Tuesday Aug. 28, 2012
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Arrived in class a little early (after having gone to vote in today's primary election) with lots of demonstration materials, a few extra sets of Expt. #1 materials, and 3 songs from Brandi Carlile ("That Year", "I Will", and "Touching the Ground").

The 1st of the 1S1P Assignments was announced in class today.  You can do 0,1, or 2 reports.  What you should be trying to do is earn 45 1S1P pts by the end of the semester.  There will be future assignments, so you don't have to do any reports this time.  But I would suggest you write at least one report just so you can get a feel for how the reports are graded.

All of the names on the various experiment signup sheets should now be online.  You can check by clicking on the Report Signup Lists link.

We started by looking quickly at some information that was stuck onto the end of the Thursday Aug. 23 notes concerning some of the causes of death both in the US and around the world.  This was just to get an idea of how serious a threat air pollution is.

Today and on Thursday we'll be looking at four main air pollutants, carbon monoxide, ozone,  sulfur dioxide, and particulate matter.  They all have a unique "personality".  One way of distinquishing between these pollutants is make a table and to list the main characteristics or properties of each pollutant.



We'll cover carbon monoxide and ozone today and you'll find this same chart at the end of today's notes with the 1st two columns filled in.

You've probably heard about carbon monoxide, we'll start with that.  You'll find additional information on carbon monoxide and other air pollutants at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.

We will mostly be talking about carbon monoxide found outdoors, where it would rarely reach fatal concentrations.  CO is a serious hazard indoors also where it can (and does) build up to deadly concentrations.  (several people were almost killed in Tucson in December 2010)

Carbon monoxide is insidious, you can't smell it or see it and it can kill you (Point 1).  Once inhaled, carbon monoxide molecules bond strongly to the hemoglobin molecules in blood and interfere with the transport of oxygen throughout your body.  The article above mentions that the CO poisoning victims were put inside a hyperbaric (high pressure) chamber filled with pure oxygen.  This must force oxygen into the blood and displace the carbon monoxide.

CO is a primary pollutant (Point 2 above).  That means it goes directly from a source into the air,  CO is emitted directly from an automobile tailpipe into the atmosphere for example. The difference between primary and secondary pollutants is probably explained best in a series of pictures.




In addition to carbon monoxide, nitric oxide (NO) and sulfur dioxide (SO2), are also primary pollutants.  They all go directly from a source (automobile tailpipe or factory chimney) into the atmosphere.  Ozone is a secondary pollutant (and here we are referring to tropospheric ozone, not stratospheric ozone).  It doesn't come directly from an automobile tailpipe.  It shows up in the atmosphere only after a primary pollutant has undergone a series of reactions.



Point 3 explains that 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 in Tucson.

Vehicles must now be fitted with a catalytic converter that will change CO into CO2 (and also NO into N2 and O2 and hydrocarbons into H2O and CO2).  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 (Point 4).  The reason for this is surface radiation inversion layers.  They are most likely to form on cold winter mornings.

In an inversion layer (Point 5) air temperature actually increases with increasing altitude which is just the opposite of what we are used to.  This produces stable atmospheric conditions which means there is little up or down air motion.  Air at the surface can't mix with cleaner air above.

During the night in the winter, the ground cools more quickly than the air above.  Air in contact with the ground cools and can end up warmer than the air just above.  This kind of occurrence is shown in the left figure above.  The ground has a temperature of 40F.  The air warms to 50 F above the ground then starts cooling again.  The stable inversion layer is where air temperature increases with increasing altitude.  When CO is emitted into the thin stable layer, the CO remains in the layer and doesn't mix with cleaner air above.  CO concentrations build.

By afternoon the ground and air in contact with the ground has warmed.  Temperature decreases with increasing altitude above the ground, the inverison layer is gone.  Pollutants are emitted into a much larger volume of air and the concentration doesn't get as high.


Thunderstorms contain strong up (updraft) and down (downdraft) air motions.  Thunderstorms are a sure indication of unstable atmospheric conditions. 

We have a little more information to cover about carbon monoxide but we'll postpone it until Thursday.
You are able to see a lot of things in the atmosphere (clouds, fog, haze, even the blue sky) because of scattering of light.  I'm going to try to make a cloud of smog in class later todeay.  The individual droplets making up the smog cloud are too small to be seen by the naked eye.  But you will be able to see that they're there because the droplets scatter light.  So we took some time for a demonstration that tried to show you exactly what light scattering is.

In the first part of the demonstration a narrow beam of intense red laser light was directed from the middle of the classroom toward the wall
 
Looking down on the situation in the figure above.  Neither the students or the instructor could see the beam of light.  To be able to see something rays of light must travel from the object straight toward you.  Nobody could see the beam because there weren't any rays of light pointing from the laser beam toward the students or toward the instructor.




The instructor would have been able to see the beam if he had stood at the end of the beam of laser light and looked back along the beam of light toward the laser.  That wouldn't have been a smart thing to do, though, because the beam was strong enough to possibly damage his eyes (there's a warning on the side of the laser). 

Everybody was able to see a bright red spot where the laser beam struck the wall.
 



This is because when the intense beam of laser light hits the wall it is scattered (splattered is a more descriptive term).  The original beam is broken up into a myriad of weaker rays of light that are sent out in all directions.  There is a ray of light sent in the direction of every student in the class.  They see the light because they are looking back in the direction the ray came from.  It is safe to  look at this light because the original intense beam is split up into many much weaker beams.

Next we clapped some erasers together so that some small particles of chalk dust fell into the laser beam.



Now instead of a single spot on the wall, students saws lots of points of light coming from different positions along a straight segment of the laser beam.  Each of these points of light was a particle of chalk, and each piece of chalk dust was intercepting laser light and sending light out in all directions.  Each student saw a ray of light coming from each of the chalk particles.

We use chalk because it is white, it will scatter rather than absorb visible light.  What would you have seen if black particles of soot had been dropped into the laser beam?

In the last part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water.  The cloud droplets are much smaller than the chalk particles but are much more numerous.  They make very good scatterers.



 
The beam of laser light really lit up as it passed through the small patches of cloud.  The cloud droplets are small but there are many of them.  So much light was scattered that the spot on the wall fluctuated in intensity (the spot dimmed when lots of light was being scattered, and brightened when not as much light was scattered).  Here's a photo I took back in my office.


The laser beam is visible in the left 2/3 rds of the picture because it is passing through cloud and light is being scattered toward the camera.  There wasn't any cloud on the right 1/3rd of the picture so you can't see the laser beam over near Point 1.

There's something else going on in this picture also.  We're not just seeing the narrow beam of laser light but some of the cloud outside the laser beam is also visible.

Up to this point we've just considered single scattering.  A beam of light encounters a cloud droplet or a particle of chalk and gets redirected and then travels all the way to your eye or to a camera.  That's what's happening at Point 2.  You just see the narrow laser beam.  But sometimes the scattered ray of light runs into something else and gets scattered again.  This is called multiple scattering.  And that is what is illuminating the cloud alongside the beam of laser light at Point 3.  Light is first scattered by a cloud droplet in the beam.  As it leaves the beam it runs into another droplet and gets scattered again.  So now it looks like it is coming from the cloud surrounding the laser beam rather than from the beam itself.



Here's a comment that wasn't mentioned in class  Air molecules are able to scatter light too, just like cloud droplets.  Air molecules are much smaller than cloud droplets and don't scatter much light.  That's why you couldn't see the laser beam as it was traveling from one side of the classroom to the other through the air.  Outdoors we are able to see sunlight scattered by air molecules.  This is true for a couple of reasons.  The sunlight is much stronger than the laser beam and its shining through a lot more air.  That means there is more scattered light.
Sunlight is white light which means it's made up of a mixture of violet, blue, green, yellow, orange, and red light.  Air molecules have an unusual property: they scatter the shorter wavelengths (violet, blue, green) much more readily than the longer wavelength colors in sunlight (yellow, orange, and red).  When you look away from the sun and look at the sky, the blue color that you see are the shorter wavelengths in sunlight that are being scattered by air molecules.

You shouldn't look directly at the sun.  Direct sunlight is too intense just as was true with the laser.  But it is OK to look at the blue sky.  That's scattered sunlight and is much weaker than direct sunlight and safe to look at.

We'll come back to this concept of scattering of light in the next couple of lectures.


Now back to air pollutants - ozone.
Ozone has a kind of Dr. Jekyll and Mr Hyde personality.


The figure above can be found on p. 14a in the photocopied ClassNotes.  The ozone layer (ozone in the stratosphere) is beneficial, it absorbs dangerous high energy ultraviolet light (which would otherwise reach the ground and cause skin cancer, cataracts, etc.  There are some types of UV light that would quite simply kill us).

Ozone in the troposphere is bad, it is toxic and 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 in a class demonstration.  To do this we'll first need some ozone; we'll make use of the simple stratospheric recipe (shown above) for making what we need instead of the more complex tropospheric process (the 4-step process in the figure below).  You'll find more details a little further down in the notes.



At the top of this figure (p. 15 in the packet of ClassNotes) 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 in air are heated (in an automobile engine for example) and react.  The NO can then react with oxygen in the air to make nitrogen dioxide, the poisonous brown-colored gas that I've been thinking about making in class.  Sunlight can dissociate (split) the nitrogen dioxide molecule producing atomic oxygen (O) and NO.  O and O2 react in a 4th step to make ozone (O3) just like happens in the stratosphere.  Because ozone does not come directly from an automobile tailpipe or factory chimney, but only shows up after a series of reactions in the air, it is a secondary pollutant.   Nitric oxide (NO) 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.  Because sunlight is needed in step #3 and because sunlight is usually most intense at noon, the highest ozone concentrations are usually found in the afternoon.  Ozone concentrations are also usually higher in the summer when the sunlight is most intense.


Once ozone is formed, the ozone can react with a hydrocarbon of some kind to make a product gas.  The ozone, hydrocarbon, and product gas are all invisible, but the product gas sometimes condenses to make a visible smog cloud or haze.  The cloud is composed of very small droplets or solid particles.  They're too small to be seen but they are able to scatter light - that's why you can see the cloud.

Here's a pictorial summary of the photochemical smog demonstration.

We started by putting a small "mercury vapor" lamp inside a flash.  The bulb produces a lot of ultraviolet light (the bulb produced a dim bluish light that we could see, but the UV light is invisible so we had no way of really telling how bright it was).  The UV light and oxygen in the air produced a lot of ozone (you could easily have smelled it if you had taken the cover off the flask).

After a few minutes we turned off the lamp and put a few pieces of lemon peel into the flash.  Part of the smell that comes from lemon peel is limonene, a hydrocarbon.  The limonene gas reacted with the ozone to produce a product gas of some kind.  The product gas condensed, producing a visible smog cloud (the cloud was white, not brown as shown above).  I meant (but forgot)  to shine the laser beam through the smog cloud to reinforce the idea that we are seeing the cloud because the drops or particles scatter light.

Here are the main points that we covered so far, for carbon monoxide (CO) and ozone.  We'll cover sulfur dioxide and particulate matter later in the week and finish the table.