The Ozone Layer

From Discovery to Recovery

Guy P. Brasseur

• Publisher: American Meteorological Society
• Year: 2020
• ISBN:
  9781944970550
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

From the discovery of ozone in the eighteenth century, through the late twentieth-century international agreements to protect humanity from the destruction of ozone in the stratosphere, Guy P. Brasseur traces the evolution of our scientific knowledge on air quality issues and stratospheric chemistry and dynamics. The history of ozone research is marked by typical examples of the scientific method at work, perfectly illustrating how knowledge progresses. Hypotheses are contested and then eventually accepted or rejected; truths once believed to be universal and permanent can be called into question; and debates and disagreements between scientists are settled by information from laboratory and field experiments. Of course, the scientific method can also lead to new observations—in this case, the discovery of the ozone hole. This finding took researchers by surprise, leading to new investigations and research programs.This first complete study of ozone research demonstrates the key role fundamental research plays in solving global environmental, climate, and human health problems. More importantly, it shows that the scientific method works. Convincing decision makers of research results that do not correspond to their values, or to the interests of certain business groups, stands to be the highest hurdle in using science to benefit humanity. Students, early-career scientists, and even specialists who do not know much about the history of their field will benefit from this big picture view, offered by a researcher who has played leadership roles in stewarding this science through decades of discovery.

CHAPTER ONE

The First Steps

The Discovery of Ozone

Christian Friedrich Schönbein (1799–1868), a prominent professor of chemistry at the University of Basel (Figure 1.1), was a specialist in electrolysis. He invented the fuel cell and discovered the explosive properties of nitrocellulose. Passionate about chemistry already at a young age, he started working as an apprentice at a pharmaceutical company in Böblingen, Germany, at the age of 13. In order to satisfy his ambition to explore the complex processes of chemistry, he passed several examinations organized by a professor of chemistry in Tübingen. In 1820, he did an apprenticeship training in a textile dyer company before enrolling as a student at the universities of Erlangen and Tübingen. In 1826, he moved to England to become a teacher in a reformatory house and later completed his studies in physics and chemistry at the Sorbonne in Paris. He joined the University of Basel in 1828, where he was promoted to “Ordinary Professor” in 1835. Schönbein became one of Europe’s most renowned chemists while being involved in the political affairs of the city of Basel. The famous chemist Justus von Liebig (1803–1873), professor at the University of Giessen and later of Munich, one of the founders of industrial agronomy, who met Schönbein for the first time in 1853, seemed to appreciate the company of his Basel colleague as in a letter to Friedrich Wöhler (1800–1882), professor at the University of Göttingen,¹ he wrote:

Schönbein’s sense of humor is priceless; if only I had his stomach . . .²

Figure 1.1. Christian Friedrich Schönbein, professor at the University of Basel. Source: Photogravure after a painting by Beltz, c. 1860; original photo from the University of Basel Library.

During an experiment on the electrolysis of acidic water conducted in early 1839, Schönbein noticed a peculiar odor, similar to that produced by the oxidation of white phosphorus. It was very similar to the smell of “electric material” that Martin van Marum³ (1750–1837), a physician in the city of Harlem in the Netherlands, had reported 40 years earlier and that accompanied the production of electric arcs by a friction electrostatic generator that the Dutch scientist had built. In fact, as highlighted in the Odyssey and Iliad by Greek poet Homer (born sometimes between the twelfth and eighth century BC), the “divine odour of Zeus” had already been observed during Antiquity in relation to “Jupiter’s thunders” (lightning flashes).

Figure 1.2. (Left) Arthur de la Rive. Source: Wikipedia. (Right) Jean Charles Galissard de Marignac. Source: Orden pour le mérite für Wissenschaften und Künste. These two scientists showed that ozone is produced in an enclosure containing only oxygen.

On March 13, 1839, Schönbein went to a meeting of the Natural Sciences Society (Naturforschung Gesellschaft) in Basel and made his discovery public; the minutes of the meeting state, “Professor Schönbein told the Society of an odour that develops during the electrolysis of water.”⁴

It was only a few months later, however, in 1840, during a lecture he gave at the Academy of Sciences in Bavaria, that Schönbein suggested that the odor produced during the electrolysis of water should be attributed to a chemical substance, but he had not yet been able to identify this substance. Was it a particular molecule or a mixture of several chemical elements? Or simply, as suggested by the professor of Physics of Geneva, Arthur de la Rive (1801–1873; Figure 1.2), an odor produced by fine particulates released from the material constituting the electrode of the electrolysis cell? Schönbein refutes this last hypothesis by arguing that the same odor, detected in the vicinity of thunderstorms, was probably due to lightning strokes. He therefore argued that the odor recorded in his laboratory must be linked to the electrical arc produced during his experiment (see Box 1.1).

Box 1.1. The characteristic smell of ozone is observed near thunderstorms

M. Buchwalder, a Swiss engineer, was one day on the summit of the Senlis, near Appenzell, with his servant. They were reclining in a little tent erected in the snow, when they were suddenly enveloped in a thunderstorm. The servant was killed on the spot, and, immediately after, the tent was filled with a very powerful and remarkable odor. One day when M. Schönbein was experimenting with ozone, his laboratory being filled with its odor, he received a visit from M. Buchwalder, who recognized the odor as precisely similar to that which he had perceived in his tent on the mountain.

Incident narrated by C. Schönbein and reproduced from the text published in a book by C. Fox entitled Ozone and Antozone with reference to an article published in the Union Médicale, p. 82, 1853.

During a lunch at the university, Schönbein shared his discovery with his colleague Wilhelm Vischer-Bilfinger (1808–1874), a philologist and specialist in ancient Greek literature. Vischer suggested to name this odorous property “ozon” after the Greek word όζειν (ozein, to smell).⁵ Schönbein adopted this idea, which he communicated in 1840 to French academician François Arago⁶ (1786–1853) in a letter on the “nature of the odor that manifests itself in certain chemical actions.” This letter was read at the meeting of the Academy of Sciences in Paris on April 27, 1840. In an article published in German on April 8, 1840, and after having exposed the results of his work, Schönbein wrote,

After these remarks, I still make the proposal to name ozon the odorant principle if future research shows that it behaves as an elemental or composed halogen.⁷

A similar letter in which he mentioned the presence of the new odorous substance “accompanying electricity” was sent to his friend Michael Faraday (1791–1867) and read at a session of the Royal Society in London.

Identification of the Ozone Molecule

The nature of the “electric smell” was hotly debated because it was highly controversial. Schönbein was initially convinced that the electric discharge produced during the electrolysis of the water produces a gaseous substance that he first regarded as a halogen (chlorine or bromine). In his letter to Arago, he wrote,

Since I am pretty sure that the odorant principle must be classified according to the kind of body to which chlorine and bromine belong, i.e. elemental and halogenated substances, I propose to call it ozone. As I am convinced that this body is always released into the air in fairly noticeable quantities, when the weather is stormy, I propose to do a series of experiments this year to highlight the presence of ozone in our atmosphere.⁸

Yet seven years later, he revised himself and, in an article published in 1847, concluded that the substance emitted should contain oxygen and hydrogen atoms, and that it would probably be a high-order oxide HO₃ rather than the hydrogen peroxide H₂O₂, a chemical substance discovered by Louis Jacques Thénard⁹ (1777–1857). Later, Schönbein suggested that ozone was actually the peroxy radical HO₂. De la Rive was of a different opinion based on the experiments he had carried out in his laboratory and of which he reported in a letter to Arago in 1845:

We passed through a tube a stream of oxygen perfectly pure and perfectly dried up [. . .] Thus [. . .] ozone comes only from oxygen and, in order to have the manifestation of it, the simplest and most direct way is to produce a succession of electric sparks through a volume of pure oxygen.¹⁰

Encouraged by these experimental results, Jean Charles Galissard de Marignac (1817–1894; Figure 1.2), professor of chemistry at the Geneva Academy, who had also found that the characteristic smell of ozone appeared when an electric discharge was produced in a chamber containing pure oxygen, suggested that ozone is an allotropic form of oxygen. This conclusion was immediately rejected by Schönbein who considered that the enclosure used by de Marignac probably contained impurities and in particular traces of water vapor. In an article, “On the Nature and Production of the Ozone,” published in the Electricity Archives in 1845, de Marignac wrote,

The attention of chemists and physicists has been excited to a high degree by the remarkable phenomena which have been studied by Mr. Schönbein. The interest in the curious properties of ozone was further enhanced by the ingenious hypothesis through which the learned professor of Basel believed that he could explain these properties. However, chemists cannot easily adopt a theory that overturns all their doctrines on the simple nature of nitrogen, without collecting fully convincing evidence. [. . .] When the air is subject to a series of electric discharges [. . .], ozone develops, without the presence of water vapor being necessary. It seems to us from these facts that the oxygen is likely, in certain circumstances, to undergo a particular modification which enhances its chemical affinities, and makes it able to combine directly with bodies on which it is without action in its ordinary state. Ozone is nothing else than oxygen in this particular state of chemical activity, which results from the influence of an electric current.¹¹

Until 1850, Schönbein remained convinced that ozone was indeed a compound of oxygen and hydrogen. But with the laboratory measurements made in Geneva, he finally had to realize that ozone contains only oxygen atoms. But in what form?

Two theories confronted each other immediately. The first theory expressed by Schönbein was based on the fact that the amount of ozone produced by an electric discharge in an oxygen-containing enclosure is unexpectedly low. It implies therefore that a chemical destruction process involving an unidentified substance reduces the net formation rate of ozone. In 1858, Schönbein imagined that ozone was a negatively electrified form of oxygen, and that its abundance was balanced by the presence of a similar quantity of positively electrified oxygen. In a letter to Michael Faraday, he used the name antozone to describe the positively charged oxygen substance, and considered that the “ordinary” oxygen molecule (O₂) resulted from the combination of ozone and antozone. The contradictory hypotheses that Schönbein made over the years to characterize “the odor of electricity” puzzled Faraday who underlined his deep confusion in a letter sent to Schönbein in 1858 (see Box 1.2). At this point, it remained to characterize the chemical nature and the properties of antozone and to first isolate this compound in the laboratory.

Box 1.2. Letter from Michael Faraday to Christian Schönbein highlighting the disarray in which he finds himself after hearing the different theories presented successively by Schönbein

Royal Institution Nov. 13, 1858
My Dear Schönbein,

Daily and hourly am I thinking about you and yours, and yet with as unsatisfactory a result as it is possible for me to have. I think about Ozone, about Antozone, [. . .] and it all ends in a giddiness and confusion of the points that ought to be remembered.

I want to tell our audience what your last results are upon this most beautiful investigation, and yet am terrified at the thoughts of trying to do so, from the difficulty of remembering from the reading of one letter to that of another, what the facts in the former were. I have never before felt so seriously the evil of loss of memory and of clearness in the head; and though I expect to fail some day at the lecture table, as I get older, I should not like to fail in ozone, or...