Paul Jozef Crutzen was born in Amsterdam on 3 December 1933 and passed away in Mainz on 28 January 2021. He was the son of Anna Gurk and Jozef Crutzen. In Amsterdam, on 14 February 1958, he married Terttu Soininen; Paul and Terttu have two daughters, Ilona and Sylvia, and three grandchildren . There is no doubt that Paul was a very hard-working man. He once said “see, this is the life of a scientist, always working”. He was very dedicated and demanded the same of his collaborators and students. If the only opportunity to talk science with him was on a Saturday afternoon, you had no choice but to accept his invitation and come to the office. However, this intensity also meant that he was always very interested in the work of colleagues and students; you could count on a well-elaborated reply from Paul in a very short time frame to any scientific text you sent him, be it a paper draft, parts of a doctoral thesis, or any other kind of text.

Nevertheless, no matter how much Paul concentrated on his scientific work, he always had time for his family (Fig. ). He himself mentioned that weekends were reserved for family, especially during the time when his daughters were young. The entire Crutzen family has fond memories of family gatherings, weekends, holidays, and vacations with Paul. For his colleagues, Paul was a very pleasant person to be with, and science was not necessarily the main subject of conversation. One could chat with Paul over a coffee or an evening meal on many other topics that he was interested in, such as sports. For example, he was always well-informed about the performance of the Dutch ice-skating team and was interested in football,

Football is the game referred to in American English as soccer.

Paul's scientific achievements are too numerous and their scope too broad to be covered in detail in this brief note for more information, see e.g.. However, stratospheric ozone chemistry, in general, and NOx chemistry, in particular, were what started his scientific work, even before his doctoral research . He proposed the ground-breaking idea that reactions catalysed by NO and NO2 control the ozone concentration in the middle stratosphere , according to the following catalytic cycle: 1NO+O3→NO2+O2,2NO2+O→NO+O2. Here, the sum of NO and NO2 is defined as NOx=NO+NO2 and O indicates that an oxygen atom in its ground state (O(3P)).

The discovery of this mechanism in 1970 was a crucial step towards facilitating a quantitative description of the chemistry of the stratospheric ozone layer; prior to Paul's discovery, the dominant catalytic loss cycle of stratospheric ozone (DN, through Reactions  and ) was not known (Fig. ). It is now known that DO, the reaction O3+O→2O2, originally proposed by , is only a minor sink of stratospheric ozone and that HOx-induced catalysis is important only close to the tropopause and above about 45 km (e.g. ; see also Fig. ). The recognition that chlorine (DCl) also catalytically contributes to stratospheric ozone loss came a few years after the discovery of the NOx-induced cycle .

The recognition of the strong effect of NOx on stratospheric ozone had a major impact, as emissions of NOx caused by a possible fleet of supersonic planes was the first stratospheric ozone depletion issue that was studied . Paul also investigated how tropospheric nitrogen-containing compounds (like N2O) can enter the stratosphere and cause the formation of stratospheric NOx . He initiated the first studies on the budget of N2O in the atmosphere and how it is influenced by human activity .

Tropospheric chemistry, in particular the chemical production of ozone in the troposphere, was of great importance to Paul . The OH radical is responsible for the oxidation of CH4 (and many other compounds emitted into the atmosphere). It was found that, in environments containing sufficient NO, the methane oxidation chain could produce ozone in large quantities – compared with the downward flux of ozone from the stratosphere to the troposphere . At that time, very little was known about the homogeneous and heterogeneous reactions affecting the methane oxidation chain in the troposphere; therefore, initial conclusions remained uncertain . However, some years later, along with Jack Fishman and Susan Solomon, Paul presented observational evidence of strong in situ tropospheric ozone production . Later, Paul's work on tropospheric ozone led him to realise the importance of biomass burning for the chemistry of the atmosphere .

Paul's work on smoke from fires after a possible nuclear war and the absorption of sunlight by the smoke introduced the concept that the use of nuclear weapons would have global impacts that go far beyond the more obvious direct effects. This work inspired substantial research activity: and , soon followed by and , calculated the surface temperature response to fires after a nuclear war and introduced the term nuclear winter see also. Paul's original intention, however, at the outset of these studies was to explore the impact of nitrogen oxides, which might form as a result of a possible nuclear war, on stratospheric ozone, a concept that had been discussed earlier e.g.. Paul counted his contribution to this field as important from a political point of view. Indeed, along with John Birks, Jeannie Peterson, Alan Robock, Carl Sagan, Georgiy Stenchikov, Brian Toon, and Richard Turco, he was presented with the 2022 Future of Life Award (https://futureoflife.org/project/future-of-life-award/, last access: 14 December 2023) by the Future of Life Institute. The award was presented to this team for reducing the risk of nuclear war by developing and popularising the science of nuclear winter.

After the impact of chlorofluorocarbons (CFCs) on stratospheric ozone was identified , Paul published a modelling study on this topic in the same year . In 1985, the ozone hole was discovered by . A year later, Paul – along with Frank Arnold – showed that the formation of stratospheric particles (well above the temperature threshold for ice formation) and the nitric acid uptake into these particles are crucial aspects of ozone hole chemistry . Heterogeneous chemistry and an ozone loss cycle specific for ozone hole conditions turned out to be further key processes explaining the chemical processes responsible for the formation of the ozone hole. Furthermore, Paul suggested that carbonyl sulfide (COS) constitutes the major non-volcanic source of aqueous sulfuric acid aerosol particles in the stratosphere .

In recognition of the importance of the multi-phase chemistry on atmospheric aerosol particles and the many unknown processes regarding their microphysics, Paul initiated a junior research group in Mainz in the early 1990s. The name of the group was “Heterogene Chemie und Mikrophysik atmosphärischer Aerosolteilchen” (“Heterogeneous chemistry and microphysics of atmospheric aerosol particles” in English). An important starting point of this research was the paper by , who reported on the homogeneous and heterogeneous freezing rates of sulfuric acid droplets under stratospheric conditions, which has implications for the theory of the formation of nitric acid trihydrate particles in the polar stratosphere. The three authors of that paper are shown in Fig. . This photograph was taken by Thomas Koop. At the same time, the scene was photographed by A. R. Ravishankara, who also wanted to take a picture of the authors of the paper.

Further work of the junior research group showed the occurrence and importance of a new type of polar stratospheric cloud (Type Ib) that consisted of liquid rather than crystalline particles ; other groups also investigated liquid polar stratospheric clouds . It later became clear that these liquid clouds, and not the frozen nitric acid hydrate or ice particles, are the main hosts of heterogeneous chemical reactions responsible for chlorine activation and, thus, polar ozone depletion e.g.. In another study , the junior research group showed that the composition and freezing behaviour of the liquid particles depends on small-scale temperature fluctuations in the atmosphere. As it turned out, the smallest droplets reached higher HNO3 concentrations than larger ones, thus counterintuitively increasing the smaller droplets' likelihood to crystallise. When Paul, who was himself rather short in stature, first learnt about this result during a discussion, he commented on it with a pinch of self-irony and a big smile: “Never neglect the small ones!”.

A discussion about Paul as a scientist is not complete without talking about his role in the scientific community. Throughout his career, Paul interacted and collaborated with many important, influential scientists. These collaborations are easily noticeable upon investigation of his list of publications. However, perhaps even more impressive is the educational impact that he had. There is a very large number of outstanding scientists who had – and have – an extraordinary career that started with a PhD, a postdoc, or an early scientific interaction with Paul. The inspiration for these careers clearly came from Paul, and they started at the various institutions at which he was active.