Thursday Aug. 31, 2006

The Practice Quiz is one week from today.  In anticipation of that event, a preliminary version of the Practice Quiz Study Guide is now available online.  There may be some small changes made between now and early next week.  Locations of the reviews will be added to the study guide once they are known.  Next Tuesday I'll distribute a photocopy of the Study Guide.

The packet containing old quizzes and an old final exam is now available for purchase ($2.50).  You'll find sample questions on the Study Guide that come from quizzes in this collection.

Tropical storm Ernesto is moving through Florida and into the SE United States.  Meanwhile in the Pacific, off the west coast of Mexico, Hurricane John is now a category 3 hurricane (it reached category 4 strength yesterday).  Hurricane John may influence our weather by the end of the week.

A short new reading assignment was made.
We covered sulfur dioxide, the first of three air pollutants, on Tuesday.  You'll find lots of detailed information about air pollutants in Tucson and Pima County at the Pima County Department of Environmental Quality webpage.  The US Environmental Protection Agency also has a large amount of information about this topic.  Today we'll start with carbon monoxide and then cover tropospheric ozone and photochemical smog.



Carbon monoxide (CO) is a colorless, odorless, toxic gas.  It is a primary pollutant that results from incomplete combustion (complete combustion would produce carbon dioxide).  The highest CO concentrations are observed on winter mornings.  CO is trapped in stable morning surface inversion layers that form on winter mornings.


Concentrations of several pollutants are measured daily in many cities (particulate matter, ozone, and carbon monoxide are monitored in Tucson) and measured values are reported in the newspaper or on television using the Air Quality Index (formerly the pollutant standards index).  This is basically the measured value divided by the allowed value multiplied by 100%.   Current Air Quality Index values for Tucson are available online.


While CO concentrations in the atmosphere are of concern, even higher, potentially fatal, levels of carbon monoxide can quickly build up inside a house or apartment if gas-burning appliances aren't operating properly or aren't adequately vented to the outside.  You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.



This rather busy and confusing picture just illustrates how small changes in how air temperature changes with increasing altitude can determine whether the atmosphere will be stable or unstable.   Just for the purposes of illustration we imagine riding a bicycle from Swan and River Rd up a hill to Swan and Sunrise (fhe figure shows an elevation change of 1000 ft, it is actually quite a bit less than that)

At far left the air temperature drops 6o F.  This is a fairly rapid drop with increasing altitude and would make the atmosphere absolutely unstable.  The atmosphere wouldn't remain this way.  Air at the ground would rise, air above would sink, and the temperature profile would change.  In some ways it would be like trying to pour vinegar on top of oil in a glass.  The lower density oil would rise because it would "want" to float on top of the higher density vinegar.

The next picture shows air temperature decreases a little more slowly with increasing altitude.  This small change makes the atmosphere conditionally unstable (we won't go into the conditions).  The atmosphere is frequently in this state. 

The atmosphere cools only 2o F in the next picture.  This creates an absolutely stable atmosphere.  Air at the ground will remain at the ground and won't rise and mix with air higher up.  Compare this with the glass containing vinegar and a layer of oil on top.  The two layers won't mix.

Air temperature in the last figure actually increases with increasing altitude   This is a temperature inversion and produces very stable conditions.


It is relatively easy to make ozone in the statosphere.  We will make use of this simple two step reaction for our demonstration in class today.

At this point, to prepare for the photochemical smog demonstration, a small mercury vapor lamp was inserted into a large 4 liter flask.  The lamp emits a lot of (invisible) ultraviolet radiation and is used to produce ozone inside the flask.  The flask was sealed with foil so that the ozone couldn't escape.  The glass walls of the flask should absorb the dangerous UV radiation.  But just to play it safe the flask was covered with a black cloth.  The ozone will be used later in the class to make photochemical smog.

We will see in the next figure that ozone production in the troposphere is a little more complicated.




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).
The following figure wasn't shown in class.  It is included here to motivate the next section of material that we will be covering.


For the remainder of today's class and also next Tuesday 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.