Friday August 26, 2011
click here to download a copy of today's notes in a more printer friendly format

Iron and Wine provided the background music while Experiment #1 materials were distributed before class today ("The Devil Never Sleeps", "Jezebel" and "Freedom Hangs Like Heaven").

Coming next Monday TA office hours and (hopefully) the first optional homework assignment of the semester.

Some information about haboobs (dust storms) and the thunderstorms that cause them during the first portion of class today.  This was in response to a student question after class last Wednesday.  You'll find this material in the middle of the Wednesday Aug. 24 lecture notes.  The section includes two time lapse videos of a pretty spectacular haboob that moved through the Pheonix area earlier this summer (July 5).

Next we finished up the short section that we started on Wednesday concerning the origin and evolution of the atmosphere.  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.

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, located on the western tip of 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.

Here are a couple of pictures of the 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 (Point 3).  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!  And thanks for returning them by the way.

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).

Now we're ready to start a section on air pollutants.  We'll spend the rest of today, Monday and Wednesday next week on this topic.

We listed the 5 most abundant gases in the atmosphere at the beginning of class.  Several more important trace gases were added to the list in class.  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 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.

(iii)  Ozone is also a greenhouse gas.

I like lists.  Here are lists of the major causes of death in the US and worldwide.
Air Pollution is a serious health hazard in the US and around the world.  Click here to download a copy of the statistics shown below.



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.  I'm not sure how the researchers determine that 150,000 people are killed by climate change every year.

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 had time to start a section on carbon monoxide.  We'll finish this next Monday.  You'll find additional information on carbon monoxide and other air pollutants at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.

We will mostly be talking about carbon monoxide found outdoors, where it would rarely reach fatal concentrations.  Indoors is a serious hazard indoors also where it can (and does) build up to deadly concentrations.  ( several people were almost killed in Tucson last December)

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.  The article above mentions that the CO poisoning victims were put inside a hyperbaric (high pressure) chamber filled with pure oxygen.  This must force oxygen into the blood and displace the carbon monoxide.

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.





In addition to carbon monoxide, nitric oxide (NO) and sulfur dioxide (SO2), are also primary pollutants.  They all go directly from a source (automobile tailpipe or factory chimney) into the atmosphere.  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 surface radiation inversion layers.  They are most likely to form on cold winter mornings.

In an inversion layer (Point 5) air temperature actually increases with increasing altitude which is just the opposite of what we are used to.  This produces stable atmospheric conditions which means there is little up or down air motion.

 There is very little vertical mixing in a stable air layer.



In the left figure above, notice how temperature increases from 40 F to 50 F in the thin air layer next to the ground (it then decreases with altitude above that).  This is the stable inversion layer.  When CO is emitted into the thin stable layer, the CO remains in the layer and doesn't mix with cleaner air above.  CO concentrations build.

In the afternoon, the ground warms, and the atmosphere becomes more unstable.  Temperatures decrease with increasing altitude in the right figure above.  CO emitted into air at the surface mixes with cleaner air above.  The CO concentrations are effectively diluted.


Thunderstorms contain strong up (updraft) and down (downdraft) air motions.  Thunderstorms are a sure indication of unstable atmospheric conditions.