Wed., Jan. 17, 2007

The Experiment #1 materials were handed out today.  Reports will be due on Monday Feb. 5.  It may take several days to collect the data, so don't wait until the last minute to begin this experiment.  Once you have collected the data you need, return the materials and pick up the supplementary information sheet that will help with the analysis portion of your report.  You'll find a little more information about Experiment #1 at the end of today's online notes.

Some additional reading was assigned from Chapter 1. 

We finished up the material on CO2, global warming and climate change that was started last Friday.  You'll find that material together with the rest of the Fri., Jan. 12 notes.

The next section in Chapter 1 deals with the vertical structure of the atmosphere.  We will look at how characteristics such as air temperature, pressure, and density vary with changing altitude in the atmosphere.  We'll start with temperature because that is the property that you are most familiar with.

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.  (the numbers 1 - 6 were added after class to aid with the discussion  of this figure)

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.

2. 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).  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). 

3.  Temperature remains constant between 10 and 20 km and then increases with increasing altitude between 20 and 50 km.  These two sections comprise 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.  Thinner inversion layers are sometimes found at ground level. When pollutants are emitted into a shallow surface inversion layer, the pollutant concentrations can build and reach unhealthy levels.

4.  10 km (kilometers) is approximately 30,000 feet.    At nearly just over 29,000 feet altitude, the summit of Mt. Everest is near the top of the troposphere.  Commercial aircraft fly at cruising altitudes between 30,000 and 40,000 feet.  This is right at the boundary between the top of the troposphere and the bottom of the stratosphere.

Most 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 higher up and further from the ground (in the troposphere anyway). 

6.  How do you explain increasing temperature with increasing altitude in the stratosphere.  The ozone layer is found in the stratosphere (peak concentrations are found near 25 km altitude).  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.

This is probably a good place to learn more about ozone in the stratosphere and in the troposphere.

Ozone plays both beneficial and detrimental roles in our atmosphere.  Most of the ozone is found in the stratosphere where it absorbs dangerous high energy ultraviolet (UV) light. 

Ozone is toxic and, when found in the troposphere (where we live), it is hazardous (particularly to people with existing respiratory disease).  Tropospheric ozone also reacts with hydrocarbons in the air to form photochemical smog.

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.

Three ways in which is destroyed naturally are shown in the figure above.  The absorption of UV light by either oxygen or ozone is what protects us from this dangerous component in sunlight.

Once you understand how stratospheric ozone is formed you can appreciate why the peak concentrations 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.

As mentioned earlier, the Experiment #1 materials were handed out today.  We spent the last 10 minutes of class learning a little bit about Experiment #1.  This should be helpful to the students actually working on the experiment and will give other students a better idea of what lies ahead when they will be performing an experiment and writing a report.

With this and the other experiments you will receive most or all of the materials you need to complete the experiment, a description of what should go into your report, instructions that tell you how to perform the experiment, and a data collection sheet.

The object of Experiment #1 is to measure the percentage concentration of the oxygen in air (key words are underlined and should appear in the report title).  Basically you moisten a piece of steel wool, stick the steel wool into a graduated cylinder, and turn the cylinder upside down and immerse the open end in a cup of water.  In that way you seal off the air sample (in the cylinder) from the rest of the air in the atmosphere.

As the next figure shows air pressure will keep water out of the cylinder if you just try to lower the open end of the cylinder into the cup of water.

You need to insert a flexible piece of tubing into the cylinder and then lower the cylinder into the water.  The water will now rise into the cylinder.  You want the water to go just far enough into the cylinder that its level can be read on the cylinder scale.  Then be sure to remove the tubing.

With time the oxygen in the air sample in the cylinder will react with the wet piece of steel wool to form rust.  This removes oxygen from the air sample.  As oxygen is removed, the water level will rise (the air sample volume will decrease).  You will need to use the ideal gas law to explain (in your report) why this occurs.

The reaction between the oxygen and the steel wool sometimes happens in a day or two.  Other times it may take several days.  You will periodically need to record the time and the air sample volume ( you read the water level on the cylinder scale).  Be sure you do not lift the open end of the cylinder out of the water.  That would break the seal and you would need to restart the experiment.

Eventually the air sample volume will stop changing; all of the oxygen has been removed from the air sample and the experiment is over.   You will receive a supplementary information sheet when you have returned your materials.  You don't have to return the rusty piece of steel wool - throw it away.  Don't worry about trying to clean the rust stains off the inside of the cylinder.