lunar eclipse March
3, '07 imaged in Germany by Eva Seidenfaden (atmospheric
optics site) . The blue at the edge of the Earth's
umbral shadow is produced by ozone in Earth's stratosphere.
In addition to being a strong ultraviolet absorber, ozone absorbs
red light. The
upper part of the moon is illuminated by refracted rays that
have made a long slanting passage through our stratosphere. The
reds towards the
umbra centre are from light refracted through the denser troposphere. Image
©Eva seidenfaden, shown with permission.
As we climb beyond the tropopause,
temperatures start to increase again. We are now in the stratosphere,
a region extending from a nominal 15 km (9 mile) up to the stratopause
at 50 km (31 mile).
Oxygen molecules, O2,
iIn the upper stratosphere absorb short wavelength ultraviolet
radiation (<200 nm) and
dissociate into highly reactive oxygen atoms*.
The atoms diffuse through the stratosphere and at heights of
mostly 30-50 km many eventually combine** with
more oxygen molecules to produce the reactive oxygen allotrope,
Ozone, O3 ,is
a strong absorber of longer wavelength (200-340 nm) UV radiation
and the absorbed energy heats the atmosphere. The ozone
layer is responsible for the stratosphere's increasing temperature
Without ozone, mixing between the troposphere and stratosphere
would be much faster and the structure of our atmosphere
The ozone layer prevents harmful UV from reaching
surface and is partly responsible for the deep blue-violet
beauty of the twilight sky.
|Oxygen atoms have
quite different properties to oxygen molecules.
of O atoms with an oxygen molecule requires a collision
involving a third body to remove excess energy and momentum
otherwise the newly formed ozone molecule would almost
immediately decompose. The third body, 'M' in the diagram,
can be a nitrogen or another oxygen molecule. In the rarified
conditions of the stratosphere two body collisions are
infrequent and three body ones even more so. Ozone formation
is therefore slow.
maximum is at ~50km where the low density air requires
very little energy to raise its temperature. The greatest
ozone densityis at 20-25km because the ozone
destruction processes are slower there. Ozone is destroyed
by several catalytic chain reactions involving O2,
NO and, at lower altitudes, HO2 radicals. Man-made
chlorine, bromine and nitrogen compounds are also potent
ozone destruction catalysts.