Wednesday Aug. 29, 2007

The Practice Quiz is one week from today.  A preliminary version of the Practice Quiz Study Guide is now available online (there probably won't be many changes made between now and next week).  Note the location of the reviews held before the practice quiz aren't yet known.

The collection of old quizzes from a previous semester of this course are now available for purchase ($2.50).

You should expect to see the first 1S1P Assignment and the first Optional (Homework) Assignment soon.
We'll spend much of the period today covering stratospheric ozone.
But first there are a few things you should probably know about sulfur dioxide.

Sulfur dioxide is produced by the combustion of sulfur containing fuels such as coal.  Combustion of fuel also produces carbon dioxide and carbon monoxide.  People probably first became aware of sulfur dioxide because it has an unpleasant smell.  Carbon dioxide and carbon monoxide are odorless.

Volcanoes are a natural source of sulfur dioxide.

The Great London smog is still the deadliest air pollution event in history.  Because the atmosphere was stable, SO2 emitted into air at ground level couldn't mix with cleaner air above.  The SO2 concentration was able to build to dangerous levels.  4000 people died during this 4 or 5 day period.  The sulfur dioxide didn't kill them directly.  They had a prexising condition of some kind.  The SO aggravated their condition and contributed to their death. 

London type smog which contains sulfur dioxide and is most common during the winter is very different from photochemical or Los Angeles type smog.  Los Angeles type smog contains ozone and is most common in the summer.

Some other air pollution disaster also involved high SO2 concentrations.  The Donora Pennsylvania event is described on p. 346 in the textbook.

We'll come back to sulfur dioxide briefly in class on Friday.


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. The figure above and the figure below are found on p. 17 in the photocopied classnotes.

One way in which is destroyed naturally are shown in the figure above.  The ozone molecule is destroyed when it absorbs UV light.  The reactions in brackets shown how ozone can be destroyed by reacting with atomic oxygen or with another ozone molecule.

Once you understand how stratospheric ozone is formed you can appreciate why the peak concentrations (the ozone layer) are 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.

There is lots of UV light above 25 km but not much oxygen.  There is plenty of oxygen below 25 km but not enough UV light.  The optimum amounts of both ingredients are found near 25 km.

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.  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 [point (a)  in the figure above] remain in the atmosphere long enough that they can eventually make their way up into the stratophere.  UV light can break chlorine atoms off the CFC molecule [ Figure (b) 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 might serve as condensation nuclei for cloud droplets (the small water drops that clouds are composed of) or these gaseous products 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 following information about the ozone hole was not covered in class.


The ozone hole that forms above the S. Pole every year around 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, rather the ozone layer thins (concentration drops) significantly.


The discussion above explains how extremely cold temperatures and 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.  Basically a new series of reactions (that take place on the surface of cloud particles) interfere with the interference reactions.  The interference reactions would ordinarily keep chlorine from reacting with and destroying ozone.  Interfering with those interference reactions makes the chlorine available again to react with and destroy ozone.  Chlorine containing compounds build up during the winter and are able to destroy ozone once the sun returns in the spring.
The middle portion of Chapter 1 looks at how atmospheric characteristics such as air temperature, air pressure, and air density change with altitude.  In the case of air pressure we will spend some time trying to understand what pressure is and what can cause it to change.

We will start by looking at how air temperature changes with altitude because that is a property that were are able to feel and are probably most familiar with.


Here is a picture we started but didn't quite finish in class. (the highlighted numbers 1 - 3 were added after class to aid with the discussion  of this figure)

The atmosphere can be split into layers depending on whether temperature is increasing or decreasing with increasing altitude.  The two lowest layers are shown in the figure above.  There are additional layers (the mesosphere and the thermosphere) above 50 km but we won't worry about them.

1. We live in the troposphere.  The troposphere is found, on average, between 0 and about 10 km altitude, and is where temperature usually decreases with increasing altitude.

The troposphere contains most of the water vapor in the atmosphere and is where most of the weather occurs.  The troposphere can be stable or unstable (tropo means to turn over and refers to the fact that air can move up and down in the troposphere).

2a. The thunderstorm shown in the figure indicates unstable conditions, meaning that strong up and down air motions are occurring.  When the thunderstorm reaches the top of the troposphere, it runs into the stable stratosphere.  The air can't continue to rise into the stable stratosphere so the cloud flattens out and forms an anvil (anvil is the name given to the flat top of the thunderstorm).   The flat anvil top is something that you can see and often marks the top of the troposphere.

2b.  The summit of Mt. Everest is nearly 30.000 ft. tall and is close to the top of the troposphere.

2c.   Cruising altitude in a passenger jet is usually between 30,000 and 40,000, near or just above the top of the troposphere.

3.  Temperature remains constant between 10 and 20 km and then increases with increasing altitude between 20 and 50 km.  These two sections form the stratosphere.  The stratosphere is a very stable air layer.  Increasing temperature with increasing altitude is called an inversion.  This is what makes the stratosphere so stable.

The figure was redrawn and some additional information was added after class.


4.  10 km (kilometers) is approximately 30,000 feet or about 6 miles.

5a. Much of the sunlight arriving at the top of the atmosphere passes through the atmosphere and is absorbed at the ground.  This warms the ground.  The air in contact with the ground is warmer than air just above.  As you get further and further from the warm ground, the air is colder and colder.  This explains why air temperature decreases with increasing altitude.

5b. How do you explain increasing temperature with increasing altitude in the stratosphere.    Absorption of ultraviolet light by ozone warms the air in the stratosphere and explains why the air can warm.  The air in the stratosphere is much less dense (thinner) than in the troposphere.  It doesn't take as much energy to warm this thin air as it would to warm denser air closer to the ground.

6.  The ozone layer is found in the stratosphere (peak concentrations are found near 25 km altitude).

Point ?!  That's a manned balloon, Auguste Piccard and Paul Kipfer are inside.  They were to first men to travel into the stratosphere (see pps 31 & 32 in the photocopied Class Notes).  We'll see a short video showing part of their adventure in the next week or so.