Friday Aug. 27, 2010
click here to download today's notes in a more printer friendly format

Three songs from Patty Griffin ("Stay on the Ride", "Getting Ready", and "You'll Remember") from her Children Running Through CD

The Experiment #1 materials were handed out in class today.  You can read a little bit more about the experiment here.  Don't wait until the last moment to do this experiment.  When you have finished collecting your data, return your materials, and pick up the supplementary information sheet.

You can follow the movement of Hurricane Danielle (off the east coast) and Tropical Storm Frank (off the coast of Baja California) at the National Hurricane Center website.

You can find the latest Tucson weather forecast at the Tucson office of the National Weather Service.


We'll spend most of the class today and a class or two next week learning about several of the main air pollutants (listed below).

 Before getting into the details, we had a look at the following statistics.  Air Pollution is a serious health hazard in the US and around the world (we'll mainly discuss outdoors pollution, but indoors air pollution is also a problem).  Click here to download a copy of this handout.



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 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.

The Blacksmith Institute 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.


We started with carbon monoxide.  Some basic information found on p. 7 in the photocopied ClassNotes is shown below.  You'll find additional information at the Pima County Department of Environmental Quality website and also at the US Environmental Protection Agency website.


We will be talking about carbon monoxide found both outdoors (where it would rarely reach fatal concentrations) and indoors (where it can easily and rapidly build up to deadly concentrations).

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.

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.






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 that this is when a surface temperature inversion layer is likely to form. 

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.

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



When CO is emitted into the 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 contain strong up (updraft) and down (downdraft) air motions.  Thunderstorms are a sure indication of unstable atmospheric conditions.  When the downdraft winds hit the ground they spread out horizontally.  These surface winds can sometimes reach 100 MPH, stronger than many tornadoes. 


The concentrations of several of the main pollutants are monitored in large cities in the US and around the world.  Six pollutants are listed below (p. 8 in the photocopied ClassNotes).  In Tucson, carbon monoxide, ozone, and particulate matter are of primary concern and daily measurements are reported in the city newspaper.  The Air Quality Index value is reported instead of the actual concentration.  The AQI is the ratio of the measured to accepted concentrations multiplied by 100%.  Air becomes unhealthy when the AQI value exceeds 100%.



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. We'll talk about them in a little more detail next week.

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. 

Here are a couple of example calculations:

If the observed CO concentration were 4.5 ppm averaged over an 8 hour period the AQI would be
AQI = 100% x (4.5ppm / 9ppm) = 50%

and the air quality would be considered good.

What would the measured CO 8 hr average concentration be for an AQI value of 33%?



Current Air Quality Index values for Tucson are available online.


We'll come back to carbon monoxide briefly next Monday.

On Monday this week you were able to see a cloud form when moist air came into contact with liquid nitrogen.  If everything goes according to plan you'll see a cloud of photochemical smog in a demonstration next Monday.  In both cases, the droplets making up the clouds are probably too small to be seen by the naked eye.  You are able to see the clouds because the cloud droplets scatter light.  We spent the last 15 minutes or so of class with a demonstration that will try to show you exactly what light scattering is.

In the first part of the demonstration a narrow beam of intense red laser light was shined from one side of the classroom to the other. 



The students couldn't see the laser beam because the light rays weren't pointing straight at them.  The instructor would have been able to see the beam if he had walked to the wall and looked back along the beam of light (that wouldn't have been a smart thing to do because the beam is strong enough to damage his eyes). 

Students were 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).  Weaker rays of light 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 rays are weaker than the initial beam.

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 in a straight line along 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 made very good scatterers.




The laser light really lit up and turned the small patches of cloud red. The cloud 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).

A comment that may not have been mentioned in class (if it was mentioned it certainly wasn't emphasized).  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 weren't able to see light being scattered by air before we put chalk particles or cloud droplets into the beam.  Outdoors you are able to see sunlight (much more intense than the laser beam used in the class demonstration) scattered by air molecules.  Sunlight is white and is made up 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.

We'll come back to the topic of light scattering next week. when we cover particulate matter and its effect on visibility.