Thursday Aug. 26, 2010
click here to download today's notes in a more printer friendly format

A couple of songs  Tamacun and Diablo Rojo from Rodrigo y Gabriela before class today.  They also have a very nice version of Stairway to Heaven.

The Experiment #1 materials and the Experiment signup sheets were handed out in class today.

The only class assignment at this point is to read through the Lecture Notes as they appear online.

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 (I'm looking into the question of why the original atmosphere was hydrogen and helium and hope to have a little more complete answer by next Tuesday).  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). 

Most of our present atmosphere is though to have come from volcanic eruptions.




Try to remember the three gases listed at left.  Don't worry about remembering the gases at right.  As the earth began to cool the water vapor condensed and began to create oceans.  Carbon dioxide dissolved in the oceans and was slowly turned into rock.  Smaller amounts of nitrogen (N2) are emitted by volcanoes.  Nitrogen is relatively unreactive and remained in the air.  Nitrogen concentration built up over time.

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



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

It is sometimes easier and clearer to show or explain a reaction in formulas instead of words.  I don't expect you to remember the chemical formulas in the example above.  You might just remember that the earth's original oxygen came from other gases in the air.  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 it can react with atomic oxygen (O) to form ozone (O3). This is an example of two formulas that you probably should remember. 

Once formed, ozone in the atmosphere began to absorb ultraviolet light and life forms could safely move from the oceans (which would absorb UV light in the absence of ozone) onto land.  Eventually plants and photosynthesis would become the main source of atmospheric oxygen.

Photosynthesis is a source of oxygen, it removes CO2 from the air.  Combustion is really just the opposite of photosynthesis.  We burn fossil fuels to generate energy.  Water vapor and carbon dioxide are by products.  Combustion is a source of CO2.
The following figure is the first page in the packet of photocopied ClassNotes.

This 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.
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 (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 been found. 



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




Living stromatolites are found in a few locations today.  The picture above is from Coral Bay Australia, located on the western tip of the continent.  The picture was probably taken at low tide, the stromatolites would normally be covered with ocean water.

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 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 (Point 3).  A couple of small polished pieces of banded iron rock (actually "tiger iron")  were passed around class (thanks for returning them).   In addition to the red and black layers, the tiger iron contains yellow layers made of fibers of quartz.   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!



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 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.  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 Monday.  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. 

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants.  We'll cover these 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 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.  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.

We'll spend the rest of class today and the next class or two learning about several of the main air pollutants (listed below).

 Before getting into the details, we had a look at the following statistics.  Air Pollution is a serious health hazard in the US and around the world (we'll mainly discuss outdoors pollution, but indoors air pollution is also a problem).  Click here to download a copy of this handout.



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 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.

The Blacksmith Institute 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.

We started with carbon monoxide.  Some basic information found on p. 7 in the photocopied ClassNotes is shown below.  You'll find additional information at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.


We will be talking about carbon monoxide found both outdoors (where it would rarely reach fatal concentrations) and indoors (where it can easily and rapidly build up to deadly concentrations).

Carbon monoxide is insidious, you can't smell it or see it and it can kill you (Point 1).  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.


Nitric oxide, NO, and sulfur dioxide, SO2, are also primary pollutants.  Ozone is a secondary pollutant (and here we are referring to tropospheric ozone, not stratospheric ozone).  It doesn't come directly from an automobile tailpipe.  It shows up in the atmosphere only after a primary pollutant has undergone a series of reactions.

Point 3 explains that CO is produced by incomplete combustion of fossil fuel (insufficient oxygen).  Complete combustion would produce carbon dioxide, CO2.   Cars and trucks produce much of the CO in the atmosphere in Tucson.

Vehicles must now be fitted with a catalytic converter that will change CO into CO2 (and also NO into N2 and O2 and hydrocarbons into H2O and CO2).  In Pima County vehicles must also pass an emissions test every year and special formulations of gasoline (oxygenated fuels) are used during the winter months to try to reduce CO emissions. 

In the atmosphere CO concentrations peak on winter mornings (Point 4).  The reason for this is that this is when a surface temperature inversion layer is likely to form.  In an inversion layer air temperature actually increases with increasing altitude which is just the opposite of what we are used to. 

We were running out of time at this point and temperature inversions are a reasonably important concept.  I didn't want to rush through that section, so we'll come back to Point 5 on Tuesday.

We have been discussing the composition of air.  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. 
Here are some of the colored gases that I wish I could bring to class (the information in quotes is from the webelements.com webpage)
 Bromine
"It is a heavy, volatile, mobile, dangerous reddish-brown liquid. The red vapour has a strong unpleasant odour and the vapour irritates the eyes and throat. When spilled on the skin it produces painful sores. It is a serious health hazard, and maximum safety precautions should be taken when handling it."  Since I don't really know what maximum safety precautions entail, I probably shouldn't bring any bromine to class.
Chlorine
You shouldn't mix household bleach with another household cleaner because the mixture might release chlorine gas.  Chlorine and mustard gas were used during World War I.
Iodine
Nitrogen Dioxide
"Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure."

I sometimes make nitrogen dioxide in class.  But it isn't a particularly educational demonstration (though it does emphasize the fact that many air pollutants are toxic).  I have a couple much better demonstrations planned for next week, so we'll probably skip the NO2 demonstration this semester.