Friday Jan. 13, 2006

In the first few minutes of class I reviewed the information on dew point temperature and evolution of the earth's atmosphere.  This material was not covered in class on Wednesday but was stuck on the end of the Jan. 11 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.

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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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The following information was not covered in class.



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 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 and 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.



CO₂ conentration peaks in the winter when plants are dormant and are not removing as much CO₂ from the atmosphere during photosynthesis.  In the summer when plants are growing the concentration of CO₂ concentration decreases slighly.