|1/ What is ozone?
is a gas* made of oxygen
atoms. Usually oxygen atoms hang around in pairs
- this is the sort of oxygen that we breathe and that
helps things to burn. Oxygen sometimes however will
form a molecule with three oxygen atoms, this is what
we call ozone:
- two oxygen
atoms - ordinary common or garden oxygen
three oxygen atoms - Ozone
has a the particularly useful characteristic that it
can absorb large quantities of ultra-violet (uv) light
- more of that soon.
Jack Flash is also a gas (ask your Dad)
2/ Where is the ozone?
the ozone on earth isn't on earth at all, but in the
layer of the earth's atmosphere called the stratosphere.
This is the upper layer of the atmosphere and starts
between 12.9 to 19.3 km (8 to 12 miles) above our heads
and goes upwards to almost 50 km (30 miles). The stratosphere
has virtually no clouds or other form of weather, it's
thinnest at the equator and thickest at the poles.
is formed in the stratosphere by the action of sunlight
on oxygen molecules. In particular it is the high
energy ultra-violet light in sunlight that is effective,
it causes an oxygen molecule to split into two oxygen
---------> O + O
these then joins with another oxygen molecule to form
a molecule of ozone:
O + O2
may also be destroyed by joining with a lone oxygen
atom to get back to oxygen again. Ultra-violet light
is required for ozone to form in the stratosphere, but
then the ozone absorbs the ultra-violet light so stopping
it reaching deeper into the earth's atmosphere. The
result is that levels of ozone are greatest at around
20km up. This is good news for us as it stops lots
of ultra-violet light getting through to us and also
keeps the ozone high up in the atmosphere out of the
3/ Isn't ozone bad
news? I'm sure I saw something on the weather forecast....
Ironically, at ground level ozone
is very bad news. It is a major component of photochemical
smog. It is caused by the effect of ultra-violet
light on nitrogen oxide from vehicle exhausts and so
particularly affects built up areas in regions of high
Ozone affects lung function, it can
aggravate asthma and other chronic respiratory tract
and lung diseases and can reduce lung function in the
short term or even permanently on repeated exposure.
Ozone has an effect like sunburn on the lining of the
respiratory tract damaging the cells.
4/ Why is a ozone hole
Ozone in the stratosphere is nicely
out of the way and has the wonderful benefit to life
on earth that it specifically absorbs the harmful ultra-violet
light from the sun while letting other light wavelengths
talk about a "hole" in the ozone layer,
it's not really a hole at all, just a thin
bit. Ozone is spread thinly throughout the
stratosphere - and in low quantities too
- if all the ozone above your head was
collected together in a continuous layer,
it would only be about 3-5mm (1/8 of an
to think of the ozone as being like orange
squash in a glass of water where the water
is the rest of the atmosphere. When we talk
about an "ozone hole" we actually mean a
region where the squash is more diluted
than we'd like it to be.
If there is a hole in the ozone layer
then this means that more harmful ultra-violet rays
get through than are good for us or many other life
forms, plant or animal. Too much ultra-violet light
can result in:
- Eye damage such as cataracts
- Immune system damage
in phytoplankton in the oceans that forms the basis
of all marine food chains including those in Antarctica.
to the DNA in various life-forms So far this has
been as observed in Antarctic ice-fish that
lack pigments to shield them from the ultra-violet
light (they've never needed them before)
other things too that we don't know about at the
5/ Why does a ozone
hole form over Antarctica?
The ozone hole
is caused by the effect of pollutants in the atmosphere
destroying stratospheric ozone. During the Antarctic
winter something special happens to the Antarctic weather.
winds blowing around the continent form, this is
known as the "polar vortex" - this isolates the
air over Antarctica from the rest of the world.
Secondly, special clouds
form called Polar Stratospheric Clouds. Clouds don't
normally form in the stratosphere and these turn out to
have the effect of concentrating the pollutants that break
down the ozone, so speeding the process up.
stratospheric clouds at about 80,000 feet altitude.
These are the highest flying of all clouds and only
occur in polar regions where the temperature in
the upper atmosphere dips below minus 100F. They
are sometimes called "nacreous clouds" as they are
coloured like the nacre of mother-of-pearl with
coloured bands that move the position of cloud and
More pictures of Polar Stratospheric Clouds
By the time
spring arrives and the sun comes back after the long polar night,
the ozone levels are severely depleted around the Antarctic
continent causing the "ozone hole". Unfortunately, there then
follows a particularly long period of high sunshine and long
days, just to make the effect of the ozone hole worse.
of ozone in the atmosphere is measured in "Dobson Units",
the average concentration of ozone in the atmosphere is about
300 Dobson Units. The ozone hole is considered to be wherever
the concentration drops below 220 Dobson Units.
following pictures are provided courtesy of NASA. They show
the extent of ozone thinning. Dark blue and purple colors
correspond to the thinnest ozone, while light blue, green, and
yellow pixels indicate progressively thicker ozone.
October 1999 (average)
the Antarctic ozone hole was largest during October.
In recent years however, September has been the peak
September 7th 2000
The ozone hole grew quicker than usual and exceptionally
large. By the first week in September the hole was the
largest ever at that time. For the first time it reached
towards South America and to regions of high population.
September 2006 average
From September 21 to 30, 2006, the average area
of the ozone hole was the largest ever observed, at
10.6 million square miles. A little over a week later,
instruments recorded the lowest concentrations of ozone
ever observed over Antarctica, making the ozone hole
the deepest it had ever been.
The ozone hole builds up over
the winter months, peaking at around September and breaking
up again by December, this data set from 2005.
6/ What causes the ozone depletion?
is mainly broken down by chemicals called ChloroFluoroCarbons
CFC's and also by nitrogen
oxides. CFC's ironically were first used in large quantities
because they were thought to be safe and inert (unreactive)
chemicals. They are a group of chemically similar gases
used in refrigeration systems, air conditioners, aerosols,
solvents and in the production of some types of packaging.
Nitrogen oxides are a by-product of fuel burning, e.g. aircraft
CFC's don't occur naturally,
they are man-made chemicals. They are very useful when they
are where they are supposed to be, and doing what they are supposed
to be doing. But once released into the atmosphere they are
a serious pollutant. The problem is it took us many years to
realise this during which we thought they were perfectly harmless,
but in fact were building up to levels that will take decades
for them to disappear again even if we stop producing them altogether.
actual reactions that destroy ozone are very complicated. They
take place on the surface of the ice particles of the Polar
Stratospheric Clouds and it takes
only a small amount of CFC to destroy an awful lot of ozone.
7/ Is the ozone hole
going to stay over Antarctica?
the annual thinning of the ozone layer over Antarctica was first
discovered, measurements have been carried out in all regions.
has been measured everywhere in the world except in the tropics.
Depletion is usually worse the further from the equator and
recently an Ozone hole (as defined by a distinct area of very
low ozone levels) has been detected above the North Pole in
There is a lot
to learn about the breakdown of ozone in the atmosphere. Warmer
region, non polar depletion of ozone in particular is not properly
So for the time being the "ozone hole"
seems to be an Antarctic phenomena, but a less severe thinning
of the ozone layer is pretty much a world-wide thing. How acute
and important it will be in the future is not known.
8/ Can the
ozone hole recover?
The way to stop the formation, growth and
spread of ozone thinning is to reduce the production of those
chemicals that cause the destruction of ozone, namely CFC's
and nitrogen oxides.
In 1987, the Montreal Protocol
was signed by many nations whereby those nations that signed
agreed to reduce their emissions of CFC's to a half (of the
1987 levels) by 2000.
Potential problems come from
nations that do not see the reduction of CFC's to be a priority,
and also from the huge quantity of refrigeration and air conditioning
systems in the world that still contain CFC's. If they are not
disposed of correctly, then the CFC's will escape into the atmosphere
and continue to destroy ozone.
problem is far from settled and is under investigation by research
teams all over the world. The latest estimates are that as long
as production and release of CFC's is regulated properly, global
ozone levels should recover by 2050.