Tue., Jan. 24 notes

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

Some gypsy jazz (Caravan at the Django Reinhardt "Live at Birdland" Festival in New York in 2004) to get things rolling this week.

Something new in class today, an Optional In-class Assignment (you work on the assignment and turn it in at the end of class). If you weren't in class and would like to do the assignment and earn a little extra credit, you can download the assignment and turn it in at the start of class on Thursday.

Also the first 1S1P Assignment is now online. You can write 0, 1, or 2 reports (I would encourage you to do at least one report). They are due on or before Tue., Feb. 7. See the assignment page and/or the Writing Requirements handout for more information.

We'll finish up our coverage of air pollutants today starting with sulfur dioxide, which is considered to be the 1st air pollutant that people became aware of. The following information is on p. 11 in the photocopied ClassNotes.

Sulfur dioxide is produced by the combustion of sulfur-containing fuels such as coal. The combustion also produces carbon dioxide and carbon monoxide. People probably became aware of sulfur dioxide because it has an unpleasant and irritating smell. Carbon dioxide and carbon monoxide are odorless.

Volcanoes are a natural source of sulfur dioxide.

Sulfur dioxide has been involved in some of the world's worst air pollution disasters. If not the deadliest, The Great London Smog of 1952 is in the top two or three. Because the atmosphere was stable, SO₂ emitted into air at ground level couldn't mix with cleaner air above. The SO₂ concentration was able to build to dangerous levels. 4000 people died during this 4 or 5 day period. As many as 8000 additional people died in the following weeks and months. A report prepared by NPR (National Public Radio in the US) states that visibility at one point during the smog fell to 1 foot!

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
from: http://news.bbc.co.uk/2/hi/uk_news/england/2545759.stm	from: http://www.martinfrost.ws/htmlfiles/great_smog.html

The sulfur dioxide didn't kill people directly. Rather it would aggravate an existing condition of some kind. The SO₂ probably also made people susceptible to bacterial infections such as pneumonia. Here's a link that discusses the event and its health effects in more detail.

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." The photograph below shows some of the mills that were operating in Donora at the time. The factories were not only emitted pollutants into the air but probably also discharging pollutants into the river.
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 you can check out, read, and report on to fulfill part of the writing requirements in this class (though I would encourage you to do an experiment instead). 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 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 natural "pristine" 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 described below and done in class should make this point clearer.

Some of the problems associated with acid rain.

Click on this acid rain demonstration link for a detailed description 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 (particulates are 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 (µm) is one millionth of a meter (10^-6 m). 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). You'll find some actual pictures and more information at this source. Red blood cells are 6-10 µm in diameter. A nanometer (nm) is 1000 times smaller than a micrometer (10^-9 m). An atom is apparently 0.1 to 0.3 nm across, depending on the particular element.

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. Gases sometimes react in the atmosphere to make small drops or particles (this is what happened in the photochemical smog demonstration). Just the smallest, weakest gust of wind is enough to keep particles this small suspended in the atmosphere.

One of the main concerns with particulate pollution is that the small particles might be a health hazard ( a health advisory was issued on Sunday because of the dusty conditions in Tucson)

Particles with dimensions of 10 µm and less can be inhaled into the lungs (larger particles get caught in the nasal passages). These inhaled particles may be poisonous, might cause cancer, damage lung tissue, or 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/cubic meter 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 summer 2008 Olympic Games in Beijing that the polluted air would keep athletes from performing at their peak. Chinese authorities restricted transportation and industrial activities before and during the games in an attempt to reduce pollutant concentrations. Rainy weather during the games may have done the greatest amount of good.

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.

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

This could be seen last weekend.

Here's a photograph of the Catalina mountains taken just before noon on Sunday from the Gould Simpson Building on campus.

This visibility Sunday morning was about as bad as I can remember. Apparently it had been windy west of Tucson on Saturday. This wind stirred up a lot of dust that was then carried into town.

Here's the same view taken earlier on Tuesday earlier in the week and after some rainy weather on Monday. The air on Tuesday was much clearner than it was on Sunday and the visibility on Tuesday was much better.

Now we will try to understand how particulates affect visibility (beating a concept to death again). We need to first learn a little bit more about scattering. You can find all kinds of things in the sky: air, particulates, clouds, etc. Let's first assume there isn't anything in the sky, not even air.

The picture above shows rays of sunlight streaming in from the upper left. Sunlight is white light which means it is a mixture of all the colors. I'll be using yellow to represent white light in this and the following figures. What would you see if you looked back along one of the rays of sunlight (the direction of the green arrow above). You'd be looking straight at the sun and would see the sun (of course you shouldn't do that)

What would you see if you looked away from the sun. If there really weren't anything in the sky (no clouds, no air, no particulates, nothing) there'd be nothing to scatter sunlight and the sky would appear black.

Now we'll put a cloud in the sky. The small water droplets or ice crystals in a cloud scatter and reflect sunlight. All of the colors in the beam of sunlight are scattered equally, so the scattered light is white. That's why clouds are (usually) white.

Air molecules also scatter light, but in a very different way. Because they are so small (much smaller than the wavelength of visible light) they scatter the shorter wavelengths (violet, blue, green) in greater amounts that the longer wavelengths (red, orange, yellow).

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. A deep blue in this case. The response of our eyes also plays a role. Here's a little more explanation of why the sky appears blue.

What happens when you add particles to the air?

Particles are relatively large (compared to air molecules) which means they scatter all of the colors in sunlight in equal amounts just like cloud droplets and ice crystals do. The scattered light from particles is white.

So the color of the sky can change from deep blue (clean air + few particles) to whitish blue (air molecules + more particles).

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 (remember white light is shown in yellow in these pictures). 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 (I'm also making the images a little smaller each time for added effect).

Here are a couple of 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 and extraneous light.

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 also 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)

One last thing not really covered in class (because I started to sense a hint of restlessness coming from the room).

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.

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. 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.

This finishes the section on the composition of the atmosphere and air pollutants. On Thursday we'll be moving into a completely different topic. You can get an idea of what that will be by navigating over to the Upcoming Topics page.