Tuesday Jan 22, 2008

As promised, experiment report signup sheets were passed around in class.  The sheets will be back on Thursday if you didn't get a chance to signup today.  The experiment #1 materials were also distributed.  If you checked out some materials in class your name should be on this list.  You can also click here to learn more about Expt. #1.

Last Thursday, students were given the opportunity to decide which of four topics should be covered first in NATS 101 this semester.  Here are the somewhat surprising results of the survey (I would have predicted CO2 & climate change or surface weather maps would have been the first choice).
air pollutants
     50%    
CO2 & climate change
 39%
pressure
 8%
surface weather maps
 3%

Now that we know which topic we will be covering first, here is a new reading assignment
We listed the 5 most abundant gases in the atmosphere in class on Wednesday.  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.

The air the past few days in Tucson has been a little colder than average and also very dry.  Dewpoint temperatures have been in the teens and single digits.  There is very little water vapor in the air in Tucson at the present time, that is why it was put in 4th place on the list.

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 more next week and learn more about how the greenhouse effect actually works when we get to Chapter 2. 

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants.  We'll cover carbon monoxide today and talk about sulfur dioxide and ozone next week.

Be careful with ozone:
(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 of the atmosphere) ozone is a pollutant and is one of the main ingredients in photochemical smog.

Some basic information about carbon monoxide is shown below (p. 7 in the photocopied Class Notes).   You'll find additional information at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.

Once inhaled, carbon monoxide molecules bond strongly to the hemoglobin molecules in blood and interferes with the transport of oxygen throughout your body.

CO is a primary pollutant.  That means it goes directly from a source into the air,  CO is emitted directly from an automobile tailpipe into the atmosphere for example. This is illustrated in the figure below.





Nitric oxide, NO, and sulfur dioxide, SO2, are also primary pollutants.  Ozone is a secondary pollutant.  It shows up in the atmosphere only after a primary pollutant has undergone a series of reactions.

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.  Vehicles must now be fitted with a catalytic converter that will change CO into CO2 (and also NO into N2 and O2).  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.  Surface temperature inversion layers form on long winter nights when the sky is clear and winds are calm.  The ground cools quickly and becomes colder than the air above.  Air in contact with the cold ground ends up colder than air above.  Air temperature increases with increasing altitude in a temperature inversion and this produces a very stable layer of air at ground level.  A typical wintertime temperature profile in Tucson is shown at the top of p. 9 in the photocopied Classnotes.

The inversion extends from Point A at the ground to Point B about 1000 feet above the ground.  Temperature increases from 47o F at the ground to about 60o F at 1000 feet altitude.

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




When CO is emitted into a thin stable layer (left figure above), 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.  CO emitted into air at the surface mixes with cleaner air above.  The CO concentrations are effectively diluted.


Thunderstorms occur when the atmosphere is unstable.  Strong up and down air motions are found in thunderstorms.  The downdraft can sometimes produce damaging, surface winds.

Six main air pollutants are listed at the top of this page.  Concentrations of some or all of these pollutants are monitored daily in many cities.  The atmospheric concentration of lead has decreased significantly since the introduction of unleaded gasoline.  PM stands for particulate matter.  These small particles are invisible, remain suspended in the air, and may be made of harmful materials.

CO, O3 and particulate matter are the pollutants of most concern in Tucson and pollutant concentrations are reported in the newspaper or on television using the Air Quality Index (formerly the pollutant standards index).  This is basically the measured value divided by the allowed value multiplied by 100%.  For carbon monoxide concentrations up to 35 ppm (parts per million) for a 1 hour period and 9 ppm for an 8 hour period are allowed.   Current Air Quality Index values for Tucson are available online.

Yearly changes in the AQI values for ozone and carbon monoxide in 1993 are plotted at the bottom of p.9 in the photocopied Classnotes.
So are we have been talking about carbon monoxide found in the atmosphere.  Carbon monoxide is also a serious hazard indoors where is can build to much higher levels than would ever be found outdoors.  You may remember having heard about an incident earlier near the beginning of the school year last August.  Carbon monoxide from a malfunctioning hot water heater sickened 23 Virginia Tech students in an apartment complex.  The CO concentration is thought to have reached 400 ppm.  You can get an idea of what kinds of health effects concentrations this high could cause from the figure below (from p. 9 in the photocopied Classnotes)

Follow the 400 ppm line (shaded orange) from left to right.  At exposure times less than 1 hour you should experience no symptoms.  Beginning at 1 hour you might experience headache, fatique, and dizziness.  Exposures of a few hours will produce throbbing headache, nausea, convulsions, and collapse.  The 400 ppm trace level approaches the level where CO would cause coma and death.  At Virginia Tech several students were found unconscious and one or two had stopped breathing.

Carbon monoxide alarms are relatively inexpensive and readily available at any hardware store.  They will monitor CO concentrations indoors and warn you when concentrations reach hazardous levels. Indoors CO can be produced by gas furnaces and water heaters (and other things) that are either operating improperly or aren't being adequately vented outdoors.  A few hundred people are killed indoors by carbon monoxide every year in the United States.  You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.

The following figure wasn't covered in class

This rather busy and confusing picture just illustrates how small changes in how air temperature changes with increasing altitude can determine whether the atmosphere will be stable or unstable.   Just for the purposes of illustration imagine riding a bicycle north from Swan and River Rd up the hill to Swan and Sunrise (fhe figure shows an elevation change of 1000 ft, it is actually quite a bit less than that)

At far left the air temperature goes from 47o F to 41o F, a drop of 6o F.  This is a fairly rapid rate of decrease with increasing altitude and would make the atmosphere absolutely unstable.  The atmosphere wouldn't remain this way.  Air at the ground would rise, air above would sink, and the temperature profile would change.  In some ways it would be like trying to pour vinegar on top of oil in a glass.  The lower density oil would rise because it would "want" to float on top of the higher density vinegar.

The next picture shows air temperature decreasing a little more slowly with increasing altitude.  This small change makes the atmosphere conditionally unstable (we won't go into the conditions).  The atmosphere is frequently in this state. 

The atmosphere cools only 2o F in the next picture.  This creates an absolutely stable atmosphere.  Air at the ground will remain at the ground and won't rise and mix with air higher up.  Compare this with the glass containing vinegar and a layer of oil on top.  The two layers won't mix.

Air temperature in the last figure actually increases with increasing altitude.  This is a temperature inversion and is very common on winter mornings.  If you ever find yourself heading north on Swan Rd. early in the morning you will pass through some pretty cold air as you cross the Rillito River.  By the time you get to Sunrise, the air can be 10 degrees warmer and will seem balmy compared to the cold air at the bottom of the hill.  If you're up for a real challenge continue north on Swan past Skyline.  You'll find a short but very steep section of road at the far north end of Swan.

Next we turned our attention to another of the main air pollutants - ozone.  

Ozone has a Dr. Jekyll and Mr. Hyde personality.

Ozone in the stratosphere is beneficial, it absorbs dangerous high energy ultraviolet light (which would otherwise reach the ground and cause skin cancer, cataracts, and many other problems).

Ozone in the troposphere is bad, it is a pollutant.  Tropospheric ozone is also a key component of photochemical smog (also known as Los Angeles-type smog)

We'll be making some photochemical smog as a class demonstration.  This will require ozone (and a hydrocarbon of some kind).  We'll use the simple stratospheric recipe for making ozone in the demonstration rather than the more complex tropospheric process.


At the top of this figure you see that a more complex series of reactions is responsible for the production of tropospheric ozone.  The production of tropospheric ozone begins with nitric oxide (NO).  NO is produced when nitrogen and oxygen are heated (in an automobile engine for example) and react.  The NO can then react with oxygen to make nitrogen dioxide, a poisonous brown-colored gas.  Sunlight can dissociate (split) the nitrogen dioxide molecule producing atomic oxygen (O) and NO.  O and O2 react (just as they do in the stratosphere) to make ozone (O3).  Because ozone does not come directly from an automobile tailpipe or factory chimney, but only shows up after a series of reactions, it is a secondary pollutant.   Nitric oxide would be the primary pollutant in this example.

NO is produced early in the day (during the morning rush hour).  The concentration of NO2 peaks somewhat later.  Peak ozone concentrations are usually found in the afternoon.  Ozone concentrations are also usually higher in the summer than in the winter.  This is because sunlight plays a role in ozone production and summer sunlight is more intense than winter sunlight.

As shown in the figure below, invisible ozone can react with a hydrocarbon of some kind which is also invisible to make a product gas.  This product gas sometimes condenses to make a visible smog cloud or haze.

The class demonstration of photochemical smog is summarized below (a flash was used instead of the aquarium shown on the bottom of p. 16 in the photocopied class notes).  We begin by using the UV lamp to fill the flask with ozone.  Then a few pieces of fresh lemon peel were added to the flask.  A whitish cloud quickly became visible (colored brown in the figure below).