Thu., Jan. 21 notes

Thursday Jan. 21, 2010
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Today's music featuring Amadou et Mariam comes from a student (in the MWF section) recommendation:

You heard "Sabali", "Ce n'est Pas Bon," and part of "Beaux Dimanches" (if there's time)
Experiment #1 materials were handed out in class on Tuesday, a few more sets were distributed today. If you checked out materials you should find your name and more information about the experiment on the Expt. #1 Report Signup List.

We started by finishing up the secton on carbon monoxide.
Air temperature increases with increasing altitude in a temperature inversion layer.

A very reasonable wintertime morning temperature profile in Tucson is shown at the top of p. 9 in the photocopied Classnotes.

Temperature increases from 47^o F at the ground (Point A) to about 60^o F at 1000 feet altitude (Point B), that's the stable inversion layer. Temperature begins to decrease with increasing altitude above Point B.

There is very little vertical mixing in a stable air layer.

When CO is emitted into the thin stable layer (left figure above), the CO remains in the layer and doesn't mix with cleaner air above. CO concentrations build. You'll find more information about what makes the atmosphere stable or unstable at the end of today's notes.

The concentrations of several of the main pollutants are monitored in large cities in the US and around the world. Six pollutants are listed below (p. 8 in the photocopied ClassNotes). In Tucson, carbon monoxide, ozone, and particulate matter are of primary concern and daily measurements are reported in the city newspaper. The Air Quality Index value is reported instead of the actual concentration. The AQI is the ratio of the measured to accepted concentrations multiplied by 100%. Air becomes unhealthy when the AQI value exceeds 100%.

The atmospheric concentration of lead has decreased significantly since the introduction of unleaded gasoline. PM stands for particulate matter. These small particles are invisible, remain suspended in the air, and may be made of harmful materials. We'll talk about them in a little more detail next week.

For carbon monoxide, concentrations up to 35 ppm (parts per million) for a 1 hour period and 9 ppm for an 8 hour period are allowed. If the observed CO concentration were 4.5 ppm averaged over an 8 hour period the AQI would be

AQI = 100% x (4.5ppm / 9ppm) = 50%

and the air quality would be considered good. Current Air Quality Index values for Tucson are available online.

So are we have been talking about carbon monoxide found in the atmosphere. Carbon monoxide is also a serious hazard indoors where is can build to much higher levels than would ever be found outdoors. You may remember having heard about an incident at the beginning of the school year in 2007. Carbon monoxide from a malfunctioning hot water heater sickened 23 Virginia Tech students in an apartment complex. The CO concentration is thought to have reached 500 ppm. You can get an idea of what kinds of health effects concentrations this high could cause from the figure below (from p. 9 in the photocopied Classnotes).

To get an idea of what effects 500 pm CO concentrations could cause, we will follow the 400 ppm line (shaded orange) from left to right. At exposure times less than 1 hour you should experience no symptoms. Beginning at 1 hour you might experience headache, fatique, and dizziness. Exposures of a few hours will produce throbbing headache, nausea, convulsions, and collapse. The 400 ppm trace level approaches the level where CO would cause coma and death. At Virginia Tech several students were found unconscious and one or two had stopped breathing but they were revived.

Carbon monoxide alarms are relatively inexpensive and readily available at any hardware store. They will monitor CO concentrations indoors and warn you when concentrations reach hazardous levels. Indoors CO is produced by gas furnaces and water heaters that are either operating improperly or aren't being adequately vented to the outdoors. A few hundred people are killed indoors by carbon monoxide every year in the United States. You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.

In the next few minutes of class we did a short demonstration illustrating the scattering (splattering) of light. Last week we were able to see a cloud when moist air came into contact with liquid nitrogen. We will be making a smog cloud in class later in class today. In both the case of the water cloud and smog, being able to see the cloud depends on the fact that the cloud droplets scatter light. We would probably not be able to see the clouds otherwise, the cloud droplets are just too small.

In the first part of the demonstration a narrow beam of intense red laser light was shined from one side of the classroom to the other.

The students couldn't see the laser beam because the light rays weren't pointing toward them. The instructor would have been able to see the beam if he had walked to the far wall and looked back along the beam of light (that wouldn't have been a smart thing to do because the beam is strong enough to damage his eyes).

Students were able to see a bright red spot where the laser beam struck the wall.

This is because when the intense beam of laser light hits the wall it is scattered (splattered is a more descriptive term). Weaker rays of light are sent out in all directions. There is a ray of light sent in the direction of every student in the class. They see the light because they are looking back in the direction the ray came from. It is safe to look at this light because the rays are weaker than the initial beam.

Next we clapped some erasers together so that some small particles of chalk dust fell into the laser beam.

Now instead of a single spot on the wall, students saws lots of points of light coming from different positions along the laser beam. Each of these points of light was a particle of chalk, and each piece of chalk dust was intercepting laser light and sending light in all directions. Each student saw a ray of light coming from each of the chalk particles.

We use chalk because it is white, it will scatter rather than absorb visible light. What would you have seen if black particles of soot had been dropped into the laser beam?

In the last part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water. The numerous little water droplets made very good scatterers.

The laser light really lit up and turned the small patches of cloud red. The cloud did a very good job of scattering laser light. So much light was scattered that the spot on the wall fluctuated in intensity (the spot dimmed when lots of light was being scattered, and brightened when not as much light was scattered).

A comment not mentioned in class. Air molecules are able to scatter light too, just like cloud droplets. Air molecules are much smaller than cloud droplets and don't scatter much light. That's why you weren't able to see light being scattered by air before we put chalk particles or cloud droplets into the beam. Outdoors you are able to see sunlight (much more intense than the laser beam used in the class demonstration) scattered by air molecules. Sunlight is white and is made up of violet, blue, green, yellow, orange, and red light. Air molecules have an unusual property: they scatter the shorter wavelengths (violet, blue, green) much more readily than the longer wavelength colors in sunlight (yellow, orange, and red). When you look away from the sun and look at the sky, the blue color that you see are the shorter wavelengths in sunlight that are being scattered by air molecules. We'll come back to this again in a week or two when we cover particulate matter.

We discussed another gaseous pollutant today, tropospheric ozone.

Ozone has a Dr. Jekyll and Mr. Hyde personality (The Jeckyl and Hide reference was added after class). Ozone in the stratosphere (the ozone layer) is beneficial, it absorbs dangerous high energy ultraviolet light (which would otherwise reach the ground and cause skin cancer, cataracts, and many other problems).

Ozone in the troposphere is bad, it is a pollutant. That is the stuff we will be concerned with today. Tropospheric ozone is also a key component of photochemical smog (also known as Los Angeles-type smog)

We'll be making some photochemical smog as a class demonstration. This will require ozone (and a hydrocarbon of some kind). We'll use the simple stratospheric recipe for making ozone in the demonstration rather than the more complex tropospheric process (4-step process shown below).

At the top of this figure 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 to make nitrogen dioxide, the poisonous brown-colored gas we made 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, 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. 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 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.

The class demonstration of photochemical smog is summarized below (a flask 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 create and 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).

We finished the material above a little earlier than planned so I added some information about stratospheric ozone and the ozone layer (found on pps 17-18 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. One of these reacts with O2 to make O3 (ozone).

Ozone is destroyed when it absorbes UV light and is split into O and O2 (the two pieces move away from each other and don't recombine to make ozone). O3 is also destroyed when it reacts with an oxygen atom. Two atoms of oxygen reacting to make O2 reduce the amount of one of the raw materials needed to make O3 and thereby reduce the concentration of ozone in the ozone layer.

The bottom figure attempts to show that the ozone concentration in the stratosphere will change until the 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).

Knowing that you need O2 and UV light to make ozone, you can begin to understand why the ozone layer is found not too high, not too low, but rather 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. This wasn't discussed in class.

Skin cancer and cataracts are probably the best known hazards associated with UV light. At some point in the next week or two we'll look at how man may be damaging the ozone layer by introducing chemical compounds into the atmosphere that react with and destroy stratospheric ozone.

Here's something else that we didn't cover in class (it might well have caused the students to rise up in revolt)
I have a bad habit of "beating some concepts to death." Here's an example. The rather busy and confusing picture below (p. 10 in the photocopied ClassNotes) 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 imagine riding a bicycle north from Swan and River Rd up the 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 goes from 47^o F to 41^o F, a drop of 6^o F. This is a fairly rapid rate of decrease with increasing altitude and would make the atmosphere absolutely unstable. The atmosphere wouldn't remain this way. Air at the ground would rise, air higher up 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 decreasing a little more slowly with increasing altitude. This small change makes the atmosphere conditionally unstable (we won't go into what the conditions might be). The atmosphere is frequently in this state.

The atmosphere cools only 2^o F in 1000 feet 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 is very common on winter mornings. The atmosphere is extremely stable under these conditions.

Temperature inversions are something you can check out for yourself this time of the year. Head north on Swan Rd. on your bicycle early some winter morning. You will pass through some pretty cold air as you cross the Rillito River. By the time you get to Sunrise, the air can be 10 to 15 degrees warmer and will seem balmy compared to the cold air at the bottom of the hill. If you're up for a real hill-climbing challenge continue north on Swan past Skyline. You'll find a short but very steep section of road at the far north end of Swan.

As long as we're talking about bicycles and hills here's a picture of my bicycle. I was in France last summer trying to ride up some of the famous Tour de France mountain stages in the Alps. One of the most famous is the Alpe d'Huez. That's my bicycle, a green "Gilmour" (Andy Gilmour is a local bicycle builder) at the top of the Alpe d'Huez.