Today
Tropospheric ozone and Los Angeles - type smog
Sulfur dioxide and London - type smog
Today we
turned our attention to another of the main air pollutants -
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.
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, a poisonous brown-colored
gas. 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).
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's
some basic information on sulfur dioxide, the last of the 3 main
pollutants that we will cover
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.
Volcanoes are a natural source of sulfur dioxide.
The Great
London smog is still the deadliest air pollution
event 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. The 1948 Donora Pennsylvania event is described
on p.
346 in the textbook.
"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
A limited number of students might prefer to read a book rather than
performing an experiment. "When Smoke Ran Like Water," a book
about air pollution has been added to the list of books. 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.
Click here for a discussion of the acid rain demonstration that we will
perform in class on Friday.
