Tuesday Sept. 2, 2008
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The Experiment #1 materials were handed
out before class today. A few more sets of materials will be
brought to class on Thursday in case you weren't able to pick up your
materials today. You can read a little bit more about Expt. #1 here.
Three selections from Vampire
Weekend's 2008 CD were played before
class while experiment materials were being distributed.
Incidentally, Vampire Weekend will be appearing in Tucson at
The Rialto on Sept. 23.
Some new reading in Chapter
14 has
been assigned. We will spend most of this week covering this
material in class, so you don't have to necessarily do the reading
right away. I recommend that you also read through the online
class notes once they appear on the class website.
We learned a little bit about carbon monoxide last Thursday. The
information on p. 9 in the photocopied ClassNotes was not covered in
class and was added to the Thu., Aug. 28 online notes.
Here are some of the key things you should remember about carbon
monoxide. (We will also list main points for ozone, sulfur
dioxide, and particulate matter as we cover them. I suggested you
put this
summary on a single sheet of paper and stick that page in front of all
your
notes on air pollutants, where it can act as a sort of table of
contents.)
I have a bad habit of "beating certain topics to death." The
concept of stable and unstable atmospheric conditions and temperature
inversions is an example. The following figure is from p. 10 in
the photocopied ClassNotes and was redrawn after class for improved
clarity.
This rather
busy and confusing picture just illustrates how small changes in how
air temperature changes with increasing altitude can determine whether
the atmosphere will be stable or unstable. Just for
the
purposes of illustration imagine riding a bicycle north from Swan and
River Rd up the hill to Swan and Sunrise (fhe figure shows an elevation
change of 1000 ft, it is actually quite a bit less than that)
At far left the air temperature goes from 47o F to 41o
F, a drop of 6o
F. This is a
fairly
rapid rate of decrease with increasing altitude and would make the
atmosphere
absolutely unstable. The atmosphere wouldn't remain this
way. Air at the ground would rise, air above would sink, and the
temperature profile would change. In some ways it would be like
trying to pour vinegar on top of oil in a glass. The lower
density oil would rise because it would "want" to float on top of the
higher density vinegar.
The next picture shows air temperature decreasing a little more slowly
with increasing altitude. This small change makes the atmosphere
conditionally unstable (we won't go into what the conditions might
be). The
atmosphere is frequently in this state.
The atmosphere cools only 2o F in the next picture.
This creates
an absolutely stable atmosphere. Air at the ground will remain at
the ground and won't rise and mix with air higher up. Compare
this with the glass containing vinegar and a layer of oil on top.
The two layers won't mix.
Air temperature in the last figure actually increases with increasing
altitude. This is a temperature inversion and is very common on
winter mornings. If you ever find yourself heading north on Swan
Rd. early in the morning you will pass through some pretty cold air as
you cross the Rillito River. By the time you get to Sunrise, the
air can be 10 degrees warmer and will seem balmy compared to the cold
air at the bottom of the hill. If you're up for a real challenge
continue north on Swan past Skyline. You'll find a short but very
steep section of road at the far north end of Swan. We will be
making a visible smog cloud in class later today. You will be
able to see the smog cloud because the smog cloud
droplets scatter light. We took a little detour at this point to
learn exactly what is meant by scattering of light.
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 instructor would have been able to see the beam if he
had
walked to the far 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). The students in the class weren't
able to see the beam because they were looking at it from the
side. To be able to see the beam, you need to look back along a
beam of laser light.
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 blackboard 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 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
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 numerous little water
droplets 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).
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 were able to see light being scattered by air before we
put chalk particles or cloud droplets into the beam.
The following wasn't mentioned in
class, it is a topic that we will come back to later in the
semester. 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.
Now we can turn our attention to ozone.
Ozone has a Dr. Jekyll and Mr. Hyde personality. Ozone in
the stratosphere is beneficial, it absorbs dangerous high
energy ultraviolet light (which would otherwise reach the ground and
cause skin cancer, cataracts, and many other problems).
Ozone in the troposphere is bad, it is a pollutant.
That is the stuff we will be concerned with today. Tropospheric
ozone is also a key component of photochemical smog (also known as Los
Angeles-type smog)
We'll be making some photochemical smog as a
class
demonstration. This will require ozone (and a hydrocarbon of some
kind). We'll use the simple stratospheric recipe for making
ozone in the demonstration rather than the more complex tropospheric
process.
At the top of this figure you see that a more complex
series
of
reactions is responsible for the production of tropospheric
ozone. The production of tropospheric
ozone begins with nitric
oxide
(NO). NO is produced when nitrogen and oxygen are heated (in an
automobile engine for example) and react. The NO can then react
with oxygen to make nitrogen dioxide, the poisonous brown-colored
gas we made in class last week. Sunlight can dissociate (split)
the nitrogen dioxide
molecule producing atomic oxygen (O) and NO. O and O2
react (just
as they do in the stratosphere) to make ozone (O3).
Because ozone
does not come directly from an automobile tailpipe or factory chimney,
but only shows up after a series of reactions, it is a secondary
pollutant. Nitric oxide would be the primary pollutant in
this example.
NO is produced early in the day (during the morning rush hour).
The concentration of NO2
peaks
somewhat later. Peak ozone concentrations are usually found in
the afternoon. Ozone concentrations are also usually higher in
the summer than in the winter. This is because sunlight plays a
role in ozone production and summer sunlight is more intense than
winter sunlight.
As shown in the figure below,
invisible ozone can react with a hydrocarbon of some kind which is also
invisible to make a
product
gas. This product gas sometimes condenses to make a visible smog
cloud or haze.
The class demonstration of photochemical smog is summarized
below (a flash was used instead of the aquarium shown on the bottom of
p. 16 in the photocopied class notes). We begin by using the UV
lamp to fill the flask with
ozone. Then a few pieces of fresh lemon peel were added to the
flask. A whitish cloud quickly became visible (colored brown in
the figure below).
Here are
some key points concerning tropospheric ozone.
We were
able to start the section on sulfur dioxide, another of the air
pollutants we will cover. Here's some basic information from p.
11 in the photocopied ClassNotes.
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. 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 weeks and months following the December event.
Some
of the photographs below come from articles published in 2002 on the
50th anniversary of the event.
The sulfur dioxide didn't
kill people directly. 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
"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
(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 on p. 12 in the photocopied Class Notes. We will
come back to this topic in class on Thursday.
