrate to sunlight, leading to the surface of the recent decades of the Holocene (a geo- 15 chlorine gas (Cl2) and nitric acid earth becoming dark and cold . There logical epoch which started ~12,000 throughout the winter. Then in spring- would be widespread starvation. Imme- time, when solar radiation becomes diate deaths due to the atomic bomb available again, photolysis of chlorine would be soon exceeded by deaths due to releases reactive chlorine atoms that the indirect climatic consequences, even trigger the ozone depletion events. The in regions that did not go to war. Paul work of Crutzen, Molina and Rowland considered this idea to be amongst the provided the scientific basis for the Mon- most important he ever had in his life16. treal Protocol, which was signed in 1987. From 1980 to 2000, he set up an active The treaty led to a ban on emissions of international research programme at the CFCs into the atmosphere. As of 2021, it Max Planck Institute of Chemistry. He has been ratified by all 197 member conducted numerous atmospheric field countries of the United Nations, making measurement studies in diverse ecosys- it the most successful global treaty. tems of the world and used the data in From 1974 to 1980, Paul Crutzen advanced numerical models. Many of Paul Crutzen with his grand-daughter Lea worked at the National Centre of Atmos- these studies provided the first findings/ in 2004. Photo credit: Mrs Terttu Crutzen.

1104 CURRENT SCIENCE, VOL. 120, NO. 6, 25 MARCH 2021 PERSONAL NEWS years ago after the last Ice Age), Paul felt reviewed scientific journal articles, 135 a first-hand experience of the meticulous exasperated and he suddenly remarked scientific publications in research jour- and friendly manner in which he contri- aloud: ‘We are no longer in the Holocene nals, and 15 books. He was one of the buted to his co-authored scientific but in (with pause) – the Anthropocene.’ most highly cited scientists in the world, papers. When I got back the draft of the The spontaneous improvisation caught having been bestowed numerous awards manuscript from Paul, I found that Paul the attention of the audience, crystalliz- and honours, as a member of many scien- had checked each and every reference ing the growing realization that the Earth tific academies. He has a street in Hol- manually by striking out the citation in system had recently begun to change at a land and comet named after him. He was the reference list whenever he first read much more dramatic rate and scale than a recipient of the Jawaharlal Nehru Birth it in the main text. The manuscript was through many previous millennia of Centenary Medal 2006, awarded by the published as a preprint23 and received slowly growing human occupation of the Indian National Science Academy, New critical scrutiny. The most valuable planet. Crutzen invited Stoermer, an Delhi, India. In addition to his scientific lesson however was while preparing the ecologist who had first used the term in body of work, another lasting legacy are revised submission, I realized that a pre- an ecological context to co-publish the the large number (46) of Ph.D. students viously unrecognized temperature effect term and concept in the newsletter of the and Postdocs who trained directly under in the inlet could explain most of the ob- International Geosphere and Biosphere him, many of whom like Jos Lelieveld served night-time methane variability, Program19. Two years later he published (who succeeded him as Director of Max which had been used to derive the flux. I the article entitled Geology of mankind – Planck Institute for Chemistry in Mainz, was rather crestfallen as it was only my The Anthropocene in Nature20. The con- Germany), Frank Dentener (at European second first author paper, but Paul put cept of a planet that is human-dominated Commission’s Joint Research Centre, me completely at ease by saying, on a geological scale had been suggested Italy), Mark Lawrence (Director at Insti- ‘Vinayak I should have also thought previously too, but never considered se- tute for Advanced Sustainability Studies, about it before we submitted the paper riously by mainstream geologists. For his Potsdam, Germany), John Burrows (Di- but it is good that we now know about 80th birthday celebrations in 2013 in rector of the Institutes of Environmental how strong this effect can be so we can Mainz, at the scientific symposium Physics and Remote Sensing at the Uni- be more careful in the future.’ My Ph.D. organized in his honour, he chose Anth- versity of Bremen) and Susan Solomon advisor and the other senior co-author ropocene as the theme. When the Anth- (Lee and Geraldine Martin Professor of were also amazingly supportive. The les- ropocene Working Group members met Environmental Studies at MIT, USA) son learnt was that it is ok to make ‘hon- him in Mainz in 2018, even though he have gone on to become famous leaders est’ mistakes and the process of science had not been keeping well for many of the field and continue to reshape and allows for acknowledging past errors and weeks, he still engaged in two long days redefine research pertaining to atmos- moving on after being the wiser for it. of detailed evidence and disputation and pheric chemistry, global environmental Paul dedicated his Nobel lecture to the in his characteristic fashion did not show change, climate of the earth and air pol- generation of his grandchildren and the least attachment to his brainchild. As lution. future generations hoping that they stated by one of the members of the Crutzen cared for and continued to would get to celebrate the recovery of group, ‘He was unfailingly supportive actively support outreach and training of the Antarctic ozone hole. He also tended and encouraging about the work being early career scientists across the world to end his lectures with a picture of one done, even when the procedural nitpick- well after his retirement through training of his grandchildren and himself to re- ing of geological timescale work went schools, special talks and events includ- mind people of their duty to hand over a against the grain of his own quickness ing for school children. There are hun- safe planet to subsequent generations. and clarity. Not all great scientists are dreds of students spanning several I would like to end this tribute to him likable and good company – Crutzen generations in developing nations of by quoting from one of the condolences24 was.’ Asia, Africa and South America whose posted in memory of Paul J. Crutzen: ‘It In 2006, he wrote a provocative edi- own research careers have been touched is very sad that Paul has died but we all torial on climate geo-engineering21, and benefitted by Paul Crutzen’s warm have reason to be thankful that he lived.’ which appeared to be rather controver- and generous scientific feedback and

sial. Speaking about his motivation in inspirational mentoring. I still recollect 16 this regard in an interview he noted: ‘I how in 2006, while presenting measure- 1. Crutzen, P. J. and Brauch, H. G. (eds), wanted to give people a wake‐up call. I ments on methane emissions from Springer briefs on pioneers in science wanted to make it clear what serious forested ecosystems during the depart- and practice No. 50, Springer Interna- compromises we are making to the cli- mental meeting held at Schloss Ring- tional Publishing, London, 2016; https:// mate and to atmospheric chemistry. If we berg, Paul got as excited as the Ph.D. www.springer.com/gp/book/978331927- carry on this way the consequences can- student author about the ambient me- 4591. not be overseen. The most important thane data collected from the boreal for- 2. Crutzen, P. J., https://www.nobelprize. thing is that we reduce CO emissions. est in Finland and tropical rain forest org/uploads/2018/06/crutzen-lecture.pdf 2 (accessed on 8 December 1995). That is the number one issue. But we ha- in Suriname. Earlier that year, Frank 3. Chapman, S., Mem. Roy. Soc., 1930, 3, ven’t seen anything of it yet. And that is Keppler had reported the surprising find- 22 103–125. why I wanted to make it quite clear how ing in Nature , that terrestrial plants 4. Hunt, B. G., J. Geophys. Res., 1966, 71, dangerous the situation is.’ could emit methane under aerobic condi- 1385. During his illustrious career, Paul J. tions. The experience of working with 5. Rhine, P. E., Tubbs, L. D. and Williams, Crutzen published over 360 peer- Paul is deeply treasured by me as it gave D., Appl. Opt., 1969, 8, 1.501.

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6. Crutzen, P. J., Q.J.R. Meteorol. Soc., 15. Crutzen, P. J. and Birks, J. W., Ambio, 24. The Max Planck Institute for Chemistry 1970, 96, 320–325. 1982, 11(2/3), 114–125. mourns the loss of its former director and 7. Crutzen, P. J., J. Geophys. Res. 1971, 76, 16. Crutzen, P. J., Lax, G. and Reinhardt, C., Nobel Laureate Paul J. Crutzen, 2021; 731. Angew. Chem. Int. Edn, 2013, 52, 48–50. https://www.mpic.de/4677594/trauer-um- 8. Johnston, H., Science, 1971; doi: 17. Lelieveld, J. et al., Science, 2001, paul-crutzen. 10.1126/science.173.3996.517. 291(5506), 1031–1036. 9. Molina, M. and Rowland, F., Nature, 18. https://www.scientificamerican.com/article/ 1974, 249, 810–812; https://doi.org/ remembering-the-extraordinary-scientist- ACKNOWLEDGEMENTS. I thank the Max 10.1038/249810a0. paul-crutzen-1933-2021/ Planck Institute for Chemistry in Mainz and 10. Farman, J., Gardiner, B. and Shanklin, J., 19. Crutzen, P. J. and Stoermer, E. F., IGBP the Nobel Prize foundation for freely and Nature, 1985; https://doi.org/10.1038/ Newsl., 2000, 41. openly providing access to resources covering 315207a0. 20. Crutzen, P. J., Nature, 2002, 415, 23. Paul Crutzen’s glorious scientific life. 11. Levy, H. 2nd, Science, 1971; doi: 21. Crutzen, P. J., Climatic Change, 2006, 10.1126/science.173.3992.141. 77, 211–220. 12. Fishman, J. and Crutzen, P. J., Nature, 22. Keppler, F., Hamilton, J. T., Brass, M. VINAYAK SINHA 1978; https://doi.org/10.1038/274855a0. and Röckmann, T., Nature, 2006; doi: 13. Fishman, J. et al., Nature, 1979; 10.1038/nature04420. https://doi.org/10.1038/282818a0. 23. Sinha, V., Williams, J., Crutzen, P. J. Indian Institute of Science Education and 14. Fishman, J., Solomon, S. and Crutzen, P. and Lelieveld, J., Atmos. Chem. Phys. Research Mohali, J., Tellus, 1979; https://doi.org/10.3402/ Discuss, 2007; https://doi.org/10.5194/ Mohali 140 306, India tellusa.v31i5.10458. acpd-7-14011-2007. e-mail: [email protected]

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