Monday Aug.27, 2007

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.
We'll finish the section on carbon monoxide today.  Last week we learned that carbon monoxide is a primary pollutant produced by incomplete combustion.  Peak CO concentrations are observed on winter mornings.


Six main pollutants are listed at the top of this page.  Concentrations of some or all of these pollutants are measured 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%.   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 unconscious when found by rescue personnel.

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 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, 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 section 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 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).

On Wednesday we will look at stratospheric ozone in a little more detail and at anthropogenic destruction of stratospheric ozone (thinning of the ozone layer).  We'll briefly learn about sulfur dioxide and London-type smog.  Then we will begin the middle portion of Chapter 1 that deals with the vertical structure of the atmosphere (changes of air pressure, air temperature, and air density with altitude).  Some new reading will be assigned on Wednesday.