Monday Feb. 18, 2008

The 2nd Optional Assignment was collected in class today.  A handout with answers to the questions was distributed in class.

The Experiment #1 reports have been graded and were returned.  You now have two weeks to revise your report if you wish to.  Revised reports are due on Mon., Mar. 3.  Please return your original report with your revised report.  You only need to rewrite sections where you want to earn additional credit.

The 1S1P Topic #1 reports were returned today.  Topic #2 reports are starting to come back from the Teaching Assistants so they should be returned soon. 

The first real quiz of the semester is on Wednesday this week.  Reviews are scheduled for Mon. and Tue. afternoons from 4-5 pm in FCS 225. The quiz will cover material from both the Practice Quiz and the Quiz #1 Study Guides.
We'll spend much of the period today covering stratospheric ozone.
Don't spend a lot of time studying this new material.  At most there might only be a part of a question or perhaps an extra credit question on stratospheric ozone or the ozone hole on this week's quiz.  The figure below is from p. 17 in the photocopied Classnotes.


Stratospheric ozone forms naturally when UV light splits oxygen molecules (O2) into two oxygen atoms (photodissociation).  The O atoms can then react with unsplit O2 to make O3 ozone.  This is the reaction we used to make ozone in the photochemical smog demonstration.

There are also natural processes that destroy ozone.  The ozone molecule is destroyed when it absorbs UV light and prevents the UV light from reaching the ground.  Ozone can also be destroyed by reacting with atomic oxygen or with another ozone molecule.

The ozone layer concentration would fluctuate up and down until the natural processes of production and destruction balance each other.  Once balance occurs the ozone layer concentration would remain constant.  The green box at the bottom of the figure above represents this natural ozone layer concentration.  Man is adding additional processes of destruction.  These will have the effect of lowering the ozone layer concentration (symbolized above by a smaller orange box drawn within the green box).

Once you understand how stratospheric ozone is formed you can appreciate why the ozone layer is found not at the bottom or top of the atmosphere but at some level in between (at around 25 km), where there are optimal amounts of oxygen and UV light, the two ingredients needed to make ozone.

The ozone layer is centered around 25 km in the middle of the stratosphere.  There is plenty of UV light at higher altitudes, but not enough oxygen.  Oxygen is plentiful at lower altitudes, but UV light is in short supply.

Stratospheric ozone, the ozone layer, absorbs much (but not all) of the dangerous high energy ultraviolet light from the sun.  Listed above are some of the serious hazards or problems associated with exposure to ultraviolet light.

Human activities add substances to the atmosphere that can potentially reduce ozone concentration in the ozone layer (which would result in increased exposure to UV light at the ground). 

The first set of reactions above involve nitric oxide, NO.  First, NO reacts with O3 to form NO2 and O2 (ordinary molecular oxygen).  Then notice the NO2 reacts with an oxygen atom (which might otherwise react with O2 to make O3) to form NO again and O2. The NO is available again to react with and destroy another ozone molecule.

At one time many countries were considering building fleets of supersonic aircraft that would fly into the stratosphere.  The plans were scrapped partly due to concern that the NO emissions from these aircraft would damage the ozone layer.

The main threat now comes from chlorofluorocarbons (CFCs).  The reactions involving CFCs have been copied onto the next figure.

CFCs were at one time thought to be an ideal industrial chemical and had a variety of uses.  CFCs are unreactive, non toxic, and stable.  Once they get into the atmosphere they remain there a long time, as much as 100 years.

CFCs released at ground level [lower left corner in the figure above] remain in the atmosphere long enough that they can eventually make their way up into the stratophere.  UV light can then break chlorine atoms off the CFC molecule [ upper left portion of the figure above].  The resulting "free chlorine" can react with and destroy ozone.  This is shown in (1) above.  Note how the chlorine atoms reappears at the end of the two step reaction.  A single chlorine atom can destroy 100,000 ozone molecules.

There are ways of removing chlorine from the atmosphere.  A couple of these so called "interference reactions" are shown in (2) above.    The reaction products, reservoir molecules (because they store chlorine),  might serve as condensation nuclei for cloud droplets (the small water drops that clouds are composed of) or might dissolve in the water in clouds.  In either event the chlorine containing chemical is removed from the atmosphere by falling precipitation.  Clouds are probably the most effective way of cleaning the atmosphere.

The ozone hole that forms above the S. Pole every year in late September-early October was one of the first real indications that CFCs could react with and destroy stratospheric ozone.  The hole is not really a hole in the ozone layer, just a temporary thinning of the ozone layer above the S. Pole and the continent of Antarctica.  The ozone concentration decreases to perhaps 30% of its normal value.

It is unusual to find clouds in the stratosphere.  It gets very cold above the S. Pole in the winter and polar stratospheric clouds do sometimes form (they are made from water and other materials). This together with an unusual wind pattern above the S. Pole in the winter are thought to create the ozone hole when the sun returns in the spring.

The ozone destruction reactions are shown in purple above.  Cl reacts with O3 to make ClO.  This reacts with O to produce Cl and O2.  The Cl is now available to react again with other ozone molecules.

In green are "interference" reactions.  ClO reacts with NO2 to make ClNO3.  The Cl in this "reservoir" molecule can't react with any more ozone.

Now what happens above the S. Pole in the winter is that the reservoir molecules react on the surfaces of the polar stratospheric cloud particles to make some kind of new compound.  This reaction is shown in orange above.  The new compound HOCl accumulates in the air during the winter.  When the sun reappears in the spring, the UV light splits off all the Cl molecules which react with ozone.  A lot of chlorine suddenly becomes available and the ozone concentration takes a nosedive.
We spent the last few minutes of the period viewing the first successful non-stop trip around the globe in a balloon.  Bertrand Piccard (Jacques' son, Auguste's grandson) was part of the two man that flew the balloon.  The next page are some notes written down during that video.

There were two balloons in the air at the same time.  The lead balloon (the Cable and Wireless balloon) crashed in the Sea of Japan (the balloon became coated with ice in a snow storm and lost its bouyancy).  The second balloon (the Breitling Orbiter 3) balloon avoided any mishaps.  The Breitling Orbiter 3 was launched on Mar. 1, 1999 and completed its trip on Mar. 20, so it took nearly three weeks to circle the globe.