Tuesday Aug. 28, 2007

We'll finish up air pollutants today.  What we aren't able to cover in class will be added to today's online notes. we will begin the middle portion of Chapter 1 on Thursday.  that deals with the vertical structure of the atmosphere (changes of air pressure, air temperature, and air density with altitude).  Some new reading was assigned.

The remaining Expt. #1 kits have been checked out.  If you are signed up to do Expt. #1 you will now have to wait until students begin to return the materials that they have checked out (some additional materials have also been ordered and should arrive soon).  If you have your materials please start the experiment as soon as you can
so that you can return the materials for someone else to use.

At least one student in class needs a note-taker.  If you feel you take clear, complete, concise notes and are willing to share a copy with a student with a disability, please come see me at the beginning or end of class.  You will be provided with carbonless copy paper to take notes and will be formally recognized through a letter of volunteer service for your porfolio or resume.
Carbon monoxide is the most abundant of the air pollutants we will cover.

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.



Carbon monoxide molecules bond strongly to the hemoglobin molecules in blood and interfere with the transport of oxygen through your body.  CO is a primary pollutant.  That means it goes directly from a source into the air (nitric oxide, NO, and sulfur dioxide, SO2, are also primary pollutants).  CO is emitted directly from an automobile tailpipe into the atmosphere for example

CO is produced by incomplete combustion of fossil fuel.  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 which will change CO into CO2 (and also NO into N2 and O2).  In Pima County vehicles must 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.  See if you can figure out why carbon monoxide is often a problem in cities at high altitude (the answer is found at the bottom of today's online notes)

Carbon monoxide is also a serious hazard indoors.  Because it is odorless, concentrations can build to dangerous levels without you being aware of it.  You can purchase a carbon monoxide alarm that will monitor CO concentrations indoors and warn you when concentrations reach hazardous levels. Indoors CO is produced by gas furnaces and water heaters that are either operating improperly or aren't being adequately vented outdoors.  Many people are killed indoors by carbon monoxide every year.  You can learn more about carbon monoxide hazards and risk prevention at the Consumer Product Safety Commission web page.

In the atmosphere CO concentrations peak on winter mornings.  Surface temperature inversion layers form on long winter night when the ground becomes colder than the air above.  Air in contact with the cold ground cools and 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.

The figure above wasn't shown in class.
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 atmosphere becomes more unstable.  CO emitted into air at the surface mixes with cleaner air above.  The CO concentrations are effectively diluted and don't get as high as they do in the morning.

A portion of a time lapse cloud move was shown at the end of class.  Thunderstorms were developing over the Catalina mountains.  Thunderstorms are a visible indication of unstable atmospheric conditions.  The figure below wasn't shown in class either.

You could see the clouds growing vertically in the movie, evidence of rising air motions.  Falling precipitation also produces a downdraft that contains sinking air.  This downdraft is the source of the strong, often damaging, surface winds that accompany thunderstorms.

Six main pollutants are listed at the top of this page.  Concentrations of some or all of these pollutants are monitored throughout the day 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%.   Current Air Quality Index values for Tucson are available online.

The first graphs shows a typical atmospheric temperature profile near the ground in the winter.  The inversion is the bottom portion of the plot where temperature increases from 47 F to near 60  F with 1000 feet of altitude gain.  The 1000 foot deep layer is a stable layer.

The middle figure shows some of the health effets and symptoms of CO poisoning.  The effect of CO depends on both the concentration and the length of exposure.  The NAAQS values are shown at bottom  of the chart.  Exposure to CO concentrations of these levels shouldn't cause any symptons in a healthy individual.  Concentrations reached 500 ppm in the apartment building near the campus of Virginia Tech.  Several students were discovered unconcious.

The bottom figure shows average monthly AQI values for CO and O3 in Tucson.  CO concentrations (blue curve) tend to peak on winter mornings. 

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 we imagine riding a bicycle from Swan and River Rd up a 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 drops 6o F.  This is a fairly rapid drop 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 decreases 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, common on winter mornings in Tucson (and worth bicycling up the hill on Swan Rd. just to experience on a cool winter morning).   This is a temperature inversion and produces very stable conditions.   If you do find yourself on a bicycle at Swan and Sunrise, check out the very steep portion at the far northern end of Swan.
Next we will turn our attention to 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 Los Angeles type or photochemical smog.

We'll eventually 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 process 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).

Sulfur dioxide was covered briefly in class (we may come back to this on Thursday).

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.

Volcanoes are a natural source of sulfur dioxide.


The Great London smog is still the deadliest air pollution event in history.  Because the atmosphere was stable, SO2 emitted into air at ground level couldn't mix with cleaner air above.  The SO2 concentration was able to build to dangerous levels.  You'll find the Donora, PA, disaster described in more detail on p. 346 in the text.

London type smog which contains sulfur dioxide and is most common during the winter is very different from photochemical or Los Angeles type smog.  Los Angeles type smog contains ozone and is most common in the summer.

Stratospheric ozone forms naturally when UV light splits oxygen molecules (O2) into two oxygen atoms (photodissociation).  The O atoms can then react with unsplit O2 to make O3 ozone. The figure above and the figure below are found on p. 17 in the photocopied classnotes.

One way in which is destroyed naturally are shown in the figure above.  The ozone molecule is destroyed when it absorbs UV light. 

Once you understand how stratospheric ozone is formed you can appreciate why the peak concentrations (the ozone layer) are found not at the bottom or top of the atmosphere but at some level in between (at around 25 km), where there are optimal amounts of oxygen and UV light.


There is lots of UV light above 25 km but not much oxygen.  There is plenty of oxygen below 25 km but not enough UV light.  The optimum amounts of both ingredients are found near 25 km.

Stratospheric ozone, the ozone layer, absorbs much (but not all) of the dangerous high energy ultraviolet light from the sun.  Listed above are some of the serious hazards or problems associated with exposure to ultraviolet light.  This is about as far as we were able to get in class.


The figure above wasn't shown in class.
Human activities add substances to the atmosphere that can potentially reduce ozone concentration in the ozone layer (which would result in increased exposure to UV light at the ground). 

The first set of reactions above involve nitric oxide, NO.  First, NO reacts with O3 to form NO2 and O2 (ordinary molecular oxygen).  Then notice the NO2 reacts with an oxygen atom (which might otherwise react with O2 to make O3) to form NO again and O2. The NO is available again to react with and destroy another ozone molecule.

At one time many countries were considering building fleets of supersonic aircraft that would fly into the stratosphere.  The plans were scrapped partly due to concern that the NO emissions from these aircraft would damage the ozone layer.

Chlorofluorocarbons now probably represent the greatest threat to the ozone layer.  Free chlorine (Cl from a CFC molecule) reacts with ozone to form chlorine monoxide (ClO).  The ClO reacts with an oxygen atom and to O2 and Cl. The free chlorine is available to react again and destroy another ozone molecule.   It is thought that a single Cl atom could destroy 100,000 ozone molecules before being removed from the stratosphere.
Answer to the question found earlier in the notes:
The air in high altitude cities is thinner (less dense) than at lower altitude.  There isn't as much oxygen in a volume of air.  With oxygen in short supply, combustion of fuels will more likely be incomplete and will produce CO rather than carbon dioxide.