Friday Aug. 24, 2012
click here to download today's notes in a more printer friendly format

A couple of songs from Johnny Hollow before class today.  "Stone Throwers" was followed by "People Are Strange".

About 40 sets of Experiment #1 materials were handed before class today.  Those of you that were lucky enough to get one of the sets of materials will find you name on this online list.  This weekend would be a perfect time to start the experiment.  It's a good idea to check the experiment fairly frequently early in the experiment.  It slows down somewhat as it progresses.

Once you've completed the experiment and return the materials you'll receive a supplementary information handout that will help with the analysis portion of the experiment.  You should try to return the materials well before the report due date.  This will make materials available to students on the waiting list for Expt. #1.

If you signed up for Experiment #1 but didn't get any materials you should find your name on the waiting list.  I'll bring in a few more sets of materials on Monday and additional sets after that as they are returned.  Don't worry, you'll be given the same amount of time as everyone else to perform the experiment.

We spend the first few minutes finishing up the following figure

(you'll find the complete discussion near the end of Wednesday's online notes)

Then we got started on air pollutants, a topic we'll spend most of next week on.
At the top of the list below are the 5 most abundant gases in the atmosphere.  Several more important trace gases were added to the bottom of the figure.  Trace gases are gases found in low concentrations (and often the concentrations vary with time and location).  Low concentrations doesn't mean they aren't important, however.

Carbon monoxide, nitric oxide, nitrogen dioxide, ozone, and sulfur dioxide are some of the major air pollutants.  We'll cover 3 of these in more detail next week.

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 and learn more about how the greenhouse effect actually works later in the course.

Ozone has sort of a Dr. Jeckyl and Mr. Hyde personality
(i)  Ozone in the stratosphere (a layer of the atmosphere between about 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.  It was only after ozone started to buildup in the atmosphere that life could move from the oceans onto land.  Chlorofluorocarbons are of concern in the atmosphere because they destroy stratospheric ozone.

(ii)  In the troposphere (the bottom 10 kilometers or so of the atmosphere) ozone is a pollutant and is one of the main ingredients in photochemical smog.

(iii)  Ozone is also a greenhouse gas.

Air Pollution is a serious health hazard in the US and around the globe  (click here to download a copy of the statistics shown below).  The lists below give some idea of how serious a threat it is.

The top list shows the external or environmental agent that causes death.  The second list is the internal body function that ultimately leads to your demise.  Keep in mind that many of these numbers are difficult to measure and some may contain a great deal of uncertainty.  The row that is highlighted, toxic agents, contains estimates of deaths caused by indoor and outdoor air pollution, water pollution, and exposure to materials such as asbestos and lead both in the home and at the work place.  It is estimated that 60% of the deaths are due to exposure to particulate matter, something that we will examine in a little more detail late next week.

Air pollution is a serious hazard worldwide.  Interestingly indoor air pollution is, in many places, a more serious threat than outdoor air pollution.  I'm not sure how the researchers determine that 150,000 people are killed by climate change every year.

The Blacksmith Institute (mentioned above) listed the Top 10 polluted places in the world in a 2007 report.  The report has received a lot of worldwide attention.  If you go to this address (click on 2007 at the top left edge of the page) you can view the report online or download and print a copy of the report.  This is just in case you are interested (click on some of the other years also if you do go to the site).

The two photographs below (not shown in class) show residents of Linfen, China, which was, in 2006 was considered (by hte Blacksmith Institute) to be the most polluted city on earth.

The Photograph at left comes from an article in The Guardian.  The photograph at right from an article from The Blacksmith Institute.  Pollutants of concern included: Fly-ash, carbon monoxide, nitrogen oxides, PM-2.5, PM-10, sulfur dioxide, volatile organic compounds, arsenic, and lead (PM stands for particulate matter).

We had time to just start a section on carbon monoxide.  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.

This is about as far as we got in the discussion of carbon monoxide, we'll come back to it next Monday.  But I've added a little more information below about temperature inversions.  Then you'll find the light scattering demonstration that was done in class at the end of today's notes.

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

In the left figure above, notice how temperature increases from 40 F to 50 F in the thin air layer next to the ground (it then decreases with altitude above that).  This is the stable inversion layer.  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.

In the afternoon, the ground warms, and the atmosphere becomes more unstable.  Temperatures decrease with increasing altitude in the right figure above.  CO emitted into air at the surface mixes with cleaner air above.  The CO concentrations are effectively diluted.

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

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 next week.  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 one side of the classroom to the

Looking down on the situation in the figure above.  Neither the students or the instructor could see the beam of light.  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 did a very good job of scattering laser light.  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. 

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.