ÐÓ°ÉÔ­´´

Over the past 20 months, researchers have been making ready for the
start of probably the largest ever European environmental science project.
Next week over 250 scientists from more than 60 research groups will begin
taking and interpreting measurements from some 20 ground sites, balloons,
rockets and aircraft. Nearly all the countries of the European Community
and the European Free Trade Association are represented; scientists from
the US, Japan, New Zealand and the Soviet Union are also involved. In addition,
there will be collaboration with the second US Airborne Arctic Stratospheric
Expedition, and with the Canadian Arctic Research Programme. Combined with
data from NASA’s Upper Atmosphere Research Satellite, launched in September
this year, this research will give the teams a chance to observe the lower
stratosphere of the Arctic in unprecedented detail.

The goal of the European Arctic Stratospheric Ozone Experiment (EASOE)
is a better understanding of stratospheric ozone and how it is destroyed
around the Arctic. In the past few years, ozone levels in the northern hemisphere
have decreased, both around the Arctic and closer to the Equator. The loss
is not as severe as the 60 per cent loss found in the springtime Antarctic
stratosphere, where ozone loss was first documented, but it is happening
over areas where a lot of people live.

In the 1970s, researchers had speculated that certain compounds – nitrogen
oxides from the exhausts of supersonic planes and chlorine from CFCs used
in aerosol sprays, for example – might destroy ozone in the stratosphere
(see Box for an explanation of these early theories). No evidence was forthcoming
until 1985, when observations made by Joe Farman, Jonathan Shanklin and
Brian Gardiner of the British Antarctic Survey demonstrated that this disquieting
possibility was indeed fact. Satellite data from NASA’s Total Ozone Mapping
Spectrometer (TOMS) confirmed that the loss came each Antarctic spring.

When the Antarctic ozone hole was discovered, many researchers thought
that chlorine was to blame. But as details emerged, the atmospheric chemists
had to think again as they realised that other cycles of reactions could
be more effective at destroying ozone. Ozone was found to be depleted in
the lower stratosphere (between altitudes of 15 and 20 kilometres), while
theory had suggested that the greatest loss would occur at around 40 kilometres.
The depletion was also happening much faster than predicted. Since 1985,
several US expeditions to the Antarctic have established how the ozone has
been lost.

It is now clear that ozone in the Antarctic is destroyed mainly in a
catalytic cycle involving the chlorine monoxide dimer (ClO)2, which consists
of two chlorine monoxide radicals bound together. This cycle of reactions
does not involve oxygen atoms, which have very low concentrations in the
stratosphere. Another cycle involving both chlorine monoxide and bromine
oxide is probably also important.

In the Antarctic, ozone is particularly vulnerable to destruction by
the dimer cycle because of the unique weather. In winter and spring, the
atmospheric circulation there is dominated by westerly winds blowing parallel
to the lines of latitude in what is called zonal flow. These winds effectively
isolate the area around the pole, so that the Antarctic winter is particularly
cold, with temperatures often as low as -85 °C in the lower stratosphere.
A further consequence of this weather pattern is that molecules that contain
chlorine molecules build up in the skies above the Antarctic.

These low temperatures favour the formation of the chlorine monoxide
dimer and so help to generate reactions that destroy ozone in the lower
stratosphere. When sunlight returns in the spring, the cycles of reactions
involving the dimer and bromine and chlorine monoxide quickly destroy ozone.
The cold also helps the formation of a particular type of cloud – known
as a polar stratospheric cloud – that plays an important part in ozone loss.
Reactions on the surfaces of cloud particles convert less reactive chlorine
compounds, such as hydrogen chloride and chlorine nitrate, into more reactive
species that can destroy ozone.

The tiny particles in the clouds also remove water vapour and oxides
of nitrogen from the stratosphere – processes called dehydration and denitrification.
Denitrification is an important part of the ozone story; without it, the
oxides of nitrogen could react with chlorine monoxide and stop the dimer
cycle from operating efficiently. The zonal Antarctic winds have another
effect on ozone: there is little mixing of air from south to north, so that
ozone destroyed in the spring is not replenished to any great degree by
air from further north.

The circulation and temperature of the stratosphere in the northern
hemisphere differ from the south, mainly due to interhemispheric differences
in the distribution of mountains and of land and sea. In the north, the
atmospheric circulation is much less zonal, so that the polar stratosphere
does not cool as much; on average, it is about 10 degrees warmer than the
Antarctic in the winter. Ozone destruction will in consequence be less efficient
in the north. And because there is far more movement of air from north to
south around the Arctic, loss of ozone at the pole could be ameliorated
by less-depleted air moving in from farther south.

On the other hand, air at low temperatures in the Arctic night could
still be primed for destruction by polar stratospheric clouds, then move
south into sunlight where chemical reactions could destroy the ozone. So
the meteorology of the lower stratosphere in the northern hemisphere may
lead to less ozone destruction at the pole itself than in the south, but
there may also be a decrease spread over more of the hemisphere.

SATELLITE CONFIRMATION

These expectations are borne out by recent measurements of ozone levels
by the TOMS on the Nimbus 7 satellite. The data show much smaller ozone
losses over the North Pole than over the South Pole (perhaps 10 per cent
compared to 60 per cent), but the losses at middle latitudes in each hemisphere
are comparable. Indeed, the largest ozone losses at middle latitudes – around
30 to 60 degrees north or south – are in the northern hemisphere. At 40
degrees north, roughly the latitude of Madrid and New York, stratospheric
ozone has dropped by about 8 per cent in the past decade during the late
winter and early spring.

Why are ozone levels changing in the northern hemisphere? As in the
south, the finger points towards chlorine compounds; there are, for example,
high concentrations of active chlorine compounds in the stratosphere above
the Arctic. But researchers do not know exactly how the losses at lower
latitudes arise. It could be because air that has lost ozone moves to middle
latitudes from the Arctic or because air primed for ozone loss at the pole
moves, then the destruction reactions happen locally. Ozone could be destroyed
at middle latitudes through priming reactions on aerosols there, perhaps
produced after volcanic eruptions, or there could be other processes. Some
combination of processes is most likely.

But whatever the cause, the decrease in ozone in the north represents
a worrying trend: despite controls on the production and emission of chlorine
compounds such as the CFCs, concentrations of these compounds in the stratosphere
will continue to increase until at least the end of the century. If chlorine
chemistry is indeed to blame, we can look forward to depletions significantly
greater than the 8 per cent already reported.

With these observations as their basis, the EASOE teams will conduct
a large number of different but complementary studies. They will concentrate
on various aspects of the problem, together comprising a campaign that will
run for nearly five months and cover a large area of the northern hemisphere.

One set of measurements will focus on the movement of the atmosphere
around the North Pole. This circulation carries air from the middle stratosphere
slowly down to the lower stratosphere, then spreads away from the pole.
As indicated above, the concentration of ozone at any one point varies,
partly because of this continual movement of air. If we can determine the
circulation at high latitudes, we may be able to disentangle the dynamics
of the atmosphere – which alter ozone concentration by rearranging packets
of air – from the chemical processes which may be destroying the ozone itself.
The circulation is very slow and so cannot be measured directly. Instead,
researchers will infer the strength of the circulation from measurements
of long-lived gases which are good tracers of atmospheric motion.

Many of these measurements will made from balloons launched into the
stratosphere from the Swedish Space Corporation’s base near Kiruna in northern
Sweden, the operational centre for the campaign. A team from France’s National
Space Studies Centre in Aire sur l’Adour will launch the balloons, some
as big as 100 000 cubic metres and capable of carrying payloads of 500 kilograms,
up to heights of 30 kilometres. The great advantage of these balloons is
that they allow measurements to be made throughout the lower stratosphere,
between 10 and 30 kilometres up. This is far higher than the most advanced
research aircraft can travel. But the strength of the stratospheric winds
means that flights can last only a few hours; otherwise researchers would
never be able to recover the instruments.

Although balloons are invaluable sources of data, they do present some
problems. The frequency of launches is at the mercy of the weather on the
ground, which can be severe at Kiruna. And balloons cannot be steered, but
must follow the prevailing wind. The data that the balloons will provide
once launched, however, makes them worthwhile. They can carry a range of
instruments collecting data that would otherwise be unobtainable. For example,
steel flasks that collect samples of gases at various altitudes will be
collected from the balloons after their flight, so that researchers can
analyse their contents using mass spectrometry back in the laboratory. This
will yield concentrations of unreactive gases, including carbon disxide
and nitrous oxide; reactive gases such as nitrogen dioxide and chlorine
monoxide will have reacted in the flask, so measured concentrations will
be meaningless, but the balloons can use other techniques to measure concentrations
of this type of gas.

One class of instrument measures the radiation from the Sun in a part
of the spectrum where several species known to be in the atmosphere absorb
radiation. By making measurements at different solar elevations, the instruments
find their concentrations. Other instruments measure the minute amounts
of infrared radiation emitted by molecules in the atmosphere. Some balloons
will also carry mass spectrometers able to make measurements of concentration
in situ. Another technique, chemiluminescence, will assess the concentration
of a particular species from the intensity of the light emitted in a reaction
between the target and a compound carried by the balloon.

Some instruments will be carried into the stratosphere each week, giving
regular measurements of long-lived gases such as nitrous oxide and CFCs,
which are good tracers of atmospheric transport, and of water vapour, nitrogen
dioxide and ozone. In addition, information about how atmospheric dynamics
and chemistry evolve over the five months will come from measurements at
ground stations scattered in and around the Arctic and Europe, covering
an area from Greenland to the northern USSR and stretching as far south
as Greece. Ozone and nitrogen dioxide will be measured at 12 sites, for
example, using ultraviolet and visible light spectrometers. The data collected
can be used to infer the abundance of these species in the atmosphere above
the site of the instrument. Some of these spectrometers will also measure
absorption by chlorine dioxide and bromine monoxide, good indicators of
the extent of the reactions thought to be responsible for the destruction
of ozone. Concentrations of ozone low in the Arctic stratosphere will be
monitored in unprecedented detail throughout the campaign. We will supplement
the usual ground-based measurements of ozone with more than a thousand measurements
of its profile through the atmosphere from 21 ground stations. Each site
will launch ozonesondes – balloons roughly a metre across that carry detectors
able to measure ozone through an electrochemical reaction. This type of
instrument responds quickly to changes in ozone levels. It provides accurate
ozone readings, precisely linked to height. Combined with the analyses of
weather and information on atmospheric circulation provided by other measurements,
the ozonesonde data should, at the very least, provide persuasive clues
to whether ozone is being chemically depleted.

Ultimately, we hope to launch as many as 50 large balloons from Kiruna.
Some of the particularly complex instruments, such as those that measure
the very small amounts of radiation characteristic of certain trace molecules,
will need longer periods floating at their assigned altitude. Collaboration
with Soviet scientists means that for the first time the balloon instruments
will be allowed to fly over the Soviet Union, where they will be recovered
from the Kola Peninsula. This gives perhaps twice as much time for data
collection, making possible some measurements that would otherwise be of
only marginal value. Soviet scientists are also working with a number of
instruments included on the payloads of the balloons, to measure water vapour,
ozone and properties of polar stratospheric clouds.

Polar stratospheric clouds will be the focus of another major component
of the campaign. This will involve observations from instruments based on
the ground in the Arctic circle, from balloons and rockets launched at Kiruna,
and from aircraft. Among the ground-based instruments will be lidars, systems
that direct laser light into the atmosphere and measure the light scattered
back, in this case by polar stratospheric clouds. The time taken for a pulse
of laser light to be scattered and returned gives the clouds’ altitude.
Other instruments can pick up the concentrations of water vapour, nitric
acid and oxides of nitrogen, believed to be important in reactions on polar
stratospheric clouds, as well as in dehydration and denitrification.

Besides being the centre of balloon activities, Kiruna will be the base
for a number of airborne missions using three research aircraft, two supplied
by Germany and one by France. These will carry an array of remote-sensing
instruments. The big advantage of using aircraft is the mobility and flexibility
they offer. ÐÓ°ÉÔ­´´s can fly them when and where they think that the most
exciting measurements can be made, for example, in regions where polar stratospheric
clouds may be forming when it is particularly cold. Once researchers have
found these clouds, they can send the planes upwind and downwind of them,
to see what changes happen before and after they form.

Researchers at Kiruna will direct the day-to-day running of the mission.
All the experimental results and detailed meteorological information will
be assembled at a data centre at the Norwegian Institute for Air Research
in Oslo, and sent to Kiruna by a fast electronic link. The group at Kiruna
will suggest periods when, for example, more frequent ground-based measurements
would be useful, and indicate the best times for balloon flight or the most
useful routes for the aircraft.

Theoretical studies form the final component of the project. A number
of groups based at Kiruna will run models of atmospheric chemistry and transport
as winter proceeds. The experiments should provide the modellers with detailed
information about the processes in the Arctic atmosphere as they happen.
As a result, they hope to develop a new generation of models capable of
predicting the evolution of the atmospheric ozone layer through a season
and over time periods lasting decades.

While the experiments end in March next year, that is not the end of
the project. Analysis of the data will continue for months and, in some
cases, years afterwards. The experiments will provide new data that should
reveal valuable and exciting information about the processes controlling
the evolution of the lower stratosphere. It will be a period of sustained,
concerted effort to monitor all the processes contributing to ozone loss
in the northern hemisphere. The scientists involved hope for many advances
from this innovative combination of data, experiment and theory.

But by its very nature, the EASOE project already marks a significant
success: the coming together of European atmospheric scientists in an endeavour
in which national and European Community funds have been pooled to build
an enterprise well beyond the scope of any single country.

John Pyle is head of the European Ozone Research Coordinating Unit based
at the chemistry department of the University of Cambridge and the British
Antarctic Survey in Cambridge.

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TIPPING THE OZONE BALANCE

The abundance of ozone in the atmosphere arises from a balance between
its production and destruction. Ozone forms when ultraviolet radiation in
sunlight splits oxygen molecules (O₂) into reactive oxygen atoms.
These promptly combine with another oxygen molecule to make ozone, O₃.

Many reactions destroy ozone in the stratosphere. Nitric oxide, the
hydroxyl radical, chlorine and bromine atoms all behave as catalytic species
that destroy ozone in a two stage process. First, the catalyst removes one
oxygen atom from ozone to form its oxide and an oxygen molecule. If the
catalyst is, for example, chlorine, this reaction produces chlorine monoxide.
Then the oxide loses its oxygen to another oxygen atom, and the catalyst
is reformed; chlorine monoxide reverts to chlorine and oxygen.

The net effect of these two reactions is to convert an oxygen atom and
an ozone molecule into two oxygen molecules, with a molecule of the catalyst
ready to start the cycle again. Here lies the problem with adding such compounds
to the atmosphere; each molecule can destroy ozone many times over.

Because the balance between ozone destruction and creation is dynamic,
if there is an increase in the concentration of a catalyst, the rate of
destruction of ozone increases until a new balance is achieved at a lower
ozone concentration.