Review of Stratospheric Ozone:  why its important and its measurement from space

Announcement of the 1995 Nobel Prize in Chemistry went to three atmospheric chemists who pioneered the work in understanding the chemical mechanisms which result  in ozone depletion in the stratosphere.

Kungl. Vetenskapsakademien The Royal Swedish Academy of Sciences

The Royal Swedish Academy of Sciences has decided to award the 1995 Nobel Prize in Chemistry to

Professor Paul Crutzen, Max-Planck-Institute for Chemistry, Mainz, Germany (Dutch citizen),

Professor Mario Molina, Department of Earth, Atmospheric and Planetary Sciences and Department of Chemistry, MIT, Cambridge, MA, USA and

Professor F. Sherwood Rowland, Department of Chemistry, University of California, Irvine, CA, USA

for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone.

	Professor Paul Crutzen	Professor Mario Molina	Professor F. Sherwood Rowland

The ozone layer - the Achilles heel of the biosphere
The atmosphere surrounding the earth contains small quantities of ozone - a gas with molecules consisting of three oxygen atoms (O₃). If all the ozone in the atmosphere were compressed to a pressure corresponding to that at the earth's surface, the layer would be only 3 mm thick. But even though ozone occurs in such small quantities, it plays an exceptionally fundamental part in life on earth. This is because ozone, together with ordinary molecular oxygen (O₂), is able to absorb the major part of the sun's ultra-violet radiation and therefore prevent this dangerous radiation from reaching the surface. Without a protective ozone layer in the atmosphere, animals and plants could not exist, at least upon land. It is therefore of the greatest importance to understand the processes that regulate the atmosphere's ozone content.

Paul Crutzen, Mario Molina and Sherwood Rowland have all made pioneering contributions to explaining how ozone is formed and decomposes through chemical processes in the atmosphere. Most importantly, they have in this way showed how sensitive the ozone layer is to the influence of anthropogenic emissions of certain compounds. The thin ozone layer has proved to be an Achilles heel that may be seriously injured by apparently moderate changes in the composition of the atmosphere. By explaining the chemical mechanisms that affect the thickness of the ozone layer, the three researchers have contributed to our salvation from a global environmental problem that could have catastrophic consequences.

How this knowledge evolved
Ozone is formed in the atmosphere through the splitting of ordinary oxygen molecules (O₂) by ultra-violet radiation from the sun. The oxygen atoms thereby liberated react with the molecular oxygen according to :

O₂ + uv-light =>2O
O + O₂ + M =>O₃ + M

where M is a random air molecule (N₂ or O₂).

The English physicist Sidney Chapman formulated in 1930 the first photochemical theory for the formation and decomposition of ozone in the atmosphere. This theory, which describes how sunlight converts the various forms of oxygen from one to another, explains why the highest contents of ozone occur in the layer between 15 and 50 km, termed the ozone layer (Fig. 1). Later measurements, however, showed appreciable deviations from Chapman's theory. The calculated ozone contents were considerably higher than the observed ones. Thus, there must be other chemical reactions contributing to the reduction of the ozone content. Some years later the Belgian Marcel Nicolet contributed important knowledge of how the decomposition of ozone was enhanced by the presence of the hydrogen radicals OH and HO₂.

Fig. 1. Variation in temperature and ozone concentration up through the atmosphere

The scientist to take the next fundamental step towards a deeper understanding of the chemistry of the ozone layer was Paul Crutzen. In 1970 he showed that the nitrogen oxides NO and NO₂ react catalytically (without themselves being consumed) with ozone, thus accelerating the rate of reduction of the ozone content.

NO + O₃ => NO₂ + O₂
NO₂ + O => NO + O₂
O₃ + uv-light =>O₂ + O
________________________
Net result: 2O₃ => 3O₂

These nitrogen oxides are formed in the atmosphere through the decay of the chemically stable nitrous oxide N₂O, which originates from microbiological transformations at the ground. The connection demonstrated by Crutzen between microorganisms in the soil and the thickness of the ozone layer is one of the motives for the recent rapid development of research on global biogeochemical cycles.

The first threat noted: supersonic aircraft
The power of nitrogen oxides to decompose ozone was also noted early by the American researcher Harold Johnston, who carried out extensive laboratory studies of the chemistry of nitrogen compounds. In 1971 he pointed out the possible threat to the ozone layer that the planned fleet of supersonic aircraft and supersonic travel (SST) might represent. These aircraft would be capable of releasing nitrogen oxides right in the middle of the ozone layer at altitudes of 20 km. Crutzen's and Johnston's work gave rise to a very intensive debate among researchers as well as among technologists and decision-makers. This was also the start of intensive research into the chemistry of the atmosphere which has made great progress during the past several years. (The subsequent cancellation of plans for a large SST fleet had other reasons than the environmental risks they involved.)

Spray cans and refrigerators damage the ozone layer
The next leap in our knowledge of ozone chemistry was in 1974, when Mario Molina and Sherwood Rowland published their widely noted Nature article on the threat to the ozone layer from chlorofluorocarbon (CFC) gases - "freons" - used in spray bottles, as the cooling medium in refrigerators and elsewhere and plastic foams. Molina and Rowland based their conclusions on two important contributions by other researchers:
- James Lovelock (England) had recently developed a highly sensitive device of measuring extremely low organic gas contents in the atmosphere, the electron capture detector. Using this he could now demonstrate that the exclusively man-made, chemically inert, CFC gases had already spread globally throughout the atmosphere.
- Richard Stolarski and Ralph Cicerone (USA) had shown that free chlorine atoms in the atmosphere can decompose ozone catalytically in similar ways as nitrogen oxides do.

Molina and Rowland realised that the chemically inert CFC could gradually be transported up to the ozone layer, there to be met by such intensive ultra-violet light that they would be separated into their constituents, notably chlorine atoms. They calculated that if human use of CFC gases was to continue at an unaltered rate the ozone layer would be depleted by many percent after some decades. Their prediction created enormous attention. For the CFC gases were used in many technical processes and their very chemical stability and non-toxicity were thought to render them environmentally ideal. Many were critical of Molina's and Rowland's calculations but yet more were seriously concerned by the possibility of a depleted ozone layer. Today we know that they were right in all essentials. It was to turn out that they had even underestimated the risk.

Ozone content over Antarctica
Molina's and Rowland's report led to certain restrictions on CFC release during the late 1970s and early 1980s. Not until 1985, when the real shock came, was there any real urgency in the international negotiations on release restrictions. Then the Englishman Joseph Farman and his colleagues noted a drastic depletion of the ozone layer over the Antarctic, the "ozone hole" (Fig. 2). The depletion was, at least periodically, far greater than expected from earlier calculations of the CFC effect. The debate among researchers now intensified. Was this a natural climatic variation or was it chemical decomposition brought about by mankind? Thanks to pioneering research by many researchers, among them Crutzen, Molina and Rowland, as well as Susan Solomon and James Anderson, both from the USA, the picture has now cleared. The depletion is caused chiefly by ozone reacting chemically with chlorine and bromine from industrially manufactured gases.

Fig. 2. Thickness of the ozone layer (mean monthly value for October) over Halley Bay, Antarctica. Note the drastic depletion since the end of the 1970s.

The surprisingly rapid depletion of the ozone layer over Antarctica could not be explained by transport processes or by gas phase chemical reactions. An alternative mechanism must exist which could accelerate the decomposition of ozone. Crutzen and colleagues identified this mechanism as chemical reactions on the surface of cloud particles in the stratosphere. 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" (PSCs). 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.

The ozone layer and the climate
The ozone problem also has interesting connections with the issue of how mankind is affecting the climate. Ozone, like carbon dioxide and methane, is a greenhouse gas that contributes to high temperatures at the surface of the earth. (CFC gases have a similar effect). Model calculations have shown that the climate is specially sensitive to changes in the ozone content in the lower layers, the troposphere. Here the ozone content has increased markedly during the past century, chiefly because of the release of nitric oxide, carbon monoxide and gaseous hydrocarbons from vehicles and industrial processes and from the combustion of biomass in the tropics. The elevated ozone content in lower atmospheric layers is itself an environmental problem for the damage it can cause to crops and human health. Paul Crutzen has been the world's leading researcher in mapping the chemical mechanisms that determine the ozone content at these levels.

What can we expect in the future?
Thanks to our good scientific understanding of the ozone problem (and very largely to Crutzen, Molina and Rowland) it has been possible to make far-reaching decisions on prohibiting the release of gases that destroy ozone. A protocol on the protection of the ozone layer was negotiated under the auspices of the United Nations and signed in Montreal, Canada, in 1987. Under the latest tightening-up of the Montreal Protocol, the most dangerous gases will be totally banned from 1996 (developing countries have a few years' grace to introduce substitutes that do not harm the ozone layer). Since it takes some time for the ozone-destroying gases to reach the ozone layer we must expect the depletion, not only over Antarctica but also over parts of the Northern Hemisphere, to worsen for some years to come. Given compliance with the prohibitions, the ozone layer should gradually begin to heal after the turn of the century (Fig. 3). Yet it will take at least 100 years before it has fully recovered.

Fig. 3. Change in the chlorine content in the stratosphere up to the present and three different future scenarios:
a) Without restrictions on release,
b) Limitations according to the original Montreal Protocol of 1987
c) The release limitations now internationally agreed. (Chlorine content is a measure of the magnitude of ozone depletion.)

Rodhe, H. och Bolin, B., Luftföroreningar, I Tidens miljöbok - En forskarantologi, Tidens förlag, 1992.

Rowland, F. S. and Molina, M. J., Ozone depletion: 20 years after the alarm, Chemical and Engineering News 72, 8-13, 1994
Scientific assessment of ozone depletion 1994, WMO Report 37, World Meteorological Organization and United Nations Environment Programme, Geneva, 1995.
Toon, O. B. and Turco, R. P., Polar stratospheric clouds and ozone depletion, Scientific American 264, 68-74, 1991.
Wayne, R. P. Chemistry of Atmospheres, Oxford Science Publications, 1993.
WMO and the ozone issue, World Meteorological Organization, Report 778, 1992

Professor Paul Crutzen
Max-Planck-Institute for Chemistry
P.O. Box 3060
D-55020 Mainz, Germany.

Mario Molina was born in 1943 in Mexico City, Mexico. U.S. citizen. PhD in physical chemistry, University of California, Berkeley. Member of the US National Academy of Sciences.

Professor Mario Molina
Department of Earth, Atmospheric and Planetary Sciences
MIT 54 - 1312
Cambridge MA 02139, USA

F. Sherwood Rowland was born in Delaware, Ohio, USA, 1927. Doctor's degree in chemistry, University of Chicago, 1952. Member of the American Academy of Arts and Sciences and of the US National Academy of Sciences, where he is currently Foreign Secretary.

Professor F. Sherwood Rowland
Department of Chemistry
University of California
Irvine CA 92717, USA

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The Ozone Depletion Process

The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) leak from equipment (1). Winds efficiently mix the troposphere and evenly distribute the gases. CFCs are extremely stable, and they do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere, about 10 kilometers above the Earth's surface (2).

Strong UV light breaks apart the ODS molecule. CFCs release chlorine atoms, and halons release bromine atoms (3). It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before finally being removed from the stratosphere (4).

Ozone is constantly being produced and destroyed in a natural cycle. However, the overall amount of ozone is essentially stable. This balance can be thought of as a stream's depth at a particular location. Although individual water molecules are moving past the observer, the total depeth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades.

Large increases in stratospheric chlorine and bromine, however, have upset that balance. In effect, they have added a siphon downstream, removing ozone faster than natural ozone creation reactions can keep up. Therefore, ozone levels fall.

Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the surface. The more depletion, the larger the increase in incoming UVB (5). UVB has been linked to skin cancer, cataracts, damage to materials like plastics, and harm to certain crops and marine organisms. Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem (6).

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Ozone Depletion Over North America

Ozone depletion occurs outside Antarctica. In fact, ground-based and satellite data show clear depletion over most of the world. The graph below shows average ozone levels over North America in March 1979 and March 1994. Three cities, Seattle, Los Angeles, and Miami, have been highlighted.

Source: NASA Goddard Space Flight Center

Note that in 1979, column ozone over the Seattle area was 391 dobson units (DU). In 1994, however, ozone levels had dropped to 360 DU. Los Angeles saw a similar drop, from 368 DU to 330 DU. Finally, the Miami area ozone levels fell from 303 DU to 296 DU.

In general, ozone depletion is greater at higher latitudes. Thus, the decrease near Seattle will be greater than near Los Angeles, while Miami will see the smallest depletion of the three cities. However, southern cities also have much higher incidence of UVB light; even with less depletion, the net increase in UVB can be greater. While exact calculations cannot be made from this graph, it demonstrates that the ozone layer is being damaged over much of the globe, not just over Antarctica.

It is important to note that this graph is simply an example of the kinds of observations that scientists use when they conclude that ozone depletion is occurring at midlatitudes. Different months, different years, and different regions would show different amounts of depletion. Furthermore, daily variations also change how much ozone there is over any one site. But the graph is a good example of the general downward trend observed in the protective ozone shield.

A different graph from the National Oceanic and Atmopsheric Administration (NOAA) shows the percent drop between 1979 and 1995 over the Northern Hemisphere. Large areas have seen decreases in ozone levels of as much as 40%. NOAA has a great deal of information from their 1995 Winter Bulletins. Ozone Depletion over Arosa, Switzerland

Written by EPA's Stratospheric Protection Division

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