Tue., Jan. 17, 2006

In the first few minutes of class I reviewed some information on the evolution of the earth's atmosphere.  This material was not covered in class last Thursday but was stuck on the end of the Jan. 12 notes nonetheless.

Signup sheets for the experiments and book report were also circulated through class.  Names will be transferred to the online Report Signup Lists.  Experiment #1 materials were distributed in class.

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We'll spend a couple of class periods covering some of the principal atmospheric pollutants.  We started today with sulfur dioxide.  You'll find sulfur dioxide discussed on pps 11-13 in the packet of photocopied class notes.

Sulfur dioxide is produced by the combustion of sulfur containing fuels such as coal.  Combustion of fuel also produces carbon dioxide and carbon monoxide.  People probably first became aware of sulfur dioxide because it has an unpleasant smell.  Carbon dioxide and carbon monoxide are odorless.

Sulfur dioxide concentrations lower than the National Ambient Air Quality Standards listed above (established by the Environmental Protection Agency) should not present a health risk.  Note however that levels of 1 ppm (part per million, one SO₂ molecule mixed in with one million air molecules) does present a health risk to certain people.  SO₂ concentration did exceed this value in some of the air pollution events listed in the next figure.



The Great London smog is still the deadliest air pollution event in history.  A stable air layer next to the ground can't mix with cleaner air above. 



Acid rain often falls hundreds or thousands of miles away from the source of the SO₂.  Coal fired factories and electric power plants in the Ohio River Valley could produce acid rain in New England and Canada.  Acid rain in Scandinavia could be the result of SO₂ emissions in England and Belgium.

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An acid rain demonstration was performed in the last 15 minutes of class to give you a general idea of how acid rain is produced.  Carbon dioxide rather than SO₂ was bubbled through Tucson tap water.  The tap water is initially slightly basic (pH > 7).  Dissolved CO₂ however turned the tap water acidic.

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Small drops of sulfuric acid that formed in the stratosphere following the Mt. Pinatubo eruption reflected incoming sunlight.  With less sunlight arriving at the ground this lowered average temperatures at the ground slightly for a period of a few years.

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The acid rain demonstration involved carbon dioxide.  Carbon dioxide is an important greenhouse gas.  There is worldwide concern about increasing atmospheric concentrations of CO₂ and other greenhouse gases.  The following figure is found on page 1 in the class notes.



We'll cover the greenhouse effect in detail in Chapter 2.  The natural greenhouse effect raises the overall average surface temperature on the earth.  The average annual global average would be about 0^o F without greenhouse gases in the atmosphere.  With greenhouse gases the average is a much more pleasant 60^o F.  The natural greenhouse effect is beneficial

The concentration of CO₂ (and other greenhouse gases) is increasing.  The Keeling curve shown above (and in Fig. 1.3 in the text) clearly shows this.  The concentration has increased from about 315 ppm in 1958 when the measurements were started to about 370 ppm at present.

There is concern that increasing greenhouse gas concentrations may strengthen or enhance the greenhouse effect and increase the global average surface temperature.  This could have a variety of consequences that we will examine later. 

We will first look at what is causing atmospheric CO₂ concentrations to increase.  Before we do that we need to see how CO₂ is added to and removed from the atmosphere.



Natural processes such as respiration and decay add CO₂ to the atmosphere.  Volcanoes are an additional natural source.  Combustion and deforestation are human activities that add CO₂ to the air.

Photosynthesis removes CO₂ from the air and is the main source of atmospheric oxygen.  We saw how easily CO₂ gas dissolved in water in the acid rain demonstration.  CO₂ is removed from the atmosphere when it dissolves in the oceans. 

Knowing something about the sources and sinks of atmospheric CO₂ we can  explain the wavy appearance in the Keeling curve.  It takes one year to complete one cycle.

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The carbon cycle shows the rates at which carbon (mostly in the form of CO2) is added to an removed from the atmosphere.  It also shows various places where carbon is stored (underlined numbers).

The atmosphere contains only about 700 units of carbon (mostly CO2 but also some CH4, methane).  The deep oceans contain 38,000 units of carbon, 50 million units are stored in sedimentary rock.  There are 7500 units of carbon in the form of fossil fuels waiting to be dug up. 

Natural processes such as respiration and decay add 113 units of carbon to the atmosphere every year.  113 units are removed by photosynthesis.  The oceans add and remove 90 units of carbon per year.  Notice the natural processes are in balance - they would not change the atmospheric CO2 concentration.

Activities of man such as burning fossil fuels and deforestation add a total of 6 to 7 units (5 + 1 or 2 units) of CO2 to the atmosphere every year.  There is fairly small compared to the added by natural processes, however the manmade contributions are not balanced by equal rates of removal.  About one-half of what man adds every year is removed.  Exactly how this is done is not known.  It is the imbalance that is causing atmospheric CO2 to increase.

There are 7500 units of carbon in the form of fossil fuels that will probably be burned in the next 100 years or so.  This is 7500 units of carbon that will be added to the atmosphere.  You can see that this could have a big effect on atmospheric CO2 concentration.  There is a lot of research being done to try to figure out how the atmospheric concentration will change, and also how changing atmospheric concentration of CO2 (and other greenhouse gases) will change climate.