nearingzero (nz288.jpg)

1. http://www.nearingzero.net (nz288.jpg)

2. Physics 6 Schedule Lab meets Friday! “Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.”—A. Einstein

3. Announcements • Homework #7: let the Wayback Machine take you to January 17, 1999. Homework #7 can be turned in Wednesday.

4. Reviewing the last few slides from the previous lecture…

5. Yes, I see the 1973-1993 decreasing trend, but let’s go back to 1984 (and try to remove the possibly misleading trend line).

6. Sending myself back to 1984, I say “this needs watched…still.”

7. 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.

8. Many resources show the Arosa data (which I also showed you) because it represents the longest continuously-monitored series of ozone measurements. Remember, the Arosa spectrometer measures the total column of ozone above the observing station, so it is sensitive to stratospheric ozone. Ozone-monitoring stations were also established in Antarctica, and satellite measurements began in the 1970’s.

9. As we monitored the ozone layer during the late 1970’s and early 1980’s, new research suggested it was premature to believe there was a problem. Large amounts of nitrogen oxides, such as NO2, have been released by the combustion of fossil fuels. Nitrogen oxides also participate in catalytic reactions that should destroy ozone. Early calculations (ca. 1970) showed that ozone depletion should have been far more severe than was then observed. Clearly, some aspect of atmospheric chemistry was not well understood.

10. Another puzzle is the neutralization of chlorine by nitrogen oxides. Chlorine and nitrogen oxides react and short-circuit the ozone-destroying process. The interaction reduces, but does not completely eliminate, the ability of chlorine to destroy ozone. As research continued, more chlorine reactions were discovered. These reactions result in “chlorine reservoirs,” in which chlorine is tied up for long periods of time, reducing the destructive power of chlorine.

11. Examples of reactions that produce chlorine reservoirs: ClO + NO2 ClONO2 Cl- + CH4  HCl (For the chemists in the audience, these are not meant to be balanced reactions!) After the chlorine reservoirs were discovered, it was concluded that CFC’s should have minimum effect on the ozone.

12. But in 1984, British Antarctic Survey scientists Jonathon Shanklin, Joseph Farman, and Brian Gardiner reported the alarming discovery that Antarctic ozone levels were 35% below the 1960’s average.

13. Their report was initially dismissed. “Your data are clearly wrong. Go back and measure it again.” Their data were not supported by satellite measurements. Shanklin and co-workers double-checked. They were right. The satellite data were also right. The analysis of the satellite data was flawed!

14. The computer programs that analyzed the satellite ozone data were programmed to discard “outliers.” Actual ozone levels were so low that they were treated as outliers and “neglected!”* The 1985 discovery of the ozone “hole” was such a shock that it triggered incredibly rapid international reaction. *The actual story is a bit more complicated. See here and here for starters.

15. In 1987 the Montreal Protocol required CFC production to be cut by 50% by 1998. As the evidence for ozone depletion mounted, so did the response; in 1990 the London Amendment to the Montreal Protocol required CFC production to cease by 2000. The 1992 Copenhagen amendment required CFC production to cease by 1996. I think it is safe to say that most of those who have been following the ozone situation believe the ozone layer will recover if the nations of the world abide by these protocols.

16. A look ahead: Global Warming. Define “Global Warming.” What will be necessary to convince you that global warming is happening?

17. Click here for a link to the full-sized picture.

19. It is easy to find skeptics who believe there is no ozone “hole,” or that there is no reason to be alarmed by ozone depletion. I suggest that it is healthy to practice skepticism. I suggest that it is unhealthy to practice skepticism in the face of data to the contrary. Some ozone depletion *myths: *See the link for details.

20. “Volcanoes and sunspots are the cause of ozone depletion.” The claim is not supported by data!

21. “CFC’s are heavier than air, and can’t reach the stratosphere.” The claim might be valid if there were no motion in the atmosphere (which is untrue), and is not supported by data!

22. “Ozone depletion occurs only over Antarctica.” False. Period.

23. “There is no link between ozone depletion and higher uv levels.” False!

24. There is a massive amount of data showing ozone depletion.

25. The British Antarctic Survey is an excellent source of data. You can also read NASA’s Ozone Watch.

26. One more subject before I go back to Energy. Why is “the” ozone hole over Antarctica? What about the Arctic? The chemistry of polar atmospheric clouds is complex and still not fully understood. However, enough is known to understand the mechanism for ozone depletion. Polar stratospheric clouds are not like the tropospheric clouds that we all know.

27. The unique geography of Antarctica results in the formation of a “polar vortex.” During the Antarctic winter, the cold air circulates like a giant bathtub vortex. Polar stratospheric clouds do not form by uplifting of warm, moist air. Rather, they form slowly, during the long polar winters, and require long periods of isolation of their air masses before temperatures become low enough for them to form. Here is a movie. Here is an external link (if you are viewing this lecture from a computer other than mine).

28. Although much remains to be learned, it is evident that large quantities of the ClOOCl dimer (notice it is made of two molecules of ClO) build up during the Antarctic winter. When spring returns and sun shines on the Antarctic, the ClOOCl is rapidly converted into chlorine ions: ClOOCl + UV  Cl- + ClO2 ClO2  Cl- + O2. These reactions rapidly produce huge concentrations of Cl-, which triggers massive destruction of ozone.

29. As spring proceeds and the polar vortex breaks down, the stratospheric clouds vaporize, and the ozone-destroying reactions slow down. The ozone “hole” dissipates as ozone-containing air from the warmer latitudes mixes with the polar air. Because this mixing dilutes ozone concentrations from the rest of the atmosphere, and carries ozone-destroying chlorine away from the polar regions to the rest of the atmosphere, the antarctic ozone hole can effect the atmosphere elsewhere. You can find hundreds of images illustrating the Antarctic ozone hole. (Birth and death of the 1997 Ozone Hole.)

30. “We” got a wake-up call when an Arctic hole appeared.

31. More animations. Arctic, 2000. Antarctic, 2003. (External link.) (External link.)

32. Final thoughts: Where are we now? • The year’s Antarctic hole recently closed. This year’s Arctic hole is likely growing.

33. Wild Cards for the future? Global Warming? • Ozone depletion and global warming are two different issues, except… • The same greenhouse gases that may (or may not) cause global warming may (or may not) have caused a measurable decrease in stratospheric temperatures. • Decreasing stratospheric temperatures enhances the formation of stratospheric clouds, and leads to increased ozone depletion.

34. Why all this time spent on Ozone depletion? • The Process is interesting. • The Process is important. • Human choices will determine who lives, and under what conditions. • We were lucky this time. Will we be lucky next time?