Friday Jan. 22, 2010
click here to download today's notes in
a more printer friendly format
Today's musical selection: about the best version of Stand By Me that
I've ever heard.
All of the names on the Experiment, Scientific Paper, and Book Report Signup Lists are now
online.
Today:
Tropospheric ozone and
photochemical smog (aka Los Angeles-type smog) with a demonstration
and probably a little bit about
natural production and destruction of stratospheric ozone
Sulfur dioxide and London-type smog
We'll finish up air pollution next Monday
Before discussing tropospheric ozone, which is
a pollutant, a quick reminder that there is both "good" and "bad"
ozone.
Ozone has a Dr. Jekyll and Mr. Hyde
personality. Ozone
in
the stratosphere (the ozone layer) 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 first be concerned with today.
Tropospheric
ozone is 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 (4-step process shown below).
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 in air 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. Sunlight can dissociate (split)
the nitrogen dioxide
molecule producing atomic oxygen (O) and NO. O and O2
react in a 4th step 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 cloud is composed of very small droplets or
solid particles. They're too small to be seen but they are able
to scatter light - that's why you can see the cloud.
The class demonstration of
photochemical smog is summarized
below (a flask 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 create and 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).
Next we
moved on to the 2nd air pollutant that we will be discussing -
sulfur dioxide. Here's some basic information from the left hand
of 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 following weeks and months.
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.
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 will see how this happens in a class
demonstration next Monday.
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.
There was a litle time left in the period so we
covered a little information about stratospheric ozone and the ozone
layer (found
on pps 17-18 in the photocopied ClassNotes).
The
top
two
equations
show
how ozone is produced in the stratosphere.
Ultraviolet (UV) light splits an O2 molecule into two O atoms.
One of these reacts with O2 to make O3 (ozone).
Ozone is destroyed when it absorbes UV light and is split into O and O2
(the two pieces move away from each other and don't recombine to make
ozone). O3 is also destroyed when it reacts with an oxygen
atom. Two atoms of oxygen reacting to make O2 reduce the amount
of one of the raw materials needed to make O3 and thereby reduce the
concentration of ozone in the ozone layer.
The bottom figure attempts to show that the ozone concentration in the
stratosphere will change until the rates of production and destruction
balance each other (analogous to your bank account not changing when
the amount of money deposition and withdrawn are equal).
Knowing that you need O2 and UV light to make ozone, you can begin
to understand why the ozone layer is found not too high, not too low,
but rather in the middle of the atmosphere.
There is plenty of UV light high in the atmosphere but not much
oxygen (air gets thinner at higher and higher altitude). Near the
ground there is plenty of oxygen but not as much UV light (it is
absorbed by gases above the ground). You find the optimal amounts
of UV light and oxygen somewhere in between, near 25 km altitude.
This next figure lists some of the problems associated with
exposure to UV light. Thinning of the ozone layer will result in
increased amounts of UV light reaching the ground. This wasn't
discussed in class.
Skin cancer and cataracts are probably the best known hazards
associated with UV light. At some point in the next week or two
we'll look at how man may be damaging the ozone layer by introducing
chemical compounds into the atmosphere that react with and destroy
stratospheric ozone.
