Announcements • I’m gradually getting caught up on grades and course material.
Hole In The Sky: Ozone Part II What did you get out of Monday’s lecture?
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 an old web link to that file. For 2012 try this.
Chapman Reactions The presence of ozone in the atmosphere was first explained in a 1930 paper by Sydney Chapman. We’ll discuss these reactions in detail in the slides that follow. uvc: 100-280 nm uvb: 280-315 nm
Chapman Reactions A uvc photon strikes an oxygen molecule in the stratosphere, and “splits” it into two oxygen atoms. uvc: 100-280 nm
Chapman Reactions The two oxygen atoms combine with two oxygen molecules in the atmosphere to form two ozone molecules. uvc: 100-280 nm
Chapman Reactions A uvb photon strikes an ozone molecule and “splits” it into an oxygen molecule and an oxygen atom. uvc: 100-280 nm uvb: 280-315 nm
Chapman Reactions The “freed” oxygen atom can combine with an ozone molecule to form two oxygen molecules. uvc: 100-280 nm uvb: 280-315 nm
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. uvc: 100-280 nm uvb: 280-315 nm
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-dependent, with different temperature dependencies for each. About 90% of atmospheric ozone is in a narrow band at an altitude of about 25-30 km.
Because each reaction has a different temperature dependence, ozone ends up concentrated in a thin layer in the stratosphere. There’s not much ozone up there! If you brought all the ozone from the stratosphere down to the surface of the earth… you’d have a layer of ozone gas only 3 mm thick!
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.
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.
In the 1920’s, 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 Buisson’s 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.
Because ozone concentration is highly temperature-dependent, it depends on the season and on geographical location. Dobson’s 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.
At about the time of Dobson’s 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 next class). Go to http://www.archive.org. In the WAYBACK MACHINE box, type http://www.word.com/machine/dray. Click “Take Me Back.” Read the February 24, 2001 article. Print out ONLY the last page and turn it in (with your name on it, of course). The pages may be 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 can’t go “deeper!” Be sure to click on all the links.
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.
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 1970’s. Earth-based measurements were supplemented beginning in the 1970’s by data from the Nimbus satellites.
You may recall from the Hole in the Sky video that CFC’s (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). You’ll learn more about CFC’s when you work on the homework assignment I just gave you. British scientist James Lovelock devised a detector to measure atmospheric CFC’s, and began research using it in 1970.
For environmental issues, I believe it’s 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,” and “The Revenge of Gaia.” He also supports the use of nuclear energy. *But don’t let your disagreement with their personal issues cause you to automatically discount their science!
Whether or not you “like” what Lovelock believes in, you may owe him your life. Lovelock discovered CFC’s 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 CFC’s there too. There’s only “one” source of CFC’s. Guess who? You can’t blame nature for CFC’s. You can’t say they’ve been around for thousands of years.
Lovelock asked the British government for a small amount of money to fund CFC studies in Antarctica.
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
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 can’t imagine a more useless bit of knowledge than finding the atmospheric concentration of CFC-11.” So, using his own money, Lovelock measured CFC’s in the Antarctic atmosphere—and found they were everywhere… …even though there weren’t great masses of humans there to put CFC’s in the Antarctic atmosphere. Conclusion: if you spray CFC’s into the atmosphere, they could end up anywhere…and stay there for a long time.
But that’s OK, because CFC’s 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 CFC’s and chlorocarbons (CC’s) were dissociated by ultraviolet light in the stratosphere to produce ozone-destroying chlorine atoms and chlorine monoxide.
As your homework reading points out, CFC manufacturers quickly (and correctly) pointed out that there was no proof that CFC’s depleted the ozone… …and Rowland and Molina also (and correctly) pointed out that CFC manufacturers were hypothesizing it was safe to put CFC’s 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.
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… Where’s the beef? Sorry, you young folks won’t appreciate that… but so far I’ve been telling a story. Is there any data?
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?
I’ll get back to the data in a minute, but first I’ll remind you of the smoking/cancer controversy (nothing personal if you choose to smoke). None of us here remember, but during World War II the US government strongly encouraged smoking. Cigarettes and chocolate—the 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 …don’t 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.
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.
In the case of ozone depletion, the mechanism was established (Rowland and Molina) before the depletion data were* observed. CFC’s 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 don’t when it sounds awkward.
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. Here’s a little movie that shows what Rowland and Molina discovered. (Here used to be an external link.) Is there data suggesting that such ozone destruction is actually taking place? Let’s go back to the data I showed earlier.
I see periodic peaks and valleys (a few peaks indicated). They seem to happen once a year each. I also see random (???) fluctuations.
Yes, I see the 1973-1993 decreasing trend, but let’s go back to 1984 (and try to remove the possibly misleading trend line).
Sending myself back to 1984, I say “this needs watched…still.”
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. 2008 & 2012: got to here