In this lecture we will conclude our discussion of air pollutants.  Here's some basic information about sulfur dioxide.

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.  That is why sulfur dioxide was the first pollutant people became aware of.

Volcanoes are a natural source of sulfur dioxide.



The Great London smog is still one of the two or three deadliest air pollution events in history.  Because the atmosphere was stable, SO2 emitted into air at ground level couldn't mix with cleaner air above, and the SO2 concentration was able to build to dangerous levels.  4000 people died during this 4 or 5 day period and as many as 8000 additional people died in the following weeks and months. 

Some of the photographs below come from articles published in 2002 on the 50th anniversary of the event. 
 

from:
http://news.bbc.co.uk/1/hi/uk/2542315.stm

from:
http://news.bbc.co.uk/1/hi/health/2545747.stm

from:
http://news.bbc.co.uk/1/hi/england/2543875.stm

from:
http://www.npr.org/templates/story/story.php?storyId=873954

The sulfur dioxide didn't kill people directly.  Rather the SO2 aggravated an existing condition of some kind and hastened their death.  The SO2 probably also made people susceptible to bacterial infections such as pneumonia.   This link discusses the event and its health effects in more detail.
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.

Some other air pollution disasters also involved high SO2 concentrations.  One of the deadliest events in the US occurred in 1948 in Donora, Pennsylvania.



"This eerie photograph was taken at noon on Oct. 29, 1948 in Donora, PA as deadly smog enveloped the town. 20 people were asphyxiated and more than 7,000 became seriously ill during this horrible event." (from: http://oceanservice.noaa.gov/education/kits/pollution/02history.html)


(from: http://www.eoearth.org/article/Donora,_Pennsylvania)

"When Smoke Ran Like Water," a book about air pollution is among the books that students can check out, read, and report on to fulfill part of the writing requirements for the classroom version of this course (instead of doing an experiment report).  The author, Devra Davis, lived in Donora Pennsylvania at the time of the 1948 air pollution episode.

Sulfur dioxide is one of the pollutants that can react with water in clouds to form acid rain (some of the oxides of nitrogen can react with water to form nitric acid).  The formation and effects of acid rain are discussed below.


Note that clean unpolluted rain has a pH less than 7 and is slightly acidic.  This is because the rain contains dissolved carbon dioxide gas.  We'll see how this happens in a classroom demonstration.  Acid rain is often a problem in regions that are 100s even 1000s of miles from the source of that sulfur dioxide that forms the acid rain.  Acid rain in Scandinavia came from industrialized areas in other parts of Europe.


Some of the problems associated with acid rain.

The last pollutant that we will cover is Particulate Matter (PM) - small solid particles or drops of liquid (but not gas) that remain suspended in the air (sometimes referred to as aerosols).  The designations PM10 and PM25 refer to particles with diameters less than 10 micrometers and 2.5 micrometers, respectively.  A micrometer is one millionth of a meter.  The drawing below might give you some idea of what a 1 micrometer particle would look like (actually it would probably be too small to be seen without magnification).


Particulate matter can be produced naturally (wind blown dust, clouds above volcanic eruptions, smoke from lightning-caused forest and brush fires).  Human activities also produce particulates.

Particles with dimensions of 10 micrometers and less can be inhaled into the lungs (larger particles get caught in the nasal passages).  Inhaled particulates are a health threat.  The particles can cause cancer, damage lung tissue, and aggravate existing repiratory diseases.  The smallest particles can pass through the lungs and get into the blood stream (just as a molecule of oxygen does) and damage other organs in the body.

The figure below identifies some of the parts of the human lung mentioned in the figure above.

 
Crossectional view of the human lungs
from: http://en.wikipedia.org/wiki/Lung

1 - trachea
2 - mainstem bronchus
3 - lobar bronchus
4 - segmental bronchi
5 - bronchiole
6 - alveolar duct
7 - alveolus
from http://en.wikipedia.org/wiki/Image:Illu_quiz_lung05.jpg

Note the PM10 annual National Ambient Air Quality Standard (NAAQS) value of 50 micrograms/meter3 at the bottom of the figure before last.  The following list shows that there are several cities around the world where PM concentrations are 2 or 3 times higher than the NAAQS value.




There was some concern during the most recent summer Olympic Games that the polluted air in Beijing would keep athletes from performing at their peak.  Chinese authorities restricted transportation and industrial activities both before and during the games in an attempt to reduce pollutant concentrations.  Rainy weather during the games may have had the greatest effect, however. 


Clouds and precipitation are the best way of cleaning pollutants from the air.
Before beginning this section, you should familiarize yourself with the concept of light scattering by looking at the Light Scattering Activity.

The second main concern with particulates is the effect they may have on visibility (esthetics in the figure below should actually be spelled aesthetics - i.e. qualities that might make something appear beautiful or not).

Here's a photograph of the Catalina mountains just north of town taken in early January year from the Gould Simpson Building on campus.


Some rainy weather had occurred just a day to two earlier and the visibility was very good.

Windy weather a few days later stirred up a lot of dust that was carried into town. 




This photo was taken the day after the windy weather.  There is still a lot of dust in the air and the visibility is pretty bad.



Next we we will try to understand how very small particulates can have such a big effect on visibility.  We need to first learn a little bit more about scattering (we're really going to beat this concept to death). 

You can find all kinds of things in the sky: air, particulates, clouds, etc.  But first let's imagine there isn't an atmosphere.  No air, clouds, particulates, nothing.

If you went outside and looked at the sun (you shouldn't do that of course) you'd see a bright sun against a black background.  You'd see the sun because you're looking back in the direction of one of the rays of light coming from the sun. 

If you look away from the sun and toward the sky you wouldn't see anything.  The sky would appear black.  That's because there's nothing to scatter the sunlight.  If you remember the light scattering demonstration with the laser this is just like when you couldn't see the laser beam as it traveled across the front of the classroom.  You couldn't see the beam unless something was put into the beam to scatter some of the laser light.

In the next picture we'll add an atmosphere.  Just air molecules, no particles, or clouds.


Air molecules scatter light.  We didn't see this light in the laser demonstration because the laser light scattered by the air was too weak.  But when you're dealing with intense sunlight traveling through a lot more air in the atmosphere you can see the scattered light.

The incoming sunlight is white.  White light is a mixture of all the colors.  Air molecules scatter the shorter wavelengths (violet blue green) more than the longer wavelengths (yellow orange red).  This is depicted above.  Air molecules scatter light in this way because they are very small (much smaller than the wavelength of visible light).

Violet has the shortest wavelength and is scattered the most.  However there isn't as much violet in sunlight as there is blue and green.  There's a lot of green light in sunlight (more than any other color as a matter of fact) but it isn't scattered as readily as blue.  So the end result is that we see blue light coming from the sky.  This is why the sky is blue.  When the air is clean (from of particulates), the sky has a deep blue color.  Here's a little more detailed explanation of why the sky appears blue.

Next we'll add a cloud to the picture.  As we saw in the laser demonstration, cloud droplets and ice crystals are good scatters of light.  Cloud droplets and ice crystals though are much larger than air molecules.  Because of this they scatter all the colors in equal amounts.


When white light strikes a cloud, white light is scattered and reflected.  This is why clouds are white (with some shades of grey mixed in if the cloud is thick).

What about particles?  Particulates are much bigger than air molecules and a little bit smaller than cloud droplets.  They scatter light is the same way that cloud droplets and ice crystals do.  The scattered light from particles is white.

What do you see now when you look at the sky.  It depends on how much particulate matter is in the air.  When the air is clean and doesn't contain much particulate matter the sky is a deep blue.  As the concentration of particulates increases you mix in more and more white light.  The color of the sky can change to a whitish blue when the particulate concentration is high.

OK now let's look at how the appearance of some nearby mountains might change as more and more particles are added to the air.  We're going to try to understand why increasing amounts of particles can reduce visibility.




In this first picture we start out with clean air.  When we look at a mountain we see the light that is reflected off the soil and trees on the mountain (shown at left above).  I've colored this reflected light green and brown.  When you look at the mountain it's green and brown (right figure above).




Now we'll add some particles.  When you look at the mountain you see brown and green light plus some white light that is coming from sunlight being scattered by the particles.  Some white specks of light have been superimposed on the view of the mountain at right.

More particles, more scattered light, and more white light being mixed in with the brown and green reflected light.



Even more particles.  Now the white light from scattering from particles begings to dominate.  Eventually it becomes difficult to even make out the mountain because of all the scattered light.  Light from the mountain also runs into particles on its way toward your eyes and gets redirected so that you don't see it.  Of course there was considerable artistic license used in this explanation.

One last thing.

You might think that when the air is clean visibility would be unlimited.  That isn't the case.  Scattering of sunlight by air molecules alone puts a limit on visibility.  The following figure tries to explain why this is so.


The nearby mountain appears green and brown.  You are mostly seeing sunlight reflected off the mountain.

As the mountain gets further away you start seeing increasing amounts of blue light (sunlight scattered by air molecules in between you and the mountain) being added to the brown and green reflected light.  This is because there is more air between you and the mountain.  The mountain at medium range now appears brown, green, and blue.    As the mountain gets even further away the amount of this blue light from the sky increases.  The most distant mountain in the picture above is now blue.  Eventually the mountain gets so far away that you only see blue light from the sky and none of the light reflected by the mountain itself.  The mountain has faded from view.

Here's a photograph of the Blue Mountains in Australia (source of this image)



If you look closely I think you can see 5 mountain ranges in this picture (labeled in order of increasing distance in the picture above)  Notice how they became fainter and fainter and lighter and lighter blue.  It is becoming hard to distinquish mountain range 5 from the blue color of the sky.

And with that were finished with this first module on the composition of the atmosphere and air pollutants.  Thanks for your patience and for persevering to the end.