Thursday Aug. 23, 2012
click here to download today's notes in a more printer friendly format

Time for 4 songs before class today:  "Don't Think Twice, It's Alright", "Places to Go", "At Last", "Smile Big" from the Leftover Cuties.  They're a recent discovery.  I heard a song I liked during a Samsung Galaxy commercial on TV during the Olympics.  Turned out it was one of theirs.

We had time for 4 songs because I arrived in class a little early to distribute Experiment #1 materials.  Those of you that were lucky enough to get one of the sets of materials will find you name on this online list.  This weekend would be a perfect time to start the experiment.  It's a good idea to check the experiment fairly frequently early in the experiment.  It slows down somewhat as it progresses.

Once you've completed the experiment and return the materials you'll receive a supplementary information handout that will help with the analysis portion of the experiment.  You should try to return the materials well before the report due date.  This will make materials available to students on the waiting list for Expt. #1.

Signup sheets for the remaing experiments were circulated in class today also.  If you signed up for Experiment #1 but didn't get any materials you should find your name on the waiting list.

We learned a little bit about the dew point on Tuesday.

The air is currently about as moist as it ever gets in Tucson.  You can feel the effect that has.  The air should dry out a little bit by the weekend (dew points might drop into the 50s perhaps the 40s).  You might be able to sense the difference.  By mid to late September the summer monsoon will end and dew points will drop back into the 25 to 45 degree range where they are most of the year.  You should definitely notice the difference.  My swamp cooler will begin to work much better and I hope the mosquitoes will decide to move somewhere else.

In addition to giving you an idea of how much water vapor is in the air, the dew point has another job.  The following notes were stuck onto the end of Tuesday's online lecture notes but weren't covered in class.

If you cool air next to the ground to its dew point, water vapor will condense and coat the ground with water.  The ground will be covered with dew.  If a little thicker layer of air is cooled fog will form.

The following demonstration wasn't done in class on Tuesday.  It's important enough that we did it in class today.


First of all you can see liquid nitrogen (it's clear not purple as shown in the figure).  Once the liquid evaporates and turns into nitrogen gas it is invisible.

Water vapor (water in gaseous form) is also invisible.  When moisture in the air comes into contact with the cold liquid nitrogen in the cup, the water vapor condenses and forms a visible cloud of very small water droplets (liquid water) and ice crystals (solid water).

Here's a summary



Liquid nitrogen evaporates and turns into invisible nitrogen gas.  Nitrogen is the most abundant gas in the atmosphere.  Invisible water vapor (a gas) in the air is cooled to its dew point temperature and condenses to form a visible cloud composed of water droplets and ice crystals.


Next it was on to the main topic of the day - 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 (you might find slightly different lists depending on the source you check).  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.

Here's something I didn't mention in class.  Two or three 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 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.  There were 5 main points I wanted you to take from this figure.

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.  This isn't a geology class, we're learning about stromatolites because the cyanobacteria on them were a very early form of life on the earth and were 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 of oxygen in the atmosphere 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).

Finally a look at what we will be covering next week.
At the top of the list below are the 5 most abundant gases in the atmosphere.  Several more important trace gases were added to the bottom of the figure.  Trace gases are gases found in low concentrations (and often the concentrations vary with time and location).  Low concentrations doesn't mean they aren't important, however.


Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants.  We'll cover 3 of these in more detail next week.

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.

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 is as far as we went in class on Thursday.  I've added a little additional information below

Air Pollution is a serious health hazard in the US and around the globe  (click here to download a copy of the statistics shown below).  The lists below give some idea of how serious a threat it is.



The top list shows the external or environmental agent that causes death.  The second list is the internal body function that ultimately leads to your demise.  Keep in mind that many of these numbers are difficult to measure and some may contain a great deal of uncertainty.  The row that is highlighted, toxic agents, contains estimates of deaths caused by indoor and outdoor air pollution, water pollution, and exposure to materials such as asbestos and lead both in the home and at the work place.  It is estimated that 60% of the deaths are due to exposure to particulate matter, something that we will examine in a little more detail late next week.

Air pollution is a serious hazard worldwide.  Interestingly indoor air pollution is, in many places, a more serious threat than outdoor air pollution.  I'm not sure how the researchers determine that 150,000 people are killed by climate change every year.

The Blacksmith Institute (mentioned above) listed the Top 10 polluted places in the world in a 2007 report.  The report has received a lot of worldwide attention.  If you go to this address (click on 2007 at the top left edge of the page) you can view the report online or download and print a copy of the report.  This is just in case you are interested (click on some of the other years also if you do go to the site).

The two photographs below show residents of Linfen, China, which was, in 2006 was considered (by hte Blacksmith Institute) to be the most polluted city on earth.

The Photograph at left comes from an article in The Guardian.  The photograph at right from an article from The Blacksmith Institute.  Pollutants of concern included: Fly-ash, carbon monoxide, nitrogen oxides, PM-2.5, PM-10, sulfur dioxide, volatile organic compounds, arsenic, and lead (PM stands for particulate matter).