Nobel prize in chemistry 1995

From nobelsrv@www.nobel.ki.seThu Oct 12 21:03:26 1995
Date: Thu, 12 Oct 1995 20:08:39 +0100
From: Nobel Foundation WWW Server 

              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.

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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 consequenses.

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.

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
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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.

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Further reading
Dotto, L. and Schiff, H. I., The ozone war, Doubleday, Garden City, New York,
1978.
Graedel, T. E. and Crutzen, P.J., Atmosphere, climate and change. Scientific
American Library, 1995.
Gribbin, J., The hole in the sky, Corgi Books, London, 1988.
Ozonskiktet, Faktablad fren Naturverdsverket, Februari 1992

Rodhe, H. och Bolin, B., Luftfvroreningar, I Tidens miljvbok - En
forskarantologi, Tidens fvrlag, 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

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Paul Crutzen was born in 1933 in Amsterdam. Dutch citizen. Doctor's degree in
meteorology, Stockholm University, 1973. Member of the Royal Swedish Academy of
Sciences, the Royal Swedish Academy of Engineering Sciences and Academia
Europaea.

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. 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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