Tuesday, Jan. 17, 2012
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3 songs from Pistolera to get the blood circulating before class this morning.  You heard "La Despedida", "La Espera", and "Laberinto".

40 sets of Experiment #1 materials were checked out before class today (something you accomplished in a very efficient and orderly way).  I will try to bring more materials to class next Thursday.  Signup sheets for the remaining experiments were passed around in class.  Those names will eventually be transferred to the online Report Signup Lists.




This picture shows methane, aka natural gas and a potent greenhouse gas, coming from melting permafrost in Alaska (photo credit: Todd Paris/The Associated Press. You'll find the photo in an article that appeared in USA Today).  You can learn more about methane in a short video.

Three main goals today:



The largest portion of the class was spent learning a little bit about the origin and evolution of our present day atmosphere.


The atmosphere we have today (mostly nitrogen, oxygen, water vapor, and argon) is very different from the earth's original atmosphere which was mostly hydrogen and helium.  This original atmosphere either escaped (the earth was hot and the gases were moving around with enough speed that they could overcome the pull of the earth's gravity) or was swept into space by the solar wind (click on the link if you are interested in learning more about the solar wind, otherwise don't worry about it). 

With the important exception of oxygen, most of our present atmosphere is though to have come from volcanic eruptions.



Don't worry about remembering all of the gases listed above.  Volcanoes emit a lot of water vapor and carbon dioxide.  As the earth began to cool the water vapor condensed and began to create and fill oceans.  Carbon dioxide dissolved in the oceans and was slowly turned into rock.  Smaller amounts of nitrogen (N2) are also emitted by volcanoes.  Because nitrogen is relatively unreactive it remained in the air and its concentration began to built up over time.  There are lots of poisonous gases such as sulfur dioxide emitted by volcanoes.  We'll learn a little more about sulfur dioxide, in particular, next week when we cover air pollutants.

Two years ago, air travel to Europe was being severely disrupted by the eruption of the the Eyjafjallajökull volcano in Iceland.  Here are some really amazing pictures published in the Boston Globe.  Here's another set of photos also from the Boston Globe.

Volcanoes didn't add any of the oxygen that is the atmosphere.  Where did that come from?


The oxygen is thought to have first come from photodissociation of water vapor and carbon dioxide by ultraviolet (UV) light (the high energy UV light is able to split the H20 and CO2 molecules into pieces).  The O and OH then react to form O2 and H.

By the way I don't expect you to remember the chemical formulas in the example above.  It's often easier and clearer to show what is happening in a chemical formula than to write it out in words.  If I were to right the equations down, however, you should be able to interpret them.  Ultraviolet is a high energy form of light and it's probably also good to remember that ultraviolet light is capable of breaking molecules apart.


Once molecular oxygen (O2) begins to accumulate in the air UV light can split it apart to make atomic oxygen (O).  The atoms of oxygen can react with molecular oxygen to form ozone (O3). 



Ozone in the atmosphere began to absorb ultraviolet light (another photodissociation reaction, shown above) and life forms could then begin to safely move from the oceans onto land.  Prior to the buildup of ozone, ocean water offered protection from UV light. 

Photosynthesis is the 2nd and now the main source of atmospheric oxygen. 

Photosynthesis in its most basic form is shown in the chemical equation above.  Combustion is really just the opposite of photosynthesis and is shown below.


We burn fossil fuels (dead, undecayed plant material) to generate energy.  Water vapor and carbon dioxide are by products.  Combustion is a source of CO2.  We'll see these two equations again when we study the greenhouse effect (CO2  is a greenhouse gas ) and global warming.

The following figure is the first page in the packet of photocopied ClassNotes.


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.

First, Point 1: the earth is thought to be between 4.5 and 4.6 billion years old.  If you want to remember the earth is a few billion years old that is probably close enough.  Something I didn't mention in class, it's in small type above.  The formation of a molten iron core was important because it gave the earth a magnetic field.  The magnetic field deflects the solar wind and keeps it from blowing away our present atmosphere.

Stromatolites (Point 2) 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 also been found. 



Blue green algae grows at the top of the column, under water but near the ocean surface where it can absorb sunlight.  Sediments begin to accumulate on top of the algae and start to block the sunlight.  The cyanobacteria would then move to the top of this sediment layer and the process would repeat itself.  In this way the stromatolite column would grow a layer at a time.  We're learning about stromatolites because the cyanobacteria on them were one of the earliest forms of life able to produce oxygen using photosynthesis.





Living stromatolites are found in a few locations today.  The two pictures above are from Coral Bay (left) and Shark's Bay (right) in Western Australia.  The picture was probably taken at low tide, the stromatolites would normally be covered with ocean water.  It doesn't look like a good place to go swimming, I would expect the top surfaces of these stromatolites to be slimy.

Point 3 refers to banded iron rock.  These rocks are 3 billion years old (maybe older) and are evidence of oxygen being produced in the earth's oceans.  Here are a couple of pictures of samples of banded iron formation rock that was passed around in class.



The main thing to notice are the alternating bands of red and black rock.  The next paragraph and figure explain how these formed.

Rain would first of all wash iron ions from the earth's land surface into the ocean (at a time before there was any oxygen in the atmosphere).  Oxygen from the cyanobacteria living in the ocean water reacted with the dissolved iron (the iron ions) to form hematite or magnetite.  These two minerals precipitated out of the water to form a layer on the sea bed.  This produced the black layers.



Periodically the oxygen production would decrease or stop (rising oxygen levels might have killed the cyanobacteria or seasonal changes in incoming sunlight might have slowed the photosynthesis).  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.  In addition to the red and black layers, you see yellow layers made of fibers of quartz in the samples passed around class.   The rocks are fairly heavy because they contain a lot of iron, but the most impressive thing about them in my opinion is their age - they are a few billion years old!  And thanks for returning them by the way.

Eventually the oxygen in the ocean reacted with all of the iron ions and was free to move 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.  These are called "Red Beds" (Point 4).  None of these so-called 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 million years ago (Point 5 in the figure).

Here's an example of a question that you might find on a quiz (it's actually from the Final Exam from class last Fall).


Equation (c) is the first step in the natural production of ozone.  This is followed by the reaction in Eqn. (a).  Together Eqns. (c) and (a) produce ozone.  Equation (b) shows the natural destruction of ozone. 

On Thursdeay we'll start a fairly short section on air pollutants.  The list below gives you a little bit of an idea what we'll be covering. 

At the top of the list are the 5 most abundant gases in the atmosphere mentioned last week.  Several more important trace gases were added to the list today.  Trace gases are gases found in low concentrations (and often time the concentrations are variable).  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 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.  We'll cover some of these in more detail today and early next week.

Ozone has sort of a Dr. Jeckyl and Mr. Hyde personality
(i)  Ozone in the stratosphere (a layer of the atmosphere between about 10 and 50 km altitude) is beneficial because it absorbs dangerous high energy ultraviolet (UV) light coming from the sun.  Without the protection of the ozone layer, life as we know it would not exist on the surface of the earth.  It was only after ozone started to buildup in the atmosphere that life could move from the oceans onto land.  Chlorofluorocarbons are of concern in the atmosphere because they destroy stratospheric ozone.

(ii)  In the troposphere (the bottom 10 kilometers or so of the atmosphere) ozone is a pollutant and is one of the main ingredients in photochemical smog.

(iii)  Ozone is also a greenhouse gas.

This was a good point for a demonstration, one that was once voted the prettiest demonstration of the semester.

You are able to see a lot of things in the atmosphere (clouds, fog, haze, even the blue sky) because of scattering of light.  We'll try to make a cloud of smog in class later this week.  The individual droplets making up the smog cloud are too small to be seen by the naked eye.  But you will be able to see the smog cloud because the droplets scatter light.  So we took some time for a demonstration that tried to show you and explain exactly what light scattering is.

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.



This is a view from above.    Neither the students or the instructor could see the beam of light.  Nobody could see the beam because there weren't any rays of light pointing from the laser beam toward the students or toward the instructor.


The instructor would have been able to see the beam if he had stood at the end of the beam of laser light and looked back along the beam of light toward the laser.  That wouldn't have been a smart thing to do, though, because the beam was strong enough to possibly damage his eyes (there's a warning on the side of the laser). 

Everybody was 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).  The original beam is broken up into a myriad of weaker rays of light that 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 their ray came from.  It is safe to  look at this light because the original intense beam is split up into many much weaker beams.

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 a straight segment of 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 out 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 water into a cup of liquid nitrogen.  The cloud droplets are much smaller than the chalk particles but are many more of them.  They make very good scatterers.






The beam of laser light really lit up as it passed through the small patches of cloud.  The cloud droplets 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).  Here's a photo I took back in my office.




The laser beam is visible in the left 2/3 rds of the picture because it is passing through cloud and light is being scattered toward the camera.  There wasn't any cloud on the right 1/3rd of the picture so you can't see the laser beam over near Point 1.

There's something else going on in this picture also.  We're not just seeing the narrow beam of laser light but some of the cloud outside the laser beam is also visible.

Up to this point we've just considered single scattering.  A beam of light encounters a cloud droplet or a particle of chalk and gets redirected and then travels all the way to your eye or to a camera.  That's what's happening at Point 2 (it's also shown below in Path 1).  You just see the narrow laser beam.  But sometimes the scattered ray of light runs into something else and gets scattered again.  This is called multiple scattering.  And that is what is illuminating the cloud alongside the beam of laser light at Point 3.  Light is first scattered by a cloud droplet in the beam.  As it leaves the beam it runs into another droplet and gets scattered again (Path 2 below).  So now it looks like it is coming from the cloud surrounding the laser beam rather than from the beam itself.





Here's a comment that wasn't 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 couldn't see the laser beam as it was traveling from one side of the classroom to the other through the air.  Outdoors we are able to see sunlight scattered by air molecules.  This is true for a couple of reasons.  The sunlight is much stronger than the laser beam and its shining through a lot more air. 

Sunlight is white light which means it's made up of a mixture 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 the topic of light scattering next week. when we cover particulate matter and its effect on visibility.