Fri., Sep. 3 notes

Friday Sep. 3, 2010
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The start of a 3-day weekend seemed like a good place for a couple of songs from the Flobots ("Handlebars" and "By the Time You Get This Message").

All of the names on the Report Signup sheets should now be on the Online Lists. If you name isn't there and you think it should be, let me know. Sorry about any misspelled names.

Next Monday is a holiday. There'll be a Practice Quiz review next Tuesday afternoon from 4-5 pm in Haury 129 (aka Anthropology 129). The Practice Quiz is next Wednesday (it'll start around 2:20 pm).

Now that we have finished the section on air pollutants, here's a list of the key points for each of the pollutants that we covered.

carbon monoxide (CO) colorless, odorless primary pollutant incomplete combustion winter, morning pollutant temperature inversion layer	tropospheric ozone (O₃) secondary pollutant summer, afternoon pollutant Los Angeles - type (photochemical smog)
sulfur dioxide (SO₂) 1st pollutant London - type smog acid rain	particulate matter (PM) health hazard affects visibility

Before moving into a new and important section on air pressure, we took a moment to cover a little new material on stratospheric ozone. Stratospheric ozone (the ozone layer) absorbs dangerous high-energy ultraviolet light. Earlier this week we covered tropospheric ozone which is a pollutant and a key ingredient in photochemical smog.

This topic is covered on pps. 17-19 in the photocopied ClassNotes.

The top two equations show how ozone is produced in the stratosphere. Ultraviolet (UV) light splits an O2 molecule into two O atoms (photodissociation). Each of the O atoms can react with O2 to make O3 (ozone).

Ozone is destroyed when it absorbs UV light and is split into O and O2 (the two pieces move away from each other and don't recombine and remake ozone). O3 is also destroyed when it reacts with an oxygen atom (thereby removing one of the "raw ingredients" used to make ozone). Two molecules of ozone can also react to make 3 molecules of O2.

The bottom part of the figure attempts to show that the ozone concentration in the stratosphere will shift up and down until the natural rates of production and destruction balance each other (analogous to your bank account not changing when the amount of money deposition and withdrawn are equal). If an additional man-caused destruction process is added (orange) that will lower the ozone layer concentration (if someone else starts spending some of your money, you balance will decrease).

Knowing that you need O2 and UV light to make ozone, you can begin to understand why the ozone layer is found in the middle of the atmosphere.

There is plenty of UV light high in the atmosphere but not much oxygen (air gets thinner at higher and higher altitude). Near the ground there is plenty of oxygen but not as much UV light (it is absorbed by gases above the ground). You find the optimal amounts of UV light and oxygen somewhere in between, near 25 km altitude.

This next figure lists some of the problems associated with exposure to UV light. Thinning of the ozone layer will result in increased amounts of UV light reaching the ground.

Skin cancer and cataracts are probably the best known hazards associated with UV light. There was a question about whether tanning booths are safe. You can find online about this question. Here is an example from the US Food and Drug Administration.

Human activities add substances to the atmosphere that can potentially reduce ozone concentration in the ozone layer (which would result in increased exposure to UV light at the ground).

The first set of reactions above involve nitric oxide, NO. First, NO reacts with O₃ to form NO₂ and O₂ (ordinary molecular oxygen). Then notice the NO₂ reacts with an oxygen atom (which might otherwise react with O₂ to make O₃) to form NO again and O₂. The NO is available again to react with and destroy another ozone molecule.

At one time many countries were considering building fleets of supersonic aircraft that would fly into the stratosphere. The plans were scrapped partly due to concern that the NO emissions from these aircraft would damage the ozone layer.

The main threat now comes from chlorofluorocarbons (CFCs). CFCs were at one time thought to be an ideal industrial chemical and had a variety of uses. CFCs are unreactive, non toxic, and stable. Once they get into the atmosphere they remain there a long time, as much as 100 years. The reactions involving CFCs are shown on the next figure.

CFCs released at ground level [lower left corner in the figure above] 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 [a]. The resulting "free chlorine" can react with and destroy ozone. This is shown in (b) 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 (c) above. The reaction products, reservoir molecules (because they store chlorine), might serve as condensation nuclei for cloud droplets (the small water drops that clouds are composed of) or 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.

An iron bar was passed around at the beginning of class. You were supposed to guess how much it weighed.

We came back to this later in the period.

Bottles containing approximately equal volumes of water and mercury were passed around in class (thanks for being careful with the mercury). There is a lot more mass in the bottle of mercury than in the bottle of water. Because it has more mass the bottle of mercury also weighs more than the bottle of water (that's something you can feel). Mercury is much denser than water.

Before we can learn about atmospheric pressure, 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 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 than the Volkswagen once it is moving).

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 where the pull of gravity never changes, any three objects that all have the same mass (even if they had different volumes and were made of different materials) would always have the same weight. Conversely:

When gravity is always the same, three objects with the same weight would also 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.

On the earth a brick with a mass of about 2 kg weighs about 5 pounds. If you carried the brick to the moon it would have the same mass. But gravity on the moon is weaker than on the earth. Objects on the moon weigh less than on the earth.

In the first example there is more mass (more dots) 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.

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.

Pressure is defined as force divided by area. Air pressure is the weight of the atmosphere overhead divided by the area the air is resting on. Atmospheric pressure is determined by and tells you something about the weight of the air overhead. This is one way, a sort of large scale representation, of understanding air pressure.

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).

Now here's where the steel bar comes in. The steel bar also weighs exactly 14.7 pounds (many people think it is heavier than that). Steel is a lot denser than air, so a steel bar only needs to be 52 inches tall to have the same weight as an air column that is 100 miles or more tall.

14.7 psi is one weigh of expressing average sea level pressure. Here are average sea level pressure values in different units.

Typical sea level pressure is 14.7 psi or about 1000 millibars (the units used by meterologists and the units that we will use in this class most of the time) or about 30 inches of mercury (refers to the reading on a mercury barometer). If you ever find yourself in France needing to fill your automobile tires with air (I lived in France for a while and owned a Peugeot 404) remember that the air compressor scale is probably calibrated in bars. 2 bars of pressure would be equivalent to 30 psi.