Wednesday Jan. 26, 2011
click here to download today's notes in a more printer friendly format.

Three songs by Adele before class this morning ("Hometown Glory", "My Same", and "Make You Feel My Love")

The Practice Quiz is next Wednesday.  A preliminary version of the Practice Quiz Study Guide is now available (study guides will appear roughly one week before all of the quizzes this semester).

Today's pictures of the day are examples of excellent to very poor visibility in the Pheonix area.  Decreases in visibility are caused largely by increasing concentrations of particulate matter.

We'll finish our coverage of air pollutant with a quick look at tropospheric ozone and photochemical smog (aka Los Angeles type smog)

The figure above can be found on p. 14a in the photocopied ClassNotes.  Ozone has a Dr. Jekyll (good) and Mr. Hyde (bad) personality.  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.  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.  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 I decided not to make 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 one of the reactions 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.

                 

Once the cloud had formed we shined the laser beam through the flask.  Laser light wasn't visible to the left or the right of the flask, only in the flask where smog droplets were present and scattering laser light.  The smog droplets (and they may well be solid particles, I don't know) are very small and even the weakest air current is able to keep them suspended.
As long as we're talking about ozone it seemed reasonable to learn a little bit more about stratospheric ozone.  Stratospheric ozone (the ozone layer) absorbs dangerous high-energy ultraviolet light. 

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


The top two equations show how ozone is produced naturally 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 change 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 exposure to UV light. 

Three different types of UV light were mentioned in class: UVA, UVB, and UVC.  UVA is a relatively long wavelength (0.315 to 0.4 micrometers), low energy form of UV light; it is the light emitted by a "black light".  UVB has shorter wavelength (0.28 to 0.315 micrometers).  UVC has even shorter wavelength (0.1 to 0.28 micrometers) and is the most dangerous of the three types of UV light.  Germicidal bulbs emit UVC light and are used to sterilize and purify air, food, and water.  All of the UVC in sunlight is absorbed by the upper atmosphere. 

There is some question about whether tanning booths, which emit mostly UVA, are safe.  You can find information 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 (which was not shown in class).



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.

Next we will be learning about some of the physical properties of the atmosphere 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 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.