This somewhat confusing figure shows some of the important events in the history of the earth and evolution of the atmosphere.  The numbered points were emphasized. Point 5 was added after class.
First, Point 1: the earth is thought to be between  4.5 and  4.6 billion years old.
The iron catastrophe was an important event (but wasn't discussed in class).  Circulation of liquid metal in the core of the earth gives the earth a magnetic field.  The magnetic field deflects the solar wind around the earth.  Remember the solar wind may have swept away the earth's original atmosphere.

Stromatolites (Points 2 and 3) are column-shaped structures made up of layers of sedimentary rock, that are created by microorganisms living at the top of the stromatolite (I've never actually seen a stromatolite, so this is all based on photographs and written descriptions).  Fossils of the very small microbes (cyanobacteria = blue green algae) have been found in stromatolites as old as 2.7 B years and are some of the earliest records of life on earth.  Much older (3.5 to 3.8 B years old) stromatolites presumably also produced by microbes, but without microbe fossils, have been found. 

We're learning about stromatolites because the cyanobacteria were able to produce oxygen using photosynthesis.

Once cyanobacteria began to produce oxygen in ocean water, the oxygen reacted with dissolved iron (iron ions in the figure below) to form hematite or magnetite.  These two minerals precipitated out of the water to form a layer on the sea bed.

Periodically the oxygen production would decrease or stop (rising oxygen levels might have killed the cyanobacteria or seasonal changes might have slowed the photosynthesus).  During these times of low dissolved oxygen concentrations, layers of jasper would form on the ocean bottom.  Eventually the cyanobacteria would recover, begin producing oxygen again, and a new layer of hematite or magnetite would form.  The rocks that resulted, containing alternating layers of black hematite or magnetite and red layers of jasper are known as the banded iron formation.  A small polished piece of banded iron rock (actually "tiger iron")  was passed around class.   In addition to the red and black layers, the tiger iron contains yellow layers made of fibers of quartz.   The most impressive thing about them in my opinion is their age - they are around 3 billion years old!

Eventually the dissolved iron in the ocean was used up (Point 4 in the timeline figure above).  Oxygen produced by cyanobacteria no longer reacted with iron and was free to diffuse from the ocean into the atmosphere.  Once in the air, the oxygen could react with iron in sediments on the earth's surface.  This produced red colored (rust colored) sedimentary rock.  None of these socalled red beds are older than about 2 B years old.  Thus it appears that a real buildup up oxygen began around 2 B years ago. Oxygen concentrations reached levels that are about the same as today around 500 to 600 years ago (Point 5 in the figure).

We listed the 5 most abundant gases in the atmosphere in class on Tuesday.  Several more important trace gases were added to the list in class today.  Trace gases are gases found in low concentrations.  Low concentrations doesn't mean they aren't important, however.

Water vapor, carbon dioxide, methane, nitrous oxide (N₂O = laughing gas), chlorofluorocarbons, and ozone are all greenhouse gases.  Increasing atmospheric concentrations of these gases are responsible for the current concern over climate change and global warming.  We'll discuss this topic more next week and learn more about how the greenhouse effect actually works later in the course.

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants. 

Be careful with ozone:

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Many of the gases in the atmosphere are invisible.  Some are colored.  We'll make some nitrogen dioxide at the end of the class today.  Nitrogen dioxide has a reddish brown color and is visible.  We are able to see clouds, smog, fog, and haze even though the particles that make them up are too small to be seen.  We can see them because they scatter light.  The blue color of the sky is produced by scattering of sunlight by air molecules. 

We did a short demonstration to explain and illustrate light scattering.

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 were 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 later in the semester.

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We started to learn a little bit about carbon monoxide before moving to the last demonstration of the day.  We will be talking about carbon monoxide found both outdoors (where it rarely would reach fatal concentrations) and indoors (where it can be deadly).  The following is found on the top of p. 7 in the photocopied ClassNotes.


Carbon monoxide is insidious, you can't smell it or see it and it can kill you.  Once inhaled, carbon monoxide molecules bond strongly to the hemoglobin molecules in blood and interfere with the transport of oxygen throughout your body.

CO is a primary pollutant (Point 2 above).  That means it goes directly from a source into the air,  CO is emitted directly from an automobile tailpipe into the atmosphere for example. The difference between primary and secondary pollutants is probably explained best in a series of pictures.

Air is mostly composed on invisible gases.  I've often thought it might be interesting to bring in several examples of gases that you can actually see (the gases are colored, not clear; you can't of course see the individual gas atoms or molecules).  Once I started to do some research I found that many of these gases are very poisonous.  In some cases a sample large enough for you to be able to see would be a potentially fatal dose if it were to be released accidentally into the classroom.  You're going to have to settle for pictures of chlorine (a gas with a yellow-green color), and bromine (a liquid that evaporates, the resulting gas has a very vivid reddish color).  The caution on the www.webelements.com website: "Bromine is a serious health hazard and maximum safety precautions should be taken when handling it" worried me a little bit.  I will bring in some iodine (a solid that sublimates producing a gas with a faint pink color) later in the semester (it's poisonous but not nearly as scary as some of these other gases).

We did however make some nitrogen dioxide, a toxic pollutant.  We did this by putting an ordinary copper penny (Cu(s) in the equation below) into a large 4 liter glass flask that contained a small amount of concentrated nitric acid ( HNO3(aq) ).