Friday Sep. 2, 2011
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Three songs before class this afternoon.  The first two were from Alison Krauss and Union Station ("My Love Follows You Where You Go" and "Dimming of the Day") from their 2011 Paper Airplane CD.  The 3rd song was "Sister Rosetta Goes Before Us" and is from the Robert Plant and Alison Krauss "Raising Sand" CD.

Optional Assignment #1 was collected in class today.  I'll post some answers sometime early next week (probably Tuesday morning after the T Th section of the class has turned in their papers).

Here's an idea of what we'll be doing today.

We'll finish up the section on composition of air and air pollutants today by learning about tropospheric ozone.  Then we'll start a new block of material: mass, weight, density and pressure.  Rather than covering stratopheric ozone in class, I think I will make that one of the One-Side-of-One-Page report topic choices (the assignment will probably be posted next week and announced in class before the Practice Quiz).  There were a couple of suggestions I had for people that might be looking for something to do this weekend but I erased them.


The figure above can be found on p. 14a in the photocopied ClassNotes.  Ozone has a Dr. Jekyll (good) and Mr. Hyde (bad) personality.  The ozone layer (ozone in the stratosphere) is beneficial, it absorbs dangerous high energy ultraviolet light (which would otherwise reach the ground and cause skin cancer, cataracts, etc.  There are some types of UV light that would kill us).

Ozone in the troposphere is bad, it is a pollutant.  Tropospheric ozone is a key component of photochemical smog (also known as Los Angeles-type smog)

We'll be making some photochemical smog as a class demonstration.  To do this we'll first need some ozone; we'll make use of the simple stratospheric recipe (shown above) for making ozone in the demonstration rather than the more complex tropospheric process (the 4-step process in the figure below).


At the top of this figure (p. 15 in the packet of ClassNotes) you see that a more complex series of reactions is responsible for the production of tropospheric ozone.  The production of tropospheric ozone begins with nitric oxide (NO).  NO is produced when nitrogen and oxygen in air are heated (in an automobile engine for example) and react.  The NO can then react with oxygen in the air to make nitrogen dioxide, the poisonous brown-colored gas that I've been thinking about making in class.  Sunlight can dissociate (split) the nitrogen dioxide molecule producing atomic oxygen (O) and NO.  O and O2 react in a 4th step 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 in the air, it is a secondary pollutant.   Nitric oxide would be the primary pollutant in this example.

NO is produced early in the day (during the morning rush hour).  The concentration of NO2 peaks somewhat later.  Because sunlight is needed in step #3 and because peak sunlight normally occurs at noon, the highest ozone concentrations are usually found in the afternoon.  Ozone concentrations are also usually higher in the summer when the sunlight is most intense.


Once ozone is formed, the ozone can react with a hydrocarbon of some kind to make a product gas.  The ozone, hydrocarbon, and product gas are all invisible, but the product gas sometimes condenses to make a visible smog cloud or haze.  The cloud is composed of very small droplets or solid particles.  They're too small to be seen but they are able to scatter light - that's why you can see the cloud.

Here's a pictorial summary of the photochemical smog demonstration.

We started by putting a small "mercury vapor" lamp inside a flash.  The bulb produces a lot of ultraviolet light (the bulb produced a dim bluish light that we could see but UV light is invisible so we had no way of really telling how bright it was).  The UV light and oxygen in the air produced a lot of ozone (you could easily have smelled it if you took off the cover of the flask).

After a few minutes we turned off the lamp and put a few pieces of lemon peel into the flash.  Part of the smell that comes from lemon peel is limonene, a hydrocarbon.  The limonene gas reacted with the ozone to produce a product gas.  The product gas condensed, producing a visible smog cloud (the cloud was white, not brown as shown above).  I also shined the laser beam through the smog cloud to reinforce the idea that we are seeing the cloud because the drops or particles scatter light.

Next one of the brutal breaks and changes of topic that will periodically occur in this class.  Now that we've learned about the composition of the atmosphere we will be learning about some of its other physical properties such as temperature, air density, and air pressure.  We'll also be interested in how they change with altitude.

Before we can learn about atmospheric pressure in particular, we need to review the terms mass and weight.  In some textbooks you'll find mass defined as "amount of stuff" or "amount of a particular material."  Other books will define mass as inertia or as resistance to change in motion (this comes from Newton's 2nd law of motion, we'll cover that later in the semester).  The next picture illustrates both 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 would be much harder to get moving than the VW, it has a larger inertia (it would also be harder to slow down than the Volkswagen once it is moving).

As long as we're talking about cars, here's a picture of my vehicle.  It's a 1980 Toyota Celica.



I think it was considered a sports car at one time.  I'm showing it to you so that you'll be extra careful if you see it on the road.  The windshield is often very dirty and that makes it hard to see other cars and people crossing the street (and the windshield wipers need to be replaced so I can't see very well when its raining either).



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.  On the earth's surface you determine the weight of an object by multiplying the object's mass by g.  As long as you're on the surface of the earth g has a constant value; it's called the gravitational acceleration.  I probably didn't emphasize this enough in class.



I asked a question at this point.  I would say the people that responded to the question were about evenly divided.  Equal numbers answered yes (the objects would have the same weight) and no (the weights would be different).


To determine the weight you multiply the mass by the gravitational acceleration.  Since all three objects have the same mass and g is a constant you get the same weight for each object.  Here was a follow up question:

A student responded that the two objects would have different weights if they were on different planets.  That's correct.

The example above looks at bricks on the earth and the moon.  The value of the gravitational acceleration on the moon is about 1/6th the value on the earth.  The brick would weigh a lot more than 5 pounds if you were to take it to Jupiter.

Here's a little more information (not covered in class) about what determines the value of the gravitational acceleration (Newton's Law of Universal Gravitation).

Density is the next term we need to look at.



In the first example there is more mass (more dots, which symbolize air molecules) in the right box than in the left box.  Since the two volumes are equal the box at right has higher density.  Equal masses are squeezed into different volumes in the bottom example.  The box with smaller volume has higher density.

Now we're ready to define (and hopefully understand) pressure.  It's a pretty important concept.  A lot of what happens in the atmosphere is caused by pressure differences.  Pressure differences cause wind.  Large pressure difference (such as you might find in a tornado or a hurricane) create powerful and destructive storms.



The air that surrounds the earth has mass.  Gravity pulls downward on the atmosphere giving it weight.  Galileo conducted (in the 1600s) a simple experiment to prove that air has weight.  The experiment wasn't mentioned in class.

Atmospheric pressure is determined by the weight of the air overhead.  This is one way, a sort of large scale representation, of understanding air pressure.

Pressure is defined as force divided by area.  In the case of atmospheric pressure the weight of a column of air divided by the area at the bottom of the column (as illustrated above). 

Under normal conditions a 1 inch by 1 inch column of air stretching from sea level to the top of the atmosphere will weigh 14.7 pounds.  Normal atmospheric pressure at sea level is 14.7 pounds per square inch (psi, the units you use when you fill up your car or bike tires with air).

We'll come back to this topic (and probably this picture) before the Practice Quiz next Wednesday.