Thursday Sep. 3, 2009
click here to download today's notes in a more printer friendly format

Two songs from Patty Griffin were played before class today ("Up to the Mountain (MLK Song)" and "Heavenly Day").  Both are from her "Children Running Through" CD.

The Practice Quiz Study Guide is now available online.  There will be a review next Tuesday and Wednesday (location is currently unknown).  Try to study for the Practice Quiz like you would for a real quiz so that you can get an honest idea of how you would do on a real quiz.

We looked briefly at the predicted path of Tropical Storm Jimena issued by the National Hurricane Center.  The predicted path has changed and the storm has weakened a lot since we looked at it on Tuesday.  The clouds we are seeing in Tucson today are from the outer edges of this storm and from tropical moisture moving into Arizona (click here for the latest satellite imagery).  We have a chance for some badly needed rain this week and during the weekend (though I'm still not too optimistic given the track record so far this summer).  You can find the latest local forecasts at the Tucson National Weather Service website.

The Report Signup sheets should now be complete.

We'll discuss another gaseous pollutant today, tropospheric ozone.


Ozone has a Dr. Jekyll and Mr. Hyde 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.  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 (equation above circled in yellow) 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).

So here's a short recap: key characteristics of the 3 pollutants we've talked about so far.


The last pollutant that we will cover is Particulate Matter (PM) - small solid particles or drops of liquid (but not gas) that remain suspended in the air (sometimes refer to as aerosols).  The designations PM10 and PM25 refer to particles with diameters less than 10 micrometers and 2.5 micrometers, respectively.  A micrometer is one millionth of a meter.  The drawing below might give you some idea of what a 1 micrometer particle would look like (actually it would probably be too small to be seen without magnification).
Particulate matter can be produced naturally (wind blown dust, clouds above volcanic eruptions, smoke from lightning-caused forest and brush fires).  Human activities also produce particulates.

Particles with dimensions of 10 micrometers and less can be inhaled into the lungs (larger particles get caught in the nasal passages).  Inhaled particulates are a health threat.  The particles can cause cancer, damage lung tissue, and aggravate existing repiratory diseases.  The smallest particles can pass through the lungs and get into the blood stream (just as oxygen does) and damage other organs in the body.
The figure below identifies some of the parts of the human lung mentioned in the figure above.

 
Crossectional view of the human lungs
from: http://en.wikipedia.org/wiki/Lung

1 - trachea
2 - mainstem bronchus
3 - lobar bronchus
4 - segmental bronchi
5 - bronchiole
6 - alveolar duct
7 - alveolus
from http://en.wikipedia.org/wiki/Image:Illu_quiz_lung05.jpg

Note the PM10 annual National Ambient Air Quality Standard (NAAQS) value of 50 micrograms/meter3 at the bottom of p. 13c in the photocopied ClassNotes (above).  The following list shows that there are several cities around the world where PM concentrations are 2 or 3 times higher than the NAAQS value. 


There was some concern last summer that the polluted air in Beijing would keep athletes from performing at their peaks during the Olympic Games.  Chinese authorities restricted transportation and industrial activities both before and during the games in an attempt to reduce pollutant concentrations.  Rainy weather during the games may have had the greatest effect, however.  Clouds were mentioned but the figure below was not shown in class.

Clouds and precipitation are the best way of cleaning pollutants from the air.

Particulates can affect visibility and can make the sky appear hazy.  To understand this we need to look at how air molecules and particles scatter sunlight.




Air molecules scatter sunlight (an individual air molecule doesn't scatter much light, but there are lots of air molecules in the atmosphere).  Sunlight is basically white, which tells you it is a mixture of all the colors.  Because air molecules are small (relative to the wavelength of visible light) they scatter shorter wavelengths more readily than longer wavelengths.  When you look away from the sun and toward the sky you see this scattered light, it has a deep blue color.  This is basically why the sky is blue.  If the earth didn't have an atmosphere (or if air molecules didn't scatter light) the sky would be black.

Scattering of sunlight by air molecules turns distant mountains blue and eventually makes them fade from view




A nearby mountain might appear dark green or brown.  You are mostly seeing light reflected off the mountain.  As the mountain gets further away you start seeing increasing amounts of blue light (sunlight scattered by air molecules in between you and the mountain) being added to the brown and green reflected light.  As the mountain gets even further the amount of this blue light from the sky increases.  Eventually the mountain gets so far away that you only see blue sky light and none of the light reflected by the mountain itself. 




Particles also scatter light (remember the chalk dust used in the laser demonstration).  But because the particle size is about equal to or somewhat greater than the wavelength of visible light the particles scatter all the colors equally.  The light scattered by particles is white.  This is basically why clouds are white.

As the amount of particulate matter in the air increases the color of the sky changes from deep blue to whitish blue.  The higher the particle concentration, the whiter the sky becomes.

I didn't explain the effect of particulates on visibility very well in the MWF class.  I went back to my office, banged my head against the wall a few times, and tried to come up with a new approach.  I'll try the following explanation on you T Th people.


The air is free of particles in this first picture.  You're looking at a relatively nearby mountain.  The "side view" at left explains that you are able to see the mountain because it reflects sunlight back toward you.  The picture at right is what you see when you look at the mountain.


Now some particulates have been added to the air.  They scatter sunlight, the scattered light is white (it's highlighted in yellow in the picture at left for emphasis).  So now you still see the brown and green reflected light but also some white scattered light.  Some (fairly big) spots of white light have been added to the picture at right.



More particles have been added to the air.  That means there will be more scattered light.

We'll add even more particles to the air in the next picture.  Because there is more scattered light (more spots of white light in the picture of the mountain) you have a harder time seeing the mountain.


It's getting harder and harder to see the mountain.

We spent the last part of the period today and will spend much of next Tuesday looking at the current concern over increasing greenhouse concentrations in the earth's atmosphere, global warming, and climate change.  This is a big, complex, and contentious subject and we will only scratch the surface.  I've reorganized the material presented in class into what is hopefully a much clearer introduction to this topic.

Quite a bit of information needs to be added to p. 3a in the photocopied ClassNotes; we will break it up into several smaller pieces.

Point 1.  Carbon dioxide is probably the best known of several greenhouse gases.  Much of what we say about CO2 applies to the other greenhouse gases as well.

Point 2.   Atmospheric CO2 concentrations are increasing.  This is pretty generally accepted as fact.  We'll look at some of the evidence below.

Point 3.   The basic worry is that increasing greenhouse gas concentrations will cause global warming.  This is a hypothesis though many (perhaps the vast majority of) scientists regard this as fact.

      Before we look at enhancement of the greenhouse effect, it is important to first understand that the greenhouse effect has a beneficial side.   You might refer to this as the natural greenhouse effect (i.e. one that has not been affected or influenced by human activities)

Point 3a.  If the earth's atmosphere didn't contain any greenhouse gases, the global annual average surface temperature would be about 0o F.  That's pretty cold
Point 3b.  The presence of greenhouse gases raises this average temperature to about 60o F.


Point 4.   The concern is that increasing atmospheric greenhouse gas concentrations might cause some additional warming.  This might not sound like a bad thing.  However a small change in average temperature might melt polar ice and cause a rise in sea level and flood coastal areas.  Warming might change weather patterns and bring more precipitation to some areas and prolonged drought to places like Arizona. 

Now some of the data that show atmospheric carbon dioxide concentrations are increasing.


The "Keeling" curve shows measurements of CO2 that were begun (by a graduate student named Charles Keeling) in 1958 on top of the Mauna Loa volcano in Hawaii.  Carbon dioxide concentrations have increased from 315 ppm to about 385 ppm between 1958 and the present day.  The small wiggles (one wiggle per year) show that CO2 concentration changes slightly during the course of a year (it also probably changes slightly during the course of a day). 

You'll find an up to date record of atmospheric CO2 concentration from the Mauna Loa observatory at the Scripps Institution of Oceanography site.

Once scientists saw this data they began to wonder about how CO2 concentration might have been changing prior to 1958.  But how could you now,  in 2009 say, go back and measure the amount of CO2 in the atmosphere in the past?  Scientists have found a very clever way of doing just that.  It involves coring down into ice sheets that have been building up in Antarctica and Greenland for hundreds of thousands of years.


As layers of snow are piled on top of each other year after year, the snow at the bottom is compressed and eventually turns into a thin layer of solid ice.  The ice contains small bubbles of air trapped in the snow, samples of the atmosphere at the time the snow originally fell.  Scientists are able to date the ice layers and then take the air out of these bubbles and measure the carbon dioxide concentration.  This isn't easy, the layers are very thin, the bubbles are small and it is hard to avoid contamination.

Using the ice core measurements scientists have determined that atmospheric CO2 concentration was fairly constant at about 280 ppm between 1000 AD and the mid-1700s when it started to increase.  The start of rising CO2 coincides with the beginning of the "Industrial Revolution."   Combustion of fossil fuels needed to power factories began to add significant amounts of CO2 to the atmosphere.  Concentrations of several of the other greenhouse gases have been increasing in much the same way CO2 has.