The Ozone Hole 1995

WMO Antarctic Ozone Summary for 1995

Remarks

 An important source of column ozone measurements from space is the Total Ozone Mapping Spectrometer (TOMS), which when combined with measurements from NOAA's TIROS Operational Vertical Sounder (TOVS) and ground-based measurements results in highly accurate images with complete global coverage. Unfortunately, TOMS data were not available during the 1995 ozone hole period therefore the resulting ozone analysis for 1995 is somewhat less comprehensive.

 Meteorological conditions

 While the minimum temperatures between mid September and November were somewhat lower than in the previous decade, the area sufficiently low to produce PSCs was very similar to recent years. However, in October the PSC area was somewhat larger than usual and in general PSC temperatures continued longer than any year since 1987. From June to mid July the vortex area was smaller than usual, particularly on the lower levels where the area remained quite small until the end of October. In November the vortex area remained nearly constant rather than the usual decrease observed during this period.

 Ozone observations

 This year the maximum area of the ozone hole was about 23 million km², practically identical to 1992 and 1994, and again the largest on record. Furthermore, the monthly averages of October and November were the greatest on record. The ozone hole dissipated in early December, similar to the long lasting ozone holes of 1987, 1990, 1992 and 1993, and reflects the continued strength of the vortex in November. In early October there was a large area with column ozone values up to 60% below pre-ozone hole norms. The maximum area with ozone values more than 50% below pre-ozone hole norms was 10 million km², somewhat below the maximum level of 1994 (12 million km²). Nevertheless, the area in 1995 was at least twice as great as all other years. Some ground stations measured their lowest daily ozone values ever, with South Pole measuring column ozone nearly 70% below pre-ozone hole norms (85 DU). This remains the lowest ozone value ever observed in Antarctica as of 2002. The ozone hole shape through late October was rather circular but later became more elongated.

 Conditions over the populated regions

 Until mid October the southern tip of South America experienced ozone values up to 20% below norms several times, and Ushuaia measured ozone hole values for two days in mid October. During November, minimum column ozone values observed in this region were 30% below pre-ozone hole norms. Over New Zealand and southern Australia nearly normal ozone values were observed during the entire ozone hole period.  

British Antarctic Survey summary 1995-Preliminary mean daily ozone values dropped from values of around 280 Dobson Units (DU) at the beginning of August to around 175 DU at the end of September (50% depletion). Individual daily values dropped as low as 132 DU. The first week of October saw a major spring warming event, with a rise in mean total ozone to over 300 DU as the circumpolar high ozone belt moved across the station. Mean values then fell back to around 190 DU in mid October before rising again to over 300 DU in a second warming event at the end of October. Mean values dropped back again to 200 DU in early November before slowly rising to around 315 DU in the final warming event of the year in mid December. Values slowly declined from the December peak to around 260 DU at the end of April.

Generally values in the 95/96 season were a little above those reached in 1993 and 1994 during the first half of the season and below during the second half. The final spring warming was much later than in either year. Values were significantly below those of the long term mean throughout the season.

Press Release

11 October 1995

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.

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 ultraviolet 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 an 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.

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.)