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Physics 6 Schedule
Feb. 13 Ozone Depletion Video (104 Phys.) Feb. 15 Video Discussion Energy Ozone
Feb. 20 Ozone II Feb. 22 Cool Activity
Feb. 27 Ozone III Issue Topic Due March 1 Energy Thermodynamics
March 6 Video (104 Physics) March 8 Video Discussion Thermodynamics
No Lab Friday!
If youre making mistakes less than 10 of the
time, youre not making enough mistakes.unknown
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Hole In The Sky Ozone Part II
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Ozone Creation and Destruction in the Stratosphere
O2 molecule absorbs uv radiation and splits into
two oxygen atoms.
Oxygen atoms are highly reactive.
Oxygen combines with O2 molecules to form O3,
ozone.
It takes lots of energy (uv radiation, high
voltage discharge) to break up an O2 molecule,
and O3 forms naturally when an oxygen atom
encounters an O2 molecule. Movie here.
The movie link above requires a file in the same
folder as this lecture. Here is a web link to
that file.
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Chapman Reactions
The presence of ozone in the atmosphere was first
explained in a 1930 paper by Sydney Chapman.
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Chapman Reactions
A uvc photon strikes an oxygen molecule in the
stratosphere, and splits it into two oxygen
atoms.
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Chapman Reactions
The two oxygen atoms combine with two oxygen
molecules in the atmosphere to form two ozone
molecules.
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Chapman Reactions
A uvb photon strikes an ozone molecule and
splits it into an oxygen molecule and an oxygen
atom.
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Chapman Reactions
The freed oxygen atom can combine with an ozone
molecule to form two oxygen molecules.
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Chapman Reactions
There are two formation reactions, one of which
results in production of ozone,
There are two formation reactions, one of which
results in production of ozone, and two
depletion reactions, which remove ozone.
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Chapman Reactions
O2 uvc ? 2 O (oxygen sacrifices
itself for us)
2 O 2O2 ? 2 O3 (ozone production)
O3 uvb ? O2 O (ozone sacrifices
itself for us)
O O3 ? 2O2 (ozone stolen from us)
The third and fourth reactions remove ozone from
the stratosphere.
All four reactions are highly temperature-dependen
t, with different temperature dependencies for
each. About 90 of atmospheric ozone is in a
narrow band at an altitude of about 25-30 km.
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Because each reaction has a different temperature
dependence, ozone ends up concentrated in a thin
layer in the stratosphere.
Theres not much ozone up there!
If you brought all the ozone from the
stratosphere down to the surface of the earth
youd have a layer of ozone gas only 3
mm thick!
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Alternative views of the ozone profile. The
picture on the left came directly off a long-lost
(to me) web page. The pictures are not drawn to
the same scale.
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Between 1913 and 1930, French scientists Fabry
and Buisson developed a technique for measuring
the amount of ozone in a vertical column of the
atmosphere.
This can be done by measuring the absorption of
uvb and uvc radiation by the atmosphere.
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In the 1920s, British experimentalist G M B
Dobson set out to explain why the temperature in
the atmosphere decreases as you go up through the
troposphere, but increases in the upper part of
the stratosphere.
He hypothesized that the temperature increase was
due to heat generated when O2 and O3 absorbed uv
photons.
Starting from Fabry and Buissons ideas, he
developed a spectrophotometer to measure ozone
concentration.
He hoped to be able to predict weather, but
instead became interested in seasonal ozone
concentrations.
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Because ozone concentration is highly
temperature-dependent, it depends on the season
and on geographical location.
Dobsons measurements established that the ozone
in a column of the atmosphere is equivalent, on
the average, to a layer about 3 mm thick at the
surface of the earth.
Ozone concentration is reported in Dobson units.
A DU is one milli-atmosphere-centimeter
(unconventional units!). Dobson measured the
average atmospheric ozone concentration to be 300
DU.
3 mm is about half the diameter of your pen or
pencil.
In liquid form, this O3 layer would be about
0.003 mm thick.
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At about the time of Dobsons work, Chapman was
figuring out his reactions (which I showed you
earlier).
Life was good (except for the Great Depression).
We understood how the ozone layer formed, and how
it protects us from extinction
and clever people were inventing many of the
modern conveniences of life which we take for
granted.
Homework Assignment 7 (due in one week). Go to
http//www.archive.org. In the WAYBACK MACHINE
box, type http//www.word.com/machine/dray. Click
Take Me Back. Read the October 21, 2000
article. Print out ONLY the last page and turn it
in (with your name on it, of course).
The pages are very slow to load. A few pictures
and animations are missing. Be patient! How do
you know when your are on the last page? When you
cant go deeper!
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Did anybody wonder why I started out these
lectures on ozone by mentioning Antarctica?
In 1957, scientists around the world participated
in a remarkable series of observations as part of
the International Geophysical Year.
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The effort was remarkable because it was an
international effort focused on science and not
torn apart by international politics.
Research stations were established in Antarctica
and used to study worldwide ice conditions,
weather, and the atmosphere.
A number of research stations began making
regular ozone measurements. They observed
seasonal variations in atmospheric ozone, with an
annual pattern that was unchanged through the
late 1970s.
Earth-based measurements were supplemented
beginning in the 1970s by data from the Nimbus
satellites.
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You may recall from the Hole in the Sky video
that CFCs (chlorofluorocarbons) are wonderfully
useful compounds.
They are especially wonderful because they are
harmless to living organisms, and last a long
time (like decades or even hundreds of years).
Youll learn more about CFCs when you work on
the homework assignment I just gave you.
British scientist James Lovelock devised a
detector to measure atmospheric CFCs, and began
research using it in 1970.
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For environmental issues, I believe its worth
knowing about scientists as people, and not just
look at their work by itself.
James Lovelock is one of the main ideological
leaders in the history of the development of
environmental awareness.
He has written books such as The Age of Gaia,
in which he argues the earth is a superorganism.
He also supports the use of nuclear energy.
But dont let your disagreement with their
personal issues cause you to automatically
discount their science!
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Whether or not you like what Lovelock believes
in, you may owe him your life.
Lovelock discovered CFCs in the air over Ireland
coming from the direction of London.
He studied air over the North Atlantic
(uncontaminated at the time by recent urban
pollution, and found CFCs there too.
Theres only one source of CFCs. Guess who?
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Lovelock asked the British government for a small
amount of money to fund CFC studies in Antarctica.
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Those of you who end up as scientists will find
that the peer review process for papers is
friendly, but the process for getting money is
a bit more ... rigorous
Reviewer
You
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Lovelock asked the British government for a small
amount of money to fund CFC studies in Antarctica.
One of the reviewers commented Even if such a
measurement succeeds, I cant imagine a more
useless bit of knowledge than finding the
atmospheric concentration of CFC-11.
So, using his own money, Lovelock measured CFCs
in the Antarctic atmosphereand found they were
everywhere
even though there werent great masses of humans
there to put CFCs in the Antarctic atmosphere.
Conclusion if you spray CFCs into the
atmosphere, they could end up anywhereand stay
there for a long time.
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But thats OK, because CFCs are (really!)
harmless to humans.
Now comes an astonishingly rapid chain of events.
1971 Two papers by Crutzen proposing that
catalytic reactions involving nitric oxide and
nitrogen dioxide are a major ozone destruction
mechanism. This was part of the Supersonic
Transport debate.
1973 Stolarski and Cicerone discover chlorine
species that catalytically destroy ozone.
1974/1975 Mario Molina and F. Sherwood Rowland
propose that the nearly inert CFCs and
chlorocarbons (CCs) were dissociated by
ultraviolet light in the stratosphere to produce
ozone-destroying chlorine atoms and chlorine
monoxide.
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As your homework reading points out, CFC
manufacturers quickly (and correctly) pointed out
that there was no proof that CFCs depleted the
ozone
and Rowland and Molina also (and correctly)
pointed out that CFC manufacturers were
hypothesizing it was safe to put CFCs in the
stratosphere, without any proof they were right.
1976 the National Academy of Science releases
its report verifying the Rowland-Molina finding.
Remarkably, the theoretical work was taken
seriously enough (even though data was spotty)
that rapid changes followed, starting with
elimination of trivial CFC uses.
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1976 The Food and Drug Administration and the
EPA announce a phaseout of CFCs in aerosols.
1978 CFCs used in aerosols are banned in the
United States.
1978-1984 we (scientists) watched and waited
Wheres the beef?
Sorry, you young folks wont appreciate that but
so far Ive been telling a story. Is there any
data?
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A Dobson spectrometer (as shown on the previous
slide) has been in use continually at Arosa,
Switzerland, since 1926, measuring atmospheric
ozone. What do YOU make of this?
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Ill get back to the data in a minute, but first
Ill remind you of the smoking/cancer controversy
(nothing personal against those students I see
taking their last puffs before our class).
None of us here remember, but during World War II
the US government strongly encouraged smoking.
Cigarettes and chocolatethe two greatest treats
a GI in the foxhole or a sailor in the engine
room could enjoy in those dark times.
Those of you who smoke today could be dont take
this choice of words the wrong way byproducts of
that encouragement.
But later, we found correlations between smoking
and cancer.
Occurrences of cancer were far greater in the
smoking population than the non-smoking
population.
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The tobacco companies argued, and I have to admit
that at the time their argument was valid that
there was no scientific connection between
smoking and cancer.
I walked through the door, and the chandelier
fell down.
Did my act of walking through the door cause the
chandelier to fall?
If you want to prove the answer is yes, you
must establish a mechanism that connects walking
through the door with the falling chandelier.
Convincing mechanisms connecting smoking and
cancer have since been thoroughly established.
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In the case of ozone depletion, the mechanism was
established (Rowland and Molina) before the data
were observed.
CFCs in the stratosphere can be split by the
uv radiation which is abundant there
CFC uv ? Cl- other atoms
Chlorine is highly reactive, and participates in
the following reactions
Cl- O3 ? ClO O2 O ClO ? Cl- O2
Data is plural. I should always use data are
and are there data. I dont when it sounds
awkward.
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Cl- O3 ? ClO O2 O ClO ? Cl- O2
Notice that chlorine removes both ozone molecules
and oxygen atoms (which is also bad) but is
itself unchanged by the sequence of reactions.
Chlorine atoms act as catalysts and a single
chlorine atom can destroy thousands of ozone
molecules.
Heres a little movie that shows what Rowland and
Molina discovered. (Here is an external link.)
Is there data suggesting that such ozone
destruction is actually taking place? Lets go
back to the data I showed earlier.
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I see periodic peaks and valleys (a few peaks
indicated). They seem to happen once a year each.
I also see random (?) fluctuations.
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Yes, I see the 1973-1993 decreasing trend, but
lets go back to 1984 (and try to remove the
possibly misleading trend line).
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Sending myself back to 1984, I say this needs
watchedstill.
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Why does this situation need watched?
We have a dangerous situation.
We have a mechanism causing the situation.
We have taken steps to correct the situation.
(Banning of trivial CFC uses, such as
propellants.)
Are our steps working?
We must watch and wait.