Friday, Sept. 1, 2006

A copy of the Practice Quiz Study Guide was handed out in class.

Hurricane John is near the southern tip of Baja California.  The National Hurricane Center is currently predicting it will travel along the west coast of Baja California and then turn toward the west.  Some models are predicting paths that would carry the storm and remnants of the storm into the Gulf of California and even into the western edge of Arizona.  There is a chance that moisture from Hurricane John could move into Arizona and produce heavy rain and flooding.  The chances of this occurring are very uncertain at the present time.  You can keep track of the latest weather advisories at the Tucson National Weather Service web page.
Tropospheric ozone is a pollutant.  Tropospheric ozone is also a key component of photochemical smog.



The production of tropospheric ozone begins with nitric oxide (NO).  NO is produced when nitrogen and oxygen are heated (in an automobile engine for exampe) and react.  The NO can then react with oxygen to make nitrogen dioxide, a poisonous brown-colored gas.  Sunlight can dissociate (split) the nitrogen dioxide molecule producing atomic oxygen (O) and NO.  O and O2 react (just as they do in the stratosphere) to make ozone (O3).  Because ozone does not come directly from an automobile tailpipe or factory chimney, but only shows up after a series of reactions, it is a secondary pollutant.   The nitric oxide would be an example of a primary pollutant.

NO is produced early in the day.  The concentration of NO2 peaks somewhat later.  Peak ozone concentrations are usually found in the afternoon.  Ozone concentrations are also usually higher in the summer than in the winter.  This is because sunlight plays a role in ozone production and summer sunlight is more intense than winter sunlight.

As shown in the figure below, invisible ozone can react with a hydrocarbon of some kind which is also invisible to make a product gas.  This product gas sometimes condenses to make a visible smog cloud or haze.


The class demonstration of photochemical smog is summarized below (a flash was used instead of the aquarium shown on the bottom of p. 16 in the photocopied class notes).  We begin by using the UV lamp to fill the flask with ozone.  Then a few pieces of fresh lemon peel were added to the flask.  A whitish cloud quickly became visible (colored brown in the figure below).


For the remainder of today's class and also next Friday (after the Practice Quiz on Wednesday) we return to the middle part of Chapter 1 and will look at how characteristics such as air temperature, pressure, and density vary with changing altitude in the atmosphere.  We'll start with temperature.


 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 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 using decreases with increasing altitude.

 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.

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 in the stable stratosphere so the cloud flattens out and forms an anvil. 

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.

4.  10 km (kilometers) is approximately 30,000.    At nearly 30,000 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.

5.   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.

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, it has a larger inertia (it would also be harder to slow down if it were already moving).

We tend to use weight and mass interchangeably because we spend all our lives on earth where gravity never changes.  Next we can explore the concept of mass a little more by considering two equal volumes of different materials.


One student suggested that the bottle of mercury above (bottles of water and mercury will be passed around in class next Friday) might contain more atoms than the bottle of water even though the volumes are the same.  We will see that this is actually true.  But even if the two bottles contained the same volumes and the same numbers of atoms or molecules of water and mercury, the masses would be very different.  There is another more basic difference between water and mercury.