Thu., Aug. 30 notes

Thursday Aug. 30, 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.

Here is a little more material concerning stratospheric ozone.

This figure illustrates destruction of stratospheric ozone (thinning of the ozone layer) by chlorofluorocarbons (CFCs). 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 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. 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 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.

Now we really need to move into the middle portion of Chapter 1. Before doing that we will learn go back to sulfur dioxide and learn a little bit about acid rain.

Some of what we learned about sulfur dioxide in class last Tuesday is listed above. Some more information about acid rain (from p. 12 in the photocopied Class Notes) is given below.

Note that clean unpolluted rain has a pH less than 7 and is slightly acidic. This is because the rain contains dissolved carbon dioxide gas. Acid rain is often a problem in regions that are 100s even 1000s of miles from the source of that sulfur dioxide that forms the acid rain.

Some of the problems or consequences of acid rain.

A short colorful acid rain demonstration was done in class. Carbon dioxide gas was used instead of sulfur dioxide.

Now we're finally ready to start the middle portion of Chapter 1 and look 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.

There's a lot of information on this figure. We'll work through it number by number (the numbers were added after class). With a little work you should be able to start with a blank sheet of paper and recreate this figure on your own.

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 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.

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).

7. That is a manned balloon. Auguste Piccard and Paul Kipfer were the 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 two.

What follows is a little more detailed discussion of the basic concepts of mass, weight, and density than was done in class.

Before we can learn about atmospheric pressure, we need to review the terms mass and weight. In some textbooks you'll find mass defined at the "amount of stuff." Other books will define mass as inertia or as resistance to change in motion. The next picture illustrates both these definitions. A Cadillac and a volkswagen have both stalled in an intersection. Both cars are made of steel. The Cadillac is larger and has more steel, more stuff, more mass. The Cadillac is also much harder to get moving than the VW, it has a larger inertia (it would also be harder to slow down if it were already moving).

It is possible to have two objects with the same volume but very different masses. Here's an example:

Bottles containing equal volumes of water and mercury were passed around in class (thanks for being careful with the bottles of mercury). The bottle of mercury was quite a bit heavier than the bottle of water.

Weight is a force and depends on both the mass of an object and the strength of gravity.

We tend to use weight and mass interchangeably because we spend all our lives on earth where gravity never changes.

Any three objects that all have the same mass would necessarily have the same weight. Conversely

Three objects with the same weight would have the same mass.

The difference between mass and weight is clearer (perhaps) if you compare the situation on the earth and on the moon.

If you carry an object from the earth to the moon, the mass remains the same (its the same object, the same amount of stuff) but the weight changes because gravity on the moon is weaker than on the earth.

Now back to the bottles of water and mercury. The different weights told us there were different masses. The volumes were equal, how can we account for the differences in mass?

Mercury atoms are built up of many more protons and neutrons than a water molecule (also more electrons but they don't have nearly as much mass as protons and neutrons). The mercury atoms have 11.1 times as much mass as the water molecule. This doesn't quite account for the 13.6 difference in density. Despite the fact that they contain more protons and neutrons, the mercury atoms must also be packed closer together than the molecules in water.

Definition and illustrations of high and low density.