Day 011 - 12 Jul 94 - Page 16
1 ozone layer?
A. The mechanism is the following: As has been -- I just
2 said, in the troposphere the CFCs are very unreactive
which means that once they are released into the
3 troposphere they will stay in the troposphere, essentially
not doing anything.
4
Q. They do not release their chlorine in the troposphere?
5 A. No. However, particularly in the equatorial regions
of the earth, there are strong storms and the result of
6 this is there is a weak interaction between the lower bit
of the atmosphere, the troposphere, and the upper part --
7 the next bit of the atmosphere, the stratosphere, such
that some of the material in the troposphere eventually
8 finds its way into the stratosphere, particularly close to
the equator. In other words, a long way from where many
9 of these chemicals have been initially released.
10 Once in the stratosphere they are slowly moved to very
high altitudes. Once they are above the ozone layer they
11 then are subject to the full radiation from the sun, which
is the short wave length radiation, which can then break
12 them up. Once this happens the chlorine atoms can in fact
react with other chemicals, and these chemicals are
13 eventually transported to the polar regions where they are
trapped.
14
It is in the polar regions where the chemistry occurs
15 which primarily causes ozone destruction. That chemistry
is now known to be very complicated. Basically, in the
16 polar regions, particularly the South Pole, the atmosphere
acts like a giant refrigerator. During the winter time
17 any chemicals which are transported in the way which
I have described are trapped in what is called "the polar
18 vortex", which is a rotating, basically a rotating --
almost like a column or hollow column in the poles. If you
19 were to actually look at it, it looks rather like some of
the pictures you used to get on television of a clay pot
20 on a potter's wheel. It is something which appears to be
like a cylinder going up which wobbles a little bit.
21
Once it is in there nothing very much happens until the
22 spring, when the sunshine can then start to make chemistry
happen in the polar regions.
23
The chemistry turns out to occur primarily on a particular
24 type of cloud, which is known as a polar stratospheric
cloud, which has been abbreviated as a PSC. These are
25 peculiar clouds in that they are made primarily of nitric
acid and water; in fact, it is one nitric acid with three
26 molecules of water, so it is known as nitric acid
trihydrate or NAT.
27
On the surface of these clouds, which are formed at very
28 low temperatures below approximately 200 Kelvin, which, if
you like, is minus 70 degrees celcius and below, peculiar
29 chemistry can occur which forms chlorine monoxide; it is
the chlorine monoxide and the chlorine liberated from the
30 chlorine monoxide dimer which start to attack the ozone in
the Antarctic spring.
