Wednesday Sep. 3, 2008
click here to download
these notes in a more printer friendly Microsoft WORD format.
The Experiment #1 materials
were handed
out before class today. A few more sets of materials may be
available in class on Friday in case you weren't able to pick up your
materials today. You can read a little bit more about Expt. #1 here.
The Story, a song by Brandi
Carlile song was played before class while the experiment materials
were being handed out.
Last Friday we finished the first of the air pollutants that
we will be covering in class. 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.)
Next we turn our attention to another air pollutant, 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.
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. 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.
Some of the problems or consequences of acid rain. Here are
the key points that go with sulfur dioxide
Click here for a sneak peek at the acid
rain demonstration that we will
perform in class on Friday.
