Wednesday Sep. 1, 2010
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Two or three songs ("La Vida Tombola" and "Me Gustas Tu" are the two I remember) from Manu Chao before class today.

The Practice Quiz is one week from today.  A Study Guide is now available online. 

Hurricane Earl, now a Category 4 hurricane (on a scale that goes to 5), is forecast to brush the East Coast this week and weekend.  You can find the latest news and predicted storm path at the National Hurricane Center website.

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 on p. 12 in the photocopied Class Notes.


Acid rain is often a problem in regions that are 100s even 1000s of miles from the source of the sulfur dioxide that forms the acid rain.  Acid rain in Canada could come from sources in the US, acid rain in Scandinavia came from industrialized areas in other parts of Europe. 

Note at the bottom of the figure above that clean unpolluted rain has a pH less than 7 and is slightly acidic.  This is because the rain contains dissolved carbon dioxide gas.  The acid rain demonstration done in class today should make this point clearer.


Some of the problems associated with acid rain.  Click on this acid rain demonstration link for a detailed discussion of the demonstration done in class.

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 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 p. 13c in the photocopied ClassNotes (above).  The following list (p. 13d in the ClassNotes) 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.   We'll see later in the semester that cloud droplets form on small particles in the air called condensation nuclei.  The cloud droplets then form raindrops and fall to the ground carrying the particles with them.

Particulates can affect visibility and can make the sky appear hazy.  To understand this we need to look at how air molecules and particles scatter sunlight.



First try to recall the laser demonstration from a few days ago.  You couldn't see the laser beam as it passed across the front of the classroom.  It was only after putting chalk dust or a cloud of water droplets in the beam that you could see scattered laser light.  The situation outdoors illustrated above and described below is similar.

Rays of sunlight are passing through clean air above.  To see the sunlight you would need to look back toward the sun along one of the rays (direction a) in the figure (this would be like standing next to the spot on the wall during the laser demonstration and looking back at the laser).  You'd see the sun in that case (you shouldn't of course look directly at the sun).  If you look away from the sun and at the sky (direction b) you see blue light.  This is sunlight that is being scattered by air molecules.

Sunlight is white light; white light is a mixture of all the colors.  Because air molecules are small (relative to the wavelength of visible light) they scatter shorter wavelengths more readily than longer wavelengths.  When you look away from the sun and toward the sky you see this scattered light, it has a deep blue color.  This is basically why the sky is blue.  If the earth didn't have an atmosphere (or if air molecules didn't scatter light) the sky would be black.

We've added some particles to the air in the picture below.  Particles also scatter light (remember the chalk dust used in the laser demonstration).  Particles are bigger than air molecules.  When the particle size is about equal to or somewhat greater than the wavelength of visible light the particles scatter all the colors equally.  The light scattered by particles is white.  This is basically why clouds are white.





When you look at the sky now you see blue light from sunlight scattered by air molecules mixed together with some white light that is sunlight being scattered by particles.  The color of the sky changes from deep blue to bluish white.  The higher the particle concentration, the whiter the sky becomes.

Have a look at the color of the sky before and after a rainstorm.  Before the storm, the air will be full of particulate pollution and will appear whitish blue.  After the storm, after the rain has removed a lot of the pollutants, the sky often has a much deeper blue color. 

The next set of figures tries to explain how particles in the air can affect visibility.



The air is free of particles in this first picture.  You're looking at a relatively nearby mountain.  The "side view" at left explains that you are able to see the mountain because it reflects sunlight back toward you (the green trees reflect green light, the rocks and soil reflect brown light).  The picture at right is what you see when you look at the mountain.  We are ignoring any light scattered by air molecules because the mountain is close by.




Now some particulates have been added to the air.  They scatter sunlight, the scattered light is white (it's highlighted in yellow in the picture at left for emphasis).  So now you still see the brown and green reflected light but also some white scattered light.  Some (fairly big) spots of white light have been added to the picture at right. 



More particles have been added to the air.  That means there will be more scattered light.

We'll add even more particles to the air in the next picture.  Because there is more scattered light (more spots of white light in the picture of the mountain) you have a harder time seeing the mountain.



Eventually all you see is the scattered light.  The mountain fades from view.

Here are some analogous situations that might help understand how/why light scattered by particles in the air reduce visibility.

Driving with a dirty windshield at night.  Light from oncoming traffic is scattered by dirt on the wind shield producing glare.  It is hard to see the other car and even harder to see a pedestrian or a bicycle on the side of the road because of all the glare.

Trying to understand a student in the back of the room asking a question if lots of students in the middle and front of the room are talking.  The students voice from the back of the room is "drowned out" by all the noise coming from the rest of the front (note I'm not implying there has been a lot of noise in the classroom, quite the opposite so far this semester)

You might think that when the air is clean that visibility might 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.


A nearby mountain would appear dark green or brown.  You are mostly seeing light reflected off the mountain with just a little light scattered by air molecules.

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 and more air between you and the mountain.  As the mountain gets even further the amount of this blue light from the sky increases.  Eventually the mountain gets so far away that you only see blue sky light and none of the light reflected by the mountain itself.